Predicted responses to coapplication of two orthosteric agonists. The effects of coapplication of GABA with low-efficacy agonist P4S (A) or high-efficacy agonist β-alanine (B) on receptor open probability. The concentration-response data for P4S and β-alanine in the absence of GABA are shown as open circles. The concentrations of GABA were held at 5.0, 7.6, 12.2, or 29.8 μM, to elicit responses with Popen values of 0.05, 0.1, 0.2, and 0.5, respectively (filled circles, bottom to top in the graphs). The simulations were done using eq. 2 and the parameters provided in Table 1. The approach assumes that two shared sites mediate the actions of GABA + P4S and GABA + β-alanine.