Anticonvulsant, anxiolytic, and non-sedating actions of imidazenil and other imidazo-benzodiazepine carboxamide derivatives

Recent evidence suggests that α1-containing GABAA receptors mediate the sedative, amnestic, and to some extent the anticonvulsant actions of non-selective benzodiazepine (BZ) receptor ligands, such as diazepam (DZ). Anxiolytic and in part, anticonvulsant actions of BZ ligands are mediated by α2-, α3-, and α5-containing GABAA receptors. This has resulted in increasing interest in developing BZ ligands with selective actions at GABAA receptors, including α2-, α3-, and α5-subunits, but devoid of efficacy at α1-containing receptors. To refine their spectrum of pharmacological actions, efforts are being made to minimize unwanted effects such as sedation, amnesia, and tolerance liabilities. A prototype for such BZ ligands is imidazenil (IMD), an imidazo-benzodiazepine carboxylic acid derivative that elicits potent anticonvulsant and anxiolytic actions at doses virtually devoid of sedative, cardio-respiratory depressant and amnestic effects, and anticonvulsant tolerance liability. To define the pharmacological profile of IMD and its derivatives, we compared the anticonflict (anxiolytic), anti-proconflict (antipanic), anti-bicuculline (BIC), and maximal electroshock seizure (MES) effects, and the suppression of locomotor activity by imidazo-benzodiazepine carboxylic acid derivatives to those of DZ and bretazenil (BTZ). We report here that IMD and one of its derivatives (RO 25-2775) possess dose-dependent anticonflict, anti-proconflict, and anti-BIC actions but failed to suppress locomotor activity. Like DZ, the other IMD derivatives (enazenil, RO 25-2776, and RO 25-2847) not only elicit dose-dependent anticonflict, anti-proconflict, anti-BIC, anti-MES effects but also suppress locomotor activity. In contrast, none of the IMD derivatives studied shows any similarity to BTZ, which elicits anticonflict, anti-proconflict actions and suppresses locomotor activity but is virtually inactive against BIC-induced tonic–clonic convulsions.