SYNAPTIC CONTACTS BETWEEN GONADOTROPIN-RELEASING HORMONE-CONTAINING FIBERS AND NEURONS IN THE SUPRACHIASMATIC NUCLEUS AND PERICHIASMATIC AREA - AN ANATOMICAL SUBSTRATE FOR FEEDBACK-REGULATION

The suprachiasmatic nucleus (SCN) is critically involved in the genera tion and entrainment of circadian rhythms in mammalian species. Both t he occurrence and the timing of the luteinizing hormone surge on the a fternoon of proestrus in the female rodent are critically dependent on the integrity of the SCN. Recently, we demonstrated the presence of a monosynaptic pathway from the SCN to the gonadotropin releasing hormo ne (GnRH) neurons in the preoptic area. In addition, we found that int eraction between the SCN and the GnRH system may be found close to the SCN, since we observed apposition of SCN efferents and GnRH fibers at the ultrastructural level in that region. The aim of the present stud y was to investigate the presence of synaptic contacts between GnRH fi bers and structures in the SCN and surrounding perichiasmatic area (pe riSCN). At the light microscopical level, the immunoreactivity for GnR H showed a considerable overlap with the immunoreactivity for vasopres sin and vasoactive intestinal peptide, two neuropeptides synthesized b y SCN neurons. At the ultrastructural level, we demonstrated synaptic input of GnRH-containing axons on immunocytochemically unidentified st ructures in the SCN/peri-SCN region. The present results clearly demon strate that the SCN and periSCN are postsynaptic tar ets of GnRH fiber s. It is hypothesized that the GnRH input in the SCN region represents an anatomical substrate for feedback-control between these systems. ( C) 1997 Elsevier Science B.V.