Effects of the GluN2B antagonist, Ro 25-6981, on extinction consolidation following adolescent- or adult-onset methamphetamine self-administration in male and female rats

Previous work suggests adolescent rats have deficient extinction consolidation relative to adults. Although the mechanisms underlying this age difference are currently unknown, studies in adult rats have implicated GluN2B-containing NMDA receptor function in extinction consolidation of drug-associated memory. Importantly, GluN2B neurotransmission emerges during adolescent development, and drugs of abuse during adolescence may delay the development of extinction consolidation by disrupting the ontogeny of GluN2B function. Here, we trained Sprague-Dawley rats of both sexes to self-administer methamphetamine (METH, 0.1 mg/kg/infusion i.v.) beginning during adolescence [postnatal (P) day 41] or adulthood (P91). Rats were given short access (2 h) to self-administer METH in seven daily sessions followed by fourteen sessions with long access (6 h). Subsequently, rats underwent four daily 30-min extinction sessions with immediate post-session injections of either a GluN2B antagonist (Ro25-6981; 6 mg/kg, i.p.) or a vehicle solution. After four daily 2-h extinction sessions, a priming injection (1 mg/kg METH, i.p.) was given prior to a final 2-h reinstatement session. During LgA, adolescent-onset rats earn more METH than adult-onset rats and display greater drug-loading behavior. Rats reduced their drug-seeking behavior across extinction sessions, with no significant group differences. Rats reinstated drug-seeking following the METH priming injection, with females displaying greater reinstatement than males. These results do not support our a priori hypothesis that adolescent-onset METH use disrupts the ontogeny of GluN2B transmission and contributes to age-of-onset differences in extinction of METH-seeking. However, our findings suggest that age-of-onset contributes to excessive METH-taking, while sex confers vulnerability to relapse to METH-seeking.