GABA-Mediated Inhibition in Cultured Neurons of the Rat Brain Cortex

In cultured pyramidal neurons of the rat brain cortex, we recorded (in the whole-cell configuration) postsynaptic currents (PSC) evoked by direct electrical microstimulation of an axon of the interneuron adjacent to the pyramidal cell. Application of 5 M bicuculline rapidly, entirely, and reversibly blocked these currents. Linear changes in the holding potential on the membrane of the postsynaptic cell resulted in linear changes in the amplitude of averaged currents. The currents underwent reversion when the holding potential was –16 mV, which was close to the reversal potential for Cl^- ions at their respective concentrations in the extra- and intracellular solutions. We conclude that the recorded currents are inhibitory PSC (IPSC) mediated by GABA release. The amplitudes of the recorded currents varied from a measurable minimum (8 pA) to more than 150 pA at a holding potential on the postsynaptic cell membrane of –80 mV. Times to peak of the high- and low-amplitude currents showed no significant differences, being about 6.4 msec on average. Decays of the current could be satisfactorily approximated by a monoexponential function with a mean time constant of 17 msec. The time constants of IPSC decay were distributed accordingly to the Gaussian law. In some cases, the amplitude distributions of IPSC were unimodal ((with a rightward asymmetry), but in most cases they were clearly polymodal. The amplitude distribution can be described by the sum of several Gaussian distributions; the distance between modes of the Gaussians was 25 ± 6 pA, on average. The obtained estimates of the amplitude of monoquantal GABA-induced IPSC in neurons of the brain cortex allow us to conclude that in various CNS regions the dimension of the vesicles in GABA-ergic synapses formed by inhibitory interneurons is identical.