NK-1R Antagonism suppresses the expression of pain -associated neuronal activity markers following corneal mechanical injury

Purpose : Corneal injury is associated with neuronal activation potentially causing long-term pain in patients. Substance P mediates ocular surface nociception by primarily activating neurokinin 1 receptor (NK-1R). Our purpose is to determine the effect of NK-1R antagonism on the expression levels of signature molecular markers for pain-associated neuronal activity. Methods : Corneal mechanical injury was induced in mice with Algerbrush II. L-733,060 (1μg/μl), an NK1-R antagonist, or PBS was administered topically twice for 21 days. Hyperalgesia and allodynia were quantified by eye wipe test (EWT) and palpebral ratio (PR). Using qPCR, we assessed the gene expression levels of activating transcription factor 3 (ATF3), cFos, glial fibrillary acidic protein (GFAP), transient receptor potential cation channel subfamily M Member 8 (TRPM8) and subfamily V member 1 (TRPV1) in the trigeminal ganglia (TG) of the mice before injury and on days 4 and 21 post-injury. Results : Injured mice showed significantly worsened EWT and PR scores following injury, which were significantly improved in L-733,060 treated group compared to controls on days 7(p=0.008), 14(p=0.008; p= 0.0004) and 21(p= 0.01; p=0.05). L-733,060 treatment resulted in significantly lower expression levels of ATF3 in TG compared to PBS treated mice on day 4(p=0.0031) and 21(p=0.0084), comparable to non-injured mice levels. The expression levels of Fos in TG were significantly reduced in L-733,060 treated mice on days 4(p=0.004) and 21(p=0.001) compared to controls. Fos expression levels in L-733,060 treated mice were comparable to non-injured mice at both time points. The expression levels of GFAP in TG of L-733,060 treated mice were significantly lowered to non-injured mice expression levels on days 4(p=0.011) and 21 (p=0.004) compared to controls. Similarly, L-733,060 treated group showed significantly lower TG expression levels of TRPV1 and TRPM8 on day 4(p=0.009 and p=0.015, respectively) and day 21(p=0.032 and p=0.003, respectively) compared to controls. NK1R antagonism restored TRPV1 and TRPM8 expression levels to that observed in non-injured mice at both 4 and 21 days. Conclusions : Our data suggest that antagonism of NK-1R leads to reduction in expression levels of pain-associated neuronal activity markers. Therefore, topical NK-1R inhibition represents a potential disease-modifying therapy of corneal injury-related pain.