Laminar distribution of GABA-immunoreactive neurons and processes in area 18 of the cat.

Intracortical inhibition mediated by the neurotransmitter gamma-amino butyric acid (GABA) plays a crucial role in the formation of the physiological response properties of neurons in mammalian visual cortex. Using a potent antibody developed against the amino acid neurotransmitter. GABA (Immunoclear Co., USA), we have identified GABA-immunoreactive neuronal somata and processes in area 18 of the cat. The GABA-positive neurons included multipolar, bipolar and bitufted but not pyramidal cell types. The density of GABA-immunoreactive neurons was higher in Layers I-IV (range 182-205 cells/mm2) than in Layers V-VI (range 78-92 cells/mm2). The mean areal measurement of GABA-immunoreactive somatic profiles was 135 micron2 (s.d. = 66). We observed numerous GABA-immunoreactive fiber fragments predominantly in Layers I, III, IV and V. Most of the fibers were oriented with their long axis tangential to the cortical surface, but, vertically-oriented fibers were observed as well. Many of the fibers were axonal-like and appeared beaded in Layer I and occasionally in II, while most of the fibers in the other layers appeared myelinated. A dense GABA-immunoreactive neuropil was present in Layer V. Results from our studies provide immunohistochemical evidence for a system of GABA-immunoreactive neurons and axonal collaterals which presumably mediate inhibition in the visual cortex. Since many of the GABA-immunoreactive fibers were oriented tangential to the cortical surface, the structural elements required for inhibition between functional columns are present. GABA-mediated inhibition both within and between functional columns likely assists in the formation of receptive field properties in the visual cortex.