Effects of chronic cocaine on impulsivity: relation to cortical serotonin mechanisms

Drug addiction can be considered an impulse control disorder in that addicts exhibit increased impulsivity on both behavioural and self-report measures. We investigated whether chronic cocaine affects delay of gratification and/or behavioural disinhibition in rats using the delayed reinforcement and Go/No-go paradigms. Animals were treated with saline or cocaine (15 mg/kg) three times per day for 14 days; all behavioural tests occurred prior to daily injections. To assess the effectiveness of the cocaine treatment, sucrose intake, behavioural sensitization and serotonin (5-HT)-dependent (dorsal raphe-stimulated) cortical activation were also measured. Chronic cocaine caused a transient (days 7-8) increase in impulsivity in the delayed reinforcement paradigm, but did not influence behaviour in the Go/No-go paradigm. As expected, chronic cocaine increased behavioural sensitization scores, although it did not affect sucrose consumption. Although, cocaine treatment did not affect dorsal raphe-stimulated clectrocorticographic activation, the serotonergic receptor antagonist methiothepin (0.1 mg/kg) was more effective in blocking cortical activation in cocaine- than in saline-treated animals. The electrocorticographic changes may be the result of a pre-synaptic 5-HT deficit and the compensatory supersensitivity of post-synaptic 5-HT receptors. Given the differential time courses of the behavioural and electrocorticographic data, however, this change probably does not mediate the effects of chronic cocaine in the delayed reinforcement paradigm. (C) 2003 Elsevier B.V. All rights reserved.