Abstract
Purpose :
Pathological ocular sensations can arise from damage of the three types of corneal nociceptors- polymodal, cold, and mechanosensitive neurons. Benzalkonium chloride (BAK) is the most common preservative in ophthalmic drops and has been demonstrated to cause gross structural disruption of corneal epithelium and nerves upon repeated exposure at clinically-relevant concentrations. The purpose of this study is to combine electrophysiological and immunohistochemical techniques in order to evaluate the time course and properties of corneal epithelial and neuronal damage resultant from a single, acute exposure to BAK-containing solutions in rodents.
Methods :
To evaluate the functional changes in corneal neurons, we used in vivo extracellular electrophysiology to record from the cell bodies of single corneal neurons of anesthetized rats. We continuously evaluated the electrical activity of these neurons before, during, and after ocular instillation of 0.01% BAK or unpreserved ophthalmic solutions. To assess the morphological changes induced by this treatment, we performed immunostaining for TUJ1 (neuron) and ZO-1 (tight junction) markers from corneal wholemounts of mouse and rat eyes treated with either normal saline and 0.01% BAK solutions for one hour. FIJI plug-ins were used to calculate the percent area occupied by TUJ1 or ZO-1 signal, and the results were analyzed using paired, two-tailed t-tests.
Results :
Within 30 minutes of BAK ocular instillation the activity of cold and mechanosensitive neurons either completely disappears or significantly decreases (n=3). After the loss of electrical signal, a subsequent hour-long washout with artificial tears or normal saline is unable to restore the neurons' basal activity, suggesting possible destruction rather than desensitization of corneal nerve terminals. Additionally, wholemount immunostaining reveals a significant reduction in epithelial tight junctions and corneal nerve density in treated eyes (p=0.026 (tight junctions), p=0.025 (nerves); n=3).
Conclusions :
Our preliminary findings provide a time course of functional changes in individual corneal nerve subtypes during a single acute exposure to the ubiquitous ophthalmic preservative BAK. Taken together with our immunohistochemical results, our findings add to the growing body of evidence that exposure to BAK-preserved drops may cause corneal epithelial and nerve toxicity.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.