Immunocytochemical staining of GABA in the insular lobe of the savanna baboon: a light microscopic study.

The free floating peroxidase antiperoxidase (PAP) technique has been applied to sections of the baboon insular cortex using an antibody for gamma-aminobutyric acid (GABA). Immunostaining was localized to neuronal processes, punctate structures in the neuropil, and neuroglial cells in the subcortical white matter. GABAergic neurons were present in all cortical layers (especially layers II, III, and V/VI), in the subcortical white matter, and at all insular levels. Individual GABA-immunostained nerve cell bodies were non-pyramidal in type, often vertically oriented, round or pear-shaped, and 7.5-12.5 microns in their major transverse diameter. In the deepest cortical layers larger GABA-positive neurons were present. Horizontal GABA-positive cells were rarely identified. Immunostained neurons with apically oriented processes, basally directed processes, bipolar neurons, and multipolar neurons (10-12.5 microns in major transverse diameter) were also identified. Pyramidal shaped cells (measuring 17.5-18.5 micron) and the proximal portions of their processes were often outlined by puncta. GABA-immunostained cells in the subcortical white matter typically had a long but narrow shape. These GABAergic neurons are considered to be intrinsic or local circuit neurons.