Effects of Opioid Pure Agonists on the Excitibility of Frog Sciatic Nerve Fibers

opioid pure agonists, morphine, meperidine and methadone, were used to investigate the effect on the opioid receptor of fron sciatic nerve fibers using sucrose gap apparatus. When applied extracellularly by perfusion, morphine, methadone and meperidine significantly depressed the amplitude of the action potential in frog sciatic nerve fibers as a dose-dependent $(10^{-10}\;M-10^{-2}\;M)$ manner. The depression with morphine or methadone was partially antagonized by the simultaueous treatment with a lower $(10^{-10}\;M-10^{-8}\;M)$ concentration of naloxone, but that of meperidine was not blocked. When the three opioid agonists were applied intracellularly by placing it in a compartment with a cut end of the sciatic nerve fibers, all of themn depressed the amplitude of the action potentials by similar potency, and these reductions significantly blocked by pretreatment of lower concentration $(10^{-10}\;M-10^{-8}M)$ of naloxone. These results support the previous findings by other workers that the stereospecific opioid receptors of this preparation are located on or near the intracellular opening of the sodium channels which are sensitive to naloxone.