Effect of extracellular pH on recombinant α1β2γ2 and α1β2 GABAA receptors

Recently, we have reported that extracellular protons allosterically modulated neuronal GABAA receptors [Mozrzymas, J.W., Zarnowska, E.D., Pytel, M., Mercik, K., 2003a. Modulation of GABA(A) receptors by hydrogen ions reveals synaptic GABA transient and a crucial role of desensitiztion process. Journal of Neuroscience 23, 7981–7992]. However, GABAARs in neurons are heterogeneous and the effect of hydrogen ions depends on the receptor subtype. In particular, γ2 subunit sets the receptor sensibility to several modulators including protons. However, the mechanisms whereby protons modulate γ2-containing and γ2-free GABAARs have not been fully elucidated. To this end, current responses to ultrafast GABA applications were recorded for α1β2γ2 and α1β2 receptors at different pH values. For both receptor types, increase in pH induced a decrease in amplitudes of currents elicited by saturating [GABA] but this effect was stronger for α1β2 receptors. In the case of α1β2γ2 receptors, protons strongly affected the current time course due to a down regulation of binding and desensitization rates. This effect was qualitatively similar to that described in neurons. Protons strongly influenced the amplitude of α1β2 receptor-mediated currents but the effect on their kinetics was weak suggesting a predominant direct non-competitive inhibition with a minor allosteric modulation. In conclusion, we provide evidence that extracellular protons strongly affect GABAA receptors and that, depending on the presence of the γ2 subunit, the modulatory mechanisms show profound quantitative and qualitative differences.