Synthesis and Metabolism of Dopamine

The family of catecholamines includes norepinephrine, epinephrine, and dopamine, of which dopamine is the most abundant in the central nervous system. It was not until the 1950s that dopamine was recognized as a critical neurotransmitter, and not simply an intermediate in the single biosynthetic pathway it shares with norepinephrine and epinephrine. In the first and rate-limiting step of the pathway, L-tyrosine is hydroxylated via the enzyme tyrosine hydroxylase (TH) to form L-dihydroxyphenylalanine (DOPA).¹ Removal of a carboxyl group from L-DOPA by DOPA decarboxylase then produces dopamine. In subsequent steps, dopamine can then be further converted to norepinephrine and then to epinephrine by dopamine β-hydroxylase and phenylethanolamine-N-methyltrasferase, respectively (Fig. 11-1).

FIGURE 11-1. Schematic of a prototypical dopaminergic synapse. Pre- and post-synaptic components of a dopaminergic synapse summarizing molecular pathways for dopamine synthesis, dopamine metabolism, and second messenger pathways following D_1-like or D_2-like receptor activation.

Activation of TH is the rate-limiting step in the production of dopamine, and it is under strict regulatory control by a variety of factors, including inhibitory feedback by the catecholamine products (e.g., dopamine). To convert tyrosine to L-DOPA, TH requires the binding of iron to the catalytic domain at the C terminus. Catalytic activity of TH also requires (6R)-(L-erythro-1′,2′-dihydroxypropyl)-2-amino-4-hydroxy-5, 6, 7, 8-tetrahydropteridine (6R-tetrahydrobiopterin [6RBPH4], more commonly known as tetrahydrobiopterin [BH₄], a naturally occurring pteridine cofactor), to reduce the iron to the ferrous form (Fe²⁺). This allows the binding of the substrates (e.g., L-tyrosine and molecular oxygen) to the C terminus.² Following a catalytic cycle, the molecular oxygen can oxidize a fraction of the iron to the ferric form, thus increasing the binding affinity for dopamine and L-DOPA. When either is bound to the regulatory domain of the N terminus, the complex is inactivated by preventing the binding of BH₄. Biosynthesis of L-DOPA, and consequently dopamine, can be restored by phosphorylation of the tyrosine hydroxylase enzyme at serine 40 by cAMP-dependent protein kinase phosphorylation, thus decreasing the binding affinity for dopamine 300-fold and increasing the binding affinity for the pteridine cofactor.^3,4 Meanwhile, endogenous levels of BH₄ are regulated by guanosine triphosphate (GTP) cyclohydrolase activity as its synthesis is downstream of the rate-limiting GTP enzyme.⁵ Mutations in the GTP cyclochydrolase I gene contribute to hereditary L-DOPA responsive dsytonia,⁶ which manifests a dopamine-responsive circadian distribution of symptoms with greater penetrance in women, sharing these two features in common with restless legs syndrome (RLS).^7,8

Following the release of dopamine, the primary mode of removal from the synapse is reuptake into the presynaptic neuron via the dopamine transporter (DAT). DAT is dependent on the energy created by the Na⁺/K⁺ pump and is a member of the Na⁺/Cl⁻-dependent plasma membrane transporter family, as are the norepinephrine and γ-aminobutyric acid (GABA) transporters. Imaging studies using compounds with highly specific affinity for DAT [i.e., 3β-(4-iodo-phenyl)tropane-2β-carboxylic acid (β-CIT)] permit visualization of the integrity of the dopamine system.

Once returned to the presynaptic terminal, dopamine is repackaged in synaptic vesicles via the vesicular monoamine transporter (VMAT) or metabolized to dihydroxyphenylacetic acid (DOPAC) by monoamine oxidase (MAO). Two alternative pathways are available for dopamine catabolism in the synapse, depending on whether the first step is catalyzed by MAO or catechol-O-methyltransferase (COMT). Thus, dopamine can be either deaminated to 3,4-dihydroxyphenylacetic acid (DOPAC) or methylated to 3-methoxytyramine (3-MT). In turn, deamination of 3-MT and methylation of DOPAC lead to homovanillic acid (HVA). In humans, cerebrospinal fluid levels of HVA have been used as a proxy for levels of dopaminergic activity within the brain.

Physiological Effects of Dopamine (Cellular and Subcellular)

The physiological effects of the dopaminergic system are best characterized as “neuromodulatory.” Rather than eliciting excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs) in a manner similar to glutamate and GABA, for example, dopamine allows the gating of inputs via alteration of membrane properties and specific ion conductances.⁹ This enhanced or decreased response to other inputs affects the intensity, duration, and timing of output commensurate with environmental and homeostatic demands.^10–12 The multivariate control of dopamine is provided through five subtypes of seven-transmembrane domain G protein–coupled receptors (D1 through D5), which, based on similarities in pharmacology, biochemistry, and amino acid homology, are divided into two classes, D_1-like (D1, D5) and D_2-like (D2, D3, and D4).¹³ D3 demonstrates the highest affinity for endogenous dopamine, followed, in decreasing order of affinity, by D4, D2, D5, and D1.^14,15 Furthermore, each receptor subtype has unique patterns of localization throughout the brain that increase the array of the behavioral effects of dopamine.

D1_-like receptors activate the G_s transduction pathway stimulating the production of adenylyl cyclase, which increases the formation of cyclic adenosine monophosphate (cAMP) and ultimately increases the activity of cAMP-dependent protein kinase (PKA). PKA activates DARPP-32 (dopamine and cyclic adenosine 3′,5′-monophosphate-regulated phosphoprotein [32 kDa]) via phosphorylation, permitting phospho-DARPP-32 to then inhibit protein phosphatase-1 (PP-1). The downstream effect of decreased PP-1 activity is an increase in the phosphorylation states of assorted downstream effector proteins regulating neurotransmitter receptors and voltage-gated ion channels. Ultimately, this results in increased activity of glutamate receptors (N-methyl-D-aspartic acid [NMDA] and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid [AMPA]), Ca²⁺ channels (L, N, and P types), and CREB, as well as decreased activity of GABA_A receptors, Na⁺ channels, and Na⁺/K⁺-ATPase.¹⁶ Alternatively, D2_-like receptors stimulate the G_i transduction pathway, which is negatively coupled to the production of adenylyl cyclase. Activation of D_2-like receptors also leads to an increase in intracellular calcium concentrations. These two pathways can act independently, through decreased PKA and increased calcineurin, respectively, to return phospho-DARPP-32 to the inactive DARPP-32. Additionally, calcineurin activity can be stimulated by the increase in intracellular calcium concentrations caused by glutamatergic activation of NMDA receptors. Other mechanisms of action mediated by D_2-like receptors include increasing K⁺ conductance and inhibiting Ca²⁺ entry via voltage-gated Ca²⁺ channels.

Both D_1-like and D_2-like receptors are found postsynaptically, exerting their effect on nondopaminergic neurons targeted by dopaminergic projections. D_2-like receptors can also be localized presynaptically on the dendrites, soma, and presynaptic terminals of dopaminergic cells. The presynaptic localization of the autoreceptors enables them to provide an inhibitory feedback mechanism. The regulation in the somatodendritic region includes modulation of the firing rate of the dopaminergic cell, and in the nerve terminal autoreceptors control the synthesis and release of dopamine. In addition, dopamine appears to act on receptors present on endothelial cells lining the brain’s microvasculature promoting vasoconstriction.¹⁷ Although the exact mechanisms for the regulation of dopamine synthesis and for dopamine release remain to be elucidated, evidence does exist that these are distinct mechanisms.¹⁸ For example, in the prefrontal and cingulated cortices, activation of autoreceptors regulates the release, but not the synthesis, of dopamine.¹⁸ Low-dose effects of dopaminomimetics are mediated by autoreceptor activation, as opposed to postsynaptic receptors, due to their 10-fold higher affinity for dopamine.

Functional Anatomy of Central Dopamine Systems

In the 1960s, a group of Swedish scientists first described the nigrostriatal, mesocorticolimbic, and tuberoinfundibular dopamine neurons as giving rise to the three most conspicuous and behaviorally relevant dopamine circuits in the brain.¹⁹ Using histofluorescence and subsequently immunohistochemical identification of TH,^20,21 16 unique monoaminergic cell groups were identified and given designations A1 through A16, of which dopamine has been identified as a major transmitter in a subpopulation of A2 dorsal motor vagal neurons, and in the A8 through A17 cell groups.²² There is generally marked conversation in the cellular and receptor distributions and major pathways across species with limited exceptions.²³ Operationally, these groups can be characterized as long projection versus local circuit neurons with unique functions (Table 11-1).

The nigrostriatal pathway originates in the midbrain, from catecholamine cell groups A8 and A9, and projects to the caudate nucleus and putamen (collectively, often referred to as the dorsal striatum). This pathway is traditionally taught to modulate voluntary (waking) movement, and its destruction or degeneration (as in Parkinson’s disease [PD]) results in impairments in the planning, initiation, and execution of movement and motor engrams.^24–27 Heuristic models of this major dopaminergic circuit focus themselves on dopamine’s indirect actions (i.e., via dorsal striatal pathways to the internal segment of the globus pallidus [GPi] and substantia nigra pars reticulata [SNr]) and a series of parallel, segregated striatopallidal-thalamocortical recurrent loops centered on functionally distinct cortical regions.^28,29 The anatomy of the input and output connections of the A8 and A9 neurons and associated behaviors are most often considered in isolation with the basal ganglia nuclei and the thalamus. Moreover, wakefulness has been the “default” medium through which the behavioral correlates of dopamine dysfunction are believed to play out in this major pathway. It is less well established or understood what relevance this dopaminergic system might have to the modulation of normal and pathological wake/sleep states such as RLS/periodic limb movements in sleep (RLS/PLMS) (see, however, Rye^30,31).

The mesocorticolimbic pathway arises from the midbrain catecholamine cell group A10, within the ventral tegmental area, and targets the ventral striatum (nucleus accumbens), subcortical limbic nuclei such as the septum and amygdala, the hippocampus, and prefrontal cortex.³² Activation of this pathway is known to modulate various cognitive and emotive functions including reward, the psychomotor effects associated with drugs of abuse, and working memory.^33–35 Disruption of this pathway is believed to modify schizophrenia, attention-deficit/hyperactivity disorder, Tourette’s syndrome, and major depression. Together, the nigrostriatal and mesocorticolimbic systems account for nearly 80% of the brain’s dopamine content.

The third major circuit, the tuberoinfundibular and tuberohypophyseal pathways, originate in the hypothalamic arcuate/periarcuate nuclei and periventricular hypothalamus (catecholamine cell groups A12 and A14, respectively). Activation of the A12 cluster modulates the release of numerous hormones in very complex ways—often in opposing ways at the cellular versus presynaptic level. Dopamine in the tuberoinfindibular system/anterior pituitary tonically inhibits release of prolactin, and luteinizing and thyroid-stimulating hormones, and promotes growth hormone release, primarily via effects on releasing hormones.^36–40 The effects of dopamine on the tuberohypophyseal system include, generally, inhibitory modulation of vasopressin release and facilitation of oxytocin release.²¹

The little-studied A11 catecholamine cell group in the subparafascicular thalamus is the largest, likely sole, source of spinal dopamine.^41–44 Within the spinal cord, these axons target the intermediolateral column (IML) housing preganglionic sympathetic neurons, dorsal horn regions related to afferent nerve processing, interneurons (e.g., Renshaw cells), and somatic motoneurons,⁴³ where they likely dampen spinal nociceptive processing and sympathetic outflow and enhance motor output predominantly via D_2-like receptor mechanisms.^45,46

The axons of major projecting brain dopamine systems have a proclivity to collateralize extensively; that is, individual axons branch and innervate two or more physically, and perhaps functionally, unique regions.^47–49 In addition to innervating the striatum and frontal cortex, for example, A8 through A10 neurons also target the thalamus (principally the midline, intralaminar, and reticular nuclei that modulate thalamocortical excitability), the extended amygdala, the noradrenergic locus ceruleus, and the serotonergic raphe system. Axons of individual A11 cells branch extensively to all spinal cord levels along their course in the dorsolateral funiculi, as well as to the prefrontal cortex, amygdala, and nucleus of the solitary tract.⁵⁰