Post-tetanic depression of spinal reflexes in the rabbit and the possible involvement of opioid peptides.

In spinally transected, decerebrated rabbits, the reflex evoked in the ankle extensor gastrocnemius medialis by stimulation of the ipsilateral sural nerve was depressed for periods of 12-45 min following tetanic stimulation of the high-threshold afferents of the ipsilateral tibial, common peroneal, gastrocnemius medialis and sural nerves. It was unaffected by similar tetanic stimulation of ipsilateral saphenous or semitendinosus nerves, or of any nerve in the contralateral limb. The extension reflex between saphenous nerve and vastus lateralis was depressed after tetanic stimulation of saphenous nerve. Post-tetanic depression was partially reversed by the opioid antagonist (-)-naloxone in a dose of 50 micrograms/kg I.V., implicating the involvement of endogenous opioids. The sural-semitendinosus flexion reflex was depressed only by stimulation of the homonymous muscle nerve. It was enhanced for up to 20 min after iterative stimulation of the high-threshold fibres of the ipsilateral sural nerve. This reflex was unaffected by tetanic stimulation of any of the other ipsilateral or contralateral nerves tested. Tetanic stimulation of high-threshold afferents of hind-limb nerves apparently stimulates the release of endogenous opioids within the spinal cord. It is proposed that this is brought about by the activation of enkephalin-containing neurones in the superficial laminae of the dorsal horn.