Antagonizing NK-1R modulates pain perception following corneal injury

Substance P (SP) expressed by corneal nerves, is an 11-amino acid long neuropeptide from the tachykinin family, encoded by the Tac1 gene, and binds to neurokinin receptors. SP overexpression is associated with various pathological responses in the cornea including vasodilation, pain, inflammation, and angiogenesis in the normally avascular tissue. This study investigates the role of neurokinin-1 receptor (NK-1R) mediated signaling in nociception, nerve regeneration, and neuronal activation following mechanical corneal injury in mice. Corneal injuries were induced in age- and sex-matched C57BL/6 mice by removing corneal epithelium and partial anterior stroma. Following injury, mice were treated with either L-733,060, an NK-1R antagonist, or vehicle, administered topically twice daily for 21 days. Corneal SP levels were measured using ELISA, and nerve regeneration was assessed by quantifying corneal nerve fiber density (CNFD) via β-Tubulin III staining. Gene expression of neuronal markers (ATF3, GFAP, cFos, TRPV1, and TRPM8) in the trigeminal ganglia was measured using qPCR. Pain responses were evaluated using the eye-wiping test (EWT) and palpebral ratio (PR). Results indicated a persistent increase in corneal SP post-injury, significantly reduced by NK-1R antagonism. At 21 days, NK-1R antagonist-treated mice showed higher CNFD, reduced expression of neuronal activation markers, and lower pain perception compared to controls. These findings suggest that SP/NK-1R signaling is critical in corneal nociception post-injury, and its inhibition reduces pain, prevents neuronal hyperactivation, and supports nerve regeneration.