Outstanding issues surrounding vomeronasal mechanisms of pregnancy block and individual recognition in mice

Male mouse urine contains a complex mixture of chemosignals, which signal the presence of a reproductively active male. These have powerful effects as primer pheromones on female reproductive state, including the ability to block pregnancy. Male mouse urine also contains individuality chemosignals that enable the female to recognise her mate and prevent his pheromones from eliciting the pregnancy block effect. A range of neurochemical and electrophysiological evidence suggests that memory formation to the mating male’s pheromones involves synaptic changes in the accessory olfactory bulb, at the first stage of the vomeronasal pathway. This results in a selective inhibition of the mate’s pheromonal signal, preventing it from activating neural circuits in the corticomedial amygdala and hypothalamus that mediate the endocrine changes responsible for pregnancy block. This article reviews the current state of knowledge regarding the neurobiological basis of this mate recognition, highlighting important gaps in our current understanding. Despite recent findings of the ability of peptides associated with the major histocompatibility complex to influence mate recognition, the pregnancy blocking and individuality chemosignals remain to be identified. Recent research has also shed doubt on our understanding of the mechanism by which noradrenaline imprints the male pheromonal signal. Finally, the effect of learning on the transmission of information in the vomeronasal system and its relationship to chemosensory information processed by the main olfactory system remain to be elucidated.