Morphine and supraspinal inhibition of spinal neurones: evidence that morphine decreases tonic descending inhibition in the anaesthetized cat.

1 A study was made in cats anaesthetized with barbiturate or alpha-chloralose, of the excitation of dorsal horn neurones by impulses in unmyelinated (C) primary afferent fibres of the tibial nerve. 2 Block of conduction in the first lumbar segment by cooling produced large increases in the number of action potentials evoked by C fibre afferents in neurones of more caudal segments. 3 Morphine (0.3 to 1.0 mg/kg) reduced the excitation of neurones by C fibre afferents and also reduced the increase produced by blocking conduction in the spinal cord. Naloxone (0.1 to 0.3 mg/kg) reversed these effects of morphine. 4 This decrease in descending inhibition supports findings in the decerebrate cat but gives no support to the hypothesis that an important component of morphine analgesia is an activation of descending inhibitory pathways.