The Dry Eye Digest

ABSTRACT: Acute corneal epithelial debridement unmasks the corneal stromal nerve responses to ocular stimulation in rats: Implications for abnormal sensations of the eye.

J Neurophysiol. 2017 Mar 1:jn.00925.2016. doi: 10.1152/jn.00925.2016. [Epub ahead of print]
Hirata H1, Mizerska KK2, Dallacasagrande VA2, Guaiquil VH3, Rosenblatt MI3.

Abstract 

It is widely accepted that the mechanisms for transducing sensory information reside in the nerve terminals. Occasionally, however, studies have appeared demonstrating that similar mechanisms may exist in the axon to which these terminals are connected. We examined this issue using the cornea where nerve terminals in the epithelial cell layers are easily accessible for debridement, leaving the underlying stromal (axonal) nerves undisturbed. In isoflurane-anesthetized rats, we recorded extracellularly from single trigeminal ganglion neurons innervating the cornea that are excited by ocular dryness and cooling: low threshold (<2 and="" cooling="" high="" oc="" threshold="">2 oC) cold-sensitive plus dry sensitive (LT-CS+DS and HT-CS+DS) neurons, playing possible roles in tearing and ocular pain. We found that the responses in both types of neurons to dryness, wetness, and menthol stimuli were effectively abolished by the debridement, indicating that their transduction mechanisms lie in the nerve terminals. However, some responses to the cold, heat and hyperosmolar stimuli in LT-CS+DS neurons still remained. Surprisingly, the responses to heat in ~ half of the neurons were augmented after the debridement. We were also able to evoke these residual responses and follow the trajectory of the stromal nerves, which we subsequently confirmed histologically. The residual responses always disappeared when the stromal nerves were cut at the limbus, suggesting that the additional transduction mechanisms for these sensory modalities originated mostly likely in stromal nerves. The functional significance of these residual and enhanced responses from stromal nerves may be related to the abnormal sensations observed in ocular disease. 

Copyright © 2016, Journal of Neurophysiology.