Allosteric modulation of GABAA receptors by extracellular ATP

Background: The gamma-aminobutyric acid type A receptor (GABA(A)R) is the primary receptor mediating fast synaptic inhibition in the brain and plays a critical role in modulation of neuronal excitability and neural networks. Previous studies have demonstrated that ATP and its nucleotide analogs may regulate the function of GABA(A)Rs via Ca2+-dependent intracellular mechanisms, which require activation of purinergic 2 (P2) receptors or cross-talk between two receptors. Results: Here, we report a potentiation of GABA(A)Rs by extracellular ATP via a previously un-recognized allosteric mechanism. Using cultured hippocampal neurons as well as HEK293 cells transiently expressing GABA(A)Rs, we demonstrate that extracellular ATP potentiates GABA(A)R mediated currents in a dose-dependent manner with an EC50 of 2.1 +/- 0.2 mM. The potentiation was mediated by a postsynaptic mechanism that was not dependent on activation of either ecto-protein kinase or P2 receptors. Single channel recordings from cell-free excised membrane patches under outside-out mode or isolated membrane patches under cell-attached mode suggest that the ATP modulation of GABA currents is achieved through a direct action of ATP on the channels themselves and manifested by increasing the single channel open probability without alteration of its conductance. Moreover, this ATP potentiation of GABA(A)R could be reconstituted in HEK293 cells that transiently expressed recombinant rat GABA(A)Rs. Conclusions: Our data strongly suggest that extracellular ATP allosterically potentiates GABA(A)R-gated chloride channels. This novel mode of ATP-mediated modulation of GABA(A)Rs may play an important role in regulating neuronal excitability and thereby in fine-tuning the excitation-inhibition balance under conditions where a high level of extracellular ATP is ensured.