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Journal of neurochemistry

Interconvertible kinetic states of t-butylbicycloorthobenzoate binding sites of the gamma-aminobutyric acidA ionophores.


PMID 8245964

Abstract

The kinetics of t-[3H]butylbicycloorthobenzoate (TBOB) binding to the convulsant sites of the gamma-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 microM GABA and decelerated by 1 microM 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 multi-affinity states of the convulsant sites depending on the presence of GABA or bicuculline methochloride. Apparent association rate constants do not obey the equation k(app) = k(off) + k(on) [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 microM) 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 microM DIDS) leaving the enhancement by AVM B1a unaffected.

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