Sex comparison of neuronal Fos immunoreactivity in the rat vomeronasal projection circuit after chemosensory stimulation.

In rodents, reproductively relevant pheromonal cues are detected by receptors in the vomeronasal organ, which in turn transmit this information centrally via the accessory olfactory bulb, the medial nucleus of the amygdala, the posterior medial bed nucleus of the stria terminalis and the medial preoptic area. In the rat, more neurons are present in males than in females at virtually every relay in this vomeronasal projection circuit. Using Fos immunoreactivity as a marker of neuronal activation, we compared the ability of pheromonal cues derived from the urine and feces of estrous or anestrous female rats to activate neurons in this vomeronasal projection circuit in sexually experienced, gonadectomized male and female rats which were chronically treated in adulthood with a high dose of testosterone propionate (5 mg/kg). When compared with rats killed after 2 h of exposure to clean bedding, male and female subjects exposed for 2 h to bedding from estrous females had similar and significant increments in the number of Fos-immunoreactive neurons at each level of the vomeronasal projection circuit, including the granular layer of the accessory olfactory bulb, the posterior dorsal portion of the medial amygdaloid nucleus, the posterior medial portion of the bed nucleus of the stria terminalis and the medial preoptic area. Exposure to bedding from anestrous females stimulated similar and significant increments in Fos immunoreactivity in most of these same brain regions. Chemosensory stimulation failed to augment Fos immunoreactivity in neurons located in the ventrolateral subregion of the ventromedial nucleus of the hypothalamus or in the midbrain central tegmental field, sites at which mating has previously been shown to augment Fos immunoreactivity in both sexes. Finally, chemosensory stimulation augmented Fos immunoreactivity in the nucleus accumbens shell and core, two regions receiving dopaminergic afferents which have been implicated in sexual reward. On two occasions all subjects were given simultaneous access to bowls containing bedding from estrous versus anestrous females. Both males and females spent significantly more time investigating the estrous bedding, although the total time spent investigating either type of bedding was significantly greater in males. The results suggest that the previously established sexual dimorphism in the morphology of the rat's vomeronasal projection circuit is not reflected in the functional responsiveness of neurons in this circuit to chemosensory cues emitted by female conspecifics.