Differences in corticosterone level due to inter-food interval length: implications for schedule-induced polydipsia

The purpose of this study was to determine the effects of different food-reinforcement schedules on plasma corticosterone (CORT), and its possible involvement in the acquisition and maintenance of schedule-induced polydipsia (SIP). In Experiment 1, three groups of rats were submitted to two different fixed-interval (FI) schedules with inter-food intervals of 30 and 120 s, and to a massed-feeding presentation for 40 days until SIP was well stabilized. In Experiment 2, six groups of rats were exposed to the same schedules, FI 30s and FI 120s, and to the massed-feeding condition, but no water bottles were presented. CORT levels were determined on Days 3 and 40. Results of Experiment 1 indicated that FI 30s schedule, but not FI 120s or the massed-feeding condition, induces excessive drinking from Day 3. Results in Experiment 2 indicated that CORT levels were similar for all the groups on Day 3. However, only animals on the FI 30s schedule did increase their CORT levels on Day 40, with no variation in the hormone in the other two conditions, FI 120s and massed-feeding presentations. The data are discussed in terms of the implications of these results for hypotheses of SIP as anxiolitic behavior.