Responses to GABA recorded from identified rat visual cortical neurons.

Responses to GABA were recorded from 87 neurons of rat visual cortical slices. Pyramidal and nonpyramidal cells were identified by intracellular dye injection, and their responses were compared. All identified pyramidal and nonpyramidal cells, as well as unstained cells, responded to GABA ejected from a pipette that was positioned within 300 micron of the soma. Their responses were similar, regardless of their morphology. In addition, GABA responses of visual cortical neurons could not be distinguished from those of other areas of the neocortex, or pyramidal cells in area CA1 of hippocampus. Depending on the site of application, there appeared to be two types of GABA responses that were present in all cells. The first was generated by application of GABA to the soma (GABAs response; mean reversal potential = -71.7 mV). The second occurred when GABA was applied on dendrites (GABAD response; mean reversal potential = -49.3 mV). When GABA was ejected on proximal dendritic regions, both responses could be observed in combination. Both GABAs and GABAD responses were accompanied by extremely large increases in conductance. In some cells, a third type of GABA response was elicited following somatic or dendritic GABA application. This response was a relatively small-amplitude, long-lasting hyperpolarization which followed a GABAS and GABAD response (late hyperpolarization, mean reversal potential = -79.8 mV).