Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) is a prominent feature of the anterior hypothalamus. In virtually all mammals, the SCN is a compact group of small neurons dorsal to the optic chiasm along most of its length and just lateral to the periventricular nucleus and third ventricle (Figure 1). The SCN was recognized in early cytoarchitectonic studies (Brockhaus, 1942; Gurdjian, 1927), but its function remained obscure until the early 1970s. Perhaps the first hint was a report by Pate (1937), who observed what must be considered transneuronal atrophy of the SCN after eye removal in the cat. The issue of retinohypothalamic projections in mammals was contentious in that era and the great neuroanatomist W. J. H. Nauta concluded in the late 1960s that there were no compelling data to support such a projection. This conclusion, however, was contrary to the situation in lower vertebrates, where a projection from the retina to the suprachiasmatic area was well established (Ebbeson, 1970). As often happens, new technical advances provided the basis for new understanding. The discovery of anterograde transport by Weiss and colleagues (Weiss, 1972) was dependent upon the fact that tritiated amino acids are incorporated into proteins in neuronal perikarya and then transported through axons to sites of terminal arbors. This phenomenon formed the basis for an axoplasmic tracing method to analyze connections in brain (Cowan, Gottlieb, Hendrickson, Price, & Woolsey, 1972) and this was applied to the study of retinal projections, providing new evidence for a retinohypothalamic tract in mammals (Hendrickson, Wagoner, & Cowan, 1972; Moore, 1973; Moore & Lenn, 1972). In these studies, silver grains were found distributed over the ventral SCN, strongly suggesting that terminals of retinal axons were present in that location. This was confirmed by the electron microscopic finding of degenerating axon terminals synapsing on SCN neuron dendrites after eye removal (Moore & Lenn, 1972). These data, then, provided definitive evidence for a retinohypothalamic tract terminating in the SCN.