Metabolic activity changes in the rat spinal cord during adjuvant monoarthritis

The development of chronic pain is associated with activity-dependent plast ic changes in neuronal structures in the peripheral and central nervous sys tem. In order to investigate the time-dependent processing of afferent noxi ous stimuli in the spinal cord we employed the quantitative autoradiographi c 2-deoxyglucose technique in a model of chronic monoarthritic pain in the rat. Spinal metabolic activity was determined at various time-points (two, four and 14 days) after the injection of complete Freund's adjuvant into th e left tibiotarsal joint. In addition, the effect of acute noxious mechanic al stimulation of the arthritic joint was investigated at 14 days of monoar thritis. Local glucose utilization was determined in lumbar segments L-2-L- 5, ipsi- and contralateral to the inflamed hindpaw, and compared with salin e-injected controls, in general, monoarthritic animals had bilaterally incr eased metabolic activity in all laminae of the spinal cord. Detailing the t ime-course showed that in rats with two days of monoarthritis metabolic act ivity was significantly increased to a similar extent on both sides of all spinal laminae. In contrast, at four days, glucose utilization in deep lami nae of the dorsal horn (laminae V-VI), the central gray area (laminae X) an d the ventral horn (laminae W-M) tended to return to control levels. At 14 days of monoarthritis, however, metabolic activity showed a further increas e in all laminae of the spinal cord. This increase was more pronounced on t he side ipsilateral to inflammation, reaching 65% above corresponding contr ol levels in laminae V, VI. Animals with 14 days of monoarthritis which wer e subjected to mechanical noxious stimulation of the arthritic joint displa yed clear behavioral signs of acute pain. Although in this group metabolic activity was above control levels, it was lower than in animals with 14 day s of monoarthritis that were not additionally stimulated. The data show not only a general increase of spinal cord metabolic activity during the time-course of the development of a chronic pain state, but als o show a region-specific non-linear time profile. This may reflect the comp lexity of transducing and suppressive transmitter systems involved in the c entral processing of ongoing pain. (C) 1999 IBRO. Published by Elsevier Sci ence Ltd.