Phototoxicity causes a transient downregulation of melanopsin in the rat retina.

Abstract

Purpose : Intrinsically photosensitive retinal ganglion cells (ipRGCs) are a special subtype of RGCs that express melanopsin and thus respond to light. It has been proposed that ipRGCs are more resistant than other RGCs to different lesions. Here we study the effects of acute light exposure on the population of ipRGCs.

Methods : Adult female albino Sprague Dawley (SD) rats (2 months old; n=22) were exposed to light (3000 lux) for 48 hours following our previously described methods (García-Ayuso et al., Mol Vis 2011; 17:1716-1733) and processed 0 hours, 3, 7 or 30 days later. Whole-mounted retinas were incubated with antibodies against Brn3a, melanopsin and rhodopsin and photographed using a motorized fluorescence microscope. Rod density was analyzed qualitatively. The total numbers of Brn3a⁺RGCs and melanopsin positive RGCs (m⁺RGCs) were automatically determined in the whole mount reconstructions using Image-Pro Plus 5.0 for Windows® following previously described methods (García-Ayuso et al., IOVS 2015; 56:4592-4604). Co-expression of Brn3a and melanopsin by RGCs was also assessed.

Results : Three days after light exposure (ALE) rods were scarce in the arciform “light sensitive” area of the superior retina. With time ALE rod loss was widespread throughout the retina. Light exposure did not affect the numbers of Brn3a⁺RGCs that remained similar to those observed in control retinas during the first month ALE. However, immediately ALE there was a significant decrease in the mean number of m⁺RGCs. From 3 days ALE, the numbers of m⁺RGCs increased again progressively and reached normal values one month ALE. Co-expression analysis showed that while in control animals the vast majority of m⁺RGCs were Brn3a negative, in the light exposed animals the numbers of m⁺RGCs that expressed also Brn3a increased immediately ALE and decreased later to reach numbers similar to those found in control animals by one month ALE.

Conclusions : This study demonstrates that during the first month ALE there is a transient downregulation of melanopsin expression by ipRGCs and a transient upregulation of Brn3a expression by m⁺RGCs. Transient downregulation of melanopsin expression has been previously reported following optic nerve injury (Nadal-Nicolás et al., IOVS 2015; 56:4309-4323). Further studies are needed to understand the significance of these regulatory changes.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.