title: Analgesia produced by normal doses of opioid antagonists alone and in combination with morphine.
abstract: 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.
authors.affiliation: Department of Psychology, McGill University, 1205 Dr. Penfield Avenue, Montreal, Que. H3A 1B1 Canada Department of Anatomy and Physiology, Atlantic Veterinary College, University of Prince Edward Island, P.O. Box 2400, Charlottetown, PE1 C1A 8C1 Canada.
authors.forename: Anthony L
authors.initials: AL
authors.lastname: Vaccarino
authors.forename: R A R
authors.initials: RAR
authors.lastname: Tasker
authors.forename: Ronald
authors.initials: R
authors.lastname: Melzack
mesh_terms: D000698:Analgesia; D000818:Animals; D004305:Dose-Response Relationship, Drug; D005260:Female; D051379:Mice; D008807:Mice, Inbred BALB C; D009020:Morphine / Q000008:administration & dosage*; D009270:Naloxone / Q000008:administration & dosage*; D009271:Naltrexone / Q000008:administration & dosage*; D010147:Pain Measurement; D011957:Receptors, Opioid / Q000187:drug effects / Q000502:physiology*
chemical_list: D011957:Receptors, Opioid; D009270:Naloxone; D009271:Naltrexone; D009020:Morphine