Differential effects of spinal cord lesions on narcotic and non-narcotic supression of nociceptive reflexes: Further evidence for the physiologic multiplicity of pain modulation

These studies examined the effects of bilateral lesions of the dorsolateral funiculus (DLF) of the rat spinal cord on the inhibition of a nociceptive reflex produced either by a systemic injection of 4 mg/kg of morphine or by a 20 sec exposure to 1.0 mA of transcutaneous electric shock. Reflex inhibition was quantified and analgesia inferred by use of a modified version of the D'Amour-Smith tail-flick test. Lesions which included only the DLF reduced morphine-produced analgesia (MA) by 73% but had no effect on shock-produced analgesia (SA) observed in the same rats. Baseline tail-flick latencies of this group were not affected by the lesions. Control lesions restricted to the dorsal columns attenuated neither MA nor SA. Lesions which included both the dorsal columns and DLF did not affect SA and produced no greater reduction in MA than lesions of the DLF alone. Previous work indicates that both MA and SA result, at least in part, from supraspinal activity. The current data indicate that: (1) supraspinal modulation participating in two different types of analgesic induction involves separate descending spinal pathways and (2) the maximal expression of analgesia produced by administration of narcotics requires the integrity of a supraspinal neural system projecting in the DLF.