Search

Several short-duration tests have demonstrated that the surgical removal of the vomeronasal organ (VNX) from sexually-inexperienced male rodents results in a reduction in copulatory behavior, compared to the effects of sham surgery (SHAM). We extended these studies to adult male prairie voles, Microtus ochrogaster, and substantially increased the duration of the tests. During the initial interactions with females, VNX males spent significantly less time with their noses in close proximity to the females than did SHAM males. Moreover, only two of nine VNX males sired offspring after having been paired with females for 8 weeks, whereas nine of 12 SHAM males sired offspring in that interval. We also found that VNX and SHAM males were equivalently non-aggressive to an anesthetized stimulus-male prior to being paired with females. However, after spending 2 weeks paired with a female, the VNX males were significantly less aggressive than were the SHAM males, possibly as a result of having copulated less often. In a later test, nearly all of the VNX and SHAM males that sired offspring were vigorously aggressive to a stimulus male. We conclude that the stimulation of the vomeronasal system in sexually-inexperienced male prairie voles is important for maximal reproductive performance and that the VNX-induced impairment in reproduction is associated with a decrease in inter-male aggression. The possible sensory effects of the vomeronasal system on the neural and endocrine control of reproduction and behavior are discussed.

Chemical cues from male voles activate reproduction in female prairie voles (Microtus ochrogaster). Twelve hours of contact with a male, followed by exposure to his soiled bedding for 2 days, is sufficient to initiate follicular maturation and induce uterine hypertrophy. Our recent work indicates that the chemosensory vomeronasal organ (VNO) can mediate this response. Here, we examined whether other sensory systems can acquire the ability to activate female reproduction as a result of learning or experience. To explore this issue, the VNO was removed (VNX) from nulliparous and primiparous females who were then exposed to cues from males. In Experiment 1, we found that nulliparous VNX females had lower uterine and ovarian weights than did sham-operated females. In Experiment 2, we determined that sexual experience did not ameliorate the reproductive deficits normally induced by VNX. The present results contrast with those of previous studies suggesting that males of some rodent species, when allowed reproductive experience prior to VNX, can utilize other sensory systems to mediate subsequent reproductive responses. We conclude that the role of the VNO in transducing chemosensory information is crucial for coordinating the reproductive efforts of male and female prairie voles.

In the last decade, research in our laboratories has focused on the effects of deafferentation of the mammalian chemosensory vomeronasal organ (VNX). Many different assays have been conducted and the results of some are briefly reviewed in this contribution, including the effects of VNX on neuroanatomical assessments using histochemistry (lectin binding) and immunohistochemistry (LHRH), male mouse and prairie vole ultrasonic vocalizations and hormone surges in response to cues from females, male mouse courtship and sexual behavior, territorial marking and inter-male aggression, the production of a puberty-altering substance found in mice, activation of reproduction in female voles (who generally do not exhibit estrous cycles) and maternal behaviors by female mice, including aggression directed toward intruder males. In some instances, the otherwise detrimental effects of VNX can be overcome by experience prior to deafferentation, especially in assays that are dependent upon expressions of behavior. In other situations, experience may have little impact on amelioration of the effects of VNX. The essential conclusions of this work focus our attention on reproductive physiology and behavior and a role for the vomeronasal organ in the perception of pheromones that modulate these functions.

The reproductive system of female prairie voles remains quiescent in the absence of stimulation from males; however, chemosignals from males are capable of at least partially activating female reproduction. In other species, the vomeronasal system mediates some of the reproductive responses of females to males. We found that surgical removal of the vomeronasal organ (VNX) from adult female prairie voles impeded reproductive activation in response to pairing with stud males: ovarian and uterine weights of VNX females paired with stud males for 24 or 60 hours were significantly less than those of normal (NORMAL) or sham-operated (SHAM) females. Furthermore, 8 of 9 NORMAL, 10 of 13 SHAM, but only 4 of 9 VNX females paired with stud males for 60 hours mated. VNX females, however, were still able to use chemosensory cues to locate food. Behavioral observations of females encountering stud males were similar for VNX, SHAM and NORMAL females. We conclude that vomeronasal chemoreception may be a primary component of reproductive activation in female prairie voles, presumably by mediating neuroendocrine responses to chemosignals.