Barbiturate-Benzodiazepine Interactions at the ??-Aminobutyric AcidA Receptor in Rat Cerebral Cortical Synaptoneurosomes

Abstract

Combinations of benzodiazepines (midazolam and diazepam) with barbiturates (pentobarbital and phenobarbital) exhibit synergistic (supra-additive) hypnotic interactions in rats. Because both benzodiazepines and barbiturates interact with the γ-aminobutyric acidA (GABAA) receptor complex, we have tested the hypothesis that these supra-additive hypnotic interactions are due to a synergistic effect on 36CI− conductance subsequent to binding at allosterically coupled sites on the GABAA receptor ionophore complex. Equilibrium binding and 36CI− flux measurements were performed under nearly identical conditions using rat brain cerebrocortical synaptoneuroomes. The benzodiazepines and barbiturates alone both allosterically enhance binding of [3H]muscimol to comparable, but modest, extents (range = 18%–32% enhancement). Isobolographic analysis reveals that combinations of benzodiazepines and barbiturates do in fact produce a synergistic enhancement of [3H]muscimol binding. Paradoxically, this effect is not translated into a synergistic enhancement of muscimolstimulated 36CI− flux. Because the positively cooperative interactions between benzodiazepines and barbiturates, as demonstrated both behaviorally and by binding measurements, are not reflected in enhanced CI− conductance, the mechanistic basis for hypnotic synergism may involve other non-GABAergic components.