Lateral nucleus of the rat amygdala is reciprocally connected with basal and accessory basal nuclei: a light and electron microscopic study.

Information flow within the intra-amygdaloid circuitry has been generally believed to be unidirectional rather than reciprocal, in which case sensory inputs entering the amygdala via the lateral nucleus would not be modulated by inputs from other amygdaloid regions. In the present study we extend our earlier findings which indicated that the lateral nucleus of the rat amygdala is reciprocally connected with the basal and accessory basal nuclei. The type of synaptic contacts made by these connections is also characterized at the ultrastructural level. An anterograde tracer, Phaseolus vulgaris leucoagglutinin, was injected into the basal (n=22) or accessory basal nuclei (n=12) of the rat amygdala. The results demonstrate that the ventrolateral division of the lateral nucleus receives projections from the basal nucleus, while the medial division receives projections from the accessory basal nucleus. Electron microscopic analyses revealed that axons projecting from the basal nucleus formed both asymmetric and symmetric contacts within the ventrolateral division of the lateral nucleus, whereas axons projecting from the accessory basal nucleus to the medial division of the lateral nucleus formed only asymmetric synapses with their targets. These findings suggest that the lateral nucleus receives both inhibitory and excitatory intra-amygdaloid projections and indicate that information flow within the amygdala is not unidirectional as previously thought. The results of this study provide evidence that the early phase of sensory processing within the amygdala is already modified by inputs from other amygdaloid nuclei.