Defensive Burying

Defensive burying48 refers to the rodent behavior of displacing bedding material with treading and shoveling movements of their heads and forepaws towards a stimulus that they perceive to be a near and/or imminent threat (e.g., introduction of a wall-mounted electrified probe into the cage). The animal is placed into the cage with the probe exposed, and following the initial contact with the probe behavior is observed for 10-15 min. Although initial studies recorded only probe directed burying time, later studies included measurement of several concurrent behavioral indices of fear/anxiety, reactivity, and exploration (for review see 49). In addition to defensive burying of the probe, animals also display increases in freezing postures away from the probe indicating both active and passive avoidance components of the test. The predictive validity of this test is better than other tests of anxiety, with GABAA and serotonergic compounds showing dose-related suppression of defensive burying and freezing behavior. Further, the relative potency of clinically used compounds in this test is roughly comparable to those used in the therapeutic setting.49 Suppression of burying has also been reported for putative anxiolytics with novel mechanisms of action including CRF antagonists,50 neurosteroids,51 and angiotensin II antagonists.52

In addition to the defensive burying of electrified probes, mice also bury harmless objects like glass marbles. This latter test is relatively simple to set up and requires placing mice into a cage containing a layer of marbles evenly spaced on top of the bedding. Thirty minutes later, the animal is removed and the number of buried marbles recorded. Inhibition of marble-burying behavior in mice is considered to be a correlative, rather than a direct, measure of anxiety behavior since the marbles themselves are not a fear-provoking stimulus as in the case of the electrified probe. SSRIs, buspirone, and neurokinin (NKi) receptor antagonists are effective in this test.53'54 Given this pharmacological profile, it has been suggested that the marble burying behavior by mice may be more reflective of OCD rather than anxiety although the validity of this test as a model for OCD remains to be determined.

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