Fixed-ratio schedules of oral ethanol self-administration in inbred mouse strains

Previous studies of ethanol reinforcement in BALB/cJ and C57BL/6J mice have shown that over a range of concentrations oral ethanol appeared to serve as a reinforcer only for the C57BL/6J mice. In the previous studies BALB/cJ mice maintained rates of responding for ethanol that only slightly exceeded the rates maintained by the vehicle, water. However, the quantity of ethanol consumed with the continuous reinforcement schedule (fixed ratio one) may have led to pharmacologically significant effects, given the high sensitivity to ethanol of this genotype. The present study tested whether and to what extent ethanol would maintain responding under increasing fixed ratio size in these two strains of mice at ethanol concentrations of 0%, 8%, and 16% (w/v). For the C57BL/6J mice, as fixed-ratio size increased from 1 to 2, 4, and 8, there were almost directly proportional increases in response rate at ethanol concentrations of 8% and 16% (w/v), but not at 0%. Post-session blood ethanol levels confirmed intake of pharmacologically significant quantities. The volume consumed per unit of body weight decreased as fixed-ratio size increased. For the BALB/cJ mice, at no condition did ethanol maintain responding at levels that significantly exceeded vehicle maintained responding. BALB/cJ mice did not differ from C57BL/6J mice as fixed-ratio size was increased during vehicle conditions. These results, along with earlier findings, demonstrate that ethanol can serve as a reinforcer for C57BL/6J mice but not in BALB/cJ mice over a range of schedule conditions. They further support the conclusion that genotype is an important determinant of ethanol reinforced behavior.