Opioid antagonists and spinal reflexes in the anaesthetized cat

In barbiturate-anaesthetized cats, intravenous naloxone (0.025-0.10 mg/kg) increased the amplitude of monosynaptic reflexes produced by 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 neurones with cutaneous receptive fields to the same stimuli. The results suggest that, in anaesthetized cats, inhibition involving opioid peptides at some stage is present on many motoneurones. This inhibition may have relevance to animal behaviour after injury.