Interconvertible Kinetic States of t-Butylbicycloorthobenzoate Binding Sites of the γ-Aminobutyric AcidA Ionophores |
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Authors: | Gabor Maksay Clementina M van Rijn |
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Institution: | Department of Molecular Pharmacology, Central Research Institute for Chemistry, Hungarian Academy of Sciences, Budapest, Hungary;and Department of Comparative and Physiological Psychology, University of Nijmegen, The Netherlands |
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Abstract: | The kinetics of t-3H]butylbicycloorthobenzoate (TBOB) binding to the convulsant sites of the γ-aminobutyric acidA (GABAA) receptor-ionophore complex were examined in synaptosomal membrane preparations of rat brain. On and off rates of TBOB binding were accelerated by 1 μM GABA and decelerated by 1 μM bicuculline methochloride, a GABAA antagonist. The presence of GABA and bicuculline methochloride created rapid and slow phases of dissociation, respectively. The three groups of rate constants distinguished for the dissociation of 4 nM and 30 nM 3H]TBOB represent multiaffinity states of the convulsant sites depending on the presence of GABA or bicuculline methochloride. Apparent association rate constants do not obey the equation kapp=koff±kon TBOB] without assuming interconvertibility of the kinetic states during binding. Avermectin B1a (AVM B1a), a chloride channel opening agent, accelerated the association and dissociation of TBOB and resulted in a biphasic effect on TBOB binding, i.e., enhancement at low concentrations (EC50, 7.8 nM) followed by displacement at high concentrations (IC50 6.3 μM) of AVM B1a. AVM B1a resulted in similar biphasic effects on t- 35S]butylbicyclophosphorothionate binding. DIDS, an isothiocyanatostilbene derivative with irreversible anion channel blocking effect, selectively inhibited basal 3H]TBOB binding (IC50 125 μM DIDS) leaving the enhancement by AVM B1a unaffected. |
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Keywords: | GABAA receptor complex Kinetics t-Butylbicycloorthobenzoate binding Bicuculline methochloride Avermectin B1a Interconvertible binding sites of cage convulsants |
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