![Figure 6. An elements-based kinetic model for BZD-positive modulation of GABAA receptors. (A) Binary elements model representation, which extends scheme I (Fig. 4 A) by inserting intermediate elements (F) involved in transducing agonist binding at two agonist binding site elements (A) to gating of both main and fast/slow desensitization gates (m, df, ds). In addition, a drug-binding site (D) is coupled to one of the intermediate elements. Elements are depicted as labeled circles, and interactions as lines connecting the pair of interacting elements (e.g., see Fig. 3). All model parameters are referenced with respect to the symbol of their associated elements such that αi and βi are the rate constants for element i, and Θij, Λij, and Λji are the interaction factors between elements i and j (see Materials and methods and Eqs. 7–12). Model parameters reflect simulations at a temperature of 298.15 K. Optimized rate constants ([M−1]s−1) across patches are αA = 3.2 ± 0.1 × 106, βA = 1,836 ± 124, αF = 5.5 ± 0.4, βF = 1124 ± 25, αdf = 0.25 ± 0.02, βdf = 31 ± 2, αds = 3.3 ± 0.3 × 10−3, and βds = 0.19 ± 0.01. Optimized interaction factors are ΛFA = 3,191 ± 25, ΘDF = 18.6 ± 1.1, and ΛDF = 2.6 ± 0.1. All other model parameters were constrained as described in the Materials and methods. (B) Simulated open probability in response to 4-ms (left) or 500-ms (right) pulses of 10 mM GABA either in the absence (control) or presence of 1 µM of drug for scheme VI, overlaid on current recordings from outside-out patches (gray). Current recordings in the presence and absence of 1 µM of the BZD-positive modulator diazepam (DZ) were interleaved on the same patch. The top trace is the solution exchange time course at the pipet tip after the experiment (see Materials and methods). (C) Same as B, except for 20-ms pulses of 10 µM GABA. (D) Family of simulated current responses to 500-ms pulses of varying GABA concentrations in the absence (blue traces) or presence of 1 µM drug (red traces). (E) Peak open probability dose–response relationships for both full (e.g., GABA) and partial agonists in the absence and presence of 1 µM drug. Partial agonism was modeled by a reduced interaction energy between agonist-binding sites and the intermediate elements (see Results). (F) Increase in simulated unliganded channel open probability upon application of 1 µM drug (red bar) for scheme VI.](https://cdn.rupress.org/rup/content_public/journal/jgp/144/1/10.1085_jgp.201411183/4/jgp_201411183_fig6.jpeg?Expires=1767803139&Signature=MxU1FkOjgpvN38oemOgmfAuXao-VaWSNjrFd1S0XN4XHY47b7kxU8QkOHna45rcK9eMHINswiihkjZb-wBfiIi3GvyMePvwa0PzeLPoLYzfPfO2C0V62V~iuEQ2SUCMKzWWsTEg6TeF3ebSZRqi0Io7VhDm2mJLrMPy85fuHS1Pb765ikdHWL3NQrMKPlPfFF7gFDGMUdmD38650-y3~uDKgAMzWyomUivMGT1dqV-c~dwHxuOBbHGlJDWuHattozZRB9BytjAFamEGlgGSDwbq2oKJEGfNqHf8Gm5FRen1uDvhkeq2iI0CYavEfnDhUzCASQy0VS3xaIHT4q39j6A__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA)
An elements-based kinetic model for BZD-positive modulation of GABAA receptors. (A) Binary elements model representation, which extends scheme I (Fig. 4 A) by inserting intermediate elements (F) involved in transducing agonist binding at two agonist binding site elements (A) to gating of both main and fast/slow desensitization gates (m, df, ds). In addition, a drug-binding site (D) is coupled to one of the intermediate elements. Elements are depicted as labeled circles, and interactions as lines connecting the pair of interacting elements (e.g., see Fig. 3). All model parameters are referenced with respect to the symbol of their associated elements such that αi and βi are the rate constants for element i, and Θij, Λij, and Λji are the interaction factors between elements i and j (see Materials and methods and Eqs. 7–12). Model parameters reflect simulations at a temperature of 298.15 K. Optimized rate constants ([M−1]s−1) across patches are αA = 3.2 ± 0.1 × 106, βA = 1,836 ± 124, αF = 5.5 ± 0.4, βF = 1124 ± 25, αdf = 0.25 ± 0.02, βdf = 31 ± 2, αds = 3.3 ± 0.3 × 10−3, and βds = 0.19 ± 0.01. Optimized interaction factors are ΛFA = 3,191 ± 25, ΘDF = 18.6 ± 1.1, and ΛDF = 2.6 ± 0.1. All other model parameters were constrained as described in the Materials and methods. (B) Simulated open probability in response to 4-ms (left) or 500-ms (right) pulses of 10 mM GABA either in the absence (control) or presence of 1 µM of drug for scheme VI, overlaid on current recordings from outside-out patches (gray). Current recordings in the presence and absence of 1 µM of the BZD-positive modulator diazepam (DZ) were interleaved on the same patch. The top trace is the solution exchange time course at the pipet tip after the experiment (see Materials and methods). (C) Same as B, except for 20-ms pulses of 10 µM GABA. (D) Family of simulated current responses to 500-ms pulses of varying GABA concentrations in the absence (blue traces) or presence of 1 µM drug (red traces). (E) Peak open probability dose–response relationships for both full (e.g., GABA) and partial agonists in the absence and presence of 1 µM drug. Partial agonism was modeled by a reduced interaction energy between agonist-binding sites and the intermediate elements (see Results). (F) Increase in simulated unliganded channel open probability upon application of 1 µM drug (red bar) for scheme VI.
