A: MOR (μ-opioid receptor), CCK1R (CCK1 receptor), CCK2R (CCK2 receptor) and CCK mRNA are co-expressed in SH-SY5Y cells. The PCR products were separated by 2% agarose gel electrophoresisi and imaged. Lane 1: MOR; Lane 2: CCK1R; Lane 3: CCK2R; Lane 4: CCK. B: The effects of chronic morphine exposure (10 μM) for 48 h on CCK1R, CCK2R and endogenous CCK expression. The data are obtained from the analysis of the fold of relative expression level compared to the saline control group, and expressed as the mean ± S.D. of three independent experiments. **P<0.01, ***P<0.001 compared with the control by t-test.