Targeted mutation of CCK2 receptor gene modifies the behavioural effects of diazepam in female mice

Rationale Evidence suggests that GABA and CCK have opposite roles in the regulation of anxiety. Objective The aim of the present work was to study diazepam-induced anxiolytic-like action and impairment of motor co-ordination, and the parameters of benzodiazepine receptors in mice lacking CCK₂ receptors. Methods The action of diazepam (0.5–3 mg/kg IP) was studied in the elevated plus-maze model of anxiety and rotarod test using mice lacking CCK₂ receptors. The parameters of benzodiazepine receptors were analysed using ³H]-flunitrazepam binding. Results In the plus-maze test, the exploratory activity of the homozygous (−/−) mice was significantly higher compared to their wild-type (+/+) littermates. However, the wild-type (+/+) mice displayed higher sensitivity to the anxiolytic-like action of diazepam. Even the lowest dose of diazepam (0.5 mg/kg) induced a significant increase of open arm entries in the wild-type (+/+) mice. A similar effect in the homozygous (−/−) mice was established after the administration of diazepam 1 mg/kg. The highest dose of diazepam (3 mg/kg) caused a prominent anxiolytic-like effect in the wild-type (+/+) mice, whereas in the homozygous (−/−) animals suppression of locomotor activity was evident. The performance of the homozygous (−/−) mice in the rotarod test did not differ from that of the wild-type (+/+) littermates. However, a difference between the wild-type (+/+) and homozygous (−/−) animals became evident after treatment with diazepam. Diazepam (0.5 and 3 mg/kg) induced significantly stronger impairment of motor co-ordination in the homozygous (−/−) mice compared to their wild-type (+/+) littermates. The density of benzodiazepine binding sites was increased in the cerebellum, but not in the cerebral cortex and hippocampus, of the homozygous (−/−) mice. Conclusions Female mice lacking CCK₂ receptors are less anxious than their wild-type (+/+) littermates. The reduced anxiety in homozygous (−/−) mice probably explains why the administration of a higher dose of diazepam is necessary to induce an anxiolytic-like action in these animals. The highest dose of diazepam (3 mg/kg) induced significantly stronger suppression of locomotor activity and impairment of motor co-ordination in the homozygous (−/−) mice compared to the wild-type (+/+) littermates. The increase in the action of diazepam is probably related to the elevated density of benzodiazepine receptors in the cerebellum of homozygous (−/−) mice. The present study seems to be in favour of increased tone of the GABAergic system in mice without CCK₂ receptors.