TY - JOUR T1 - NEUROMUSCULAR EFFECTS OF MORPHINE AND NALOXONE JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 136 LP - 142 VL - 184 IS - 1 AU - GEORGE C. SOTEROPOULOS AU - FRANK G. STANDAERT Y1 - 1973/01/01 UR - http://jpet.aspetjournals.org/content/184/1/136.abstract N2 - The effects of morphine and naloxone were studied on the in vivo soleus neuromuscular preparation of the cat. Both drugs changed the pattern of the tetanic contraction and depressed post-tetanic potentiation (PTP) and twitch tension. Naloxone was found to be 10 times more potent than morphine in depressing PTP and about equipotent in depressing twitch tension. Preadministration of 5 or 10 mg/kg of naloxone i.v. caused subsequent doses of morphine to produce less PTP depression but had no effect on maximal twitch depression. A possible role of histamine liberated by morphine or naloxone in these phenomena was sought and not found. Since PTP is caused by post-tetanic repetitive activity originating in the motor nerve terminals, it is concluded that morphine and naloxone exert their effects on neuromuscular function by selectively stabilizing the motor nerve terminals and that naloxone, which competitively antagonizes the depression produced by morphine, is a partial agonist of morphine in this system. © 1973 by The Williams & Wilkins Company ER -