Projections of the basal retrochiasmatic area: a neural site involved in the photic control of pineal metabolism.

It has been shown that the basal retrochiasmatic area (RCHb), situated immediately ventral to the third ventricle behind the suprachiasmatic nucleus and in front of the arcuate nucleus, is implicated in the nocturnal inhibitory process of melatonin production induced by short-term retinal photo-stimulation. In the present study, the projections of the RCHb have been examined using the Phaseolus vulgaris leucoagglutinin (PHA-L) method in the rat. Considering the putative role of the RCHb in contributing to the short-term photo-inhibition of the pineal gland during the night, it is reasonable to suppose that the RCHb may ultimately inhibit the sympathetic outflow of the upper thoracic segments, known to be critically involved in the control of melatonin secretion. Of particular interest, the present anterograde tract-tracing study indicates all possible paths from the RCHb which may conceivably be involved in influencing the sympathetic outflow and, therefore, melatonin production. Thus, apart from a direct projection to the intermediolateral column at thoracic levels of the spinal cord, the RCHb is in a position to control the sympathetic outflow through other potential routes, such as the dorsal parvicellular part of the paraventricular nucleus of the hypothalamus, lateral hypothalamic area, ventromedial nucleus of the hypothalamus, lateral part of the periaqueductal gray and Barrington's nucleus.