Nalorphine: actions at an opiate receptor on frog skeletal muscle fibers

Intracellular microelectrode studies were conducted to investigate the actions of the partial agonist – antagonist nalorphine at an opiate receptor on functional frog skeletal muscle fiber membranes. In high bath concentrations (≥ 10−4 M), nalorphine alone produces agonist actions similar to the "full" opiate agonists. These actions were (i) to depress both the sodium and potassium (gNa and gK) conductance increases due to electrical stimulation by a nonspecific local anestheticlike mechanism and (ii) to depress gNa by a specific opiate receptor mediated mechanism. In a much lower bath concentration (1 × 10−8 M) nalorphine acts to antagonize the specific opiate receptor mediated depression of gNa produced by the "full" agonist meperidine. Thus in this preparation nalorphine, "the partial antagonist," has the same actions as naloxone, which is often considered to be a full antagonist. The quantitative differences observed in the effects of these two opiate antagonists are discussed.