Differential effects on substance P immunoreactivity in rat suprachiasmatic and olivary pretectal nuclei

We examined the effects of constant dark or constant light on substance P (SP) and/or neuropeptide Y (NPY) immunoreactive fibres in the rat suprachiasmatic nucleus (SCN) and olivary pretectal nucleus (OPT) by immunohistochemistry. After constant dark, SP immunoreactive fibres and terminals decreased slightly in the SCN, while they increased markedly in the OPT. After constant light, they increased markedly in the SCN, but were little changed in the OPT. NPY immunoreactive fibres and terminals in the SCN decreased slightly after after constant light, but there were no effects on these fibres after constant dark. These findings suggest that SP immunoreactive fibres are involved in mediating illumination discrimination in the SCN and/or OPT.