Recent observations have implicated the vomeronasal (accessory olfactory) system in the chemosensory control of rodent social behaviors. The purpose of this study was to observe the effects of peripheral vomeronasal organ extirpation on sexual behavior, aggression, and urine marking in male mice. Relative to sham-operated control animals, mice lacking vomeronasal organs displayed significantly reduced levels of copulatory behavior and intermale aggression. Urine marking rates were not reduced. The peripheral removal of the vomeronasal organ resulted in complete bilateral deafferentation of the accessory olfactory bulbs but spared the peripheral input to the main olfactory bulbs as evidenced by the lack of anterograde vomeronasal nerve transport but normal anterograde olfactory nerve transport of intranasally applied horseradish peroxidase. Neither body weights, paired testes weights, nor seminal vesicle weights of mice with vomeronasal system lesions differed significantly from those of control animals. Thus, an intact vomeronasal organ is important for the normal display of sexual behavior and aggression in male mice, and the reductions in these androgen-dependent behaviors following peripheral deafferentation of the vomeronasal system cannot be attributed to a chronic reduction of gonadal hormone secretion.Chemosensory stimuli and the olfactory system influence reproductive physiology and behavior in a broad range of mammalian species.
Evidence has accumulated demonstrating that the vomeronasal (accessory olfactory) system mediates intraspecific chemosensory communication in several mammals. For example, the neuroendocrine effects of priming pheromones in females and the behavioral responses to signaling pheromones in males are disrupted in mice with damage to the vomeronasal system. The experiment reported here examined the potential involvement of the vomeronasal system in the neuroendocrine reflexes observed in male mice following exposure to female and chemosensory stimuli. Excision of the vomeronasal organ (VNO) or sham VNO ablation was performed on sexually experienced males. Next, consecutive blood samples were withdrawn through chronic, intracardiac cannulas while the males were exposed to female mouse urine and then to an ovariectomized female. Plasma levels of luteinizing hormone (LH) were measured in the sequential samples by radioimmunoassay. Removal of the VNO did not affect the spontaneous pattern of episodic LH release that is characteristic of male mice. Reflexive release of LH following the urine stimulus was blocked in males lacking the VNO, but the female stimulus did cause LH responses in these mice. Our results therefore demonstrate that the VNO mediates pheromonally induced release of LH in male mice and that additional cues which emanate from behaving females also effectively stimulate a hormonal response in sexually experienced males.
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