Substrates and inhibitors display different sensitivity to expression level of the dopamine transporter in heterologously expressing cells

Hypocretin / Orexin Receptor 1 Knockdown in GABA or Dopamine Neurons in the Ventral Tegmental Area Differentially Impact Mesolimbic Dopamine and Motivation for Cocaine

The hypocretins/orexins (HCRT) have been demonstrated to influence motivation for cocaine through actions on dopamine (DA) transmission. Pharmacological or genetic disruption of the hypocretin receptor 1 (Hcrtr1) reduces cocaine self-administration, blocks reinstatement of cocaine seeking, and decreases conditioned place preference for cocaine. These effects are likely mediated through actions in the ventral tegmental area (VTA) and resulting alterations in DA transmission. For example, HCRT drives VTA DA neuron activity and enhances the effects of cocaine on DA transmission, while disrupting Hcrtr1 attenuates DA responses to cocaine. These findings have led to the perspective that HCRT exerts its effects through Hcrtr1 actions in VTA DA neurons. However, this assumption is complicated by the observation that Hcrtr1 are present on both DA and GABA neurons in the VTA and HCRT drives the activity of both neuronal populations. To address this issue, we selectively knocked down Hcrtr1 on either DA or GABA neurons in the VTA and examined alterations in DA transmission and cocaine self-administration in female and male rats. We found that Hcrtr1 knockdown in DA neurons decreased DA responses to cocaine, increased days to acquire cocaine self-administration, and reduced motivation for cocaine. Although, Hcrtr1 knockdown in GABA neurons enhanced DA responses to cocaine, this manipulation did not affect cocaine self-administration. These observations indicate that while Hcrtr1 on DA versus GABA neurons exert opposing effects on DA transmission, only Hcrtr1 on DA neurons affected acquisition or motivation for cocaine - suggesting a complex interplay between DA transmission and behavior.

Dopamine Transporter, PhosphoSerine129 α-Synuclein and α-Synuclein Levels in Aged LRRK2 G2019S Knock-In and Knock-Out Mice

The G2019S mutation in leucine rich-repeat kinase 2 (LRRK2) is a major cause of familial Parkinson’s disease. We previously reported that G2019S knock-in mice manifest dopamine transporter dysfunction and phosphoSerine129 α-synuclein (pSer129 α-syn) immunoreactivity elevation at 12 months of age, which might represent pathological events leading to neuronal degeneration. Here, the time-dependence of these changes was monitored in the striatum of 6, 9, 12, 18 and 23-month-old G2019S KI mice and wild-type controls using DA uptake assay, Western analysis and immunohistochemistry. Western analysis showed elevation of membrane dopamine transporter (DAT) levels at 9 and 12 months of age, along with a reduction of vesicular monoamine transporter 2 (VMAT2) levels at 12 months. DAT uptake was abnormally elevated from 9 to up to 18 months. DAT and VMAT2 level changes were specific to the G2019S mutation since they were not observed in LRRK2 kinase-dead or knock-out mice. Nonetheless, dysfunctional DAT uptake was not normalized by acute pharmacological inhibition of LRRK2 kinase activity with MLi-2. Immunoblot analysis showed elevation of pSer129 α-syn levels in the striatum of 12-month-old G2019S KI mice, which, however, was not confirmed by immunohistochemical analysis. Instead, total α-syn immunoreactivity was found elevated in the striatum of 23-month-old LRRK2 knock-out mice. These data indicate mild changes in DA transporters and α-syn metabolism in the striatum of 12-month-old G2019S KI mice whose pathological relevance remains to be established.

Chemogenetic Manipulation of Dopamine Neurons Dictates Cocaine Potency at Distal Dopamine Transporters

The reinforcing efficacy of cocaine is largely determined by its capacity to inhibit the dopamine transporter (DAT), and emerging evidence suggests that differences in cocaine potency are linked to several symptoms of cocaine use disorder. Despite this evidence, the neural processes that govern cocaine potency in vivo remain unclear. In male rats, we used chemogenetics with intra-VTA microinfusions of the agonist clozapine-n-oxide to bidirectionally modulate dopamine neurons. Using ex vivo fast scan cyclic voltammetry, pharmacological probes of the DAT, biochemical assessments of DAT membrane availability and phosphorylation, and cocaine self-administration, we tested the effects of chemogenetic manipulations on cocaine potency at distal DATs in the nucleus accumbens as well as the behavioral economics of cocaine self-administration. We discovered that chemogenetic manipulation of dopamine neurons produced rapid, bidirectional modulation of cocaine potency at DATs in the nucleus accumbens. We then provided evidence that changes in cocaine potency are associated with alterations in DAT affinity for cocaine and demonstrated that this change in affinity coincides with DAT conformation biases and changes in DAT phosphorylation state. Finally, we showed that chemogenetic manipulation of dopamine neurons alters cocaine consumption in a manner consistent with changes in cocaine potency at distal DATs. Based on the spatial and temporal constraints inherent to our experimental design, we posit that changes in cocaine potency are driven by alterations in dopamine neuron activity. When considered together, these observations provide a novel mechanism through which GPCRs regulate cocaine's pharmacological and behavioral effects.SIGNIFICANCE STATEMENT Differences in the pharmacological effects of cocaine are believed to influence the development and progression of cocaine use disorder. However, the biological and physiological processes that determine sensitivity to cocaine remain unclear. In this work, we use a combination of chemogenetics, fast scan cyclic voltammetry, pharmacology, biochemistry, and cocaine self-administration with economic demand analysis to demonstrate a novel mechanism by which cocaine potency is determined in vivo These studies identify a novel process by which the pharmacodynamics of cocaine are derived in vivo, and thus this work has widespread implications for understanding the mechanisms that regulate cocaine consumption across stages of addiction.

Role of RGS12 in the differential regulation of kappa opioid receptor-dependent signaling and behavior

Kappa opioid receptor (KOR) agonists show promise in ameliorating disorders, such as addiction and chronic pain, but are limited by dysphoric and aversive side effects. Clinically beneficial effects of KOR agonists (e.g., analgesia) are predominantly mediated by heterotrimeric G protein signaling, whereas β-arrestin signaling is considered central to their detrimental side effects (e.g., dysphoria/aversion). Here we show that Regulator of G protein Signaling-12 (RGS12), via independent signaling mechanisms, simultaneously attenuates G protein signaling and augments β-arrestin signaling downstream of KOR, exhibiting considerable selectivity in its actions for KOR over other opioid receptors. We previously reported that RGS12-null mice exhibit increased dopamine transporter-mediated dopamine (DA) uptake in the ventral (vSTR), but not dorsal striatum (dSTR), as well as reduced psychostimulant-induced hyperlocomotion; in the current study, we found that these phenotypes are reversed following KOR antagonism. Fast-scan cyclic voltammetry studies of dopamine (DA) release and reuptake suggest that striatal disruptions to KOR-dependent DAergic neurotransmission in RGS12-null mice are restricted to the nucleus accumbens. In both ventral striatal tissue and transfected cells, RGS12 and KOR are seen to interact within a protein complex. Ventral striatal-specific increases in KOR levels and KOR-induced G protein activation are seen in RGS12-null mice, as well as enhanced sensitivity to KOR agonist-induced hypolocomotion and analgesia-G protein signaling-dependent behaviors; a ventral striatal-specific increase in KOR levels was also observed in β-arrestin-2-deficient mice, highlighting the importance of β-arrestin signaling to establishing steady-state KOR levels in this particular brain region. Conversely, RGS12-null mice exhibited attenuated KOR-induced conditioned place aversion (considered a β-arrestin signaling-dependent behavior), consistent with the augmented KOR-mediated β-arrestin signaling seen upon RGS12 over-expression. Collectively, our findings highlight a role for RGS12 as a novel, differential regulator of both G protein-dependent and -independent signaling downstream of KOR activation.

Hypocretin receptor 1 involvement in cocaine-associated behavior: Therapeutic potential and novel mechanistic insights

Since its discovery in 1998, the hypocretin/orexin system has been identified as a critical modulator of behavior. Through interactions with dopamine neurons of the ventral tegmental area, this system is poised to regulate motivation for drug rewards by impacting dopamine neurotransmission in target structures including the nucleus accumbens. Across numerous experiments, we and others have identified a critical influence of hypocretin receptor 1 in mediating the behavioral and physiological effects of cocaine which positions this receptor as a potential target for the treatment of cocaine addiction. Here we discuss evidence for hypocretin receptor 1 involvement in driving cocaine-associated behavior and how hypocretin receptor 1 in the ventral tegmental area are critical for supporting dopamine neuron activity and dopamine neurotransmission. We then present new data supporting the novel hypothesis that in addition to exerting acute actions on dopamine systems, pharmacological hypocretin manipulations also produce lasting adaptations to dopamine terminals that impact sensitivity to cocaine, and ultimately, future behavior.

Amphetamine Reverses Escalated Cocaine Intake via Restoration of Dopamine Transporter Conformation

Cocaine abuse disrupts dopamine system function, and reduces cocaine inhibition of the dopamine transporter (DAT), which results in tolerance. Although tolerance is a hallmark of cocaine addiction and a DSM-V criterion for substance abuse disorders, the molecular adaptations producing tolerance are unknown, and testing the impact of DAT changes on drug taking behaviors has proven difficult. In regard to treatment, amphetamine has shown efficacy in reducing cocaine intake; however, the mechanisms underlying these effects have not been explored. The goals of this study were twofold; we sought to (1) identify the molecular mechanisms by which cocaine exposure produces tolerance and (2) determine whether amphetamine-induced reductions in cocaine intake are connected to these mechanisms. Using cocaine self-administration and fast-scan cyclic voltammetry in male rats, we show that low-dose, continuous amphetamine treatment, during self-administration or abstinence, completely reversed cocaine tolerance. Amphetamine treatment also reversed escalated cocaine intake and decreased motivation to obtain cocaine as measured in a behavioral economics task, thereby linking tolerance to multiple facets of cocaine use. Finally, using fluorescence resonance energy transfer imaging, we found that cocaine tolerance is associated with the formation of DAT-DAT complexes, and that amphetamine disperses these complexes. In addition to extending our basic understanding of DATs and their role in cocaine reinforcement, we serendipitously identified a novel therapeutic target: DAT oligomer complexes. We show that dispersion of oligomers is concomitant with reduced cocaine intake, and propose that pharmacotherapeutics aimed at these complexes may have potential for cocaine addiction treatment.SIGNIFICANCE STATEMENT Tolerance to cocaine's subjective effects is a cardinal symptom of cocaine addiction and a DSM-V criterion for substance abuse disorders. However, elucidating the molecular adaptions that produce tolerance and determining its behavioral impact have proven difficult. Using cocaine self-administration in rats, we link tolerance to cocaine effects at the dopamine transporter (DAT) with aberrant cocaine-taking behaviors. Further, tolerance was associated with multi-DAT complexes, which formed after cocaine exposure. Treatment with amphetamine deconstructed DAT complexes, reversed tolerance, and decreased cocaine seeking. These data describe the behavioral consequence of cocaine tolerance, provide a putative mechanism for its development, and suggest that compounds that disperse DAT complexes may be efficacious treatments for cocaine addiction.

The correlation between DNA methylation and transcriptional expression of human dopamine transporter in cell lines

This study aims to investigate the relationship between DNA methylation and expression of human dopamine transporter (hDAT). We examined methylation status of hDAT in cells with various hDAT expression levels, including two dopaminergic neural cell lines (SK-N-AS and SH-SY-5Y) and one non-dopaminergic cell line (HEK293) by bisulfite sequencing PCR(BSP). The effects of DNA methyltransferase inhibitor 5-aza-dC or/and histone deacetylase inhibitor (HDACi, sodium butyrate, NaB) on the DNA methylation status and mRNA expression levels of hDAT were examined. The results revealed marked hypomethylation of the two promoter regions (-1214 to -856bp and -48 to 439bp, the first base of exon 1 was taken as +1 bp)of hDAT in SK-N-AS (4.7%±2.0^mC and 3.5%±1.0^mC, respectively) compared with SH-SY-5Y (88.0%±4.4%^mC and 81.1%±8.8%^mC) and HEK293 (90.7%±2.4^mC and 84.4%±8.6% ^mC) cell lines, indicating a cell-specific methylation regulation of hDAT. 5-aza-dC and NaB decreased hypermethylation,while increase hDAT expression in SH-SY-5Y cells and recovered hDAT mRNA expression in HEK293 cells. DNA methylation enabled the cell-specific differential expression of the hDAT gene. hDAT silencing was reversed by the introduction of DNA hypomethylation via 5-aza-dC or/and NaB.

Hypocretin receptor 1 blockade produces bimodal modulation of cocaine-associated mesolimbic dopamine signaling

RATIONALE

Cocaine addiction is a chronic psychiatric disorder characterized by pathological motivation to obtain cocaine and behavioral and neurochemical hypersensitivity to cocaine-associated cues. These features of cocaine addiction are thought to be driven by aberrant phasic dopamine signaling. We previously demonstrated that blockade of the hypocretin receptor 1 (HCRTr1) attenuates cocaine self-administration and reduces cocaine-induced enhancement of dopamine signaling. Despite this evidence, the effects of HCRTr1 blockade on endogenous phasic dopamine release are unknown.

OBJECTIVE

In the current studies, we assessed whether blockade of HCRTr1 alters spontaneous and cue-evoked dopamine release in the nucleus accumbens core of freely moving rats.

METHODS

We first validated the behavioral and neurochemical effects of the novel, highly selective, HCRTr1 antagonist RTIOX-276 using cocaine self-administration and fast-scan cyclic voltammetry (FSCV) in anesthetized rats. We then used FSCV in freely moving rats to examine whether RTIOX-276 impacts spontaneous and cue-evoked dopamine release. Finally, we used ex vivo slice FSCV to determine whether the effects of RTIOX-276 on dopamine signaling involve dopamine terminal adaptations.

RESULTS

Doses of RTIOX-276 that attenuate the motivation for cocaine reduce spontaneous dopamine transient amplitude and cue-evoked dopamine release. Further, these doses attenuated cocaine-induced dopamine uptake inhibition at the level of dopamine terminals.

CONCLUSION

Our results provide support for the standing hypothesis that HCRTr1 blockade suppresses endogenous phasic dopamine signals, likely via actions at dopamine cell bodies. These results also elucidate a second process through which HCRTr1 blockade attenuates the effects of cocaine by reducing cocaine sensitivity at dopamine terminals.

Susceptibility to traumatic stress sensitizes the dopaminergic response to cocaine and increases motivation for cocaine

Patients with post-traumatic stress disorder have a heightened vulnerability to developing substance use disorders; however, the biological underpinnings of this vulnerability remain unresolved. We used the predator odor stress model of post-traumatic stress disorder with segregation of subjects as susceptible or resilient based on elevated plus maze behavior and context avoidance. We then determined behavioral and neurochemical differences across susceptible, resilient, and control populations using a panel of behavioral and neurochemical assays. Susceptible subjects showed a significant increase in the motoric and dopaminergic effects of cocaine, and this corresponded with heightened motivation to self-administer cocaine. Resilient subjects did not show differences in the motoric effects of cocaine, in dopamine signaling in vivo, or in any measure of cocaine self-administration. Nonetheless, we found that these animals displayed elevations in both the dopamine release-promoting effects of cocaine and dopamine autoreceptor sensitivity ex vivo. Our results suggest that the experience of traumatic stress may produce alterations in dopamine systems that drive elevations in cocaine self-administration behavior in susceptible subjects, but may also produce both active and passive forms of resilience that function to prevent gross changes in cocaine's reinforcing efficacy in resilient subjects.

Potential Drug Interactions Mediated by Renal Organic Anion Transporter OATP4C1

Organic anion-transporting polypeptide 4C1 (OATP4C1) is an organic anion transporter expressed in the basolateral membrane of the renal proximal tubules. It plays a major role in the urinary excretion of both exogenous drugs and endogenous compounds. Our previous studies have indicated the importance of OATP4C1 in pathologic and physiologic conditions; however, the majority of its pharmacologic characteristics remained unclear. Therefore, to provide essential information for clinical drug therapy decisions and drug development, we clarified drug interactions mediated by OATP4C1. To elucidate potential drug interactions via OATP4C1, we screened 53 representative drugs commonly used in clinical settings. Next, we evaluated the IC50 values of drugs that inhibited OATP4C1 by more than 50%. To apply our results to clinical settings, we calculated the drug-drug interaction (DDI) indices. The screening analysis using an OATP4C1-expressing cell system demonstrated that 22 out of 53 therapeutic drugs inhibited OATP4C1-mediated triiodothyronine transport. In particular, OATP4C1-mediated transport was strongly inhibited by 10 drugs. The IC50 values of 10 drugs-nicardipine, spironolactone, fluvastatin, crizotinib, levofloxacin, clarithromycin, ritonavir, saquinavir, quinidine, and verapamil-obtained in this study were 51, 53, 41, 24, 420, 200, 8.5, 4.3, 100, and 110 µM, respectively. The IC50 values of these drugs were higher than the plasma concentrations obtained in clinical practice. However, ritonavir showed the highest DDI index (1.9) for OATP4C1, suggesting that it may strongly influence this transporter and thus cause drug interactions seen in clinical settings. Our finding gives new insight into the role of OATP4C1 in clinical DDIs.

Cocaine Potency at the Dopamine Transporter Tracks Discrete Motivational States During Cocaine Self-Administration

Although the dopamine transporter (DAT) is the primary site of action for cocaine, and the dopamine system is known to mediate the reinforcing effects of cocaine, the dopaminergic variations underlying individual differences in cocaine self-administration behaviors are not fully understood. Recent advances in the application of economic principles to operant tasks in rodents have allowed for the within-subject, within-session determination of both consummatory and appetitive responding for reinforcers. Here we combined a behavioral economics approach with cocaine self-administration and ex vivo voltammetric recording of dopamine signaling in the core of the nucleus accumbens of rats to determine the relationship between dopamine signaling and discrete aspects of cocaine taking and seeking. We found neither dopamine release or uptake tracked individual differences in cocaine consumption or the reinforcing efficacy of cocaine. Cocaine potency at the DAT was correlated with reinforcing efficacy, but was not related to cocaine consumption. Further, we introduce a novel analysis that determines perseverative responding within the same procedure, and find that cocaine potency at the DAT also tracks differences in perseverative responding. Together, we demonstrate that cocaine effects at the DAT determine the reinforcing efficacy of cocaine, and perseverative responding for sub-threshold doses of cocaine that do not maintain responding when presented in isolation. Surprisingly, we find that variations in cocaine potency do not account for differences in cocaine consumption, suggesting that satiation for cocaine is determined by other targets or mechanisms. Finally, we outline a novel approach for relating drug-target interactions and potency to discrete motivational states during a single self-administration session.

Age-dependent dopamine transporter dysfunction and Serine129 phospho-α-synuclein overload in G2019S LRRK2 mice

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common genetic cause of Parkinson’s disease. Here, we investigated whether the G2019S LRRK2 mutation causes morphological and/or functional changes at nigro-striatal dopamine neurons. Density of striatal dopaminergic terminals, nigral cell counts, tyrosine hydroxylase protein levels as well as exocytotic dopamine release measured in striatal synaptosomes, or striatal extracellular dopamine levels monitored by in vivo microdialysis were similar between ≥12-month-old G2019S knock-in mice and wild-type controls. In vivo striatal dopamine release was insensitive to the LRRK2 inhibitor Nov-LRRK2-11, and was elevated by the membrane dopamine transporter blocker GBR-12783. However, G2019S knock-in mice showed a blunted neurochemical and motor activation response to GBR-12783 compared to wild-type controls. Western blot and dopamine uptake analysis revealed an increase in dopamine transporter levels and activity in the striatum of 12-month-old G2019S KI mice. This phenotype correlated with a reduction in vesicular monoamine transporter 2 levels and an enhancement of vesicular dopamine uptake, which was consistent with greater resistance to reserpine-induced hypolocomotion. These changes were not observed in 3-month-old mice. Finally, Western blot analysis revealed no genotype difference in striatal levels of endogenous α-synuclein or α-synuclein bound to DOPAL (a toxic metabolite of dopamine). However, Serine129-phosphorylated α-synuclein levels were higher in 12-month-old G2019S knock-in mice. Immunohistochemistry confirmed this finding, also showing no genotype difference in 3-month-old mice. We conclude that the G2019S mutation causes progressive dysfunctions of dopamine transporters, along with Serine129-phosphorylated α-synuclein overload, at striatal dopaminergic terminals, which are not associated with dopamine homeostasis dysregulation or neuron loss but might contribute to intrinsic dopaminergic terminal vulnerability. We propose G2019S knock-in mice as a presymptomatic Parkinson’s disease model, useful to investigate the pathogenic interaction among genetics, aging, and internal or environmental factors leading to the disease.

Impact of disruption of secondary binding site S2 on dopamine transporter function

The structures of the leucine transporter, drosophila dopamine transporter, and human serotonin transporter show a secondary binding site (designated S2 ) for drugs and substrate in the extracellular vestibule toward the membrane exterior in relation to the primary substrate recognition site (S1 ). The present experiments are aimed at disrupting S2 by mutating Asp476 and Ile159 to Ala. Both mutants displayed a profound decrease in [(3) H]DA uptake compared with wild-type associated with a reduced turnover rate kcat . This was not caused by a conformational bias as the mutants responded to Zn(2+) (10 μM) similarly as WT. The dopamine transporters with either the D476A or I159A mutation both displayed a higher Ki for dopamine for the inhibition of [3H](-)-2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane binding than did the WT transporter, in accordance with an allosteric interaction between the S1 and S2 sites. The results provide evidence in favor of a general applicability of the two-site allosteric model of the Javitch/Weinstein group from LeuT to dopamine transporter and possibly other monoamine transporters. X-ray structures of transporters closely related to the dopamine (DA) transporter show a secondary binding site S2 in the extracellular vestibule proximal to the primary binding site S1 which is closely linked to one of the Na(+) binding sites. This work examines the relationship between S2 and S1 sites. We found that S2 site impairment severely reduced DA transport and allosterically reduced S1 site affinity for the cocaine analog [(3) H]CFT. Our results are the first to lend direct support for the application of the two-site allosteric model, advanced for bacterial LeuT, to the human DA transporter. The model states that, after binding of the first DA molecule (DA1 ) to the primary S1 site (along with Na(+) ), binding of a second DA (DA2 ) to the S2 site triggers, through an allosteric interaction, the release of DA1 and Na(+) into the cytoplasm.

Glucagon-like peptide 1 receptor activation regulates cocaine actions and dopamine homeostasis in the lateral septum by decreasing arachidonic acid levels

Agonism of the glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) has been effective at treating aspects of addictive behavior for a number of abused substances, including cocaine. However, the molecular mechanisms and brain circuits underlying the therapeutic effects of GLP-1R signaling on cocaine actions remain elusive. Recent evidence has revealed that endogenous signaling at the GLP-1R within the forebrain lateral septum (LS) acts to reduce cocaine-induced locomotion and cocaine conditioned place preference, both considered dopamine (DA)-associated behaviors. DA terminals project from the ventral tegmental area to the LS and express the DA transporter (DAT). Cocaine acts by altering DA bioavailability by targeting the DAT. Therefore, GLP-1R signaling might exert effects on DAT to account for its regulation of cocaine-induced behaviors. We show that the GLP-1R is highly expressed within the LS. GLP-1, in LS slices, significantly enhances DAT surface expression and DAT function. Exenatide (Ex-4), a long-lasting synthetic analog of GLP-1 abolished cocaine-induced elevation of DA. Interestingly, acute administration of Ex-4 reduces septal expression of the retrograde messenger 2-arachidonylglycerol (2-AG), as well as a product of its presynaptic degradation, arachidonic acid (AA). Notably, AA reduces septal DAT function pointing to AA as a novel regulator of central DA homeostasis. We further show that AA oxidation product γ-ketoaldehyde (γ-KA) forms adducts with the DAT and reduces DAT plasma membrane expression and function. These results support a mechanism in which postsynaptic septal GLP-1R activation regulates 2-AG levels to alter presynaptic DA homeostasis and cocaine actions through AA.

Classic Studies on the Interaction of Cocaine and the Dopamine Transporter

The dopamine transporter is responsible for recycling dopamine after release. Inhibitors of the dopamine transporter, such as cocaine, will stop the reuptake of dopamine and allow it to stay extracellularly, causing prominent changes at the molecular, cellular, and behavioral levels. There is much left to be known about the mechanism and site(s) of binding, as well as the effect that cocaine administration does to dopamine transporter-cocaine binding sites and gene expression which also plays a strong role in cocaine abusers and their behavioral characteristics. Thus, if more light is shed on the dopamine transporter-cocaine interaction, treatments for addiction and even other diseases of the dopaminergic system may not be too far ahead. As today's ongoing research expands on the shoulders of classic research done in the 1990s and 2000s, the foundation of core research done in that time period will be reviewed, which forms the basis of today's work and tomorrow's therapies.

A Single Amphetamine Infusion Reverses Deficits in Dopamine Nerve-Terminal Function Caused by a History of Cocaine Self-Administration

There are ∼ 1.6 million people who meet the criteria for cocaine addiction in the United States, and there are currently no FDA-approved pharmacotherapies. Amphetamine-based dopamine-releasing drugs have shown efficacy in reducing the motivation to self-administer cocaine and reducing intake in animals and humans. It is hypothesized that amphetamine acts as a replacement therapy for cocaine through elevation of extracellular dopamine levels. Using voltammetry in brain slices, we tested the ability of a single amphetamine infusion in vivo to modulate dopamine release, uptake kinetics, and cocaine potency in cocaine-naive animals and after a history of cocaine self-administration (1.5 mg/kg/infusion, fixed-ratio 1, 40 injections/day × 5 days). Dopamine kinetics were measured 1 and 24 h after amphetamine infusion (0.56 mg/kg, i.v.). Following cocaine self-administration, dopamine release, maximal rate of uptake (Vmax), and membrane-associated dopamine transporter (DAT) levels were reduced, and the DAT was less sensitive to cocaine. A single amphetamine infusion reduced Vmax and membrane DAT levels in cocaine-naive animals, but fully restored all aspects of dopamine terminal function in cocaine self-administering animals. Here, for the first time, we demonstrate pharmacologically induced, immediate rescue of deficits in dopamine nerve-terminal function in animals with a history of high-dose cocaine self-administration. This observation supports the notion that the DAT expression and function can be modulated on a rapid timescale and also suggests that the pharmacotherapeutic actions of amphetamine for cocaine addiction go beyond that of replacement therapy.

Dopamine transporter oligomerization: impact of combining protomers with differential cocaine analog binding affinities

Previous studies point to quaternary assembly of dopamine transporters (DATs) in oligomers. However, it is not clear whether the protomers function independently in the oligomer. Is each protomer an entirely separate unit that takes up dopamine and is inhibited by drugs known to block DAT function? In this work, human embryonic kidney 293 cells were co-transfected with DAT constructs possessing differential binding affinities for the phenyltropane cocaine analog, [³H]WIN35,428. It was assessed whether the binding properties in co-expressing cells capable of forming hetero-oligomers differ from those in preparations obtained from mixed singly transfected cells where such oligomers cannot occur. A method is described that replaces laborious 'mixing' experiments with an in silico method predicting binding parameters from those observed for the singly expressed constructs. Among five pairs of constructs tested, statistically significant interactions were found between protomers of wild-type (WT) and D313N, WT and D345N, and WT and D436N. Compared with predicted Kd values of [³H]WIN35,428 binding to the non-interacting pairs, the observed affinity of the former pair was increased 1.7 fold while the latter two were reduced 2.2 and 4.1 fold, respectively. This is the first report of an influence of protomer composition on the properties of a DAT inhibitor, indicating cooperativity within the oligomer. The dopamine transporter (DAT) can exist as an oligomer but it is unknown whether the protomers function independently. The present results indicate that protomers that are superpotent or deficient in cocaine analog binding can confer enhanced or reduced potency to the oligomer, respectively. In this respect, positive or negative cooperativity is revealed in the DAT oligomer.

Differential influence of dopamine transport rate on the potencies of cocaine, amphetamine, and methylphenidate

Dopamine transporter (DAT) levels vary across brain regions and individuals, and are altered by drug history and disease states; however, the impact of altered DAT expression on psychostimulant effects in brain has not been systematically explored. Using fast scan cyclic voltammetry, we measured the effects of elevated DAT levels on presynaptic dopamine parameters as well as the uptake inhibition potency of the blockers cocaine and methylphenidate (MPH) and the releaser amphetamine (AMPH) in the nucleus accumbens core. Here we found that increases in DAT levels, resulting from either genetic overexpression or MPH self-administration, caused markedly increased maximal rates of uptake (V_max) that were positively correlated with the uptake inhibition potency of AMPH and MPH, but not cocaine. AMPH and MPH were particularly sensitive to DAT changes, with a 100% increase in V_max resulting in a 200% increase in potency. The relationship between V_max and MPH potency was the same as that for AMPH, but was different from that for cocaine, indicating that MPH more closely resembles a releaser with regard to uptake inhibition. Conversely, the effects of MPH on stimulated dopamine release were similar to those of cocaine, with inverted U-shaped increases in release over a concentration–response curve. This was strikingly different from the release profile of AMPH, which showed only reductions at high concentrations, indicating that MPH is not a pure releaser. These data indicate that although MPH is a DAT blocker, its uptake-inhibitory actions are affected by DAT changes in a similar manner to releasers. Together, these data show that fluctuations in DAT levels alter the potency of releasers and MPH but not blockers and suggest an integral role of the DAT in the addictive potential of AMPH and related compounds.

Intermittent cocaine self-administration produces sensitization of stimulant effects at the dopamine transporter

Previous literature investigating neurobiological adaptations following cocaine self-administration has shown that high, continuous levels of cocaine intake (long access; LgA) results in reduced potency of cocaine at the dopamine transporter (DAT), whereas an intermittent pattern of cocaine administration (intermittent access; IntA) results in sensitization of cocaine potency at the DAT. Here, we aimed to determine whether these changes are specific to cocaine or translate to other psychostimulants. Psychostimulant potency was assessed by fast-scan cyclic voltammetry in brain slices containing the nucleus accumbens following IntA, short access, and LgA cocaine self-administration, as well as in brain slices from naive animals. We assessed the potency of amphetamine (a releaser), and methylphenidate (a DAT blocker, MPH). MPH was selected because it is functionally similar to cocaine and structurally related to amphetamine. We found that MPH and amphetamine potencies were increased following IntA, whereas neither was changed following LgA or short access cocaine self-administration. Therefore, whereas LgA-induced tolerance at the DAT is specific to cocaine as shown in previous work, the sensitizing effects of IntA apply to cocaine, MPH, and amphetamine. This demonstrates that the pattern with which cocaine is administered is important in determining the neurochemical consequences of not only cocaine effects but potential cross-sensitization/cross-tolerance effects of other psychostimulants as well.

Neuroprotective effects of metabotropic glutamate receptor group II and III activators against MPP(+)-induced cell death in human neuroblastoma SH-SY5Y cells: the impact of cell differentiation state

Recent studies have documented that metabotropic glutamate receptors from group II and III (mGluR II/III) are a potential target in the symptomatic treatment of Parkinson's disease (PD), however, the neuroprotective effects of particular mGluR II/III subtypes in relation to PD pathology are recognized only partially. In the present study, we investigated the effect of various mGluR II/III activators in the in vitro model of PD using human neuroblastoma SH-SY5Y cell line and mitochondrial neurotoxin MPP(+). We demonstrated that all tested mGluR ligands: mGluR II agonist - LY354740, mGluR III agonist - ACPT-I, mGluR4 PAM - VU0361737, mGluR8 agonist - (S)-3,4-DCPG, mGluR8 PAM - AZ12216052 and mGluR7 allosteric agonist - AMN082 were protective against MPP(+)-evoked cell damage in undifferentiated (UN-) SH-SY5Y cells with the highest neuroprotection mediated by mGluR8-specific agents. However, in retinoic acid- differentiated (RA-) SH-SY5Y cells we found protection mediated only by mGluR8 activators. We also demonstrated the cell proliferation stimulating effect for mGluR4 and mGluR8 PAMs. Next, we showed that the protection mediated by mGluR II/III activators in UN-SH-SY5Y was not accompanied by the modulation of caspase-3 activity, however, a decrease in the number of apoptotic nuclei was found. Finally, we showed that the inhibitor of necroptosis, necrostatin-1 blocked the mGluR III-mediated protection. Altogether our comparative in vitro data add a further proof to neuroprotective effects of mGluR agonists or PAMs and point to mGluR8 as a promising target for neuroprotective interventions in PD. The results also suggest the participation of necroptosis-related molecular pathways in neuroprotective effects of mGluR III activation.

Rats classified as low or high cocaine locomotor responders: a unique model involving striatal dopamine transporters that predicts cocaine addiction-like behaviors

Individual differences are a hallmark of drug addiction. Here, we describe a rat model based on differential initial responsiveness to low dose cocaine. Despite similar brain cocaine levels, individual outbred Sprague-Dawley rats exhibit markedly different magnitudes of acute cocaine-induced locomotor activity and, thereby, can be classified as low or high cocaine responders (LCRs or HCRs). LCRs and HCRs differ in drug-induced, but not novelty-associated, hyperactivity. LCRs have higher basal numbers of striatal dopamine transporters (DATs) than HCRs and exhibit marginal cocaine inhibition of in vivo DAT activity and cocaine-induced increases in extracellular DA. Importantly, lower initial cocaine response predicts greater locomotor sensitization, conditioned place preference and greater motivation to self-administer cocaine following low dose acquisition. Further, outbred Long-Evans rats classified as LCRs, versus HCRs, are more sensitive to cocaine's discriminative stimulus effects. Overall, results to date with the LCR/HCR model underscore the contribution of striatal DATs to individual differences in initial cocaine responsiveness and the value of assessing the influence of initial drug response on subsequent expression of addiction-like behaviors.

Mice expressing markedly reduced striatal dopamine transporters exhibit increased locomotor activity, dopamine uptake turnover rate, and cocaine responsiveness

Variations in the expression levels of the dopamine transporter (DAT) can influence responsiveness to psychostimulant drugs like cocaine. To better understand this relationship, we studied a new DAT-low expresser (DAT-LE) mouse model and performed behavioral and biochemical studies with it. Immunoblotting and [(3) H]WIN 35,428 binding analyses revealed that these mice express ∼35% of wildtype (WT) mouse striatal DAT levels. Compared to WT mice, DAT-LE mice were hyperactive in a novel open-field environment. Despite their higher basal locomotor activity, cocaine (10 or 20 mg/kg, i.p.) induced greater locomotor activation in DAT-LE mice than in WT mice. The maximal velocity (Vmax ) of DAT-mediated [(3) H]DA uptake into striatal synaptosomes was reduced by 46% in DAT-LE mice, as compared to WT. Overall, considering the reduced number of DAT binding sites (Bmax ) along with the reduced Vmax in DAT-LE mice, a 2-fold increase in DA uptake turnover rate (Vmax /Bmax ) was found, relative to WT mice. This suggests that neuroadaptive changes have occurred in the DAT-LE mice that would help to compensate for their low DAT numbers. Interestingly, these changes do not include a reduction in tyrosine hydroxylase levels, as was previously reported in DAT knockout homozygous and heterozygous animals. Further, these changes are not sufficient to prevent elevated novelty- and cocaine-induced locomotor activity. Hence, these mice represent a unique model for studying changes of in vivo DAT function and regulation that result from markedly reduced levels of DAT expression.

Enduring increases in anxiety-like behavior and rapid nucleus accumbens dopamine signaling in socially isolated rats

Social isolation (SI) rearing, a model of early life stress, results in profound behavioral alterations, including increased anxiety-like behavior, impaired sensorimotor gating and increased self-administration of addictive substances. These changes are accompanied by alterations in mesolimbic dopamine function, such as increased dopamine and metabolite tissue content, increased dopamine responses to cues and psychostimulants, and increased dopamine neuron burst firing. Using voltammetric techniques, we examined the effects of SI rearing on dopamine transporter activity, vesicular release and dopamine D2-type autoreceptor activity in the nucleus accumbens core. Long-Evans rats were housed in group (GH; 4/cage) or SI (1/cage) conditions from weaning into early adulthood [postnatal day (PD) 28-77]. After this initial housing period, rats were assessed on the elevated plus-maze for an anxiety-like phenotype, and then slice voltammetry experiments were performed. To study the enduring effects of SI rearing on anxiety-like behavior and dopamine terminal function, another cohort of similarly reared rats was isolated for an additional 4 months (until PD 174) and then tested. Our findings demonstrate that SI rearing results in lasting increases in anxiety-like behavior, dopamine release and dopamine transporter activity, but not D2 activity. Interestingly, GH-reared rats that were isolated as adults did not develop the anxiety-like behavior or dopamine changes seen in SI-reared rats. Together, our data suggest that early life stress results in an anxiety-like phenotype, with lasting increases in dopamine terminal function.

Methylphenidate amplifies the potency and reinforcing effects of amphetamines by increasing dopamine transporter expression

Methylphenidate (MPH) is commonly diverted for recreational use, but the neurobiological consequences of exposure to MPH at high, abused doses are not well defined. Here we show that MPH self-administration in rats increases dopamine transporter (DAT) levels and enhances the potency of MPH and amphetamine on dopamine responses and drug-seeking behaviours, without altering cocaine effects. Genetic overexpression of the DAT in mice mimics these effects, confirming that MPH self-administration-induced increases in DAT levels are sufficient to induce the changes. Further, this work outlines a basic mechanism by which increases in DAT levels, regardless of how they occur, are capable of increasing the rewarding and reinforcing effects of select psychostimulant drugs, and suggests that individuals with elevated DAT levels, such as ADHD sufferers, may be more susceptible to the addictive effects of amphetamine-like drugs.

Site-directed mutations near transmembrane domain 1 alter conformation and function of norepinephrine and dopamine transporters

The human dopamine and norepinephrine transporters (hDAT and hNET, respectively) control neurotransmitter levels within the synaptic cleft and are the site of action for amphetamine (AMPH) and cocaine. We investigated the role of a threonine residue within the highly conserved and putative phosphorylation sequence RETW, located just before transmembrane domain 1, in regulating hNET and hDAT function. The Thr residue was mutated to either alanine or aspartate. Similar to the inward facing T62D-hDAT, T58D-hNET demonstrated reduced [(3)H]DA uptake but enhanced basal DA efflux compared with hNET with no further effect of AMPH. The mutations had profound effects on substrate function and binding. The potency of substrates to inhibit [(3)H]DA uptake and compete with radioligand binding was increased in T→A and/or T→D mutants. Substrates, but not inhibitors, demonstrated temperature-sensitive effects of binding. Neither the functional nor the binding potency for hNET blockers was altered from wild type in hNET mutants. There was, however, a significant reduction in potency for cocaine and benztropine to inhibit [(3)H]DA uptake in T62D-hDAT compared with hDAT. The potency of these drugs to inhibit [(3)H](-)-2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane-1,5-napthalenedisulfonate (WIN35,428) binding was not increased, demonstrating a discordance between functional and binding site effects. Taken together, these results concur with the notion that the T→D mutation in RETW alters the preferred conformation of both hNET and hDAT to favor one that is more inward facing. Although substrate activity and binding are primarily altered in this conformation, the function of inhibitors with distinct structural characteristics may also be affected.

Cocaine-insensitive dopamine transporters with intact substrate transport produced by self-administration

BACKGROUND

Psychomotor stimulant drugs such as cocaine and amphetamine activate brain dopamine (DA) neurotransmission and support self-administration in humans and laboratory animals. Cocaine amplifies DA signaling by blocking the DA transporter (DAT), and this has been described as the most important mechanism underlying cocaine's reinforcing effects. Amphetamine has the added mechanism of reverse transport of intracellular DA through the DAT.

METHODS

We used cocaine and amphetamine self-administration under a fixed-ratio 1 schedule followed by microdialysis in freely moving rats to measure extracellular DA levels and fast scan cyclic voltammetry in brain slices to measure subsecond DA release and uptake parameters.

RESULTS

Following a high dose (1.5 mg/kg intravenous) cocaine self-administration paradigm (40 injections/day × 5 days), the DAT was markedly less sensitive to cocaine, as measured by microdialysis and voltammetry in the nucleus accumbens core. In contrast, the DAT substrate amphetamine retained the same efficacy at the DAT in cocaine self-administering animals, and amphetamine did not mimic cocaine's effect on the DAT when self-administered. A single session of cocaine self-administration caused a significant decrease in the ability of cocaine to inhibit the DAT, a finding that may provide a neurochemical basis for rapid tolerance. The effects of cocaine returned to normal within a few weeks following cessation of self-administration.

CONCLUSIONS

Here, we, for the first time, demonstrate an in vivo, pharmacologically induced alteration in the sensitivity of the DAT to cocaine that is specific to cocaine, spares DAT and substrate/releaser interactions, and is independent of maximal rate of DA uptake (V(max)).

Is slow-onset long-acting monoamine transport blockade to cocaine as methadone is to heroin? Implication for anti-addiction medications

The success of methadone in treating opiate addiction has suggested that long-acting agonist therapies may be similarly useful for treating cocaine addiction. Here, we examined this hypothesis, using the slow-onset long-acting monoamine reuptake inhibitor 31,345, a trans-aminotetralin analog, in a variety of addiction-related animal models, and compared it with methadone's effects on heroin's actions in the same animal models. Systemic administration of 31,345 produced long-lasting enhancement of electrical brain-stimulation reward (BSR) and extracellular nucleus accumbens (NAc) dopamine (DA). Pretreatment with 31,345 augmented cocaine-enhanced BSR, prolonged cocaine-enhanced NAc DA, and produced a long-term (24-48  h) reduction in cocaine self-administration rate without obvious extinction pattern, suggesting an additive effect of 31,345 with cocaine. In contrast, methadone pretreatment not only dose-dependently inhibited heroin self-administration with an extinction pattern but also dose-dependently inhibited heroin-enhanced BSR and NAc DA, suggesting functional antagonism by methadone of heroin's actions. In addition, 31,345 appears to possess significant abuse liability, as it produces dose-dependent enhancement of BSR and NAc DA, maintains a low rate of self-administration behavior, and dose-dependently reinstates drug-seeking behavior. In contrast, methadone only partially maintains self-administration with an extinction pattern, and fails to induce reinstatement of drug-seeking behavior. These findings suggest that 31,345 is a cocaine-like slow-onset long-acting monoamine transporter inhibitor that may act as an agonist therapy for cocaine addiction. However, its pattern of action appears to be significantly different from that of methadone. Ideal agonist substitutes for cocaine should fully emulate methadone's actions, that is, functionally antagonizing cocaine's action while blocking monoamine transporters to augment synaptic DA.

Interrelation of dopamine transporter oligomerization and surface presence as studied with mutant transporter proteins and amphetamine

Our previous work suggested a role for oligomerization in regulating dopamine transporter (DAT) internalization, with d-amphetamine dissociating DAT oligomers and monomers being endocytosed. This model was put to detailed testing in the present work with the use of DAT constructs differentially tagged with Myc or Flag, reversal of tags in co-immunoprecipitation and cross-linking assays, and application of antibodies against different tags in biotinylation experiments. Upon pairing wild-type (WT) DAT with W84L mutant, effects of d-amphetamine on oligomerization (decrease) but not surface DAT are observed. Internalization of W84L monomers appears to be slow as inferred from the inability of d-amphetamine to reduce surface Myc upon co-expressing Flag-WT with Myc-W84L but not Myc-WT with Flag-W84L, and from the sluggish Myc-W84L endocytosis rate (both with or without d-amphetamine). Results obtained for D313N, D345N, or D436N mutants can all be accommodated by a model in which D-amphetamine is unable to dissociate mutant protomers from oligomers (tetramers or higher-order assemblies) that contain them; this interpretation is confirmed in experiments with both tag reversal in co-expression and antibody reversal in western blotting. Upon co-transfecting Myc- and Flag-tagged constructs, resulting tetramers can be calculated to be composed of different species (MycMycMycMyc, MycMycMycFlag, MycMycFlagFlag, MycFlagFlagFlag, and FlagFlagFlagFlag), but it is shown that outcomes predicted by models based on MycMycFlagFlag oligomers are not changed in a major way by the occurrence of the additional species.

Ethanol alters endosomal recycling of human dopamine transporters

Dynamic membrane trafficking of the monoamine dopamine transporter (DAT) regulates dopaminergic signaling. Various intrinsic and pharmacological modulators can alter this trafficking. Previously we have shown ethanol potentiates in vitro DAT function and increases surface expression. However, the mechanism underlying these changes is unclear. In the present study, we found ethanol directly regulates DAT function by altering endosomal recycling of the transporter. We defined ethanol action on transporter regulation by [(3)H]DA uptake functional analysis combined with biochemical and immunological assays in stably expressing DAT HEK-293 cells. Short-term ethanol exposure potentiated DAT function in a concentration-, but not time-dependent manner. This potentiation was accompanied by a parallel increase in DAT surface expression. Ethanol had no effect on function or surface localization of the ethanol-insensitive mutant (G130T DAT), suggesting a trafficking-dependent mechanism in mediating the ethanol sensitivity of the transporter. The ethanol-induced increase in DAT surface expression occurred without altering the overall size of DAT endosomal recycling pools. We found ethanol increased the DAT membrane insertion rate while having no effect on internalization of the transporter. Ethanol had no effect on the surface expression or trafficking of the endogenously expressing transferrin receptor, suggesting ethanol does not have a nonspecific effect on endosomal recycling. These results define a novel trafficking mechanism by which ethanol regulates DAT function.

Low and high cocaine locomotor responding male Sprague-Dawley rats differ in rapid cocaine-induced regulation of striatal dopamine transporter function

Adult outbred Sprague-Dawley rats can be classified as either low or high cocaine responders (LCRs or HCRs, respectively). Importantly, LCRs and HCRs are distinguished by their differential responsiveness to acute cocaine-induced (but not baseline) locomotor activity, inhibition of the dopamine transporter (DAT) and resulting extracellular DA (HCR > LCR), as well as by repeated cocaine-induced locomotor sensitization and measures of cocaine's rewarding and reinforcing effects (LCR > HCR). Curiously, 30 min after acute cocaine HCRs exhibit greater DAT-mediated [(3)H]DA uptake into striatal synaptosomes than LCRs. To investigate this finding further, we measured locomotor activity, striatal [(3)H]DA uptake kinetics and DAT cell surface expression in LCRs and HCRs over an extended period (25-180 min) after a single relatively low-dose of cocaine (10 mg/kg, i.p.). HCRs exhibited the "predicted" locomotor response: a marked initial activation that returned to baseline by 120 min post-injection. While LCRs exhibited a >50% lower maximal locomotor response, this increase was sustained, lasting approximately 33% longer than in HCRs. At 25 min post-cocaine, maximal velocity (V(max)) of [(3)H]DA uptake was significantly higher by 25% in HCRs than LCRs, with no difference in affinity (K(m)). Despite the DAT V(max) difference, however, DAT surface expression did not differ between LCRs and HCRs. There was a similar trend (HCR > LCR) for DAT V(max) at 40 min, but not at 150 or 180 min. These findings suggest that, compared to LCRs, HCRs have an enhanced ability to rapidly up-regulate DAT function in response to acute cocaine, which may contribute to their more "normal" cocaine-induced locomotor activation.

Low or high cocaine responding rats differ in striatal extracellular dopamine levels and dopamine transporter number

Both humans and animals exhibit marked individual differences in cocaine responsiveness. By using the median split of cocaine-induced locomotor activity, we have classified outbred male Sprague-Dawley rats as either low or high cocaine responders (LCRs or HCRs, respectively). LCR/HCR classification predicts differences in cocaine inhibition of striatal dopamine (DA) transporters (DATs), cocaine-induced locomotor sensitization, cocaine-conditioned place preference, and motivation to self-administer cocaine. In this study, we used in vivo microdialysis to investigate whether the differential cocaine inhibition of DATs in LCRs and HCRs is translated into differential extracellular DA levels. Paralleling their locomotor profiles, LCRs and HCRS had similar basal extracellular DA levels in dorsal striatum (dSTR) and nucleus accumbens (NAc); after acute cocaine injection (10 mg/kg i.p.), HCRs showed greater cocaine-induced increases in DA than LCRs, with more pronounced differences in NAc. After repeated cocaine injection, LCRs and HCRs no longer differed in cocaine-induced locomotor activity or extracellular DA. To further explore the differential susceptibility of LCR/HCR DATs to cocaine, we used in vitro [(3)H]2-carbomethoxy-3-(4-fluorophenyl)tropane ([(3)H]WIN 35,428) binding and quantitative autoradiography to measure the number of DAT binding sites and cocaine's affinity for them. After acute cocaine administration, HCRs had fewer DAT binding sites in dSTR and NAc shell, compared to LCRs. No LCR/HCR differences were observed in DAT number after repeated cocaine injection or in cocaine's affinity. Our findings suggest that levels of striatal extracellular DA and DATs both make important contributions to initial differences in cocaine activation, which in LCRs/HCRs predict differential cocaine reward and reinforcement.

A dopamine transport inhibitor with markedly low abuse liability suppresses cocaine self-administration in the rat

RATIONALE

N-substituted benztropine analogs are potent dopamine uptake inhibitors that display pharmacokinetic/dynamic properties consistent with the profile of a substitute medication for cocaine addiction.

OBJECTIVES

The purpose of the present experiments was to characterize in rats the addictive-like properties of one such analog, 3 alpha-[bis(4'-fluorophenyl)methoxy]-tropane (AHN-1055), incorporating probes of its stimulant and incentive/motivational effects and of its ability to influence cocaine self-administration.

METHODS

We used open field activity and drug self-administration assays. To examine the effects of AHN-1055 on locomotor behavior, the analog was administered alone (0, 1, 3, and 10 mg/kg intraperitoneally) and in combination with cocaine (15 mg/kg i.p.). The influence of AHN-1055 on cocaine's intake was studied by administering the analog (0, 3, and 10 mg/kg i.p.) before the start of the self-administration sessions. To compare the addictive-like properties of AHN-1055 and cocaine, progressive ratio performance and abstinence-induced context-conditioned relapse were evaluated.

RESULTS

AHN-1055 evoked robust and sustained locomotor activity when administered alone and increased cocaine-induced locomotor stimulation. Notably, the analog showed by comparison to cocaine weak reinforcing efficacy in a modified progressive ratio schedule of drug reinforcement, and contrary to cocaine, it showed no ability to promote context-conditioned relapse to drug seeking following stable self-administration and abstinence. Further, AHN-1055 treatment blocked cocaine intake dose-dependently in rats with a steady history of cocaine self-administration without reducing responding for sucrose, a natural reward.

CONCLUSIONS

These findings demonstrate essential psychopharmacological differences between AHN-1055 and cocaine and highlight important properties of the analog as a possible pharmacotherapy in cocaine addiction.

The dopamine uptake inhibitor 3 alpha-[bis(4'-fluorophenyl)metoxy]-tropane reduces cocaine-induced early-gene expression, locomotor activity, and conditioned reward

Benztropine (BZT) analogs, a family of high-affinity dopamine transporter ligands, are molecules that exhibit pharmacological and behavioral characteristics predictive of significant therapeutic potential in cocaine addiction. Here, we examined in mice the effects of 3 alpha-[bis(4'-fluorophenyl)metoxy]-tropane (AHN-1055) on motor activity, conditioned place preference (CPP) and c-Fos expression in the striatum. AHN-1055 produced mild attenuation of spontaneous locomotor activity at a low dose (1 mg/kg) and weak stimulation at a higher dose (10 mg/kg). In parallel, the BZT analog significantly increased c-Fos expression in the dorsolateral caudoputamen at the high dose, whereas producing marginal decreases at low and moderate doses (1, 3 mg/kg) in both dorsal and ventral striatum. Interaction assays showed that cocaine's ability to stimulate locomotor activity was decreased by AHN-1055 treatment, but not by treatment with D-amphetamine. Such reduced ability did not result from an increase in stereotyped behavior. Another dopamine uptake inhibitor, nomifensine, decreased cocaine-induced locomotor activity but evoked by itself intense motor stereotypies. Remarkably, the BZT analog dose-dependently blocked cocaine-induced CPP without producing CPP when given alone, and blocked in conditioned mice cocaine-stimulated early-gene activation in the nucleus accumbens and dorsomedial striatum. These observations provide evidence that AHN-1055 does not behave as a classical psychomotor stimulant and that some of its properties, including attenuation of cocaine-induced striatal c-Fos expression, locomotor stimulation, and CPP, support its candidacy, and that of structurally related molecules, as possible pharmacotherapies in cocaine addiction.

Homozygous loss-of-function mutations in the gene encoding the dopamine transporter are associated with infantile parkinsonism-dystonia

Genetic variants of the SLC6A3 gene that encodes the human dopamine transporter (DAT) have been linked to a variety of neuropsychiatric disorders, particularly attention deficit hyperactivity disorder. In addition, the homozygous Slc6a3 knockout mouse displays a hyperactivity phenotype. Here, we analyzed 2 unrelated consanguineous families with infantile parkinsonism-dystonia (IPD) syndrome and identified homozygous missense SLC6A3 mutations (p.L368Q and p.P395L) in both families. Functional studies demonstrated that both mutations were loss-of-function mutations that severely reduced levels of mature (85-kDa) DAT while having a differential effect on the apparent binding affinity of dopamine. Thus, in humans, loss-of-function SLC6A3 mutations that impair DAT-mediated dopamine transport activity are associated with an early-onset complex movement disorder. Identification of the molecular basis of IPD suggests SLC6A3 as a candidate susceptibility gene for other movement disorders associated with parkinsonism and/or dystonic features.

Ethanol potentiates dopamine uptake and increases cell surface distribution of dopamine transporters expressed in SK-N-SH and HEK-293 cells

Ethanol increases dopaminergic release in the reward and reinforcement areas of the brain. The primary protein responsible for terminating dopamine (DA) neurotransmission is the plasma membrane-bound dopamine transporter (DAT). In vitro electrophysiological and biochemical studies in Xenopus laevis oocytes have previously shown ethanol potentiates DAT function and increases transporter-binding sites. The potentiating effect of ethanol on the transporter is eliminated in Xenopus oocytes by the DAT mutation glycine 130 to threonine. However, ethanol's action on DAT functional regulation has yet to be examined in mammalian cell expression systems. To further understand the molecular mechanisms of ethanol's action on DAT, we determined the direct mechanistic action of short-term (< or =2 h) ethanol exposure on transporter function and cell surface distribution in non-neuronal human embryonic kidney cells-293 (HEK-293) and neuronal SK-N-SH neuroblastoma cells expressing the transporter. Wild-type or G130T mutant DAT were overexpressed in HEK-293 and SK-N-SH cells. Ethanol potentiated DAT mediated [(3)H]DA uptake in a dose (25, 50, 100 mM), but not time dependent manner in cells expressing wild-type DAT. Ethanol-induced potentiation of uptake was significantly reduced in cells expressing the G130T mutant. Analysis of DA uptake kinetic parameters indicates 100-mM ethanol exposure increased [(3)H]DA uptake velocity (V(max)), while affinity for DA (K(m)) remained unchanged. The effect of ethanol on wild-type DAT surface expression was measured by biotinylation cell surface labeling. DAT surface expression increased 40%-50% after 1-h, 100-mM ethanol exposure. These studies show ethanol potentiates DAT functional regulation in both neuronal and non-neuronal cells, suggesting a direct mechanistic action of ethanol on transporter trafficking in mammalian systems. Our findings demonstrate ethanol's action on DAT function and regulation is consistent across multiple model systems.