Local infusion of nerve growth factor attenuates myelinated nerve fiber sprouting into lamina II of the spinal dorsal horn and reduces the increased responsiveness to mechanical stimuli in rats with chronic constriction nerve injury

Purpose. To clarify the relationship between allodynia and the sprouting of myelinated fibers, we examined whether the administration of nerve growth factor (NGF) affected the paw withdrawal response to non-noxious mechanical stimuli and the sprouting of myelinated fibers into lamina II of the spinal dorsal horn, using a chronic constriction injury model of the sciatic nerve. Methods. Mechanical allodynia was determined as the threshold of the withdrawal response stimulated by von Frey filaments. Sprouting was examined using horseradish peroxidase conjugated to the B fragment of cholera toxin (B-HRP). NGF was continuously infused into the site of nerve injury for 14 days after nerve ligation. Results. With vehicle infusion, significantly increased responsiveness to mechanical stimuli was observed on postoperative days (PODs) 5, 7, and 14 after ligation, compared with before surgery, and B-HRP-positive fibers were newly localized in lamina II on PODs 7 and 14. Infusion of NGF reduced the responsiveness to mechanical stimuli on 5, 7, and 14 PODs and B-HRP-positive fibers in lamina II on PODs 7 and 14. Conclusion. We propose that the suppression of the increased responsiveness to mechanical stimuli produced by NGF could be related to the disappearance of B-HRP-positive fibers in lamina II.