Neuronal configurations in lateral and basolateral amygdala

The lateral and basolateral nuclei of the rat amygdala have been studied with the rapid Golgi method. Both nuclei have similar cell types, which closely resemble cells in the cerebral cortex. Therefore, we suggest that what is known about cortical circuitry can be used as a guide for studying synaptic circuitry in the lateral and basolateral nuclei.

The most abundant neurons that are impregnated in both nuclei are pyramidal cells. They have conical cell bodies and easily recognizable apical and basilar dendrites. The ones in the center of each nucleus have long axes that roughly parallel the long axis of the nucleus. Towards the periphery, pyramidal cells have apical dendrites that either stick directly across the nucleus or follow along a nuclear border. The peripheral dendrites tend to enclose the nuclei. There is considerable overlap among the dendritic trees and the dendrites of one nucleus extend into the territory of the other. Pyramidal cells have extensive axonal systems. The principal axon of basolateral cells usually projects rostrally but long collaterals leave the nucleus in other directions. The axons of lateral nucleus pyramidal cells are also widely distributed. The major thrust of their axons is caudal and lateral.

Stellate cells are the most common variety of the non-pyramidal cells. They occur in both nuclei and have round cell bodies, 10–15 μm diameter, and spherical dendritic trees that are confined to a limited region of the nucleus. Their axons form dense terminal fields that remain within the vicinity of the parent cell's dendritic tree. Another type of non-pyramidal cell is the cone cell, whose non-spiny, varicose dendrites describe cones. These neurons are found mainly in the apex of the lateral nucleus. The most rare non-pyramidal cells are the extended neurons, which have long, straight dendrites that reach beyond the nucleus into surrounding neuropil. They are mostly in the rostral part of the basolateral nucleus but also occur in the lateral nucleus, near the ventricular border. The axons of cone cells and the extended neurons have been only partially impregnated.

We also have examined stellate cells in the guinea-pig lateral and basolateral nuclei. They have many of the same features as those in the rat brain, except that their dendritic trees and axonal systems are more complicated.

There are two large groups of afferents: one consists of longitudinally running axons and the other of transversely coursing fibers. The axons in both groups are thin, have a rather straight course through the nucleus and have only a few collaterals. We have attempted to relate these groups to the known afferent supply of the nuclei. In addition, axons from neurons in the intercalated cell masses and in the claustrum-dorsal endopiriform area enter at least the periphery of the lateral and basolateral nuclei.

Finally, we propose that the lateral and basolateral nuclei are functionally interrelated and that they link exteroceptive sensory data (‘percepts’) with the output of the basolateral nucleus, which is seen to coordinate forebrain areas that direct motivationally-guided behavior. Furthermore, this linked input-output pattern is communicated to the entorhinal cortex and, hence, to the hippocampus where it is encoded.