Neuropeptide expression following ligation of the ferret lingual nerve.

Previous studies on the ferret inferior alveolar nerve found a close association between the spontaneous neural activity generated at a site of nerve injury, and the accumulation of neuropeptides in the injured axons. More recent electrophysiological studies on the lingual nerve revealed high levels of spontaneous activity 3 days after injury, a decline at 3 weeks and a late rise at 3 months. In the present study we have used immunocytochemical techniques to see whether this time course of spontaneous activity is again paralleled by an accumulation of neuropeptides. In 20 anaesthetised adult ferrets the left lingual nerve was ligated and sectioned distally, and the animals left to recover for 3 days, 3 weeks or 3 months. The tissue was processed using indirect immunofluorescence and image analysis was used to quantify levels of the neuropeptides; calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal polypeptide (VIP), galanin (GAL), enkephalin (ENK) and neuropeptide Y (NPY). Immunoreactivity to all of the neuropeptides was present proximal to the ligature 3 days after injury, and these high levels of expression had decreased considerably by 3 weeks. By 3 months ENK and NPY expression had almost disappeared proximal to the ligature, but levels of CGRP, SP, VIP and GAL had increased slightly. This was also accompanied by an accumulation of all of the neuropeptides, except NPY, in the portion of nerve immediately distal to the ligature. This late accumulation of certain neuropeptides coincides with the increase in spontaneous activity seen in our previous electrophysiological studies and supports the suggestion that neuropeptides may play a role in the aetiology of sensory disorders after nerve injury.