GABAergic neuronal populations in monkey primary auditory cortex defined by co-localized calcium binding proteins and surface antigens.

The primary auditory cortex (A1) of monkeys was investigated by immunohistochemistry, using antibodies to gamma-aminobutyric acid (GABA), to the calcium binding proteins parvalbumin and calbindin, and to certain proteoglycan epitopes. The two calcium binding proteins were found to be localized in subpopulations of GABAergic neurons. Parvalbumin immunoreactive cells were mostly found in the middle layers of the cortex. Parvalbumin immunoreactivity was found in fibres in the white matter underlying A1 and a particularly dense concentration of parvalbumin immunoreactive fibers and terminals occurred in layer IV suggesting that a significant population of geniculocortical fibers is also parvalbumin positive. Calbindin positive cells were mostly located in superficial layers and in these layers the neuropil staining was also dense. Two monoclonal antibodies (MAbs) raised against monkey brain tissue and which had previously been shown to recognize neuronal surface antigens stained overlapping subpopulations of GABAergic cells. Occasional pyramidal cells were also immunoreactive. Most of the MAb positive cells were found in the middle layers and all were parvalbumin but not calbindin immunoreactive. Although the physiological roles in the brain for calcium binding proteins and the relevant cell surface markers have not yet been clarified, the presence of these markers in selected subpopulations of cells suggests the existence of functionally distinct circuits in AI cortex.