Steady-state activation and modulation of the synaptic-type α1β2γ2L GABAA receptor by combinations of physiological and clinical ligands

The synaptic alpha 1 beta 2 gamma 2 GABA(A) receptor is activated phasically by presynaptically released GABA. The receptor is considered to be inactive between synaptic events when exposed to ambient GABA because of its low resting affinity to the transmitter. We tested the hypothesis that a combination of physiological and/or clinical positive allosteric modulators of the GABA(A) receptor with ambient GABA generates measurable steady-state activity. Recombinant alpha 1 beta 2 gamma 2L GABA(A) receptors were expressed in Xenopus oocytes and activated by combinations of low concentrations of orthosteric (GABA, taurine) and allosteric (the steroid allopregnanolone, the anesthetic propofol) agonists, in the absence and presence of the inhibitory steroid pregnenolone sulfate. Steady-state activity was analyzed using the three-state cyclic Resting-Active-Desensitized model. We estimate that the steady-state open probability of the synaptic alpha 1 beta 2 gamma 2L GABA(A) receptor in the presence of ambient GABA (1 mu mol/L), taurine (10 mu mol/L), and physiological levels of allopregnanolone (0.01 mu mol/L) and pregnenolone sulfate (0.1 mu mol/L) is 0.008. Coapplication of a clinical concentration of propofol (1 mu mol/L) increases the steady-state open probability to 0.03. Comparison of total charge transfer for phasic and tonic activity indicates that steady-state activity can contribute strongly (similar to 20 to >99%) to integrated activity from the synaptic GABA(A) receptor.