For dependence determinations, mice were assessed 18 h after the final dose of morphine when spontaneous dependence-related hyperalgesia was maximal [16]. Naloxone (Sigma Chemical) 10 mg/kg was injected s.c. in 50 μl of NaCl as described previously by our laboratory and others [8,9]. After naloxone administration, mice were placed in clear plastic cylinders (10 cm in diameter and 40 cm in height), and the number of jumps during the following 15 min was counted. Naloxone-precipitated jumping behavior is the most robust response reflecting physical dependence on opioids observed across strains of inbred mice [17].
