Changes in Melanopsin expression in the iris after corneal abrasion in an experimental rat model

Melanopsin is a photoreceptive protein known to be important for non-image forming functions in the retina, where it is expressed in a small subset of retinal ganglion cells. Melanopsin is also found in the iris where it contributes to the pupillary light reflex in rodents, and in other tissues where its functions are not fully understood. This study assessed temporal changes in pain behavior (orbital tightening) and melanopsin immunofluorescence in the retina and iris in a corneal abrasion model. Pain behaviors peaked at 24 h after corneal abrasion and returned to near baseline levels at 1 week. Changes in melanopsin were examined at both 24 h and 1 week in both abraded and control rats. In the retina, there was no difference in the number of melanopsin-positive cells 24 h after corneal abrasion and a small decrease at 1 week post-abrasion. In the iris, melanopsin labeling in the layers surrounding the constrictor muscle was higher in abraded animals at 24 h after injury and returned to baseline levels at 1 week, in both males and females. Morphological changes at the edge of the pupillary ruff were also observed 24 h following abrasion and returned to baseline at 1 week. Our data indicate that melanopsin expression in the iris, but not the retina, has a dynamic response to corneal abrasion that correlates with abrasion-induced changes in pain behavior and morphological changes in the pupillary ruff. Thus, both molecular and morphological changes in the iris may contribute to ocular pain and photoallodynia.