Brain processing of the mammary pheromone in newborn rabbits

Chemosignals strongly contribute to social interactions in mammals, including mother-young relationships. In the European rabbit, a volatile compound emitted by lactating females in milk, the 2-methylbut-2-enal, has been isolated. Carrying the properties of a pheromone, in particular the spontaneous ability to release critical sucking-related movements in newborns, it has been called the mammary pheromone (MP). Lesion of the vomeronasal organ and preliminary 2-deoxyglucose data suggested that the MP could be processed by the main olfactory system. However, the neuronal substrate that sustains the MP-induced response of neonates remained unknown. Here, we evaluated Fos expression in 4-day-old-rabbits exposed to the MP (in comparison with control neonates exposed to non-relevant odorant, no odorant or unmanipulated pups) both at the level of the olfactory bulb and central brain regions. Evidence of high and widespread Fos immunoreactivity in the main olfactory bulb appear in MP pups while the accessory olfactory bulb exhibits a negligible staining. However, no obvious bulbar pattern of Fos expression is observed, when in contrast a certain pattern emerges with the neutral odorant. Compared to this latter, the MP exposure increases Fos expression in the anterior piriform cortex, the organum vasculosum of the lamina terminalis and the habenula, with a tendency in the lateral preoptic region. For the first time, a pheromone essential for mother-young interaction is thus highlighted for its processing by the main olfactory system, the whole olfactory bulb, and by brain regions involved in osmoregulation, thirst and motivation-guided motor responses.