ACUTE AND CHRONIC EFFECTS OF OPIATES ON SINGLE NEURONS OF THE MYENTERIC-PLEXUS

  • 1 January 1981
    • journal article
    • research article
    • Vol. 217  (1) , 70-80
Abstract
Extracellular recordings were made from single neurons in the myenteric plexus of the guinea-pig ileum in vitro. Tissue was removed from morphine-naive guinea pigs and maintained in vitro for up to 24 h; the firing rate of the neurons was reduced by opiate agonists and unaffected by opiate antagonists. Tissues were also removed from animals which had been pretreated with morphine during 3 days and placed into an in vitro solution which contained morphine. An increase in the concentration of morphine did not inhibit neuronal firing and naloxone caused a pronounced excitation of such cells. Cross-tolerance among different opiate agonists was apparent. In a 3rd experiment, tissues were removed from morphine-naive guinea pigs and incubated in vitro for 24 h in a solution which contained an opiate agonist. An increase in the concentration of agonist did not inhibit cell firing and opiate antagonists caused marked excitations of neurons incubated with morphine. Incubation with opiate agonists induced a much reduced sensitivity to the inhibitory effect of morphine on cell firing (tolerance) and also sensitized the cells to a marked excitatory effect of opiate antagonists (dependence) which was similar to the changes induced by in vivo opiate administration. The changes induced by morphine during 24 h in vitro were not affected by the concomitant presence of sufficient lidocaine to prevent neuronal activity. [The other drugs used were normorphine, levorphanol, dextrorphan, ketocyclazocine, cyclazocine, buprenorphine, hyoscine and hexamethonium].

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