Intraplantarly (i.pl.)-injected morphine showed a peripheral analgesia in experiments to assess the blockade of bradykinin (i.pl.)-induced nociceptive flexor response in mice. The peripheral morphine analgesia in mice which developed central analgesic tolerance to chronic morphine (10 mg/kg s.c., 5 days), was equivalent to that in vehicle-treated mice in any doses between 0.1 and 1 nmol (i.pl.). These findings suggest that morphine tolerance in the central analgesia may use unique mechanisms deficient in the peripheral nervous system.
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