Pituitary adenylate cyclase-activating polypeptide induces period1 and period2 gene expression in the rat suprachiasmatic nucleus during late night

The suprachiasmatic nucleus generates circadian rhythms which are synchronized to the environmental light–dark cycle via the retinohypothalamic tract. Pituitary adenylate cyclase-activating polypeptide and glutamate, two neurotransmitters co-stored in the retinohypothalamic tract of the rat, are able to phase shift the endogenous rhythm similar to light. The “clock genes” period1 ( per1) and per2, which show circadian oscillation within the suprachiasmatic nucleus, have been attributed a role in light-induced resetting of the mammalian circadian clock due to rapid induction of the period ( per) genes after light stimulation at night. Using a rat in vitro brain slice model, we demonstrate by quantitative in situ hybridization histochemistry that the diurnal alteration in expression of both per genes in the suprachiasmatic nucleus was retained in vitro. In the model, we examined the effects of pituitary adenylate cyclase-activating polypeptide and glutamate alone and in combination on per1 and per2 gene expression at late subjective night (circadian time 19). Glutamate administration (10 −3 M) induced both per1 and per2 gene expression in the suprachiasmatic nucleus of the brain slice within 1 h. The per gene responses were similar to the induction of gene expression observed after light stimulation in vivo at late night. Pituitary adenylate cyclase-activating polypeptide (10 −6 M) administered alone had no effect on the per gene expression, but when pituitary adenylate cyclase-activating polypeptide in micromolar concentration was applied before glutamate, the neuropeptide blocked the glutamate-induced per1 and per2 gene expression in the suprachiasmatic nucleus. In contrast to the lack of effect of pituitary adenylate cyclase-activating polypeptide itself in micromolar concentration, pituitary adenylate cyclase-activating polypeptide (10 −9 M) induced both per1 and per2 gene expression, an effect which was not augmented by co-application of glutamate. Our results provide the molecular substrate for the previous electrophysiological findings that pituitary adenylate cyclase-activating polypeptide in high concentration is able to block glutamate-induced phase advance at late night, and that the peptide in low concentration can induce a phase advance similar to light and glutamate.