Morphine antagonizes inhibitory controls of nociceptive reactions, triggered by visceral pain in the rat.
The interaction between morphine analgesia and the heterotopic hypoalgesic effects induced by visceral pain was investigated using a behavioral approach in the rat. Both systemic morphine and a dull pain of peritoneal-visceral origin (induced by i.p. phenylbenzoquinone, PBQ) increase the threshold for vocalization to tail-shock. The question of the interaction between these two factors was assessed by administering morphine during the period of increase in threshold induced by PBQ. Not only did we find that morphine did not increase the threshold further, but, indeed, very significantly decreased it, with the more obvious effect being observed for a moderate dose (0.33 mg/kg, i.v.). It is concluded that morphine antagonizes the heterotopic hypoalgesic effects of visceral pain. These results do not support the hypothesis that morphine increases inhibitory controls of pain triggered by noxious inputs.
D000818:Animals; D016227:Benzoquinones; D004305:Dose-Response Relationship, Drug; D007274:Injections, Intraperitoneal; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D009433:Neural Inhibition / Q000187:drug effects*; D009619:Nociceptors / Q000187:drug effects*; D010146:Pain / Q000503:physiopathology*; D011809:Quinones / Q000008:administration & dosage; D051381:Rats; D013997:Time Factors; D014828:Vocalization, Animal / Q000187:drug effects
D016227:Benzoquinones; D011809:Quinones; C029322:phenylbenzoquinone; D009020:Morphine