Proton sensitivity of rat cerebellar granule cell GABA(A) receptors: dependence on neuronal development

1. The effect of GABA(A) receptor development in culture on the modulation of GBA-induced currents by external H+ was examined in cerebellar granule c ells using whole-cell and single- channel recording. 2. Equilibrium concentration-response curves revealed a lower potency for G ABA between 11 and 12 days in vitro (DIV) resulting in a shift of the EC50 from 10.7 to 2.4 muM. 3. For granule cells before 11 DIV, the peak GABA-activated current was inh ibited at low external pH and enhanced at high pH with a pK(a) of 6.65. For the steady-state response, low pH was inhibitory with a pK(a) of 5.56. 4. After 11 DIV, the peak GABA-activated current was largely pH insensitive ; however, the steady-state current was potentiated at low pH with a pK(a) of 6.84. 5. Single GABA-activated ion channels were recorded from outside-out patche s of granule cell bodies. At pH 5.4-9.4, single GABA channels exhibited mul tiple conductance states occurring at 22-26, 16-17 and 12-14 pS. The conduc tance levels were not significantly altered over the time period of study, nor by changing the external H+ concentration. 6. Two exponential functions were required to fit the open-time frequency h istograms at both early (< 11 DIV) and late (> 11 DIV) development times at each H+ concentration. The short and long open time constants were unaffec ted either by the extracellular H+ concentration or by neuronal development . 7. The distribution of all shut times was fitted by the sum of three expone ntials designated as short, intermediate and long. At acidic pH, the long s hut time constant decreased with development as did the relative contributi on of these components to the overall distribution. This was concurrent wit h an increase in the mean probability of channel opening. 8. In conclusion, this study demonstrates in cerebellar granule cells that external pH can either reduce, have no effect on, or enhance GABA-activated responses depending on the stage of development, possibly related to the s ubunit composition of the GABA(A) receptors. The mode of interaction of Hat the single-channel level and implications of such interactions at cerebe llar granule cell GABA(A) receptors are discussed.