UI - 000217

AU - Vaccarino AL

AU - Tasker RA

AU - Melzack R

TI - Analgesia produced by normal doses of opioid antagonists alone and in combination with morphine

AB - AB - In a recent study [30] it was reported that naloxone, at doses normally employed for opioid

antagonism, produced a dose-dependent analgesia in BALB/c mice in the formalin test. We report here that

another opioid antagonists, naltrexone, also produces analgesia under these conditions. Female BALB/c

mice were injected subcutaneously with naltrexone (0.01-1.0 mg/kg) or saline alone and tested for analgesia

using the formalin test. Naltrexone produced a statistically significant dose-dependent analgesia, with an

ED50 of 0.05 mg/kg and almost total analgesia at doses of 0.1 mg/kg or greater. To determine the

relationship between naloxone analgesia and better documented forms of opioid analgesia, BALB/c mice

were injected with naloxone or saline following the administration of a pre-determined ED50 for morphine

and tested for analgesia using the tail-flick and formalin tests. Naloxone antagonized morphine analgesia in

the tail-flick test at both doses used (0.3 and 10 mg/kg). In the formalin test, however, naloxone attenuated

morphine analgesia at the lower doses (0.1 and 0.3 mg/kg) and potentiated morphine analgesia at the highest

dose (10 mg/kg). The implications of this finding are discussed AD - Department of Psychology AD -McGill

University AD - Montreal AD - Que AD - Canada UI - 89144631

SO - Pain 1989;36:103-109