The anxiolytic like responses produced by anti SVG suggests

The anxiolytic-like responses produced by anti-SVG-30 suggests that CRF2 receptors mediate not only the elicitation of anxiety behavior induced by prior stress, i.e. conditioned freezing, but also unconditioned anxiety behavior induced by exposure to the unfamiliar environment of the elevated plus maze and defensive-withdrawal test. Thus, behavioral differences reported in mice tested in the elevated plus maze after administration of anti-SVG-30 (see Introduction) [42] do not appear to be a consequence of whether anxiety testing occurred with or without prior exposure to stress. It should be indicated that we failed to detect an anti-SVG-30 induced increase in anxiety using the elevated plus maze test as reported previously (see Fig. 3a in Ref. [27]). In that study, mice treated with an i.c.v. injection of 400 ng of anti-SVG-30 showed a decrease in open arm exploration. In comparison, our lowest tested dose of 500 ng had no significant behavioral effects. Species differences may account for these conflicting results. An anti-SVG-30 dose–response study performed in mice may provide insight into the behavioral differences obtained from studies conducted between the two species. An issue underlying the interpretation of the present results is whether the anxiolytic-like effects produced by anti-SVG-30 are due entirely to antagonism of CRF2 receptors. The initially reported Kd of anti-SVG-30 at CRF2β and CRF1 receptors was 1.4 and 153.6 nM, respectively [43]. This demonstrated binding to the CRF1 Calpain Inhibitor XII opens the possibility that microgram doses of anti-SVG-30 also antagonized CRF1 receptors which may have promoted an anxiolytic-like behavioral profile [18], [31], [35], [45]. However, a recent study using a different binding protocol reported that anti-SVG-30 shows an even greater selectivity (1000–10,000-fold) towards vertebrate CRF2 receptors [20], suggesting a lack of interaction between the CRF1 receptor and anti-SVG-30. It is notable that administration of a CRF2 antisense oligonucleotide, which produced ∼70% suppression of CRF2 receptors in the rat’s lateral septum, was highly effective in attenuating conditioned freezing [21]. This result suggests that disrupting the actions of CRF2 receptors may be sufficient to produce a reduction in anxiety behavior. As indicated in the Introduction, several research groups reported that the CRF2 knockout mouse exhibits alterations in anxiety behavior [2], [9], [27]. However, the direction of the behavioral alterations was not always consistent across laboratories. Although the present results do not reconcile these differences, it is notable that one study found that CRF2 knockout mice had elevated levels of anxiety behavior as well as increased expression of CRF mRNA in the central nucleus of the amygdala [2]. The central amygdala plays a major role in the activation of diverse responses associated with stress [10], [25], [28]. In addition, CRF mRNA increases in the central amygdala after exposure to stress [23], and corticosteroid implants located in the dorsal margin of the central nucleus elevates CRF mRNA expression and induces anxiety behavior in rats tested in the elevated plus maze [46]. Therefore, the heightened anxiety behavior reported in some CRF2 knockout mouse studies may be due, in part, to an increase in CRF mRNA expression occurring in the central nucleus of the amygdala. The effective dose of anti-SVG-30 required to produce anxiolytic-like behavioral effects appears to depend on the anxiety test. Whereas a 1.0-μg dose was effective in reducing anxiety behavior in the elevated plus maze, 2.0-μg or higher doses were required to produce significant anxiolytic-like effects in the freezing and defensive-withdrawal tests. These differences may be due to the recruitment of different brain regions that are activated during exposure to specific anxiety provoking situations. For example, fos-like immunoreactivity in the rat hypothalamic paraventricular nucleus appears after exposure to an avoidance stress procedure but not after testing in the elevated plus maze [14]. Test-specific activation of different brain regions may place varying degrees of constraints upon the effectiveness of anti-SVG-30 to alleviate the distress.