petevaluationofthedopaminesystemofthe humanbrainjnm.snmjournals.org/content/37/7/1242.full.pdf ·...

in three neuronal groups: the retrobulbar, the substantia nigra(SN) and the ventral tegmental area (VTA). Dopamine neuronsfrom the SN project predominantly to the dorsal striatum andare mainly concerned with initiation and execution of movements (9). Those from the VTA project predominantly to limbicand limbic-connected regions including nucleus accumbens,orbital and cingulate cortices, amygdala and hippocampus andare involved with reinforcement, motivation, mood and thoughtorganization (1, 10â€”13). Dopamine neurons from the retrobulbar area project to the hypothalamus where they regulatehormone secretion from the pituitary (14). Dopamine cells inthe SN and in the VTA and their major projections are shownin Figure 1.

Dopamine is synthesized in the dopamine neurons where it isstored within vesicles which protect it from oxidation bymonoamine oxidase (MAO). Dopamine is released into thesynapse in response to an action potential and interacts withpostsynaptic dopamine receptors. The concentration of dopamine in the synapse is regulated primarily by its reuptake by thedopamine transporters, to maintain low (nanomolar) steadystate concentrations (15). Dopamine is also removed by oxidation by MAO A in neurons and by MAO B in glia whichsurround the dopaminergic nerve terminals and by catecholamine 0-methyltransferase (COMT). Brain dopamine release isregulated by autoreceptor as well as by other neuroanatomicallydistinct neurotransmitters through interactions with the dopamine neuron (16).

PET is an imaging method used to track the regionaldistribution and kinetics of chemical compounds labeled withshort-lived positron-emitting isotopes in the living body (1 7). Itwas the first technology that enabled direct measurement ofcomponents of the dopamine system in the living human brain.Dopamine was labeled with Iâ€˜C25 yr ago for the purpose ofimaging catecholamine metabolism in peripheral organs such asthe adrenals and the heart (18). Because dopamine does notcross the blood-brain barrier, however, imaging studies ofdopamine in the living brain have been indirect, relying on thedevelopment of radiotracers to label dopamine receptors, dopamine transporters, precursors of dopamine or chemical compounds which have specificity for the enzymes which degradesynaptic dopamine (Fig. 2). Table 1 gives the brain concentration and activity of the dopamine-related elements which havebeen measured with PET. Additionally, through the use oftracers that provide information on regional brain activity (i.e.,brain glucose metabolism and cerebral blood flow) and ofappropriate pharmacological interventions, it has been possibleto assess the functional consequences of changes in braindopamine activity.

Dopamine plays a pivotal role in the regulation and control ofmovement, motivation and cognition. It also is closely linked toreward, reinforcement and addiction. Abnormalities in brain dopamine are associated with many neurological and psychiatric disorders including Parkinson's disease, schizophrenia and substanceabuse. This close association between dopamine and neurologicaland psychiatric diseases and wfth substance abuse make ft animportant topic in research in the neurosciences and an importantmolecular target in drug development. PET enables the directmeasurement of components of the dopamine system in the livinghuman brain. It relies on radiotracers which label dopamine receptors, dopamine transporters, precursors of dopamine or compounds which have specificity for the enzymes which degradedopamine. Additionally, by using tracers that provide information onregional brain metabolism or blood flow as well as neurochemicallyspecific pharmacological interventions, PET can be used to assessthe functional consequences of changes in brain dopamine activity.PET dopamine measurements have been used to investigate thenormal human brain and its involvement in psychiatric and neurological diseases. It has also been used in psychopharmacologicalresearch to investigate dopamine drugs used in the treatment ofParkinson's disease and of schizophrenia as well as to investigatethe effects of drugs of abuse on the dopamine system. Sincevarious functional and neurochemical parameters can be studied inthe same subject, PET enables investigation of the functional integrity of the dopamine system in the human brain and investigation ofthe interactions of dopamine with other neurotransmitters. Throughthe parallel development of new radiotracers, kinetic models andbetter instruments, PET technology is enabling investigation ofincreasingly more complex aspects of the human brain dopaminesystem. This paper summarizes the different tracers and experimentel strategies developed to evaluate the various elements of thedopamine system in the human brain with PET and their applicationsto clinical research.KeyWords:imaging;neurotransmitters;pharmacology;psychiatricillnesses; neurological illnesses.

J NucIMed1996;37:1242â€”1256

Thedopaminesystemisinvolvedintheregulationofbrainregions that subserve motor, cognitive and motivational behaviors (1â€”3).Disruptions of dopamine function have been implicated in neurological (4) and psychiatric illnesses includingsubstance abuse (5 ), as well as on some of the deficitsassociated with aging ofthe human brain (6). This has made thedopamine system an important topic in research in the neurosciences and neuroimaging as well as an important moleculartarget for drug development (4, 7,8).

Dopamine cells reside predominantly in the mesencephalon

Rec@vod Sept. 12, 1995; revision accepted Oct. 18, 1995.For correspondence or reprints contact: Nora D. volkow, MD, Medic@Department,

Bldg. 490, Brookhaven National Laboratory, Upton, NY 11973.

1242 THE JOURNALOFNUCLEARMEDICINEâ€¢Vol. 37 â€¢No. 7 â€July 1996

PET Evaluation of the Dopamine System of theHuman Brain

Nora D. Volkow, Joanna S. Fowler, S. John Gatley, Jean Logan, Gene-Jack Wang, Yu-Shin Ding and Stephen DeweyMedical and Chemistry Departments, Brookhaven National Laboratory, Upton, New York and Department of Psychiatry,SUNY-Stony Brook, Stony Brook, New York

by on June 30, 2020. For personal use only. jnm.snmjournals.org Downloaded from

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ConcentrationActivityBindingsite or activity (pmole/g)(pmole/g/sec)

TABLE IBinding Site Concentrations and Enzymatic Activities in the

Human Striatum*

ReceptorsDl (82)D2 (238)

TransportersDopaminetransporter(239) 400

EnzymesTyrosinehydroxylase(240)AromaticL-aminoacid decarboxylase

Monoamineoxidase A (240,241)Monoamineox@aseB (240)Catechol-O-methyftransferase(242,243)

â€˜ValuescitedarefairlyrepresentaThieofvalues reported inotherstudies ofpostmortem brains.However,considerablevariationisfound inthe literature,associated withdifferences in methodologiesand in indMdualsubjects. Adetailed review of in vitro studins isfar beyond the scope ofthis article. Sinceenzymaticactivitiesare usuallymeasured withsaturatingsubstrate concentrations,the cited values probab@@do not reflectmetabolicfluxes in vn,o.

t@ermined in rat striatum.1Cornbined MAO A and MAO B activities assayed using tyramine as

substrate.@Determinedin human cerebral cortex.

dopamine receptors which are grouped into two major families:those which stimulate adenyl cyclase (Dl, D5) and those whichinhibit adenyl cyclase (D2, D3, D4) (21 ). The concentrations aswell as the locations of these receptors in human brain differand the most ubiquitous are the Dl (50 pmole/g) and D2receptors (20 pmole/g). The highest concentration for Dl andD2 receptors occurs in striatum (22 ), where D 1 receptorsappear to be predominantly expressed by striatal output neuronsprojecting to the SN; and D2 receptors appear to reside mainlyin output neurons projecting to the globus pallidus (23).Extrastriatal regions have much lower densities (0.3â€”4pmole/g) of D2 (22,24) and Dl receptors (25). D3 receptorsexist in low concentrations (1 pmole/g) in the shell of thenucleus accumbens and in the islands of Calleja (26). Theconcentration of D4 receptors is also very low (2. 1 pmole/gtissue) and they are localized in several limbic and corticalregions with relatively lower levels in striatum (27,28). D5receptors are located in limbic areas (29). PET tracers tomeasure D2 and Dl receptors have been developed; howeverthere are currently no specific PET ligands to differentiallyevaluate D3, D4 and D5 receptors.

The D2 receptors were the first to be imaged with PET(30â€”33).Several D2 receptor antagonists have been labeled foruse with PET (34). These ligands differ with respect to theiraffinity for the D2 receptors, their specificity and their kinetics(Table 2). More recent studies have started to focus on labelingdopamine receptor agonists (34). The most widely used D2PET ligands are the dopamine receptor antagonists [I â€˜C]raclopride (33) and [â€˜1C] or [18FJ-labeled N-methylspiropendol(NMS) (30,31 ). Raclopride is a ligand with moderate affinityfor D2 receptors (Kd = 1000â€”2000 pM) (35). It has a highselectivity in terms of affinity for receptors for other neurotransmitters but also binds to D3 receptors. NMS has a higheraffinity for D2 receptors than raclopride (Kd = 50â€”300pM) butit also binds to 5HT2 as well as D4 receptors (27). Because the

5020

200200

650t

25200

4@Q*

40@

FIGURE 1. Simplifieddiagramfor the dopaminergicprojectionsof thesubstantia nigra(SN)and the ventraltegmental area Ã§VTA).CG = CingulateGyrus, PreF = PrefrOntalCortex, OFC = OrbitofrontalCortex, NAc =NucleusAccumbens, CA = Caudate, PUT = Putamen, AM = Amygdala,Hipp = Hippocampus.

This article summarizes the different tracers and experimental strategies developed to evaluate the various elements of thedopamine system in the human brain with PET and theirapplications in clinical research. Although SPECT methodology can also be used to measure some of the same components(19), this chapter will not explicitly discuss this methodology.

DOPAMINE RECEPTORSDopamine receptors are present both pre- and postsynapti

cally at dopaminergic synapses (Fig. 2). At the postsynapticside they function in cell-to-cell communication, and at thepresynaptic side they modulate the release and synthesis ofdopamine (16). The dopamine receptors have been the maintherapeutic targets in the treatment of psychotic symptoms inschizophrenics and of extrapyramidal motor symptoms inParkinson's disease patients (20). There are five subtypes of

FiGURE2@Simpkfieddiagramof a typicalstriataldopaminesynapse.Inpractice, electron microscopic studies show great structural heterogeneity.There are approximately1011dopamine terminalsper gram of stilatum,outofa totalnumberofabout1012terminalspergram.Theiraveragevolumeisthat of a sphere of radius 0.55 @,and there are about 5000 terminalsperstviatalcell.The spontaneous flnngrate of dopaminecells is about 3 Hz.Thevolumeof each synaptic cleft is about 1017 I,@@ total dopamine cleftvolumeisabout 1 @.dper gram.Theaverage distance between clefts isabout2 @.For convenience, Dl and D2 receptors are shown on the samepostsynaptic cell, whereas they are almost certainly localized on differentcells.

100 _â€”!@.@____.[ i.â€”i: 1_:@::â€”.-_ 200D2 receptors f@ \ Dl receptors

Ic0MT

I Poit@ceIl

PET DOPAMINETRACERSâ€¢Volkow et al. 1243

ICAPreF

: OFC..



RadioligandKiinvitro

(nm)HumST/CBan

data

Time max(mm)RodenST/CBt

data

Time max(mm)DlSCH0.3392023390(244,245)NNC7560.17530-60(244,43)NNC1128-12(34)D2Raclopride1.1420-4075(246,247)NMS0.0512>50010>60(60,168,247)IBZM0.451.66(78,248-250)Epidepride0.06>10>20020120(249)

TABLE 2Radioligandsfor Dopamine Receptors I'1CINNC 112

IIIFiGURE3. Brainimagesobtained

with the Dl receptorradioligandr1c]NNC112(50)inahealthycontrol.Theimagecorrespondsto theaverageactivityobtainedbetween9and68mm(courtesyof Halldinandcollaborators).

The Dl receptors have also been measured with PET thoughthey have been much less investigated. Several Dl receptor

antagonists have been labeled with positron emitters and indude [â€œC]SCH23390 (47), [â€œC]SCH39166 (48), [â€œC]NNC687, [1â€˜C}NNC756 (49) and NNC 112 (50) (Table 2). Aproblem for most of these radioligands is that they also haveconsiderable affinity for 5HT2 receptors (49). [1â€˜CISCH23390has been the most widely used (25). High uptake of theseradioligands occurs in striatum which is the area with thehighest Dl receptor density (25). The concentration of Dlreceptors in cortex is relatively low but is higher than that forD2 receptors (25) and has been measured with PET. Ofthe PETDl radiotracers the one that fives the highest cortical tocerebellar ratios (1 .8â€”2.2)is [â€C]NNC 112 which makes itprobably the best of the Dl radiotracers for extrastriatalimaging (Fig. 3) (34). The dopamine Dl receptor agonist SKF75670 was labeled with â€˜â€˜Cand proposed as a ligand that maybe potentially useful for measuring high-affinity Dl receptorsites (51).

Radioligands for Dl and D2 receptors have been used toinvestigate their involvement in aging, in psychiatric andneurological illnesses and to assess receptor occupancy byantipsychotic drugs. Ofthe brain changes associated with aging,those in the dopamine system are among the most conspicuousand are probably responsible for some of the motor andbehavioral changes in the elderly. PET studies evaluating theeffects of age on D2 receptors have consistently reported asignificant decline in D2 receptors with age. The estimates forD2 losses obtained from PET studies range between 4â€”8% perdecade of life (52â€”55) and are slightly higher than thosereported for postmortem studies which range between 2â€”5%perdecade (56). Dopamine Dl receptors in striatum and frontalcortex have also been found to decrease with age (57).

Increased brain dopamine activity is an important contributorto the symptomatology observed in schizophrenic patients (8).PET studies in schizophrenic patients measuring D2 receptorshave yielded inconsistent results with some studies reportingelevations, others no changes and others documenting elevations in some patients but not in others (41 ). Several reasonshave been given for these discrepancies related to differences inchoice of tracers and subject populations (58), including thesuggestion that the elevations observed with NMS could be dueto increases in D4 receptors in schizophrenic patients (27).Studies in schizophrenic patients have also been done to assessthe relationship between receptor occupancy by antipsychoticdrugs and therapeutic efficacy. For typical neuroleptics it wasestimated that 70â€”80%ofthe D2 receptors need to be occupiedfor therapeutic efficacy and that higher occupancies are asso

dissociation of NMS at the 5HT2 receptors is significantlyfaster than at the D2 receptors, tracer kinetic modeling differentiates these two components of NMS binding (36). Since theratio of D2/(D3 + D4) receptor binding is quite high in mostbrain regions (37,38) the PET measurements from these twotracers predominantly reflect binding to D2 receptors. Also theirbinding, at least for NMS, appears to be predominantly topostsynaptic receptors (39,40). For most PET studies D2receptor measurements have been limited to the striatum whereneurons containing D2 receptors are predominantly GABAergicor cholinergic.

The use of PET to measure D2 receptors in areas other thanstriatum has been limited by the relatively low concentration ofD2 receptors in extrastriatal regions as well as the smallvolumes of some of the areas (41 ). Although D2 receptors inextrastriatal areas have been imaged with PET and SPECT(41â€”44), the limited sensitivity of these instruments do notallow accurate quantification. The development of D2 ligandswith higher affinity for D2 receptors and of PET instrumentswith a higher sensitivity and spatial resolution may enableimproved quantitation of D2 receptors in extrastriatal areas.Some ligands with very high affinity for D2 receptors haverecently been developed that may facilitate these measurements. Imaging of the human brain with one of these highaffinity ligands [â€˜â€˜C]FLB457(Kd = 20 pM), showed significant accumulation in thalamus, substantia nigra, colliculus andtemporal cortex (41,45).

Initially the development of D2 ligands was targeted towardstracers with very high affinity for D2 receptors. However, tracerkinetic modeling of these tracers is limited by tracer deliverywhich makes them sensitive to cerebral blood flow (CBF) inaddition to receptor concentration (46). This is particularlyproblematic for areas of high receptor concentration sincebinding equilibrium is not reached within the time period whenthe measurements can be made. However, these tracers may beuseful to quantify D2 receptors in areas with relatively low D2concentrations as discussed for [â€˜â€˜CJFLB457. On the otherhand, recent interest in the development of tracers with loweraffinities has been stimulated by the feasibility of using them tomeasure relative changes in synaptic dopamine concentration.

1244 THEJOURNALOFNUCLEARMEDICINEâ€¢Vol. 37 â€¢No. 7 â€¢July 1996



I@inHuman

dataRodentdataTimeBmax/TimePeak

ST@maxbKd@CST/maxbRadioligand(nm)(nCVcc/mCi) ST/CBS (mm)(mVg)PIdST/CB@(mm)

Â°Ratstha@ membranes.@â€˜Timeof maximumuptake in striatum.@CaIcUIatedusingtheratioofdistributionvolume(DV)instriatumtothat inreferencetissue,obtainedusinggraphical(209)forcocaineanddtMP;Bmax/Kd

from two-compartment model for nomifensine;calculatedas k@,/k@from three-compartmentmodel for WiN 35,428and R11-55.dAt time of maximal striatal binding; not corrected for plasma protein binding.

Â°Metabolitecorrection based on plasma/erythrocytepartitioning.@Maximumvalueobserved;at endof measurementperiodif fOllOWedby +.@Representatjveradioactivityconcentrationin striatum (nCi/cc/mCiinjected)at peak or at indicatedtime after injection.

@ from MBq (g tissue)-1/MBq (9 body wt)-l.

1@ermedV3, or equilibriumpartition coeffident (251).

TABLE 3Radioligandsfor DopamineTransporters

RTI-55(90,251)1.5130(500')12+>12006.T25010@120WIN35428(10,93)12150(60')4+>1205?460dtMP

(96,252)401002.330-401.625215Nomifensine(84)[email protected]â€”300.9Â°10Â°â€”â€”Cocalne(86,209,253,254)100851.75â€”70.641.55

ciated with side effects (80â€”90%)(59â€”60). In contrast, foratypical neuroleptics, such as clozapine, the average D2 occupancy was significantly lower and ranged between 20â€”67%(61 ). Levels of D2 receptor occupancy by typical neurolepticswere linearly relatedto drugplasmaconcentrations(62,63). ForDl receptors, levels of occupation by typical neurolepticsranged between 16â€”44%and occupation by atypical neuroleptics ranged between 36â€”59%(59). These findings may suggestthat D2 receptors are a main target for typical but not foratypical neuroleptics. Though for the atypical neuroleptics,blockade of Dl receptors is larger than that of D2 receptors,therapeutic efficacy may also involve blockade ofD4 and 5HT2receptors (64). Studies done to determine if the lack oftherapeutic response seen in nonresponding schizophrenic patients was a result of inadequate D2 receptor blockade by theantipsychotic drug showed that there were no differences inreceptor blockade by neuroleptics in responders and nonresponders patients (65â€”66). However, the two groups ofpatients differed in their baseline D2 receptor measures suggesting that there may be underlying biological differences thatmay predict responsiveness to antipsychotic agents (62). Thesefmdings also question the common practice of increasingantipsychotic doses beyond the therapeutic window in nonresponding patients.

The dopamine systemhas been linked with addictive disorders. Imaging studies have shown significant reductions in D2receptor availability in cocaine abusers which persist aftercocaine withdrawal and which were correlated with self ratingsof dysphoria (67,68). Abnormalities in D2 receptor measureshave also been documented in alcoholics (69). No drug abusestudies have been done with PET Dl radioligands.

In psychotic depressed patients increases in striatal D2receptors were recently reported. Because the relation wasbetween D2 and psychosis and not mood this finding wasinterpreted as most likely reflecting the psychotic state ratherthan the mood abnormality (70). Studies with the Dl receptorligand [â€˜â€˜C]SCH23390 in depressed patients showed reductions in the binding potential in frontal cortex but no changes instriatum (71 ). These findings provide further evidence that thedopamine system may be involved in affective disorders(72,73).

Studies with Parkinson's disease patients have shown thatwhile there may be initial increasesduring early stagesof thedisease, for the most part D2 receptor concentrations do notdiffer from those of age-matched controls (74â€”77). BecauseL-DOPA requires the presence of dopamine receptors in orderto exert a therapeutic effect, imaging with D2 receptors hasbeen used to predict responsiveness in patients with movementdisorders (78). In patients with Huntington's chorea, where thepathology is localized in striatal neurons, significant reductionsin striatal D2 (79â€”81) and Dl (82) receptors have beendocumented. Furthermore, because Huntington's disease affectspredominantly medium-sized spiny neurons in striatum, whereDl receptors are located, it has been suggested that Dl ligandsmay be more sensitive than D2 ligands in detecting degeneration in this disease (82). There is also some evidence ofreductions in Dl receptors in the frontal cortex of patients withHuntington's chorea (82).

DOPAMINETRANSPORTERSInterest in the dopamine transporter (DAT) has been stimu

lated, in part, by the fact that it constitutes the main target sitefor the reinforcing properties of cocaine (83). Additionally,because transporters are localized on the presynaptic terminalthey serve as markers of dopamine neurons. Several radioligands have been developed for their suitability as PET andSPECT probes of the DAT. These include [â€˜â€˜C]nomifensine(84â€”85), [â€œC]cocaine(86), [18F] GBR 13119 (87,88), [â€˜8F]GBR 12909 (89), Ã§â€•C]and [â€˜23I]RTI-55(90, 91), [â€œC]WIN35428 (92â€”94), [â€C]methylphenidate (95) and [â€˜â€˜C]d-threomethylphenidate (96). These radioligands differ with respect totheir affinities for the DAT, their speciflc-to-nonspecificbinding ratios and their specificity for the DAT as well as theirkinetics. Table 3 summarizes this information for DAT ligandswhich have been examined in reasonable detail in humansubjects. The cocaine analogs WIN 35,428 (also known asCFT) and RTI-55 (alsoknown as a-CIT), have affinities for theDAT approximately 10 and 100 times higher, respectively, thanthose of cocaine and S-(+)-nomifensine and d-threo-methylphenidate have affinities intermediate between those of cocaineand WIN 35,428. Their times to maximum uptake vary by afactor of over 200, that is between about 6 mm for cocaine, and




over 1200 mm for the high-affinity cocaine analog RTI-55. Theinitial uptake in brain for the five radioligands in Table 3 is highand corresponds to approximately 7â€”10%of the injected dose.

Carbon-i 1-nomifensine was the first DAT ligand developedfor PET (97). Its uptake in brain is relatively fast and peakconcentrations are achieved approximately 20 mm after administration which allows for proper modeling and quantification(84). A potential disadvantage of this ligand is that it bindsmore tightly to the norepinephrine transporter than to the DAT.Carbon-i 1-cocaine exhibits faster kinetics than[1 â€˜C]nomifensine, with a half-life in striatum of approximately

20 mm. Though in vitro studies show binding of cocaine toserotonin as well as norepinephrine transporters, as a PETligand it shows specificity ofbinding to DAT (98). A limitationfor this tracer is that its specific-to-nonspecific binding ratio isrelatively low and that its fast kinetics limit the statisticalquality of PET images. The specific binding of [l â€˜C]methylphenidate and of [â€˜â€˜C]d-threomethylphenidate in the brain ismainly due to DAT. Its clearance is slower than that of[I â€˜C]cocaine which is an advantage for kinetic analysis in that

it is fast enough to allow for proper quantification but is slowenough to permit appropriate counting statistics. A disadvanta@e of racemic [l â€˜CJmethylphenidate when compared with[I â€˜C]d-threo methylphenidate is that the 1 enantiomer in the

racemic mixture contributes to nonspecific binding and confounds its quantification. An advantage for [â€˜â€˜C]d-threomethylphenidate and for [I 1C}methylphenidate is that its pharmacology in humans is well studied so that unlabeledmethylphenidate can be administered to humans to assessnonspecific binding.

RTI-55 has highly desirable properties which are optimizedin the â€˜23I-labeledcompound used with SPECT (i.e., very highaffinity for the DAT and a high specific-to-nonspecific bindingratio) (90). The SPECT measurements with [1231]RTI-55 aremade when the radioligand distribution corresponds to anequilibrium situation, which is thought to be the case after about18 hr. However, PET measurements with [l 1C]RTI-55 (halflife = 20 mm) must be conducted very far from equilibrium(91 ). A danger under these conditions is that tissues with somethreshold concentration of binding sites may trap essentiallyevery molecule of radiotracer which is delivered. This isexpected to diminish the sensitivity of radiotracer binding tosmall decreases in binding site concentration. Furthermore,radiotracer binding will become more sensitive to alterations intissue perfusion, which proportionately alters delivery of radioligand. Carbon-i i-WIN 35,428 also does not achieve a maximum value of striatal binding within the time constraints ofPET experiments; its striatal uptake is still rising after morethan 5 half-lives for Iâ€˜Chave elapsed (1 10 mm) (93). Theextent to which this may compromise its ability to detect smalldecreases in DAT concentration, or its usefulness in circumstances of altered CBF, is presently unclear. PET studies arerequired to determine under what circumstances one DATligand may be more accurate than another. For example, underconditions of decreased DAT availability and/or to examineregions with low DAT densities, ligands with very highaffinities such as [l â€˜C]W[N35428 may be desirable. However,for quantification in patients with decreased CBF, li@ands witha relatively lower affinity for the DAT such as [â€CJd-threomethylphenidate may be more appropriate.

Though most of the radioligands developed for monoaminetransporters have been targeted to the cellular transporters,[1 â€˜C}tetrabenazine has been proposed as a PET ligand to

measure the vesicular monoamine transporter (99). The advantage for this type of ligand is that it does not appear to up- or

Pivldnson's

DopatrineTransporters(Iâ€•CId-threo-MP)

Dopanine02 Receptors( tlIq radopride)

l@ucoseMetabolism(â€˜@DG)

FIGURE4@Brainimages from a mumpletmcer study comparing a patientwithParkinson'sdisease and a control.The imagesfor D2receptors and fordopamine transporters correspond to the distribution volumes of r1C]rackpride and of C1C]d-threomethyiphenidate,respectively.Metabolicimageswere obtained v@th18FD0.

down-regulate in response to treatment and hence may be lesssensitive to the confounding effects of medication (100). Thedisadvantage is a lack of specificity for the different monoamine transporters.

It is to be noted that most studies in humans using DATradioligands have shown age-related decreases in binding(85, 101, 102) which are consistent with the reduction in DATand in dopamine cells documented in postmortem studies(103, 104). In patient populations these radioligands have beenused to assess dopamine cell degeneration in subjects withParkinson disease who show marked reductions in DAT whencompared with healthy age-matched controls (Fig. 4)(85,90,91,94). Studies in cocaine abusers have shown thatwhile there are increases in DAT shortly after withdrawal (105)there are decreases or no changes with protracted withdrawal(106). A preliminary study done in violent alcoholics showedsignificant elevations of DAT when compared with nonalcoholic subjects (107). In contrast, nonviolent alcoholics hadlower DAT levels than controls (107).

DOPAMINE SYNThESISDopamine synthesis occurs within the dopamine neuron as

shown in Figure 5. Tyrosine is transported via amino acidcarriers in the blood-brain barrier and cell membranes. Once inthe intracellular space it is hydroxylated to L-3,4-dihydroxyphenylalanine (L-DOPA) by tyrosine hydroxylase (TH, E. C.1.14.16.2).L-DOPA is then decarboxylatedby aromaticLamino acid decarboxylase (AADC, E. C. 4. 1. i .28) (108) toform dopamine.

TH is the rate-limiting enzyme in the synthesis of braindopamine. PET radiotracers which would enable an assessmentof TH activity are still under development (109,110). The firstPET tracer developed for studies of dopamine metabolism inthe human brain was â€˜8F-labeledfluoro L-DOPA (111,112).Like L-DOPA, 6-['8F]fluoro-L-DOPA crosses the blood-brain

1246 THEJOURNALOFNUCLEARMEDICINEVol. 37 â€¢No. 7 â€¢July 1996

Nomt@d



radioactivity. Recently this tracer has been applied in the studyof dopamine metabolism in patients with unipolar depression(120).

The accumulation of 18F after the injection of 6-[18F]fluoroL-DOPA has been used as a measure of the integrity of thenigrostriatal dopamine system in Parkinson's disease (7) and arecent postmortem study has correlated fluoro-DOPA uptakeconstants with nigrostriatal neuronal density (121 ). The extentto which â€˜8Faccumulation is sensitive to smaller age-relatedlosses in dopamine neurons is controversial with some investigators finding an age-related loss in dopamine neurons iscontroversial with some investigators finding an age-associateddecline and others finding no change in PET studies of normalsubjects (122â€”124). Studies in the postmortem human brainsuggest that AADC may be up-regulated in dopamine neuronsthat are spared during aging and that PET measures of6-[18F]fluoro-L-DOPA uptake may overestimate the number ofdopamine nerve terminals during normal aging (125).

In patients with Parkinson's disease, a correlation has beenfound between 6-['8F]fluoro-L-DOPA uptake in putamen andmotor deficits and between 6-['8F]fluoro-L-DOPA uptake incaudate and memory (delayed recall test) (126). Though moststudies done with 6-['8F]fluoro-L-DOPA measure AADC activity in striatum, concentrations in the amygdala, mesencephalon, hippocampus and thalamus are higher than those in thecerebellum (116).

Other tracers have been developed in an attempt to improvequantitation and to reduce the spectrum of labeled metabolitesfrom 6-[18F]fluoro-L-DOPA. The best characterized is[â€˜8F]fluoro-m-tyrosine which is missing the hydroxyl group oncarbon-4 of the aromatic ring (127, 128). This molecule is asubstrate for AADC but is not a substrate for COMT. Thistracer has been investigated in monkeys and showed lowperipheral metabolism as predicted (118). Initial studies havebeen carried out in humans and it has been proposed that the useof labeled fluoro-m-tyrosine will be a considerable advanceover the use of labeled fluoro-DOPA for the measurement ofAADC activity (129). Another tracer, [â€˜8F}fluoro-@-fluoromethylene-m-tyrosine is also being investigated (130).

DOPAMINE METABOUSMMonoamine oxidase (MAO, EC i.4.3.4) formally oxidizes

amines such as dopamine to the corresponding aldehyde (131).The enzyme exists in two subtypes, MAO A and MAO B (132).Though both forms can oxidize each of the biogenic amines,MAO A preferentially oxidizes 5-hydroxytryptamine whereasMAO B oxidizes benzylamine. Dopamine is a substrate for bothsubtypes of MAO (133), and both subtypes have similaraffinities for dopamine (134). There are selective inhibitors foreach form of the enzyme, the best known of which areclorgyline for MAO A (135) and L-deprenyl for MAO B (136).In the human brain MAO B predominates (B:A = 4: 1) (137).Since MAO B is largely compartmentalized in glial cells, it hasbeen suggested that MAO B inhibition spares extraneuronaldopamine (138). Many studies support a link between increasesin MAO B (but not MAO A) and aging and neurodegenerativedisease (139).

By far the major medical interest in MAO stems from theneuropsychological effects of MAO inhibitor drugs on thehuman brain. Indeed, MAO inhibitors were among the firstantidepressant drugs. The fact that MAO inhibition spares braindopamine led to the use of the MAO B inhibitor L-deprenyl asan adjunct with L-DOPA in the treatment of Parkinson'sdisease (140). More recently it has been shown that L-deprenylslows the natural progression of Parkinson's disease (141).

DOPAMINE SYNTHESIS

Tyrosine L-DOPA Do@

FIGURE5. Pathways for the synthesis and metabolismof dopamine in theCNS.Th, tyroelnehydroxylase(EC1.14.16.2);MDC, aromaticamino aciddecarboxylase (EC 4.1.1.28); MAO,monoamine oxidase (EC i.4.3.4);AD,aldehydedehydrogenase (EC1.2.1.3);COMT,catechol-O-methyltransferase(EC2.1.1.6).

barrier resulting in an accumulation of â€˜8Fin striatum. Thisstriatal accumulation appears to be unidirectional over the first90 mm and probably reflects the synthesis of fluorodopamineand subsequent storage within vesicles (1 13). However, because L-DOPA (and fluoroDOPA) and its metabolites areextensively metabolized by MAO and by COMT, labeledmetabolites, especially 3-O-methyl-6-[18F]fluoroDOPA alsocontribute to striatal â€˜5Factivity as visualized by PET (114).This complicates the estimation of indices which reflect the rateof dopamine synthesis. To minimize the extensive peripheralmetabolism of fluoroDOPA by AADC, which limits traceravailability in brain, peripheral AADC inhibitors have beenused to spare [â€˜8F]fluoroDOPA (115). Unfortunately, AADCblockade enhances COMT-catalyzed formation of peripheral[â€˜8F@3-O-methylfluoroDOPA which enters the brain and contributes to the nonspecific signal.

It has been suggested that the accumulation of 6-['8F]fluoroL-DOPA metabolites in the striatum parallels AADC activityand that this reflects the brain's capacitj for dopamine synthesis(116). A recent PET study using 6-[ 8F]fluoro-L-DOPA employed a kinetic model that took into account labeled metabolites to calculate the relative rates of conversion of6-['8F}fluoro-L-DOPA to 6-['8F]fluorodopamine. The highestvalues for AADC activity were found in caudate and putamenwhich are consistent with in vitro measurements.

The extent to which 6-['8F]fluoro-L-DOPA parallels thebehavior of L-DOPA has been addressed by comparing6-['8F]fluoro-L-DOPA and either [j3-' â€˜C]L-DOPA, [13-'â€˜C}6-fluoro-L-DOPA or [3H]L-DOPA (11 7,118). The behavior of6-['8F]fluoro-L-DOPA qualitatively parallels that of L-DOPAbut its uptake is higher and its metabolism by COMT is faster.The brain kinetics of L-[f3-1â€˜C]DOPAhave been measured inhuman volunteers with PET (119). The use of [@3-'â€˜C}L-DOPAallows PET studies to be carried out at tracer conditions eventhough its use was reported to be limited by large statisticalvariation due to the limited time frame for quantitation of low




4It

4I

FIGURE 6. Influx constant images of human brain MAO B using r1cjH-deprenyl-d2.Notk@ethe highvalues in the subcorticalregionsand lowervalues in cortex inagreement withpostmortemassays.

Though there is controversy concerning the mechanism(s)responsible for the therapeutic actions ofL-deprenyl (142), thisfinding has stimulated interest in the development ofnew MAOinhibitor drugs.

PET studies of MAO have measured its regional distribution,its inhibition and its synthesis rate. Two general approaches tomapping MAO with PET in vivo have been developed, one usesthe labeled suicide inactivators, L-deprenyl and clorgyline, tolabel the two forms of the enzyme (143) and the other useslabeled dimethylphenethylamine to produce â€˜â€˜C-labeled dimethylamine, a MAO B-generated metabolite which is intracellularly trapped (144). The most frequently used approach isthe measurement of MAO B with [â€˜â€˜C]L-deprenyland [â€˜â€˜C]Ldeprenyl-D2 (a deuterium-substituted analog of L-deprenylwith improved sensitivity (145)).

The general approach to the in vivo labeling of MAO B with[1 â€˜C]L-deprenyl is based on the principle of suicide inhibition

in which the radiotracer becomes covalently attached to theenzyme as a result of oxidation and the formation of a veryreactive intermediate. The distribution of â€˜â€˜Cin brain after theinjection of [â€˜â€˜C]L-deprenylclosely parallels the distribution ofMAO B as determined in the postmortem human brain (Fig. 6).Carbon-i i-L-deprenyl has been used to determine the durationof MAO B inhibition by pharmacological doses of L-deprenyl.Since L-deprenyl is a suicide inactivator of MAO B, enzymerecovery after withdrawal from the drug requires the synthesisofa new enzyme. PET measurements showed that the half-timefor recovery of brain MAO B averaged about 40 days (146).Though L-deprenyl is given to patients at a dose of 10 mg/day,it is obvious from the PET study that complete MAO Binhibition can be obtained at a fraction of this dose. A similarinvestigation was carried out to study the duration of MAO Binhibition by the new reversible, MAO B inhibitor drug Ro 196327 (147, 148) which is being proposed for the treatment ofParkinson's disease. This study showed total recovery of theenzyme after 36 hr of Ro 19 6327 discontinuation.

Catechol-O-methyltransferase (COMT; EC 2.1.i.6) is one ofthe two major enzymes which metabolize dopamine (149).COMT catalyzes the transfer of a methyl group from Sadenosyl-L-methionine to the phenolic group of a substrate thatmust have a catechol structure. It is distributed throughout thebody and brain and is an important molecular target in thedevelopment of drugs to treat Parkinson's disease (150). Theproduct of dopamine metabolism by COMT is 3-methoxytyra

FIGURE7. Breln images of the distributionvolume for C1C]raclor,Ã±deobtained with no pharmacologicalinterventionand after administrationofmethyiphenidate.

mine and the concentration of this compound in the brain istaken as an index ofdopamine release (151 ). Although the mainfunction of COMT was described in the late 1950s (152), thesignificance of this enzyme has been less studied, in part,because of the lack of suitable in vivo inhibitors. The recentdevelopment of selective and potent COMT inhibitors (150)has presented the opportunity to investigate the distribution ofCOMT in the living body with PET and appropriately labeledCOMT inhibitors. In this regard Ro 41-0960 (3,4-dihydroxy-5-nitro-2'-fluorobenzophenone), a potent and selective fluorinecontaining COMT inhibitor which has been reported to crossthe blood-brain barrier, has been labeled with â€˜8Fin order tostudy brain COMT with PET (153). Though PET studies in thebaboon using â€˜8F-labeledRo 41-0960 demonstrated a negligible uptake in the brain, high uptake was observed in the kidneysand in other organs known to have high COMT activitydemonstrating the potential of PET to investigate peripheral, ifnot central, COMT activity (154).

DOPAMINERGIC RESPONSIVITY ANDDOPAMINERELEASE

The competition of endogenous dopamine with D2 receptorradioligands presents the opportunity to measure changes insynaptic dopamine in vivo by observing the degree ofchange inthe binding of a D2 receptor radiotracer by a pharmacologicalchallenge or other perturbation (155). The in vivo striatalbinding of [3H}raclopride has been shown to be sensitive topharmacologically-induced changes in endogenous dopamine(35, 156). This property in turn has been used to assess changesin endogenous synaptic dopamine concentration with PET withlabeled raclopride and with labeled NMS (157â€”162). Comparisons are made between the binding of the radiotracer atbaseline and after administering a drug that changes dopamineconcentration (i.e., methylphenidate, amphetamine) (Fig. 7).Studies in humans have shown that methylphenidate significantly reduced [â€˜â€˜C]raclopridebinding in the brain. The reductions varied among subjects and decreased with age (160).Because [â€˜â€˜C]raclopridebinding is highly reproducible (163â€”164) these reductions most likely reflect changes in synapticdopamine. The effects of amphetamine on dopamine release inthe human brain have also been measured with SPECT and theD2 radioligand [â€˜231]IBZM(165).

Investigating relative changes in dopamine concentration in


MonoamineOxidaseB ([1t]L-deprenyl-D2) [11CjRaclopride

Baseline

Methyiphenidat.



response to a drug provides a tool with which to evaluate therelationship between drug-induced changes in dopamine concentration and its behavioral effects (160,165). It may alsoenable the detection of dopamine dysfunction with a highersensitivity than when studying patients during baseline conditions. Limitations of this strategy are: the inability to measurebaseline dopamine concentration; the limited spatial resolutionof PET (measures are of large volumes of tissue which may beheterogeneous in their responses to drugs); the inability todifferentiate whether changes in binding reflect the magnitudeof the rise in dopamine or the ratio of high- versus low-affinityD2 receptors (dopamine predominantly competes with theradiotracers for the binding to high-affinity D2 receptors); andthe relatively low sensitivity ofthe technique which requires theuse of pharmacological challenges that raise dopamine levelsseveralfold. Also, because the pharmacological challenge canaffect tracer delivery (166), studies are required to assess itseffects on tracer kinetics and on the model parameter. In thecase of[' â€˜C}raclopride,the model parameter used (BmaxlKd) isinsensitive to changes in CBF (167). To our knowledge, thesensitivity of these strategies to nonpharmacological, i.e., behavior-related (168) changes in dopamine has not been demonstrated. This methodology differs from the measures obtainedwith microdialysis (169) in that it simultaneously measures allbrain regions, it measures predominantly synaptic rather thanextracellular dopamine concentration, and it is minimally invasive permitting its use in awake human subjects.

Recent studies in our laboratory using electrically-stimulatedbrain slices have suggested that the binding of Di radioligandsmay be more sensitive to changes in synaptic dopamine than toD2 radioligands. To our knowledge this has not yet beenevaluated in PET experiments (170).

INTERACtiONS OF DOPAMINE WITh OThERNEUROTRANSMITrERS

Dopamine does not function alone and behaviors that involvedopamine are also regulated by other neurotransmitters (16).The dopamine system interacts with several neurotransmittersboth in the mesencephalic area and in projection regions. Closeinteractions between dopamine and norepinephrine, serotonin,gamma-amino butyric acid (GABA), glutamate, acetylcholine,opiates and CCK among others, have been documented (16).These interactions regulate dopamine function and are ofrelevance for understanding brain pathology as well as for thetherapeutic drug development. For example, interactions ofdopamine with glutamate have been associated with schizophrenia (1 71 ) and Parkinson's disease (23); those with serotonm with schizophrenia (1 72) and those with the opiate systemwith substance abuse (1 73). Understanding how the dopaminesystem is modulated by other neurotransmitters is leading to thedevelopment of candidate drugs which exploit these interactions, e.g., glutamate antagonists for Parkinson's disease (1 74)and serotonin antagonists in schizophrenia (1 75).

The feasibility of studying neurotransmitter interactions withPET was first demonstrated in the baboon brain for theinteraction between dopamine and acetylcholine (1 76). Themethodology takes advantage of the sensitivity of receptorspecific radioligands to competition with endogenous neurotransmitters in an analogous way to the strategy described tomeasure dopamine responsivity. The difference is that insteadof changing dopamine concentration with a drug that directlyinteracts with dopamine neurons, it uses drugs which affectspecific neurotransmitters that are known to modulate dopamine (158). Using this strategy PET has been used to evaluatethe ability of acetylcholine (1 76,177), GABA (157), serotonin

(1 78) and the opiate system (1 79) to modulate striatal dopamine release. In humans the interactions between dopamine andacetylcholine have been investigated in healthy controls (1 77).Striatal dopamine D2/acetylcholine interactions have also beenstudied in primates using [â€˜8F](â€”)-4-N-ethyl-fluoroacetamidobenzovesamicol as a radioligand marking cholinergic activity(180).

The possibility that abnormal interactions between corticalglutaminergic neurons and the striatal dopamine system mayunderlie schizophrenia was also recently investigated with PETwhere it was demonstrated that DOPA decarboxylase activity iselevated in patients with psychosis as measured with6-['8F]fluoro-DOPA (181 ). This observation was interpreted assupporting the hypothesis that in schizophrenia, psychosis is theresult of insufficient cortical (glutamatergic) stimulation ofnigrostriatal terminals leading to a low baseline concentrationof extracellular dopamine in the striatum and a correspondingincrease in the activity of the enzymes involved in dopaminesynthesis (182).

REGIONAL BRAIN GLUCOSE METABOUSM AND CBFBrain metabolism is tightly coupled with brain function and

it can therefore be used to assess the regional functionalchanges associated with manipulation of the dopamine system.Local brain metabolic rates can be measured using FDG (183).Though a number of energy-requiring processes contribute tothe basal rate ofglucose utilization, it is neuronal activity whichis the major contributor to glucose utilization (184â€”186). Underphysiological conditions CBF is tightly coupled to glucosemetabolism so that it can also be used as an index of brainfunction. However, care must be taken when measuring CBF asa functional tracer in experiments that use pharmacologicalchallenges since some of the psychoactive drugs have vasoactive properties (166). In humans, manipulations of the dopamine system have been made using drugs that raise dopamineconcentration and with drugs that block D2 receptors. For themost part, drugs that increase dopamine concentrations (i.e.,cocaine and amphetamine) when given acutely have beenshown to produce a widespread decrease in brain glucosemetabolism (187, 188). Acute administration of D2 receptorantagonists was shown to have a minimal effect on regionalbrain glucose metabolism in schizophrenic patients (189). Incontrast, acute haloperidol administration in normal subjectsdecreased metabolism in frontal and limbic cortices, in thalamus and in caudate nucleus (190). Though not always consistent, most studies evaluating the effects of chronic treatmentwith D2 receptor antagonists have shown increases in striatalmetabolism and relative decreases in frontal metabolism (191â€”198). The inability to differentiate changes in metabolism thatresult from direct changes in dopamine levels from those whichreflect secondary adaptation processes limits the interpretationof these findings.

MULtiPLE TRACERS STUDIESThe relatively low dosimetry from positron-labeled tracers

permits injection of volunteers with more than one PETradiotracer. Multiple tracer studies that measure glucose metabolism and/or CBF in conjunction with specific dopaminetracers, enable one to assess the functional significance of thesedopamine elements. Such studies have been done to investigatethe relation between brain glucose metabolism and dopamineD2 receptors in cocaine abusers. A significant correlation wasreported between dopamine D2 receptors and glucose metabolism in orbitofrontal cortex, cingulate gyrus and superior frontalcortex (68). Lower values for D2 receptor concentration are

PET DOPAMINETRACERsâ€¢Volkow et al. 1249



associated with lower metabolism in these brain regions. InParkinson's disease studies that evaluated in the same subject6-['8F]fluoroDOPA and brain glucose metabolism have alsoreported an association between the decreases in [â€˜8F]DOPAuptake in striatum and decreases in metabolic activity in frontalcortical areas (199,200). Multiple tracer studies also enableinvestigation of the relationship between the different elementsof the dopamine system and may aid the evaluation of patients.An example of such a study is shown in Figure 4 from aParkinson's disease patient who was evaluated with threetracers: [I â€˜C]d-threomethylphenidate for DAT; [â€˜â€˜C]raclopridefor D2 receptors; and FDG for regional brain glucose metabolism. While this patient showed marked reductions in DAT andin frontal metabolic activity; there were no differences indopamine D2 receptors. The DAT/D2 ratio was thereforemarkedly reduced when compared with that ofcontrols. The useof multiple tracers is slowly starting to emerge as a powerfultool to investigate the relations between functional and neurochemical parameters in the normal and the diseased brain.

QUANTITATIVE MEASURES OF DOPAMINEPARAMETERSWITHPET

The models used to describe the uptake and binding of PETligands are generally simple two- or three-compartment modelswith at most four kinetic parameters even though the physiological processes underlying the PET image are complex andinvolve, among other factors, delivery via capillary network,diffusion to the binding site, competition with endogenousneurotransmitter, binding to plasma proteins and associationand dissociation with specific binding sites (of possibly morethan one kind). The models used are simply of necessity sincethere are only two measurements: plasma radioactivity due tothe labeled ligand and the radioactivity measured by the PETcamera which is the total radioactivity due to all processesoccurring within the voxel. As a result only a limited number ofmodel parameters can be uniquely determined (201 ). Threecompartment models for D2 dopamine receptors have beenproposed (33,46,202â€”206). Alternatively there are model-independent methods for the analysis of PET data which do notdepend upon a particular model but only require that linearfirst-order kinetics are applicable to the movement of theradiotracer (207â€”210).Both approaches provide some measurewhich is a function of the free receptor concentration.

A radioligand generally can be classified as either reversiblewhen there is uptake and loss of the ligand from tissue duringthe course of the experiment or irreversible, in which case theligand is taken up by receptors/enzyme and remains bound forthe duration of the experiment. The three-compartment modelwhich has been applied to both types of radiotracers consists ofthree to four parameters. Two parameters control transportbetween plasma and tissue, K1 is the plasma to tissue influxconstant and k2 is the tissue-to-plasma efflux constant. Thereare one or two receptor parameters, k3, which is proportional tothe number of free receptors or binding sites and k4, theligand-receptor dissociation constant (in the case of irreversibleligands k4 = 0) (33,46,203,206. ) The transport constants, asthey are generally used, are functions ofblood flow, permeability, plasma protein binding (K1) and nonspecific binding (k2). Insome models blood flow and nonspecific binding are considered separately (202). Values for the model parameters can bedetermined by fitting the model to PET data or by relying on apseudo equilibrium to estimate receptor number from PET datadirectly (33,204). There are many different approaches andassumptions used in the determination of the model parameterseven within the framework of the three-compartment model

(33,46,202â€”204) but the goal is to separate the effects oftransport from that of receptor binding. The parameter ofinterest is k3 a Bmax or k3/k4 a Bmax/Kd where Bmax is thetotal receptor concentration and Kd is the equilibrium dissociation constant; Bmax/Kd is sometimes referred to as thebinding potential (202). General aspects of parameter estimation and modeling techniques used in PET have beenreviewed (211â€”214).

For reversible ligands, the distribution volume (DV) canbe determined directly. The DV is the ratio of tissue radioligand concentration to plasma radioligand concentration atequilibrium.

Alternatives to the methods that require explicit modelparameter determination are the model independent methodswhich are much easier to apply. In the case of irreversiblybinding ligands, the graphical technique (Patlak plot) (207,208)has been extensively used. In this method a plot of

fTROI(T) I Cp(T) versus@ Cp(t)/Cp(T)

Jobecomes linear after some time with a slope Ki, referred to asthe influx constant. Cp(T) is the plasma radioactivity of thelabeled ligand and ROI(T) is the tissue radioactivity measuredby the PET camera at time T. K1determines the steady-state rateoftransfer ofthe ligand into the irreversible compartment whichis given by [email protected]@,a function of the number of free receptorsor binding sites, is also a function oftracer delivery (K,). In thecase of a very high rate of trapping (controlled by k3) K. K1and therefore contains little information about free receptorconcentration, a situation referred to as flow limited. Byreducing the rate of trapping, the sensitivity of K@to variationsin free receptor concentration can be improved. This was donewith deuterium-substituted L-deprenyl ([I â€˜C]L-deprenyl-D2)for which the trapping rate was reduced over that of L-deprenylin regions of high MAO B (145). Under these conditions Ki ismuch less sensitive to changes in delivery and more sensitive tochanges in enzyme/receptor concentration. The condition ofbeing flow limited is a consequence of the rapid trapping andirreversible nature of the binding and limits the ability todetermine reliable measures of free receptors whether expressedin terms of k3 of the three-compartment model or K. The slopeof a plot of radioactivity in the receptor region to that in anonreceptor region versus time as a measure of the number offree receptors has also been used (46,60). This method can bederived from the Patlak analysis for irreversibly binding ligands(46,215). The steady state (when this ratio becomes constant).Since the DV is a steady-state quantity, it is independent ofblood flow (167,216). In terms of model parameters the DV isgiven by

where

K1/DV = â€” (@i+@ Bmax' @/Kd'1

k2

Bmax'1 _ (Bmax1â€” Lâ€” L') 1

Kd'1@ Kd1 @l+NS

Eq. 1

Eq. 2

Bmax, is the receptor concentration of receptor type i and Kd@is the equilibrium dissociation constant, NS refers to the ratio ofkinetic constants for nonspecific binding (assumed to be sufficiently rapid that it is always in a steady state (202)), L is theconcentration of endogenous neurotransmitter and L' is theconcentration of receptors occupied with unlabeled ligand ordrug. The free receptor concentration is related to the DV. By




inverse agonists stabilize R favoring the inactive state andantagonists do not perturb the equilibrium (225,226). Thepresence of constitutively active receptors was recently documented for dopamine D5 receptors (227). Though the physiological significance of these constitutive dopamine receptors is

E 3 not understood there is evidence from other receptor systemsq. that abnormal expression of constitutive receptors can lead to

pathology as is the case for mutations in the rhodopsin receptorwhich lead to retinitis pigmentosa (228), and for mutations ofthe thyrotropin-stimulating hormone receptor which are associated with the presence of oncogenic lesions (229). Access toexperimental strategies with PET that would enable discrimination of R* from R would allow the assessment of thefunctional state of the receptors in the normal and the diseasedbrain. The possibility that there is increased expression ofconstitutive receptors in diseases with overactivity of thedopamine system, as has been postulated for schizophrenia,merits investigation. The PET strategy described to measuredopaminergic responsivity after a pharmacological challengecould perhaps be useful to assess the functional state ofreceptors since it reflects not only changes in intrasynapticdopamine but also the ratio of R* to R for dopamine bindspredominantly to R*. Unfortunately this strategy is confoundedby baseline measures, since in a state with increased expressionof R@, dopamine would also be more effective in blockingbinding of the receptor radioligand. Another strategy which hasbeen proposed to measure the functional state of the receptorswith PET is to differentially evaluate binding of dopaminereceptor agonists from binding of dopamine antagonists (230).

@ _& S_The binding of dopamine to its receptors results in an

intracellular response transmitted by second messenger molecules such as cAMP which in turn regulate enzymatic processeswithin the cell such as protein kinase A (16). Changes in secondmessenger systems will therefore affect the functional effects ofthe receptors on the cells expressing them and may be involvedin disease, i.e., affective disorders (231 ) and drug addiction(232). The pharmacological properties of certain therapeuticagents, such as lithium, appear to be in part mediated by theirability to change these second messenger systems (233).Development of radiotracers for second messenger systemswould be an enormous advance because there are theoreticalreasons to expect that brain diseases and therapeutic interventions will express themselves more clearly at the level ofsecond- and third-messenger systems.

Fluorine-18-labeled 1,2-diacylglycerols are currently beinginvestigated as PET ligands to image second-messenger systems (234). New tracers targeted to specific elements of thesecond-messenger systems may allow investigation of theirinvolvement in brain diseases.

Molecular Gene&sAntisense nucleotides are finding applications in many fields

and have the potential to map mRNA for various moleculartargets including elements of the dopamine system such asreceptors and transporters. The antisense nucleotide complements the coding strand of DNA and selectively binds to astrand of specific mRNA. The possibility of using this technique in nuclear medicine has been investigated primarily topermit the evaluation of oncogenes amplified in cancer cells(235). Using PET to measure regional expression of mRNA inbrain would be predicted to be limited by difficulties inpreparation of the required labeled oligonucleotides and delivery of nucleotides into the brain as well as the relatively lowrate of trapping and diffusion in tissue. Dopamine plays an

taking the DV ratio of receptor region (ROl) to a nonreceptorregion (NR), the dependence upon plasma protein bindingwhich is contained in K, is eliminated. It is assumed that theratio oftransport constants is close to the same for both regions.

DV(ROI)DV(NR)@ = @Bmax'1/Kd'1.

There are several methods for determining the DV. One ofthem involves an infusion protocol to maintain a constantplasma level and the DV is directly determined from the ratio ofradioactivity in tissue to that in plasma (21 7). However, formost PET studies the ligand is introduced as a bolus injectionwhich requires a modeling technique to determine the DV.Under these conditions the DV can be determined using atwo-compartment two-parameter model that does not explicitlycontain receptor parameters (210). The DV can also be determined from the plot of

(T CTI ROI(t)dt/ROI(T)vs@ Cp(t)Jo Jo

which becomes linear after some time with a slope equal to theDV (209).

The determination of absolute values for the receptor concentration Bmax has been reported for NMS (218) andraclopride (219), although with conflicting results. Such experiments require a loading dose of unlabeled ligand or drug toreduce the number of available receptors. However, for anâ€˜8F-labeled congener of raclopride, the three-compartmentmodel failed to give consistent results for the ligand-receptordissociation (k4) under conditions of saturation compared to thehigh specific activity nonsaturation experiments (220). Theauthors postulate that the geometry of the synapse and thephysical arrangement of receptors on the membrane are responsible for the discrepancy. In spite of this failure the threecompartment model has proved to be useful in assessingchanges in receptor availability even though it may lack thecomplexity to reproduce the observed behavior of ligandbinding under some circumstances.

FUTURE DIRECTiONS

Radiotracer Design and DevelopmentAdvances made in radiotracer design and synthesis have

played a major role in shaping the PET field as we know ittoday. These advances have made it possible to create theradiotracers required to investigate new biomolecular targetsand to improve the quantitative nature of PET measurements.Development of more selective radioligands for dopaminereceptor subtypes and radiotracers with a range of kineticproperties to maximize information from specific experimentalparadigms are important immediate goals. Also, new radiotracers which have the sensitivity to probe small changes insynaptic dopamine, as might be expected to arise from behavioral stimulation, and/or to assess baseline dopamine concentration would also be an important advance in our understanding of the functional activity of the dopamine system.

Functional State of ReceptorsThe documentation of constitutively active receptors that

couple to G proteins without the need of an agonist (221â€”224)has led to the renewed consideration of the two-state receptormodel for G-protein coupled receptors (225). The two-statereceptor model describes an equilibrium between active (R*)and inactive receptors (R) which is perturbed by the presence ofreceptor ligands; agonists stabilize R* favoring the active state,

PET DOPAMINETRACERsâ€¢Volkow et al. 1251



important role in regulating the functions of various peripheralorgans such as the heart and kidneys. The extent to whichimaging of these peripheral organs with positron-labeled antisense probes is at all feasible is currently difficult to predict.

Higher Spatial ResolutionMost studies measuring dopamine receptors and dopamine

transporters are done in the caudate and the putamen. However,these striatal regions are known to be functionally as well asneurochemically heterogeneous. It is anticipated that futurePET instruments with higher spatial resolution and sensitivitywill enable measurements in specific striatal subregions, such asthe nucleus accumbens.

Integration with Functional MRI StudiesPET has benefited from advances in MRI predominantly by

the use of co-registration procedures that improve regionalquantitation of PET images. Also, since it is difficult to locatesome brain structures in PET images due to variability in theirlocation and size between patients, the individual subject's MRIcan be used to identify them. Newer strategies are beingdeveloped for fusion imaging in which the data from bothimage modalities are fused to generate an image that has thespatial resolution of MRI with the biochemical informationfrom PET. The development of functional MRI that enablesassessment of functional brain activation with a higher spatialand temporal resolution than that of PET offers the opportunityof incorporating both technologies for investigating the temporal dynamics of the functional brain responses to acute changesin dopamine (i.e., temporal relationship between the receptoroccupancy by an agonist and/or antagonist and its functionalconsequences).

Other Clinical ApplicationsThough D2 and DAT radioligands are being used for the

evaluation of patients with movement disorders and D2 radioligands have been shown to be useful for evaluating receptorblockade by antipsychotic agents, their clinical utility has notbeen fully evaluated. For example, recent studies have documented a role of dopamine in malignant neuronal cell proliferation and have shown significant concentrations ofDl receptorsin human brain meningiomas (236). Similarly, the presence ofdopamine receptors, which are expressed on certain pituitaryadenomas and which may be related to the cellular origins ofthe neoplastic cells, has been detected by the dopamine D2receptor antagonist, â€˜â€˜CNMS (237). Thus, some of the dopamine receptor radioligands may be useful in the evaluation ofabnormal cell proliferation in the human brain.

CONCLUSIONPET, in conjunction with appropriate radiotracers, is being

used to assess the dopamine system in the normal and thediseased human brain. As a result, information which couldonly be previously investigated in animals or in postmortemhuman brains is accessible in living human subjects. This hasenabled initial investigations of the relation between changes inthe dopamine system in the human brain and its functional andclinical consequences. At the same time these studies have beenable to assess the effects of drugs in the dopamine system.Future advances in radiotracer development and in instrumentation will enable performance of more selective measurementswith respect to the parameters investigated as well as the brainregions assessed. Such studies will increase our understandingof the mechanisms by which the dopamine system regulatesmotor, cognitive and motivational behaviors in the humanbrain.

ACKNOWLEDGMENTThis work was supported by Department of Energy grant

DE-ACO2-76CH00016 and National Institute of Drug Abusegrants ROIDAO6891 and RO1DA06278.

REFERENCESI. Goldman-RakicP.Circuitryof theprefrontalcortex.In: PlumF,ed.Handbookof

psychology. Bethesda, MD: American Physiological Society; 1987:373â€”417.2. LeMoalM, SimonH. Mesocorticolimbicdopaminenetwork:functionalandregula

tory roles. Physiol Rev 1991;71:l55â€”234.3. Koob OF, Bloom FE. Cellular and molecular mechanisms of drug dependence.

Science 1988;242:715â€”723.4. Kopin Ii. The pharmacology of Parkinson's disease therapy: an update. Ann Rev

Pharmacol Toxicol 1993 ;32:467â€”495.5. Creese I, Burt DR. Snyder 5H. Dopamine receptor binding predicts clinical and

pharmacological potencies of antischizophrenic drugs. Science 1976;192:481â€”483.6. McGeer PL, McGeer EG, Suzuki iS. Aging and extrapyramidal function. Arch

Neurol 1977;34:33â€”35.7. Weeks PA, Brooks DJ. Positron emission tomography and central neurotransmitter

systems in movement disorders. Fundam Clin Pharmacol 1994;8:503â€”517.8. Volkow ND, Fowler J5. Neuropsychiatric disorders: investigation of schizophrenia

and substance abuse. Semin Nuci Med 1992;22:254â€”267.9. Di Chiara 0, Morelli M, Acquas E, Carboni E. Functions of dopamine in the

extrapyramidal and liinbic systems. Arzneim-Forsch Drug Rev 1992;42:231â€”237.10. Willner P, Muscat R, Papp M, Sampson D. In: Willner P. Scheel-Kruger J, eds. The

mesolimbicdopaminesystem:frommotivationto action. New York, NY: JohnWileyandSons;1992:387â€”400.

11. Glowinski J, Tassin JP, Thierry AM. The mesocortical-prefrontal dopaminergicneurons. TINS 1984:418â€”451.

12. Selemon LD, Goldman Rackic PS. Longitudinal topography and interdigitations ofcorticostriatal projections in the rhesus monkey. J Neurosci 1984;5:776â€”794.

13. stuss DT, Benson DF. Thefrontallobes. New York, NY: Raven Press; 1986:12â€”38.14. McGeer PL, Eccles JC, McGeer EG. Physiology and pharmacology of dopamine.

Molecular neurobiologyof the mammalian brain. New York, NY: Plenum Press;1987:294â€”308.

15. Gartis PA, Ciolkowski EL, Pastore P, Wightman RM. Efflux of dopamine from thesynaptic cleft in the nucleus accumbens ofthe rat brain. JNeurosci 1994;14:6084-6093.

16. Jackson DM, Westlind-Danielsson A. Dopamine receptors: molecular biology,biochemistry and behavioral aspects. Pharmacol Ther 1994;64:291-369.

17. Fowler J5, WoIfAP. New directions in positron emission tomography. In: Bristol JA,ed. Annual reports in medicinal chemistry. 1989;24:277â€”286.

18. Christman DR, Hoyte RM, Wolf AP. Organic radiopharmaceuticals labeled withisotopes of short half-life. I: dopamine-hydrochloride-1-' â€˜C.J Nucl Med 1970;1 1:474â€”478.

19. Chumpradit S. Kung MP, Billings J, Kung HF. Fluorinated and iodinated dopamineagents: D2 imaging agents for PET and SPECT. J Med Chem 1993;36:221â€”228.

20. Seeman P, Niznik H. Dopamine receptors and transporters in Parkinson's disease andschizophrenia. FASEB J I990;4:2737â€”2744.

21. 5ibley DR. Monsma FJ. Molecular biology ofdopamine receptors. TrendsPharmacolSci 1992;13:61â€”69.

22. Kessler RM, Whetsell WO, Ansasi Ms. et al. Identification ofextrastriatal D2 receptorsin postmortem human brain with [Wljepidepride. Brain Re.s 1993;609:237-243.

23. starr Ms. Glutamate/dopamine D1ID2 balance in the basal ganglia and its relevanceto Parkinson's disease. Synapse 1995;19:264â€”293.

24. Camps M, Cofles R, Gueye B, Probst A, Palacios JM. Dopamine receptors in humanbrain autoradiographic distribution of D2 sites. Neuroscience 1989;28:275â€”290.

25. Hall H, Sedvall 0, Magnusson 0, Kopp J, Halldin C, Farde L. Distribution ofDI- andD2-dopamine receptors, and dopamine and its metabolites in human brain. Neuropsychopharmacology 1994; 11:245â€”256.

26. Murray AM, Ryoo H, Joyce iN. vis@1i@ation of dopamine D3-like receptors inhuman brain with [â€˜251]epidepride.Eur J Pharmacol 1992;227:443â€”445.

27. Seeman P, Guan H, Van Tol H. Dopamine D4 receptors elevated in schizophrenia.Nature 1993;365:441â€”445.

28. Strange PG. Dopamine D4 receptors: curiouser and curiouser. Trends Pharmacol Sci1994;15:3 17â€”319.

29. Meador-Woodruff JH, Mansour A, et al. Distribution of D5-dopamine receptormRNA in rat brain. Neurosc Leti l992;145:209â€”212.

30. Wagner HN, Burns HD, Dannals RF, et al. Imaging DA receptors in the human brainbyPET.Sciencel983;221:1264â€”1266.

31. Ameft CD, Wolf AP, Shiue CY, et al. Improved delineation of human dopaminereceptors using [â€˜8F]-N-methylspiroperidol and PET. J Nuci Med 1986;27: I878â€”1882.

32. Maziere B, Loc'h C, Hantraye P. et al. 76Br-bromospiroperidol: a new tool forquantitative in vivo imaging of neuroleptic receptors. Life Sci 1984;35:1349â€”1356.

33. Farde L, Hall H, Ehrin E, Sedvall 0. Quantitative analyses ofD2-dopamine receptorbinding in the living human brain by positron emission tomography. Sciencel986;231:258â€”260.

34. Halldin C. Dopamine receptor radioligands. Med Chem Res 1994;5:l27â€”149.35. Seaman P. Guan C, Niznik HB. Endogenous dopaminc lowers the dopamine D2

receptor density as measured by 3H raclopride: implications for positron emissiontomography of the human brain. Synapse 1989;3:96â€”97.

36. Wang G-J, Volkow ND, Logan J, et al. Evaluation ofage-related changes in serotonin5-HT2 and dopamine D2 receptor availability in healthy human subjects. Life Sci1995;56:249â€”253.

37. VanTol HM, Bunzow JR. Guan C, et al. Cloning of the gene for a human dopamineD4 receptor with high affinity for the antipsychotic clozapine. Nature 1991;350:610â€”614.

1252 THE JOURNALOFNUCLEARMEDICINEâ€¢Vol. 37 â€¢No. 7 â€¢July 1996



38. Huntley OW, Morrison JH, Prikhozhan A, 5ealton SC. Localization of multipledopamine receptor subtype mRNAs in human and monkey motor cortex and striatum.Mo! BrainRes 1992;15:181â€”188.

39. Filloux F, Dawson TM, Wamsley 1K. Localization ofnigrostriatal dopamine receptorsubtypes and adenylate cyclase. Brain Res Bull 1988;20:447â€”459.

40. JoyceThi,MarshallJF. Quantitativeautoradiographyof dopamineD2 sitesin ratcaudate-putamen: localization to intrinsic neurons and not to neocortical afferents.Neurosci 1987;20:773â€”794.

41 . Farde L, Nordstrom A-L, Karisson P. Halldin C, Sedvall 0. Positron emissiontomography. Studies on dopamine receptors in schizophrenia. Clin Neuropharmacol1995;l8:512 1â€”S129.

42. Wang 0-i, Volkow ND, Fowler iS, et al. Comparison of two PET radioligands forimaging extrastriatal dopamine receptors in human brain. Synapse 1993;15:246â€”249.

43. Yousef K, Volkow ND, Schlyer D, et al. Inhibition of extrasttiatal dopaminereceptors by haloperidol. Synapse 1995; 19:14â€”17.

44. Kessler RM, Whetsell WO, Ansari MS. et al. Identification ofextrastriatal dopamineD2 receptors in postmortem human brain with [â€˜251]epidepride.Brain Res 1993;609:237â€”243.

45. Halldin C, Farde L, Hogberg T, et al. Carbon-I 1-FLB 457: a radioligand forextrastriatal D2 dopamine receptors. J Nuc! Med 1995;36:1275â€”128l.

46. Wong DF, Gjedde A, Wagner HN. Quantification of neuroreceptors in the livinghuman brain. I. Irreversible binding of ligands. J Cereb Blood Flow Metab1986;6: 136â€”146.

47. Farde L, Halidin C, 5tone-Elander 5, Sedvall 0. PET analysis of human dopÃ¡minereceptor subtypes using â€˜â€˜C-sCH23390 and â€˜â€˜Craclopride. Psychopharmacology1987;92:278â€”284.

48. Halldin C, Farde L, Barnett A, Sedvall 0. Synthesis of carbon-l I-labeled SCH39166, a new selective dopamine Dl receptor ligand, and preliminary PET investigations. App! Radiat Isot 199 1;42:45 1â€”455.

49. Karlsson P, Farde L, Halldin C, et al. PET examination of [â€˜â€˜C]NNC687 and[I â€˜C]NNC 756 as new radioligands for the DI-dopamine receptor. Psychopharma

cology1993;113:149â€”156.50. HalldinC, FogedC, LochC, et al. [â€˜â€˜C]NNCI 12,a selectivePETradioligandfor

examination of extrastriatal dopamine Dl receptors. J Nuci Med 1994;35:l22P.51. Da5ilva iN, Wilson AA, 5eeman P. Houle S. Synthesis of [â€˜â€˜C]SKF75670 as a

potential dopamine Dl receptor agonist imaging agent for PET. J Labeled CompdRadiopharm 1994;35:460â€”461.

52. Wong DF, Wagner HN ir, Dannals RF, et al. Effects of age on dopamine andserotonin receptors measured by positron emission tomography in the living brain.Science I986;226: I393â€”1396.

53. Baron JC, Maziere B, Loch C, et al. Loss of striatal [76Br]bromospiperone bindingsites demonstrated by positron emission tomography in progressive supranuclearpalsy. J Cereb Blood Flow Metab 1986;6:131â€”136.

54. Antonini A, Leenders KL, Reist H, et al. Effect of age on D2 dopamine receptors innormal human brain measured by positron emission tomography and â€˜â€˜Craclopride.Arch Neurol 1993;50:474â€”480.

55. Rinne JO, Hietala J, Ruotsalainen U, et al. Decrease in human striatal dopamine D2receptor density with age: a PET study with â€˜â€˜Craclopride. J Cereb Blood FlowMetab 1993;13:310â€”314.

56. 5eeman P, Bzowe NH, Gum HG, et al. Human brain dopamine receptors in childrenandagingadults.Synapse1987;l:399â€”404.

57. Suhara T, Fukuda H, lnoue 0, et al. Age-related changes in human Dl dopaminereceptors measured by positron emission tomography. Psychopharmacology 1991;103:41â€”45.

58. Andreasen NC, Carson R, Diksic M, et al. Workshop on schizophrenia, PET and thedopamine D2 receptors in the human neostriatum. Schiz Bull 1988;14:471â€”484.

59. FardeL, NordstromA-L, WieselFA,etal.Positronemissiontomographyanalysisofcentral DI and D2 receptor occupancy in patients treated with classical neurolepticsand clozapine. Relation to extrapyramidal side effects. Arch Gen Psychiatry 1992;49:538â€”544.

60. 5mith M, WolfAP, Brodie ID, et al. Serial â€˜8F-N-methylspiroperidol PET studies tomeasure changes in antipsychotic drug D2 receptor occupancy in schizophrenicpatients. Biol Psychiatry 1988;23:653â€”663.

61. Farde L, Nordstrom A, Nyberg 5, Halldin C, 5edvall 0. Dl, D2 and 5HT2 occupancyin clozapine treated patients. J Clin Psychiatry 1994;55(suppl B):67â€”69.

62. Wolkin A, Brodie JD, Rotrosen I, et al. Dopamine receptor occupancy and plasmahaloperidol levels. Arch Gen Psychiatry 1989;46:482â€”483.

63. Cambon H, Baron IC, Boulenger JP. In vivo assay for neuroleptic receptor bindingin the striatum positron tomography in humans. BrJ Psychiatry 1987;15l:824â€”830.

64. Carlsson A. Towards a new understanding of dopamine receptors. Clin Neuropharmacol 1995;18:S6â€”S13.

65. Wolkin A, Barouche F, Wolf AP, et al. Dopamine blockade and clinical response:evidence for two biological subgroups of schizophrenia. Am J Psychiatiy 1989;46:905â€”908.

66. Coppens W, 51off Ci, Paans AMJ, et al. High central D2 dopamine receptoroccupancy as assessed with positron emission tomography in medicated but therapyresistant schizophrenic patients. Biol Psychiatry 1991;29:629â€”634.

67. Volkow ND, Fowler IS, Wolf AP, et al. Effects of chronic cocaine abuse onpostsynaptic dopamine receptors. Am J Psychiatry 1990;l47:719â€”724.

68. Volkow ND, Fowler 15, Wang 0-i, et al. DecreaseddopamineD2 receptoravailability is associated with reduced frontal metabolism in cocaine abusers. Synapse1993;14:169â€”177.

69. Hietala I, West C, Syvalahti E, et al. Striatal D2 dopamine receptor bindingcharacteristics in vivo in patients with alcohol dependence. Psychopharmacology1994;1 16:285â€”290.

70. PearlsonGD, Wong DF, Tune LE, et al. In vivo dopaminereceptordensityinpsychotic and nonpsychotic patients with bipolar disorder. Arch Gen Psychiatry1995;52:471â€”477.

71. Suhara 1, Nakayama K, Inoue 0, et al. Dl dopamine receptor binding in mood

disorders measured by positron emission tomography. Psychopharmacology 1992;106:14â€”18.

72. Caldecott-Hazzard S. Morgan DO, Dc Leon-Jones F, et al. Clinical and biochemicalaspects of depressive disorders. II. Transmitter/receptor theories. Synapse 1991;9:251â€”301.

73. BrownAS, Gershon5. Dopamineand depression.J Neural Transm[Gen Sect]1993;91 :75â€”109.

74. AquiloniusSM. WhathasPETtoldusaboutParkinson'sdisease?ActaNeurolScand1991;136:37â€”39.

75. Rinne UK, Laihinen A, Rinne JO, et al. Positron emission tomography demonstratesdopamine D2 receptor supersensitivity in the striatum of patients with earlyParkinson's disease. Mov Disord l990;5:55â€”59.

76. Hassan MM, Thakar I. Dopamine receptors in Parkinson's disease. Prog Neuropsychopharmacol Bio! Psychiatry 1988; 12:173â€”182.

77. Rinne JO, Laihinen A, Lannberg P, Maramaki P, Rinne UK. A postmortem study onstriatal dopamine receptors in Parkinson's disease. Brain Res 1991;556: I 17â€”122.

78. SchwartzJ,TatschK, Arnold0, Ctal. â€˜23I-iodobenzamide-5PECTpredictsdopaminergic responsiveness in patients with de novo parkinsonism. Neurology 1992;41:556â€”561.

79. Leenders K, Frackowiak R, Quinn N. Brain energy metabolism and dopaminergicfunctions in Huntington's disease measured in vivo using positron emission tomography. Mov Disord 1986;1:69â€”77.

80. Hagglund J, Aquilonius S-M, Eckernas S-A, et al. Dopamine receptor properties inParkinson's disease and Huntington's chores evaluated by positron emission tomography using â€˜â€˜C-N-methylspiperone.Ada Neurol Scand l987;75:87â€”94.

81. Brandt I, Foistein 5, Wong D, et al. D2 receptors in Huntington's disease: positronemission tomography findings and clinical correlates. J Neuropsychiatry C/inNeurosci 1990;2:20â€”27.

82. Sedvall 0, Karlsson P. Lundin A, et al. Dopamine Dl receptor numberâ€”a sensitivePET marker for early brain degeneration in Huntington's disease. Eur ArchPsychiatry C/in Neurosci l994;243:249â€”255.

83. Ritz MC, Lamb RI, GoldbergSR. KuharMi. Cocainereceptorson dopaminetransporters are related to self administration of cocaine. Science 1987;237:1219â€”1223.

84. Salmon E, Brooks Di, Leenders KL, et al. A two-compartment description andkinetic procedure for measuring [I â€˜C]nomifensineuptake using positron emissiontomography. J Cereb Blood Flow Metab I990; 10:307â€”317.

85. TedroffI, AquiloniusS-M, HartvigP.etal. Monoaminereuptakesitesin thehumanbrain evaluated in vivo by means of@ â€˜Cnomifensine and positron emissiontomography: the effect of age and Parkinson's disease. Acta Neurol Scand 1988;77:92â€”101.

86. Fowler iS, Volkow ND, WolfAP, et al. Mapping cocaine binding sites in human andbaboon brain in vivo. Synapse I989;4:371â€”377.

87. Kilbourn MR. In vivo binding of [â€œF]GBR 131 19 to the brain dopamine uptakesystem. Life Sci 1988;42:1347â€”1351.

88.KilbournMR. HakaMS, MulhollandOK, JewettDM, Kuhl D. Synthesisofradiolabeled inhibitors of presynaptic monoamine uptake systems: [â€˜â€˜F]GBR13 119(DA), [â€˜â€˜C]nisoxetine(NE) and [â€˜â€˜C]fluoxetine(5-HT). J Lab Cmpd Radiopharm1989;26:4l2â€”414.

89.KoeppeRA, KilbournMR. FreyKA, et al. Imagingandkineticmodelingof[â€˜8FJGBR12909, a dopamine uptake inhibitor. J NucI Med 1990;31 :720.

90. Innis RB, Seibyl JP, Scanley BE, et al. Single-photon emission computed tomographic imaging demonstrates loss of striatal dopamine transporters in Parkinson'sdisease. Proc Nat! Acad Sci USA I993;90: 11965â€”11969.

91. Laihinen AO, Rinne JO, Nagren KA, et al. PET studies on brain monoaminetransporters with carbon-I 1-f3-CIT in Parkinson's disease. J Nuc! Med l995;36:1263â€”1267.

92. Madras BK, Spealman RD. Fahey MA, et al. Cocaine receptors labeled by2f3-carbomethoxy-3@3-(4-fluorophenyI)tropane. Mo! Pharm I989;36:5 I8â€”524.

93. Wong DF, Yung B, Dannals RF, et al. In vivo imaging of baboon and humandopamine transporters by positron emission tomography using (â€˜â€˜C]WIN35,428.Synapse1993;15:130â€”142.

94. Frost ii, Rosier AJ, Reich SO, et al. Positron emission tomographic imaging of thedopamine transporter with â€˜â€˜C-WIN 35,428 reveals marked declines in mildParkinson's disease. Am J Neuro! 1993;34:423â€”431.

95. Ding Y-S, Fowler IS, Volkow ND, et al. Pharmacokinetics and in vivo specificity of[I â€˜C]d-threo-methylphenidate for the presynaptic dopaminergic neuron. Synapse

1994;18:152â€”160.96. Volkow ND, Ding YS, Fowler IS, et al. [â€˜â€˜C]d-threo-methylphenidate: a new PET

ligand for the dopamine transporter. II. Studies in the human brain. J Nuc! Med1995;36:2162â€”2168.

97. Aquilonius 5-M, Bergstrom K, Eckernas 5A, et al. In vivo evaluation of striataldopamine reuptake sites using@ â€˜C-nomifensine and positron emission tomography.Acta Neurol Scand 1987;76:283â€”287.

98. Volkow ND, Fowler iS, Logan I, Ct al. Comparison of [â€˜â€˜Cicocainebinding atsub-pharmacological and pharmacological doses: a PET study. J Nuc! Med I995;36:1289â€”1297.

99. Kilboum MR. DaSilva IN, Frey KA, Koeppe RA, KuhI DE. In vivo imaging ofvesicular monoamine transporters in human brain using [â€˜â€˜C]tetrabenazineandpositron emission tomography. J Neurochem 1993;60:23 15â€”2318.

100. Kilbourn MR. Shades of grey: radiopharmaceutical chemistry in the 1990s andbeyond. NucI Med Biol l992;19:603â€”606.

101. Volkow ND, Fowler 15, Wang 0-i, et al. Decreased dopamine transporters with agein healthy human subjects. Ann Neurology l994;36:237â€”239.

102. Van Dick CH, Seibyl iP, Malison RT, et al. Age-related decline in dopaminetransporter binding in human striatum with [l-l23J@3-CIT SPECT. J Nucl Medl995;36:1175â€”1181.

103. Zelnik N, Angel I, Paul SM, Kleinman JE. Decreased density of human striataldopamine uptake sites with age. Eur J Pharmacol l986;126:175â€”176.




104. DeKeyserI, EbingerG, Vauquelin0. Age-relatedchangesin thehumannigrostriataldopaminergic system. Ann Neurol 1990;27:157â€”l6l.

105. Malison RT, Wallace EA, Best 5, et al. SPECT imaging ofdopamine transporters incocaine dependent and healthy control subjects with [â€˜23I]@3CIT.Soc Neurosci Abstr1994;20:1625.

106. Volkow ND, Wang 0-i, Fowler IS, et al. Cocaine binding is decreased in the brainof detoxified cocaine abusers. J Neuropsychopharmaco! 1996;14:159â€”168.

107. Tiihonen I, Kuikka I, Bergstrom K, et al. Altered striatal dopamine reuptake sitedensities in habitually violent and nonviolent alcoholics. Nature Medicine 1995;l:654â€”657.

108.ZhuM-Y,JuorioAV. AromaticL-aminoaciddecarboxylase:biologicalcharacterization and functional role. Gen Pharmaco! 1995;26:681â€”696.

I09. Cumming P, Venkatachalam 1K, Rajagopal 5, Diksic M, Ojedde A. Brain uptake ofa-['4C]methyl-para-tyrosine in the rat. Synapse l994;l7:125â€”l28.

I 10. Deiesus 01, Murali D, Kitchen R, et al. Evaluation of 3['8F]fluoro-a-fluoromethylp-tyrosine, a potential PET tracer for tyrosine hydroxylase activity. Nuc/ Med Bio!1994;2l:663â€”667.

I 11. Garnett ES, Firnau 0, Nabmias C. Dopamine visualized in the basal ganglia of livingman. Nature 1983;305:137â€”l38.

I 12. Garnett Es, Fimau 0, Chan PKH, Sood 5, Belbeck LW. [â€œF]Fluoro-dopa,an analogof dopa, and its use in direct external measurements of storage, degradation andturnover of intracerebral dopamine. Proc Nat Acad Sci USA 1978;75:464â€”467.

113. Hoshi H, Kuwabara H, Leger 0, Cumming P, Guftman M, Ojedde A. 6-[â€•F]FluoroL-dopa metabolism in living human brain: a comparison ofsix analytical methods. JCereb B!ood Flow Metab 1993; 13:57â€”69.

I 14. Dhawan V, Ishikawa T, Chaley T, et al. Combined FDOPA and 3OMFD PET studiesin Parkinson's disease: modeling issues. J Nuc! Med l996;in press.

115. Melega WP, Hoffman JM, Luxen A, et al. The effects ofcarbidopa on the metabolismof 6-[â€•F]fluoro-L-dopain rats, monkeys and humans. Life Sci l990;47:l49â€”l57.

116. Gjedde A, Reith I, Dyve 5, et al. Dopa decarboxylase activity in the living humanbrain. Proc Nat Acad Sci USA 1991;88:2721â€”2725.

117. Hartvig P, Lindner KJ, TedroffJ, et al. Regional brain kinetics of6-fluoro-(@-' â€˜C)L-dopa and (a-' â€˜C)-L-dopafollowing COMT inhibition. A study in vivo usingpositron emission tomography. J Neural Transm 1992;87:15â€”22.

118. Melega WP, Luxen A, Perlmutter MM, et al. Comparative in vivo metabolism of6-[â€•F]fluoro-L-dopa and [3H]L-dopa in rats. Biochem Pharm 1990;39:1853â€”l860.

119. Hartvig P, Agren H, Reibring L, et al. Brain kinetics of L-[@-'â€˜C]DOPAin humansstudied by positron emission tomography. J Neural Transm 199l;86:25â€”41.

120. Agren H, Reibring L, Hartvig P. Ctal. Monoamine metabolism in human prefrontalcortex and basal ganglia. PET studies using [a-' â€˜C]L-5-hydroxytryptophan and[p-I â€˜C]L-DOPA in healthy volunteers and patients with unipolar depression. Dc

pressionl993;l:7lâ€”8l.121. Snow BJ, Tooyama I, McGeer EG, et al. Human positron emission tomographic

[â€œF]fiuorodopastudies correlate with dopamine cell counts and levels. Ann Neurol1993;34:324â€”330.

122. Eidelberg D, Takikawa 5, Dhawan V, Ctal. Striatal â€œF-DOPAuptake: absence of anaging effect. J Cereb Blood Flow Metab l993;l3:88lâ€”888.

123. Martin WRW, Palmer MR. Patlak CS, Calne DB. Nigro-striatal function in manstudied with positron emission tomography. Ann Neurol 1989;26:535â€”542.

124. Sawle GV, Colebatch JO, Shah A, et al. Striatal function in normal aging:implications for Parkinson's disease. Ann Neurol l990;28:799â€”804.

125. Kish 51, Zhong XH, Hornykiewicz 0, et al. Striatal 3,4-dihydroxyphenylalaninedecarboxylase in aging: disparity between postmortem and positron emission tomography studies. Ann Neurol 1995;38:260â€”264.

126. HolthoffVA, Kessler I, Pietrzyk U, et al. Motor and cognitive deficits in Parkinson'sdisease are related to striatal dopamine uptake [Abstract]. J Cereb Blood F/ow Metab1993;13:S253.

127. Deiesus OT, Mukhergee J. Radiobrominated m-tyrosine analogs as potential CNSL-dopaPET tracers.BiochemBiophysRetCommun 1988;l50:l027â€”lO3l.

128. Melega WP, Perlmutter MM, Luxen A, et al. 4-['8F]Fluoro-L-m-tyrosine: AnL-3,4-dihydroxyphenylalanine analog for probing presynaptic dopaminergic functionwith positron emission tomography. J Neurochem 1989;53:31 1â€”314.

129. Nahmias C, Wahl L, Chirakal R, et al. A probe for intracerebral aromatic amino-aciddecarboxylase activity: distribution and kinetics of [â€œF]6-fluoro-L-m-tyrosine in thehuman brain. Mov Disord l995;lO:298â€”304.

130. DeiesusOT, Holden JE, EndresC, Ctal. Visualization of dopaminenerve terminalsby positron tomography using [â€œF)[email protected] Rev1992;597:l5lâ€”154.

131. Berry MD, Juorio AV, Paterson IA. The functional role of monoamine oxidases Aand B in the mammalian central nervous system. Prog Neurobiol l994;42:375â€”391.

132. Breakefield XO, Chen Z-Y, Tivol E, Shalish C. Molecular genetics and inheritanceof human monoamine oxidases A and B. Neuro! Dis Ther 1994;21:95â€”l12.

133. Youdim MBH, RiedererP. Dopamine metabolismand neurotransmissionin primatebrain in relationship to monoamine oxidase A and B inhibition. J Neural Transml993;9l:18lâ€”195.

134. O'Carroll M, Fowler Ci, Phillips JP, et a!. The deamination of dopamine by humanbrain monoamine oxidase. Specificity for the two enzyme forms in seven brainregions. Arch Pharmacol 1983;322: 198â€”202.

135. Johnston JP. Some observations upon a new inhibitor ofmonoamine oxidase in braintissue. Biochem Pharmacol 1968;l7:1285â€”l297.

136. Knoll I, Magyar K. Some puzzling effects of monoamine oxidase inhibitors. AdvBiochem Psychopharmaco/ l972;5:393â€”408.

137. Oreland L, Arai Y, Stenstrom A, Fowler Ci. Monoamine oxidase activity andlocalization in the brain and the activity in relation to psychiatric disorders. Mod ProbPharmacopsychiafty 1983; 19:246â€”254.

138. RiedererP. KonradiC, SchayV. et al. LocalizationofMAO A andMAO B in humanbrain: a step in understanding the therapeutic action of L-deprenyl. In: Yahr MD,Bergman Ki, eds. Advances in Neurology. New York, NY: Raven Press, 1986:111â€”118.

139. Strolin, Benedetti M, Dostert P. Monoamine oxidase: from physiology and pathophysiology to the design and clinical application of reversible inhibitors. Adv DrugResearch1992;23:67â€”125.

140. Birkmayer W, Knoll J, Youdim MBH, et al. Increased life expectancy resulting fromaddition of L-deprenyl to L-DOPA treatment in Parkinson's disease: a Iongtermstudy.JNeural Transm 1985;64:l13â€”127.

141. Tetrud JW, Langston 1W.The effect ofdeprenyl (selegeline) on the natural history ofParkinson's disease. Science l989;245:5l9â€”522.

142. Olanow CW, Calne D. Does selegeline monotherapy in Parkinson's disease act bysymptomatic or protective mechanisms? Neurology l99l;41 (suppl 4):l3â€”26.

143. Fowler IS, MacGregor RR, Wolf AP, et al. Mapping human brain monoamineoxidase A and B with â€˜â€˜C-suicideinactivators and positron emission tomography.Science 1987;235:48 1â€”485.

144. Shinotoh H, Inoue 0, Suzuki K, et al. Kinetics of [â€â€˜C]N,N-dimethylphenethylamine

in mice and humans: potential for measurement ofbrain MAO B activity. J NucI Med1987;28:I006â€”lOl 1.

145. Fowler 15, Wang 0-i, Logan J, et al. Selectivereductionof radiotracertrappingbydeuterium substitution: comparison of [â€˜â€˜C]L-deprenyland [â€˜â€˜C]L-deprenyl-D2 forMAO B mapping. JNuclMed 1995;36:1255â€”1262.

146. Fowler J5, Volkow ND, Logan I, et al. Slow recovery ofhuman brain MAO B afterL-Deprenyl (selegeline) withdrawal. Synapse I994; 18:86â€”93.

147. Bench Ci, Price OW, Lammertsma AA, et al. Measurement of human cerebralmonoamine oxidase B (MAO B) activity with positron emission tomography (PET):a dose ranging study with the reversible inhibitor Ro 19 6327. EurJ Clin Pharmacol1991;1O:l69â€”l73.

148. Fowler 15, Volkow ND, Logan I, et al. MAO B inhibition therapy in Parkinson'sdisease: the degree and reversibility of human brain MAO B inhibition by RO196327.Neurology1993;42:1984â€”1992.

149. Guldberg HC, Marsden CA. Catechol-O-methyltransferase: pharmacological aspectsand physiological role. Pharmacol Rev 1975;27:135â€”206.

150. Mannisto TP, Kaakkola S. New selective COMT inhibitors: useful adjuncts forParkinson's disease? TIPS 1989;I0:34â€”36.

151. Wood PL, Altar CA. Dopamine release in vivo from nigrostriatal mesolimbic andmesocortical neurons: utility of 3-methoxytyramine measurements. Pharmacol Rev1988;40:l63â€”187.

152. Axelrod I. The 0-methylation ofepinephrine and other catechols in vitro and in vivo.Sciencel957;126:1657â€”l660.

153. Mannisto PT, Kaakkola 5, Nissinen E, et al. Potent, selective and orally activeinhibitors of catechol-O-methyltransferase. L@feSci l988;32: 1465â€”1471.

154.DingYS,GatleySI, FowlerIS, et al. Mappingcatechol-O-methyltransferase(invivoâ€”initial studies with [â€œF]Ro41-0960. Life Sci 1995;58:195â€”208.

155. Friedman AM, Deiesus 01, Revenaugh I, et al. Measurements in vivo of parametersofthe dopamine system. Ann Neurol l984;15:566â€”576.

156. Ross SB, Jackson DM. Kinetic properties ofthe accumulation of3H raclopride in themouse in vivo. Arch Pharmaco! l989;340:6â€”12.

157. Dewey SL, Smith OW, Logan J, et al. GABAergic inhibition of endogeneousdopamine release measured in vivo with â€˜â€˜C-racloprideand positron emissiontomography. J Neurosci 1992;12:3773â€”3780.

158. Dewey SL, Smith 05, Logan I, et al. Striatal binding of the PET ligand â€˜â€˜C-raclopride is altered by drugs that modify synaptic dopamine levels. Synapse1993;13:350â€”356.

159. Hume 5P, Myers R, Bloomfield PM, et al. Quantitation ofcarbon-l I-labeled raclopndein rat striatum using positron emission tomography. Synapse l992;12:47â€”54.

160. Volkow ND, Wang G-J, Fowler IS, et al. Imaging endogenous dopamine competitionwith [â€œC]raclopridein the human brain. Synapse 1994;l6:255â€”262.

161. Dewey SL, Logan I, Wolf AP, et al. Amphetamine induced decreases in [â€œF]N-methylspiroperidol binding in baboon brain using positron emission tomography(PET). Synapse 1991;7:324â€”327.

162. Logan I, Dewey SL, WoIfAP, et al. Effects ofendogenous dopamine on measures of[â€œF]N-methylspiroperidol binding in the basal ganglia: comparison of simulationsand experimental results from PET studies in baboons. Synapse 1991;9:195â€”207.

163. Nordstrom AL, Farde L, Pauli 5, et al. PET analysis of central [â€˜â€˜C]raclopridebinding in healthy young adults and schizophrenic patients, reliability and age effects.Human Psychopharmacol 1992;7: I 57â€”165.

164. Volkow ND, FowlerlS, Wang 0-i, et al. Reproducibility ofrepeated measures of' â€˜Craclopride binding in the human brain. J Nuc! Med l993;34:609â€”613.

165. Laruelle M, Abi-Dargham A, van Dick C, et al. SPECT imaging ofstriatal dopaminerelease after amphetamine challenge. J Nuc! Med 1995;36:l 182â€”1190.

166. Wang 0-i, Volkow ND, Fowler iS, et al. Methylphenidate decreases regionalcerebral blood flow in normal human subjects. L@feSci l994;54:1433â€”1446.

167. Logan J, Volkow ND, Fowler IS, et al. Effects of blood flow on [â€˜â€˜C]raclopridebinding in the brain: model simulations and kinetic analysis of PET data. J CerebBlood Flow Metab 1994;14:995â€”l0l0.

168. Inoue 0, Tsukada H, Kobayashi K, et al. Swim stress alters in vivo binding of[3H]N-methylspiperone. Neuropharmacol 1991 ;30: 1101â€”1106.

169. Deterding LI, Dix K, Burka LT, Tomer KB. On-line coupling ofin vivo microdialysiswith tandem mass spectrometry. Anal Chem 1992;64:2636-264l.

170. Gifford AN, Gatley 51, Ashby CR. Endogeneously released dopamine inhibits thebinding of dopaminergic PET and SPECT uganda in superfused rat striatal slices.Synapse1990;22:232â€”238.

171. Riederer P. Lange KW, Kornhuber I, et al. Glutamatergic-dopaminergic balance inthe brain: its implications in motor disorders and schizophrenia. Arzneim-Forsch1992;42:265â€”268.

172. Meltzer H. Clinical studies on the mechanism of action of clozapine: the dopamineserotonin hypothesis of schizophrenia. Psychopharmacol 1989;99:518â€”527.

173. Schaefer GJ. Opiate antagonists and rewarding brain stimulation. Neurosci Behav Rev1988;l2:lâ€”17.

174. Girault IA, Halpain 5, Greengard P. Excitatory amino acid antagonists and Parkinson's disease. Trends Neurosci 1990;13:325â€”326.




175. Stockmeier CA, DiCarlo Ii, Zhang Y, et al. Characterization oftypical and atypicalantipsychotic drugs based on in vivo occupancy of serotonin-2 and dopamine-2receptors. J Pharmacol Exp Ther l993;266: 1374â€”1384.

176. Dewey 5L, Brodie ID, Fowler 15, et al. Positron emission tomography studies ofdopaminergic/cholinergic interactions in the baboon brain. Synapse l990;6:32lâ€”327.

177. Dewey SL, Smith OS, Logan I, et al. Effects of central cholinergic blockade onstriatal dopamine release measured with positron emission tomography in normalhuman subjects. Proc NatI Acad Sci USA l993;90:l 1816â€”11820.

178. Dewey SL, Smith OS, Logan I, et al. Serotonergic modulation of striatal dopaminemeasured with positron emission tomography and in vivo microdialysis. J Neurosci1995; 15:821â€”829.

179. Smith 05, Dewey SL, Logan I, et al. Opiate modulation of striatal dopamine releasemeasured with positron emission tomography and â€˜â€˜C-raclopride[Abstract]. SocNeurosciAbstrl993;19:128.9.

180. Ingvar M, Stone-Elander 5, Rogers 0, et al. Striatal D2/acetylcholine interactions:PET studies of the vesamicol receptor. NeuroReport l993;4:l311â€”13l4.

181. Reith I, Benkelfat C, Sherwin A, et al. Elevated dopa decarboxylase activity in livingbrain of patients with psychosis. Proc Nail Acad Sci USA 1994;9l :11651â€”11654.

I82. Grace AA. Phasic versus tonic dopamine release and the modulation ofthe dopaminesystem responsivity. Neurosci 199l;4l :1â€”24.

183. Reivich M, KuhI D, Wolf AP, et al. The [â€œF]fluorodeoxyglucose method for themeasurement oflocal cerebral glucose utilization in man. Circ Res l979;44:l27â€”137.

184.MatsM, FinkDI, GainerH.Activity-dependentenergymetabolismin ratposteriorpituitary primarily reflects sodium pump activity. J Neurochem l980;34:2l3â€”215.

185. Sokoloff L, Reivich M, Kennedy C, et al. The [â€˜4C]deoxyglucose method formeasurement of local cerebral glucose utilization. J Neurochem l977;28:897â€”916.

186. Pomno U, Crane AM. Metabolic mapping of the effects of drugs of abuse with the2-['4C]deoxyglucose method. Modern methods in pharmacology, volume 6, testingand evaluation ofdrugs ofabuse. New York, NY: Wiley-Liss; 1990:147â€”164.

187. Wolkin A, Angrist B, Wolf A, et al. Effects of amphetamine on local cerebralmetabolism in normal and schizophrenic subjects as determined by positron emissiontomography. Psychopharmacol I987;92:24 1â€”246.

188. London ED, Cascella NO, Wong DF, et al. Cocaine-inducedreductionof glucoseutilization in human brain. A study using positron emission tomography and[fluorine-l8]-fluorodeoxyglucose. Arch General Psychiatry 1990;47:567â€”574.

189.VolkowND,BrodieID, WolfAP,etal.Brainmetabolismin schizophrenicsbeforeand after acute neuroleptic administration. J Neurol Neurosurg Psychiatry 1986;49:I199â€”1202.

190.BartlettEl, BrodieID, SimkowitzP. Ctal. Effectsof haloperidolchallengeonregional cerebral glucose utilization in normal human subjects. Am J Psychiatry1994;l51:681â€”686.

191. Wolkin A, iaeger I, Brodie ID, et aI. Persistence ofcerebral metabolic abnormalitiesin chronic schizophrenia as determined by positron emission tomography. AmJ Psychiatry l985;142:564â€”571.

192. DeLisi LE, Holcomb HH, Cohen RM, Ct a]. Positron emission tomography inschizophrenic patients with and without neuroleptic medication. J Cereb B!ood FlowMetab I985;5:20lâ€”206.

193. BuchsbaumMS. Wu IC, DeLisi LE, et al. Positronemissiontomographystudiesofbasal ganglia and somatosensory cortex neuroleptic drug effects. Biol Psychiatry1987;22:479â€”494.

194.SzechtmanH,NahmiasC,GarnettES,etal.Effectofneurolepticsonalteredcerebralglucose metabolism in schizophrenia. Arch Gen Psychiatry l988;45:523â€”532.

195. Wik 0, Wiesel F-A, Sjorgen I, et al. Effects of sulpinde and chlorpromazineonregional cerebral glucose metabolism in schizophrenic patients as determined bypositron emission tomography. Psychopharmaco! 1989;97:309â€”318.

196. Buchsbaum MS. Potkin SO, Siegel BV Ir, et al. Striatal metabolic rate and clinicalresponse to neuroleptics in schizophrenia. Arch Gen Psychiatry l992;49:966â€”974.

197. Bartlett El, Wolkin A, Brodie ID, Ctal. Importance ofpharmacologic control in PETstudies: effects of thiothixene and haloperidol on cerebral glucose utilization inchronic schizophrenia. Psychiatry Res 199 1;40: 115â€”124.

198. Holcomb HH, Cascella NO, Thaker OK, et al. Functional sites of neuroleptic drugaction in the human brain: PETIFDG studies with and without haloperidol. Am JPsychiatryI996;l53:41â€”49.

199. Eidelberg D, Moeller JR. Ishikawa T, et al. Early differential diagnosis of Parkinson'sdisease with â€œF@@fluorodeoxyglucoseand positron emission tomography. Neurologyl995;45: 1995â€”2004.

200. Eidelberg D, Moeller JR. Dhawan V. et al. The metabolic anatomy of Parkinson's

disease: complementary@ and â€˜8F-fluorodopapositron emission tomography studies. Mov Disord l990;5:203â€”2l3.

201. Swan IAA, Korf I. In vivo dopaminereceptorassessmentfor clinical studiesusingpositron emission tomography. Biochem Pharmacol l987;36:2241â€”2250.

202. Mintun MA, Raichle ME, Kilbourn MR. et al. A quantitative model for the in vivoassessment of drug binding sites with positron emission tomography. Ann Neurol1984;l5:217â€”227.

203.WongDF,OjeddeA,WagnerHN,etal.Quantificationofneuroreceptorsinthelivinghuman brain. II. Inhibition studies of receptor density and affinity. J Cereb BloodFlow Metab 1986;6:147â€”153.

204. Farde L, Eriksson L, Blomquist 0, Halldin C. Kinetic analysis ofcentral [â€â€˜C]raclo

pride binding to D2-dopamine receptors studied by PETâ€”a comparison to theequilibrium analysis. J Cereb Blood Flow Metab l989;9:696â€”708.

205. PerlmutterIS, Larson KB, Raichle ME, Ctal. Strategiesfor in vivo measurementofreceptor binding using positron emission tomography. J Cereb B!ood Flow Metab1986;6: 154â€”169.

206. Logan I, Wolf AP, Shiue C-Y, Fowler IS. Kinetic modeling of a receptor-ligandbinding applied to positron emission tomographic studies with neuroleptic tracers.J Neurochem l987;48:73â€”83.

207. Patlak CS, Blasberg RG, Fenstennacher ID. Graphical evaluation of blood-to-braintransferconstants from multiple-time uptake data.JCerebBloodFlowMetab l983;3:lâ€”7.

208. Patlak CS, Blasberg RO. Graphical evaluation of blood-to-brain transfer constants

from multiple-time uptake data. Generalization. J Cereb Blood Flow Metab 1985;5:584â€”590.

209. Logan I, Fowler 15, Volkow ND, et al. Graphical analysisof reversibleradioligandbinding from time-activity measurements applied to [N-' â€˜C-methyl]-(â€”)-cocainePET studies in human subjects. J Cereb Blood Flow Metab l990;lO:740â€”747.

210. Koeppe RA, Holthoff VA, Frey KA, et al. Compartmentalanalysisof [â€˜â€˜C]flumazenil kinetics for the estimation ofligand transport rate and receptor distribution usingpositron emission tomography. J Cereb Blood Flow Metab 1991 ;l 1:735â€”744.

21 1. Carson RE. Parameter estimation in positron emission tomography. In: Phelps M,Mazziotta I, Schelbert H, eds. Positron emission tomography and autoradiography:principles and applications for the brain and heart. New York, NY: Raven Press;I986:347â€”390.

212. Huang S-C, Barrio IR, Phelps ME. Neuroreceptor assay with positron emissiontomography: equilibrium versus dynamic approaches. J Cereb Blood Flow MetabI986;6:5I5â€”521.

213. Zeeberg BR, Gibson RE, Reba RC. Accuracy of in vivo neuroreceptor quantificationby PET and review of steady-state, transient, double injection and equilibriummodels. IEEE Trans Med Imaging l988;7:203â€”2l5.

214. Delforge I, Syrota A, Mazoyer BM. Experimental design optimization: theory andapplication to estimation of receptor model parameters using dynamic positronemission tomography. Phys Med Biol 1989;34(4):419â€”435.

215. Logan I, Schlyer DI, Wolf AP, et al. Antipsychotics D-2 receptor occupancy andplasma concentration. Biol Psychiatry (Letter) 1990;28:1068â€”l070.

216. Holthoff VA, Koeppe RA, Frey KA, et al. Differentiation of radioligand delivery andbinding in the brain: validation of a two-compartment model for [â€˜â€˜C]flumazenil.JCereb Blood Flow Metab 1991 ;I 1:745â€”752.

217. Carson RE, Channing MA, Blasberg RO, Ct al. Comparison of bolus and infusionmethods for receptor quantitation: application to [â€œF]cyclofoxy and positron emission tomography. J Cereb Blood Flow Metab l993;l3:24â€”42.

218. Wong DF, Wagner HN Ir, Tune LE, et al. Positronemission tomography revealselevated D2-dopamine receptors in drug-naive schizophrenics. Science 1986:234:1558â€”1563.

219. Farde L, Wiesel F-A, Stone-Elander 5, Ctal. D2-dopamine receptors in neurolepticnaive schizophrenic patients. Arch Gen Psychiatry 1990;47:213â€”219.

220. Votaw iR, Kessler RM, Dc Paulis T. Failure of the three-compartmentmodel todescribe the pharmacokinetics in brain of a high affinity substituted benzamide.Synapse 1993;l5:177â€”190.

221. Samana P. Cottechia 5, Costa T, Lefkowitz Ri. A mutation-induced activated state ofthe @32-adrenergicreceptor. J Biol Chem l993;268:4625â€”4636.

222. Costa T, Ogino Y, Munson P1, Ct al. Drug efficacy at guanine nucleotide-bindingregulatory protein-linked receptors: thermodynamic interpretation of negative antagonism and of receptor activity in the absence of ligand. Mo! Pharmacol l992;41:549â€”560.

223. Black 1W, Shankley NP. Inverse agonists exposed. Nature 1995;374:214â€”215.224. Bond RA, Leff P, iohnson TD, et al. Physiologicaleffects of inverse agonistsin

transgenic mice with myocardial overexpression of the f32-adrenoceptor. Nature1995;374:272â€”276.

225. Leff P. The two-state model of receptor activation. Trends Pharmacol Sci 1994; 15:408â€”409.

226. Milligan 0, Bond RA, Lee M. Inverse agonists: pharmacological curiosity orpotential therapeutic strategy? Trends Pharmaco! Sci 1995;16: 10â€”13.

227. Tiberi M, Caron MC. Constitutive activity of dopamine Dl receptor subtypes. C/inNeuropharmaco! 1995;18:S43â€”548.

228. Robinson PR, Cohen GB, Zhukovsky EA, Oprian DD. Constitutively active mutantsof rhodopsin. Neuron 1992;9:719â€”725.

229. Parma I, Duprez L, Van Sande I, et al. Somatic mutations in the thyrotropin receptorgene cause hyperfunctioning thyroid adenomas. Nature 1993:365:649â€”651.

230. Yang Z-Y, Mukherjee I. Development of agonists as radiotracers for the assessmentof the functional state of the dopamine D2 receptors [Abstract]. J Nuc! Med1995;36(suppl): l52P.

231.ManjiHK,Chen0, SimonH,Ctal.Guaninenucleotide-bindingproteinsin bipolaraffective disorder. Arch Gen Psychiatry l995;52:l35â€”144.

232. Nestler El. Molecular neurobiology of drug addiction. Neuropsychopharmacol1994;!1:77â€”88.

233.ManjiHK,PotterWZ,LenoxRH.Signaltransductionpathways.Moleculartargetsfor lithium's actions. Arch Gen Psychiatry 1995;52:531â€”543.

234. Takahashi T, Ido T, Ootake A, et al. [â€œF]Labeledl,2-diacylglycerols; a new tracerfor the imaging of second messenger system [Abstract]. J Labeled Compds Radiopharm 1994;35:5l7â€”5l9.

235. Dewanjee MK, Ohafounpour AK, BootheT, et al. Gallium-67 labeling of antisensedeoxyoligonucleotide (GAASDON) probes and uptake in leukemic cells (P388)[Abstract]. J Labe!ed Compd Radiopharm 1994;35:356â€”357.

236. Schrell UM, Nomikos P. Fahlbusch R. Presence of dopamine Dl receptors andabsence of dopamine D2 receptors in human cerebral meningioma tissue. J Neurosurg l992;77:288â€”294.

237. Muhr C, Bergstrom M, Lundberg P0, Ct al. Dopamine receptors in pituitaryadenomas: PET visualization with â€˜â€˜C-N-methylspiperone. J Comp Assist Tomog1986;l0:l75â€”180.

238. Luabeya MK, Maloteaux IM, Laduron PM. Regional and cortical laminar distributions of serotonin 52, benzodiazepine, muscarinic and dopamine D2 receptors inhuman brain. J Neurochem 1984;43:1068â€”1071.

239.McElvainIS, Schenk10.A multisubstratemechanismof striataldopamineuptakeand its inhibition by cocaine. Biochem Pharmacol l992;43:2 189â€”2199.

240.MackayAVP,DaviesP.DewarCM,YatesCM.Regionaldistributionof enzymesassociated with neurotransmission by monoamines, acetylcholine and GABA in thehuman brain. J Neurochem 1978;30:827â€”839.

241. O'CarroIl AM, Anderson MC, Tobbia I, et al. Determinationof the absoluteconcentrations ofmonoamine oxidase A and B in human tissues. Biochem Pharmacol1989;38:901â€”905.




242.HuhMM,FriedhoffAJ.Multiplemolecularformsof catechol-O-methyltransferase.J Biol Chem 1979;254:299â€”308.

243. KastnerA, AngladeP. BounaixC, et al. lmmunohistochemicalstudyof catechol-Omethyltransferase in the human mesostriatal system. Neurosci 1994;62:449â€”457.

244. AndersenPH,GronvaldFC, HohlwegR, et al. NNC-l 12,NNC-687andNNC-756:new, selective and highly potent dopamine DI-receptor antagonists. Eur J Pharmaco!I992;2I9:45â€”52.

245. Inoue0, KobayashiK, SakiyamaY, SuzukiT. Theeffectof benzodiazepineson thebinding of[3H]SCH 23390 in vivo. Neuropharmacol 1992;31:ll5â€”12l.

246. Farde L, Ehrin E, Enksson L, et al. 5ubstituted benzamides as ligands forvisualization of dopamine receptor binding in the human brain by positron emissiontomography. Proc Nat! Acad Sci USA 1985:82:3863â€”3867.

247. Inoue0, KobayashiK, TsukadaH, et al. Differencesin in vivo receptorbindingbetween [3H]N-methylspiperone and [3H]raclopride in reserpine-treated mouse brain.iNeural Transm l991;85:lâ€”lO.

248.KungHF,Pan5, KungMP,etal. In vitroandin vivoevaluationof [â€˜231]IBZM:apotential CNS D2-dopamine receptor imaging agent. J Nuc! Med 1989;30:88â€”92.

249.KesslerRM,AnsariMS.SchmidtDE,et al. High-affinitydopamineD2receptorradioligands. 2. [â€˜23ljepidepride,a potent and specific radioligand for the characterization ofstriatal and extrastriatal dopamine D2 receptors. Life Sci 1991;49:6l7â€”628.

250. Pellevoisin C, Chalon 5, Zouakia A, et al. Comparison oftwo radioiodinated ligandsofdopamine D2 receptors in animal models: iodobenzamide and iodocthylspiperone.Life Sci 1993;52:185lâ€”1860.

251. LaruelleM, WallaceE, SeibylIP, et al. Graphical,kineticandequilibriumanalysesof in vivo [â€˜23I]f3-CITbinding to dopamine transporters in healthy human subjects.J Cereb Blood Flow Metab 1994; 14:982â€”994.

252. Gatley SI, Ding V-S. Volkow ND, Ctal. Binding ofd-threo-[' â€˜C]methylphenidatetothe dopamine transporter in vivo: insensitivity to synaptic dopamine. Eur J Pharmacol 1995;281:14lâ€”l49.

253. Volkow ND, Fowler IS, Wolf AP, et al. Distribution and kinetics of carbon-I I-cocaine in the human body measured with PET. J NucI Med 1992;33:52lâ€”525.

254. Scheffel U, Boja 1W, Kuhar Mi. Cocaine receptors: in vivo labeling with 3H-(â€”)cocaine, 3H-WLN 35,065-2 and 3H-WIN 35,428. Synapse 1989;4:390â€”392.

patterns are variable and not easily interpreted as being causallyrelated to a particular disease entity (e.g., mild head injury,AIDS, chronic fatigue syndrome, toxic exposures, foreign-bodyreactions, autoimmune disorders, substance abuse, violence andothers) (30,31 ). Furthermore, while specific PET and SPECTdefects appear useful to confirm a certain disease diagnosis orto support the localization of a particular clinical finding(32â€”35),there is only limited evidence that specific diseases orneurological, psychiatric or behavioral deficits can be predictedfrom specific scan patterns (36,37). While these types of studiesremain extremely important for identifying previously unrecognized brain abnormalities and potential disease mechanisms ina variety of neuropsychiatric illness, their utility in the management of individual patients is still far from clear.

As SPECT and PET become more widely available, enthusiasm tempered by a cautious attitude seems appropriate regarding their use in brain disorders as a whole, given clear evidencethat functional patterns are highly dependent on a large numberof technical, analytical and physiological variables. The purpose ofthis document is to provide recommendations and basicguidelines for brain SPECT and PET acquisition, interpretationand reporting, with particular attention to recognized andgenerally accepted clinical indications, and to urge cautionregarding applications in unstudied behavioral disorders.

BASIC METhODOLOGICAL ISSUESThe Society of Nuclear Medicine has recently published

technical guidelines on brain SPECT acquisition and imagereconstruction (38,39). However, even with adherence to theserecommendations, the quality of the study will vary frominstitution to institution and is dependent on several factors,including instrumentation, collimation, filters, behavioral stateofthe subject during tracer uptake, timing ofthe scan relative totracer injection, scan duration, patient movement, attenuation,reconstruction and analytical methods, as well as quality control.

In general, disease patterns are established using specificinstruments, physiological measurements and methods of analysis. However, for some situations, the imaging technologyavailable at a particular site may not be sufficient for diagnosticpurposes. Accordingly, the degree to which subsequent studies

The development and evolutionof functionalbrain imagingtechnologies and their broad application to a wide range of neurological andpsychiatric disorders have led to their scientifically sound use inspecific clinical situations. In addition, there is a growing diversity ofempirical new applicatkxis where there is ktde prevkxis research orclinical experience. Therefore, a committee of the Brwn ImagingCouncil of the Society of Nuclear Medicine was formed to addresstheneedforspecificguidelinesregardingscaninterpretationandreporting. This committee considered the wide range of current andpotential uses of PET and SPECT, including its growing role inforensics. A set of basic guidelines for the reporting and interpretation of brain imaging studies applicable to all clinical situations,including forensics, was formulated. These gukielines were cornposed in a manner sensitive to the need for standards that arescientifically defensible now, and which willcontinue to be valid asthe field evolves. It is the intent of the committee and this summarydocument to positively influence the clinical use of brain SPECT andPET by offering guidance concerning the elements essential to acomplete and useful clinical report, defining standards to differentiate well-established clinical applications from research uses andproviding a framework in which to consider the appropriateness offunctional brain imaging used in the forensic arena.Key Words PET; SPECT; clinical applications; forensics; gukielines

J NucIMed1996;37:1256-1259

TheuseofSPECTandPETinthemanagementofpatientswith stroke, epilepsy, brain tumors and dementia, and in somecases, movement disorders and moderate-to-severe head traumais now well recognized (1â€”9).Scan abnormalities have alsobeen identified in patients with certain psychiatric diagnoses,including depression, obsessive compulsive and panic disorders, schizophrenia and substance abuse (10â€”19), but consistent patterns for these disorders have not been confirmed.Sensitivity and specificity are often unknown and in manycases, group patterns may actually be too subtle to detect inindividual patients.

More elusive or less well-characterized behavioral syndromes have also been studied (20â€”29). In many cases, the

Received Mar. 12, 1996; accepted Mar. 20, 1996.Forcorrespondenceorreprintscontact JOannaWIISOn,CouncilCoordinator,Society

ofNuclearMedicine,1850SamuelMorseDr.,Reston,VA22090.

1256 THEJOURNALOFNUCLEARMEDICINEâ€¢Vol. 37 â€¢No. 7 July 1996

Ethical Clinical Practice of Functional Brain ImagingSociety of Nuclear Medicine Brain Imaging CouncilReston, Virginia



1996;37:1242-1256.J Nucl Med. Nora D. Volkow, Joanna S. Fowler, S. John Gatley, Jean Logan, Gene-Jack Wang, Yu-Shin Ding and Stephen Dewey PET Evaluation of the Dopamine System of the Human Brain

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