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

To examine the contribution of low threshold mechanoreceptive afferent input to the development of allodynia and the involvement of descending pathways, we investigated the effects of repeated innocuous brush on inflammation-induced spinal Fos protein expression in dorsolateral funiculus-lesioned (DLFX) rats following hindpaw inflammation. In DLF sham-operated animals, brush stimuli induced a significant increase in the number of Fos-labeled neurons in ipsilateral laminae I-IV, and a slight suppression of Fos expression in ipsilateral laminae V-VI when compared to sham-lesioned rats without brushing. 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 further facilitation of neuronal activity in laminae I-IV and inhibition of neuronal activity in laminae V-VIII. We propose that there is an unmasking of inhibitory mechanisms or a reduction in descending facilitatory effects after DLFX that alter Fos protein expression produced by innocuous mechanical stimulation.