Elevated cholecystokininergic tone constitutes an important molecular/neuronal mechanism for the expression of anxiety in the mouse.

Cholecystokinin (CCK), one of the most abundant neuropeptides in the brain, plays an important role in anxiogenesis through the activation of CCK receptor-2 (CCKR-2). Accumulating evidence, however, has suggested this role depends on endogenous CCKergic "tone," which is largely determined by the expression level of the CCKR-2. Using the tTA/tetO-inducible transgenic (tg) approach, we show here that overexpression of the CCKR-2 in neurons of the forebrain significantly increases CCKR-2 binding capacity in tg mice compared with their littermate controls. Interestingly, these tg mice consistently exhibit increased fear responses, which are generally interpreted as anxiety-like behaviors in the rodent, in a battery of behavioral tests, which represented conflict situations or delivered stress to the subjects. The inhibition of transgene expression with doxycycline treatment completely diminished both increased receptor-binding activity and all behavioral phenotypes. Furthermore, treatment of tg mice with diazepam significantly attenuated these anxiety-like behaviors. Our results directly demonstrate that the elevated CCKergic tone via overexpression of the CCKR-2 in the brain may constitute an underlying molecular/neuronal mechanism for the expression of anxiety. In addition, our study has validated a robust genetic anxiety model in the mouse in terms of their face, constructive, and predictive validity.