Selective Dopamine Transporter Inhibitor Research Articles

A Novel and Selective Dopamine Transporter Inhibitor, (S)-MK-26, Promotes Hippocampal Synaptic Plasticity and Restores Effort-Related Motivational Dysfunctions.

Dopamine (DA), the most abundant human brain catecholaminergic neurotransmitter, modulates key behavioral and neurological processes in young and senescent brains, including motricity, sleep, attention, emotion, learning and memory, and social and reward-seeking behaviors. The DA transporter (DAT) regulates transsynaptic DA levels, influencing all these processes. Compounds targeting DAT (e.g., cocaine and amphetamines) were historically used to shape mood and cognition, but these substances typically lead to severe negative side effects (tolerance, abuse, addiction, and dependence). DA/DAT signaling dysfunctions are associated with neuropsychiatric and progressive brain disorders, including Parkinson’s and Alzheimer diseases, drug addiction and dementia, resulting in devastating personal and familial concerns and high socioeconomic costs worldwide. The development of low-side-effect, new/selective medicaments with reduced abuse-liability and which ameliorate DA/DAT-related dysfunctions is therefore crucial in the fields of medicine and healthcare. Using the rat as experimental animal model, the present work describes the synthesis and pharmacological profile of (S)-MK-26, a new modafinil analogue with markedly improved potency and selectivity for DAT over parent drug. Ex vivo electrophysiology revealed significantly augmented hippocampal long-term synaptic potentiation upon acute, intraperitoneally delivered (S)-MK-26 treatment, whereas in vivo experiments in the hole-board test showed only lesser effects on reference memory performance in aged rats. However, in effort-related FR5/chow and PROG/chow feeding choice experiments, (S)-MK-26 treatment reversed the depression-like behavior induced by the dopamine-depleting drug tetrabenazine (TBZ) and increased the selection of high-effort alternatives. Moreover, in in vivo microdialysis experiments, (S)-MK-26 significantly increased extracellular DA levels in the prefrontal cortex and in nucleus accumbens core and shell. These studies highlight (S)-MK-26 as a potent enhancer of transsynaptic DA and promoter of synaptic plasticity, with predominant beneficial effects on effort-related behaviors, thus proposing therapeutic potentials for (S)-MK-26 in the treatment of low-effort exertion and motivational dysfunctions characteristic of depression and aging-related disorders.

Structure-Activity Relationships of Novel Thiazole-Based Modafinil Analogues Acting at Monoamine Transporters.

Atypical dopamine reuptake inhibitors, such as modafinil, are used for the treatment of sleeping disorders and investigated as potential therapeutics against cocaine addiction and for cognitive enhancement. Our continuous effort to find modafinil analogues with higher inhibitory activity on and selectivity toward the dopamine transporter (DAT) has previously led to the promising thiazole-containing derivatives CE-103, CE-111, CE-123, and CE-125. Here, we describe the synthesis and activity of a series of compounds based on these scaffolds, which resulted in several new selective DAT inhibitors and gave valuable insights into the structure-activity relationships. Introduction of the second chiral center and subsequent chiral separations provided all four stereoisomers, whereby the S-configuration on both generally exerted the highest activity and selectivity on DAT. The representative compound of this series was further characterized by in silico, in vitro, and in vivo studies that have demonstrated both safety and efficacy profile of this compound class.

Involvement of catecholaminergic and GABAAergic mediations in the anxiety-related behavior in long-term powdered diet-fed mice

Dietary habits are important factors which affect metabolic homeostasis and the development of emotion. We have previously shown that long-term powdered diet feeding in mice increases spontaneous locomotor activity and social interaction (SI) time. Moreover, that diet causes changes in the dopaminergic system, especially increased dopamine turnover and decreased dopamine D4 receptor signals in the frontal cortex. Although the increased SI time indicates low anxiety, the elevated plus maze (EPM) test shows anxiety-related behavior and impulsive behavior. In this study, we investigated whether the powdered diet feeding causes changes in anxiety-related behavior. Mice fed a powdered diet for 17 weeks from weaning were compared with mice fed a standard diet (control). The percentage (%) of open arm time and total number of arm entries were increased in powdered diet-fed mice in the EPM test. We also examined the effects of diazepam, benzodiazepine anti-anxiety drug, bicuculline, GABA-A receptor antagonist, methylphenidate, dopamine transporter (DAT) and noradrenaline transporter (NAT) inhibitor, atomoxetine, selective NAT inhibitor, GBR12909, selective DAT inhibitor, and PD168077, selective dopamine D4 receptor agonist, on the changes of the EPM in powdered diet-fed mice. Methylphenidate and atomoxetine are clinically used to treat attention deficit/hyperactivity disorder (ADHD) symptoms. The % of open arm time in powdered diet-fed mice was decreased by treatments of atomoxetine, methylphenidate and PD168077. Diazepam increased the % of open arm time in control diet-fed mice, but not in powdered diet-fed mice. The powdered diet feeding induced a decrease in GABA transaminase, GABA metabolic enzymes, in the frontal cortex. Moreover, the powdered diet feeding induced an increase in NAT expression, but not DAT expression, in the frontal cortex. These results suggest that the long-term powdered diet feeding may cause low anxiety or impulsivity, possibly via noradrenergic and/or dopaminergic, and GABAAergic mediations and increase the risk for onset of ADHD-like behaviors.

Interactions between Cocaine and the Putative Allosteric Dopamine Transporter Ligand SRI-31142.

Drugs that inhibit the dopamine (DA) transporter (DAT) include both therapeutic agents and abused drugs. Recent studies identified a novel series of putative allosteric DAT inhibitors, but the in vivo effects of these compounds are unknown. This study examined the abuse-related behavioral and neurochemical effects produced in rats by SRI-31142 [2-(7-methylimidazo[1,2-a]pyridin-6-yl)-N-(2-phenyl-2-(pyridin-4-yl)ethyl)quinazolin-4-amine], one compound from this series. In behavioral studies, intracranial self-stimulation (ICSS) was used to compare the effects produced by SRI-31142, the abused and nonselective DAT inhibitor cocaine, and the selective DAT inhibitor GBR-12935 [1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine]. In neurochemical studies, in vivo microdialysis was used to compare the effects of SRI-31142 and cocaine on levels of DA and serotonin in nucleus accumbens (NAc). The effects of SRI-31142 in combination with cocaine were also examined in both procedures. In contrast to cocaine and GBR-12935, SRI-31142 failed to produce abuse-related increases in ICSS or NAc DA; instead, SRI-31142 only decreased ICSS and NAc DA at a dose that was also sufficient to block cocaine-induced increases in ICSS and NAc DA. Pharmacokinetic studies suggested low but adequate brain penetration of SRI-31142, in vitro binding studies failed to identify likely non-DAT targets, and in vitro functional assays failed to confirm DA uptake inhibition in an assay of DAT-mediated fluorescent signals in live cells. These results indicate that SRI-31142 does not produce cocaine-like abuse-related effects in rats. SRI-31142 may have utility to block cocaine effects and may warrant further study as a candidate pharmacotherapy; however, the role of DAT in mediating these effects is unclear, and side effects may be a limiting factor.

Simultaneous Multiple MS Binding Assays for the Dopamine, Norepinephrine, and Serotonin Transporters.

In this work, we present label-free, mass-spectrometry-based binding assays (MS Binding Assays), targeting the human dopamine, norepinephrine, and serotonin transporters (hDAT, hNET, and hSERT) in simultaneous binding experiments. Using a validated LC-ESI-MS/MS method for quantification of the selective dopamine transporter inhibitor (R,R)-4-(2-benzhydryloxyethyl)-1-(4-fluorobenzyl)piperidin-3-ol ((R,R)-D-84), the selective norepinephrine transporter inhibitor (S,S)-reboxetine, and the selective serotonin reuptake inhibitor (S)-citalopram, binding affinities at the three monoamine transporters could be characterized simultaneously in a single binding experiment. The performed simultaneous saturation and competition experiments yielded results that are in good accordance with those determined in MS Binding Assays addressing the monoamine transporters individually. The results obtained from this study underscore the potential of MS Binding Assays for simultaneous affinity determination at different targets, which is difficult to accomplish with conventional radioligand binding assays.

S-phenylpiracetam, a selective DAT inhibitor, reduces body weight gain without influencing locomotor activity

S-phenylpiracetam is an optical isomer of phenotropil, which is a clinically used nootropic drug that improves physical condition and cognition. Recently, it was shown that S-phenylpiracetam is a selective dopamine transporter (DAT) inhibitor that does not influence norepinephrine (NE) or serotonin (5-HT) receptors. The aim of the present study was to study the effects of S-phenylpiracetam treatment on body weight gain, blood glucose and leptin levels, and locomotor activity. Western diet (WD)-fed mice and obese Zucker rats were treated daily with peroral administration of S-phenylpiracetam for 8 and 12weeks, respectively. Weight gain and plasma metabolites reflecting glucose metabolism were measured. Locomotor activity was detected in an open-field test. S-phenylpiracetam treatment significantly decreased body weight gain and fat mass increase in the obese Zucker rats and in the WD-fed mice. In addition, S-phenylpiracetam reduced the plasma glucose and leptin concentration and lowered hyperglycemia in a glucose tolerance test in both the mice and the rats. S-phenylpiracetam did not influence locomotor activity in the obese Zucker rats or in the WD-fed mice. The results demonstrate that S-phenylpiracetam reduces body weight gain and improves adaptation to hyperglycemia without stimulating locomotor activity. Our findings suggest that selective DAT inhibitors, such as S-phenylpiracetam, could be potentially useful for treating obesity in patients with metabolic syndrome with fewer adverse health consequences compared to other anorectic agents.

Potential role of dopamine transporter in behavioral flexibility

Behavioral flexibility is subserved by the prefrontal cortex and the basal ganglia. Orbitofrontal cortex (OFC) and dorsomedial striatum (DMS) form a functional frontocorticostriatal circuit crucial for the mediation of flexibility during reversal learning via dopamine (DA) neurotransmission. The regulatory control in maintaining DA homeostasis and function is provided by the dopamine transporter (DAT), which therefore likely plays a significant role in controlling the influence of DA on cognitive processes. Here we used a gene knockout mouse model to investigate the role of DAT in the performance on the Attentional Set-Shifting Task (ASST) stages dependent upon the OFC and the DMS. Additionally, behavior of mice after repeated administration of selective DAT inhibitor, GBR 12909, was examined. The animals were treated with the inhibitor to elicit a compensatory DAT up-regulation following withdrawal. Learning was slower and the number of errors during reversal learning and intra-dimensional shift stages was higher in DAT+/- mutant mice than in WT mice. GBR 12909-treated mice had deficits in reversal stages of the ASST. Neuronal activation in the OFC and DMS during the ASST was examined with early growth response proteins 1 and 2 (egr-1, egr-2) immunohistochemistry. Density of egr-2 labeled cells in the OFC was lower in mutant mice than in wild-types during reversal learning and the expression of the egr-1 was lower in mutant mice in the OFC and DMS during reversal and intra-dimensional shift stages. Mice with decreased DAT levels displayed behavioral difficulties that were accompanied by a lower task-induced activation of neurons in brain regions involved in the reversal learning. Altogether, these data indicate the role of the DAT in the behavioral flexibility.

CTDP-32476: A Promising Agonist Therapy for Treatment of Cocaine Addiction.

Agonist-replacement therapies have been successfully used for treatment of opiate and nicotine addiction, but not for cocaine addiction. One of the major obstacles is the cocaine-like addictive potential of the agonists themselves. We report here an atypical dopamine (DA) transporter (DAT) inhibitor, CTDP-32476, that may have translational potential for treating cocaine addiction. In vitro ligand-binding assays suggest that CTDP-32476 is a potent and selective DAT inhibitor and a competitive inhibitor of cocaine binding to the DAT. Systemic administration of CTDP-32476 alone produced a slow-onset, long-lasting increase in extracellular nucleus accumbens DA, locomotion, and brain-stimulation reward. Drug-naive rats did not self-administer CTDP-32476. In a substitution test, cocaine self-administration rats displayed a progressive reduction in CTDP-32476 self-administration with an extinction pattern of drug-taking behavior, suggesting significantly lower addictive potential than cocaine. Pretreatment with CTDP-32476 inhibited cocaine self-administration, cocaine-associated cue-induced relapse to drug seeking, and cocaine-enhanced extracellular DA in the nucleus accumbens. These findings suggest that CTDP-32476 is a unique DAT inhibitor that not only could satisfy 'drug hunger' through its slow-onset long-lasting DAT inhibitor action, but also render subsequent administration of cocaine ineffectual-thus constituting a novel and unique compound with translational potential as an agonist therapy for treatment of cocaine addiction.

Cocaine withdrawal alters the reward omission effect and enhances traits of negative urgency in rats across multiple days of testing

BackgroundThe personality trait of negative urgency, characterized as behaving rashly when emotionally perturbed, is gaining attention as an indicator for susceptibility to problematic substance use. How this trait is influenced by exposure to drugs of abuse is still unclear. Using an animal model of binge cocaine consumption, we tested this relationship in a reward-omission task across multiple days. MethodsAdult, male, Sprague-Dawley rats received seven daily (ip) injections of saline, cocaine (10–20mg/kg), or cocaine (20–40mg/kg). Cocaine doses increased linearly each day from the lower to the higher dose. A separate group received RTI-113 (3.0mg/kg), a selective dopamine transporter inhibitor, for 7 days. Fifteen days after their final injection, rats were trained on a reward-omission task with an operant component to earn further rewards. ResultsPrevious exposure to cocaine resulted in dose-dependent increases in negative urgency in separate behavioral variables across days of testing. The lower dose range increased negative urgency on the dimension of decreased reaction time to press a lever, while the higher dose range increased the rate of increase in lever presses made per trial. Rats receiving RTI-113 did not resemble either cocaine group and instead showed a decrease in lever pressing across days. ConclusionsOur results indicate that previous binge cocaine consumption enhances behavioral markers of negative urgency in a dose-dependent, time-sensitive manner on discrete behavioral dimensions. The results with RTI-113 suggest the relationship between cocaine exposure and negative urgency is unlikely to be explained solely by inhibition of dopamine reuptake.

Progress in the Search for New Dopamine Transporter Inhibitors.

In the present review, we provide a comprehensive summary of recent pharmacological studies on dopamine transporter (DAT) inhibitors, which are potential treatments for neurodegenerative and psychiatric disorders. Extensive structure-activity relationship studies have identified numerous tropane-based ligands with high affinity and selectivity for the DAT or with high affinity to the DAT and other monoamine transporters (dual and triple monoamine reuptake inhibitors). The review covers advances in the field in past fifteen years. Among the described compounds, many appear to be promising drug candidates, while other may serve as valuable tools for research or as prototypes for new classes of selective DAT inhibitors. Special attention is being paid to separation of the DAT inhibition from NET and SERT inhibition.

Essential Oils from the Medicinal Herbs Upregulate Dopamine Transporter in Rat Pheochromocytoma Cells.

The dopamine transporter (DAT) protein, a component of the dopamine system, undergoes adaptive neurobiological changes from drug abuse. Prevention of relapse and reduction of withdrawal symptoms are still the major limitations in the current pharmacological treatments of drug addiction. The present study aimed to investigate the effects of essential oils extracted from Elsholtzia ciliata, Shinchim, Angelicae gigantis Radix, and Eugenia caryophyllata, well-known traditional Korean medicines for addiction, on the modulation of dopamine system in amphetamine-treated cells and to explore the possible mechanism underlying its therapeutic effect. The potential cytotoxic effect of essential oils was evaluated in PC12 rat pheochromocytoma cells using cell viability assays. Quantification of DAT, p-CREB, p-MAPK, and p-Akt was done by immunoblotting. DAT was significantly reduced in cells treated with 50 μM of amphetamine in a time-dependent manner. No significant toxicity of essential oils from Elsholtzia ciliata and Shinchim was observed at doses of 10, 25, and 50 μg/mL. However, essential oils from A. gigantis Radix at a dose of 100 μg/mL and E. caryophyllata at doses of 50 and 100 μg/mL showed cytotoxicity. Treatment with GBR 12909, a highly selective DAT inhibitor, significantly increased DAT expression compared with that of amphetamine only by enhancing phosphorylation of mitogen-activated protein kinases (MAPK) and Akt. In addition, essential oils effectively induced hyperphosphorylation of cyclic-AMP response element-binding protein (CREB), MAPK, and Akt, which resulted in DAT upregulation. Our study implies that the essential oils may rehabilitate brain dopamine function through increased DAT availability in abstinent former drug users.

Increased dopamine transporter function as a mechanism for dopamine hypoactivity in the adult infralimbic medial prefrontal cortex following adolescent social stress

Being bullied during adolescence is associated with later mental illnesses characterized by deficits in cognitive tasks mediated by prefrontal cortex (PFC) dopamine (DA). Social defeat of adolescent male rats, as a model of teenage bullying victimization, results in medial PFC (mPFC) dopamine (DA) hypofunction in adulthood that is associated with increased drug seeking and working memory deficits. Increased expression of the DA transporter (DAT) is also seen in the adult infralimbic mPFC following adolescent defeat. We propose the functional consequence of this increased DAT expression is enhanced DA clearance and subsequently decreased infralimbic mPFC DA availability. To test this, in vivo chronoamperometry was used to measure changes in accumulation of the DA signal following DAT blockade, with increased DAT-mediated clearance being reflected by lower DA signal accumulation. Previously defeated rats and controls were pre-treated with the norepinephrine transporter (NET) inhibitor desipramine (20 mg/kg, ip.) to isolate infralimbic mPFC DA clearance to DAT, then administered the selective DAT inhibitor GBR-12909 (20 or 40 mg/kg, sc.). Sole NET inhibition with desipramine produced no differences in DA signal accumulation between defeated rats and controls. However, rats exposed to adolescent social defeat demonstrated decreased DA signal accumulation compared to controls in response to both doses of GBR-12909, indicating greater DAT-mediated clearance of infralimbic mPFC DA. These results suggest that protracted increases in infralimbic mPFC DAT function represent a mechanism by which adolescent social defeat stress produces deficits in adult mPFC DA activity and corresponding behavioral and cognitive dysfunction.

Pharmacological MRI response to a selective dopamine transporter inhibitor, GBR12909, in awake and anesthetized rats.

Pharmacological magnetic resonance imaging (phMRI) is a powerful tool for imaging the effects of drugs on brain activity. In preclinical phMRI studies, general anesthesia used for minimizing head movements is thought to influence the phMRI responses to drugs. In this study we investigated the phMRI responses to a selective dopamine transporter (DAT) inhibitor, GBR12909, and a dopamine (DA) releaser, d-amphetamine (AMPH), in the isoflurane anesthetized and awake rats using a relative cerebral blood volume (rCBV) method. AMPH (1 mg/kg i.p.) caused an increase in rCBV in the dopaminergic circuitry in the both anesthetized and awake rats. The striatal rCBV change was correlated with the change of the striatal DA concentration induced by AMPH in the both anesthetized and awake rats. GBR12909 (10 mg/kg i.p.) caused a positive rCBV response and showed a similar regional pattern of rCBV response to AMPH in the awake rats, and the correlation between the change of the striatal rCBV and the striatal DA concentration was observed. However, in the anesthetized rats, GBR12909 induced a widespread negative rCBV response, whereas an increase in striatal DA concentration was observed. These findings indicate that phMRI responses to activation of DA neurotransmission by GBR12909 or AMPH are overall identical in the awake state, while the phMRI response to a DAT inhibitor, GBR12909 but not to AMPH was changed by isoflurane anesthesia. For the evaluation of neuroactive drugs using phMRI, isoflurane anesthesia might be complicated the interpretation of pharmacodynamic effects of drugs in preclinical studies.

Differential effects of dopamine transporter inhibitors in the rodent Iowa gambling task

The Iowa Gambling Task (IGT) can be used to quantify impulsive and risky choice behaviors in psychiatric patients, e.g., bipolar disorder (BD) sufferers. Although developing treatments for these behaviors is important, few predictive animal models exist. Inhibition of the dopamine transporter (DAT) can model profiles of altered motor activity and exploration seen in patients with BD. The effect of DAT inhibition on impulsive choices related to BD has received limited study however. We used a rodent IGT to elucidate the effects of similarly acting drugs on risky choice behavior. We hypothesized that (1) C57BL/6 mice could adopt the "safe" choice options in the IGT and (2) DAT inhibition would alter risk preference. Mice were trained in the IGT to a stable risk-preference and then administered the norepinephrine/DAT inhibitor amphetamine, or the more selective DAT inhibitors modafinil or GBR12909. Mice developed a preference for the "safe" option, which was potentiated by amphetamine administration. GBR12909 or modafinil administration increased motor impulsivity, motivation significantly, and risk preference subtly. The rodent IGT can measure different impulse-related behaviors and differentiate similarly acting BD-related drugs. The contrasting effects of amphetamine and modafinil in mice are similar to effects in rats and humans in corresponding IGT tasks, supporting the translational validity of the task. GBR12909 and modafinil elicited similar behaviors in the IGT, likely through a shared mechanism. Future studies using a within-session IGT are warranted to confirm the suitability of DAT inhibitors to model risk-preference in BD.

Rimcazole Attenuates the Cocaine‐Induced Stimulation of Mesolimbic Dopamine related to its Abuse and Dependence

Rimcazole (RIM) is a sigma receptor (σR) antagonist that also has affinity for the dopamine (DA) transporter (DAT). Despite this DAT affinity, RIM lacks stimulant effects and dose‐dependently attenuates ambulatory effects of cocaine (COC). RIM and its analogs decrease COC self‐administration in rats (JPET 339: 662), an effect that is obtained neither with selective σR antagonists nor standard DAT blockers.Thus, we tested RIM (1–17 mg/kg i.p.) alone and in combination with increasing doses of COC that maintain self‐administration (0.1–1.0 mg/kg i.v.), on stimulation of mesolimbic DA in Sprague Dawley rats implanted with a microdialysis probes in the accumbens shell.RIM dose‐dependently attenuated COC‐induced stimulation of DA levels at doses that had no effects on DA levels when administered alone. Previous studies suggest that the attenuation of COC self‐administration produced by RIM is due to the combined actions at σRs and the DAT as combinations of selective DAT inhibitors and selective σR produced effects like those or RIM.Thus, the present study indicates that not only can combined DAT and σR inhibition decrease cocaine self administration, it also can decrease its neurochemical substrates suggesting that combined actions at these two targets may serve as a new strategy for discovery of novel compounds for development as medications for stimulant abuse.

Decreases in Cocaine Self-Administration with Dual Inhibition of the Dopamine Transporter and σ Receptors

Sigma receptor (σR) antagonists attenuate many behavioral effects of cocaine but typically not its reinforcing effects in self-administration procedures. However, the σR antagonist rimcazole and its N-propylphenyl analogs, [3-(cis-3,5-dimethyl-4-[3-phenylpropyl]-1-piperazinyl)-propyl]diphenylamine hydrochloride (SH 3-24) and 9-[3-(cis-3,5-dimethyl-4-[3-phenylpropyl]-1-piperazinyl)-propyl]carbazole hydrobromide (SH 3-28), dose-dependently decreased the maximal rates of cocaine self-administration without affecting comparable responding maintained by food reinforcement. In contrast, a variety of σR antagonists [N-phenethylpiperidine oxalate (AC927), N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine dihydrobromide (BD 1008), N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino) ethylamine dihydrobromide (BD 1047), N-[2-(3,4-dichlorophenyl) ethyl]-4-methylpiperazine dihydrochloride (BD 1063), and N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100)] had no effect on cocaine self-administration across the range of doses that decreased rates of food-maintained responding. Rimcazole analogs differed from selective σR antagonists in their dual affinities for σRs and the dopamine transporter (DAT) assessed with radioligand binding. Selective DAT inhibitors and σR antagonists were studied alone and in combination on cocaine self-administration to determine whether actions at both σRs and the DAT were sufficient to reproduce the effects of rimcazole analogs. Typical DAT inhibitors [2β-carbomethoxy-3β-(4-fluorophenyl)tropane (WIN 35,428), methylphenidate, and nomifensine] dose-dependently shifted the cocaine dose-effect curve leftward. Combinations of DAT inhibitor and σR antagonist doses that were behaviorally inactive alone decreased cocaine self-administration without effects on food-maintained responding. In addition, whereas the DAT inhibitors were self-administered at rates similar to those of cocaine, neither rimcazole analogs nor typical σR antagonists (NE-100 and AC927) maintained responding above control levels across a wide range of doses. These findings suggest that the unique effects of rimcazole analogs are due to dual actions at the DAT and σRs and that a combined target approach may have utility in development of medical treatments for cocaine abuse.

Characterization of Pharmacological and Wake-Promoting Properties of the Dopaminergic Stimulant Sydnocarb in Rats

Sydnocarb is a psychomotor stimulant structurally similar to d-amphetamine (D-AMPH) and is used in Russia for the treatment of a variety of neuropsychiatric comorbidities. The nature of sydnocarb-induced facilitation of dopamine (DA) neurotransmission [DA release versus DA transporter (DAT) inhibition] is not clear. The present study characterized the pharmacological actions and behavioral effects of intraperitoneal sydnocarb in male Sprague-Dawley rats. Where relevant, comparisons were made with intraperitoneal D-AMPH. Unlike D-AMPH, which causes release of DA from rat synaptosomes (EC(50) = 0.10 μM; 95% confidence limits, 0.06-0.18), sydnocarb (up to 100 μM) did not. Sydnocarb potently (K(i) = 8.3 ± 0.7 nM) blocked recombinant human DAT expressed in Chinese hamster ovary-K1 cells and less potently blocked the norepinephrine transporter (K(i) = 10.1 ± 1.5 μM). Sydnocarb at 10 μM did not bind to 64 other targets. In rats, 10 and 30 mg/kg sydnocarb showed a 2-fold longer half-life in plasma and brain and a 5-fold lower brain-to-plasma ratio compared with 0.3 and 1 mg/kg D-AMPH. In the Irwin assay, sydnocarb was well tolerated up to 30 mg/kg; D-AMPH-like stereotypic behaviors were evident at 100 mg/kg. Behavioral effects of 30 mg/kg sydnocarb and 0.3 mg/kg D-AMPH were comparable. In a sleep/wake assay, 10 mg/kg sydnocarb and 1 mg/kg D-AMPH increased wakefulness comparably; however, sydnocarb (up to 30 mg/kg) did not induce D-AMPH-like rebound hypersomnolence (RHS). Like D-AMPH, sydnocarb enhanced theta power, an electrophysiological measure of cognitive function. In conclusion, sydnocarb is a selective and potent DAT inhibitor that produces robust increases in the wake state without RHS, and with potential cognitive-enhancing properties.

μ-Opioid receptors in the nucleus accumbens shell region mediate the effects of amphetamine on inhibitory control but not impulsive choice.

Acute challenges with psychostimulants such as amphetamine affect impulsive behavior in both animals and humans. With regard to amphetamine, it is important to unravel how this drug affects impulsivity since it is not only a widely abused recreational drug but also regularly prescribed to ameliorate maladaptive impulsivity. Therefore, we studied the effects of amphetamine in two rat models of impulsivity, the five-choice serial reaction time task and the delayed-reward task, providing measures of inhibitory control and impulsive choice, respectively. We focused on the role of opioid receptor activation in amphetamine-induced impulsivity as there is ample evidence indicating an important role for endogenous opioids in several behavioral and neurochemical effects of amphetamine. Results showed that amphetamine-induced inhibitory control deficits were dose-dependently attenuated by the preferential μ-opioid receptor antagonist naloxone, but not by the selective δ-opioid receptor antagonist naltrindole or κ-opioid receptor antagonist nor-BNI (nor-binaltorphimine dihydrochloride). In contrast, naloxone did not affect amphetamine-induced improvements in impulsive decision making. Naloxone also completely prevented inhibitory control deficits induced by GBR 12909 [1-(2-[bis(4-fluorophenyl)methoxy] ethyl)-4-(3-phenylpropyl)piperazine dihydrochloride], a selective dopamine transporter inhibitor. Intracranial infusions of naloxone, the selective μ-opioid receptor antagonist CTAP (H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)), morphine, and the selective μ-opioid receptor agonist DAMGO ([D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin acetate salt) revealed that μ-opioid receptor activation in the shell rather than the core subregion of the nucleus accumbens (NAc) modulates inhibitory control and subserves the effect of amphetamine thereon. Together, these results indicate an important role for NAc shell μ-opioid receptors in the regulation of inhibitory control, probably via an interaction between these receptors and the mesolimbic dopamine system.

The selective dopamine uptake inhibitor, D-84, suppresses cocaine self-administration, but does not occasion cocaine-like levels of generalization

A successful replacement pharmacotherapy for treating cocaine dependency would likely reduce cocaine's abuse, support a low abuse liability, overlap cocaine's subjective effects, and have a long duration of action. Inhibitors with varying selectivity at the dopamine transporter (DAT) have approximated these properties. The objective of the present study was to characterize the behavioural effects of an extremely selective DAT inhibitor, (+) trans-4-(2-Benzhydryloxyethyl)-1-(4-fluorobenzyl) piperadin-3-ol (D-84), a 3-hydroxy substituted piperidine derivative of GBR-12935, for its cocaine-like discriminative stimulus effects, its effects on cocaine self-administration, and for its own self-administration. During cocaine discrimination tests, cocaine occasioned the 10 mg/kg cocaine training stimulus with an ED 50 value of 3.13 (1.54–6.34) mg/kg, and reduced response rates with an ED 50 value of 20.39 (7.24–57.44) mg/kg. D-84 incompletely generalized to the cocaine stimulus occasioning a maximal 76% cocaine-lever responding, while reducing response rates with lower potency than cocaine (ED 50 = 30.94 (12.34–77.60) mg/kg). Pretreatment with D-84 (9.6–30.4 mg/kg) significantly ( P < 0.05) reduced cocaine intake at 17.1 mg/kg D-84 when cocaine was self-administered at 0.5 mg/kg/infusion, and at 30.4 mg/kg D-84 when cocaine was self-administered at 0.1, 0.5 .and 1.0 mg/kg/infusion. During self-administration tests with D-84 (0.1–1 mg/kg/infusion), numbers of infusions significantly exceeded vehicle levels at 0.3 mg/kg/infusion. These results show that D-84 pretreatment can decrease cocaine intake especially when high doses of cocaine are being self-administered. This observation, combined with its incomplete generalization to the cocaine discriminative stimulus and its reported long duration of action, provides a profile consistent with a potential replacement therapy for treating cocaine-abusing patients.

The effects of d-amphetamine, methylphenidate, sydnocarb, and caffeine on prepulse inhibition of the startle reflex in DBA/2 mice

Dopamine (DA) agonists decrease prepulse inhibition (PPI) and are widely used in translational models for the sensorimotor gating deficits in schizophrenia. Reductions in PPI induced by DA agonists are routinely reversed by antipsychotics in these translational models. Nevertheless, under conditions of low-baseline PPI, DA agonists may increase PPI in humans and experimental animals. DBA/2 mice have naturally low-baseline PPI, which as in the drug-induced translational models, is increased by antipsychotics. Determine whether DBA/2 mice respond like other models of low-baseline PPI by evaluating the effect of psychostimulants (caffeine, 30-100 mg/kg IP) and the indirect DA agonists d-amphetamine (0.3-10 mg/kg IP), methylphenidate (10-100 mg/kg IP), and sydnocarb (10-30 mg/kg IP), a selective DA transporter inhibitor on PPI. Furthermore, baseline PPI in DBA/2 mice was increased by noise exposure and the effect of d-amphetamine was assessed. PPI was increased at one dose for each of the psychostimulants when baseline PPI was low in naïve DBA/2 mice. Effective doses were 3 mg/kg of d-amphetamine, 30 mg/kg of methylphenidate, 30 mg/kg of sydnocarb, and 100 mg/kg of caffeine. Higher doses of d-amphetamine (10 mg/kg) and methylphenidate (100 mg/kg IP) decreased PPI. When the baseline PPI was increased by noise exposure, 10 mg/kg of d-amphetamine only reduced PPI. Lower doses of psychostimulants increased PPI in naïve DBA/2 mice in a manner consistent with their naturally low-baseline PPI, and higher doses decreased PPI, consistent with effects observed in most mouse strains.