Dissecting the role of diazepam-sensitive γ-aminobutyric acid type A receptors in defensive behavioral reactivity to mild threat

Moderate reductions in synaptic γ-aminobutyric acidA receptors (GABAARs) have been associated with an enhanced defensive behavioral reactivity to mild threat, sensitive to diazepam. We here tested whether a deficit in α2 subunit-containing GABAergic synapses is sufficient to cause this anxiety-related phenotype and to prevent its attenuation by the benzodiazepine. Wild type (α2+/+), heterozygous (α2+/−) and homozygous (α2−/−) knock-out littermates were tested in the free-choice exploratory (FCE) and the light/dark choice (LDC) paradigms. α2−/− mice, double mutant α1H101Rα2−/− and α3H126Rα2−/− mice, which combine a lack of α2-GABAARs with point-mutated diazepam-insensitive either α1H101R or α3H126R-GABAARs, and double point-mutated α1H101Rα2H101R and α1H101Rα3H126R mice were used to uncover the GABAAR subtype(s) mediating the drug effects. Data show that in the FCE, α2−/− mice exhibited more retractions (i.e. risk assessment) and longer latencies to first occurrence into the novel compartment and less transitions and time spent inside it in comparison to α2+/− and α2+/+ mice. In the LDC, α2−/− mice visited and spent less time in the lit box and stayed longer in the tunnel than the other two groups. Minor differences were found between α2+/− and α2+/+ mice in the two paradigms. Diazepam (1.5mg/kg per os) normalized retractions and latencies in the FCE in α2−/− and α3H126Rα2−/− mice, but not in α1H101Rα2−/− mice. The same drug treatment failed to attenuate behavioral aversion in both paradigms in all mutants with impaired α2-GABAAR function. These results reveal α2-containing GABAARs as key molecular determinants in the regulation of anxiety-related responses elicited by exposure to relative novelty and mild threat. In the absence of these receptors, diazepam through activation of α1-GABAARs remains effective in reducing risk assessment, but not behavioral aversion.