Male chemosignals inhibit the neural responses of male mice to female chemosignals.

Reproductive function in mice is regulated by reproductively-stimulating and reproductively-inhibiting primer pheromones released by conspecifics. When experienced simultaneously, their responses to reproductively-inhibiting chemosignals take precedence over their responses to reproductively-stimulating chemosignals. For example, while female urine induces luteinizing hormone (LH) release in males, this response is blocked when male urine is presented in conjunction with female urine. In the present study, we examined the neural correlates of these responses to male and female urine. Sexually experienced, male CF1 mice were exposed to water, female urine, or a mixture of male and female urine. The resulting patterns of Fos-immunoreactivity (Fos-ir) were then compared between groups. Female urine induced significantly more Fos-ir within the main and accessory olfactory systems (MOS and AOS, respectively) than did water, male urine or mixed urine. Notably, within the main and accessory olfactory bulbs, male urine attenuated the responses of mitral cells, but not granule cells, to female urine. Overall, the results indicate that exposure to male urine inhibited the responses of cells within the MOS and AOS to female urine. The specific pattern of Fos-ir in the olfactory bulbs suggests that this may be due to an inhibition in the responses of mitral cells to female urine.