Anxiety-like behavior in mice lacking the angiotensin II type-2 receptor

The main biological role of angiotensin II type 2 receptor (AT2) has not been established. We made use of targeted disruption of the mouse AT2 gene to examine the role of the AT2 receptor in the central nervous system (CNS). AT2-deficient mice displayed anxiety-like behavior compared with wild-type mice. However, AT2-deficient mice showed no depressant-like activity and no change in hexobarbital-induced sleeping time as compared with findings in wild-type mice. Both noradrenergic and corticotropin-releasing factor (CRF) neuronal systems appear to be involved in this anxiety-like behavior. Diazepam, captopril (angiotensin I converting enzyme inhibitor), prazosin (alpha1 antagonist) reversed the anxiety-like behavior in these AT2-deficient mice, whereas yohimbine (alpha2 antagonist), phenylephrine (alpha1 agonist), clonidine (alpha2 agonist), isoproterenol (beta1/beta2 agonist), propranolol (beta1/beta2 antagonist) and alpha-helical CRF9-41 (CRF receptor antagonist) has no apparent effects on anxiety-like behavior in AT2-deficient mice. In addition, concentrations of plasma adrenocorticotropic hormone (ACTH) and corticosterone in AT2-deficient mice did not differ from these in wild-type mice, hence, there are probably no endocrine abnormalities involving the hypothalamic-pituitary-adrenal axis (HPA). The amygdala appears to play an important role in many of the responses to fear and anxiety. The number of [3H]prazosin but not [125I]CRF binding sites in the amygdala was significantly reduced in AT2-deficient mice. These findings indicate that the noradrenergic system is involved in mediating the anxiety-like behavior in AT2-deficient mice.