Effects of naloxone on spinal reflexes in the isolated frog spinal cord.

Abstract

Naloxone (Nix) has generally been considered to be a specific antagonist of opiates. However, other effects of Nix have been reported recently. This study demonstrated the effects of Nix using the isolated intra-arterially perfused frog spinal cord and discussed characteristics of the opiate system in the spinal cord. Nix (more than 30 mM) potentiated spontaneous discharges recorded from the ventral root (VR). Resting potentials of VR and the dorsal root (DR) recorded by sucrose-gap method were not affected or slightly depolarized by Nix. Methionine-enkephlin (Enk), 30 mM, inhibited VR-DR potential (VR-DRP) significantly, whereas morphine only slightly inhibited VR-DRP even at 100 mM. It suggests that opiate receptors mediating this inhibitory effect on VR-DR neuronal network have higher affinity for Enk. Nix (more than 10 mM) dose-dependently inhibited VR-DRP, but partially antagonized the inhibitory effect of Enk. Nix had no effect on DR-DR network. Enk increased DR-DRP, although the effect was not significant. These observations suggest that the spinal opiate system may individually modulate two presynaptic inhibitory systems, i.e., DR-DR and VR-DR neuronal networks.