INFLUENCE OF MALE-RATS ON THE LUTEINIZING-HORMONE-RELEASING HORMONE NEURONAL SYSTEM IN FEMALE RATS - ROLE OF THE VOMERONASAL ORGAN

Olfactory information processed by the vomeronasal system is reported to influence reproductive functions in a variety of mammals. The prese nt studies were designed to determine if male-associated cues affect t he luteinizing hormone-releasing hormone (LHRH) neuronal system, and, if so, to determine the extent to which these cues are processed by th e vomeronasal organ (VNO). Ovariectomized rats underwent VNO removal ( VNX) or sham surgery (VN-Sham). Forty-eight hours after estrogen primi ng (5 mug), they were subjected to one of the following treatments: re peated mating, repeated exposure to male-soiled bedding or repeated ex posure to clean bedding. In animals treated for 180 min, coronal brain sections were double labelled for Fos protein and LHRH. An intense Fo s immunoreactivity was induced following mating in the majority of LHR H neurons in the VN-Sham females, whereas removal of the VNO significa ntly suppressed the mating-induced Fos staining. Exposure of female ra ts to male-soiled bedding or clean bedding did not induce appreciable Fos immunoreactivity in LHRH neurons. Following 90 min of mating or ex posure to bedding, blood samples were assayed for luteinizing hormone (LH). Mating stimulated the release of LH in VN-Sham females, while th e removal of the VNO significantly suppressed the mating-induced LH re lease. Exposure of the females to male-soiled bedding or clean bedding did not induce an LH surge. The present results demonstrate that male -originating sensory cues (i.e. repeated mating) can influence the LHR H neuronal system, as evidenced by the presence of Fos immunoreactivit y in LHRH cell bodies, and indicate that this effect is mediated throu gh the VNO to a certain extent. The lack of Fos immunoreactivity in th e LHRH neurons of females exposed to male-soiled bedding and the inabi lity of male-soiled bedding to induce LH release suggest that male-soi led bedding is not as effective as mating in activating the vomeronasa l system in rats.