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The Journal of Neuroscience, September 3, 2003, 23(22):7981-7992

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Modulation of GABA_A Receptors by Hydrogen Ions Reveals Synaptic GABA Transient and a Crucial Role of the Desensitization Process

Jerzy W. Mozrzymas,¹ Ewa D. armowska,¹ Maria Pytel,¹ and Katarzyna Mercik^1,2

¹Department of Biophysics, Wroclaw Medical University, 50-368 Wroclaw, Poland, and ²Institute of Physics, Technical University of Wroclaw, 50-370 Wroclaw, Poland

Protons are the most ubiquitous and very potent modulators of the biological systems. Hydrogen ions are known to modulate GABA_A receptors (GABA_ARs), but the mechanism whereby these ions affect IPSCs and the gating of GABA_ARs is not clear. In the present study we examined the effect of protons on miniature IPSCs (mIPSCs) and found that hydrogen ions strongly affected both their amplitude and time course. To explore the underlying mechanisms with resolution adequate to the time scale of synaptic transmission, we recorded current responses to ultrafast GABA applications at various pH. These experiments revealed that the major effect of protons on GABA_AR gating is a strong enhancement of desensitization and binding rates at increasing pH. This analysis also indicated that desensitization rate is the fastest ligand-independent transition in the GABA_AR gating scheme. Although proton effects on the time course of mIPSCs and current responses to saturating [GABA] were similar, the pH dependencies of amplitudes were almost opposite. Our quantitative analysis, based on model simulations, indicated that this difference resulted from a much shorter receptor exposure to agonist in the case of mIPSCs. Modeling of IPSCs as current responses to brief exponentially decaying GABA applications was sufficient to reproduce correctly the pH dependence of mIPSCs, and optimal fit was obtained for peak [GABA] of 1.5-3 mM and a clearance time constant of 0.075-0.125 msec. Our analysis indicates that, for these parameters of GABA transient, in control conditions (pH 7.2) mIPSCs are not saturated.

Key words: hydrogen ions; GABA_A receptor; gating; mIPSC; synaptic GABA transient; synaptic receptor saturation; desensitization

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Received Feb 21, 2003; revised June 12, 2003; accepted June 12, 2003.

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