In vitro studies of the effects of naloxone on the root potentials in the frog spinal cord: enkephalin-like effect on the recurrent presynaptic inhibition.

Specific binding of [3H]naloxone was demonstrated in the frog spinal cord. In isolated and perfused frog spinal cord, naloxone increased the spontaneous discharges of the ventral root. Naloxone decreased the ventral root-dorsal root potential (VR-DRP) in a dose-dependent manner, and inhibited presynaptic inhibition of the ventral root reflex. Methionine-enkephalin also decreased the VR-DRP, and naloxone partially antagonized this effect. These results suggest the existence of enkephalinergic control of spinal motor activities and that naloxone has a partial agonistic effect in the frog spinal cord.