Endocrine Abstracts

According to the original definition, pheromones are substances released by an individual that have definite behavioural or physiological effects on another individual of the same species. For example, male mouse urine contains a complex mixture of chemosignals, some of which, such as brevicomin and thiazole are testosterone-dependent and signal the presence of a reproductively active male. These have powerful effects as releaser pheromones to elicit aggression from other males, as well as having effects as primer pheromones on female reproductive state, such as puberty acceleration and induction of oestrus. However, as the complexities of vertebrate chemosensory communication have become evident, the original definition of pheromones has begun to appear too restrictive. For instance, peptide chemosignals related to the major histocompatibility complex convey information about individual identity, which as signaller pheromones can influence behaviour or physiology without eliciting a definite response.

In addition to mediating individual recognition in social contexts, these individuality chemosignals enable female mice to recognise the urinary pheromones of their mate, to which they are exposed at mating. This chemosensory memory is vital for their reproductive success, as it prevents the pre-implantation pregnancy failure that is induced by exposure to urinary pheromones from an unfamiliar male. This pregnancy block effect (Bruce effect) is mediated by the vomeronasal system, via the dopaminergic suppression of prolactin production by the pituitary. A range of 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 is just one example of the way that learning can reinforce or inhibit innate pheromonal responses.