65: Disruption of the Trigeminal Myelin Transition Zone After Infraorbital Nerve Constriction Injury

Topic: Basic Science
Subtopic: Pain category (ICD-11), Chronic neuropathic pain, , , , , , , , , , Assessment and Measurement of Pain/Nociception, Animal imaging (includes Ca2+ imaging, fluorescence microscopy), Behavioral studies

Background and Aims: Chronic trigeminal neuropathic pain (CTNP) is a debilitating condition with limited effective treatment options, partly due to an incomplete understanding of the early cellular mechanisms that support the transition from acute to chronic pain1,2. This study aimed to investigate neuroglial adaptations within the central-peripheral myelin transition zone (TZ) of the trigeminal nerve (Vn) following infraorbital nerve chronic constriction injury (ION-CCI) 3,4. Using the immunohistochemical detection of glial fibrillary acidic protein (GFAP); monoamine oxidase-B (MAO-B); connexin-43 (Cx43) and aquaporin-4 (AQP4) we sought to characterise changes in astrocyte reactivity at the TZ in male and female rats, over a 28-days post-injury period.

Methods: Using a blinded experimental design, female and male Sprague–Dawley rats received either ION-CCI, sham injury, or no intervention and were examined at 2-, 7-, 14- and 28 days post-CCI. Trigeminal pain behaviours to mechanical stimuli were assessed longitudinally using von Frey filaments and individual behaviours were recorded3. Post-mortem, the TZ was analysed at each of the time points using immunofluorescence and astrocyte reactivity along the TZ border (ie., central vs peripheral) was determined from GFAP; MAO-B; Cx43 and AQP4 immunoreactivity. Statistical comparisons were made across experimental groups, time-points, and sex5.

Results: ION-CCI produced significant increases in GFAP immuno-reactivity in both the central and peripheral parts of the TZ, increased MAO-B immunoreactivity within the central part of the TZ, and enhanced co-localisation of GFAP and MAO-B across time, as well as an increased expression of Cx43 and AQP4 within the central part of the TZ. Significant differences were observed across post-injury time-points and between sexes, indicating sex-dependent mechanisms in the development and maintenance of CTNP.

Conclusions: Together, these findings provide evidence for acute and persistent changes in astrocytes in the TZ of Vn following peripheral nerve injury. Specifically, astrocyte reactivity increases, as well as signalling via Cx43, and water regulation via AQP4. These changes present potential mechanisms by which signals resulting from peripheral nerve injury could be relayed to the central nervous system. Our study provides strong support that enhanced neuro-immune signalling in the TZ contributes to the development of CTNP. This work identifies the TZ as a potential therapeutic target and establishes a methodological foundation for future mechanistic investigations.

References: 1. Romero-Reyes, M. and Uyanik, J.M. (2014) Orofacial pain management: current perspectives. Journal of Pain Research, 7, pp. 99–115.
2. Soliman, N., Moisset, X., Ferraro, M.C., de Andrade, D.C., Baron, R., Belton, J., Bennett, D.L.H., Calvo, M., Dougherty, P., Gilron, I., Hietaharju, A.J., Hosomi, K., Kamerman, P.R., Kemp, H., Enax-Krumova, E.K., McNicol, E., Price, T.J., Raja, S.N., Rice, A.S.C., Smith, B.H., Talkington, F., Truini, A., Vollert, J., Attal, N., Finnerup, N.B., Haroutounian, S. and NeuPSIG (2025) Pharmacotherapy and non-invasive neuromodulation for neuropathic pain: a systematic review and meta-analysis. The Lancet Neurology, 24, pp. 413–428.
3. Fraher, J.P. (1992) The CNS–PNS transitional zone of the rat: morphometric studies at cranial and spinal levels. Progress in Neurobiology, 38, pp. 261–316.
4. Vos, B.P., Strassman, A.M. and Maciewicz, R.J. (1994) Behavioral evidence of trigeminal neuropathic pain following chronic constriction injury to the rat’s infraorbital nerve. Journal of Neuroscience, 14, pp. 2708–2723.
5. Kang, J.W.M., Davanzo, O.I., Emvalomenos, G.M., Mychasiuk, R., Henderson, L.A. and Keay, K.A. (2024) Infraorbital nerve injury triggers sex-specific neuroimmune responses in the peripheral trigeminal pathway and common pain behaviours. Brain, Behavior, and Immunity, 118, pp. 480–498.

Presenting Author: Dylan Lugt
Submitting Author: Dylan Lugt

Senior Author: Kevin Keay

IASP 2026 Global Year theme of Neuropathic Pain: Yes

• Olivia Davanzo - Neuroscience, School of Medical Sciences, Faculty of Medicine and Health, University of Sydney; Brain and Mind Centre, University of Sydney
• Matthew Fakhoury - Sydney School of Dentistry, Faculty of Medicine & Health, University of Sydney
• Kristopher Kappatos - Sydney School of Dentistry, Faculty of Medicine & Health, University of Sydney
• Nicholas Raffoul - Sydney School of Dentistry, Faculty of Medicine & Health, University of Sydney
• Christian Sgouros - Sydney School of Dentistry, Faculty of Medicine & Health, University of Sydney
• Luke A Henderson - Neuroscience, School of Medical Sciences, Faculty of Medicine and Health, University of Sydney; Brain and Mind Centre, University of Sydney
• Kevin A Keay - Neuroscience, School of Medical Sciences, Faculty of Medicine and Health, University of Sydney; Brain and Mind Centre, University of Sydney