LESIONS OF THE SUPRACHIASMATIC NUCLEUS INDICATE THE PRESENCE OF A DIRECT VASOACTIVE INTESTINAL POLYPEPTIDE-CONTAINING PROJECTION TO GONADOTROPIN-RELEASING-HORMONE NEURONS IN THE FEMALE RAT

In non-seasonal breeders like the rat, the influence of the suprachias matic nucleus (SCN) on reproduction is most clearly expressed in the f emale. Complete lesions of the SCN induce persistent oestrus (anovulat ion) in intact female rats, whereas oestrogen implantation in ovariect omized rats results in daily luteinizing hormone surges. Vasoactive in testinal polypeptide (VIP), a peptide synthesized in cell bodies of th e SCN, inhibits the increase in pulsatile luteinizing hormone release observed in ovariectomized female rats. In search of the anatomical ba sis for these observations, the present study employs an immunocytoche mical double staining for VIP and gonadotrophin-releasing hormone (GnR H) at the light microscopical level. It was demonstrated that approxim ately 45% of the GnRH positive neurons in the diagonal band of Broca, the preoptic and anterior hypothalamic area of female rats are innerva ted by VIP-containing processes. To investigate whether these VIP-cont aining fibres represent a direct projection of the SCN to the GnRH sys tem, unilateral thermic SCN lesions were made. Lesions that unilateral ly destroyed the majority of the VIP synthesizing cells in the SCN res ulted in at least a 50% decrease of the VIP innervation of GnRH cell b odies at the lesioned side compared to the intact side. Lesions not af fecting the VIP synthesizing cell population in the SCN did not change the percentage of GnRH neurons innervated by VIP-containing fibres, w hile partial lesions resulted in intermediate effects. These results i ndicate that the majority of the light microscopical VIP-containing in put on GnRH neurons in the hypothalamus is derived from the SCN. It is suggested that the reported effects of VIP on luteinizing hormone rel ease may, at least in part, be induced via a direct effect of VIP on G nRH cell bodies. This direct SCN-GnRH pathway provides an anatomical b asis for diurnal influences on the regulation of the female reproducti ve cycle.