2.1. Animals
Experimentally naïve male Wistar rats (aged 7-8 weeks) were purchased from Charles River Laboratories Japan (Kanagawa, Japan). Upon arrival, the rats were housed at two to three per cage in a temperature (24 ± 1°C)- and humidity (45 ± 10%)-controlled room on a 12-h light-dark cycle (lights on from 8:00–20:00). Food and water were available ad libitum. In order to maintain unfamiliarity between the subjects and associates in Experiments 1 and 3, all rats in the same cage were assigned as either a subject or an associate (rats accompanying the subjects during the fear-expression test). From 3 days before fear conditioning or the open-field test, all animals were housed individually and handled for 5 min per day for 3 days. All behavioral procedures were performed during the light phase of the light-dark cycle.