Induction of NGFI-A gene expression in the rat suprachiasmatic nucleus by photic stimulation

Photic induction of NGFI-A gene expression was investigated in the rat suprachiasmatic nucleus (SCN) using in situ hybridization histochemistry. Following light exposure for 30 min, NGFI-A mRNA appeared in the ventral portion of the rostral SCN, in the ventrolateral and in part of the dorsomedial portion at the middle level, and in the lateral portion of the caudal SCN. The distribution of NGFI-A mRNA was wider than that of c-fos mRNA which was confined to the ventrolateral portion at the middle level of the SCN. By double labeling in situ hybridization, approximately half of NGFI-A mRNA-positive cells in the SCN were shown to coexpress vasoactive intestinal peptide (VIP) mRNA, while 16% of cells positive for c-fos mRNA coexpressed VIP mRNA. These findings indicate that the broadness of NGFI-A mRNA and c-fos mRNA expression after photic stimulation are different. NGFI-A gene induced in these cells of the SCN including VIP neurons may be involved in circadian entrainment by light.