Light selectively alters vasoactive intestinal peptide and peptide histidine isoleucine immunoreactivity within the rat suprachiasmatic nucleus.

The concentration of vasoactive intestinal peptide (VIP)-, peptide histidine isoleucine (PHI)-, neurotensin (NT)- and substance P (SP)-like immunoreactivity (LI) within the suprachiasmatic nucleus (SCN) were determined by radioimmunoassay in rats housed in LD 14:10 h, constant light or constant dark. No day-night differences were observed in the concentration of VIP-, PHI-, NT- or SP-LI within the SCN. Exposure to constant light significantly depressed the SCN concentrations of VIP- and PHI-LI, but had no significant effects on SCN concentrations of NT- or SP-LI, or VIP- or PHI-LI concentrations within the cortex. These data represent the first evidence that VIP/PHI-containing neurons may be involved in mediating photic information within the SCN.