Sex difference in Fos induced by male urine in medial amygdala-projecting accessory olfactory bulb mitral cells of mice

We previously reported that exposure to soiled male bedding induced Fos protein immunoreactivity (Fos-IR) in significantly more neurons of the vomeronasal organ (VNO) and medial amygdala of gonadectomized, estradiol-treated female than male mice whereas no such sex difference was seen in the intervening mitral cells of the accessory olfactory bulb (AOB). We asked whether a sexually dimorphic functional response to male urinary pheromones might be revealed in AOB mitral cells that project specifically to the medial amygdala. Gonadectomized mice of both sexes were treated with estradiol and 3 days later received bilateral injections of the retrograde tracer, Cholera toxin-B (CTB) into the medial amygdala. Five days later male urine or saline was applied nasally to each subject 90 min prior to sacrifice, and sections of the AOB were processed for double-label fluorescent immunocytochemistry for Fos protein and CTB. In both the rostral and caudal AOB, there were significantly more double-labeled mitral cells in female than in male subjects following exposure to male urine. A sex difference in the responsiveness of VNO sensory neurons seen previously to male soiled bedding is reflected in a parallel sex difference in the responsiveness of AOB mitral cells when only AOB cells that project to the amygdala are examined and when male urine as opposed to soiled male bedding is used as the activating stimulus.