Light-induced retinal degeneration causes a transient downregulation of melanopsin in the rat retina

In this work we study the effects of an acute light-induced retinal degeneration on the population of melanopsin positive retinal ganglion cells (m⁺RGCs) and the expression of the melanopsin protein in the retina. The m⁺RGCs may be more resistant than other RGCs to lesion, but the effects of an acute light exposure in this population are unknown. Albino rats were exposed to white light (3000 lux) continuously for 48 h and processed 0, 3, 7 or 30 days after light exposure (ALE). Whole-mounted retinas were immunodetected with antibodies against melanopsin, Brn3a, and rhodopsin to study the populations of m⁺RGC, Brn3a⁺RGC and rods (which are the most abundant photoreceptors in the rat retina). Three days ALE there was substantial rod loss in an arciform area of the superior retina and with time this loss expanded in the form of rings all throughout the retina. Light exposure did not affect the number of Brn3a⁺RGCs but diminished the numbers of m⁺RGCs. Immediately ALE there was a significant decrease in the mean number of immunodetected m⁺RGCs that was more marked in the superior retina. Later, the number of m⁺RGCs increased progressively and reached normal values one month ALE. Western blot analysis showed that melanopsin expression down-regulates shortly ALE and recovers thereafter, in accordance with the anatomical data. This study demonstrates that there is a transient downregulation of melanopsin expression in the RGCs during the first month ALE. Further studies would be needed to clarify the long-term effect of light exposure on the m⁺RGC population.