Dorsolateral funiculus-lesions unmask inhibitory or disfacilitatory mechanisms which modulate the effect of innocuous mechanical stimulation on spinal Fos expression after inflammation

To examine the contribution of low threshold mechanoreceptive afferent inpu t to the development of allodynia and the involvement of descending pathway s, we investigated the effects of repeated innocuous brush on inflammation- induced spinal Fos protein expression id dorsolateral funiculus-lesioned (D LFX)rats following hindpaw inflammation. In DLF sham-operated animals, brus h stimuli induced a significant increase in the number of Fos-labeled neuro ns in ipsilateral laminae I-IV, and a slight suppression of Fos expression hi ipsilateral laminae V-VI when compared to sham-lesioned rats without bru shing. In rats receiving DLFX, the brush-induced increase in Fos expression in laminae I-IV was significantly reduced. The DLFX also unmasked a brush- induced suppression of laminae VII-VIII neurons. These results suggest that innocuous mechanical stimulation of an inflamed hindpaw gives rise to furt her facilitation of neuronal activity in laminae I-IV and inhibition of neu ronal activity in laminae V-VIII. We propose that there is an unmasking of inhibitory mechanisms or a reduction in descending facilitatory effects aft er DLEX that alter Fos protein expression produced by innocuous mechanical stimulation. (C) 1999 Elsevier Science B.V. All rights reserved.