The figures show the effect of mitragynine (A, B), 7-hydroxymitragynine (C, D), and morphine (E, F) on the reward thresholds and latencies for both males and females combined. Plus signs indicate an increase or decrease in the reward thresholds or response latencies compared to rats treated with vehicle. +, p<0.05. ++, p<0.01. A,B: N=18; C,D: N=23; E,F: N=18. Data are expressed as means ± SEM.