catecholamines (dopamine [da], norepinephrine [ne], epinephrine [epi]) 1. basic neurochemistry,...

Catecholamines(dopamine [DA], norepinephrine

[NE], epinephrine [EPI])

1. Basic Neurochemistry, Chap. 12

2. The Biochemical Basis of Neuropharmacology, Chap. 8 & 9

Biosynthesis of Catecholamines

Important fetures of catecholamine biosynthesis, uptake and signaling

1. Biosynthesis

2. Release

3. Uptake (transporter)

4. Receptor-mediated signaling

5. Catabolism

Tyrosine hydrogenase: rate-limiting enzyme

1. TH is a homotetramer, each subunit has m.w. of 60,000

2. Catalyzes –OH group to meta position of tyrosine

3. Km = M range; saturation under normal condition

4. Cofactor: biopterin; competitive inhibitor: -methyl-p-tyrosine

5. Sequence homology: phenylalanine hydroxylase and tryptophan hydroxylase

6. Phosphorylation at N-terminal sites:

Phosphorylation sites of Tyrosine Hydroxylase

Modulation of catecholamine synthesis1. Neuronal activity increase would enhance the

amount of TH and DBH at both mRNA and protein levels

2. TH is modulated by end-product inhibition (catecholamine competes with pterin cofactor)

3. Depolarization would activate TH activity

4. Activation of TH involves reversible phosphorylation (PKA, PKC, CaMKs and cdk-like kinase)

Dopa decarboxylase1. Cofactor: pyridoxine; low Km but high Vmax

2. Also decarboxylate 5-HTP and other aromatic a.a.: aromatic amino acid decarboxylase (AAAD)

3. Inhibitor: -methyldopa

Dopamine -hydroxylase1. Cofactor: ascorbate; substrate: dopamine

2. Inhibitor: diethyldithiocarbamate (copper chelator)

3. DBH is a tetrameric glycoprotein (77kDa and 73kDa)

4. Store in the synaptic vesicle and releasable

Phenylethanolamine N-methyltransferase (PNMT)Substrate: S-adenosylmethionine; regulated by corticosteroids

VMAT2:

Non-selective and has high affinity to reserpine

Catecholamines packed into the synaptic vesicles

Metabolism of dopamine

Major acidic metabolites:

A. 3,4-dihydroxy phenylacetic acid (DOPAC)

B. Homovallic acid (HVA)

Inactivation of Norepinephrine

Monoamine oxidase (MAO)1. Cofactor: flavin; located on the outer membrane of mito

chondria

2. Convert amine into aldehyde (followed by aldehyde dehydrogenase to acids or aldehyde reductase to glycol)

3. MAO-A: NE and 5-HT (inhibitor: clorgyline); MAO-B: phenylethylamines (DA) (inhibitor: deprenyl)

4. Patient treated for depression or hypertension with MAO inhibitors: severe hypertension after food taken with high amounts of tyramine (cheese effect)

Catechol-O-methyltransferase (COMT) 1. Enzyme can metabolize both intra- or extracellularly

2. Requires Mg2+ and substrate of S-adenosylmethionine

Uptake of catecholamines: transporter

Uptake transporters1. Released catecholamines will be up-take back int

o presynaptic terminals (DAT, NET)2. Transporter is a Na+ and Cl+-dependent process (o

uabain [Na,K-ATPase inhibitor] and veratridine [Na channel open] block uptake process)

3. Transporter is saturable, obeys Michaelis-Menten kinetics

4. 12 transmemebrane domain: intracellular phosphorylation and extracellular glycosylation

5. Uptake is energy dependent; can be blocked by tricyclic antidepressents, cocaine, amphetamine and MPTP

Regulation of DAT by various protein kinases

Localization of catecholamine neurons

1. Immunocytochemistry (ICH): antibody against synthesis enzyme, uptake transporter and receptor

2. In situ hybridization (ISH): cDNA or cRNA probe synthesis enzyme, transporter and receptor

3. Receptor autoradiography: radiolabelled ligand ([3H] or [125I]) against receptor

Noradrenergic projection(dorsal and ventral bundle)

Dorsal bundle

(Locus ceruleus)

Spinal cordcerebellum

Cortex and hippocampus

Ventral bundle

Hypothalamus and Brainstem

Dopamine projections(nigrostriatal, mesocortical, tuberohypoph

ysial)

Nigrostriatal projection

Mesocotical projectionTuberohypophysial projection

Substantia nigra to caudate/putamen n.

Ventral tegmental area to nucleus accumbens and frontal cortexHypothalamus to median

eminence

Catecholamine receptors

1. Postsynaptic receptors locate on dendrites or cell body, axons or nerve terminals

2. Presynaptic autoreceptors locate on the same neuron:

a. terminal autoreceptor: control release

b. somatodendritic autoreceptor: synthesis control

c. major autoreceptor type: 2-adrenergic receptor in PNS/CNS; D2-dopamine receptor

d. exception: -adrenergic receptor facilitates NE release

Autoreceptor: inhibit transmitter release

Classification of Dopamine receptors

Feature of Dopamine receptors

1. Two subtypes of dopamine receptor: D-1 (short i3, long C-terminal) and D-2 like (long i3, short C-terminal) receptors

2. D2 receptors contain splicing isoform: D2L and D2S (87 bp)

3. D3 receptor has high affinity to atypical neuroleptics; D4 receptor bind tightly with clozapine

4. Chronic antagonist treatment up-regulate D2 receptors; agonist treatment might down-regulate the D2 receptor

5. Pharmacological application: anti-Parkinson (D2 agonist), anti-psychotic (D2 antagonist), addictive drugs (DA transporter)

2-D structure of dopamine D2 receptor

Classification of Adrenergic receptors

Features of Adrenergic receptors

1. Both NE and epinephrine bind to and receptors

2. 1 locates mainly in the heart and cortex; 2 predominate in the lung and cerebellum; 3 in the adipose tissue (significance in obesity)

3. -receptor stimulates AC; in turn, inactivates receptor via ARK and -arrestin

4. 1 is a post-synaptic receptor (three subtypes: 1A, 1B and 1D); while 2 is both post- and pre-synaptic receptor (three subtypes: 2A, 2B and 2C)

5. Representative ligands: propranolol ( ntgonist), yohimbine ( gonist)

propanolol

yohimbine

GPCR-mediated signal and internalization

Dynamics of catecholamine receptors

(up-regulation and down-regulation)

catecholamine receptor

agonist

antagonist