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 pacemake r housed in the suprachiasmatic nucleus (SCN). We assessed the phase-resett ing effects of microinjection of PACAP38 into the SCN region on hamster whe el-running rhythms. When administered during the middle of the subjective d ay, PACAP38 evoked large but transient phase advances (similar to 60 min), that were followed by small, steady-state phase delays. During the early su bjective 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 pha se tested. These findings show that PACAP38 has small to moderate effects o n the phase of the hamster SCN circadian pacemaker, including significant p hase delays early in the subjective night. (C) 2001 Elsevier Science Irelan d Ltd. All rights reserved.