AN ALTERNATE PATHWAY FOR VISUAL SIGNAL INTEGRATION INTO THE HYPATHALAMO-PITUITARY AXIS - RETINORECIPIENT INTERGENICULATE NEURONS PROJECT TOVARIOUS REGIONS OF THE HYPOTHALAMUS AND INNERVATE NEUROENDOCRINE CELLS INCLUDING THOSE PRODUCING DOPAMINE

Using tract tracing and immunocytochemistry, this study explored the c onnectivity between lateral geniculate efferents and neurons of the hy pothalamus, including those producing dopamine, that have direct acces s to fenestrated capsillaries. It was also determined whether the inte rgeniculate neurons that give rise to hypo thalamic projections are ta rgeted by retinal axons. Within the hypothalamus, Phaseolus vulgaris l eucoagglutinin-labeled, lateral geniculate efferents were observed in the suprachiasmatic nucleus, subparaventricular area, periventricular nuclei, medial preoptic areas, and between the arcuate and ventromedia l nuclei, In these sites, intergeniculate efferents contacted populati ons of neurons that were retrogradely labeled from fenestrated capilla ries by the intraperitoneal injection of fluorogold, Hypothalamic dopa mine neurons, a population of which was neuroendocrine, were also syna ptic targets of lateral geniculate efferents, After injection of the r etrograde tracer fluorogold into these hypothalamic projection sites i n parallel with bilateral enucleation, retrogradely labeled perikarya were restricted to the intergeniculate leaflet. All of the labeled per ikarya contained infolded nuclei, and their distal dendrites were freq uently found to be contacted by degenerated, retinal fibers. This stud y provides morphological evidence for a signaling pathway from the ret ina through the intergeniculate leaflet to hypothalamic cells that par ticipate in neuroendocrine regulations. These observations raise the p ossibility that visual signals independent of the circadian clock may also influence the hypothalamo-pituitary axis. In light of the overlap ping distribution of intergeniculate and suprachiasmatic efferents in the hypothalamus and their similar relationship with neuroendocrine ce lls, it is suggested that integration of circadian and visual signals can occur outside of the suprachiasmatic nucleus to regulate endocrine rhythms.