The temperature dependence of [35S]-t-butylbicyclophosphorothionate (TBPS) binding to the convulsant sites of the GABAA receptor complex was studied in membrane preparations of rat forebrain. Although specific [35S]TBPS binding was maximal around 20°C, the rate constants of dissociation decreased monotonously between 37°C and 2°C. The displacing potencies of the convulsant S(+) enantiomer of 1-methyl-5-phenyl-5-propyl-barbituric acid (MPPB) (IC50 = 1250 ± 30 μM) and the depressant R(-) MPPB (IC50 = 310 ± 5 μM) did not show significant changes between 19°C and 37°C. Therefore barbiturate binding seems to be driven by entropic, rather than enthalpic changes. An excess of MPPB enantiomers elicited accelerated and polyphasic dissociations of [35S]TBPS as compared to the monophasic dissociation by TBPS. Arrhenius analysis was applied to the measurable initial rate constants of dissociation. Arrhenius plots were linear between 2°C and 37°C. Activation parameters were similar when [35S] TBPS dissociation was triggered by the convulsants TBPS and S(+) MPPB. It can be attributed to similar conformations of the closed ionophore complex. In contrast, the depressant R(-) MPPB strongly decreased the activation energy of TBPS dissociation from the open ionophore ternary complex. In whole-cell patch-clamp experiments R(-) MPPB, but not S(+) MPPB, elicited chloride currents in rat primary cortical cultures with an EC50 value of 560 ± 30 μM and a Hill coefficient of 2.9 ± 0.2. These currents were similar to those elicited by GABA and blocked by TBPS. A kinetic scheme is proposed for the dissociation of TBPS and to explain the different effects of MPPB enantiomers. Submillimolar R(-) MPPB is supposed to bind to (about three) barbiturate sites on GABAA-ionophores and to open them in a cooperative manner to result in a decreased activation energy for accelerated displacement of convulsant binding.
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