Distribution of GABA immunoreactivity in the amygdaloid complex of the cat

This study describes the distribution of GABA immunoreactivity in the amygdaloid complex of cats. At the light microscopic level, immunopositive structures consisted of morphologically diverse somata and numerous small punctate elements. The latter accounted for most of the staining at low magnification and, at the electron microscopic level, were found to be axon terminals establishing symmetric synaptic contacts with a variety of postsynaptic profiles. Deep and superficial amygdaloid nuclei could be assigned to one of four groups according to (i) the intensity of immunolabeling they displayed, (ii) their density in reactive somata, and (iii) the size of the immunopositive somata they contained. Intercalated cell masses displayed the highest density of strongly immunoreactive cell bodies and presumed GABAergic terminals. However, electron microscope observations showed that intercalated somata were almost devoid of synaptic contacts. In contrast, central and medial nuclei were characterized by a low density of intensely immunoreactive somata and an elevated concentration for GABAergic terminals which contacted somatic and dendritic profiles. In addition, central and medial nuclei contained numerous neurons displaying low to moderate immunoreactivity. Superficial amygdaloid nuclei and nuclei of the basolateral complex displayed an intermediate density of immunoreactive somata and a low to moderate concentration of presumed terminals. Analysis of the distribution of soma areas within these nuclei revealed that the basolateral complex contains a distinct subpopulation of larger immunoreactive neurons. In light of recent electrophysiological findings, these results suggest that the intra-amygdaloid GABAergic system plays a major role in controlling the synaptic responsiveness and spontaneous activity of amygdaloid neurons.