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 involvin g emotion and memory. To investigate the connections that regulate the inte ractions between these regions, we injected the anterograde tracer Phaseolu s vulgaris-leucoagglutinin into various divisions of the lateral, basal, an d accessory basal nuclei of the rat amygdala. The heaviest projection to th e entorhinal cortex originates in the medial division of the lateral nucleu s which innervates layer III of the ventral intermediate and dorsal interme diate subfields. In the basal nucleus, the heaviest projection arises in th e parvicellular division and terminates in layer III of the amygdalo-entorh inal transitional subfield. In the accessory basal nucleus, the parvicellul ar division heavily innervates layer V of the ventral intermediate subfield . The most substantial projection to the hippocampus originates in the basa l nucleus. The caudomedial portion of the parvicellular division projects h eavily to the stratum oriens and stratum radiatum of CA3 and CAI. The acces sory 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 port ion of the medial division of the lateral nucleus, the caudomedial parvicel lular 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 par asubiculum in segregated rather than overlapping terminal fields. These dat a suggest that the amygdaloid complex is in a position to modulate differen t stages of information processing within the hippocampal formation. (C) 19 99 Wiley-Liss, Inc.