Effect of depletion of spinal cord norepinephrine on morphine-induced antinociception

We studied whether antinociception produced by injection of morphine into the nucleus reticularis paragigantocellularis (NRPG) or by superfusion onto the spinal cord involved norepinephrine (NE)-containing neurons that descend from brainstem into the spinal cord. Spinal cord NE concentrations were depleted with the neurotoxin, 6-hydroxydopamine, and antinociception was measured following morphine injection into NRPG or onto spinal cord. Depletion of cord NE by approximately 90% did not attenuate the antinociceptive effect of either 2 or 10 μg of morphine injected intrathecally. In contrast, the depletion did significantly attenuate the antinociceptive effect of 2.5 μg morphine injected bilaterally into the NRPG. These results suggest that NE-containing neurons descending from brainstem nuclei into the spinal cord are not important in the analgesia produced by injecting morphine directly onto the spinal cord but may be involved with analgesia produced by morphine injection into the NRPG.