Sub-Department of Animal Behaviour, University of Cambridge, High Street, Madingley, Cambridge, CB3 8AA, UK.
Pheromones are used in mammalian social communication in a wide variety of species and contexts. The best characterised examples are found in mice which not only identify individuals and their sex by odour, but the physiological reproductive state of mice is also determined largely by pheromonal cues. Thus, pheromones in male mouse urine induce female oestrus, bring forward the onset of puberty, and in females that have recently mated they induce a block to pregnancy. All of these pheromonal actions have a common similar neuroendocrine mechanisms initiated by receptors in the vomeronasal organ (VNO). The olfactory block to pregnancy requires an additional qualification of recognition, because only strange male pheromones block pregnancy: the pheromones of the male that mated with the female are ineffective in blocking his own pregnancy, but not that of other males. Extensive neurobiological studies have shown that synaptic changes at the first relay in the vomeronasal accessory olfactory system are important and sufficient for this recognition and are dependent on noradrenaline release in the accessory olfactory bulb (AOB) activated at mating. The accessory bulb is therefore not just an interface between receptor neurons and the brain, but is capable of synthesising its own message dependent on what it has expreienced with that odour on previous occasions. Pheromones are bound to a transporter in mouse urine (MUP proteins) the expression of which is under androgen regulation. There are at least 35 MUP genes with a variety of allelic variants which differ according to mouse strain, and in wild populations are sufficiently polymorphic to code for individuals. It is likely that this ligand/transporter combination conveys information relevant to individual recognition in the context of pregnancy block. It is therefore extremely likely that the VNO receptor has a protein binding domain as well as a ligand binding domain.