2.3.2. Experiment 2
The subjects were first intraperitoneally administered either naloxone (Nal group: n = 8) or saline (Sal group: n = 8). Twenty-five minutes after administration, each subject was gently placed in the center of a polypropylene box (44 × 66 × 39.8 cm) located in a dark room illuminated by dim red light. Behaviors were recorded for 15 min using a video camera (HDR-HC9, Sony) and an HDD-BD recorder (DMR-BW770; Panasonic).