Sex differences in the acquisition of IV methamphetamine self-administration and subsequent maintenance under a progressive ratio schedule in rats
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- Roth, M.E. & Carroll, M.E. Psychopharmacology (2004) 172: 443. doi:10.1007/s00213-003-1670-0
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Previous work indicates that female rats initiate cocaine use sooner than male rats and reach significantly higher break points (BPs) for a single injection of cocaine under a progressive ratio (PR) schedule compared to male rats.
The present study extends previous work examining sex differences to the acquisition of methamphetamine (METH) (0.02 mg/kg) and maintenance of METH-maintained responding under a PR schedule.
An automated priming procedure that has previously been shown to be sensitive to sex differences was used for the acquisition of drug self-administration. A PR schedule that has been shown to be sensitive in detecting sex differences in maintenance levels of cocaine-reinforced responding was used for the maintenance phase of the experiment.
A greater percentage of female rats met the acquisition criterion for METH (0.02 mg/kg) self-administration compared to male rats (55.6% versus 11.1%, respectively), and they did so at a significantly faster rate. Under stable fixed-ratio 1 (FR1) conditions (after acquisition and 5 days before the PR schedule) female rats responded for significantly more METH (0.02 mg/kg) infusions compared to males. Dose-response curves obtained under the PR schedule during maintenance indicated that female rats self-administered significantly more METH infusions compared to male rats.
These data suggest that female rats are more vulnerable to the acquisition of METH self-administration, and they are more motivated to self-administer METH compared to male rats under a PR schedule during the maintenance phase.