Social instability in female rats: the relationship between stress-related and anxiety-like consequences.

It is generally believed that anxiety and depression develop in response to stressful events that chronically increase glucocorticoid production (which in turn affects various neurotransmitter systems). In contrast to depression, however, the relationship between chronic stress and anxiety is less clear, as anxiety patients often show normal glucocorticoid levels and respond normally to dexamethasone challenge. Here we report on the interaction between symptoms of chronic stress and anxiety in female rats by making use of the social instability model of anxiety. Subjects were exposed to alternating 24 h periods of isolation and moderate crowding for 14 days. Symptoms of chronic stress and anxiety-like behaviour in the social interaction test were evaluated on the 15th day, i.e. 1 day after the last crowding phase. Social instability resulted in decreased weight gain, and chronically elevated plasma glucocorticoid levels only in rats that showed high aggressiveness during the crowding phases. In contrast, anxiety-like behaviour was increased irrespective to crowding-related aggressiveness. Thus, the development of chronic stress symptoms and anxiety-like behaviour dissociated: the former was bound to crowding-induced aggressiveness (i.e. to a higher stress load), whereas the latter occurred also when such aggressiveness was low, and symptoms of chronic stress did not develop. This finding is consistent with human data, and suggests that stressful events lead to anxiety in both cases: when stressors do or do not lead to chronic stress responses. Studying the distinctive features of anxieties associated or not with chronic endocrine stress responses would enhance our understanding of this disorder.