Activation of central CRF receptor 1 by cortagine results in enhanced passive coping with a naturalistic threat in mice

Abstract

CRF receptor subtype 1 (CRF1), abundantly expressed in the central nervous system, has been implicated in defensive behavior in rodents. Pharmacological activation of CRF1 by peptidic agonists results in enhancement of anxiety-like behavior. However, receptor specificity of commonly used agonists was confounded by significant affinity to other receptors and widely used laboratory tests of experimental anxiety suffer from artificial aversive stimulation (e.g. electric shock), and limited measures of anxiety-like behavior. We used the recently developed, CRF1-selective agonist cortagine in a mouse model of defensive behaviors under semi-natural conditions, the rat exposure test (RET). Cortagine was injected bilaterally into the cerebral ventricles (i.c.v.) of male C57Bl/6J mice, 20min before exposure to a rat in specifically designed box that evokes a wide variety of defensive behaviors such as active/passive avoidance, freezing, risk assessment, and burying. Pre-injection of the CRF receptor antagonist acidic astressin was used to test for receptor specificity of the observed cortagine effects. A control experiment with no rat present was performed to test for baseline effects of cortagine in the exposure setup. Cortagine dose-dependently enhanced passive avoidance and freezing while burying was decreased. CRF receptor antagonism reliably blocked the effects of cortagine. Our results confirm previous findings of anxiogenic-like effects of cortagine, and demonstrate the usefulness of the RET in investigating differential pattering of drug-induced anxiety-like behavior in mice. In conclusion, our results suggest that CRF1 activation in forebrain areas promotes passive coping with the natural threat presented in the RET.

Figure 1

Setup of the RET box. Chamber is connected to the surface by a tunnel section made of clear Plexiglas. Mouse surface compartment is separated from rat compartment by stainless steel mesh. Some of the mouse’s home cage bedding material (sawdust) was spread out on the surface and 60 ml were provided inside the chamber for burying or blocking of the tunnel entrance.

Figure 2

Schedule of experiments. Mice were habituated to the RET box on two subsequent days before the test day to reduce novelty-induced behavior.

Figure 3

Sums of line crossings from one compartment of the RET box (chamber, tunnel, or surface) into another in the RET. C57BL/6J mice were injected with different doses of CRF1-selective agonist cortagine, CRF receptor antagonist acidic astressin, or both, and compared with aCSF-injected controls. Effecs of cortagine were also tested in the absence of a rat. Cortagine dose-dependently decreased activity, an effect that was prevented by pre-injection of acidic astressin. n=8–10, except n=5 for 50 ng cortagine group. ** p<0.001, *** p<0.0001 vs. aCSF controls; ^## p<0.001 vs. cortagine 120 ng; ^§p<0.05 vs. aCSF controls (no rat condition).

Figure 4

Total time spent in the escape chamber compartment (upper panel) and number of entries into the chamber (lower panel) as measures of avoidance behavior indicated by location preference. C57BL/6J mice were injected with different doses of CRF1-selective agonist cortagine, CRF receptor antagonist acidic astressin, or both, and compared with aCSF-injected controls. Effecs of cortagine were also tested in the absence of a rat. While cortagine dose-dependently increased avoidance, astressin alone did not affect avoidance and blocked cortagine-induced enhancement of avoidance. With no rat present, cortagine did not enhance avoidance behavior. n=8–10, except n=5 for 50 ng cortagine group. ** p<0.001, *** p<0.0001 vs. aCSF controls; ^### p<0.0001, ^## p<0.001 vs. cortagine 120 ng. ^§p<0.05 vs. aCSF controls (no rat condition).

Figure 5

Total duration of contact with the divider mesh (upper panel) and number of individual contacts (frequency; lower panel) exhibited by C57BL/6J mice injected with different doses of CRF1-selective agonist cortagine, CRF receptor antagonist acidic astressin, or both, compared with aCSF controls. Cortagine significantly supressed duration and frequency of contact in the presence or absence of the rat. n=8–10, except n=5 for 50 ng cortagine group. * p<0.05, ** p<0.001, *** p<0.0001 vs. aCSF controls; ^# p<0.05 vs. cortagine 120 ng. ^§§p<0.001 vs. aCSF controls (no rat condition).

Figure 6

Total duration of freezing (upper panel) and number of individual freezing episodes (frequency; lower panel) of C57BL/6J mice injected with different doses of CRF1-selective agonist cortagine, CRF receptor antagonist acidic astressin, or both, compared with aCSF-injected controls. Cortagine massively enhanced freezing, an effect that was reversed by pre-injection of acidic astressin. With no rat present, cortagine did not significantly increase freezing behavior. n=8–10, except n=5 for 50 ng cortagine group. * p<0.05, ** p<0.001, *** p<0.0001 vs. aCSF controls; ^### p<0.0001, ^## p<0.001 vs. cortagine 120 ng.