Schedule-induced behavior in rats: Pellets versus powder

Abstract

Food-deprived rats develop polydipsia on an intermittent schedule (fixed time 60 sec) of food pellet delivery, but not on an identical schedule offood powder delivery. This result was demonstrated with separate groups receiving each type of food and was replicated using rats as their own controls. Powdered food not only prevented the development of polydipsia, but it abruptly terminated ongoing polydipsia in rats that were switched from the scheduled delivery of pellets to powder. Ethological analysis of the behavior showed that the rats receiving powder were not engaging excessively in some behavior other than drinking. After discounting several factors, we concluded that the amount of oral activity associated with feeding, which occurred immediately after food delivery, was reciprocally related to the level of drinking.In a review of schedule-induced behavior in general, Roper (1981) noted that schedule-induced polydipsia may be a relatively rare phenomenon. This conjecture was based on the observation that few other adjunctive behaviors have been demonstrated to be persistent and excessive relative to massed-food and no-food control conditions. This being the case, Roper suggested that an explanation of schedule-induced polydipsia should focus on the interaction between feeding and drinking.In preliminary testing, we measured schedule-induced polydipsia under a variety of conditions, one of these being to present dry powder prepared by grinding food pellets. When the powder was delivered on a fixed time (FT) 6O-secschedule, hungry rats failed to develop polydipsia. Moreover, rats that were already polydipsic, as a consequence of pellet training, abruptly lost their polydipsia when they were switched from pellets to powder. These results were somewhat surprising because the acquisition and maintenance of schedule-induced polydipsia is conceived of as being primarily dependent on food deprivation, amount of food per reinforcement, and schedule of reinforcement (Falk, 1971). In fact, scheduleinduced polydipsia is known for its robust nature and persistence under a variety of other conditions (Shearon & Allen, 1984;Wetherington, 1982). Consequently, we decided to pursue the issue, hoping that in the course of discovering why little drinking occurs in hungry rats fed powder on a schedule (powder rats), we might have a better understanding of why polydipsia occurs in hungry rats fed pellets on a schedule (pellet rats). 49In Experiment 1, we demonstrated the basic phenomenon that powder rats on an FT 6O-secschedule do not develop polydipsia, whereas pellet rats do. In Experiment 2, we buttressed the replication of the effect with the addition of massed food and omitted food control conditions for the assessment of polydipsia. In addition, we showed that changing the food from pellets to powder, and vice versa, changed the drinking behavior, in accordance with our preliminary observation. By utilizing several test and control conditions, we found that a number of factors can be discounted as contributing to the absenc...