CURRENTS ACTIVATED BY GABA AND THEIR MODULATION BY ZN2+ IN CEREBELLAR GRANULE CELLS IN CULTURE

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 muM). GABA (10 muM) induced bicuculline-sensitive whole-cell currents which desensitized. The desensitization was more rapid for higher concentrations of GABA (30 - 300 muM). 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 muM. The time constant of recovery from desensitization (tau(r)) was estimated to be 36 s. Zn2+ (30 muM) 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 muM) 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 muM) 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 GABA(A) receptor channel complex.