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 suprach iasmatic nucleus (SCN) is modulated by two types of phenomena: photic phase-shifts, mediated by the retinohypothalamic pathway and non-photi c phase-shifts mediated by the projection of the intergeniculate leafl et (IGL) to the SCN which contains the neuropeptide Y (NPY). In primat es, the retinohypothalamic pathway has been well-demonstrated but very little is known about the geniculohypothalamic tract. This prompted u s to study NPY immunoreactivity in both the SCN and the IGL in species representative of the three main primate lineages: prosimians (MIcroc ebus), New World monkeys (Callithrix) and Old World monkeys (Macacca). In species studied, we found a region in the pregeniculate nucleus co ntaining 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 ad jacent dorsal lateral geniculate nucleus. By contrast, NPY-IP fibres i n the SCN are dense in prosimians, but are sparse or absent in other p rimate species. We suggest that either the geniculohypothalamic projec tion is absent in higher primates as is the case in humans, or is abse nt in diurnal mammals, or contains a different peptide, or that NPY im munoreactivity varies according to other parameters. (C) 1998 Elsevier Science B.V. All rights reserved.