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).