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. 2013 Jun;83(6):1200–1208. doi: 10.1124/mol.112.084558

Fig. 2.

Fig. 2.

Allosteric coagonist modeling of GABA and etomidate-dependent activity in α1β2M286Cγ2L GABAA receptors. (A) Schematic of the MWC equilibrium coagonist mechanism for GABA and etomidate gating (eq. 3 in Materials and Methods). The mechanism is defined by five parameters: L0 is a dimensionless basal equilibrium gating variable for inactive/active (R/O) receptors, which was constrained as described in the text. KG and KE are equilibrium dissociation constants for GABA and etomidate binding to inactive (R) receptors, and c and d are dimensionless parameters representing the ratios of dissociation constants in active (O) versus inactive (R) states. The agonist efficacies of GABA and etomidate are inversely related to, respectively, c and d. (B) The trace depicts current recorded from a single oocyte expressing α1β2M286Cγ2L receptors, activated initially with 10 mM GABA, then with 10 mM GABA plus 2 μM alphaxalone, an allosteric modulator that does not interact directly with the etomidate site. The addition of alphaxalone increases the current elicited with GABA alone by about 50%, indicating that maximal GABA activates about 65% of receptors. (C) Two current traces are shown; these traces were recorded from the same oocyte expressing α1β2M286Cγ2L receptors; 3 mM GABA activates a large inward current. Picrotoxin (PTX, 2 mM) does not produce any discernable outward current, indicating that the spontaneous open probability of receptors is below the threshold of detection (0.1%) using this method. (D) Two current traces are shown, recorded from the same oocyte expressing α1L264Tβ2M286Cγ2L receptors. The α1L264T mutation confers spontaneous activity, evident from the apparent outward current with PTX application. The spontaneous activity is about 3% of the GABA-activated inward current. (E and F) Symbols represent estimated Popen values calculated from averaged data in Fig. 1, C and D. Lines through symbols represent fits to the MWC coagonist model (eq. 3, Materials and Methods) with parameters reported in Table 1. (E) GABA-dependent activation in the absence (solid squares) and presence of 3.2 μM etomidate (open squares). (F) Etomidate-dependent activation in the absence (solid circles) and presence of 20 μM GABA (open circles).