Most chemicals such as NBQX and MK801 were achieved from Sigma-Aldrich. Morphine hydrochloride was achieved from Shenyang Pharmaceutical Co. Ltd. under surveillance of Chinese Food and Drug Administration; sufentanil citrate was achieved from Renfu Pharmaceutical Co. Ltd. under surveillance of Chinese Food and Drug Administration; naloxone hydrochloride was achieved from Chongqing Lummy Pharmaceutical Co. Ltd.; CTOP was achieved from Abcam; naltrindole, nor-BNI, CCK8s, L365260, MK329, DAMGO, and CNO were achieved from Tocris. All drugs were first dissolved to a maximum concentration according to the manufacturer’s instructions, and the final solutions were prepared immediately before experiments.