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 imme diately ventral to the third ventricle behind the suprachiasmatic nucleus a nd in front of the arcuate nucleus, is implicated in the nocturnal inhibito ry process of melatonin production induced by short-term retinal photo-stim ulation. In the present study, the projections of the RCHb have been examin ed 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 th e upper thoracic segments, known to be critically involved in the control o f melatonin secretion. Of particular interest, the present anterograde trac t-tracing study indicates all possible paths from the RCHb which may concei vably be involved in influencing the sympathetic outflow and, therefore, me latonin production. Thus, apart from a direct projection to the intermediol ateral column at thoracic levels of the spinal cord, the RCHb is in a posit ion to control the sympathetic outflow through other potential routes, such as the dorsal parvicellular part of the paraventricular nucleus of the hyp othalamus, lateral hypothalamic area, ventromedial nucleus of the hypothala mus, lateral part of the periaqueductal gray and Barrington's nucleus. (C) 1999 Elsevier Science B.V. All rights reserved.