Regional distribution of GABAA receptor binding sites in cat somatosensory and motor cortex.

Inhibition in primary sensory cortex plays a role in neuronal responses to peripheral stimuli. For many neurons in cat primary somatosensory cortex, blockade of GABAA receptors by bicuculline results in receptive field enlargement. The magnitude of this effect varies with the neuron's adaptation characteristics and its location in particular laminae and submodality regions. To test whether these variations are correlated with the distribution of GABAA receptors, we analyzed [3H]muscimol binding in cat primary somatosensory and motor cortical areas. The highest levels of binding were in layers I-III, and the lowest levels were in layers V-VI. In somatosensory cortical areas, layer IV was distinguished by higher levels of binding than in adjacent layers. Within layer IV, levels of binding were significantly higher in posterior area 3b than in anterior area 3b. These differences may correspond to the rapidly adapting and slowly adapting submodality regions which have been described in this area. The laminar distribution of [3H]muscimol binding differed from that of [3H]flunitrazepam, and neither resembled the distribution of the magnitude of bicuculline's effects on receptive field size. The laminar distribution of [3H]muscimol binding was highly correlated with the areal density of GABA-immunoreactive neurons described in a companion study.