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

The maximal probability of being in an active state (PA,max) is a measure of gating efficacy for a given agonist acting on a given receptor channel. In macroscopic electrophysiological recordings, PA,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 PA. Here, we describe an approach to estimate the PA,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 PA,max of the test agonist is greater than the PA of the control response. Conversely, if the response to the control agonist is reduced upon exposure to the test agonist, then the PA,max of the test agonist is smaller than the PA of the control response. The exact PA,max of the test agonist can be determined by testing its effect at different concentrations of the control agonist to estimate the PA 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 GABAA receptors by combinations of high- and low-efficacy orthosteric agonists.