Neurochemical evidence for inflammation-induced activation of the coeruleospinal modulation system in the rat

By using the microdialysis technique, the concentration of noradrenaline (N A) in the dorsal horn during unilateral hindpaw inflammation was compared b etween rats receiving bilateral lesions of the locus coeruleus (LC) and non -operated control rats. Bilateral lesions of the LC were made using an anod al current one week before testing. Unilateral hindpaw inflammation was pro duced by a subcutaneous injection of carrageenan (6 mg in 0.15 mi saline). Under conditions of sodium pentobarbital anesthesia, the microdialysis prob e was inserted into the dorsal horn either ipsilateral or contralateral to the site of inflammation. The NA concentration in the dialysate was measure d by high-performance Liquid chromatography with electrochemical detection. Prior to carrageenan injection, the NA level (baseline level) did not diff er between the LC-lesioned and the non-operated groups. After carrageenan i njection, in the non-operated rats, the NA level increased significantly co mpared to the baseline level only in the dorsal horn ipsilateral to the sit e of inflammation, but not in the dorsal horn contralateral to the site of inflammation. An increase of the NA level was not observed in the LC-lesion ed rats and in rats receiving an injection of saline. The result suggests t hat unilateral hindpaw inflammation produces excitation of descending NA-co ntaining neurons from the LC, resulting in an increase of the NA level in t he dorsal horn ipsilateral to the site of inflammation. (C) 1999 Elsevier S cience B.V. All rights reserved.