Behavioral responses to social stress in noradrenaline transporter knockout mice: effects on social behavior and depression.

Noradrenaline has been implicated in the pathogenesis of depression and the noradrenaline transporter (NET) is a target for some antidepressants. Therefore, mice with disrupted NET gene expression (NET-KO) appear especially suitable for studying this behavioral disorder. We have examined the interaction between social stress (an etiological factor of depression) and the resulting depressive behaviors in NET-KO mice. Social stress was induced by daily defeats from larger resident mice while depression was assessed by the behavioral despair model. Animals subjected to repeated social stress showed reduced weight gain and a gradual shift from offensive to defensive behaviors. The latter may be considered a situation-specific depressive-like behavior. NET gene disruption did not prevent these changes that developed in a homotypic situation (i.e., during the repeated application of the same stressor). In contrast, stressed NET-KO mice showed more struggling in the behavioral despair model than stressed wild type (WT) animals. Thus, NET gene disruption inhibited depression-like behavior in chronically stressed animals tested in a situation heterotypic to the original cause of chronic stress. We suggest that the behavioral effects of NET gene disruption were overruled by experience and learning in the homotypic situation but manifested fully in the heterotypic situation. Tentatively, our data suggest that enhanced noradrenergic function does not prevent situation-specific social learning but impedes the generalization of depression to heterotypic circumstances.