(A) Changes in paw withdrawal latency before and then after seven daily injections of morphine (10 mg/kg), naloxone (1 mg/kg, intraperitoneally), naloxone methiodide (1 mg/kg, subcutaneously), or the combination of morphine (10 mg/kg) with naloxone (1 mg/kg) or naloxone methiodide (1 mg/kg). Data from vehicle-treated rats have been replotted for comparison. Lidocaine EC50 data for block of A (B) and C (C) waves evoked in sciatic nerves obtained from the rats whose behavioral data are plotted in (A). Each point represents data from a single rat, and the bars are the mean EC50. There was no significant difference between vehicle, naloxone, naloxone methiodide, or naloxone plus morphine-treated groups with respect to EC50 for A and C waves. Nor was there a significant difference between morphine and morphine plus naloxone methiodide–treated groups with respect to EC50. However, the lidocaine EC50 for both morphine and morphine plus naloxone methiodide–treated groups were significantly greater than that in all other groups for both A and C waves.
