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

Abstract

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 GABAA receptors, was evaluated on the exploratory behaviour in these mice. Expression levels of GABAA 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 GABAA receptor subunit genes were determined by means of real-time polymerase chain reaction (qRT-PCR). Results Female mice lacking CCK2 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 GABAA 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