Both α2 and α3 GABAA receptor subtypes mediate the anxiolytic properties of benzodiazepine site ligands in the conditioned emotional response paradigm

Mice with point-mutated alpha 2 GABA(A) receptor subunits (rendering them diazepam insensitive) are resistant to the anxiolytic-like effects of benzodiazepines (BZs) in unconditioned models of anxiety. We investigated the role of the alpha 2 GABAA subtype in a model of conditioned anxiety. alpha 2(H101R) and wildtype mice were trained in a conditioned emotional response (CER) task, in which lever-pressing for food on a variable interval (VI) schedule was suppressed during the presentation of a conditioned stimulus (CS+) that predicted footshock. The ability of diazepam, ethanol and pentobarbital to reduce suppression during the CS+ was interpreted as an anxiolytic response. Diazepam (0, 0.5, 1, 2, 4 and 8 mg/kg) induced a dose-dependent anxiolytic-like effect in wildtype mice. At high doses, diazepam (2, 4 and 8 mg/kg) was sedative in alpha 2(H101R) mice. Analysis of the anxiolytic properties of nonsedative diazepam doses (0.5 and 1 mg/kg), showed that alpha 2(H101R) mice were resistant to the anxiolytic effects of diazepam. Equivalent anxiolytic properties of pentobarbital (20 mg/kg) and ethanol (1 and 2 g/kg) were seen in both genotypes. These findings confirm the critical importance of the alpha 2 GABAA subtype in mediating BZ anxiolysis. However, as a compound, L-838417, with agonist properties at alpha 2, alpha 3 and alpha 5-containing receptors, gave rise to anxiolytic-like activity in a2(H101R) mice in the CER test, alpha 3-containing GABA receptors are also likely to contribute to anxiolysis. Observations that alpha 2(H101R) mice were more active, and displayed a greater suppression of lever pressing in response to fear-conditioned stimuli than wildtype mice, suggests that the alpha 2(H101R) mutation may not be behaviourally silent.