| Summary: | GABA-A receptors are ligand-gated chloride channels that mediate the majority of fast inhibitory signaling in the central nervous system. Their kinetic properties determine the charge transfer and timing of inhibitory post-synaptic currents (IPSCs), and consequently, alterations in GABA-A receptor kinetics due to mutations, allosteric modulators, or post-translational modifications can significantly influence the function of neuronal circuits. However, one of the most defining kinetic features of GABA-A receptor currents their ability to undergo extensive and multi-phasic desensitization in the continued presence of agonist remains poorly understood. Indeed, while the phenomenon of desensitization has been well characterized, neither its microscopic kinetic basis nor its physiological relevance is clear. Many investigators have actually dismissed the phenomenon as an experimental artifact, as its visualization requires receptor activation for physiologically irrelevant durations.
In the studies described herein, we challenge this notion, and argue instead that GABA-A receptor desensitization is a critical determinant of IPSC kinetics. Using a combination of patch-clamp electrophysiology and Markov modeling of GABA-A receptor function, we demonstrate that in addition to shaping the decay of individual IPSCs, desensitization is required for the phenomenon of repeated pulse inhibition, the loss of IPSC amplitude observed in the setting of high frequency stimulation. Interestingly, our results also suggest that desensitization is important for extrasynaptic signaling, where receptors are persistently activated by low concentrations of ambient GABA. Analytic solutions of Markov model equilibrium and non-equilibrium state occupancies demonstrate that the underlying desensitized state not only increases receptor affinity for GABA, but also buffers equilibrium currents from the effects of negative modulators and fluctuations in the ambient GABA concentration. Of note, the analytic solutions also exposed the microscopic kinetic determinants of desensitization, demonstrating a previously unrecognized dependence of this phenomenon on a subset of rate constants.
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