Forced swim test
The forced swim test (FST) was adapted from McLaughlin et al. (24) with minor modifications. Briefly mice were divided into 4 groups (n=6/group) and on day 1, they were injected intraperitoneally with the κ-ΟR antagonist, nor-BNI (10 mg/kg), or vehicle and placed in a 5 L beaker (40 cm tall × 25 cm in diameter) filled with 2.5 L of 30 °C water for a single swim trial of 15 min. On day 2, the animals were subjected to 4x FST trials, 6 min long, with 6 min intervals. During the last 4 min of the trial mice were recorded and the time spent immobile was counted as “stressed behavior”. After each trial, the mouse was removed from the water, dried with towels and returned to its home cage for at least 6 min before further testing. Immobility was defined as the animal remaining motionless or making only minor non-escape-related movements. To qualify as immobility each posture must be clearly visible and sustained for a minimum of 2 seconds. Immobility was measured with the specialized video tracking software Ethovision XT9.0 (Noldus, Netherlands). Difficulty in swimming or staying afloat were criteria for exclusion, however, no mice met these criteria in this study.