An Animal Model for Trigeminal Neuralgia by Compression of the Trigeminal Nerve Root

Background: Microvascular compression of the trigeminal nerve root is a major cause of most trigeminal neuralgia (TN) in patients; however, no reliable animal model to further study the pathogenesis of TN currently exists. Objective: Our objective was to establish a novel and practical animal model for TN by chronic compression of the trigeminal (CCT) nerve root in rats, which would provide a better animal model to mimic the clinical feature of TN on the research of the pathogenesis of TN. Study Design: A randomized, double blind, controlled animal trial. Methods: Sixteen adult male Sprague-Dawley rats (200-220 g) were randomly divided into 2 groups: one group that received chronic compression of the trigeminal nerve root (the CCT group, n=8) and another group that received sham operation without compression (the sham operation group, n=8). A small plastic filament was retrogressively inserted into the intracalvarium from the inferior orbital fissure until it reached the trigeminal nerve root for compression in CCT group. Animal behaviors were observed for 4 weeks after operation. Immunohistochemistry of glial fibrillary acidic protein (GFAP), isolectin B4 (IB4), substance P (SP) and calcitonin gene-related peptide (CGRP) were performed in the trigeminal root entry zone (TREZ) and medullary dorsal horn (MDH). Results: The orofacial mechanical allodynia and heat hyperalgesia in the CCT rats were obviously increased after the operation and lasted for 28 days. Increased facegrooming behavior was also observed in the CCT rats and continued for over 21 days, returning to baseline by day 28. Immunohistochemistry for GFAP in the TREZ revealed a progressive extension of astrocytic processes in the ipsilateral TREZ of rats in the CCT group. Furthermore, the IB4 positive immunoreactive nonpeptidergic C-fiber terminals in the MDH were reduced for 4 weeks after the operation. Both SP and CGRP, expressed in the peptidergic C-fiber terminals, were found to be decreased in the ipsilateral MDH of CCT animals after the trigeminal nerve root injury. Limitations: CCT animal model with a plastic filament only imitated the mechanical compression of the trigeminal root but not to display the complex vascular physiological feature as the microvascular in the TN patient. Conclusions: The chronic compression of the trigeminal nerve root in rats effectively induced persistent orofacial neuropathic pain behaviors, and it would provide a novel and practical animal model for future research on the pathogenesis of TN. Key words: trigeminal neuralgia, nerve root compression, animal model, mechanical allodynia, heat hyperalgesia, substance P, calcitonin gene-related peptide, isolectin B4