Dissociation of supraspinal and spinal actions of morphine: a quantitative evaluation.

Opiate suppression of spinal withdrawal reflexes was tested in rats with lesions of several spinal funiculi to determine the relative contribution of supraspinal descending systems. The latency of tail-flick to noxious heat was used to assess "analgesia". The effect of lesions of dorsolateral funiculus (DLF), dorsal columns (DC) and ventral quadrant (VQ) were compared to that of sham operations. None of the lesions produced a change in baseline latency. Each animal was tested with varying doses of morphine sulfate over several weeks. Only DLF lesions consistently antagonized tail-flick suppression by morphine across the dose range studied (5-15 mg/kg i.p.), although VQ lesions were somewhat effective. The reduction of morphine's action was proportionally greater for lower doses. The results indicate that both spinal and supraspinal sites contribute significantly to the analgesia produced by systemic administration of opiates.