Pharmacological characterization of dynorphin A (1-17)-induced effects on spinal cord-evoked potentials

Dynorphin A (1-17) was applied directly onto the spinal cord of rats during electrophysiologic recording of the dorsal root potential (DRP) and the ventral root potentials (VRPs), i.e., monosynaptic reflex and polysynaptic reflexes. Dynorphin application resulted in a dose-dependent depression of the DRP (ED50, 4.5 nmol) which persisted for 30 to 50 min. This effect was not antagonized by nor-binaltorphimine, a kappa-opioid receptor antagonist. During this depression we observed a potentiation of the VRPs which persisted for 4 to 5 min and preceded depression of the VRPs (ED50, 4.0-4.9 nmol). The depression of the VRPs was antagonized competitively by nor-binaltorphimine, although the potentiation was not. beta-Funaltrexamine, a mu-opioid receptor antagonist, had no influence on dynorphin-induced changes of evoked potentials. These data indicate that dynorphin-induced depression of the VRPs is mediated by kappa-opioid receptor activity, whereas neither potentiation of the VRPs nor depression of the DRP appears to be mediated by an opioid receptor effect.