Lateral and medial amygdala of anuran amphibians and their relation to olfactory and vomeronasal information

Abstract

The amygdala of anurans is currently considered as a complex of nuclei that share many features with their counterparts in amniotes. In the present study, the subdivisions of the amygdala that are directly related to olfactory and vomeronasal information, were investigated in the anurans Rana perezi and Xenopus laevis. In particular, the connectivity of the main and accessory olfactory bulbs and their related amygdaloid nuclei was studied by means of in vivo and in vitro tract-tracing with dextran amines. The projections observed from the main olfactory bulb clearly innervate the newly redefined lateral amygdala within the ventral pallium and, to a lesser extent, the rostral portion of the medial amygdala. Injections into the accessory olfactory bulb exclusively revealed projections to the medial amygdala. Tracer applications into the lateral and medial nuclei revealed abundant intra-amygdaloid connections. The dual flow of olfactory and vomeronasal projections throughout the telencephalon was not strictly segregated since the lateral pallium and the lateral amygdala, both receiving olfactory information, were found to project to the medial amygdala (the only target of vomeronasal information), which in turn projects to the lateral amygdala. Additionally, both the lateral and the medial amygdala strongly project to the hypothalamus through the anuran equivalent of the stria terminalis. The main hodological features found in the present study suggest that forerunners of the olfactory and vomeronasal amygdaloid nuclei can be distinguished in anurans. This supports the notion that all tetrapods share a common pattern of organization of the amygdaloid complex, which links environmental (olfactory/vomeronasal) information and the behavioural response of the animal.