Neuronal organization of the lateral and basolateral amygdaloid nuclei in the rat

The neuronal organization of the lateral (L) and basolateral (BL) amygdaloid nuclei was studied in the rat by using Golgi techniques. All nuclear subdivisions, which were identified in Nissl and acetylcholinesterase preparations, contain spiny class I neurons and spine-sparse class II neurons. Three of four neurogliaform class III neurons observed were located in the anterior division of BL (BLa). The exact arrangement of class I and class II neurons varies in different portions of L and BL. At the periphery of these nuclei, where L and BL border fiber bundles, major dendrites tend to be oriented parallel to nuclear borders. Many smaller dendritic branches, however, may extend into the adjacent fiber bundles. At most borders between nuclear subdivisions dendritic overlap is minimized by the fact that major dendrites tend to run parallel to subdivisional boundaries. One exception is the junction of BLa with the posterior division of BL (BLp), where unrestricted dendritic overlap of both class I and class II neurons occurs. Within most nuclear subdivisions dendrites of class I and class II neurons ramify freely and exhibit little order. In caudal portions of BLp, however, almost all class I neurons are pyramidal cells with vertically oriented apical dendrites. Dendrites of class II neurons in this region tend to be oriented horizontally, perpendicular to apical dendrites of class I cells. Class II neurons were not evenly distributed in Golgi preparations but were concentrated at the BLa-BLp border, near the boundary between the dorsolateral and ventromedial subdivisions of L and in the dorsal portion of BLp. The latter cells blend with similar spine-sparse neurons contained within the external capsule. Analysis of Nissl preparations reveals that small neurons, which correspond to small class II and class III cells, are sometimes observed in a clustered arrangement.