Imidazenil, a partial positive allosteric modulator of GABAA receptors, exhibits low tolerance and dependence liabilities in the rat.

Long-term treatment of rats with full (triazolam) or selective (diazepam) allosteric modulators of gamma-aminobutyric acid type A (GABAA) receptors rapidly induced tolerance to the protective effect of these drugs against bicuculline-induced convulsion. In contrast, long-term administration of partial allosteric modulators (imidazenil and bretazenil) of GABAA receptors, in doses equipotent to those of diazepam and triazolam that induce anticonvulsant tolerance, failed to elicit such a tolerance. Furthermore, no cross-tolerance was observed between diazepam and imidazenil. Discontinuation of long-term treatment with diazepam or triazolam, but not of long-term treatment with imidazenil or bretazenil, sensitized rats to behavioral inhibition by punishment (electric shock) in a manner that was potentiated by flumazenil. Administration of a single oral dose of [14C]diazepam or [3H] imidazenil to rats treated repeatedly with the corresponding unlabeled drug or vehicle revealed that the brain concentrations of drugs and their metabolites were similar in both groups of animals. This suggests that tolerance to the full or selective allosteric modulators of GABAA receptors may be associated with changes in the efficacy of the allosteric modulation rather than with changes in drug metabolism. Imidazenil has a longer half-life than an equipotent dose of diazepam and protects rats against bicuculline-induced convulsions for a significantly longer time than diazepam or bretazenil.(ABSTRACT TRUNCATED AT 250 WORDS)