OPIOID ANTAGONISTS AND SPINAL REFLEXES IN THE ANESTHETIZED CAT

In barbiturate-anesthetized cats, i.v. naloxone (0.025-0.10 mg/kg) increased the amplitude of monosynaptic reflexes produced electrical stimulation of the nerves to the biceps-semitendinosus and gastrocnemius muscles and the complex reflexes to electrical stimulation of myelinated afferents of the sural and tibial nerves and reflexes to electrical stimulation of unmyelinated primary afferents of the tibial nerve. Increases in reflexes were also produced by the (-)- but not the (+)-isomer of the opiate antagonist N-furylmethylnormetazocine (both isomers being given in the dose range 0.03-0.20 mg/kg). The doses of naloxone increasing reflexes to C primary afferents had no effect on the responses of some dorsal horn neurons with cutaneous receptive fields to the same stimuli. In anesthetized cats, inhibition involving opioid peptides at some stage is present on many motoneurons. This inhibition may have relevance to animal behavior after injury.