NEUROPEPTIDERGIC ORGANIZATION OF THE SUPRACHIASMATIC NUCLEUS IN THE BLIND MOLE-RAT (SPALAX-EHRENBERGI)

The blind mole rat, Spalax, is a subterranean rodent with atrophied, s ubcutaneous eyes. Whereas most of the visual system is highly degenera ted, the retino-hypothalamic pathway in this species has remained inta ct. Although Spalax is considered to be visually blind, circadian loco motor rhythms are entrained by the light/dark cycle. In the present st udy we used anterograde tracing techniques to demonstrate retinal affe rents to the suprachiasmatic nucleus (SCN) and immunohistochemistry to examine the distribution of neuropeptides that are known to be involv ed in the regulation or expression of circadian rhythmicity. Based on the localization of retinal afferents and neuropeptides, the SCN can b e divided into two subdivisions. The ventral region, which receives re tinal afferents, also contains vasoactive intestinal polypeptide (VIP) -containing neurons, and fibers that are immunopositive to neuropeptid e Y (NPY) and serotonin (5-HT), The dorsal region contains vasopressin ergic neurons, but this latter cell population is extremely sparse com pared to that described in other rodents, The dorsal region is also ch aracterized by numerous VIP-immunoreactive fibers. The presence of NPY and 5-HT fibers suggests that the SCN receives afferent projections f rom the intergeniculate leaflet and from the raphe nuclei, respectivel y. These neuroanatomical results, together with previous studies of be havior, visual tract tracing, and immediate early gene expression, con firm that an endogenous clock and the capacity for light entrainment o f circadian rhythms are conserved in the blind mole rat. (C) 1997 Else vier Science Inc.