A putative retinohypothalamic projection containing substance P in the human

The retinohypothalamic tract (RHT) is the principal pathway mediating the entraining effects of light on the circadian pacemaker, the suprachiasmatic nucleus (SCN). In the rat, the RHT has two components, one which projects to the SCN and the intergeniculate leaflet of the thalamus and has no known peptide content and one which projects to the SCN and, perhaps, to the olivary pretectal nucleus and contains substance P (SP). Both terminate predominantly in a zone of the SCN that contains vasoactive intestinal polypeptide (VIP)-producing neurons. In the human, there is a similar dense axonal plexus of SP-immunoreactive axons in the SCN located largely in the area occupied by VIP-immunoreactive neurons and distinct from other SP-immunoreactive axons in the area. We propose that this SP plexus represents a component of the RHT in the human brain.