Schedule-induced polydipsia in rats with gastric fistulas

We have investigated the development and maintenance of schedule-induced polydipsia (SIP) when ingested water (and food) was allowed to drain from the stomach. Fourteen male Long-Evans rats were prepared with permanent gastric cannulas and, after recovery, their body weight was reduced to 80%. Water intake was measured, with cannulas open or closed, during 42 daily 1-h sessions in which 45-mg food pellets were delivered one per minute. Allowing ingested material to drain from the stomach impaired the development of SIP and reduced the polydipsia in rats in which SIP had already been established. In contrast, opening the gastric fistulas increased drinking in these rats when all the food pellets were provided at once or after water deprivation. The opposite effects of gastric drainage on dehydration and schedule-induced drinking is consistent with the view that SIP is not a fluid-regulating phenomenon. It is not clear, however, how these unexpected findings fit current hypotheses to explain SIP that are based on oral, neural excitatory, or emotional mechanisms.