Role of the neutrophil elastase in facial mechanical hypersensitivity in trigeminal neuralgia via modulation of PAR2/TRPV4 signalling

Abstract Background Trigeminal nerve root compression (TNC) by surrounding blood vessels is considered a primary cause of trigeminal neuralgia. Non-neuronal cells have been reported to contribute to trigeminal nerve injury-associated orofacial neuropathic pain, but the mechanism underlying their involvement in TNC-induced orofacial neuropathic pain remains unclear. Here, we focused on neutrophils accumulating at the injury site to clarify the role of neutrophil elastase (ELA2) in TNC-induced orofacial mechanical hypersensitivity in a rat model of TNC. Methods A glass rod was inserted through the skull to compress the left trigeminal nerve root (TNC group); in the sham group, the rod was inserted without compression. Mechanical head withdrawal thresholds (MHWT) were measured in the whisker pad skin. Ultrastructural changes in compressed nerves were analysed by electron microscopy. Activating transcription factor 3 (ATF3), ELA2, protease-activated receptor-2 (PAR2), C-X-C motif chemokine ligand 1 (CXCL1), C-X-C motif chemokine receptor 2 (CXCR2) and transient receptor potential vanilloid 4 (TRPV4) expressions, and ELA2 levels, were analysed in the TG and/or trigeminal nerve root. MHWT was measured after continuous administration of an ELA2 inhibitor and a PAR2 antagonist into the TG and topical administration of a TRPV4 antagonist into the whisker pad skin. Changes in neuronal activity in TG neurons and trigeminal nerve roots were examined electrophysiologically. Results TNC induced trigeminal nerve degeneration, a persistent decrease in MHWT in whisker pad skin, and TG neuronal hyperexcitability. ELA2 and CXCL1/CXCR2 expression increased in trigeminal roots and TG after TNC. Pharmacological inhibition of ELA2 alleviated the TNC-induced decrease in MHWT. CXCL1 neutralization in the TG reduced ELA2 protein levels in the TG and trigeminal root and attenuated the TNC-induced decrease in MHWT. Furthermore, PAR2 and TRPV4 expressions increased in TG neurons innervating whisker pad skin of TNC rats, and blockade of PAR2 or TRPV4 suppressed the TNC-induced decrease in MHWT. Exogenous ELA2 administration to the TG of naïve rats decreased MHWT and enhanced TG neuronal responses to mechanical stimulation of whisker pad skin, which local TRPV4 antagonism attenuated. Conclusions TNC-induced ELA2 signalling via CXCL1/CXCR2 promotes TG neuronal sensitization by activating the PAR2/TRPV4 pathway, leading to orofacial mechanical hypersensitivity.