Phase-shifting effects of pituitary adenylate cyclase activating polypeptide on hamster wheel-running rhythms.

Pituitary adenylate cyclase activating polypeptide (PACAP38) is a putative neurochemical of the main retinal input to the mammalian circadian pacemaker housed in the suprachiasmatic nucleus (SCN). We assessed the phase-resetting effects of microinjection of PACAP38 into the SCN region on hamster wheel-running rhythms. When administered during the middle of the subjective day, PACAP38 evoked large but transient phase advances ( approximately 60 min), that were followed by small, steady-state phase delays. During the early subjective night, PACAP38 elicited small to moderate phase delays without any detectable concentration-dependence. Late in the subjective night, PACAP38 had no significant effects. Saline microinjection had no effect at any phase tested. These findings show that PACAP38 has small to moderate effects on the phase of the hamster SCN circadian pacemaker, including significant phase delays early in the subjective night.