Effects of acute social defeat on activity in the forced swim test: Parametric studies in DBA/2 mice using a novel measurement device

In the early stages of an agonistic encounter between mice, the loser of the conflict initially exhibits pronounced flight and escape attempts. When exposure to attack is prolonged, however, defeated mice display a decrease in these active defenses and become increasingly passive. The generality of such defeat-induced passivity was investigated in the present study by examining acutely defeated mice in the forced swim test, a nonsocial but threatening environment that normally engenders high levels of escape behavior. In the present experimental model, highly aggressive male C57BL/6 mice were used to defeat smaller male intruders of the DBA/2 strain in a series of brief (2-min) encounters spaced 2 min apart. In Experiment 1, DBA/2 mice were administered four defeat encounters and were then given a 10-min swim test following a postdefeat delay of 0, 30, or 60 min. Activity in the swim test was determined both by a novel, automated device that detected movement-induced perturbations in the water medium via infrared beam interruptions and by the standard observational measurement of percent time immobile. Both activity measures indicated a significant suppression effect of defeat on swim activity, the magnitude of which declined steadily as the delay between the defeat encounters and the swim test increased. In Experiment 2, DBA/2 mice experienced zero, one, two, three, or four defeat encounters spaced 2 min apart and were administered the swim test 30 min following the last defeat encounter. Activity in the swim test decreased as the number of defeat encounters increased. The present findings suggest that defeat-induced passivity in mice is of a global nature, persisting in other threatening environments in which high levels of escape activity would normally be observed. Similarities between the parameters of the present phenomenon and those of opioid-mediated defeat analgesia are discussed. (C) 1998 Wiley-Liss, Inc.