Alteration of descending modulation of nociception during the course of monoarthritis in the rat.

Diffuse noxious inhibitory controls (DNIC), which involve supraspinal structures and modulate the transmission of nociceptive signals, were investigated at different stages during the development of adjuvant-induced monoarthritis in the rat. After behavioral evaluation, recordings of trigeminal convergent neurons were performed in anesthetized animals with acute (24-48 hr) or chronic (3-4 weeks) monoarthritis of the ankle. Inhibitions of C-fiber-evoked neuronal responses during and after the application of noxious conditioning stimuli to the ankle were measured to evaluate DNIC. The conditioning stimuli consisted of mechanical (maximal flexion and graded pressures) and graded thermal stimuli and were applied alternately to normal and arthritic ankles. Behaviorally, the two groups of animals exhibited a similar increased sensitivity to mechanical stimuli applied to the arthritic joint (i.e., an increased ankle-bend score and a decreased vocalization threshold to pressure stimuli). However, they showed different electrophysiological profiles. In the animals with acute monoarthritis, the DNIC-induced inhibitions produced by mechanical or thermal stimulation of the arthritic joint were significantly increased at all intensities compared with the normal joint. In contrast, in the chronic stage of monoarthritis, the DNIC-induced inhibitions triggered by thermal or pressure stimuli were similar for both ankles, except with the most intense mechanical stimuli. This discrepancy between the behavioral and electrophysiological findings suggests that inputs activated during chronic monoarthritis may fail to recruit DNIC and may thus be functionally different from those activated in the acute stage of inflammation.