Studies on the changes of c-fos protein in spinal cord and neurotransmitter in dorsal root ganglion of the rat with an experimental peripheral neuropathy

Animal models for human chronic pain syndromes have been developed and wide ly used for pain research. One of these neuropathic pain models by Kim and Chung (1992) has many advantages for operation and pain elicitation. In thi s neuropathic model we have examined the c-fos protein, substance P, CGRP i mmunoreactivity in dorsal root ganglia and dorsal hem. 50 Sprague-Dawley ra ts were used for this study. L5 and L6 spinal nerves were ligated tightly t o produce the neuropathic pain model. After 2, 4, 8, 16, and 24 hours and 1 week of surgery, rats were anesthetized and sacrificed by perfusion. After confirmation of the roots transected by the surgery, the L5 and L6 dorsal root ganglions and spinal cord were removed and processed for immunohistoch emistry. All tissue sections were immunohistochemically stained for substan ce P, CGRP and c-fos using the peroxidase-antiperoxidase (PAP) method. The number of immunostained substance P and CGRP dorsal root ganglion cells and c-fos immunoreactive dorsal horn cells were counted and analyzed statistic ally with Mann-Whitney U test. The results are as follows. The number of c-fos protein immunoreactive neur ons in the superficial layer of dorsal horn were increased markedly 2 hours after operation, and gradually decreased to normal level 1 week after oper ation. The number of c-fos protein immunoreactive neurons in the deep layer of the dorsal horn gradually increased to a peak 24 hours after operation, then decreased to the normal level 1 week after operation. The number of s ubstance P and CGRP immunoreactive L5 and L6 dorsal root ganglion neurons w ere decreased markedly 1 week after the pain model operation. In conclusion, after neuropathic pain model operation, c-fos proteins were immediately expressed in the superficial layer of spinal dorsal hem, therea fter c-fos proteins in the deep layer of spinal dorsal horn were expressed. CORP and substance P immunoreactive neurons in DRG were decreased markedly 1 week after neuropathic pain model operation. These decrements do not coi ncide with the other chronic pain models, which show great increases in the se pain transmitting substances. Therefore, the relationship between pain a nd c-fos, SP and CORP should be investigated further.