Localization of dopamine transporter protein by microscopic histochemistry

Association of protein distribution and gene expression revealed by positron emission tomography and postmortem gene expression in the dopaminergic system of the human brain

PURPOSE

The topological distribution of dopamine-related proteins is determined by gene transcription and subsequent regulations. Recent research strategies integrating positron emission tomography with a transcriptome atlas have opened new opportunities to understand the influence of regulation after transcription on protein distribution. Previous studies have reported that messenger (m)-RNA expression levels spatially correlate with the density maps of serotonin receptors but not with those of transporters. This discrepancy may be due to differences in regulation after transcription between presynaptic and postsynaptic proteins, which have not been studied in the dopaminergic system. Here, we focused on dopamine D1 and D2/D3 receptors and dopamine transporters and investigated their region-wise relationship between mRNA expression and protein distribution.

METHODS

We examined the region-wise correlation between regional binding potentials of the target region relative to that of non-displaceable tissue (BPND) values of 11C-SCH-23390 and mRNA expression levels of dopamine D1 receptors (D1R); regional BPND values of 11C-FLB-457 and mRNA expression levels of dopamine D2/D3 receptors (D2/D3R); and regional total distribution volume (VT) values of 18F-FE-PE2I and mRNA expression levels of dopamine transporters (DAT) using Spearman's rank correlation.

RESULTS

We found significant positive correlations between regional BPND values of 11C-SCH-23390 and the mRNA expression levels of D1R (r = 0.769, p = 0.0021). Similar to D1R, regional BPND values of 11C-FLB-457 positively correlated with the mRNA expression levels of D2R (r = 0.809, p = 0.0151) but not with those of D3R (r = 0.413, p = 0.3095). In contrast to D1R and D2R, no significant correlation between VT values of 18F-FE-PE2I and mRNA expression levels of DAT was observed (r = -0.5934, p = 0.140).

CONCLUSION

We found a region-wise correlation between the mRNA expression levels of dopamine D1 and D2 receptors and their respective protein distributions. However, we found no region-wise correlation between the mRNA expression levels of dopamine transporters and their protein distributions, indicating different regulatory mechanisms for the localization of pre- and postsynaptic proteins. These results provide a broader understanding of the application of the transcriptome atlas to neuroimaging studies of the dopaminergic nervous system.

Changes in basal and cocaine-evoked extracellular dopamine uptake and release in the rat nucleus accumbens during early abstinence from cocaine: quantitative determination under transient conditions

Despite an abundance of studies on mechanisms of behavioral sensitization, considerable uncertainty exists as to whether alterations in dopamine neurotransmission underlie the exacerbated behavioral effects of cocaine observed during the early stages of abstinence. One of the factors contributing to the uncertainty and controversy may be the limitations in utilized measurement techniques (mostly conventional microdialysis). The techniques of quantitative microdialysis under transient conditions and rotating disk electrode voltammetry were used to characterize basal dopamine dynamics as well as time-related changes in extracellular dopamine concentrations and dopamine uptake that occur in response to an acute drug challenge in control animals and animals with previous history of cocaine. Basal extracellular dopamine concentrations were unaltered on abstinence day 3 from repeated cocaine administration (5 days, 20 mg/kg, i.p.). The extraction fraction of dopamine, an indirect measure of dopamine uptake, was significantly lower in cocaine-sensitized animals relative to controls. These two facts, taken together, suggest that basal dopamine release is depressed in cocaine-sensitized animals on abstinence day 3. At the same time, a cocaine challenge decreased the extraction fraction and increased the extracellular dopamine concentration in both experimental groups. The magnitude of the increase in extracellular dopamine concentration was greater in cocaine-sensitized animals, while the ability of cocaine to decrease the extraction fraction was unaltered, suggesting that the increase in extracellular dopamine concentration reflects an increase in drug-evoked dopamine release. Moreover, cocaine-pretreated rats demonstrated greater depolarization-induced dopamine release and the ability of dopamine D(2) receptor agonist, quinpirole, to inhibit release was decreased in these animals. These data demonstrate that a cocaine treatment regimen resulting in behavioral sensitization is associated with a reduction in basal dopamine release, an enhancement in both cocaine and K(+)-evoked dopamine release, and a subsensitivity of dopamine D(2) autoreceptors that regulate dopamine release in the nucleus accumbens.

Changes in the kinetics of dopamine release and uptake have differential effects on the spatial distribution of extracellular dopamine concentration in rat striatum

The objective of this study was to examine whether the limited diffusion distance of dopamine in rat striatum produces spatial heterogeneity in the extracellular dopamine concentration on a dimensional scale of a few micrometers. Such heterogeneity would be significant because it would imply that the concentration of dopamine at a given receptor depends on the receptor's ultrastructural location. Spatially resolved measurements of extracellular dopamine were performed in the striatum of chloral hydrate-anesthetized rats with carbon fiber microdisk electrodes. Dopamine was monitored during electrical stimulation of the nigrostriatal pathway before and after administration of drugs that selectively affect the kinetics of evoked dopamine release and dopamine uptake. The effects of nomifensine (20 mg/kg), L-DOPA (250 mg/kg), and alpha-methyl-p-tyrosine (250 mg/kg) on the amplitude of the stimulation responses were examined. The outcome of these experiments was compared with predictions derived from a mathematical model that combines diffusion with the kinetics of release and uptake. The results demonstrate that the extracellular dopamine concentration is spatially heterogeneous on a micrometer scale and that changing the kinetics of dopamine release and uptake has different effects on this spatial distribution. The impact of these results on brain neurochemistry is considered.

kappa-Opioid tolerance and dependence in cultures of dopaminergic midbrain neurons

Repeated cocaine exposure upregulates kappa opioids and their receptors in the mesocorticolimbic system; the ensuing kappa-mediated dysphoria appears to contribute to addiction and withdrawal. As a potential rehabilitation strategy to reverse cocaine-induced kappa sensitization, the present study used tritiated dopamine release assays to examine the induction of kappa-opioid tolerance in cultured mesencephalic neurons. Administration of the kappa agonist U69,593 inhibited tetrodotoxin-sensitive, spontaneous (EC(50) = 1.5 nM), and potassium-stimulated (EC(50) = 10 nM) release. These effects were blocked by pertussis toxin and by the kappa antagonist nor-binaltorphimine. The 2 d agonist exposure (1 microM) caused a shift in the U69,593 dose-response curve that was greater in the potassium-stimulated paradigm (140-fold) than in the spontaneous release assay (sixfold). These results were attributable to the attenuation of kappa-receptor signaling mechanisms and to dependence. In the stimulated release assay, attenuation of kappa signaling caused by 4 hr of U69,593 exposure recovered with a half-life of 1.1 hr, whereas attenuation after 144 hr of exposure recovered slowly (t(1/2) = 20 hr). In the spontaneous release assay, attenuation of kappa-opioid signaling occurred slowly (t(1/2) = 22 hr), and resensitization after a 144 hr exposure was rapid (t(1/2) < 1 hr). kappa-Opioid dependence was observed after 144 hr of U69,593 exposure. Thus multiple mechanisms of adaptation to kappa-opioid exposure occur in mesocorticolimbic neurons. These data support the idea that the administration of kappa opioids might facilitate drug rehabilitation.

Immunocytochemical localization of the dopamine transporter in human brain

The dopamine transporter (DAT) was localized in normal human brain tissue by light microscopic immunocytochemistry by using highly specific monoclonal antibodies. Regional distribution of DAT was found in areas with established dopaminergic circuitry, e.g., mesostriatal, mesolimbic, and mesocortical pathways. Mesencephalic DAT-immunoreactivity was enriched in the dendrites and cell bodies of neurons in the substantia nigra pars compacta and ventral tegmental area. Staining in the striatum and nucleus accumbens was dense and heterogeneous. Mesocortical DAT immunoreactivity in motor, premotor, anterior cingulate, prefrontal, entorhinal/perirhinal, insular, and visual cortices was detected in scattered varicose and a few nonvaricose fibers. Varicose fibers were relatively enriched in the basolateral and central subnuclei of amygdala, with sparser fibers in lateral and basomedial subnuclei. Double-labeling studies combining DAT and tyrosine hydroxylase (TH) immunostaining in the ventral mesencephalon showed two subpopulations of dopaminergic neurons differentiated by the presence or absence of DAT-immunoreactivity in the A9 and A10 cell groups. In other dopaminergic cell groups (All, A13-A15), TH-positive hypothalamic neurons showed no detectable DAT-immunoreactivity. However, fine DAT-immunoreactive axons were scattered throughout the hypothalamus, particularly concentrated along the medial border, with more coarse axons present along the lateral border. These findings demonstrate that most mesotelencephalic dopamine neurons of human brain express high levels of DAT throughout their entire somatodendritic and axonal domains, whereas a smaller subpopulation of mesencephalic dopamine cells and all hypothalamic dopamine cell groups examined express little or no DAT. These data indicate that different subpopulations of dopaminergic neurons use different mechanisms to regulate their extracellular dopamine levels.

Localization and dynamic regulation of biogenic amine transporters in the mammalian central nervous system

The monoamines, serotonin, dopamine, norepinephrine, epinephrine and histamine, play a critical role in the function of the hypothalamic-pituitary-adrenal axis and in the integration of information in sensory, limbic, and motor systems. The primary mechanism for termination of monoaminergic neurotransmission is through reuptake of released neurotransmitter by Na+, CI-dependent plasma membrane transporters. A second family of transporters packages monoamines into synaptic and secretory vesicles by exchange of protons. Identification of those cells which express these two families of neurotransmitter transporters is an initial step in understanding what adaptive strategies cells expressing monoamine transporters use to establish the appropriate level of transport activity and thus attain the appropriate efficiency of monoamine storage and clearance. The most recent advances in this field have yielded several surprises about their function, cellular and subcellular localization, and regulation, suggesting that these molecules are not static and most likely are the most important determinants of extracellular levels of monoamines. Here, information on the localization of mRNAs for these transporters in rodent and human brain is summarized along with immunohistochemical information at the light and electron microscopic levels. Regulation of transporters at the mRNA level by manipulation in rodents and differences in transporter site densities by tomographic techniques as an index of regulation in human disease and addictive states are also reviewed. These studies have highlighted the presence of monoamine neurotransmitter transporters in neurons but not in glia in situ. The norepinephrine transporter is present in all cells which are both tyrosine hydroxylase (TH)- and dopamine beta-hydroxylase-positive but not in those cells which are TH- and phenyl-N-methyltransferase-positive, suggesting that epinephrine cells may have their own, unique transporter. In most dopaminergic cells, dopamine transporter mRNA completely overlaps with TH mRNA-positive neurons. However, there are areas in which there is a lack of one to one correspondence. The serotonin transporter (5-HTT) mRNA is found in all raphe nuclei and in the hypothalamic dorsomedial nucleus where the 5-HTT mRNA is dramatically reduced following immobilization stress. The vesicular monoamine transporter 2 (VMAT2) is present in all monoaminergic neurons including epinephrine- and histamine-synthesizing cells. Immunohistochemistry demonstrates that the plasma membrane transporters are present along axons, soma, and dendrites. Subcellular localization of DAT by electron microscopy suggests that these transporters are not at the synaptic density but are confined to perisynaptic areas, implying that dopamine diffuses away from the synapse and that contribution of diffusion to dopamine signalling may vary between brain regions. Interestingly, the presence of VMAT2 in vesicles underlying dendrites, axons, and soma suggests that monoamines may be released at these cellular domains. An understanding of the regulation of transporter function may have important therapeutic consequences for neuroendocrine function in stress and psychiatric disorders.