Targeted invalidation of CCK₂ receptor gene induces anxiolytic-like action in light-dark exploration, but not in fear conditioning test

Rationale: Evidence suggests that γ-aminobutyric acid (GABA) and cholecystokinin (CCK) have opposite roles in the regulation of anxiety. Objectives: The aim of our work was to study the behaviour of CCK₂ receptor deficient mice in light-dark exploration and fear conditioning tests. Moreover, the action of diazepam and methyl-6,7-dimethoxy-4-ethyl-β- carboline-3-carboxylate (DMCM), having the opposite effect on GABA_A receptors, was evaluated on the exploratory behaviour in these mice. Expression levels of GABA_A receptor subunit genes were also measured. Methods: Light-dark exploration and fear conditioning tests were used to determine changes in anxiety of mice. The action of diazepam (0.5-2 mg/kg i.p.) and DMCM (0.25-1 mg/kg i.p.) was studied in the light-dark box. The effect of DMCM was also evaluated in the motor activity test to demonstrate that its anti-exploratory action was not related to motor suppression. Expression levels of GABA_A receptor subunit genes were determined by means of real-time polymerase chain reaction (qRT-PCR). Results: Female mice lacking CCK ₂ receptors displayed increased exploratory activity in the light-dark box compared to their wild-type (+/+) littermates. Locomotor activity in the motility boxes and the intensity of freezing did not differ in wild-type (+/+) and homozygous (-/-) mice. Treatment with diazepam (0.5 mg/kg) increased the number of transitions in wild-type (+/+) animals, whereas in homozygous (-/-) mice diazepam (0.5-2 mg/kg) reduced exploratory activity. Administration of DMCM (0.25-1 mg/kg) induced an anxiogenic-like effect in homozygous (-/-) mice, but did not change their locomotor activity. Gene expression analysis established a 1.6-fold increase in the expression of the α2 subunit of GABA_A receptors in the frontal cortex of homozygous (-/-) mice. Conclusion: Genetic invalidation of CCK₂ receptors induced an anxiolytic-like action in exploratory, but not in conditioned models of anxiety. The observed reduction in anxiety in homozygous (-/-) mice is probably related to an increased function of GABAergic system in the brain.