Imidazenil: An antagonist of the sedative but not the anticonvulsant action of diazepam

Flumazenil (FLU), a specific benzodiazepine (BZ) receptor antagonist has been used in the treatment of acute BZ intoxication or the alleviation of BZ-induced withdrawal syndrome on the basis of its weak partial agonist action at GABA(A) receptors. However, given to patients, FLU can worsen diazepam-induced withdrawal syndrome by lowering seizure threshold. We therefore investigated whether imidazenil, a selective positive allosteric modulator of GABA action at GABAA receptors containing alpha(5) subunit, can antagonize diazepam-induced sedative action and suppression of locomotor activity without affecting diazepam anti-bicuculline action. We report here that while FLU (16.5 mu mol/kg) showed no effect on locomotor activity and bicuculline-induced convulsion, it completely antagonized diazepam (10.5 mu mol/kg) anti-bicuculline action and the suppression of locomotor activity. However, imidazenil (0.76 mu mol/kg) elicited anti-bicuculline action and was dose-dependently antagonized by FLU (16.5 and 33 mu mol/kg). Furthermore, imidazenil showed no effect on path length traveled but slightly decreased (40%) horizontal activity when compared to diazepam (85%), and maintained the anti-bicuculline action of diazepam to a threshold level similar to that observed with diazepam. Whereas cross-tolerance between BZs has been reported in animals and humans, we previously reported the absence of cross-tolerance between imidazenil and diazepam. Thus, we suggest that imidazenil might be more effective than FLU at alleviating the withdrawal syndrome associated with long-term BZ administration. (c) 2005 Elsevier Ltd. All rights reserved.