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 attemp ts. When exposure to attack is prolonged, however, defeated mice displ ay 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 swi m test, a nonsocial but threatening environment that normally engender s high levels of escape behavior. In the present experimental model, h ighly aggressive male C57BL/6 mice were used to defeat smaller male in truders of the DBA/2 strain in a series of brief (2-min) encounters sp aced 2 min apart. In Experiment 1, DBA/2 mice were administered four d efeat encounters and were then given a 10-min swim test following a po stdefeat delay of 0, 30, or 60 min. Activity in the swim test was dete rmined both by a novel, automated device that detected movement-induce d perturbations in the water medium via infrared beam interruptions an d by the standard observational measurement of percent time immobile. Both activity measures indicated a significant suppression effect of d efeat on swim activity, the magnitude of which declined steadily as th e delay between the defeat encounters and the swim test increased. In Experiment 2, DBA/2 mice experienced zero, one, two, three, or four de feat 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 f indings suggest that defeat-induced passivity in mice is of a global n ature, persisting in other threatening environments in which high leve ls 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.