Assessing the contribution of the accessory olfactory system to courtship and sexual motivation in male mice

Social communication between male and female rodents occurs via volatile as well as non-volatile components of urine and other body odors which are detected and processed via the main and accessory olfactory systems. In this thesis surgical removal of the vomeronasal organ (VNOx) from male mice was used to assess the role of the accessory olfactory system in sex discrimination, mating behavior, and sexual motivation. VNO removal did not disrupt the ability of male mice to discriminate urinary odors from male vs. female conspecifics; however, VNOx eliminated males' preference to directly investigate estrous female vs. male urine. Mating performance with an estrous female was equivalent in VNO intact (VNOi) and VNOx male mice, and both groups preferred to mount an estrous female over another male. In Y-maze tests both VNOi and VNOx males acquired a preference for a distinctive goal box containing estrous female urine vs. water. However, during trials to extinguish this preference VNOx males remained significantly longer than VNOi controls in the goal box that previously contained female urine. This enhanced resistance to extinction coupled with the elimination in VNOx males of a preference to investigate female vs. male urine suggests that VNO neural inputs communicate information about the intrinsic rewarding properties of female urinary odors to reward circuits in the male's brain. Support for this view derived from the observation that VNOx blocked the significant increase in Fos immunoreactive neuronal number in the nucleus accumbens core region that occurred in VNOi males after nasal application of estrous female urine. VNO removal did not disrupt the development of a conditioned place preference response for access to an anesthetized estrous female. However, VNOx did block the induction of Fos in the AOB and other forebrain sites including the ventral tegmental area which was otherwise seen in VNOi mice after exposure to the visual and tactile cues previously associated with the anesthetized estrous female. These behavioral and anatomical results suggest that VNO inputs to neural reward circuits increase the motivation of male mice to maintain contact with opposite-sex urinary odorants so as to maximize reproductive success.