Null method to estimate the maximal P_A at subsaturating concentrations of agonist

The maximal probability of being in an active state (P_A,max) is a measure of gating efficacy for a given agonist acting on a given receptor channel. In macroscopic electrophysiological recordings, P_A,max is typically estimated by comparing the amplitude of the current response to a saturating concentration of a test agonist to that of a reference agonist with known P_A. Here, we describe an approach to estimate the P_A,max for low-efficacy agonists at subsaturating concentrations. In this approach, the amplitude of the response to a high-efficacy control agonist applied alone is compared with the amplitude of the response to a control agonist coapplied with the low-efficacy test agonist that binds to the same site(s). If the response to the combination is larger than the response to the control agonist alone, then the P_A,max of the test agonist is greater than the P_A of the control response. Conversely, if the response to the control agonist is reduced upon exposure to the test agonist, then the P_A,max of the test agonist is smaller than the P_A of the control response. The exact P_A,max of the test agonist can be determined by testing its effect at different concentrations of the control agonist to estimate the P_A at which the effect changes direction. The main advantage of this approach lies in the ability to use low, subsaturating concentrations of the test agonist. The model-based predictions are supported by observations from activation of heteromeric and homomeric GABA_A receptors by combinations of high- and low-efficacy orthosteric agonists.