Effects of morphine on ionic currents in frog node of Ranvier.

The effects of externally or internally applied morphine hydrochloride on the node of Ranvier of the frog were studied under current and voltage clamp conditions. Externally applied morphine in a concentration of 0.5-1 mM prolonged the action potential and the absolute refractory period. It inhibited the repetitive response of sensory fibres to long-lasting depolarizing pulses. Externally applied morphine was more effective in depressing the steady state K current than the peak Na current. When added to the internal medium (by diffusion from the cut ends of the fibre) morphine had a stronger effect on the peak Na current. Morphine did not alter the kinetics of Na activation but increased the time constants of development or removal of Na inactivation by a factor of about 1.2. Morphine reduced both the fast and slow phase of the inward K tail currents recorded in Ringer with 30 mM K. Of the two components (gKf1 and gKf2) of fast K conductance described by J.M. Dubois (1981, J. Physiol. 318, 297) only gKf2 was affected. The morphine antagonist naloxone in a concentration of 0.1 mM completely and reversibly abolished the component gKf2 of the fast K conductance. Naloxone did not antagonize the effect of morphine. These findings show that there are no specific opiate receptors in the membrane of peripheral nerves.