The effects of CCK-8 on saline (A) or morphine (B) pretreated, naloxone precipitated cAMP overshoot and, on forskolin stimulated cAMP accumulation (C) in SH-SY5Y cells. The SH-SY5Y cells were co-pretreated with CCK-8 (0.001-10 μM) and saline or 10 μM morphine for 48 h to observe the effect of exogenous CCK-8 on the baseline cAMP levels and morphine withdrawal-induced cAMP overshoot, respectively. The cells were then co-pretreated with CCK-8 (0.001-10 μM) and forskolin (10 μM) to rule out non-specific inhibition of CCK-8 on the cAMP overshoot. The control groups were as the cells that received no CCK-8 (0 μM CCK-8) treatment in each independent experiment. The results of the cAMP levels are represented as the percentage of cAMP content relative to the control group. The data are presented as the mean ± S.D. of three separate experiments, performed in duplicate. *P < 0.05, compared with the control by one-way ANOVA followed by Dunnett’s t-test.