After baseline nociceptive testing, morphine (Sigma Chemical Co.) was administered to mice subcutaneously (s.c.) 20 mg/kg twice per day on days 1–3 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 [12–14].
