Projections from the lateral, basal, and accessory basal nuclei of the amygdala to the entorhinal cortex in the macaque monkey.

We used the anterograde tracers Phaseolus vulgaris-leucoagglutinin (PHA-L) and biotinylated dextran amine (BDA) to examine the projections from the lateral, basal, and accessory basal nuclei of the amygdaloid complex to the entorhinal cortex in Macaca fascicularis monkeys. The heaviest amygdaloid projections originate in the lateral nucleus, which innervates the rostrally situated entorhinal fields but does not project to the caudal entorhinal cortex. The most extensive projections originate in the ventral division of the lateral nucleus. Injections in this subdivision lead to moderate to heavy fiber and terminal labeling in the entorhinal cortex, rostral levels of the rostral intermediate El (ER) and lateral fields, (ELr), and light labeling in the olfactory field EO. The projections from all portions of the lateral nucleus terminate most heavily in layer III. Layer II of EO and ER also receives a substantial input from the ventral division of the lateral nucleus. Layer II of ELr receives light innervation from all portions of the lateral nucleus that project to layer III. Projections from the basal nucleus arise mainly from the parvicellular division and are light to moderate in density. Fibers terminate predominantly in ELr, ER, EO, and the caudal portion of the lateral field (Elc); only the most rostral portion of El receives projections. While fibers from the basal nucleus innervate the same layers as the projections from the lateral nucleus, they tend to have a more vertical or radial orientation within the entorhinal cortex. Electron microscopic analysis of these fibers and terminals indicates that they overwhelmingly form asymmetrical synapses onto dendrites and dendritic spines. The accessory basal nucleus provides a light projection to the same regions of the entorhinal cortex innervated by the lateral and basal nuclei.