Reduced Activation of the Synaptic-Type GABAA Receptor Following Prolonged Exposure to Low Concentrations of Agonists: Relationship between Tonic Activity and Desensitization.

Synaptic GABAA receptors are alternately exposed to short pulses of a high, millimolar concentration of GABA and prolonged periods of low, micromolar concentration of the transmitter. Prior work has indicated that exposure to micromolar concentrations of GABA can both activate the postsynaptic receptors generating sustained low-amplitude current and desensitize the receptors, thereby reducing the peak amplitude of subsequent synaptic response. However, the precise relationship between tonic activation and reduction of peak response is not known. Here, we have measured the effect of prolonged exposure to GABA or the combination of GABA and the neurosteroid allopregnanolone, which was intended to desensitize a fraction of receptors, on a subsequent response to a high concentration of agonist in human α1β3γ2L receptors expressed in Xenopus oocytes. We show that the reduction in the peak amplitude of the post-exposure test response correlates with the open probability of the preceding desensitizing response. Curve fitting of the inhibitory relationship yielded an IC50 of 12.5 µM and a Hill coefficient of -1.61. The activation and desensitization data were mechanistically analyzed in the framework of a three-state Resting-Active-Desensitized model. Using the estimated affinity, efficacy, and desensitization parameters, we calculated the amount of desensitization that would accumulate during a long (2-minute) application of GABA or GABA plus allopregnanolone. The results indicate that accumulation of desensitization depends on the level of activity rather than agonist or potentiator concentration per se. We estimate that in the presence of 1 µM GABA, approximately 5% of α1β3γ2L receptors are functionally eliminated because of desensitization. SIGNIFICANCE STATEMENT: We present an analytical approach to quantify and predict the loss of activatable GABAA receptors due to desensitization in the presence of transmitter and the steroid allopregnanolone. The findings indicate that the peak amplitude of the synaptic response is influenced by ambient GABA and that changes in ambient concentrations of the transmitter and other GABAergic agents can modify tonically and phasically activated synaptic receptors in opposite directions.