Morphine physical dependence was measured using methods previously reported by our laboratory [11, 12]. Mice were treated with morphine for 4 days according to the standard morphine administration protocol for use in this paradigm. Dependence was assessed 18 hours after the final dose of morphine was given. To precipitate withdrawal, naloxone (Sigma Chemical Co.) 10 mg/kg was injected s.c. in 50 μl 0.9% NaCl. After naloxone administration, mice were placed in the same clear cylinders used for the mechanical nociception assays. The number of jumps made during the following 15 minutes was counted.
