Melatonin facilitates extinction, but not expression, of cued fear. (A) Schematic of the behavioral procedure used. Behavior procedure involved four training phases: habituation, fear conditioning, extinction training and extinction test. Rats were administrated melatonin 30 min before extinction training. (B) Percent freezing to the CSs, averaging across all trials, was shown for the melatonin group (n = 10) and the vehicle group (n = 10) during extinction training. (C) Percent freezing to the CSs, averaging across all trials during extinction testing. (D) Reduction in percent freezing between the extinction training and extinction test based on the data in B and C. (E) Percent freezing to the CSs during extinction training. Freezing was also scored during a 3 min acclimation period (Pre-CS). (F) Percent freezing to the CSs during extinction testing. Data used in B and C were reanalyzed for the trials of percent freezing. Mel = melatonin group, Veh = vehicle group. ** P < 0.01 for comparisons between the melatonin group and the vehicle group. All data are represented as mean ± SEM.