Differential expression of melanopsin mRNA and protein in Brown Norwegian rats

Melanopsin is expressed in a subpopulation of retinal ganglion cells rendering these cells intrinsically photosensitive (ipRGCs). The ipRGCs are the primary RGCs mediating light entrainment of the circadian clock and control of the pupillary light reflex, light regulated melatonin secretion and negative masking behaviour. Previous studies have demonstrated that melanopsin expression in albino rats is regulated by light and darkness. The present study was undertaken to study the influence of light and darkness during the circadian day and after extended periods of constant light and darkness on melanopsin expression in the pigmented retina of the Brown Norwegian rat (Rattus norvegicus). The diurnal and circadian expressions were examined in retinal extracts from rats euthanized every 4 h during a 24 h light/dark (LD) and a 24 h dark cycle (DD) using quantitative real-time PCR and Western blotting. To study whether light regulates melanopsin expression, rats were sacrificed after being placed in either constant light (LL) or darkness for 3 or 21 d. Flat mount retinas from animals kept during either LL or DD were also examined by immunohistochemistry. Melanopsin mRNA expression displayed a significant rhythmic change during the LD cycle with peak expression around dusk and nadir at dawn. Melanopsin protein also changed over the LD cycle with peak expression at the end of the night and nadir at dusk. Rhythmic expression of melanopsin mRNA but not melanopsin protein was found in constant darkness. After 3 or 21 d in either LL or DD melanopsin mRNA expression was unaltered. Melanopsin protein was at the same high level after 3 and 21 d in DD, whereas a significant decrease was found after prolonging the light period for 3 or 21 d. The change in melanopsin protein was primarily due to change in immunoreactivity in the dendritic processes. In conclusion we found that light and darkness are important for regulation of melanopsin protein expression whereas input from a retinal networks regulates melanopsin mRNA expression. It is likely to speculate that altered level of melanopsin is one way in which the retina adapts to environmental light and darkness conditions ensuring optimal light sensitivity for the transmission to the brain.