Elevations in nociceptive thresholds following locus coeruleus lesions

Serotonin depletion or lesions of the midbrain dorsal raphe nuclei attenuate both morphine-produced and stimulation-produced analgesia. In contrast, norepinephrine depletion enhances both types of analgesia. To extend these findings, the effects of destruction of the noradrenergic locus coeruleus (LC) upon nociceptive flinch-jump thresholds were investigated. after four preoperative baseline sessions, lesions were placed in the LC bilaterally and nociceptive thresholds were determined for up to five weeks postoperatively. The lesions were localized by monoamine histofluorescence procedures together with conventional histological staining techniques. In 9 of 13 animals, the LC or its ascending dorsal noradrenergic bundle sustained either bilateral or unilateral damage, evidenced by green fluorescent back-up caudal to the lesions. Eight of these animals demonstrated significantly increased jump thresholds. In the remaining four animals, both lesions spared the LC and nociceptive thresholds were either unchanged or significantly decreased. In three of the four, raphe damage was noted, evidenced by yellow fluorescent back-up. The results suggest apparently contrasting roles of norepinephrine and serotonin in nociception.