Experimental Study Of Mechanisms Underlying Sciatic Pain Induced By Herniated Lumbar Disk

The pathophysiological mechanisms underlying sciatica and back pain are not well understood. In the present study, a sciatica model was developed to investigate the contributions of inflammation and compression of the dorsal root. The procedure used autologous disk to apply direct pressure to the L5 dorsal root (disc compression group, DC group). For control, 5 additional groups were included: (1) mechanical compression of L5 dorsal root without disc (bone compression group, BC group); (2) epidurally placed disc without mechanical compression (disc group); (3) epidurally placed nucleus pulposus without mechanical compression (nucleus pulposus group, NP group); (4) epidurally placed annulus fibrosus without mechanical compression (annulus fibrosus group, AF group) and (5) sham group. The paw withdrawal threshold to mechanical stimulation, paw withdrawal latency to heat stimulation, body weight and motor function were determined pre- and post-surgery. It was observed that all experimental groups with the exception of the Sham group showed a progressive and prolonged mechanical hyperalgesia with the DC group having the strongest effect. Furthermore, the Disc group showed a greater mechanical hyperalgesia with earlier onset in comparison to the BC group and Disc, AF, and NP groups developed thermal hyperalgesia in addition to mechanical hyperalgesia following surgery. Finally, rats in all groups showed normal motor function and body weight increase. At the same time, immunohistochemistrical staining was used to observe the expression of c-Fos and vanilloid receptor 1 (VR1) in DC group 3 wk post-surgery, giving the maximum of mechanical hyperalgesia at this time point. The results showed that a lot of c-Fos protein expressions were observed in superficial layer of ipsilateral lumbar spinal dorsal hom, and that VRl-immunoreactivities were observed in superficial layer of bilateral lumbar spinal dorsal horn, but without obvious change between dorsal horns. These data suggest that (1) this new sciaticamodel is suitable to investigate the mechanisms of sciatica; (2) inflammation as well as mechanical compression is involved in the pathogenesis of mechanical hyperalgesia; (3) primary afferent fine fibres (A5 and C fibres) are possibly involved in the development of mechanical hyperalgesia; (4) annulus fibrosus and nucleus pulposus may contribute similarly to the development of sciatica and back pain.