2.3.1. Experiment 1
Fear conditioning was performed in an illuminated room, as described in our previous studies (Kodama et al. , 2011; Kiyokawa et al. , 2013). Each subject was placed in an acrylic conditioning box (28 × 20 × 27 cm) for 20 min. During this period, the subject received seven repetitions of a 3-s tone (8 kHz, 75 dB) that was terminated concurrently with a foot shock (0.5 s, 0.6 mA). Intertrial intervals varied randomly from 120 to 240 s. After fear conditioning, the subject was returned to its home cage.
Twenty-four hours after fear conditioning, a fear expression test was performed as described in our previous studies (Kiyokawa et al. , 2012; Takahashi et al. , 2013). The subjects were first intraperitoneally administered either haloperidol (Hal group: n = 8), SR49059 (SR group: n = 6), atosiban (Ato group: n = 8), naloxone (Nal group: n = 6), or saline (Sal group: n = 6). Twenty-five minutes after administration, the subjects were placed in one of two rectangular enclosures (25 × 25 × 35 cm) situated on an acrylic board (45 × 60 cm) in a dark room illuminated by dim red light. An associate was simultaneously placed in the other enclosure. Each enclosure had three acrylic walls, one wire mesh wall, and a wire mesh ceiling. The wire mesh wall was constructed of 1-cm square mesh in the lower part (20 cm) and vertical bars spaced at 1-cm intervals in the upper part (15 cm), which prevented the rats from climbing up to the ceiling. The two enclosures were positioned such that their wire mesh walls faced one another with a 5-cm intervening distance. The acrylic board within the enclosures was covered in clean bedding. We also prepared an additional control group in which the subjects were intraperitoneally administered saline and tested alone (Alo group:n = 7). After a 5-min acclimation period, a 3-s CS tone was presented five times at 1-min intervals during the first half of the 10-min experimental period. The behaviors of the subjects during the acclimation and experimental periods were recorded using a video camera (HDR-HC9, Sony, Tokyo, Japan) and an HDD-BD recorder (DMR-BW770; Panasonic, Osaka, Japan).