Currents Activated by GABA and their Modulation by Zn2+ in Cerebellar Granule Cells in Culture

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Abstract
Whole-cell and single-channel currents evoked by gamma-aminobutyric acid (GABA) were recorded from rat cerebellar granule cells in culture. The electrophysiological properties of these currents were studied in control condition and in the presence of external Zn2+ (10-30 microM). GABA (10 microM) induced bicuculline-sensitive whole-cell currents which desensitized. The desensitization was more rapid for higher concentrations of GABA (30-300 microM). The current-voltage relation of GABA currents was linear from -70 to +50 mV. Two different types of cells were found with respect to the stoichiometry for agonist binding, one with Hill coefficient 1.5 and another one with coefficient 1. The half-maximum concentration displayed more variability, with values varying from 10 to 50 microM. The time constant of recovery from desensitization (tau r) was estimated to be 36 s. Zn2+ (30 microM) blocked GABA-activated whole-cell currents in a non-competitive and voltage-independent way without a significant change in the current kinetics. In excised outside-out patches, GABA (0.5 microM) activated single-channel events of 19 and 31 pS. Kinetic analysis yielded two mean shut times (tau c1 = 2.70 ms, tau c2 = 205 ms) and one mean open time (tau o = 3.64 ms). Zn2+ (10 microM) did not affect single-channel conductances and mean open and shut times, but significantly reduced the probability of opening from 0.17 to 0.06. It is probable that Zn2+ binds to a site located on the extracellular part of the GABAA receptor channel complex.