Species- and brain region-specific dopamine transporters: immunological and glycosylation characteristics

Neurochemical organization of the ventral striatum's olfactory tubercle

The ventral striatum is a collection of brain structures, including the nucleus accumbens, ventral pallidum and the olfactory tubercle (OT). While much attention has been devoted to the nucleus accumbens, a comprehensive understanding of the ventral striatum and its contributions to neurological diseases requires an appreciation for the complex neurochemical makeup of the ventral striatum's other components. This review summarizes the rich neurochemical composition of the OT, including the neurotransmitters, neuromodulators and hormones present. We also address the receptors and transporters involved in each system as well as their putative functional roles. Finally, we end with briefly reviewing select literature regarding neurochemical changes in the OT in the context of neurological disorders, specifically neurodegenerative disorders. By overviewing the vast literature on the neurochemical composition of the OT, this review will serve to aid future research into the neurobiology of the ventral striatum.

Immunohistochemical Changes in the Mouse Striatum Induced by the Pyrethroid Insecticide Permethrin

Epidemiological studies have linked insecticide exposure and Parkinson's disease. In addition, some insecticides produce damage or physiological disruption within the dopaminergic nigrostriatal pathway of non-humans. This study employed immunohistochemical analysis in striatum of the C57BL/6 mouse to clarify tissue changes suggested by previous pharmacological studies of the pyrethroid insecticide permethrin. Dopamine transporter, tyrosine hydroxylase, and glial fibrillary acidic protein immunoreactivities were examined in caudate-putamen to distinguish changes in amount of dopamine transporter immunoreactive protein from degeneration or other damage to dopaminergic neuropil. Weight-matched pairs of pesticide-treated and vehicle-control mice were dosed and sacrificed on the same days. Permethrin at 0.8, 1.5 and 3.0 mg/kg were the low doses and at 200 mg/kg the high dose. Brains from matched pairs of mice were processed on the same slides using the avidin-biotin technique. Four fields were morphometrically located in each of the serial sections of caudateputamen, digitally photographed, and immunopositive image pixels were counted and compared between members of matched pairs of permethrin-treated and vehicle-control mice. For low doses, only 3.0 mg/kg produced a significant decrease in dopamine transporter immunostaining. The high dose of permethrin did not produce a significant change in dopamine transporter or tyrosine hydroxylase immunostaining, but resulted in a significant increase in glial fibrillary acidic protein immunostaining. These data suggest that a low dose of permethrin can reduce the amount of dopamine transporter immunoreactive protein in the caudate-putamen. They also suggest that previously reported reductions in dopamine uptake of striatal synaptosomes of high-dose mice may be due to nondegenerative tissue damage within this region as opposed to reductions of dopamine transporter protein or death of nigrostriatal terminals. These data provide further evidence that insecticides can affect the primary neurodegenerative substrate of Parkinson's disease.

A Physical Interaction between the Dopamine Transporter and DJ-1 Facilitates Increased Dopamine Reuptake

The regulation of the dopamine transporter (DAT) impacts extracellular dopamine levels after release from dopaminergic neurons. Furthermore, a variety of protein partners have been identified that can interact with and modulate DAT function. In this study we show that DJ-1 can potentially modulate DAT function. Co-expression of DAT and DJ-1 in HEK-293T cells leads to an increase in [³H] dopamine uptake that does not appear to be mediated by increased total DAT expression but rather through an increase in DAT cell surface localization. In addition, through a series of GST affinity purifications and co-immunoprecipitations, we provide evidence that the DAT can be found in a complex with DJ-1, which involve distinct regions within both DAT and DJ-1. Using in vitro binding experiments we also show that this complex can be formed in part by a direct interaction between DAT and DJ-1. Co-expression of a mini-gene that can disrupt the DAT/DJ-1 complex appears to block the increase in [³H] dopamine uptake by DJ-1. Mutations in DJ-1 have been linked to familial forms of Parkinson’s disease, yet the normal physiological function of DJ-1 remains unclear. Our study suggests that DJ-1 may also play a role in regulating dopamine levels by modifying DAT activity.

Activity-dependent regulation of the dopamine transporter is mediated by Ca(2+)/calmodulin-dependent protein kinase signaling

The mechanisms regulating expression of the dopamine transporter are poorly understood. We tested the hypothesis that neuronal activity is one of the non-genetic determinants of dopamine transporter abundance. Sustained changes in neuronal activity caused by tetrodotoxin and 4-aminopyridine altered the dopamine uptake and abundance of dopamine transporter and its mRNA in rat mesencephalic cultures. The altered neuronal activity caused by these two drugs is accompanied by changes in intracellular calcium concentrations and Ca(2+)/calmodulin-dependent protein (CaM) kinase II activity in dopamine neurons. Chronic treatment with an L-type calcium channel blocker (nifedipine) or CaM kinase inhibitor (KN93) decreased dopamine transporter-mediated uptake and occluded the effects of tetrodotoxin and 4-aminopyridine. These data suggest that neuronal activity can regulate dopamine transporter function and abundance via calcium/CaM kinase II signaling.

The comparative distributions of the monoamine transporters in the rodent, monkey, and human amygdala

The monoamines in the amygdala modulate multiple aspects of emotional processing in the mammalian brain, and organic or pharmacological dysregulation of these systems can result in affective pathologies. Knowledge of the normal distribution of these neurotransmitters, therefore, is central to our understanding of both the normal processes regulated by the amygdala and the pathological conditions associated with monoaminergic dysregulation. The monoaminergic transporters have proven to be accurate and reliable markers of the distributions of their substrates. The purpose of this review was twofold: First, to briefly recount the functional relevance of dopamine, serotonin, and norepinephrine transmission in the amygdala, and second, to describe and compare the distributions of the monoamine transporters in the rodent, monkey, and human brain. The transporters were found to be heterogeneously distributed in the amygdala. The dopamine transporter (DAT) is consistently found to be extremely sparsely distributed, however the various accounts of its subregional topography are inconsistent, making any cross-species comparisons difficult. The serotonin transporter (SERT) had the greatest overall degree of labeling of the three markers, and was characterized by substantial inter-species variability in its relative distribution. The norepinephrine transporter (NET) was shown to possess an intermediate level of labeling, and like the SERT, its distribution is not consistent across the three species. The results of these comparisons indicate that caution should be exercised when using animal models to investigate the complex processes modulated by the monoamines in the amygdala, as their relative contributions to these functions may differ across species.

Influence of polychlorinated biphenyls and turning preference on striatal dopamine metabolism

Male BALB/c mice, assessed for spontaneous nocturnal rotation that has been linked with functional differences in striatal dopamine (DA) content, were divided into right (R), left (L), and no turning preference (NP) groups. Both total turning activity and turning in the preferred direction were greatest in the R mice. To determine whether turning preference influences the response to exposure to an environmental toxicant known to reduce striatal DA function, striatal tissues from R, L, or NP mice were exposed to polychlorinated biphenyls (PCBs). In vitro exposure of striata from these mice to varying concentrations of PCBs for 4 h concentration-dependently decreased tissue DA content and increased the concentrations of DA and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the media, a phenomenon already observed, although of smaller magnitude, with rat tissues. These effects were independent of the turning preference of the mice. Although the DA content initially did not differ among the striatal tissue from R, L, and NP mice, following 4 h of incubation in control medium (without PCBs), there was significantly greater DA content in striata from R mice in comparison to that from either L or NP mice. This difference was also apparent after 6 h of incubation, and it was not due to differences in DA turnover or cytotoxicity. Rather, the greater DA content in the striata of R mice was due to increased DA synthesis, since tyrosine hydroxylase (TH) activity was greater in striata from R mice than in striata from either L or NP mice. These differences in striatal DA synthesis, if present in vivo, may explain the observed greater nocturnal turning activity of the R mice. Additionally, the in vitro analyses suggest that striata from R mice have differential responses to stress compared with striata from L or NP mice. With regard to the effects of PCBs, however, it appears that (1) striata derived from mice with different turning preference are equally sensitive to this dopaminergic toxicant and (2) mice appear more sensitive to PCBs than rats.

The Role of N-Glycosylation in Function and Surface Trafficking of the Human Dopamine Transporter

The present study addressed the role of N-linked glycosylation of the human dopamine transporter (DAT) in its function with the help of mutants, in which canonical N-glycosylation sites have been removed (N181Q, N181Q,N188Q, and N181Q,N188Q,N205Q), expressed in human embryonic kidney-293 cells. Removal of canonical sites produced lower molecular weight species as did enzymatic deglycosylation or blockade of glycosylation, and all three canonical sites were found to carry sugars. Prevention of N-glycosylation reduced both surface and intracellular DAT. Although partially or non-glycosylated DAT was somewhat less represented at the surface, no evidence was found for preferential exclusion of such material from the plasma membrane, indicating that glycosylation is not essential for DAT expression. Non-glycosylated DAT was less stable at the surface as revealed by apparently enhanced endocytosis, consonant with weaker DAT immunofluorescence at the cell surface and stronger presence in cytosol in confocal analysis of the double and triple mutant. Non-glycosylated DAT did not transport dopamine as efficiently as wild-type DAT as judged from the sharp reduction in uptake V(max), and prevention of N-glycosylation enhanced the potency of cocaine-like drugs in inhibiting dopamine uptake into intact cells without changing their affinity for DAT when measured in membrane preparations prepared from these cells. Thus, non-glycosylated DAT at the cell surface displays appreciably reduced catalytic activity and altered inhibitor sensitivity compared with wild type.

Structure and function of the dopamine transporter

The dopamine transporter mediates uptake of dopamine into neurons and is a major target for various pharmacologically active drugs and environmental toxins. Since its cloning, much information has been obtained regarding its structure and function. Binding domains for dopamine and various blocking drugs including cocaine are likely formed by interactions with multiple amino acid residues, some of which are separate in the primary structure but lie close together in the still unknown tertiary structure. Chimera and site-directed mutagenesis studies suggest the involvement of both overlapping and separate domains in the interaction with substrates and blockers, whereas recent findings with sulfhydryl reagents selectively targeting cysteine residues support a role for conformational changes in the binding of blockers such as cocaine. The dopamine transporter can also operate in reverse, i.e. in an efflux mode, and recent mutagenesis experiments show different structural requirements for inward and outward transport. Strong evidence for dopamine transporter domains selectively influencing binding of dopamine or cocaine analogs has not yet emerged, although the development of a cocaine antagonist at the level of the transporter remains a possibility.

Non-amine dopamine transporter probe [(3)H]tropoxene distributes to dopamine-rich regions of monkey brain

Drug development in psychopharmacology has adhered to the unwritten precept that compounds targeting monoamine transporters must contain an amine nitrogen in the molecular structure. A series of non-amine-bearing aryloxatropanes that are potent inhibitors of the dopamine transporter (DAT) challenged this precept. In the present study, we investigated the brain distribution of a selective, high-affinity DAT non-amine, [(3)H]tropoxene (2-carbomethoxy-3, 4dichloro-3-aryl-8-oxabicyclo[3.2.1] octene), which binds to the DAT in monkey striatum. The autoradiographic distribution of [(3)H]tropoxene was conducted in tissue sections of rhesus (Macaca mulatta) monkey brain. Highest accumulation of the radioligand was detected in the putamen and caudate nucleus, with significant levels also observed in the nucleus accumbens and substantia nigra. Moderate to low levels of [(3)H]tropoxene binding were noted in the hypothalamus, amygdala, ventral tegmental area, and thalamus. The distribution of [(3)H]tropoxene was restricted to brain regions previously identified as expressing DAT, and the relative densities of [(3)H]tropoxene binding sites in various brain regions corresponded to those observed with other selective monoamine radioligands for the DAT. This is the first report to demonstrate that transporter-selective compounds that bear no amine nitrogen in their structure bind selectively to brain regions rich in the transporter. The results support our conclusion that an amine nitrogen is not necessary for compounds to bind to monoamine transporters and distribute in brain according to the known distribution of transporters. The findings provide further incentives to investigate the pharmacological potential of transport inhibitors lacking an amine nitrogen in the molecular structure.

Multisubtrate mechanism for the inward transport of dopamine by the human dopamine transporter expressed in HEK cells and its inhibition by cocaine

Rotating disk electrode voltammetry was used to measure the time-resolved inward transport of dopamine into human embryonic kidney cells expressing the human transporter for dopamine and a kinetic mechanism of transport is hypothesized. Dopamine transport in this preparation was highly concentrative, with a 10(6)-10(7) inward bias, first order in dopamine and the K(m) and V(max) were found to be 1.6 microM and 18 pmol/sec x 10(6) cells), respectively. The hDAT turnover was estimated to be approximately 18 s(-1) and the second order rate constant of association of dopamine with hDAT was approximately 10(7) M(-1)s(-1). Dopamine transport was found to have a second order dependence on Na(+) (K(Na) approximately 100 mM) and a first order dependence on Cl(-) (K(Cl) approximately 12 mM). Multisubstrate analyses suggested that hDAT operates with an ordered kinetic mechanism in which Na(+) binds first to the transporter protein, dopamine second, and Cl(-) last before translocation of dopamine into or across the membrane. Cocaine competitively inhibited dopamine transport (reaction order of unity and K(i) approximately 0.34 microM) with no discernible effect at the Na(+) and Cl(-) binding sites. These results differ from those of previous studies conducted in preparations of the striatum and nucleus accumbens. Comparisons of the variant results are made and an analysis of the differing apparent kinetic mechanisms is presented.

Cocaine and GBR photoaffinity labels as probes of dopamine transporter structure

Several aspects of DAT structure and function have been elucidated using a combination of photoaffinity labeling, proteolysis, enzymatic deglycosylation, and epitope-specific immunoprecipitation. The two photolabels are incorporated in different regions of the protein, suggesting that the binding sites for the ligands are distinct or partially nonoverlapping, consistent with results produced by site-directed mutagenesis and analysis of chimeras. These studies have also verified several aspects of DAT structure previously hypothesized based only on theoretical considerations, including the presence of at least one transmembrane helix or other membrane-anchoring structure in two different regions of the protein, identification of the glycosylated domain, and some topological properties. It should be possible to extend and adapt these techniques to further delineate DAT structural properties and to identify other functional domains such as phosphorylation sites or active sulfhydryl moieties.

Altropane, a SPECT or PET imaging probe for dopamine neurons: I. Dopamine transporter binding in primate brain

Increasing evidence suggests that the dopamine transporter is an important marker for physiological and pathological changes in dopamine neurons. Potent dopamine transport inhibitors of the phenyltropane series (e.g., WIN 35,428 or CFT) are particularly suitable for PET (positron emission tomography) or SPECT (single photon emission computed tomography) imaging of the dopamine transporter in living brain. We investigated whether altropane, an N-iodoallyl analog of WIN 35,428 (IACFT:E-N-iodoallyl-2 -carbomethoxy-3beta-(4-fluorophenyl)tropane), displayed in vitro properties suitable for evaluation as a SPECT imaging agent. In brain striatum of cynomolgus monkey (Macaca fascicularis), the unlabeled E-isomer (IC50: 6.62 +/- 0.78 nM) was more potent than the Z-isomer (IC50: 52.6 +/- 0.3 nM) and displayed a relatively high dopamine:serotonin transporter selectivity (28-fold). In radiolabeled form, [125I]altropane bound to sites in the striatum with a single high affinity (KD: 5.33 +/- 0.55 nM) and with a site density (BMAX: 301 pmol/g original wet tissue weight) that was within the density range reported previously for the dopamine transporter in striatum. Drugs inhibited [125I]altropane binding with a rank order of potency that corresponded closely to their potencies for inhibiting [3H]WIN 35,428 binding (r2: 0.99; P < 0.0001) to the blocking dopamine transport. The favorable binding properties of altropane, together with its rapid entry into primate brain and highly localized distribution in dopamine-rich brain regions, suggest it is a suitable iodinated probe for monitoring the dopamine transporter in vitro and in vivo by SPECT or PET imaging.

Altropane, a SPECT or PET imaging probe for dopamine neurons: III. Human dopamine transporter in postmortem normal and Parkinson's diseased brain

Increasing evidence suggests that the dopamine transporter is situated almost exclusively on dopamine neurons. Accordingly, it is an valuable marker for Parkinson's disease and other pathological states of dopamine neurons. We previously demonstrated that the potent dopamine transport inhibitor [125I]altropane (IACFT:E-N-iodoallyl-2beta-carbomethoxy-3beta-(4-fluor ophenyl)tropane) is a high affinity selective probe for the dopamine transporter in monkey brain and an effective SPECT imaging agent in nonhuman primate brain. We now report the binding properties of [125I]altropane in postmortem tissue of normal human brain and compare the findings to Parkinson's diseased brain. In homogenates of human brain putamen, [125I]altropane bound with high affinity (KD: 4.96 +/- 0.38 nM, n = 4) and site density (BMAX: 212 +/- 41.1 pmol/g original wet tissue weight) well within the density range reported previously for the dopamine transporter in this brain region. Drugs inhibited [125I]altropane binding with a rank order of potency that corresponded closely to their rank order for blocking dopamine transport (r 0.98, P < 0.001). In postmortem Parkinson's diseased brain, bound [125I]altropane (1 nM) was markedly reduced (89%, 99% in putamen, depending on measures of nonspecific binding) compared with normal aged-matched controls (normal putamen: 49.2 +/- 8.1 pmol/g; Parkinson's diseased putamen: 0.48 +/- 0.33 pmol/g; n = 4). In vitro autoradiography, conducted in tissue sections at a single plane of the basal ganglia, revealed high levels of [125I]altropane binding the caudate nucleus and putamen, but lower levels (73% of the caudate-putamen) in the nucleus accumbens (n = 7). In Parkinson's diseased brains (n = 4), [125I]altropane binding was 13% of the levels detected in normal putamen, 17% of normal values in the caudate nucleus, and 25% of normal levels in nucleus accumbens. The association of [125I]altropane to the dopamine transporter in human postmortem tissue, the marked reduction of [125I]altropane binding in Parkinson's diseased brains, its rapid entry into brain and highly localized distribution in dopamine-rich brain regions, support its use as a probe for monitoring the dopamine transporter in vitro and in vivo by SPECT imaging.

Altropane, a SPECT or PET imaging probe for dopamine neurons: II. Distribution to dopamine-rich regions of primate brain

The dopamine transporter in brain, localized almost exclusively on dopamine neurons, is an effective window on dopamine neurons. SPECT or PET imaging of the transporter in brain requires selective imaging agents that display appropriate pharmacokinetic properties. We previously reported that [125I]altropane ([125I]IACFT,2beta-carbomethoxy-3beta-(4-fluorophenyl)-n-(1- iodoprop-1-en-3-yl)nortropane) bound with high affinity (Kd: 5.33 nM) to a single site on the dopamine transporter and was selective for dopamine over the serotonin transporter in homogenates of monkey striatum. To determine whether the selective binding of [125I]altropane is reflected in its brain distribution, the in vitro and ex vivo distribution of [125I]altropane in squirrel monkey (Saimiri sciureus) brain was determined by quantitative autoradiography of coronal brain sections. In vitro, [125I]altropane (2 nM) distribution was discrete and was detectable primarily in the dopamine-rich putamen, caudate nucleus, and nucleus accumbens. The resulting putamen:cerebellum ratio exceeded 120:1 (n = 3). The selective in vitro binding of [125I]altropane to the dopamine transporter, at concentrations approaching its Kd value (Kd: 5.33 nM, a single high affinity site), highlight its suitability for investigating the density of the dopamine transporter in various brain regions in vitro. Ex vivo autoradiography was conducted in monkeys to determine whether the brain distribution of [125I]altropane in vitro was predictive of its brain distribution pattern after intravenous administration. Thirty minutes after intravenous injection, highest levels of [125I]altropane (0.3 nmol/kg) were detected in the caudate-putamen and nucleus accumbens and lowest levels in the cerebellum and cortex. The putamen or caudate:cerebellum ratio was 7. SPECT imaging of the brain within 30 min of i.v. injection confirmed the rapid and selective accumulation of [123I]altropane to the striatum. The selective binding of altropane to the dopamine-rich striatum within 30 min of i.v. administration indicates that it is uniquely suited for SPECT or PET imaging of the dopamine transporter and associated dopamine neurons.

Dopamine axon varicosities in the prelimbic division of the rat prefrontal cortex exhibit sparse immunoreactivity for the dopamine transporter

The dopamine transporter (DAT) critically regulates the duration of the cellular actions of dopamine and the extent to which dopamine diffuses in the extracellular space. We sought to determine whether the reportedly greater diffusion of dopamine in the rat prefrontal cortex (PFC) as compared with the striatum is associated with a more restricted axonal distribution of the cortical DAT protein. By light microscopy, avidin-biotin-peroxidase immunostaining for DAT was visualized in fibers that were densely distributed within the dorsolateral striatum and the superficial layers of the dorsal anterior cingulate cortex. In contrast, DAT-labeled axons were distributed only sparsely to the deep layers of the prelimbic cortex. By electron microscopy, DAT-immunoreactive profiles in the striatum and cingulate cortex included both varicose and intervaricose segments of axons. However, DAT-labeled processes in the prelimbic cortex were almost exclusively intervaricose axon segments. Immunolabeling for tyrosine hydroxylase in adjacent sections of the prelimbic cortex was localized to both varicosities and intervaricose segments of axons. These qualitative observations were supported by a quantitative assessment in which the diameter of immunoreactive profiles was used as a relative measure of whether varicose or intervaricose axon segments were labeled. These results suggest that considerable extracellular diffusion of dopamine in the prelimbic PFC may result, at least in part, from a paucity of DAT content in mesocortical dopamine axons, as well as a distribution of the DAT protein at a distance from synaptic release sites. The results further suggest that different populations of dopamine neurons selectively target the DAT to different subcellular locations.

Repeated cocaine and stress increase dopamine clearance in the rat medial prefrontal cortex

The effects of repeated footshock stress or cocaine on the kinetics of dopamine clearance in the medial prefrontal cortex (mPFC) were measured by rotating disk electrode voltammetry (RDEV). Five groups of rats were used: animals were either naive (non-handled), pre-treated with five daily saline (1 ml/kg i.p.) or cocaine (15 mg/kg i.p.) injections, or pre-treated with five daily 20-min sessions of sham shock or footshock (0.05 mA/200 ms/s). Dopamine clearance was measured after a 1-week withdrawal period. No difference in Km values was present among the treatment groups, with the mean Km value at approximately 0.5 microM for all groups. However, Vmax values were approximately 50% higher in daily sham shock-, footshock- and cocaine-pre-treated animals compared to naive rats. The increased ability to remove dopamine in these animals suggests that altered dopamine clearance may serve an adaptive mechanism in the mPFC.