Previous research has identified the nucleus accumbens (NAcb) as an important brain region underlying inter-individual variation in impulsive behavior. Such variation has been linked to decreased dopamine (DA) D2/3 receptor availability in the ventral striatum of rats exhibiting spontaneously high levels of impulsivity on a 5-choice serial reaction time (5-CSRT) test of sustained visual attention. This study investigated the involvement of DA D2/3 receptors in the NAcb core (NAcbC) and the NAcb shell (NAcbS) in impulsivity. We investigated the effects of a DA D2/3 receptor antagonist (nafadotride) and a DA D2/3 partial agonist (aripiprazole) infused directly into either the NAcbC or NAcbS of rats selected for high (HI) and low (LI) impulsivity on the 5-CSRT task. Nafadotride increased significantly the level of impulsivity when infused into the NAcbS, but decreased impulsivity when infused into the NAcbC of HI rats. By contrast, intraNAcb microinfusions of aripiprazole did not affect impulsivity. Systemic administration of nafadotride had no effect on impulsive behavior but increased the number of omissions and correct response latencies, whereas systemic injections of aripiprazole decreased impulsive and perseverative behavior, and increased the number of omissions and correct response latencies. These findings indicate an opponent modulation of impulsive behavior by DA D2/3 receptors in the NAcbS and NAcbC. Such divergent roles may have relevance for the etiology and treatment of clinical disorders of behavioral control, including attention-deficit hyperactivity disorder and drug addiction.

Although high impulsivity on the 5-CSRTT has been shown to predict loss of control over cocaine intake, this does not generalize to a loss of control over heroin self-administration. These findings suggest important distinctions in vulnerability mechanisms underlying cocaine and heroin addiction with trait-like impulsivity playing a role in stimulant but not opiate addiction.

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Concomitant consumption of caffeine with recreational psychostimulant drugs of abuse can provoke severe acute adverse reactions in addition to longer term consequences. The mechanisms by which caffeine increases the toxicity of psychostimulants include changes in body temperature regulation, cardiotoxicity and lowering of the seizure threshold. Caffeine also influences the stimulatory, discriminative and reinforcing effects of psychostimulant drugs. In this review, we consider our current understanding of such caffeine-related drug interactions, placing a particular emphasis on an adverse interaction between caffeine and the substituted amphetamine, 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy'), which has been most recently described and characterized. Co-administration of caffeine profoundly enhances the acute toxicity of MDMA in rats, as manifested by high core body temperature, tachycardia and increased mortality. In addition, co-administration of caffeine enhances the long-term serotonergic neurotoxicity induced by MDMA. Observations to date support an interactive model of drug-induced toxicity comprising MDMA-related enhancement of dopamine release coupled to a caffeine-mediated antagonism of adenosine receptors in addition to inhibition of PDE. These experiments are reviewed together with reports of caffeine-related drug interactions with cocaine, d-amphetamine and ephedrine where similar mechanisms are implicated. Understanding the underlying mechanisms will guide appropriate intervention strategies for the management of severe reactions and potential for increased drug-related toxicity, resulting from concomitant caffeine consumption. AbbreviationsBAT, brown adipose tissue; BP, blood pressure; CPP, conditioned place preference; CREB, cAMP response element binding protein; CVS, cardiovascular system; CYP, cytochrome P450 enzyme; DARPP-32, cAMP-regulated phosphoprotein of 32 kDa; DPCPX, 8-cyclopentyl-1,3-dipropylxanthine; MDA, methylenedioxyamphetamine; MDMA, 3,4-methylenedioxymethamphetamine; PDE, phosphodiesterase; UCP, uncoupling protein IntroductionIn recreational drug use settings, caffeine is found in a range of commercially available products such as energy drinks that may be consumed with other drugs (Reissig et al., 2009). In addition, forensic analyses of seized illicit drug samples have reported quantities of caffeine mixed with other stimulants, including cocaine and amphetamines (Cole et al., 2011). BJPBritish Journal of Pharmacology DOI:10.1111DOI:10. /j.1476DOI:10. -5381.2012 946 British Journal of Pharmacology (2012) Such combinations may influence the toxicity of these stimulants and, consequently, investigations regarding the potential for adverse interactive effects are warranted. Interactive effects have led to concern that caffeine could have a role to play in the acute, often idiosyncratic, and long-term adverse consequences associated with the consumption of a range of psychostimulant drugs, including some agents in clinical use. Risks associated with caffeine use in recreatio...

Background and purpose: Caffeine exacerbates the hyperthermia associated with an acute exposure to 3,4 methylenedioxymethamphetamine (MDMA, 'Ecstasy') in rats. The present study investigated the mechanisms mediating this interaction. Experimental approach: Adult male Sprague-Dawley rats were treated with caffeine (10 mg·kg ) exacerbated MDMA-induced hyperthermia. Conclusions and implications:A mechanism comprising 5-HT and catecholamines is proposed to mediate MDMA-induced hyperthermia. A combination of adenosine A2A receptor antagonism and PDE inhibition can account for the exacerbation of MDMA-induced hyperthermia by caffeine.

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