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 generation and entrainment of circadian rhythms in mammalian species. Both the occurrence and the timing of the luteinizing hormone surge on the afternoon 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 hormone (GnRH) neurons in the preoptic area. In addition, we found that interaction 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 study was to investigate the presence of synaptic contacts between GnRH fibers and structures in the SCN and surrounding perichiasmatic area (periSCN). At the light microscopical level, the immunoreactivity for GnRH showed a considerable overlap with the immunoreactivity for vasopressin and vasoactive intestinal peptide, two neuropeptides synthesized by SCN neurons. At the ultrastructural level, we demonstrated synaptic input of GnRH-containing axons on immunocytochemically unidentified structures in the SCN/peri-SCN region. The present results clearly demonstrate that the SCN and periSCN are postsynaptic targets of GnRH fibers. It is hypothesized that the GnRH input in the SCN region represents an anatomical substrate for feedback-control between these systems.

Preferential induction of c-fos immunoreactivity in vasoactive intestinal polypeptide-innervated gonadotropin-releasing hormone neurons during a steroid-induced luteinizing hormone surge in the female rat

In small rodents, reproduction is critically dependent on the integrity of the circadian oscillator of the brain, the suprachiasmatic nucleus (SCN). Lesions of the SCN induce persistent estrus (anovulation) in intact female rats, whereas estrogen implantation in ovariectomized rats results in daily LH surges, which disappear after SCN lesions. Vasoactive intestinal polypeptide (VIP), a peptide synthesized in cell bodies of the SCN, has been implicated in the regulation of LH release. Recently, we have provided immunocytochemical evidence for a VIP-containing neuronal projection from the SCN to the GnRH system. This suggests that VIP from the SCN may modulate LH release via a direct influence on GnRH neurons. To investigate the involvement of VIP input on GnRH neurons and SCN neurons in the generation of a LH surge, we used immunoreactive c-fos as a marker for cell activation in ovariectomized mature rats and immature rats treated with steroids. VIP-containing fibers were observed in apposition to a substantial portion of the GnRH neurons containing c-fos. Expression of c-fos was more frequently observed in VIP-innervated GnRH neurons than in GnRH neurons in general. This difference in activation was most pronounced during the onset of the LH surge. In SCN neurons, steroid treatment did not induce c-fos immunoreactivity before or during the LH surge. The present results indicate that VIP-containing fibers, possibly originating in the SCN, are involved in the initiation of the LH surge. In view of the reported inhibitory effects of VIP on LH release, it is suggested that the role of VIP input in this respect is permissive.