Mt

Presynaptic terminal [T] = vesicular transporter

- Effector response

Postsynaptic terminal-expressing D receptors (GPCRs)

FIGURE 20-2 Dopaminergic terminal. Dopamine (DA) is synthesized in neuronal terminals from tyrosine by the sequential actions of tyrosine hydroxylase (TH), producing the intermediary l-dihydroxyphenylalanine (l-DOPA), and aromatic l-amino acid decarboxylase (AAD). In the terminal, DA is transported into storage vesicles by a vesicular membrane transporter (T). Release, triggered by depolarization and entry of Ca2+, allows dopamine to act on a variety of postsynaptic GPCRs for DA. The Dj and D2 receptors are important in brain regions involved in PD. The differential actions of DA on postsynaptic targets bearing different types of DA receptors have important implications for the function of neural circuits. The actions of DA are terminated by reuptake into the nerve terminal (where DA may be restored or metabolized) or uptake into the postsynaptic cell (where DA is metabolized). Metabolism occurs by the sequential actions of the enzymes catechol-O-methyltransferase (COMT), monoamine oxidase (MAO), and aldehyde dehydrogenase (AD). 3MT, 3-methoxytyramine; DOPAC, 3,4-dihydroxyphenylacetic acid; HVA, 3-methoxy-4-hydroxy-phenylacetic acid (see Figure 20-3). In humans, HVA is the principal metabolite of DA. (From Cooper et al., 1996, with permission.)

Norepinephrine

3-O-Methyldopa \

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