Trigeminal neuropathic pain is associated with trigeminal nerve damage. Significant remodeling of the peripheral nervous system may contribute to the pain; however, the changes and the factors that drive them have not been well described. In this study, a partial injury of the mental nerve of the rat, a purely sensory branch of the trigeminal nerve, resulted in prolonged mechanical allodynia in the lower lip skin persisting up to 4 months. Although nonpeptidergic, P2X3-immunoreactive (IR) C fibers displayed a transient decrease in density of innervation in the skin; they returned to sham levels by 4 weeks after lesioning. Ectopic sympathetic (as detected by anti-dopamine-β-hydroxylase antibodies) and parasympathetic (as detected by antibodies against the vesicular acetylcholine transporter) fibers in the upper dermis were apparent early on the following lesion (2 weeks), in close apposition with regenerating nonpeptidergic fibers. Meanwhile, the glial cell line-derived growth factor (GDNF) showed a quick upregulation in the skin after nerve lesioning, with levels peaking at 4 weeks. This suggests that an excess of GDNF in the skin drives the nonpeptidergic C-fiber regeneration and parasympathetic fiber sprouting in the upper dermis, and could be an important mechanism in trigeminal neuropathic pain. This article provides an in-depth description of the changes in nonpeptidergic fibers in the skin after nerve lesioning, and measures, for the first time, GDNF protein levels in the skin after a nerve lesion, providing strong evidence for the role of GDNF in modulating innervation of the nonpeptidergic and parasympathetic fibers in the skin after injury.
In the rat lower lip skin, peripheral nerve injury leads to an increase in GDNF levels that reflects changes in parasympathetic and nonpeptidergic sensory fiber innervation.
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aDepartment of Pharmacology and Therapeutics, McGill University, Montreal, Quebec, Canada H3G 1Y6
bAlan Edwards Center for Research on Pain, McGill University, Montreal, Quebec, Canada H3A 2B2
cDepartment of Anatomy and Cell Biology, McGill University, Montreal, Quebec, Canada H3A 2B2
*Corresponding author. Address: Department of Pharmacology and Therapeutics, McGill University, 3655 Promenade Sir-William-Osler, Montreal, Quebec, Canada H3G 1Y6. Tel.: +1 514 398 3619.
Article history: Received 19 September 2010; Received in revised form 4 February 2011; Accepted 14 February 2011.