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 integrit y of the circadian oscillator of the brain, the suprachiasmatic nucleu s (SCN). Lesions of the SCN induce persistent estrus (anovulation) in intact female rats, whereas estrogen implantation in ovariectomized ra ts results in daily LH surges, which disappear after SCN lesions. Vaso active intestinal polypeptide (VIP), a peptide synthesized in cell bod ies of the SCN, has been implicated in the regulation of LH release. R ecently, we have provided immunocytochemical evidence for a VIP-contai ning neuronal projection from the SCN to the GnRH system. This suggest s 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 immun oreactive c-fos as a marker for cell activation in ovariectomized matu re rats and immature rats treated with steroids. VIP-containing fibers were observed in apposition to a substantial portion of the GnRH neur ons 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 L H surge. In SCN neurons, steroid treatment did not induce c-fos immuno reactivity before or during the LH surge. The present results indicate that VIP-containing fibers, possibly originating in the SCN, are invo lved in the initiation of the LH surge. In view of the reported inhibi tory effects of VIP on LH release, it is suggested that the role of VI P input in this respect is permissive.