Two types of VIP neuronal components in rat suprachiasmatic nucleus

Vasoactive intestinal peptide (VIP) neurons constitute a large group in the suprachiasmatic nucleus (SCN) and it is thought that they are involved in the generation and entrainment of circadian rhythm. We have characterized these VIP-expressing neurons in rat SCN by their ability to induce the mammalian Period1 (Per1) gene in response to light exposure, innervation of retinal afferents, day-night variations in VIP mRNA, and coexpression of gastrin releasing peptide (GRP). VIP neurons in the ventrolateral SCN (SCNVL) were subdivided into two groups, light-evoked Per1-inducible main SCNVL (SCNVLmain) and non-Per1-inducible medially located SCNVL (SCNVLmed). Retinal innervation was abundant in the SCNVLmain but nearly absent in the SCNVLmed. Day-night variation in VIP mRNA expression level was observed in the SCNVLmain but not in the SCNVLmed. GRP mRNA was seen in rarely SCNVLmed but abundant in SCNVLmain, where some neurons coexpressed VIP mRNA. These findings indicate that VIP neurons in the SCN can be divided into two topographically and functionally distinct groups.