Organization of the primate circadian system.

The circadian timing system has three principal elements: the retina, the intergeniculate leaflet (IGL) of the thalamus, and the suprachiasmatic nucleus (SCN). Since the human circadian timing system cannot be studied experimentally, we have used another primate, the macaque monkey, to help provide insight into the organization of the human circadian system. The retinohypothalamic tract (RHT) in the monkey projects to the SCN, the anterior and lateral hypothalamic areas, and the retrochiasmatic area in a pattern very similar to that in the rat. The monkey SCN has a population of vasoactive intestinal polypeptide-containing (VIP+) neurons in a zone that overlaps the RHT termination and the termination of neuropeptide Y-containing (NPY+) axons arising in the IGL. This zone is surrounded by a population of vasopressin-containing (VP+) neurons. The human SCN is similar to that of other mammals with populations of VIP+ and VP+ neurons, but it differs in having a large population of neurotensin-containing (NT+) neurons that extends over the entire nucleus, and a moderate population of NPY+ neurons located centrally in the nucleus in the presumed area of RHT termination. The lateral geniculate nucleus in the monkey and human is quite different from that in rodents, but contains an area in the pregeniculate nucleus that receives bilateral retinal projections in the monkey and is characterized in both the monkey and human by a population of NPY+ neurons and a plexus of enkephalin- and substance P-containing axons. This nucleus appears homologous to the rodent IGL.