Projections from the lateral, basal, and accessory basal nuclei of the amygdala to the hippocampal formation in rat.

The amygdaloid complex and hippocampal formation mediate functions involving emotion and memory. To investigate the connections that regulate the interactions between these regions, we injected the anterograde tracer Phaseolus vulgaris-leucoagglutinin into various divisions of the lateral, basal, and accessory basal nuclei of the rat amygdala. The heaviest projection to the entorhinal cortex originates in the medial division of the lateral nucleus which innervates layer III of the ventral intermediate and dorsal intermediate subfields. In the basal nucleus, the heaviest projection arises in the parvicellular division and terminates in layer III of the amygdalo-entorhinal transitional subfield. In the accessory basal nucleus, the parvicellular division heavily innervates layer V of the ventral intermediate subfield. The most substantial projection to the hippocampus originates in the basal nucleus. The caudomedial portion of the parvicellular division projects heavily to the stratum oriens and stratum radiatum of CA3 and CA1. The accessory basal nucleus projects to the stratum lacunosum-moleculare of CA1. The subiculum receives a substantial input from the caudomedial parvicellular division. The parasubiculum receives dense projections from the caudal portion of the medial division of the lateral nucleus, the caudomedial parvicellular division of the basal nucleus, and the parvicellular division of the accessory basal nucleus. Our data show that select nuclear divisions of the amygdala project to the entorhinal cortex, hippocampus, subiculum, and parasubiculum in segregated rather than overlapping terminal fields. These data suggest that the amygdaloid complex is in a position to modulate different stages of information processing within the hippocampal formation.