Catecholamines (dopamine [DA], norepinephrine [NE], epinephrine [EPI])

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Catecholamines(dopamine DA, norepinephrine
NE, epinephrine EPI)

1. Basic Neurochemistry, Chap. 12
2. The Biochemical Basis of Neuropharmacology, Chap.
   8 9

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Biosynthesis of Catecholamines
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Important fetures of catecholamine biosynthesis,
uptake and signaling

1. Biosynthesis
2. Release
3. Uptake (transporter)
4. Receptor-mediated signaling
5. Catabolism

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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

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Phosphorylation sites of Tyrosine Hydroxylase
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Modulation of catecholamine synthesis

1. 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)

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Dopa decarboxylase

1. 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 ?-hydroxylase

1. 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
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Catecholamines packed into the synaptic vesicles
VMAT2 Non-selective and has high affinity to
reserpine
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Metabolism of dopamine

• Major acidic metabolites
• 3,4-dihydroxy phenylacetic acid (DOPAC)
• Homovallic acid (HVA)

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Inactivation of Norepinephrine
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Monoamine oxidase (MAO)

1. Cofactor flavin located on the outer membrane
   of mitochondria
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-adenosylmethionin
   e

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Uptake of catecholamines transporter
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Uptake transporters

1. Released catecholamines will be up-take back into
   presynaptic terminals (DAT, NET)
2. Transporter is a Na and Cl-dependent process
   (ouabain Na,K-ATPase inhibitor and veratridine
   Na channel open block uptake process)

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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
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Regulation of DAT by various protein kinases
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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

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Noradrenergic projection (dorsal and ventral
bundle)
Cortex and hippocampus
Dorsal bundle
Spinal cord cerebellum
(Locus ceruleus)
Hypothalamus and Brainstem
Ventral bundle
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Dopamine projections (nigrostriatal,
mesocortical, tuberohypophysial)
Nigrostriatal projection
Substantia nigra to caudate/putamen n.
Tuberohypophysial projection
Mesocotical projection
Ventral tegmental area to nucleus accumbens and
frontal cortex
Hypothalamus to median eminence
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Catecholamine receptors

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

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Autoreceptor inhibit transmitter release
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Classification of Dopamine receptors
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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)

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2-D structure of dopamine D2 receptor
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Classification of Adrenergic receptors
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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 (?
   antagonist), yohimbine (? agonist)

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propanolol
yohimbine
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GPCR-mediated signal and internalization
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Dynamics of catecholamine receptors
(up-regulation and down-regulation)
agonist antagonist
catecholamine receptor