Enkephalins and endorphins:The endogenous opiatesDANIEL MILLOY, CRNA, BSNJackson, Mississippi

The discovery of the opiate receptors inthe central nervous system of man andsubsequent identification of endogenousopiate-like ligands (enkephalins andendorphins) has provided a model for theanalgesia, euphoria and addictionproduced by the narcotics. In this article,the author reviews the background,biosynthesis and degradation, distribution,nociceptive transmission and analgesiceffects of these opioid peptides.

Anesthesia is a clinical orchestration of analgesia,amnesia, hypnosis and muscle relaxation. Of thesecomponents, analgesia is of concern to the anes-thetist in all phases of surgery: preoperatively,intraoperatively and postoperatively.

Although morphine is an effective drug forthe relief of pain, its effects are broader than justrelief. Hence, chemists have worked to create an-algesics which are devoid of morphine's side effects,which include respiratory depression, decreasedgastrointestinal motility, mood alteration, increased/decreased levels of specific hormones, toleranceand duration.

A central issue in the search for the "perfect"analgesic was to determine why and how morphineshould work in the first place. Why should a plantalkaloid have a specific binding site in the centralnervous system of man? Researchers reasoned thatperhaps there exists a morphine-like substance pro-duced by the body. This research and the subse-quent endorphin/enkephalin discovery is reviewedin this article.

Historical perspectiveThe fundamental concept in opiate activity is

understanding the function of the opiate receptor,the site which recognizes and binds opioid drugswith multiple biochemical and physiological se-quelae. The concept of an opioid receptor arosefrom research on systems that were sensitive tothe effects of narcotics. It was found that electricalstimulation of the guinea pig ileum or of themouse vas deferens caused acetylcholine release inthe ileum and norepinephrine release in the vasdeferens. Morphine was found to inhibit the re-lease of these transmitter substances, consequentlyinhibiting the electrically stimulated contraction. 1' 2

Further research showed that this action was stereo-specific and was reversible by opiate antagonists. 8

Liebeskind produced analgesia by electricalstimulation of the mesencephalic central gray andthe periventricular gray regions.4 This analgesia,shown to be reversed by naloxone", exhibited crosstolerance with morphine-induced analgesia. 6

Using highly specific opioid agonists and an-tagonists, binding sites in brain synaptic mem-branes and guinea pig ileal homogenates weredemonstrated. 7 These receptor sites were functionalonly with opioid drugs; drugs related to cholin-ergic, adrenergic, serotonergic and histaminergicsystems were ineffective. Steroids and peptide hor-mones were also ineffective. 8

Several of these morphine-like substances werenot at all like morphine. For example, their mole-cular weight was in the range of 800-1000. (Mor-phine has a molecular weight of 285.) Both leucineaminopeptidase and carboxypeptidase were found

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to destroy activity. 9 It became evident that thesesubstances were in fact peptides.

In 1975, Hughes elucidated the structure ofthe morphine-like factor which had been pre-viously named enkephalin.9 Enkephalin proved tobe a mixture of two pentapeptides. One, (Met 5)-enkephalin, had the sequence H2 N-Try-Gly-Gly-Phe-Met-OH and the other, (Leu 5)-enkephalin, hadthe sequence H2 N-Try-Gly-Gly-Phe-Leu-H. It wasdemonstrated that synthetic (Met5)- and (Leu5 )-en-kephalins produced the full spectrum of opioid-likeeffects. Hughes also made the observation that thestructure of (Met")-enkephalin was contained with-in the sequence of a 91 amino acid known as beta-lipotropin (beta-LPH) isolated by Li in 1965.10The function of this polypeptide was known onlyto be regulation of fat metabolism. Additionalstudies found this peptide to be present in allvetebrates and it is not present in any inver-tebrates.

To clarify nomenclature, the opioid peptidesrelated to beta-LPH are known as endorphins. Theterm is analogous to the term corticotropin, whichdenotes a biologic activity rather than a specificchemical structure. The enkephalins are the spe-cific pentapeptides identified by Hughes andbelong to the endorphin class.

Further research with beta-LPH beginningwith the number 61 amino acid, which starts the(Met")-enkephalin pentapeptide, has identifiedother endorphins: alpha, gamma and delta. Li re-named the entire amino acid sequence beta-LPH61-91, which was originally known as C-fragment,to beta-endorphin (for endogenous morphine). 11

Biosynthesis and degradationIn order to understand the biosynthesis of

beta-endorphin we must first look at the adreno-corticotropic hormone (ACTH). ACTH is pro-duced by both the brain and the pituitary gland.The largest form of ACTH has a molecular weightof roughly 31,000 and is known as "big ACTH" or31 kACTH. ACTH contains 39 amino acids andhas a molecular weight of about 4,000.

Several pieces of evidence support the conten-tion that big ACTH may function as a biosyntheticprecursor to both ACTH and related peptides, andto )beta-lipotropin and its related peptides. Such anexample is found in the report of a particularpituitary tumor which produces peptides withopioid agonist activity. Additionally, staining thepituitary with antibodies to both ACTH or beta-LPH shows staining in the same cells of the parsintermedia and adenohypophysis. Tliirdly, the re-lease of beta-LPH and ACTH appears to be in

parallel fashion with circulating concentrations ofbeta-IPH and ACTH rising and falling togetherin response to various physiological manipulations.Also, dexamethasone depressed both circulatingACTII and beta-IPH1. Lastly, big ACTHI has beenshown to contain (as part of its amino acid se-

tquence) the entire sequences of beta-LPH andACTH. 12

Some research has been carried out to deter-mine the metabolism of the various opioid pep-tides. The enkephalins are extremely and rapidlydegrdeed ,by enzymes in various tissue homogenatesor in blood; however, beta-endorphin is morestable. This would serve to explain the increasedantinociceptive potency of beta-endorphin over(MetO)-enkephalin. In homogenates of the brain,enkephalin appears to be degraded primarily byremoval of the N-terminal tyrosine. 1

Distribution of opioid peptidesIt is interesting to note that the globus pallidus

has a concentration of enkephalins 5-10 timeshigher than any other area of the brain. Althoughthe function of this concentration is not clear, itmay be reasonable to believe that this is involvedin the extra-pyramidal control of motor activity.Narcotics and synthetic or natural opioid peptidescan produce either hypermotility or cataleptic ef-fects, depending on dosage and other factors. 14

The limbic system also has a high concentra-tion of enkeplialin; the nucleus accumbens andamygdala contain large amounts. This distributionmay provide a physiologic basis for mood alterationassociated with morphine use. Such a role mayimplicate enkephalins in mental illness.15

In the midbrain, concentrations of enkephalinare moderate to low with the exception of theperiaqueductal gray matter (PAG) and raphenuclei. These areas are known to be important inthe transmission of pain signals. Reynolds showedelectrical stimulation of the PAG produces anal-gesia in both animals and man, and in addition,microinjection of narcotics into this region pro-duces analgesia."' Mayer and Price demonstratedthe importance of the raphe nuclei and associatedserotoninergic systems in the mediation of painsensations.1 7

Outside the central nervous system (CNS),the greatest concentration of enkephlalin is in thegastrointestinal tract.1 8 In humans, the endocrinecells that stained for enkephalin also stained forgastrin. The presence of enkephalin in the gastro-intestinal tract fits well with the pharmacology ofnarcotics, that is, with the ability of morphine toproduce constipation.19

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Neuroenldocrine effects. The pituitary appearsto be a storage/synthesis site for beta-endorphins.The effect of beta-endorphin levels on other pitu-itary hormones has been the focus of some research.It is known that morphine and other opioid drugsmay increase thle release of prolactin, growthhormone and vasopressin (ADH) following peri-pheral or intraventricular administration. 20 Theeffects of the endogenous opioids vary with theanalogue and route of administration, but ingeneral, the opioid peptides cause the release ofprolactin, growth hormone and vasopressin, al-though the release of luteinizing hormone (ILH)and thyroid-stimulating hormone (TSH) are de-pressed.

Beta-endorphin is more potent in all aspectsthan (Met5)- or (Leu 5)-enkephalin. This is consis-tent with the rapid metabolic breakdown of thepentapeptides. The effects of the opioid peptidesare naloxone reversible, indicating that the effectsare mediated by opioid binding sites.21 In the caseof prolactin, the control of prolactin release ap-pears to be due to inhibition of dopamine releasein the hypothalamus, which then has a direct effecton the pituitary.22

Nociceptive transmissionStimuli which result in tissue damage produce

pain. This damage can result from mechanical,thermal or chemical insult. Nerve terminals re-sponding to noxious stimuli are terminals of slowlyconducting myelinated axons of the A-delta groupand the unmyelinated C fibers. Stimulation of theA-delta fibers by noxious stimuli produces a sharppricking type of sensation whereas the C fiberstransmit the more prolonged burning type of pain.

These fibers enter the CNS through the dorsalroot of the spinal nerves and their terminalssynapse with cells in the superficial layers of thespinal cord dorsal horn. The dorsal horn can besubdivided into layers based on the anatomicalcharacteristics of each area. The most dorsal regionis called lamina I. This region contains neuronswhich respond specifically to noxious stimuli andis the primary region of termination of the A-deltaand C fibers.

These fibers also terminate in the underlyinglayers-laminae II and III known as the substantiagelatinosa. Lamina V contains large cells whichrespond with a low firing rate to touch and witha much higher discharge rate to pinch or pin prick.Therefore, the neurons of laminae I, II, III andV are involved in the transmission of noxiousstimuli. They send axons up the spinal cord tohigher centers in the brainstem and thalamus.

Opiate receptors have been found in the spinalcord and are concentrated in laminae I and II.Experimentation reveals that morphine could beacting postsynaptically on dorsal horn neurons oron presynaptic terminals of the A and C fiberprimary afferents.

At most locations in the brain, microinjectionsof opioids do not produce analgesia, but markedanalgesia is produced when the opioid is injectedinto the PAG and the lateral mesencephalic reticu-lar formation and nucleus gigantocellularis. ThePAG does not have a direct projection to the spinalcord; therefore, PAG-induced descending inhi-bition must be relayed via other neurons. A pro-jection from the PAG to the nucleus raphe magnus,which is rich in opioid receptors, is known toproject to the spinal cord dorsal horn.2 3-2 5

We have discussed the nociceptive pathway,but how do opioid peptides inhibit a noxiousstimulus? Neurotransmitters and agonist drugs areconventionally thought to bind to receptor sites atsynapses and trigger some change in membraneproperties such as an alteration in ion permeabilityor cyclic nucleotide formation.

Studies of ionic influences on opiate receptorbinding suggest mechanisms whereby opiaterecognition at receptors is translated into alteredion permeability. Low concentrations of sodiumselectively decrease the binding of opiate agonistsand enhance the binding of antagonists. Sodiumappears to accelerate the rate at which opiateagonists dissociate from the opiate receptor. Recentstudies have shown that opiates and the enke-phalins do indeed inhibit neuronal activity at someloci by decreasing sodium conductance, apparentlyby acting directly at the ion channel.

It would probably be misleading to think ofopiate receptors as exclusively mediating analgesia,since the characteristic effect of the opiates inhumans is less a sl)ecific blunting of pain sensationthan the production of a peculiar state of in-difference, an emotional detachment from the ex-perience of suffering. This is consistent with thehigh distribution of opioid receptors in areas ofthe brain affecting mood.26

Endogenous analgesiaSoon after Hughes elucidated the structure

of enkephalin, synthetic pentapeptides were in-jected into mouse and rat brain and some transientanalgesic effects were observed. 27 We now knowthat these transient effects were due to the rapidmetabolic breakdown of the pentapeptides. Whenstable compounds such as beta-endorphin ormodified analogues of the enkephalins were used,

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the results were more sustained. 28 Comparing anal-gesic activity of beta-endorphin to that of (Met 5)-enkephalin, the former is several thousand timesmore active in spite of the fact that the twocompounds have similar affinities for the opiatereceptor.29

In a study involving 30 patients with chronicintractable pain, intracerebral stimulators wereimplanted for pain relief. Under local anesthesia,multiple cerebral spinal fluid (CSF) samples wereobtained every five minutes for 30 minutes. Thelevel of enkephalin-like material rose from a base-line of 0.68 ± 0.2 picomoles/ml to a level of 0.88-+0.3picomoles/ml at the end of 25 minutes ofstimulation. The baseline levels exhibited by thesepatients were significantly lower than normativelevels (3.1 picomoles/ml). 30

In a recent study, Oyama selected 14 patientswith chronic intractable pain in the back, chest,abdomen, rectum and thigh secondary to meta-static malignancies. After intrathecal administra-tion of 3mg of synthetic beta-endorphin at theL2-L3 interspace, profound and long lasting anal-gesia (mean duration of pain relief 33.4 hours)was produced. No respiratory depression, hypo-tension, hypothermia or catatonia was observed. 8'

In a study involving "on demand" pain relief,CSF levels of endorphins were measured in pa-tients who were fitted with a programmable injec-tion device for delivery of pethidine (meperidine).The study showed those patients with low endor-phin levels required more analgesia; those patientswith endorphin levels of 20 picomoles/ml requiredapproximately 150 mg of pethidine, whereas pa-tients with endorphin levels of 10 picomoles/mlrequired from 250-350 mg of pethidine. 32

Beta-endorphin levels have been found toincrease during the progress of labor in a groupof women having induced labors, without systemicanalgesia during labor. Levels rose six times fromthe control period to the post-partal period. 33 On ex-amination of human placenta, two beta-endorphin-like peptides have been noted; however, these differfrom beta-endorphin by being 12 amino acidslonger than the pituitary hormone. Houck hy-pothesizes that the physiologic function maymediate the pain of parturition.3 4

The identification of endogenous opioidsprovides a physiologic basis for acupuncture. Theanalgesic effect of acupuncture appears to be theresult of interaction between impulses from thesite of pain origin and afferent impulses from theacupuncture point, with this interaction takingplace at different levels of the CNS. For acupunc-ture to be effective in relieving pain, the nucleus

raphe magnus and endogenous opiates must beinvolved, as the analgesic effect of acupunctureis abolished by the administration of naloxone.35

All potent analgesics exhibit the phenomenaof tolerance and dependence. The endogenousopiate system now provides an explanation of this.Under resting conditions, opiate receptors areexposed to a certain basal level of enkephalin.Administered morphine binds to usually unoc-cupied receptors, thereby potentiating the anal-gesic effects of the enkephalin system. On sustainedtreatment with morphine, cells that have opiatereceptors find themselves overloaded with opiate-like material, and by a hypothetical feedbackloop, they send a message to enkephalin neuronsto stop releasing enkephalin. When this happens,the binding sites are exposed only to morphineand can tolerate more morphine to make up forthe enkephalin they are no longer receiving. Whenthe administration of morphine is stopped, theopiate receptors find themselves with neither mor-phine nor enkephalin, and this lack initiates asequence of events that results in withdrawalsymptoms.36

Other effectsDepression: The opiate peptides have been

shown to decrease the turnover of serotonin anddopamine, substances which are thought to beconcerned with mood. An abnormal amount ofCSF endorphin has been measured in patients withschizophrenic and manic-depressive psychosis.

Vomiting: Opiate receptors are concentratedin the area postrema of the brain stem which in-cludes the chemoreceptor trigger zone. Morphineseems to exert some of its central actions by in-terfering with dopaminergic transmission andinitiating a mechanism which resembles denerva-tion suipersensitivity.

Sexual function: When inoculated into thelateral ventricles of rats exposed to estrous fe-males, beta-endorphin enlanced the number ofmountings; this excitatory effect can be inhibitedby naloxone.

Constipation: Exogenous and endogenousopiates can reduce or abolish peristaltic activityof the isolated intestine induced by electricalstimulation or through increased intraluminalpressure.

Oliguria: Oliguria frequently occurs before amigraine attack and terminates with compensatoryp)olyuria after the attack is over. An opiate sub-stance of unknown structure and low molecularweight has been extracted from thle plasma ofmigraine victims. This substance produces pro-

Journal of the American Association of Nurse Anesthetists572

longed analgesia, reversible with naloxone, wheninjected into rat PAG. Because opiates stimulatethe release of antidiuretic hormone (ADH) fromthe posterior pituitary gland, the morphine-likefactor could participate in the modulation of hor-monal antidiuresis and the fluid retention in mi-graine attacks could be due to a release of ADH. 37

ConclusionThe study of the endogenous opiates will have

an impact on every discipline in medicine. Theresearch has been voluminous as evidenced by themore than 3,500 articles published in the last fouryears. Conjectures of how the specialty of anes-thesia may benefit are obvious-the result could bea new class of analgesics without the annoying sideeffects of the morphine class of opiates or possibly,a new class of hypnotics. Whatever the outcome,practitioners and patients alike will profit fromthe increased knowledge and understanding of acomplex physiologic system which was virtuallyunknown six years ago.

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(16) Reynolds DV. 1969. Surgery in the rat during electricalanalgesia induced by focal brain stimulation. Science. 164:444.(17) Mayer DJ et al. 1977. Antagonism of acupuncture anal-gesia in man by the narcotic antagonist naloxone. Brain Re-search. 121:368.(18) Linnoila RF, DiBugustine RP. 1978. An immunohisto-chemical and radioimmunological study of the distribution of(Met5)- and (Leu,)-enkephalin in the gastrointestinal tract.Neuroscience. 3:1187.(19) Polak JM, et al. 1977. Enkephalin-like immunoreactivityin the human gastrointestinal tract. The Lancet. 1:972.(20) Weitzman RB, et al. 1977. Beta-endorphin stimulates se-cretion of arginine vasopressin in vivo. Endocrinology. 101:1643.(21) Meltzer HY, et al. 1978. Effects of enkephalin analogueson prolactin release in the rat. Life Sciences. 22:1931.(22) Grandison L, Guidotti A. 1977. Regulation of prolactinrelease by endogenous opiates. Nature. 270:357.(23) Deakin JFW, Dostrovsky JO. 1979. Neural mechanisms ofopiate analgesia. Scientific Progress. 66:94-101.(24) Basdaum AI, Fields HL. 1978. Endogenous pain controlmechanisms: Review and hypothesis. Annals of Neurology. 4:451.(25) Bunney WB. 1979. Basic and clinical studies of endorphins.Annals of Internal Medicine. 240-1.(26) Snyder SH. 1977. Opiate receptors in the brain. New Eng-land Journal of Medicine. 295(5):266-70.(27) Coy DH, et al. 1976. Synthesis and opioid activities ofstereoisomers and other D-amino acid analogs of methionine-enkephalin. Biochemical and Biophysical Research Communi-cation. 73:632-8.(28) Pert CB, Pert A. 1976. (D-Ala2)-Met-enkephalin-amide:A potent, long-lasting synthetic pentapeptide analgesic. Science.194:330.(29) Wir E, Loh H. 1976. Physical dependence on opiate-likepeptides. Science. 193:1262-3.(30) Akil H, et al. 1978. Endorphins, beta-l.PH and ACTH:Biochemical, pharmacological and anatomical studies. Advancesin Biochemical Psychopharmacology. 18:133-4.(31) Oyama T, Jin T, Ryuji Y. 1980. Profound analgesic effectsof beta-endorphins in man. The Lancet. 1:122-4.(32) Tamsen A, et al. 1980. Endorphins and on-demand painrelief. The I.ancet. 1:769-70.(33) Fletcher JE, Thomas TA, Hill RG. 1980. Beta-endorphinand parturition. The Lancet. 1:310.(34) Houck JC, Kembell C, Chang C. 1980. Placental beta-endorphin-like peptides. Science. 207:78-9.(35) Chung SH, Dickenson A. 1980. Pain, enkephalin and acu-puncture. Nature. 283:243.(36) Snyder SH. 1977. Opiate receptors and internal opiates.Scientific American. 236:53-4.(37) Sicuteri F. 1978. Endorphins, opiate receptors and migraineheadache. Headache. 17:253-6.

AUTHOR

Dan Milloy, CRNA, BSN, received his BSN from theUniversity of Southern Mississippi in 1976. He attended theUniversity of Mississippi Medical Center School of Nurse Anes-thesia in Jackson, where he received a BS in Nurse Anesthesi-ology. This article was prepared while he was a senior nurseanesthesia student. Mr. Milloy is currently practicing in Jackson,Mississippi.

December/1982