Characterization of the circadian system in a Brazilian species of monkey (Callithrix jacchus): Immunohistochemical analysis and retinal projections

The hypothalamic suprachiasmatic nucleus (SCN) and the thalamic pregenicula te nucleus (PGN), which appears to include the intergeniculate leaflet (IGL ), comprise circadian related centers in the primate brain. In this study, these centers were analysed in respect to their cytoarchitecture, retinal a fferents and chemical of major cells and axon terminals with tract tracers and immunohistochemical techniques to define cytoarchitecture and connectio ns, in the common marmoset. The SCN was shown to be a triangularly shaped c luster of compact cells just dorsal to the optic chiasm and lateral to the third ventricle. It is innervated in its ventral portion by terminals from the retina, and NPY-ergic fibers. Serotonergic and SP-staining processes ar e distributed throughout. VIP-neurons form a dorsolateral group of cells an d CB-immunoreactive neurons fill much of the nucleus. The PGN was shown to be a wedge-shaped cluster of cells located dorsomedially to the dorsal late ral geniculate nucleus. It appears to comprise a ventral portion which rece ives a bilateral retinal projection and contains NPY-neurons, suggesting th at this portion may correspond to IGL. The PGN also contains CB-neurons, PV -neurons and fibers, and SP- and 5-HT-fibers. These results in marmoset sho w that, beside a common plan revealed for most mammals, there are significa nt interspecific variations in the circadian timing system. Future studies are needed in order to elucidate the circadian organization in this primate species.