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FIGURE 1–4. The dopaminergic system.This figure depicts the dopaminergic projections throughout the brain (A) and various regulatory processes involved in dopaminergic neurotransmission (B). The amino acid l-tyrosine is actively transported into presynaptic dopamine (DA) nerve terminals, where it is ultimately converted into DA. The rate-limiting step is conversion of l-tyrosine to l-dihydroxyphenylalanine (l-dopa) by the enzyme tyrosine hydroxylase (TH). -Methyl-p-tyrosine (AMPT) is a competitive inhibitor of tyrosine hydroxylase and has been used to assess the impact of reduced catecholaminergic function in clinical studies. The production of DA requires that l-aromatic amino acid decarboxylase (AADC) act on l-dopa. Thus, the administration of l-dopa to patients with Parkinson's disease bypasses the rate-limiting step and is able to produce DA quite readily. DA has its action terminated in the synapse by rapidly being taken back into the presynaptic neuron through DA transporters (DATs). DA is then metabolized to dihydroxyphenylalanine (DOPAC) by intraneuronal monoamine oxidase (MAO; preferentially by the MAO-B subtype) located on the outer membrane of mitochondria, or is sequestered and stored in secretory vesicles by vesicle monoamine transporters (VMATs). Reserpine causes a depletion of DA in vesicles by interfering and irreversibly damaging uptake and storage mechanisms. -Hydroxybutyrate inhibits the release of DA by blocking impulse propagation in DA neurons. Pargyline inhibits MAO and may have efficacy in treating parkinsonian symptoms by augmenting DA levels through inhibition of DA catabolism. Other clinically used inhibitors of MAO are nonselective and thus likely elevate the levels of DA, norepinephrine, and serotonin. Once released from the presynaptic terminal (because of an action potential and calcium influx), DA can interact with five different G protein–coupled receptors (D1–D5), which belong to either the D1 or D2 receptor family. Presynaptic regulation of DA neuron firing activity and release occurs through somatodendritic (not shown) and nerve terminal D2 autoreceptors, respectively. Pramipexole is a D2/D3 receptor agonist and has been documented to have efficacy as an augmentation strategy in cases of treatment-resistant depression and in the management of Parkinson's disease. The binding of DA to G protein receptors (Go, Gi, etc.) positively or negatively coupled to adenylyl cyclase (AC) results in the activation or inhibition of this enzyme, respectively, and the production of a cascade of second-messenger and cellular effects (see diagram). Apomorphine is a D1/D2 receptor agonist that has been used clinically to aid in the treatment of Parkinson's disease. (SKF-82958 is a pharmacologically selective D1 receptor agonist.) SCH-23390 is a D1/D5 receptor antagonist. There are likely physiological differences between D1 and D5 receptors, but the current unavailability of selective pharmacological agents has precluded an adequate differentiation thus far. Haloperidol is a D2 receptor antagonist, and clozapine is a nonspecific D2/D4 receptor antagonist (both are effective antipsychotic agents). Once inside the neuron, DA can either be repackaged into vesicles for reuse or undergo enzymatic catabolism. Nomifensine is able to interfere/block the reuptake of DA. The antidepressant bupropion has affinity for the dopaminergic system, but it is not known whether this agent mediates its effects through DA or possibly by augmenting other monoamines. DA can be degraded to homovanillic acid (HVA) through the sequential action of catechol-O-methyltransferase (COMT) and MAO. Tropolone is an inhibitor of COMT. Evidence suggests that the COMT gene may be linked to schizophrenia (Akil et al. 2003).Source. Adapted from Cooper JR, Bloom FE, Roth RH: The Biochemical Basis of Neuropharmacology, 7th Edition. New York, Oxford University Press, 2001. Copyright 1970, 1974, 1978, 1982, 1986, 1991, 1996, 2001 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc.

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