Captopril alters schedule induced polydipsia, urination, and defecation in rats

Schedule induced polydipsia, urination and defecation were examined in rats that received training on a fixed interval 2 min schedule of food reinforcement. In Phase I of the experiment, animals received peripheral injections of captopril (an angiotensin conversion enzyme blocker, 0.5 or 50 mg/kg), or equivalent volumes of 0.9% saline. The results showed that low doses of captopril (0.5 mg/kg) significantly increased both operant responding and the adjunctive behaviors. High peripheral doses of captopril significantly reduced responding and schedule induced behavior. In Phase II of the experiment, animals received either low peripheral doses of captopril (sc 0.5 mg/kg), or low doses that were coupled with central injections (i.e., 0.12 mg icv + 0.5 mg/kg sc). As observed in Phase I, low peripheral doses of captopril enhanced behavior, but the enhancement effect was eliminated with low (0.12 mg) central administration. The overall results are consistent with past research examining captopril effects on non-operant, meal-induced drinking. Yet since captopril affected operant responding and adjunctive behaviors similarly, the findings suggest that angiotensin plays a common role in the motivational processes that precede and follow the arrival of food.