Central forebrain Fos responses to familiar male odours are attenuated in recently mated female mice

Exposure of recently mated female mice to the urinary odours of an unfamili ar male blocks pregnancy (the Bruce effect). The absence of a pregnancy blo ck in response to the stud male's familiar odours depends on an olfactory m emory that is formed in the accessory olfactory bulb (AOB) in response to v omeronasal organ (VNO) inputs during mating. Sexually naive Balb/c female m ice in pro-oestrus/oestrus were either placed onto soiled bedding ('bedding -only' females) from, or allowed to mate with, a Balb/c male ('recently mat ed' females). After 42 h, females were placed for 90 min onto clean bedding (controls) or onto soiled bedding from either a C57BL/6 male (unfamiliar b edding) or a Balb/c male (familiar bedding). Significant increases in Fos-i mmunoreactivity (Fos-IR, a marker of neuronal activation) occurred in the m edial amygdala and the medial preoptic area (MPA) of 'bedding only' females exposed to either unfamiliar or familiar bedding and in 'recently mated' f emales exposed to unfamiliar bedding but not to familiar bedding. This sugg ests that a mating-induced memory prevents the later activation by the fami liar stud male's odours of neurons in forebrain regions that receive inputs from the VNO-AOB. 'Bedding-only' females later exposed to either familiar or unfamiliar bedding had increased Fos-IR in the G alpha (o) protein-expre ssing basal zone of the VNO whereas no such effect occurred in 'recently ma ted' females. Familiar, as well as unfamiliar, male odours augmented Fos-IR in significantly more rostral than caudal AOB granule cells in all groups, with the effect being strongest in 'recently mated' females exposed to fam iliar male bedding. This outcome is consistent with the absence of odour-in duced Fos-IR in forebrain regions of these females and, presumably, the abs ence of a pregnancy block.