Gastrin releasing peptide in the rat suprachiasmatic nucleus: an immunohistochemical, chromatographic and radioimmunological study.

The localization of gastrin releasing peptide-immunoreactive neurons within the rat suprachiasmatic nucleus was investigated with the indirect peroxidase-antiperoxidase and avidin-biotin techniques. Gastrin releasing peptide-immunoreactive perikarya were found in the ventral part of the suprachiasmatic nucleus, which suggests a direct afferent innervation from the retina or the lateral geniculate, placing the gastrin releasing peptide neurons centrally in the regulation of circadian rhythmicity. Gastrin releasing peptide neurons gave rise to fibers and terminals within the suprachiasmatic nucleus, especially in the ventral part among the immunoreactive and non-immunoreactive neurons. The largest number of gastrin releasing peptide-immunoreactive axons leaving the suprachiasmatic nucleus could be followed in a caudodorsal direction to the subparaventricular zone and the dorsal hypothalamic area. Moreover, minor projections could be traced from the suprachiasmatic nucleus rostrally along the dorsal surface of the optic chiasm to the prechiasmatic area, rostrodorsally into the periventricular area and laterally along the dorsal surface of the optic chiasm and tract. Gel filtration and high performance liquid chromatography profiles of the tissue extracts from the suprachiasmatic area revealed the presence of two GRP-immunoreactive peptides, co-eluting with synthetic gastrin releasing peptide18-27 and gastrin releasing peptide1-27 in almost equivalent concentrations. Gastrin releasing peptide18-27 or gastrin releasing peptide1-27 might therefore play a role in regulation of circadian rhythms in hypothalamic nuclei generated by the suprachiasmatic nucleus.