No evidence that wheel-running exercise impacts cocaine conditioned place preference in male C57BL/6J mice

Intermittent voluntary wheel running promotes resilience to the negative consequences of repeated social defeat in mice

A novel approach to reduce the incidence of substance use disorders is to promote resilience to stress using environmental resources such as physical exercise. In the present study we test the hypothesis that Voluntary Wheel Running (VWR) during adolescence blocks the negative consequences of stress induced by intermittent repeated social defeat (IRSD). Four groups of adolescent male C57BL/6 mice were employed in the experiment; two groups were exposed to VWR (1 h, 3 days/week) from postnatal day (PND) 21 until the first social defeat (PND 47), while the remaining two groups did not have access to activity wheels (controls). On PND 47, 50, 53 and 56 mice, who had performed VWR, were exposed to an episode of social defeat by a resident aggressive mouse (VWR+IRSD group) or allowed to explore an empty cage (VWR+EXPL group). The same procedure was performed with control mice that had not undergone VWR (CONTROL+IRSD and CONTROL+EXPL groups). On PND 57, all the mice performed the Elevated Plus Maze (EPM), Hole-Board, Social Interaction, Tail Suspension and Splash tests. After an interval of 3 weeks, all mice underwent a conditioned place preference (CPP) procedure with 1 mg/kg of cocaine. Exposure to VWR prevented the negative consequences of social stress in the EPM, splash test and CPP, since the VWR+IRSD group did not display anxiety- or depression-like effects or the potentiation of cocaine reward observed in the Control+IRSD group. Our results support the idea that physical exercise promotes resilience to stress and represents an excellent target in drug abuse prevention.

A three-compartment apparatus alters the brain concentration of cytokines and neurotrophic factors in cocaine-induced CPP in mice

Cocaine-induced neuroinflammation plays an important role in the pathophysiology of drug addiction. Evidence suggests that the immune response contributes for memory consolidation related to place preference behavior underlying cocaine administration in mice. Conditioned place preference (CPP) is a protocol extensively used to study the rewarding and/or aversive motivational effects of drug abuse in rodents, reproducing cocaine-seeking behavior in humans. Besides the variety of apparatus used in the CPP protocol, whether different types of apparatus are able to induce the same conditioned behavior response and neurobiological changes remains to be fully explored. We hypothesize that the immune response is involved in the cocaine-induced CPP and that the type of apparatus might influence this response. Herein, two- and three-compartment apparatuses were tested using the behavioral model of CPP. Cocaine-induced CPP was demonstrated in both apparatuses. However, mice injected with cocaine had decreased levels of IL-1β, IL-6, IL-10, and GDNF in the pre-frontal cortex, and decreased CX3CL1 in the striatum, in the CPP protocol using three compartments compared to controls. While similar levels were seen in the CPP protocol using two compartments. In conclusion, the current study demonstrated that the type of apparatus might influence the investigation of neurobiological mechanisms associated with cocaine-induced CPP. Our data also suggest that the three compartment-apparatus seems to be a more appropriate model to investigate the neuroinflammatory response related to cocaine addiction.

Repeated exposure to cocaine during adolescence enhances the rewarding threshold for cocaine-conditioned place preference in adulthood

Previous studies have shown that adolescent exposure to cocaine increases drug use in adulthood, albeit incubation of cocaine seeking was found to be attenuated in rats trained to self-administer cocaine during adolescence. We here hypothesize that adolescent exposure to cocaine could alter the rewarding properties of the psychostimulant in adulthood. By employing two of the most widely used animal-experimental-preclinical models to investigate drug addiction, we evaluated whether contingent versus non-contingent cocaine self-administration during adolescence modulates its rewarding threshold in adulthood evaluated by conditioned place preference (CPP). Cocaine self-administration during adolescence increases the rewarding threshold in adulthood; CPP for cocaine was observed at the higher (20 mg/kg), but not at the lower (10 mg/kg), dose employed. Rats exposed to either contingent or non-contingent cocaine during adolescence exhibited the same behavior in the CPP paradigm suggesting that, under our experimental conditions, cocaine rewarding properties are shaped by the psychostimulant itself and not by its motivational effects. From a mechanistic standpoint, the preference for the 20 mg/kg cocaine-paired side in a CPP paradigm appears to depend, at least partially, upon the formation of GluA2-lacking Ca²⁺ -permeable AMPA receptors and the consequent increase of αCaMKII activity in the NAc, both of which are instead reduced when the 10 mg/kg dose was used. In conclusion, contingent or non-contingent cocaine exposure during adolescence desensitizes adult animals to a rewarding dose of cocaine (10 mg/kg) elevating the rewarding threshold necessary (20 mg/kg) to drive conditioned place preference, an effect that may predispose to higher consumption of cocaine during adulthood.

Therapeutic efficacy of environmental enrichment for substance use disorders

Addiction to drug and alcohol is regarded as a major health problem worldwide for which available treatments show limited effectiveness. The biggest challenge remains to enhance the capacities of interventions to reduce craving, prevent relapse and promote long-term recovery. New strategies to meet these challenges are being explored. Findings from preclinical work suggest that environmental enrichment (EE) holds therapeutic potential for the treatment of substance use disorders, as demonstrated in a number of animal models of drug abuse. The EE intervention introduced after drug exposure leads to attenuation of compulsive drug taking, attenuation of the rewarding (and reinforcing) effects of drugs, reductions in control of behavior by drug cues, and, very importantly, relapse prevention. Clinical work also suggests that multidimensional EE interventions (involving physical activity, social interaction, vocational training, recreational and community involvement) might produce similar therapeutic effects, if implemented continuously and rigorously. In this review we survey preclinical and clinical studies assessing the efficacy of EE as a behavioral intervention for substance use disorders and address related challenges. We also review work providing empirical evidence for EE-induced neuroplasticity within the mesocorticolimbic system that is believed to contribute to the seemingly therapeutic effects of EE on drug and alcohol-related behaviors.

Adult hippocampal neurogenesis as a target for cocaine addiction: a review of recent developments

Basic research in rodents has shown that adult hippocampal neurogenesis (AHN) plays a key role in neuropsychiatric disorders that compromise hippocampal functioning. The discovery that dependence-inducing drugs regulate AHN has led to escalating interest in the potential involvement of AHN in drug addiction over the last decade, with cocaine being one of the most frequently investigated drugs. This review argues that, unlike other drugs of abuse, preclinical studies do not, overall, support that cocaine induces a marked or persistent impairment in AHN. Nevertheless, experimental reduction of AHN consistently exacerbates vulnerability to cocaine. Interestingly, preliminary evidence suggests that, on the contrary, increasing AHN might help both to prevent and treat addiction.