LIMITED INVOLVEMENT OF CENTRAL NORADRENERGIC PATHWAYS IN MORPHINE-INDUCED ANTINOCICEPTION

Lesioning of cerebral and spinal noradrenergic terminals by the neurotoxin DSP4 [N-2-chloro-ethyl-N-ethyl-2-bromobenzylamine] (50 mg/kg, 7 days prior to testing) significantly attenuated the effect of morphine (2.5-7.5 mg/kg) in rats tested with the hot-plate test. The effect of DSP4 was prevented by pretreatment with the selective inhibitor of uptake of NA [norepinephrine], desipramine. Treatment with DSP4 did not attenuate the effect of morphine (5 mg/kg) in the flinch-jump and tail-flick tests, and did not by itself change the nociceptive thresholds in any of the tests. Apparently noradrenergic structures in the CNS play a limited role in analgesia induced by morphine.