title: Methionine-enkephalin antagonism and endorphin potentiation of narcotic-induced analgesia.
abstract: Intracerebroventricular (i.c.v.) administration of subanalgesic doses of beta- and leucine-endorphin in mice 15 min before subcutaneous morphine injection, significantly enhanced the analgesic effects of the morphine as measured by the tail-flick assay. A similar effect was seen with levorphanol analgesia but not enhancement of leucine- or methionine-enkephalin analgesia by beta-endorphin was observed. The same doses of the endorphins did not affect the development of single-dose morphine tolerance and dependence. Leucine-enkephalin failed to affect morphine analgesia, tolerance or dependence development, while low doses of methionine-enkephalin administered i.c.v. were observed to have an antagonistic effect on morphine analgesia without affecting tolerance or dependence development. Neither endephalin had any effect on beta-endorphin analgesia.
mesh_terms: D000818:Animals; D004347:Drug Interactions; D004361:Drug Tolerance; D004723:Endorphins / Q000494:pharmacology*; D004744:Enkephalin, Methionine; D004745:Enkephalins / Q000494:pharmacology*; D006168:Guinea Pigs; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D051379:Mice; D008813:Mice, Inbred ICR; D009020:Morphine / Q000494:pharmacology*; D009021:Morphine Dependence / Q000503:physiopathology; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects; D011930:Reaction Time / Q000187:drug effects
chemical_list: D004723:Endorphins; D004745:Enkephalins; D004744:Enkephalin, Methionine; D009020:Morphine