Neural Substrate to Associate Odorants and Pheromones: Convergence of Projections from the Main and Accessory Olfactory Bulbs in Mice

In rodents olfactory stimuli are essential for socio-sexual behaviour. Volatile stimuli mainly activate the main olfactory system, whereas non-volatile ones, some of them considered as pheromones, activate the accessory olfactory system. Traditionally, it has been considered that the efferent projections of the main and the accessory olfactory bulbs innervate different telencephalic areas. Recent studies performed in rats and mice have challenged this view. In this work we explore in mice, through the use of anterograde tracer injections in the olfactory bulbs, the brain centres where olfactory and vomeronasal information might converge. Our results show that the olfactory and vomeronasal projections overlap mainly in the cortex-amygdaloid transition area, the anterior cortical amygdala and the medial amygdala (anterior and posterodorsal divisions), as well as, to a minor extent, in the anterior amygdaloid area and a restricted zone of the anterior piriform cortex. This indicates the existence of amygdaloid areas where the association between the olfactory and vomeronasal stimuli coming from the main and accessory olfactory bulb can take place. The chemosensory associative areas described in the amygdala might play a role in emotional learning involving the association of odours and pheromones.