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Lesions of the Suprachiasmatic Nucleus Indicate the Presence of a Direct Vasoactive Intestinal Polypeptide‐Containing Projection to Gonadotrophin‐Releasing Hormone Neurons in the Female Rat

In non‐seasonal breeders like the rat, the influence of the suprachiasmatic nucleus (SCN) on reproduction is most clearly expressed in the female. Complete lesions of the SCN induce persistent oestrus (anovulation) in intact female rats, whereas oestrogen implantation in ovariectomized rats results in daily luteinizing hormone surges. Vasoactive intestinal polypeptide (VIP), a peptide synthesized in cell bodies of the SCN, inhibits the increase in pulsatile luteinizing hormone release observed in ovariectomized female rats. In search of the anatomical basis for these observations, the present study employs an immunocytochemical double staining for VIP and gonadotrophin‐releasing hormone (GnRH) at the light microscopical level. It was demonstrated that approximately 45% of the GnRH positive neurons in the diagonal band of Broca, the preoptic and anterior hypothalamic area of female rats are innervated by VIP‐containing processes. To investigate whether these VIP‐containing fibres represent a direct projection of the SCN to the GnRH system, unilateral thermic SCN lesions were made.