After baseline nociceptive testing, morphine (Sigma Chemical, St. Louis, Missouri, USA) was administered to mice subcutaneously (s.c.) 10 mg/kg twice per day on day 1, 20 mg/kg on days 2−3, and 40 mg/kg twice per day on day 4 in 50−100 μl volumes of 0.9% NaCl similar to our previous protocols for generating opioid-induced hyperalgesia (OIH), tolerance and dependence [9,13,14].
