Distribution and proportions of GABA-immunoreactive neurons in cat primary somatosensory cortex

Certain receptive field properties of cortical neurons depend upon inhibitory, GABAergic inputs. In the somatosensory cortex, iontophoresis of bicuculline, a GABAA receptor blocker, results in enlargement of receptive fields. However, bicuculline's effectiveness in changing receptive field size varies with the neuron's adaptation characteristics, location within a particular submodality region, and laminar location. To test whether regional differences in the effectiveness of bicuculline are correlated with the distribution of cortical GABAergic neurons, we determined the numbers and proportions of GABA-immunoreactive [GABA(+)] neurons within cat primary somatosensory cortex. The laminar distribution of GABA(+) neurons was similar across all four cytoarchitectonic areas of primary somatosensory cortex, with layer II containing the highest areal density of GABA(+) neurons. Numerical proportions of GABA(+) neurons in the total neuron population were similar in areas 3b and 2 (29.8% and 22.6%, respectively). Laminar distributions of the proportions of GABA(+) neurons were also similar in these two areas; in both areas, layer I contained the highest proportion of GABA(+) neurons. The laminar distributions of GABA(+) neuron densities as well as GABA(+) neuron proportions differed from the reported laminar distribution of bicuculline effects on receptive field size. Moreover, within area 3b, these measures showed no evident patterns that might correspond to rapidly adapting and slowly adapting submodality regions.