Rationale: Interactions among compounds at GABA(A) receptors might have important implications for the therapeutic and other effects of positive GABA(A) modulators (e.g. benzodiazepines).
Objectives: This study examined whether a midazolam discriminative stimulus is modified by GABA(A) agonists that act at sites other than benzodiazepine sites.
Methods: Rhesus monkeys discriminating midazolam (0.32 mg/kg SC) received direct-acting GABA(A) receptor agonists (e.g. muscimol and gaboxadol), an indirect-acting GABA(A) receptor agonist (progabide), ethanol, another benzodiazepine (triazolam), a barbiturate (pentobarbital), or a neuroactive steroid (pregnanolone) alone and in combination with midazolam.
Results: When administered alone, triazolam (0.1 mg/kg), pentobarbital (17.8 mg/kg) and pregnanolone (5.6 mg/kg) occasioned high levels of midazolam lever responding, ethanol (1-3 g/kg) occasioned intermediate levels of midazolam lever responding, and muscimol (0.32-1 mg/kg), gaboxadol (3.2-10 mg/kg) and progabide (10-32 mg/kg) occasioned low levels of midazolam lever responding. When combined with less-than-fully effective doses of midazolam, progabide (32 mg/kg) and ethanol (1 g/kg), but not muscimol and gaboxadol, enhanced the midazolam discriminative stimulus. Triazolam, pregnanolone and pentobarbital increased the potency of midazolam to occasion midazolam lever responding and the effects of these combinations were additive.
Conclusions: Direct-acting GABA(A) receptor agonists are qualitatively different from positive GABA(A) modulators in rhesus monkeys trained to discriminate midazolam. Although GABA(A) receptor agonists and modulators can enhance the actions of benzodiazepines at the GABA(A) receptor complex, the same drugs do not necessarily modify the discriminative stimulus effects of benzodiazepines. These results underscore the importance of the mechanism by which drugs alter Cl(-) flux at the GABA(A) receptor complex as a determinant not only of drug action but also of drug interaction and whether any particular drug enhances the behavioral effects of a benzodiazepine.