Localization of GABA-like immunoreactivity in the monkey amygdala.

Neurons exhibiting GABA-like immunoreactivity were identified in the monkey amygdala using an avidin-biotin immunohistochemical technique. The pattern of GABA immunoreactivity was very similar in the basolateral and superficial amygdaloid nuclei. In these regions GABA-positive cells were nonpyramidal neurons that were often arranged in clusters or curvilinear rows. These GABA-positive nonpyramidal neurons constituted about 25% of the total neuronal population of the basolateral and superficial amygdaloid nuclei. Numerous GABA-positive puncta resembling axon terminals were observed both in the neuropil and encapsulating the perikarya of GABA-negative pyramidal cells. The pattern of GABA-like immunoreactivity was different in the central and medial amygdaloid nuclei. These regions contained a very dense array of GABA-positive puncta. There were numerous GABA-positive neurons in the lateral subdivision of the central nucleus and fewer cells in the medial nucleus and medial subdivision of the central nucleus. Many immunoreactive puncta were observed contacting the perikarya and dendrites of GABA-positive cells in these regions. The intercalated nuclei consisted of numerous, small, GABA-positive neurons and a few, larger, GABA-negative cells. Both cell types were contacted by GABA-positive puncta. This study indicates that neuronal subpopulations in each of the amygdaloid nuclei of the monkey are GABAergic. The pattern of immunoreactivity varies in different amygdaloid regions and is very similar to that described in the rat. Certain aspects of the functional organization of this rich GABAergic circuitry can be elucidated by correlating the findings of the present investigation with previous anatomical, physiological, and pharmacological studies of the amygdala.