Sex differences in the behavioral effects of 24-h/day access to cocaine under a discrete trial procedure

Although more men than women are addicted to cocaine, it has been suggested that women may have an accelerated transition to addiction, and that once addicted they may be more vulnerable to relapse. Here we investigate the effects of extended access to cocaine under a 24-h/day discrete trial procedure on patterns of intake and subsequent motivation to use cocaine as assessed by responding under a progressive-ratio schedule in male and female rats. Rats were initially trained to self-administer cocaine (1.5 mg/kg/infusion) under a fixed-ratio 1 schedule until acquisition occurred, and then responding was assessed under a progressive schedule for three sessions. Subsequently, rats had 24-h access to intravenous cocaine infusions (1.5 mg/kg) that were available in discrete trials (4, 10 min trials/h) for 7 consecutive days. At 10 days after the last discrete trial session, responding was reassessed under a progressive-ratio schedule for three additional sessions to investigate changes in motivation to obtain cocaine. Prior to cocaine self-administration under the 24-h access discrete trial procedure, males and females did not differ on cocaine self-administration under the fixed-ratio or progressive-ratio schedules. However, sex differences emerged under the 24-h access discrete trial procedure with females self-administering higher levels of cocaine, for longer initial periods of time, and showing a greater disruption in the diurnal control over intake than did males. Additionally, following a 10-day forced abstinence period, females responded at higher levels under the progressive-ratio schedule to obtain cocaine infusions than did males. These findings suggest that extended access to cocaine under the discrete trial cocaine self-administration procedure produces sex-dependent patterns of intake and sex-specific changes in motivation to obtain cocaine as measured by progressive-ratio responding.