Transcript Catecholamines (dopamine, norepinephrine, epinephrine)

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. Receptormediated
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 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 cdklike kinase)
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
Catecholamines packed into
the synaptic vesicles
VMAT2:
Non-selective
and has high
affinity to
reserpine
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
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-adenosylmethionine
Uptake of catecholamines: transporter
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)
3. Transporter is saturable, obeys MichaelisMenten 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)
Cortex and
hippocampus
Dorsal bundle
Spinal cord
cerebellum
Hypothalamus
and Brainstem
(Locus ceruleus)
Ventral bundle
Dopamine projections
(nigrostriatal, mesocortical,
tuberohypophysial)
Nigrostriatal
projection
Substantia nigra to
caudate/putamen n.
Tuberohypophysial
projection
Hypothalamus to median
eminence
Mesocotical projection
Ventral tegmental area to nucleus
accumbens and frontal cortex
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 Cterminal) 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 ( antagonist),
yohimbine ( agonist)
propanolol
yohimbine
GPCR-mediated signal and internalization
Dynamics of catecholamine receptors
(up-regulation and down-regulation)
agonist
antagonist
catecholamine receptor