Is there a geniculohypothalamic tract in primates? A comparative immunohistochemical study in the circadian system of strepsirhine and haplorhine species.

In rodents, the circadian rhythm generated by the hypothalamic suprachiasmatic nucleus (SCN) is modulated by two types of phenomena: photic phase-shifts, mediated by the retinohypothalamic pathway and non-photic phase-shifts mediated by the projection of the intergeniculate leaflet (IGL) to the SCN which contains the neuropeptide Y (NPY). In primates, the retinohypothalamic pathway has been well-demonstrated but very little is known about the geniculohypothalamic tract. This prompted us to study NPY immunoreactivity in both the SCN and the IGL in species representative of the three main primate lineages: prosimians (Microcebus), New World monkeys (Callithrix) and Old World monkeys (Macacca). In species studied, we found a region in the pregeniculate nucleus containing both NPY immunopositive cells and substance P immunopositive fibres that we identified as the IGL. During evolution, this structure has moved from a ventral to a dorsomedial position relative to the adjacent dorsal lateral geniculate nucleus. By contrast, NPY-IP fibres in the SCN are dense in prosimians, but are sparse or absent in other primate species. We suggest that either the geniculohypothalamic projection is absent in higher primates as is the case in humans, or is absent in diurnal mammals, or contains a different peptide, or that NPY immunoreactivity varies according to other parameters.