2.2. Antagonists
Haloperidol (1.0 mg/kg; Sigma Aldrich, MO, USA), atosiban (1.0 mg/kg; Cayman Chemical Company, MI, USA), SR49059 (1.0 mg/kg; AOBIOUS, MA, USA), and Naloxone (4.0 mg/kg; Sigma Aldrich) were used in this study. The doses of antagonists were determined based on previous studies (Bardo et al. , 1989; Coria-Avila et al. , 2008; Ramoset al. , 2016; da Cruz et al. , 2017). Sterile saline was used as a vehicle, except for SR49059, for which a mixture of DMSO (15%; Sigma Aldrich), Tween 80 (2%; Tokyo Chemical Industry, Tokyo, Japan), and sterile saline (83%) was used. Test substances were prepared in a volume of 0.3 ml and administered intraperitoneally. After injections, the subjects were returned to their home cage and left undisturbed until the fear expression test or open-field test.