[Local anesthetic effects of morphine and naloxone]

The effects of both morphine (Mo) and naloxone (Nal), topically applied to the desheathed saphenous nerve, were studied in Sprague-Dawley rats during 22 min of superfusion at various concentrations (Mo: 0.086, 0.28, 0.86, 2.8, 8.6 mM/l; Nal: 0.1, 0.33, 1.0 mM/l) by recording single C-fibres. C-fibres were electrically stimulated and the latencies of action potentials were measured. Both drugs impaired nerve conduction in a dose-dependent, fully reversible manner. At high concentrations, conduction was blocked in 5 of 11 (Mo 2.8 mM/l) and 3 of 8 (Nal 1 mM/l) fibres. Mo (8.6 mM/l) instantly blocked all C-fibres investigated. Subsequent washing out of the drug resulted in complete recovery. The findings indicate that Mo and Nal also show local anaesthetic properties; a non-opiate receptor-specific mechanism is postulated. In spite of their blocking action, at present neither Mo nor Nal seems to provide distinct advantages over other agents that have been used successfully to induce reversible nerve blockade at peripheral sites. Due to the rather high dosage necessary (1-2 mg/ml), well-known side-effects of these opiates must also be taken into consideration.