Effect of acute treatment with ketamine and melatonin and their combination on the forced swimming and tail suspension tests in mice. Animals (n = 8 per group) were injected with the VEH or a single dose of KET (A,B) (1.5 mg/kg, i.p.) or the combination of KET with either 4 or 16 mg/kg (i.p.) of MEL at ZT 18, 30 min before the start of either the forced swimming (C) or the tail suspension tests (D) (see Methods section). Data are expressed as the mean ± standard error of the mean (SEM) of independent groups (n = 8). Data were analyzed using a one-way ANOVA followed by Bonferroni’s post-test. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001 compared with vehicle (VEH) control group.