FOS immunoreactivity after exposure to conspecific or heterospecific urine: where are chemosensory cues sorted?

Female prairie voles (Microtus ochrogaster) typically require an intact vomeronasal system and exposure to a chemical signal found in urine from male prairie voles to induce uterine growth necessary for reproduction. Urine from male mice (Mus musculus) does not contain an effective cue for activation of female vole reproduction: after 4 days of exposure to stimuli, voles exposed to urine from female or water had still uteri whereas voles exposed to urine from male voles had large uteri. The initial response to urine from male voles included neuronal activity in the vomeronasal system as indicated by FOS immunocytochemistry. Stimuli (urine from a male vole or a male mouse, or water) were painted on the nose of naive female voles 1 h before sacrifice. Female voles exposed to urine from male voles had more FOS-immunoreactive cells in the accessory olfactory bulb than voles exposed to mouse urine or to water. We conclude that exposure to urine from male voles stimulates the vomeronasal pathway (as measured by FOS immunoreactivity) and induces uterine growth in female voles, whereas exposure to urine from male mice (or water) does not. This suggests that some degree of functional specificity of the clinical cue is determined at or before the accessory olfactory bulb, perhaps in the expression of specific receptors within the vomeronasal organ, rather than entirely within the central nervous system.