Effects of naloxone, beta-endorphin and ACTH on acquisition of schedule-induced polydipsia.

A series of three experiments examined the possible involvement of endogenous opioid peptides in the development of schedule-induced polydipsia in rats. Repeated pretraining treatment with 2 mg/kg naloxone impaired acquisition of schedule-induced polydipsia, whereas the same treatment injected after training increased drinking. This later effect was time dependent, since a 30-min delay in the injection of naloxone resulted in a disappearance of its effect. Post-training injections of 10 micrograms/kg beta-endorphin or ACTH delayed the development of drinking. These findings are consistent with the hypothesis that endogenous opioid peptides modulate the development of schedule-induced polydipsia.