Immunogold localization of the dopamine transporter: an ultrastructural study of the rat ventral tegmental area

Amphetamine Induces Sex-Dependent Loss of the Striatal Dopamine Transporter in Sensitized Mice

Dopamine transporter (DAT) controls dopamine signaling in the brain through the reuptake of synaptically released dopamine. DAT is a target of abused psychostimulants such as amphetamine (Amph). Acute Amph administration induces transient DAT endocytosis, which, among other Amph effects on dopaminergic neurons, elevates extracellular dopamine. However, the effects of repeated Amph abuse, leading to behavioral sensitization and drug addiction, on DAT are unknown. Hence, we developed a 14 d Amph-sensitization protocol in knock-in mice expressing HA-epitope-tagged DAT (HA-DAT) and investigated the effects of Amph challenge on sensitized HA-DAT animals. The Amph challenge resulted in the highest locomotor activity on Day 14 in both sexes, which was sustained for 1 h in male but not female mice. Strikingly, significant (by 30-60%) loss of the HA-DAT protein in the striatum was caused by the Amph challenge of sensitized males but not females. Amph also reduced V max of dopamine transport in the striatal synaptosomes of males without changing K m values. Consistently, immunofluorescence microscopy revealed a significant increase of HA-DAT colocalization with the endosomal protein VPS35 only in Amph-challenged males. Amph-induced loss of striatal HA-DAT in sensitized mice was blocked by chloroquine, vacuolin-1, and inhibitor of Rho-associated kinases ROCK1/2, indicative of the involvement of endocytic trafficking in the DAT protein loss. Interestingly, an apparent degradation of HA-DAT protein was observed in the nucleus accumbens and not in the dorsal striatum. We propose that Amph challenge in sensitized mice triggers Rho-mediated endocytosis and post-endocytic trafficking of DAT in a brain-region-specific and sex-dependent manner.

Actions and Consequences of Insulin in the Striatum

Insulin crosses the blood–brain barrier to enter the brain from the periphery. In the brain, insulin has well-established actions in the hypothalamus, as well as at the level of mesolimbic dopamine neurons in the midbrain. Notably, insulin also acts in the striatum, which shows abundant expression of insulin receptors (InsRs) throughout. These receptors are found on interneurons and striatal projections neurons, as well as on glial cells and dopamine axons. A striking functional consequence of insulin elevation in the striatum is promoting an increase in stimulated dopamine release. This boosting of dopamine release involves InsRs on cholinergic interneurons, and requires activation of nicotinic acetylcholine receptors on dopamine axons. Opposing this dopamine-enhancing effect, insulin also increases dopamine uptake through the action of insulin at InsRs on dopamine axons. Insulin acts on other striatal cells as well, including striatal projection neurons and astrocytes that also influence dopaminergic transmission and striatal function. Linking these cellular findings to behavior, striatal insulin signaling is required for the development of flavor–nutrient learning, implicating insulin as a reward signal in the brain. In this review, we discuss these and other actions of insulin in the striatum, including how they are influenced by diet and other physio-logical states.

Overview of the structure and function of the dopamine transporter and its protein interactions

The dopamine transporter (DAT) plays an integral role in dopamine neurotransmission through the clearance of dopamine from the extracellular space. Dysregulation of DAT is central to the pathophysiology of numerous neuropsychiatric disorders and as such is an attractive therapeutic target. DAT belongs to the solute carrier family 6 (SLC6) class of Na⁺/Cl^- dependent transporters that move various cargo into neurons against their concentration gradient. This review focuses on DAT (SCL6A3 protein) while extending the narrative to the closely related transporters for serotonin and norepinephrine where needed for comparison or functional relevance. Cloning and site-directed mutagenesis experiments provided early structural knowledge of DAT but our contemporary understanding was achieved through a combination of crystallization of the related bacterial transporter LeuT, homology modeling, and subsequently the crystallization of drosophila DAT. These seminal findings enabled a better understanding of the conformational states involved in the transport of substrate, subsequently aiding state-specific drug design. Post-translational modifications to DAT such as phosphorylation, palmitoylation, ubiquitination also influence the plasma membrane localization and kinetics. Substrates and drugs can interact with multiple sites within DAT including the primary S1 and S2 sites involved in dopamine binding and novel allosteric sites. Major research has centered around the question what determines the substrate and inhibitor selectivity of DAT in comparison to serotonin and norepinephrine transporters. DAT has been implicated in many neurological disorders and may play a role in the pathology of HIV and Parkinson's disease via direct physical interaction with HIV-1 Tat and α-synuclein proteins respectively.

Up and Down γ-Synuclein Transcription in Dopamine Neurons Translates into Changes in Dopamine Neurotransmission and Behavioral Performance in Mice

The synuclein family consists of α-, β-, and γ-Synuclein (α-Syn, β-Syn, and γ-Syn) expressed in the neurons and concentrated in synaptic terminals. While α-Syn is at the center of interest due to its implication in the pathogenesis of Parkinson's disease (PD) and other synucleinopathies, limited information exists on the other members. The current study aimed at investigating the biological role of γ-Syn controlling the midbrain dopamine (DA) function. We generated two different mouse models with: (i) γ-Syn overexpression induced by an adeno-associated viral vector and (ii) γ-Syn knockdown induced by a ligand-conjugated antisense oligonucleotide, in order to modify the endogenous γ-Syn transcription levels in midbrain DA neurons. The progressive overexpression of γ-Syn decreased DA neurotransmission in the nigrostriatal and mesocortical pathways. In parallel, mice evoked motor deficits in the rotarod and impaired cognitive performance as assessed by novel object recognition, passive avoidance, and Morris water maze tests. Conversely, acute γ-Syn knockdown selectively in DA neurons facilitated forebrain DA neurotransmission. Importantly, modifications in γ-Syn expression did not induce the loss of DA neurons or changes in α-Syn expression. Collectively, our data strongly suggest that DA release/re-uptake processes in the nigrostriatal and mesocortical pathways are partially dependent on substantia nigra pars compacta /ventral tegmental area (SNc/VTA) γ-Syn transcription levels, and are linked to modulation of DA transporter function, similar to α-Syn.

A Novel Biotinylated Homotryptamine Derivative for Quantum Dot Imaging of Serotonin Transporter in Live Cells

The serotonin transporter (SERT) is the primary target for selective serotonin reuptake inhibitor (SSRI) antidepressants that are thought to exert their therapeutic effects by increasing the synaptic concentration of serotonin. Consequently, probes that can be utilized to study cellular trafficking of SERT are valuable research tools. We have developed a novel ligand (IDT785) that is composed of a SERT antagonist (a tetrahydro pyridyl indole derivative) conjugated to a biotinylated poly ethylene glycol (PEG) via a phenethyl linker. This compound was determined to be biologically active and inhibited SERT-mediated reuptake of IDT307 with the half-maximal inhibitory concentration of 7.2 ± 0.3 μM. We demonstrated that IDT785 enabled quantum dot (QD) labeling of membrane SERT in transfected HEK-293 cultures that could be blocked using the high affinity serotonin reuptake inhibitor paroxetine. Molecular docking studies suggested that IDT785 might be binding to the extracellular vestibule binding site rather than the orthosteric substrate binding site, which could be attributable to the hydrophilicity of the PEG chain and the increased loss of degrees of freedom that would be required to penetrate into the orthosteric binding site. Using IDT785, we were able to study the membrane localization and membrane dynamics of YFP-SERT heterologously expressed in HEK-293 cells and demonstrated that SERT expression was enriched in the membrane edge and in thin cellular protrusions.

Contribution of analog signaling to neurotransmitter interactions and behavior: Role of transporter-mediated nonquantal dopamine release

Neuronal networks cause changes in behaviorally important information processing through the vesicular release of neurotransmitters governed by the rate and timing of action potentials (APs). Herein, we provide evidence that dopamine (DA), nonquantally released from the cytoplasm, may exert similar effects in vivo. In mouse slice preparations, (+/-)-3,4-methylenedioxy-methamphetamine (MDMA, or ecstasy) and β-phenylethylamine (β-PEA)-induced DA release in the striatum and nucleus accumbens (NAc), two regions of the brain involved in reward-driven and social behavior and inhibited the axonal stimulation-induced release of tritiated acetylcholine ([3 H]ACh) in the striatum. The DA transporter (DAT) inhibitor (GBR-12909) prevented MDMA and β-PEA from causing DA release. GBR-12909 could also restore some of the stimulated acetylcholine release reduced by MDMA or β-PEA in the striatum confirming the fundamental role of DAT. In addition, hypothermia could prevent the β-PEA-induced release in the striatum and in the NAc. Sulpiride, a D2 receptor antagonist, also prevented the inhibitory effects of MDMA or β-PEA on stimulated ACh release, suggesting they act indirectly via binding of DA. Reflecting the neurochemical interactions in brain slices at higher system level, MDMA altered the social behavior of rats by preferentially enhancing passive social behavior. Similar to the in vitro effects, GBR-12909 treatment reversed specific elements of the MDMA-induced changes in behavior, such as passive social behavior, while left others including social play unchanged. The changes in behavior by the high level of extracellular DA-- a significant amount originating from cytoplasmic release--suggest that in addition to digital computation through synapses, the brain also uses analog communication, such as DA signaling, to mediate some elements of complex behaviors, but in a much longer time scale.

Cannabidiol prevents priming- and stress-induced reinstatement of the conditioned place preference induced by cocaine in mice

BACKGROUND

Cocaine dependence is an important problem without any effective pharmacological treatment. Some preclinical studies have suggested that cannabidiol (CBD), a component of the Cannabis sativa plant, could be useful for the treatment of cocaine use disorders.

AIMS

This work aims to evaluate the ability of CBD to reduce priming- and stress-induced reinstatement of the conditioned place preference (CPP) induced by cocaine.

METHODS

Young adult CD-1 male mice were allocated to 10 groups (n = 12/group), conditioned with cocaine (10 mg/kg) and exposed to extinction of CPP (two sessions per week). When extinction was achieved, each group received the corresponding treatment before the reinstatement test. In experiment 1, six groups were used: vehicle+saline (Veh+Sal), 5 mg/kg cocaine alone (Veh+Coc) or with CBD 30 or 60 mg/kg (CBD30+Coc, CBD60+Coc) and CBD alone (CBD30+Sal, CBD60+Sal). In experiment 2, four groups were used: exploration (Veh+Expl), social defeat (Veh+SD) and social defeat with CBD (CBD30+SD and CBD60+SD). Furthermore, the relative gene expression of the dopamine transporter (DAT) in the ventral tegmental area was measured.

RESULTS

All mice acquired cocaine CPP and extinguished it after three or four weeks. Only the groups treated with cocaine priming (Veh+Coc) or exposed to social defeat (Veh+SD) showed reinstatement of CPP. Interestingly, CBD itself did not induce reinstatement and blocked the reinstating effects of cocaine priming and social defeat. Furthermore, cocaine priming increased DAT gene expression in the ventral tegmental area and CBD completely reversed this effect.

CONCLUSION

These results suggest that CBD could reduce reinstatement to cocaine seeking after a period of abstinence.

Activity-dependent somatodendritic dopamine release in the substantia nigra autoinhibits the releasing neuron

Somatodendritic dopamine (DA) release from midbrain DA neurons activates D2 autoreceptors on these cells to regulate their activity. However, the source of autoregulatory DA remains controversial. Here, we test the hypothesis that D2 autoreceptors on a given DA neuron in the substantia nigra pars compacta (SNc) are activated primarily by DA released from that same cell, rather than from its neighbors. Voltage-clamp recording allows monitoring of evoked D2-receptor-mediated inhibitory currents (D2ICs) in SNc DA neurons as an index of DA release. Single-cell application of antibodies to Na+ channels via the recording pipette decreases spontaneous activity of recorded neurons and attenuates evoked D2ICs; antibodies to SNAP-25, a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein, also decrease D2IC amplitude. Evoked D2ICs are nearly abolished by the light chain of botulinum neurotoxin A, which cleaves SNAP-25, whereas synaptically activated GABAB-receptor-mediated currents are unaffected. Thus, somatodendritic DA release in the SNc autoinhibits the neuron that releases it.

Autophagy status as a gateway for stress-induced catecholamine interplay in neurodegeneration

The catecholamine-containing brainstem nuclei locus coeruleus (LC) and ventral tegmental area (VTA) are critically involved in stress responses. Alterations of catecholamine systems during chronic stress may contribute to neurodegeneration, including cognitive decline. Stress-related catecholamine alterations, while contributing to anxiety and depression, might accelerate neuronal degeneration by increasing the formation of toxic dopamine and norepinephrine by-products. These, in turn, may impair proteostasis within a variety of cortical and subcortical areas. In particular, the molecular events governing neurotransmission, neuroplasticity, and proteostasis within LC and VTA affect a variety of brain areas. Therefore, we focus on alterations of autophagy machinery in these nuclei as a relevant trigger in this chain of events. In fact, these catecholamine-containing areas are mostly prone to autophagy-dependent neurodegeneration. Thus, we propose a dynamic hypothesis according to which stress-induced autophagy alterations within the LC-VTA network foster a cascade towards early neurodegeneration within these nuclei.

General Overview of Organic Cation Transporters in Brain

Inhibitors of Na⁺/Cl^- dependent high affinity transporters for norepinephrine (NE), serotonin (5-HT), and/or dopamine (DA) represent frequently used drugs for treatment of psychological disorders such as depression, anxiety, obsessive-compulsive disorder, attention deficit hyperactivity disorder, and addiction. These transporters remove NE, 5-HT, and/or DA after neuronal excitation from the interstitial space close to the synapses. Thereby they terminate transmission and modulate neuronal behavioral circuits. Therapeutic failure and undesired central nervous system side effects of these drugs have been partially assigned to neurotransmitter removal by low affinity transport. Cloning and functional characterization of the polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2), OCT3 (SLC22A3) and the plasma membrane monoamine transporter PMAT (SLC29A4) revealed that every single transporter mediates low affinity uptake of NE, 5-HT, and DA. Whereas the organic transporters are all located in the blood brain barrier, OCT2, OCT3, and PMAT are expressed in neurons or in neurons and astrocytes within brain areas that are involved in behavioral regulation. Areas of expression include the dorsal raphe, medullary motoric nuclei, hypothalamic nuclei, and/or the nucleus accumbens. Current knowledge of the transport of monoamine neurotransmitters by the organic cation transporters, their interactions with psychotropic drugs, and their locations in the brain is reported in detail. In addition, animal experiments including behavior tests in wildtype and knockout animals are reported in which the impact of OCT2, OCT3, and/or PMAT on regulation of salt intake, depression, mood control, locomotion, and/or stress effect on addiction is suggested.

Involvement of the endogenous opioid system in the beneficial influence of physical exercise on amphetamine-induced addiction parameters

Psychostimulant drugs addiction is a chronic public health problem and individuals remain susceptible to relapses increasing public expenses even after withdrawal and treatment. Our research group has focused on finding new therapies to be employed in drug addiction treatment, suggesting the physical exercise as a promising tool. This way, it is necessary to know the mechanisms involved in the beneficial influences of physical exercise observing the pathway that could be explored in drug addiction treatment. Male Wistar rats were conditioned with amphetamine (AMPH) following the conditioned place preference (CPP) protocol and subsequently submitted to swimming for 5 weeks (1 h per day, 5 days per week). Half of the animals were injected with Naloxone (0.3 mg/mL/kg body weight, i.p.) 5 min prior each physical exercise day. After AMPH-CPP re-exposure, our outcomes showed that physical exercise, in addition to minimizing the relapse behavior in the CPP, it increased D1R, D2R and DAT in the Ventral Tegmental Area (VTA), but not in the Nucleus accumbens (NAc). Interestingly, while naloxone inhibited the partial beneficial influence of the exercise on drug-relapse behavior, exercise-induced changes in the dopaminergic system were not observed in the group administered with naloxone as well. Based on these evidences, besides reinforcing the beneficial influence of the physical exercise on AMPH-induced drug addiction, we propose the involvement of endogenous opioid system activation, not as a single one, but as a possible mechanism of action resulting from the physical activity practice, thus characterizing an important therapeutic approach, which may contribute to drug withdrawal consequently preventing relapse.

Quantum dots reveal heterogeneous membrane diffusivity and dynamic surface density polarization of dopamine transporter

The presynaptic dopamine transporter mediates rapid reuptake of synaptic dopamine. Although cell surface DAT trafficking recently emerged as an important component of DAT regulation, it has not been systematically investigated. Here, we apply our single quantum dot (Qdot) tracking approach to monitor DAT plasma membrane dynamics in several heterologous expression cell hosts with nanometer localization accuracy. We demonstrate that Qdot-tagged DAT proteins exhibited highly heterogeneous membrane diffusivity dependent on the local membrane topography. We also show that Qdot-tagged DATs were localized away from the flat membrane regions and were dynamically retained in the membrane protrusions and cell edges for the duration of imaging. Single quantum dot tracking of wildtype DAT and its conformation-defective coding variants (R60A and W63A) revealed a significantly accelerated rate of dysfunctional DAT membrane diffusion. We believe our results warrant an in-depth investigation as to whether compromised membrane dynamics is a common feature of brain disorder-derived DAT mutants.

Corticostriatal plasticity in the nucleus accumbens core

Small fluctuations in striatal glutamate and dopamine are required to establish goal-directed behaviors and motor learning, while large changes appear to underlie many neuropsychological disorders, including drug dependence and Parkinson's disease. A better understanding of how variations in neurotransmitter availability can modify striatal circuitry will lead to new therapeutic targets for these disorders. Here, we examined dopamine-induced plasticity in prefrontal cortical projections to the nucleus accumbens (NAc) core. We combined behavioral measures of male mice, presynaptic optical studies of glutamate release kinetics from prefrontal cortical projections, and postsynaptic electrophysiological recordings of spiny projection neurons within the NAc core. Our data show that repeated amphetamine promotes long-lasting but reversible changes along the corticoaccumbal pathway. In saline-treated mice, coincident cortical stimulation and dopamine release promoted presynaptic filtering by depressing exocytosis from glutamatergic boutons with a low-probability of release. The repeated use of amphetamine caused a frequency-dependent, progressive, and long-lasting depression in corticoaccumbal activity during withdrawal. This chronic presynaptic depression was relieved by a drug challenge which potentiated glutamate release from synapses with a low-probability of release. D1 receptors generated this synaptic potentiation, which corresponded with the degree of locomotor sensitization in individual mice. By reversing the synaptic depression, drug reinstatement may promote allostasis by returning corticoaccumbal activity to a more stable and normalized state. Therefore, dopamine-induced synaptic filtering of excitatory signals entering the NAc core in novice mice and paradoxical excitation of the corticoaccumbal pathway during drug reinstatement may encode motor learning, habit formation, and dependence.

Methylenedioxypyrovalerone (MDPV) impairs working memory and alters patterns of dopamine signaling in mesocorticolimbic substrates

Knowledge remains limited about how chronic cathinone exposure impacts dopamine systems in brain reward circuits. In the present study, a binge-like MDPV exposure that impaired novel object recognition (NOR) dysregulated dopamine markers in mesocorticolimbic substrates of rats, with especially profound effects on D1 and D2 receptor's and VMAT gene expression. Our data suggested that dopamine receptivity was reduced in the NAc but increased in the PFC and dopamine-producing VTA. The MDPV-induced impairment of NOR was prevented by a D1 receptor antagonist, suggesting that chronic MDPV exposure produces site-specific dysregulation of dopamine markers in the mesocorticolimbic circuit and memory deficits in the NOR test that are influenced by D1 receptors.

Presynaptic regulation of dopamine release: Role of the DAT and VMAT2 transporters

The signaling dynamics of the neurotransmitter dopamine has been established to have an important role in a variety of behavioural processes including motor control, cognition, and emotional processing. Key regulators of transmitter release and the signaling dynamics of dopamine are the plasma membrane reuptake transporter (DAT) and the vesicular monoamine transporter (VMAT2). These proteins serve to remove dopamine molecules from the extracellular and cytosolic space, respectively and both determine the amount of transmitter released from synaptic vesicles. This review provides an overview of how these transporter proteins are involved in molecular regulation and function together to govern the dynamics of vesicular release with opposing effects on the quantal size and extracellular concentration of dopamine. These transporter proteins are both focal points of convergence for a variety of regulatory molecular cascades as well as targets for many pharmacological agents. The ratio between these transporters is argued to be useful as a molecular marker for delineating dopamine functional subsystems that may differ in transmitter release patterns.

Dendritic Release of Neurotransmitters

Release of neuroactive substances by exocytosis from dendrites is surprisingly widespread and is not confined to a particular class of transmitters: it occurs in multiple brain regions, and includes a range of neuropeptides, classical neurotransmitters, and signaling molecules, such as nitric oxide, carbon monoxide, ATP, and arachidonic acid. This review is focused on hypothalamic neuroendocrine cells that release vasopressin and oxytocin and midbrain neurons that release dopamine. For these two model systems, the stimuli, mechanisms, and physiological functions of dendritic release have been explored in greater detail than is yet available for other neurons and neuroactive substances. © 2017 American Physiological Society. Compr Physiol 7:235-252, 2017.

Somatodendritic dopamine release: recent mechanistic insights

Dopamine (DA) is a key transmitter in motor, reward and cogitative pathways, with DA dysfunction implicated in disorders including Parkinson's disease and addiction. Located in midbrain, DA neurons of the substantia nigra pars compacta project via the medial forebrain bundle to the dorsal striatum (caudate putamen), and DA neurons in the adjacent ventral tegmental area project to the ventral striatum (nucleus accumbens) and prefrontal cortex. In addition to classical vesicular release from axons, midbrain DA neurons exhibit DA release from their cell bodies and dendrites. Somatodendritic DA release leads to activation of D2 DA autoreceptors on DA neurons that inhibit their firing via G-protein-coupled inwardly rectifying K(+) channels. This helps determine patterns of DA signalling at distant axonal release sites. Somatodendritically released DA also acts via volume transmission to extrasynaptic receptors that modulate local transmitter release and neuronal activity in the midbrain. Thus, somatodendritic release is a pivotal intrinsic feature of DA neurons that must be well defined in order to fully understand the physiology and pathophysiology of DA pathways. Here, we review recent mechanistic aspects of somatodendritic DA release, with particular emphasis on the Ca(2+) dependence of release and the potential role of exocytotic proteins.

Amphetamine activates Rho GTPase signaling to mediate dopamine transporter internalization and acute behavioral effects of amphetamine

Acute amphetamine (AMPH) exposure elevates extracellular dopamine through a variety of mechanisms that include inhibition of dopamine reuptake, depletion of vesicular stores, and facilitation of dopamine efflux across the plasma membrane. Recent work has shown that the DAT substrate AMPH, unlike cocaine and other nontransported blockers, can also stimulate endocytosis of the plasma membrane dopamine transporter (DAT). Here, we show that when AMPH enters the cytoplasm it rapidly stimulates DAT internalization through a dynamin-dependent, clathrin-independent process. This effect, which can be observed in transfected cells, cultured dopamine neurons, and midbrain slices, is mediated by activation of the small GTPase RhoA. Inhibition of RhoA activity with C3 exotoxin or a dominant-negative RhoA blocks AMPH-induced DAT internalization. These actions depend on AMPH entry into the cell and are blocked by the DAT inhibitor cocaine. AMPH also stimulates cAMP accumulation and PKA-dependent inactivation of RhoA, thus providing a mechanism whereby PKA- and RhoA-dependent signaling pathways can interact to regulate the timing and robustness of AMPH's effects on DAT internalization. Consistent with this model, the activation of D1/D5 receptors that couple to PKA in dopamine neurons antagonizes RhoA activation, DAT internalization, and hyperlocomotion observed in mice after AMPH treatment. These observations support the existence of an unanticipated intracellular target that mediates the effects of AMPH on RhoA and cAMP signaling and suggest new pathways to target to disrupt AMPH action.

Long-term health of dopaminergic neuron transplants in Parkinson's disease patients

To determine the long-term health and function of transplanted dopamine neurons in Parkinson's disease (PD) patients, the expression of dopamine transporters (DATs) and mitochondrial morphology were examined in human fetal midbrain cellular transplants. DAT was robustly expressed in transplanted dopamine neuron terminals in the reinnervated host putamen and caudate for at least 14 years after transplantation. The transplanted dopamine neurons showed a healthy and nonatrophied morphology at all time points. Labeling of the mitochondrial outer membrane protein Tom20 and α-synuclein showed a typical cellular pathology in the patients' own substantia nigra, which was not observed in transplanted dopamine neurons. These results show that the vast majority of transplanted neurons remain healthy for the long term in PD patients, consistent with clinical findings that fetal dopamine neuron transplants maintain function for up to 15-18 years in patients. These findings are critically important for the rational development of stem-cell-based dopamine neuronal replacement therapies for PD.

Restoration of cocaine stimulation and reward by reintroducing wild type dopamine transporter in adult knock-in mice with a cocaine-insensitive dopamine transporter

In previous studies, we generated knock-in mice with a cocaine-insensitive dopamine transporter (DAT-CI mice) and found cocaine does not stimulate locomotion or produce reward in these mice, indicating DAT inhibition is necessary for cocaine stimulation and reward. However, DAT uptake is reduced in DAT-CI mice and thus the lack of cocaine responses could be due to adaptive changes. To test this, we used adeno-associated virus (AAV) to reintroduce the cocaine-sensitive wild type DAT (AAV-DATwt) back into adult DAT-CI mice, which restores cocaine inhibition of DAT in affected brain regions but does not reverse the adaptive changes. In an earlier study we showed that AAV-DATwt injections in regions covering the lateral nucleus accumbens (NAc) and lateral caudate-putamen (CPu) restored cocaine stimulation but not cocaine reward. In the current study, we expanded the AAV-DATwt infected areas to cover the olfactory tubercle (Tu) and the ventral midbrain (vMB) containing the ventral tegmental area (VTA) and substantia nigra (SN) in addition to CPu and NAc with multiple injections. These mice displayed the restoration of both locomotor stimulation and cocaine reward. We further found that AAV-DATwt injection in the vMB alone was sufficient to restore both cocaine stimulation and reward in DAT-CI mice. AAV injected in the VTA and SN resulted in DATwt expression and distribution to the DA terminal regions. In summary, cocaine induced locomotion and reward can be restored in fully developed DAT-CI mice, and cocaine inhibition of DAT expressed in dopaminergic neurons originated from the ventral midbrain mediates cocaine reward and stimulation.

Dopamine D₂ and acetylcholine α7 nicotinic receptors have subcellular distributions favoring mediation of convergent signaling in the mouse ventral tegmental area

Alpha7 nicotinic acetylcholine receptors (α7nAChRs) mediate nicotine-induced burst-firing of dopamine neurons in the ventral tegmental area (VTA), a limbic brain region critically involved in reward and in dopamine D2 receptor (D2R)-related cortical dysfunctions associated with psychosis. The known presence of α7nAChRs and Gi-coupled D2Rs in dopamine neurons of the VTA suggests that these receptors are targeted to at least some of the same neurons in this brain region. To test this hypothesis, we used electron microscopic immunolabeling of antisera against peptide sequences of α7nACh and D2 receptors in the mouse VTA. Dual D2R and α7nAChR labeling was seen in many of the same somata (co-localization over 97%) and dendrites (co-localization over 49%), where immunoreactivity for each of the receptors was localized to endomembranes as well as to non-synaptic or synaptic plasma membranes often near excitatory-type synapses. In comparison with somata and dendrites, many more small axons and axon terminals were separately labeled for each of the receptors. Thus, single-labeled axon terminals were predominant for both α7nAChR (57.9%) and D2R (89.0%). The majority of the immunolabeled axonal profiles contained D2R-immunoreactivity (81.6%) and formed either symmetric or asymmetric synapses consistent with involvement in the release of both inhibitory and excitatory transmitters. Of 160 D2R-labeled terminals, 81.2% were presynaptic to dendrites that expressed α7nAChR alone or together with the D2R. Numerous glial processes inclusive of those enveloping either excitatory- or inhibitory-type synapses also contained single labeling for D2R (n=152) and α7nAChR (n=561). These results suggest that classic antipsychotic drugs, all of which block the D2R, may facilitate α7nAChR-mediated burst-firing by elimination of D2R-dependent inhibition in neurons expressing both receptors as well as by indirect pre-synaptic and glial mechanisms.

Epitope-tagged dopamine transporter knock-in mice reveal rapid endocytic trafficking and filopodia targeting of the transporter in dopaminergic axons

The plasma membrane dopamine (DA) transporter (DAT) is essential for reuptake of extracellular DA. DAT function in heterologous cells is regulated by subcellular targeting, endocytosis, and intracellular trafficking, but the mechanisms regulating neuronal DAT remain poorly understood. Hence, we generated a knock-in mouse expressing a hemagglutinin (HA)-epitope-tagged DAT to study endogenous transporter trafficking. Introduction of the HA tag into the second extracellular loop of mouse DAT did not perturb its expression level, distribution pattern, or substrate uptake kinetics. Live-cell fluorescence microscopy imaging using fluorescently labeled HA-specific antibody and a quantitative HA-antibody endocytosis assay demonstrated that in axons HA-DAT was primarily located in the plasma membrane and internalized mostly in growth cones and varicosities, where synaptic vesicle markers were also concentrated. Formation of varicosities was frequently preceded or accompanied by highly dynamic filopodia-like membrane protrusions. Remarkably, HA-DAT often concentrated at the tips of these filopodia. This pool of HA-DATs exhibited low lateral membrane mobility. Thus, DAT-containing filopodia may be involved in synaptogenesis in developing DA neurons. Treatment of neurons with amphetamine increased mobility of filopodial HA-DAT and accelerated HA-DAT endocytosis in axons, suggesting that chronic amphetamine may interfere with DA synapse development. Interestingly, phorbol esters did not accelerate endocytosis of axonal DAT.

Differential subcellular distribution of endosomal compartments and the dopamine transporter in dopaminergic neurons

Dopamine (DA) transporter (DAT) functions at the surface of dopaminergic neurons to clear extracellular DA. DAT surface levels are regulated by endocytosis. However, the endosome-lysosome system is not well characterized in dopaminergic neurons and the endocytic trafficking of endogenous DAT is poorly studied. Hence we analyzed the distribution of endocytic compartments and DAT localization in cultured rat embryonic and postnatal neurons using fluorescence microscopy. Early Rab5 and EEA.1 containing endosomes were mostly found in somatodendritic regions of neurons, whereas endosomes containing recycling markers were primarily found in axons. In axons, DAT was located mainly in recycling endosomes and plasma membrane whereas in cell bodies and dendrites DAT was detected in early, late and recycling endosomal compartments. Subcellular fractionation of adult rat striatal synaptosomes demonstrated that DAT is enriched in fractions containing plasma membrane and recycling endosomes. This pattern of DAT distribution was not altered upon activation of protein kinase C in postnatal DA neurons. Altogether, our data suggest that axonal DAT mainly shuttles between the plasma membrane and recycling endosomes, whereas in the somatodendritic region of neurons DAT traffics through all conventional endosomal pathways.

The dopamine system in idiopathic generalized epilepsies: identification of syndrome-related changes

The present study tests the hypothesis that the dopamine system is altered in idiopathic generalized epilepsy (IGE), and that the pattern of possible changes differs between juvenile myoclonic epilepsy (JME) and epilepsy with tonic-clonic seizures only (GTCS). The dopamine (DA) system was investigated with PET and a DA transporter (DAT) ligand [(11)C]PE2I in 13 patients with JME, 13 with GTCS, and 12 healthy controls. The binding potential (BP) to DAT was quantified in the caudate, putamen, and midbrain. The possible impact on function was evaluated by correlating regional BP with test performance in a battery of neuropsychological tests. Both patient groups showed a reduced BP compared to controls, albeit in different locations. JME patients had a lower tracer binding than controls in the midbrain (0.8+/-0.1 vs. 1.0+/-0.2, p=0.019), whereas GTCS patients had reduced tracer binding in the putamen (5.9+/-1.6 vs. 7.1+/-1.2, p=0.023). While GTCS patients showed impaired performance in motor functions and on one test of executive function, JME patients performed poorly also in tests of working memory and several tests of executive function. Alterations in the DA system seem to exist in both GTCS and JME. However, the regional distribution of these changes differs between the two syndromes, as does their association with psychomotor and working memory performance. The present data suggest that the two forms of IGE have different neuronal substrates.

Non-synaptic receptors and transporters involved in brain functions and targets of drug treatment

Beyond direct synaptic communication, neurons are able to talk to each other without making synapses. They are able to send chemical messages by means of diffusion to target cells via the extracellular space, provided that the target neurons are equipped with high-affinity receptors. While synaptic transmission is responsible for the 'what' of brain function, the 'how' of brain function (mood, attention, level of arousal, general excitability, etc.) is mainly controlled non-synaptically using the extracellular space as communication channel. It is principally the 'how' that can be modulated by medicine. In this paper, we discuss different forms of non-synaptic transmission, localized spillover of synaptic transmitters, local presynaptic modulation and tonic influence of ambient transmitter levels on the activity of vast neuronal populations. We consider different aspects of non-synaptic transmission, such as synaptic-extrasynaptic receptor trafficking, neuron-glia communication and retrograde signalling. We review structural and functional aspects of non-synaptic transmission, including (i) anatomical arrangement of non-synaptic release sites, receptors and transporters, (ii) intravesicular, intra- and extracellular concentrations of neurotransmitters, as well as the spatiotemporal pattern of transmitter diffusion. We propose that an effective general strategy for efficient pharmacological intervention could include the identification of specific non-synaptic targets and the subsequent development of selective pharmacological tools to influence them.

The reverse operation of Na(+)/Cl(-)-coupled neurotransmitter transporters--why amphetamines take two to tango

Sodium-chloride coupled neurotransmitter transporters achieve reuptake of their physiological substrate by exploiting the pre-existing sodium-gradient across the cellular membrane. This terminates the action of previously released substrate in the synaptic cleft. However, a change of the transmembrane ionic gradients or specific binding of some psychostimulant drugs to these proteins, like amphetamine and its derivatives, induce reverse operation of neurotransmitter:sodium symporters. This effect eventually leads to an increase in the synaptic concentration of non-exocytotically released neurotransmitters [and - in the case of the norepinephrine transporters, underlies the well-known indirect sympathomimetic activity]. While this action has long been appreciated, the underlying mechanistic details have been surprisingly difficult to understand. Some aspects can be resolved by incorporating insights into the oligomeric nature of transporters, into the nature of the accompanying ion fluxes, and changes in protein kinase activities.

Regulation of dopamine transporter activity by carboxypeptidase E

Background

The dopamine transporter (DAT) plays a critical role in terminating the action of dopamine by rapid reuptake into the presynaptic neuron. Previous studies have revealed that the DAT carboxyl terminus (DAT-CT) can directly interact with other cellular proteins and regulate DAT function and trafficking.

Results

Here, we have identified that carboxypeptidase E (CPE), a prohormone processing exopeptidase and sorting receptor for the regulated secretory pathway, interacts with the DAT-CT and affects DAT function. Mammalian cell lines coexpressing CPE and DAT exhibited increased DAT-mediated dopamine uptake activity compared to cells expressing DAT alone. Moreover, coexpression of an interfering DAT-CT minigene inhibited the effects of CPE on DAT. Functional changes caused by CPE could be attributed to enhanced DAT expression and subsequent increase in DAT cell surface localization, due to decreased DAT degradation. In addition, CPE association could reduce the phosphorylation state of DAT on serine residues, potentially leading to reduced internalization, thus stabilizing plasmalemmal DAT localization.

Conclusion

Taken together, our results reveal a novel role for CPE in the regulation of DAT trafficking and DAT-mediated DA uptake, which may provide a novel target in the treatment of dopamine-governed diseases such as drug addiction and obesity.

Dopamine spillover after quantal release: rethinking dopamine transmission in the nigrostriatal pathway

The predominance of dopamine (DA) receptors at extrasynaptic vs. synaptic sites implies that DA signaling is by diffusion-based volume transmission. In this review, we compare characteristics that regulate extracellular DA behavior in substantia nigra pars compacta (SNc) and striatum, including regional differences in structure (a 40% greater extracellular volume fraction in SNc vs. striatum) and in dynamic DA uptake (a 200-fold greater DA uptake rate in striatum vs. SNc). Furthermore, we test the assumption of diffusion-based volume transmission for SNc and striatum by modeling dynamic DA behavior after quantal release using region-specific parameters for diffusion and uptake at 37 degrees C. Our model shows that DA uptake does not affect peak DA concentration within 1 mum of a release site in either SNc or striatum because of the slow kinetics of DATs vs. diffusion. Rather, diffusion and dilution are the dominant factors governing DA concentration after quantal release. In SNc, limited DAT efficacy is reflected in a lack of influence of uptake on either amplitude or time course of DA transients after quantal release up to 10 mum from a release site. In striatum, the lack of effect of the DAT within 1 mum of a release site means that perisynaptic DATs do not "gate" synaptic spillover. This contrasts with the conventional view of DA synapses, in which DATs efficiently recycle DA by re-uptake into the releasing axon terminal. However, the model also shows that a primary effect of striatal uptake is to curtail DA lifetime after release. In both SNc and striatum, effective DA radius after quantal release is ~2 mum for activation of low-affinity DA receptors and 7-8 mum for high-affinity receptors; the corresponding spheres of influence would encompass tens to thousands of synapses. Thus, the primary mode of intercellular communication by DA, regardless of region, is volume transmission.

Presynaptic nicotinic receptors: a dynamic and diverse cholinergic filter of striatal dopamine neurotransmission

The effects of nicotine on dopamine transmission from mesostriatal dopamine neurons are central to its reinforcing properties. Only recently however, has the influence of presynaptic nicotinic receptors (nAChRs) on dopaminergic axon terminals within striatum begun to be understood. Here, rather than simply enhancing (or inhibiting) dopamine release, nAChRs perform the role of a presynaptic filter, whose influence on dopamine release probability depends on presynaptic activity in dopaminergic as well as cholinergic neurons. Both mesostriatal dopaminergic neurons and striatal cholinergic interneurons play key roles in motivational and sensorimotor processing by the basal ganglia. Moreover, it appears that the striatal influence of dopamine and ACh cannot be fully appreciated without an understanding of their reciprocal interactions. We will review the powerful filtering by nAChRs of striatal dopamine release and discuss its dependence on activity in dopaminergic and cholinergic neurons. We will also review how nicotine, acting via nAChR desensitization, promotes the sensitivity of dopamine synapses to activity. This filtering action might provide a mechanism through which nicotine promotes how burst activity in dopamine neurons facilitates goal-directed behaviour and reinforcement processing. More generally, it indicates that we should not restrict our view of presynaptic nAChRs to simply enhancing neurotransmitter release. We will also summarize current understanding of the forms and functions of the diverse nAChRs purported to exist on dopaminergic axons. A greater understanding of nAChR form and function is imperative to guide the design of ligands with subtype-selective efficacy for improved therapeutic interventions in nicotine addiction as well as Parkinson's disease.

Pancreatic innervation in mouse development and beta-cell regeneration

Pancreatic innervation is being viewed with increasing interest with respect to pancreatic disease. At the same time, relatively little is currently known about innervation dynamics during development and disease. The present study employs confocal microscopy to analyze the growth and development of sympathetic and sensory neurons and astroglia during pancreatic organogenesis and maturation. Our research reveals that islet innervation is closely linked to the process of islet maturation-neural cell bodies undergo intrapancreatic migration/shuffling in tandem with endocrine cells, and close neuro-endocrine contacts are established quite early in pancreatic development. In addition, we have assayed the effects of large-scale beta-cell loss and repopulation on the maintenance of islet innervation with respect to particular neuron types. We demonstrate that depletion of the beta-cell population in the rat insulin promoter (RIP)-cmyc(ER) mouse line has cell-type-specific effects on postganglionic sympathetic neurons and pancreatic astroglia. This study contributes to a greater understanding of how cooperating physiological systems develop together and coordinate their functions, and also helps to elucidate how permutation of one organ system through stress or disease can specifically affect parallel systems in an organism.

Monoamine transporters and psychostimulant addiction

Psychostimulants are a broadly defined class of drugs that stimulate the central and peripheral nervous systems as their primary pharmacological effect. The abuse liability of psychostimulants is well established and represents a significant public health concern. An extensive literature documents the critical importance of monoamines (dopamine, serotonin and norepinephrine) in the behavioral pharmacology and addictive properties of psychostimulants. In particular, the dopamine transporter plays a primary role in the reinforcing and behavioral-stimulant effects of psychostimulants in animals and humans. Moreover, both serotonin and norepinephrine systems can reliably modulate the neurochemical and behavioral effects of psychostimulants. However, there is a growing body of evidence that highlights complex interactions among additional neurotransmitter systems. Cortical glutamatergic systems provide important regulation of dopamine function, and inhibitory amino acid gamma-aminobutyric acid (GABA) systems can modulate basal dopamine and glutamate release. Repeated exposure to psychostimulants can lead to robust and enduring changes in neurobiological substrates, including monoamines, and corresponding changes in sensitivity to acute drug effects on neurochemistry and behavior. Significant advances in the understanding of neurobiological mechanisms underlying psychostimulant abuse and dependence have guided pharmacological treatment strategies to improve clinical outcome. In particular, functional agonist treatments may be used effectively to stabilize monoamine neurochemistry, influence behavior and lead to long-term abstinence. However, additional clinical studies are required in order to identify safe and efficacious pharmacotherapies.

Difference in magnitude of psychostimulant-induced extracellular norepinephrine in the ventral tegmental area contributes to discrepant prefrontal dopamine outflow

The dopamine (DA) efflux in the medial prefrontal cortex (mPFC) can be modulated by the interaction between afferent norepinephrine (NE) and somatodendritic DA in the ventral tegmental area (VTA). However, it is unclear how locally administered amphetamine (AMPH) or cocaine in the VTA results in discrepant response of DA efflux in the mPFC. In this study, intra-VTA infusion of AMPH (1000 microM) or cocaine (200 microM) in anesthetized rats was employed to study the concurrent profile of extracellular DA level in the VTA and mPFC. In addition, the extracellular NE levels during the intra-VTA infusion of these two psychostimulants were analyzed to compare their effects on prefrontal DA efflux. During the intra-VTA infusion of AMPH, both extracellular DA and NE increased significantly in the VTA (270 +/- 12% and 819 +/- 40%, respectively). Meanwhile, the DA efflux in the mPFC elevated significantly. During the intra-VTA infusion of cocaine, the extracellular DA and NE in the VTA also increased (271 +/- 21% and 150 +/- 15%, respectively). However, the DA efflux decreased significantly in the mPFC. Noteworthy, the increase of extracellular NE in the VTA was much more robust via AMPH infusion, as compared with cocaine. It is suggested that AMPH and cocaine enhance the extracellular NE concentrations in the VTA in different magnitudes, which in turn contribute to discrepant profiles of distal DA efflux in the mPFC.

Kappa opioid inhibition of somatodendritic dopamine inhibitory postsynaptic currents

In the midbrain, dopamine neurons can release dopamine somatodendritically. This results in an inhibitory postsynaptic current (IPSC) within adjacent dopamine cells that occurs by the activation of inhibitory D(2) autoreceptors. Kappa, but not mu/delta, opioid receptors inhibit this IPSC. The aim of the present study was to determine the mechanism by which kappa-opioid receptors inhibit the dopamine IPSC. In both the ventral tegmental area (VTA) and substantia nigra compacta (SNc) the kappa-receptor agonist U69593 inhibited the IPSC, but not the current induced by the exogenous iontophoretic application of dopamine. The endogenous peptide dynorphin A (1-13) also inhibited IPSCs in the VTA and SNc, but also the dopamine iontophoretic current in the VTA. Although both kappa agonists induced a postsynaptic outward current in the VTA, the current induced by dynorphin was dramatically larger. This suggests that the decrease in iontophoretic dopamine current was the result of occlusion. Occlusion alone, however, could not completely account for suppression of the IPSC. The kappa opioid inhibition of the IPSC was not affected by global increases or decreases in dopamine cell activity within the slice. These findings suggest that, although kappa opioid receptors can hyperpolarize dopamine neurons, they also suppress dopamine release by direct actions at the release site. The results thus demonstrate both pre- and postsynaptic actions of kappa receptor agonists. The actions of dynorphin indicate that VTA dopamine cells are selectively regulated by kappa receptors.

Three ubiquitin conjugation sites in the amino terminus of the dopamine transporter mediate protein kinase C-dependent endocytosis of the transporter

Dopamine levels in the brain are controlled by the plasma membrane dopamine transporter (DAT). The amount of DAT at the cell surface is determined by the relative rates of its internalization and recycling. Activation of protein kinase C (PKC) leads to acceleration of DAT endocytosis. We have recently demonstrated that PKC activation also results in ubiquitylation of DAT. To directly address the role of DAT ubiquitylation, lysine residues in DAT were mutated. Mutations of each lysine individually did not affect ubiquitylation and endocytosis of DAT. By contrast, ubiquitylation of mutants carrying multiple lysine substitutions was reduced in cells treated with phorbol ester to the levels detected in nonstimulated cells. Altogether, mutagenesis data suggested that Lys19, Lys27, and Lys35 clustered in the DAT amino-terminus are the major ubiquitin-conjugation sites. The data are consistent with the model whereby at any given time only one of the lysines in DAT is conjugated with a short ubiquitin chain. Importantly, cell surface biotinylation, immunofluorescence and down-regulation experiments revealed that PKC-dependent internalization of multilysine mutants was essentially abolished. These data provide the first evidence that the ubiquitin moieties conjugated to DAT may serve as a molecular interface of the transporter interaction with the endocytic machinery.

Aberrant accumulation of serotonin in dopaminergic neurons

Gene targeting approaches greatly facilitate insight into the functioning of monoamine transporters, the targets of potent antidepressants. The serotonin transporter (5-HTT) is the molecular target of a large number of antidepressants. To assess the clearance of serotonin (5-HT) in the absence of the 5-HTT, we have generated double knockout mice lacking both the 5-HTT and the catabolizing enzyme monoamine oxidase A (MAOA). We found aberrant 5-HT accumulation in the striatum of these MAOA/5-HTT double knockout mice. By additional ablation of the dopamine transporter (DAT), this aberrant 5-HT accumulation was abolished in MAOA/5-HTT/DAT triple knockout mice. Thus, aberrant uptake of 5-HT occurs in dopaminergic terminals under conditions of elevated 5-HT levels, and this aberrant uptake is mediated by the DAT. These findings have important consequences for antidepressant therapy, since during treatment of depression with selective serotonin reuptake inhibitors, clearance of 5-HT by dopaminergic neurons may reduce the desired therapeutic elevation of extracellular 5-HT levels. This provides a molecular rationale for improving antidepressant efficacy by additional pharmacological inhibition of the DAT.

Alcohol consumption alters dopamine transporter sites in Wistar–Kyoto rat brain

Even though animal and human studies show alterations in dopamine transporter (DAT) sites after alcohol withdrawal, the role of DAT in influencing either alcoholic or depressive behavior has not been examined extensively. Given that the Wistar-Kyoto (WKY) rat is a putative animal model of depressive behavior, the present study examined the effects of chronic alcohol consumption on DAT sites in WKY versus Wistar (WIS) rats. Brains from both strains were sectioned for autoradiographic analysis of [3H]-GBR12935 binding to DAT sites after 24 days of alcohol exposure. The results indicated that WKY rats consumed a greater amount of alcohol (P < 0.001) than WIS rats did throughout the experiment. Autoradiographic analyses of discrete brain regions indicated that alcohol consumption increased DAT sites in a greater number of brain areas in WKY compared to WIS rats. In WKY rats, the binding of [3H]-GBR12935 to DAT sites was increased in the basolateral, central and lateral nuclei of the amygdala, lateral nucleus of the hypothalamus, olfactory tubercle, caudate-putamen, nucleus accumbens and substantia nigra (P < 0.05) and decreased in the ventromedial nucleus of the hypothalamus and the CA1 region of the hippocampus. In WIS rats, alcohol consumption increased DAT sites in the CA1 region of the hippocampus, basolateral nucleus of the amygdala, ventral tegmental area and substantia nigra, and decreased DAT sites in the lateral and ventromedial hypothalamus and dentate gyrus. These results indicate a strain dependent alteration in DAT sites which may be related to altered dopamine neurotransmission in select brain regions following alcohol consumption.

Regulated Expression and Function of the Somatodendritic Catecholamine Neurotransmitter Transporters

Termination of neurotransmission at catecholaminergic synapses is well documented by the transporters for dopamine and norepinephrine, members of the Na(+)/Cl(-)-dependent neurotransmitter transporter family, which accumulates released transmitters within their nerve endings, respectively. Although somatodendritic expression of the transporters and the effects of cocaine and amphetamine on those have been reported, their role is still obscure. Recent findings of the transporter function as an ion channel and/or its reverse transport property provide a clue to identify the role of these transporters in the somatodendrites and their consequential interaction with uptake inhibitors. Differences in ionic environment and maturity of the release machinery in the somatodendrites at developmental stages influence the transporter functions, resulting in the formation of both positive and negative feedback loop of catecholaminergic neurons.

Constitutive and Protein Kinase C-Induced Internalization of the Dopamine Transporter is Mediated by a Clathrin-Dependent Mechanism

The amount of dopamine transporter (DAT) present at the cell surface is rapidly regulated by the rates of DAT internalization to endosomes and DAT recycling back to the plasma membrane. The re-distribution of the transporter from the cell surface to endosomes was induced by phorbol ester activation of protein kinase C in porcine aortic endothelial cells stably expressing the human DAT. Inhibition of DAT recycling with the carboxylic ionophore monensin also caused significant accumulation of DAT in early endosomes and a concomitant loss of DAT from the cell surface, due to protein kinase C-independent constitutive internalization of DAT in the absence of recycling. Such monensin-induced relocation of DAT to endosomes was therefore utilized as a measure of the constitutive internalization of DAT. Knock-down of clathrin heavy chain or dynamin II by small interfering RNAs dramatically inhibited both constitutive and protein kinase C-mediated internalization of DAT. In contrast, neither monensin-dependent nor phorbol ester-induced re-distribution of DAT were affected by inhibitors of endocytosis through cholesterol-rich membrane microdomains. Mutational analysis revealed the potential importance of amino acid residues 587-597 in DAT internalization. Altogether, the data suggest that both constitutive and protein kinase C-mediated internalization of DAT utilize the clathrin-dependent endocytic pathway, but likely involve unconventional mechanisms.

Rapid regulation of the dopamine transporter: role in stimulant addiction?

Dopamine (DA) and the DA transporter (DAT) play important roles in psychomotor stimulant behavioral activation and reward. By understanding how DAT activity is regulated, we will better appreciate its contribution to normal neurotransmission and to brain diseases like drug addiction. DAT is regulated long-term by chronic drug administration. It is also regulated in a rapid, dynamic fashion by many factors--including brief exposure to DAT substrates, e.g. DA and amphetamine, and inhibitors, e.g. cocaine. We found that individual differences in the initial and sensitized locomotor responsiveness of rats to cocaine reflect differences in in vivo DAT function. Our ex vivo studies have further suggested that differences in basal and/or rapid cocaine-induced expression of functional DATs in striatum contribute to the differences in initial responsiveness. Studies in model systems have demonstrated that short-term DAT regulation occurs by altered transporter trafficking, and thereby cell surface expression. For example, a rapid, complex regulation of DAT by DA is suggested. Amphetamine causes DAT internalization into early endosomal compartments whereas cocaine appears to up-regulate surface expression of DAT. Future studies are needed to confirm these observations in neurons, as well as to elucidate the mechanisms of rapid DAT endocytic trafficking at neuronal synapses.

The somatodendritic release of dopamine in the ventral tegmental area and its regulation by afferent transmitter systems

The release of dopamine in the ventral tegmental area (VTA) plays an important role in the autoinhibition of the dopamine neurons of the mesocorticolimbic system through the activation of somatodendritic dopamine D2 autoreceptors. Accordingly, the intra-VTA application of dopamine D2 receptor agonists reduces the firing rate and release of dopamine in the VTA, and this control appears to possess a tonic nature because the corresponding antagonists enhance the somatodendritic release of the transmitter. In addition, the release of dopamine in the VTA is increased by potassium or veratridine depolarization and abolished by tetrodotoxin and calcium omission. Overall, it appears that the somatodendritic release of dopamine is consistently lower than that in nerve endings. Apart from intrinsic dopaminergic mechanisms, other transmitter systems such as serotonin, noradrenaline, acetylcholine, GABA and glutamate play a role in the control of the activity of dopaminergic neurons of the VTA, although the final action depends on the particular receptor involved as well as the neuronal type where it is localized. Given the involvement of the mesocorticolimbic dopaminergic systems in the pathogenesis of severe neuropsychiatric disorders such as schizophrenia, the knowledge of the factors that regulate the release of dopamine in the VTA could provide new insight into the ethiogenesis of the disease as well as its implication on the mechanisms of action of therapeutic drugs.

Multiple molecular determinants in the carboxyl terminus regulate dopamine transporter export from endoplasmic reticulum

The plasma membrane dopamine transporter (DAT) has an essential role in terminating dopaminergic neurotransmission by reuptake of dopamine into the presynaptic neurons. Therefore, the amount of DAT at the cell surface is a critical determinant of DAT function. In this study, we examined the role of the carboxyl terminus of DAT in trafficking of the transporter through the biosynthetic pathway to the plasma membrane. Live cell fluorescence microscopy and cell surface biotinylation were used to study the effects of systematic deletions and alanine substitutions in the carboxyl terminus on DAT localization. It was found that alanine substitutions of Lys-590 and Asp-600 significantly delayed the delivery of DAT to the plasma membrane because of retention of DAT in the endoplasmic reticulum (ER). Most surprising, mutation of Gly-585 to alanine completely blocked the exit of DAT from the ER and surface expression of the transporter. The effect of these three mutations on ER export of DAT was demonstrated in porcine aortic endothelial cells and the immortalized neuronal cell line 1RB3AN27. In primary cultures of rat embryonic midbrain neurons, DAT G585A, K590A, and D600A mutants were restricted to the cell soma and did not traffic to the dendrites or axonal processes. These data are consistent with the model whereby the local conformation and/or intramolecular interactions of the sequences of the DAT carboxyl terminus proximal to the last transmembrane domain are essential for the ER export of the transporter.

Dopamine transporter: basic science and human variation of a key molecule for dopaminergic function, locomotion, and parkinsonism

We review the basic science of the dopamine transporter (DAT), a key neurotransmitter for locomotor control and reward systems, including those lost or deranged in Parkinson's disease (PD). Physiology, pharmaceutical features, expression, cDNA, protein structure/function relationships, and phosphorylation and regulation are discussed. The localization of DAT provides the best marker for the integrity of just the pre-synaptic dopaminergic systems that are most affected in PD. Its function is key for the actions of several toxins that provide some of the best current models for idiopathic parkinsonism, and its variation can clearly alter movement. The wealth of information about this interesting molecule that has been developed over the last 12 years has led to increased interest in DAT among workers interested in both normal and abnormal movement.

Designing human m1 muscarinic receptor-targeted hydrophobic eigenmode matched peptides as functional modulators

A new proprietary de novo peptide design technique generated ten 15-residue peptides targeting and containing the leading nontransmembrane hydrophobic autocorrelation wavelengths, "modes", of the human m(1) muscarinic cholinergic receptor, m(1)AChR. These modes were also shared by the m(4)AChR subtype (but not the m(2), m(3), or m(5) subtypes) and the three-finger snake toxins that pseudoirreversibly bind m(1)AChR. The linear decomposition of the hydrophobically transformed m(1)AChR amino acid sequence yielded ordered eigenvectors of orthogonal hydrophobic variational patterns. The weighted sum of two eigenvectors formed the peptide design template. Amino acids were iteratively assigned to template positions randomly, within hydrophobic groups. One peptide demonstrated significant functional indirect agonist activity, and five produced significant positive allosteric modulation of atropine-reversible, direct-agonist-induced cellular activation in stably m(1)AChR-transfected Chinese hamster ovary cells, reflected in integrated extracellular acidification responses. The peptide positive allosteric ligands produced left-shifts and peptide concentration-response augmentation in integrated extracellular acidification response asymptotic sigmoidal functions and concentration-response behavior in Hill number indices of positive cooperativity. Peptide mode specificity was suggested by negative crossover experiments with human m(2)ACh and D(2) dopamine receptors. Morlet wavelet transformation of the leading eigenvector-derived, m(1)AChR eigenfunctions locates seven hydrophobic transmembrane segments and suggests possible extracellular loop locations for the peptide-receptor mode-matched, modulatory hydrophobic aggregation sites.

Fusion-related release of glutamate from astrocytes

Although cell culture studies have implicated the presence of vesicle proteins in mediating the release of glutamate from astrocytes, definitive proof requires the identification of the glutamate release mechanism and the localization of this mechanism in astrocytes at synaptic locales. In cultured murine astrocytes we show an array of vesicle proteins, including SNARE proteins, and vesicular glutamate transporters that are required to fill vesicles with glutamate. Using immunocytochemistry and single-cell multiplex reverse transcription-PCR we demonstrate the presence of these proteins and their transcripts within astrocytes freshly isolated from the hippocampus. Moreover, immunoelectron microscopy demonstrates the presence of VGLUT1 in processes of astrocytes of the hippocampus. To determine whether calcium-dependent glutamate release is mediated by exocytosis, we expressed the SNARE motif of synaptobrevin II to prevent the formation of SNARE complexes, which reduces glutamate release from astrocytes. To further determine whether vesicular exocytosis mediates calcium-dependent glutamate release from astrocytes, we performed whole cell capacitance measurements from individual astrocytes and demonstrate an increase in whole cell capacitance, coincident with glutamate release. Together, these data allow us to conclude that astrocytes in situ express vesicle proteins necessary for filling vesicles with the chemical transmitter glutamate and that astrocytes release glutamate through a vesicle- or fusion-related mechanism.