Special characteristics of peripheral dopamine
As discussed in Chapter 1, the synthesis, storage, release, reuptake and metabolism of DA are dynamic processes that differ in many respects between the "closed" system of the brain dopaminergic neurons, and the "open-ended" system of peripheral DA-producing cells (see Figure 1.2). Within the closed system of neuron/synapse/neuron, the concentration of the released DA can be as high as 5-10 pM, whereas the concentration of circulating DA reaching peripheral target cells does not exceed 20-30 nM. This large differential in the effective concentrations of DA impinging upon peripheral organs has not always been taken into consideration in many studies using cultured cells/organs, resulting in the use of pharmacological, rather than physiological, doses of DA.
All monoaminergic neurons, including catecholamines and indoleam- ines, have intrinsic processes that guard against neuronal overstimulation by the released neurotransmitter. They also have specific mechanisms for maintaining an adequate intraneuronal storage/secretory capacity capable of prompt responses to frequent stimuli. Except for the tuberoinfundibular (TIDA) neurons, which release DA into the hypophysial portal vasculature, all other brain dopaminergic neurons are linked via synapses to recipient neurons in a "closed" system configuration. The latter is briefly summarized below so as to compare it with the distinct properties of the "open-ended" system of peripheral DA.
In a typical dopaminergic neuron, DA is synthesized in the trans-Golgi network and is packed into secretory vesicles (Figure 6.2). Within the vesicles, the concentration of DA can be 100-1,000 times higher than its levels in the cytoplasma. The vesicles also provide protection for DA from degradation by mitochondrial MAO. DA loaded within the vesicles exists in a dynamic equilibrium, whereby a passive outward leakage of DA into the cytoplasm is counterbalanced by an inward active transport. This active transport is controlled by the vesicular monoamine transporter (VMAT), which maintains an H+-electrochemical gradient between the cytoplasm and vesicles.
As shown in Figure 6.2, in a typical neuron, DA is released into the synaptic cleft by a calcium-mediated exocytosis and binds to DARs, which are localized either presynaptically (autoreceptors) or postsynaptically (recipient
Figure 6.2 Homeostasis of dopamine in a typical neuronal system. The synthesis, storage, release and actions of dopamine (DA) in neurons. Tyrosine hydroxylase (TH) is localized both in the cytoplasm and in storage vesicles, which also contain Dopa decarboxylase (DDC) and vesicular monoamine transporter (VMAT2). DA is released by a calcium-dependent mechanism into the synaptic space. From there, DA can either bind to postsynaptic or presynaptic dopamine receptors (DARs), can be taken back into the secreting cell by the DA transporter (DAT), or it can be degraded by COMT (catechol-O-methyltransferase). Intracellular DA that is not protected by storage vesicles can be degraded by mitochondrial monoamine oxidase (MAO).
receptors). After triggering action potentials in the postsynaptic neurons and activating intracellular signaling pathways, DA rapidly disassociates from the receptors and can undergo one or more of the following actions: It can (1) be taken back ("reuptake") into the presynaptic terminal by the DA transporter (DAT), (2) repackaged into storage vesicles by VMAT, (3) become deaminated by intracellular MAO to dihydroxyphenylacetic acid (DOPAC), and/or (4) become O-methylated by membranous or soluble COMT to 3-methoxytyramine. Homovanylic acid (HVA) is the final product of O-methylation of DOPAC.
Although the behavior of peripheral DA-producing cells follows the general scheme described above, there are several distinct dissimilarities. DA synthesis in peripheral organs can be either independent or dependent on their sympathetic innervation. Thus, chemical sympathectomy with 6-hydroxydopamine (6- OH-DA) in rats can cause a marked decrease in DA in some peripheral organs, but its preservation in others, providing proof for a nonneuronal origin . When DA synthesis is independent of sympathetic innervation, the question is whether DA in the producing cells is stored and released in a regulated manner from secretory vesicles or it passively diffuses out soon after synthesis. Unlike neural or endocrine cells, which have an abundance of secretoiy vesicles, most other tissues do not have a well-developed storage and secretoiy capacity for endocrine-like releasable factors, ft has been proposed that in some peripheral organs, DAT, which usually brings DA into the cell, acts as an outward facing transporter and facilitates the diffusion of DA out of the cells .
Without the restricted space of a synapse, DA is released from peripheral nonneuronal cells into the extracellular space and diffuses away from the producing cells into the blood. DA can then reach its target cells via the circulation and can also become inactivated by metabolic enzymes located either at adjacent or at remote sites. Deamination of peripheral DA can be done by two isoenzymes, MAO-A and MAO-B, which are highly expressed in the liver, with a lower expression in the myocardium, lung, kidney, and duodenum . O-methylation is carried out by COMT, also having the highest activity in the liver, followed by the kidney, stomach, and intestine .