Descending modulation from the region of the locus coeruleus on nociceptive sensitivity in a rat model of inflammatory hyperalgesia

The aim of the present study was to evaluate the action of the descending modulation system from the locus coeruleus (LC) in a rat model of unilateral hyperalgesic inflammation. Unilateral hindlimb inflammation was produced by a subcutaneous injection of carrageenan (6 mg in 0.15 ml saline). One week before testing, rats received bilateral lesions of the LC using anodal current. Nociception was assessed by measuring withdrawal of the paw from a noxious thermal stimulus. Four hours after carrageenan injection, paw withdrawal latencies (pWLs) in the inflamed paw of the LC-lesioned rats were significantly shorter than those of the sham-operated rats. This difference in PWL between the two groups was not observed at 7 days, whereas edema and hyperalgesia still remained in the inflamed paw. At 4 h, systemic naloxone produced a further decrease of the PWL in the LC-lesioned rats but not in the sham-operated rats. These results suggest that inflammation-induced activation of the descending modulation system from the LC occurs in only the acute phase of inflammation and that a decrease in the extent of the development of hyperalgesia in the acute phase of inflammation might depend on the interaction between the descending modulation system from the LC and the opioid inhibitory system.