The distribution of neurons immunoreactive for gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD) was examined in four regions of the primate cortex that receive somatosensory input. In the first study, the distribution of neurons immunoreactive for GAD, the synthesizing enzyme of GABA, was examined in the first (SI) and second (SII) somatosensory cortex. GAD-like immunoreactive (GAD-LI) somata and puncta were present in all layers of SI and SII and were classified as nonpyramidal neurons. GAD-LI somata were most numerous in layer IV of SI and in layer III or SII. Additionally, a dense band of GAD-LI punta was present in layer IV of SI, but was not observed in any layer of SII. The major diameters of GAD-LI somata in SII were significantly smaller than those in SI in layers I, II and IV. In the second study, the distribution of neurons immunoreactive for GABA, a major inhibitory neurotransmitter, was examined in the retroinsular cortex (Ri) and area 7b of the monkey. As with GAD- LI neurons in SI and SII, GABA-like immunoreactive (GABA-LI) somata in Ri and area 7b were observed in all layers. However, the majority of somata in Ri and area 7b were observed in the superficial cortical layers (I and II). Moreover, in contrast to the high numbers of large (greater than 15 um) labelled somata in SI and SII, less than 1% of all the somata in Ri and area 7b had major diameters greater than 15 um.
