Glial and Immune Cell Responses following Rat Infraorbital Nerve Lesion

Peripheral trigeminal nerve lesions may result in neuropathic pain that is resistant to pharmacologic management. Neuropathic pain mechanisms are varied and include contributions from both immune and glial cells that can lead to an activation of pain pathways. We are using a combined partial axotomy/chromic suture lesion of the rat infraorbital nerve (ION) to study trigeminal neuropathic pain mechanisms. This lesion produces neuropathic pain behavior and an increase in sodium channel expression within axons at the site of injury, but the effect on immune and glial cells at the lesion site is unknown. Objective: Evaluate the response of immune and glial cells at ION lesion site. Methods: Alternate ION sections from lesioned (2 weeks survival; n=5) and normal (n=5) subjects were triple-stained with the indirect immunofluorescence method using antibodies against; ED-1 (macrophage marker) or caspr (nodal/myelinated fiber marker), glial fibrillary acidic protein (GFAP; marker for nonmyelinating Schwann cells), and TO-PRO-3 (nuclear marker). Images were obtained with confocal microscopy and evaluated with NIH ImageJ to calculate GFAP and ED-1 expressions within nerve bundles, and to characterize GFAP and caspr relationships. Results: Axon bundles adjacent to the lesion site were occupied by macrophages (7.8% ±2.7 of nerve area in lesioned subjects vs. 0.0% in normal subjects; p<0.01) and showed increased GFAP expression when compared to normal samples (nerve area: 54.7% ±6.4 vs. 7.0% ±3.4; p<0.001). This seven-fold increased expression of GFAP included associations with myelinated fibers including those with altered caspr relationships that were both rarely seen in normal samples. Conclusion: ION lesion produces broad effects on both immune and glial cells. Importantly, altered GFAP expression occurs on myelinated fibers, including sites where sodium channel expression is increased, thus demonstrating important immune-glial-neuronal interactions that can contribute to neuropathic pain mechanisms following trigeminal nerve injury.

Supported by NIDCR #DE013942/M.Henry.