Effects of acute and chronic morphine treatment on methadone analgesia and metabolism

Morphine sulfate (5 mg/kg s.c.) given 30 min prior to administration of methadone prolonged methadone analgesia and increased the brain level of methadone measured 60, 120 and 180 min after administration of methadone. Rats rendered tolerant to morphine analgesia by subcutaneous implantation of two pellets, each containing 75 mg of morphine base, for 1-3 days showed cross-tolerance to methadone analgesia regardless of the presence or absence of morphine pellets. Decreases in the brain concentrations of methadone measured at 60 and 120 min time points accompanied the decreased analgesia. Neither acute nor chronic morphine pretreatment affects the biotransformation of methadone. The results suggest that the cross-tolerance to methadone analgesia seen in chronic morphine-implanted rats was partly associated with a decrease in the brain concentration of methadone occurring by a mechanism not directly related to a change in the biotransformation of methadone. In view of the known inhibitory effect of chronic morphine pretreatment on drug metabolism, our findings might demonstrate a unique phenomenon between morphine and methadone.