The effect of L-364,718 or LY-288,513 on naloxone precipitated cAMP overshoot in saline (A) or morphine (B) pretreated SH-SY5Y cells. The SH-SY5Y cells were first co-pretreated with L-364,718 or LY-288,513 (0-10 μM) and saline for 48 h to evaluate the effect of CCK receptor antagonists on the baseline cAMP level, and then incubated with L-364,718 or LY-288,513 plus morphine (10 μM) to investigate the influence of CCK receptor antagonists on the naloxone (10 μM, 15 min) precipitated cAMP overshoot. The control groups were considered as no CCK receptor antagonist-treated cells (0 μM L-364,718/LY-288,513) 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 < 0.01, ***P < 0.01, compared with the control by one-way ANOVA followed by Dunnett’s t-test.