A correlated light and electron microscopic immunocytochemical study of cholinergic terminals and neurons in the rat amygdaloid body with special emphasis on the basolateral amygdaloid nucleus.

The cholinergic innervation of the rat basolateral amygdaloid nucleus (BL) was determined by the immunocytochemical localization of the acetylcholine biosynthetic enzyme, choline acetyltransferase (ChAT). ChAT-immunoreactive (ChAT-IR) elements were observed throughout the BL in the form of fine puncta and varicose fibers. Electron microscopy revealed that the immunoreactive puncta represented small terminals (0.3-1.2 micron), most of which formed synaptic contacts with unlabeled dendritic shafts or spines. Less frequently, ChAT-IR terminals established synaptic contacts with large neuronal cell bodies, which had all the characteristics of projection neurons as defined on the basis of axonal projections to the ventral striatum. ChAT-IR terminals were sometimes seen to form synaptic contacts with small neuronal cell bodies, including those of ChAT-IR neurons. The ChAT-IR boutons contained pleomorphic clear vesicles of varying size, and the large majority of the synapses were of the symmetric type. Small ChAT-IR neurons were observed in all parts of the BL. Although the ChAT-IR cell bodies varied widely in shape from typical fusiform to round, most had a more or less oval shape with a major diameter of 10-14 micron. Most of the ChAT-IR neurons seemed to display a radial bipolar dendritic pattern, but multipolar cells were also observed. The ChAT-IR neurons contained an indented nucleus, which was often eccentrically located and surrounded by a thin or moderately thin rim of cytoplasm. The results obtained are discussed in relation to a quasi-cortical organization of the BL.