Drugs that inhibit dopamine (DA) reuptake through actions at the dopamine transporter (DAT) have been proposed as candidates for development as pharmacotherapies for cocaine abuse. Accordingly, it is important to understand the potential pharmacological interactions of cocaine with other drugs acting at the DAT. Effects of combinations of cocaine with a cocaine analog, 2␤-carbomethoxy-3␤-(4-fluorophenyl)tropane (WIN 35,428), were compared quantitatively with the combinations of cocaine with the N-butyl,4Ј,4Љ-diF benztropine analog, 3-(bis(4-fluorophenyl)methoxy)-8-butyl-8-azabicyclo[3.2.1]octane (JHW 007), to determine whether their effects on DA levels in the shell of the nucleus accumbens (NAC) in mice differed. Each of the drugs alone produced dose-related elevations in NAC DA levels. In contrast to the other drugs, JHW 007 was less effective, producing maximal effects that approached 400% of control versus ϳ700% with the other drugs. In addition, the JHW 007 dose-effect curve was not as steep as those for cocaine and WIN 35,428. Combinations of cocaine with its analog, WIN 35,428, were most often greater than those predicted based on dose additivity. In contrast, combinations of cocaine with JHW 007 were most often subadditive. This outcome is consistent with recent studies suggesting that structurally divergent DA uptake inhibitors bind to different domains of the DAT, which can result in different DAT conformations. The conformational changes occurring with JHW 007 binding may result in functional outcomes that alter its abuse liability and its effects in combination with cocaine.

The endocannabinoid system has been implicated in the development of synaptic plasticity induced by several drugs abused by humans, including cocaine. However, there remains some debate about the involvement of cannabinoid receptors/ligands in cocaine-induced plasticity and corresponding behavioral actions. Here we show that a single cocaine injection in Swiss-Webster mice produces behavioral and neurochemical alterations that are under the control of the endocannabinoid system. This plasticity may be the initial basis for changes in brain processes leading from recreational use of cocaine to its abuse and ultimately to dependence. Locomotor activity was monitored with photo-beam cell detectors, and accumbens shell/core microdialysate DA levels were monitored by HPLC with electrochemical detection. Development of single-trial cocaine-induced behavioral sensitization, measured as increased distance traveled in sensitized mice compared to control mice, was paralleled by a larger stimulation of extracellular dopamine (DA) levels in the core but not the shell of the nucleus accumbens. Both the behavioral and neurochemical effects were reversed by CB1 receptor blockade produced by rimonabant pretreatments. Further, both behavioral and neurochemical cocaine sensitization were facilitated by pharmacological blockade of endocannabinoid metabolism, achieved by inhibiting the fatty acid amide hydrolase enzyme. In conclusion, our results suggest that a single unconditioned exposure to cocaine produces sensitization through neuronal alterations that require regionally specific release of endocannabinoids. Further, the present results suggest that endocannabinoids play a primary role from the earliest stage of cocaine use, mediating the inception of long-term brain-adaptive responses, shaping central pathways, and likely increasing vulnerability to stimulant abuse disorders.

RationalePrevious studies have shown that blockade of metabotropic glutamate 5 receptors (mGluR5) results in inhibition of nicotine self-administration in experimental animals. However, these studies have not established the behavioural mechanisms which mediate these effects or the extent to which the effects of mGluR5 antagonism on nicotine self-administration reflect a selective attenuation of nicotine reinforcement.ObjectivesTo investigate the effects of antagonising mGluR5 receptors on psychopharmacological responses to nicotine measured using conditioned and unconditioned behaviours.Results2-Methyl-6-(phenylethynyl)-pyridine (MPEP) significantly (P < 0.01) reduced nicotine self-administration and attenuated (P < 0.01) the ability of non-contingent nicotine to enhance the reinforcing properties of a weak reinforcer (extinguishing the house light in an operant chamber). It also attenuated (P < 0.05) the much lower levels of responding for this reinforcer measured in control animals treated with saline. MPEP did not attenuate the increase in locomotor activity induced by acute and repeated nicotine in animals habituated on the test day to the test environment. Furthermore, it had no significant effects on responding for a palatable food reward. By contrast, MPEP significantly reduced (P < 0.001) conditioned locomotor stimulation evoked by pairing nicotine with a specific environment.ConclusionThe results are consistent with the hypothesis that mGluR5 receptors play an important role in mediating the effects of contextual cues in conditioned behavioural responses to nicotine.

Background Subjective effects related to cocaine abuse are primarily mediated by blockade of the dopamine (DA) transporter (DAT). The present study assessed the hypothesis that different conformational equilibria of the DAT regulate differences in extracellular DA induced by structurally diverse DA uptake inhibitors (DUI) and their cocaine-like subjective effects. Methods The relationship between cocaine-like subjective effects and stimulation of mesolimbic-DA levels by standard-DUIs (cocaine, methylphenidate, WIN35,428), and atypical-DUIs (benztropine analogs: AHN1-055, AHN2-005, JHW-007) was investigated using cocaine-discrimination and DA-microdialysis procedures in rats. Results All drugs stimulated DA-levels showing different time-courses and maximal effects. Standard-DUIs, which preferentially bind to the outward-facing DAT-conformation, fully substituted for cocaine, consistently producing those subjective effects at DA levels of 100-125% over basal values, regardless of dose or pretreatment time. The atypical-DUIs, with DAT binding minimally affected by DAT conformation, produced inconsistent cocaine-like subjective effects. Full effects were obtained, if at all, only at a few doses and pretreatment times, and at DA-levels 600-700% greater than basal values. Importantly, the linear, time-independent, relationship between cocaine-like subjective effects and stimulation of DA-levels, obtained with standard DUIs was not obtained with the atypical-DUIs. Conclusions These results suggest a time-related desensitization process underlying the reduced cocaine subjective effects of atypical-DUIs that may be differentially induced by the binding modalities identified using molecular approaches. Since the DAT is the target of several drugs for treating neuropsychiatric disorders, such as ADHD, these results help to identify safe and effective medications with minimal cocaine-like subjective effects that contribute to abuse liability.

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Rationale Dopamine transporter (DAT) conformation plays a role in the effectiveness of cocaine-like and other DAT-inhibitors. Cocaine-like stimulants are intolerant to DAT conformation changes having decreased potency in cells transfected with DAT constructs that face the cytosol compared to wild-type DAT. In contrast, analogs of benztropine (BZT) are among compounds that are less affected by DAT conformational change. Methods We compared the displacement of radioligand binding to various mammalian CNS sites, acute stimulation of accumbens shell dopamine levels, and place-conditioning in rats among cocaine and four BZT analogs with Cl-substitutions on the diphenyl-ether system including two with carboalkoxy substitutions at the 2-position of the tropane ring. Results Binding assays confirmed high-affinity and selectivity for the DAT with the BZT analogs which also produced significant stimulation of mesolimbic dopamine efflux. Because BZT analogs produced temporal patterns of extracellular dopamine levels different from those by cocaine (3-10 mg/kg, IP), the place conditioning produced by BZT analogs and cocaine was compared at doses and times at which both the increase in dopamine levels and rates of increase were similar to those produced by an effective dose of cocaine. Despite this equilibration, none of the BZT analogs tested produced significant place conditioning. Conclusions The present results extend previous findings suggesting that cocaine-like actions are dependent on a binding equilibrium that favors the outward conformational state of the DAT. In contrast BZT analogs with reduced dependence on DAT conformation have reduced cocaine-like behavioral effects and may prove useful in development of medications for stimulant abuse.

Rationale The rewarding effects of alcohol have been attributed to interactions between opioid and dopaminergic system within the mesolimbic reward pathway. We have previously shown that ablation of β-arrestin 2 (Arrb2), a crucial regulator of μ-opioid receptor function, attenuates alcohol-induced hyperlocomotion and c-fos activation in the nucleus accumbens. Objectives Here, we further investigated the role of Arrb2 in modulating alcohol-induced dopamine (DA) release and conditioned place preference (CPP). We also assessed the functional importance of Arrb2 for μ-opioid receptor surface expression and signaling following an acute alcohol challenge. Methods Alcohol-evoked (0.375, 0.75 and 1.5 g/kg intraperitoneally, i.p.) DA release was measured by in vivo microdialysis in the shell of nucleus accumbens. Reward was assessed by the CPP paradigm. Receptor function was assessed by μ-receptor binding and [35S]GTP-γ-S autoradiography. Results In Arrb2 knockout mice accumbal DA levels reach maximum response at a lower dose compared to wild-type (wt) animals. In line with these results, Arrb2 knockout mice display increased CPP for alcohol as compared to wt mice. Finally, Arrb2 mutant mice display increased μ-opioid receptor signaling in the ventral and dorsal striatum and amygdala in response to a low dose of alcohol, indicating impaired desensitization mechanisms in these mice. Conclusions Our results show that Arrb2 modulates the response to low doses of alcohol on various levels including μ-opioid receptor signaling, DA release, and reward. They also reveal a clear dissociation between the effects of Arrb2 on psychomotor and reward behaviors.

Repeated psychostimulant exposure produces behavioral sensitization resulting from synaptic plasticity which may play a role in addiction. It has been suggested a role for the endocannabinoid system in synaptic plasticity, but little is known about its role in psychostimulant‐induced behavioral effects. We examined the involvement of CB1 cannabinoid receptors, using SR141716A (SR) a selective CB1 antagonist, on cocaine‐induced locomotor activity in Swiss Webster mice exposed 24 hr prior to saline or cocaine (20 mg/kg). Mice were tested in chambers equipped with motion‐sensitive detectors. A single cocaine injection 24 hr before the test produced increases in cocaine effects on locomotor activity, compared to saline 24 hr prior. SR (0.3–3.0 mg/kg i.p.) 40 min before cocaine dose‐dependently blocked the pretreatment effect of cocaine. A trend toward significance was also found when SR 1 mg/kg was administered in naïve mice 40 min before 10 or 20 mg/kg cocaine. The results suggest that endogenous cannabinoids released by cocaine might play a role in the development of behavioral sensitization induced by single cocaine pre‐exposure in mice. Since behavioral sensitization might play a role in the long‐term behavioral effects of cocaine, this study provides further evidence that the cannabinoid system could be an important target in the search for medications to treat cocaine abuse. Supported by NIDA/NIH/DHHS