CIRCADIAN REGULATION OF C-FOS EXPRESSION IN THE SUPRACHIASMATIC PACEMAKER BY LIGHT

The primary objective of this research was to examine expression of th e 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 effe ct on c-fos expression in the SCN, but only at critical times when lig ht is capable of phase-shifting circadian rhythms. This evidence for c orrelative relations between the effects of light signals in inducing c-fos gene expression in the SCN and modulating the circadian period o f the SCN pacemaker suggests that immediate-early genes may be compone nts of the signal transduction cascade by which light entrains circadi an rhythms. In addition, dual-immunostaining methods were utilized to examine neurochemical identity of SCN cells that exhibit this circadia n induction of c-fos expression in response to light. Within the ventr olateral SCN, the photic induction of Fos expression occurred in neuro ns expressing gastrin-releasing peptide (GRP), but not in those contai ning vasoactive intestinal polypeptide (VIP). This finding suggests th at SCN neurons containing GRP may be involved in the transduction of p hotic signals mediating circadian entrainment.