Light Stimulation of the Hypothalamic Neuroendocrine System

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This paper demonstrates that, in the mediation of light, the suprachiasmatic nucleus (SCN) functionally associates with the anterior periventricular and parvocellular paraventricular neuron systems in rats. Intact rats (group 1) and rats undergoing a hemicomplete cutting of the SCN (group 2) were housed in a dark room (2-3 weeks) and killed after an exposure to light for 10, 30 or 60min. Other intact animals (group 3) kept in a dark room (2 weeks) were exposed to light for 10min, then stored 60min in the dark room, and killed in darkness. The SCN, anterior periventricular nucleus, and parvocellular paraventricular nucleus were examined immunohistochemically using antisera for vasoactive intestinal polypeptide (VIP), arginine vasopressin, somatostatin, rat corticotropin releasing factor (rCRF), and c-fos protein. In comparison with animals kept in darkness, animals exposed for 10 and 30min to light indicated a remarkable reduction of VIP immunoreactivity in the SCN and some increase of CRF immunoreactivity in the parvocellular paraventricular nucleus. The diminution of VIP immunoreactivity did not occur in the isolated SCN of group 2 animals. In group 3, a 10 min-light exposure induced a remarkable enhancement of nuclear c-fos immunoreactivity in neurons in the ventrolateral region of the SCN, in the anterior periventricular nucleus, and in the parvocellular paraventricular nucleus, most strongly in the SCN. Double immunolabeling methods have shown that VIP, somatostatin, and CRF neurons in the respective nuclei were c-fos positive.