GABA type a receptor activation in the allosteric coagonist model framework: Relationship between EC₅₀ and basal activity

The concerted transition model for multimeric proteins is a simple formulation for analyzing the behavior of transmitter-gated ion channels.Weused the model to examine the relationship between the EC₅₀ for activation of the GABA type A (GABA_A) receptor by the transmitter GABA and basal activity employing concatemeric ternary GABA_A receptors expressed in Xenopus oocytes. Basal activity, reflecting the receptor function in the absence of the transmitter, can be changed either by mutation to increase constitutive activity or by the addition of a second agonist (acting at a different site) to increase background activity. The model predicts that either mechanism for producing a change in basal activity will result in identical effects on the EC₅₀. We examined receptor activation by GABA while changing the level of basal activity with the allosterically acting anesthetics propofol, pentobarbital, or alfaxalone. We found that the relationship between EC₅₀ and basal activity was well described by the concerted transition model. Changes in the basal activity by gain-of-function mutations also resulted in predictable changes in the EC₅₀. Finally, we altered the number of GABA-binding sites by a mutation and again found that the relationship could be well described by the model. Overall, the results support the idea that interactions between the transmitter GABA and the allosteric agonists propofol, pentobarbital, or alfaxalone can be understood as reflecting additive and independent free energy changes, without assuming any specific interactions.