Opiate drug receptors on excitable cell membranes

In drug interaction studies on intact mice it was found that naloxone (3mumoles/kg) antagonized the general central nervous system depressant effects of morphine or meperidine. Higher naloxone doses (0.3 and 0.4 mmoles/kg), like the fullagonists, potentiated the loss of the righting reflex produced by phenobarbital. Naloxone (0.3 to 14 muM) antagonized the depressant effects of morphine or meperidine on the compound action potential of frog's sartorius muscles in vitro. Higher naloxone concentration (ED20=0.3 mM, ED100=3.0mM), like the full agonists, depressed action potential production. These results show that high doses of naloxone can duplicate some effects of morphine or meperidine but at low doses it antagonizes the effects of the full agonists. This would suggest that naloxone is a partial agonist with a very low intrinsic activity. These results also indicate that there are opiate drug receptors on excitable cell membranes and that drug activation of these receptors inhibits action potential production.