Effect of the lack of light impulses on the hypothalamic PACAP and C-fos immunoreactivities in rats.

Pituitary adenylate cyclase activating polypeptide (PACAP) is a member of the secretin family. It is widely distributed in the central and peripheral nervous systems. The highest concentration of PACAP was found in the hypothalamus. In the present work it has been studied whether PACAP is involved in the mediation of photic stimuli to the anterior pituitary gland. We have examined the effect of the lack of light impulses on the hypothalamic PACAP and C-fos immunoreactivities. In adult rats 10 days after the removal of the eyes (surgical enucleation) and in those received monosodium glutamate treatment neonatally (chemical enucleation). The PACAP immunostaining enhanced in the hypothalamic magnocellular nuclei and in the extemal zone of the median eminence. C-fos immunoreactivity also enhanced in a few hypothalamic nuclei 2 hours after the surgical enucleation indicating that the lack of light impulses activated hypothalamic neurons which, in turn, might stimulate the release of PACAP into the portal circulation. It has been concluded that PACAP may be involved in photoendocrine regulations.