Anxiety enhancement in the murine elevated plus maze by immediate prior exposure to social stressors.

Anxiety has been implicated in the acute nonopioid analgesic reaction seen in defeated mice. In the present study, behavioural responses to the elevated plus-maze test were examined in male DBA/2 mice immediately following defeat by an experienced aggressive conspecific. Compared to home-cage controls, defeat reduced total arm entries and rearing, although anxiety enhancement was indicated by decreases in percent open-arm entries and percent time spent on the open arms. These effects were accompanied by significant increases in nonexploratory behaviour (movement arrest and grooming) and risk assessment (closed arm returns, protected head dipping, and stretch-attend postures). This anxiogenic effect of social defeat was partially replicated in mice merely exposed to the scent of an aggressive male conspecific. The specificity of present findings to socially relevant stressors was confirmed by the general lack of effect on plus-maze behaviour of prior exposure to a novel cage or to interaction with a nonaggressive male. Present results are not only consistent with the anxiety hypothesis of defeat analgesia but also show that the elevated plus-maze test is sensitive to alterations in anxiety produced by ecologically relevant stimuli. Possible implications for pharmacological studies are discussed.