Benzodiazepines bind to and act on α1–3 and α5-containing GABA(A) receptors. Previous studies suggest that different GABA(A) receptor α-subtypes mediate the various behavioral effects of benzodiazepines, which raises the possibility of combining benzodiazepines with subtype-selective GABA(A) receptor antagonists to improve the therapeutic profiles of benzodiazepines. This study examined the GABA(A) receptor subtype mediation of the tolerance to midazolam-induced antinociception in rats. Midazolam (3.2 mg/kg) significantly reduced the locomotion in rats which was prevented by the selective α1-preferring GABA(A) receptor antagonist β-carboline-3-carboxylate-t-butyl ester (βCCt) (3.2 mg/kg). Midazolam increased the paw withdrawal threshold as tested by the von Frey filament assay in the complete Freund’s adjuvant-induced inflammatory pain model in rats, and this effect was not altered by βCCt or another α1-preferring GABA(A) receptor antagonist 3-propoxy-β-carboline hydrochloride (3PBC). Repeated treatment with midazolam in combination with vehicle, βCCt or 3PBC (twice daily) for 7 days led to progressive increase of the ED(50) values in the midazolam- and vehicle-treated rats, but not in other rats, suggesting the development of tolerance to midazolam but not to the combination of midazolam with α1-preferring GABA(A) receptor antagonists. These results suggest the essential role of the α1-subtype of GABA(A) receptors in mediating the development of tolerance to midazolam-induced antinociceptive effects and raise the possibility of increasing therapeutic profiles of benzodiazepines by selectively blocking specific α-subtypes of GABA(A) receptors.
