Spinal antinociceptive action of three representative opioid peptides in frogs.

Spinal administration of low doses of dynorphin, beta-endorphin or Met-enkephalin produces a potent, dose-dependent increase in the nociceptive threshold in the unanesthetized frog, Rana pipiens. Nociceptive thresholds were determined by using the acetic acid test, previously shown to be a sensitive indicator of antinociception in this amphibian species. Of particular interest, spinally administered dynorphin produces a potent antinociception in frogs without any signs of motor dysfunction seen after spinally administered dynorphin in mammalian species.