2.2. Pharmacological chemicals
Fentanyl (purity, >99%) was obtained from the National Institute of Food and Drug Control (Beijing, China). Morphine hydrochloride was purchased from Qinghai Pharmaceutical Factory (Xining, China). A GRK2 inhibitor was purchased from Sigma-Aldrich (St. Louis, MO, USA). All chemicals were dissolved in distilled water. DAPI was purchased from ServiceBio Technology Co., Ltd. (Wuhan, China), and 4% paraformaldehyde-fixed solution was purchased from ApplyGen Technology Co., Ltd. (Beijing, China). Triton X-100 was purchased from SeaskyBio Innovation Technology Co., Ltd. (Beijing, China). Fetal bovine serum was purchased from Excell Bio-Technology Co., Ltd. (Taicang, China).