A Novel Objective Assay for Evaluating Rodent Behavioral Responses Associated With Corneal Irritation and Injury

Abstract

Purpose: : Present evaluations of rodent behavioral responses to corneal irritation are essentially limited to counting of irritant–evoked blinks and eye wipes. These measures are subjective, labor intensive and time consuming. In the present study, we developed an objective method of evaluating behavioral responses evoked by an irritant applied to the cornea. We also tested the effect of a standard corneal injury model on these behavioral responses.

Methods: : Male Sprague Dawley rats were utilized and all procedures were approved by IACUC. Under deep isoflurane anesthesia, injury was produced by a 2–s application of silver nitrate (SN) to the left cornea. Sham treatment involved anesthesia without SN injury. Rats were habituated for 2 min in a modified conical restrainer attached to a platform that transduces force from rat motion into voltage (i.e. startle apparatus). Voltage recordings were collected continuously for ∼1.5 minutes prior to corneal instillation of a drop of an irritant stimulus, 327 µM capsaicin (or vehicle), and for an additional ∼1.5 minutes following instillation. Behavioral testing occurred only once per rat at 1, 3 or 7 days following SN injury or sham. Activity–induced voltage output was analyzed.

Results: : At 1, 3 and 7 days following SN injury or sham treatment, capsaicin instillation elicited significantly greater poststimulus activity relative to vehicle (p<0.05). Overall, the response to capsaicin was significantly greater in SN injured rats compared with the uninjured sham group (p<0.05), with maximal differences occurring at day 7. Unexpectedly, by day 7 SN injured rats displayed less basal (prestimulus) activity compared with uninjured rats.

Conclusions: : We have developed a robust, novel assay for objectively evaluating activity/agitation in rodents following corneal irritation and injury. Using this assay, we are able to detect injury–related hyperalgesia evoked by the ocular irritant, capsaicin. Future studies will evaluate the ability of analgesic and anesthetic compounds to alter these behaviors associated with ocular surface sensation.

Keywords: cornea: basic science • ocular irritants • inflammation