Abstract
The primary objective of this research was to examine expression of the immediate–early gene c-fos within the suprachiasmatic nucleus (SCN) for evidence of circadian regulation by photic stimuli. In vivo and in vitro analyses demonstrate that photic signals have an inductive effect on c-fos expression in the SCN, but only at critical times when light is capable of phase-shifting circadian rhythms. This evidence for correlative relations between the effects of light signals in inducing c-fos gene expression in the SCN and modulating the circadian period of the SCN pacemaker suggests that immediate-early genes may be components of the signal transduction cascade by which light entrains circadian rhythms. In addition, dual-immunostaining methods were utilized to examine neurochemical identity of SCN cells that exhibit this circadian induction of c-fos expression in response to light. Within the ventrolateral SCN, the photic induction of Fos expression occurred in neurons expressing gastrin-releasing peptide (GRP), but not in those containing vasoactive intestinal polypeptide (VIP). This finding suggests that SCN neurons containing GRP may be involved in the transduction of photic signals mediating circadian entrainment.