Light Regulation Of Melanopsin–positive Retinal Ganglion Cells In The Albino Hamster

Abstract

Abstract: : Purpose: Pigmented male golden hamsters (Mesocricetus auratus) are long–day breeders, being reproductively competent in long photoperiods and constant light (LL). Albino male golden hamsters, however, become reproductively incompetent in LL as though they were housed in constant dark (DD). Melanopsin–containing intrinsically photosensitive retinal ganglion cells have been shown to be involved in the regulation of non–visual photophysiology including photosuppression of pineal melatonin production. We hypothesized that constant light may downregulate melanopsin in the albino retinas. Methods: Immunocytochemistry with anti–melanopsin antiserum UF007 was used as previously described (Panda et. al, 2002, Science 298, pp 2213–2216) to quantify melanopsin–positive cells in hamster retinal flat–mounts. In each retinal quadrant 3 images were captured sequentially from the periphery to the central retina. Melanopsin–positive cells were counted and expressed as cell number/mm². Real–time PCR was done as previously described (Panda et. al, 2003, Science 301, pp 525–527) to quantitate melanopsin mRNA. Hamster melanopsin–specific primers were designed to span the putative first intron. The presence of a single amplimer of appropriate size was confirmed by melting curve analysis. Results: We demonstrate that hamster melanopsin can be regulated by light. Pigmented golden hamsters raised in LL (c. 250 µW/cm²) for 2 months exhibited significantly fewer melanopsin–positive ganglion cells (30 ± 7.21 cells/mm²) relative to animals raised in DD for two months (46.25 ± 2.06 cells/mm²) versus a light/dark cycle (14L:10D; c. 250 µW/cm²) (46.85 ± 5.12 cells/mm²). However, a dramatic reduction was seen in albino golden hamsters subjected to equivalent light conditions (DD, 42.33 ± 4.44 cells/mm²; LD, 0 cells/mm²; LL, 0 cells/mm²). Comparison of retinal melanopsin mRNA by quantitative PCR showed no difference between DD–, bright LD– or bright LL–raised animals, suggesting that the synthesis or degradation of the protein is light regulated. A decrease in melanopsin–immunoreactive cell number in DD–raised animals is observed after only 24 h of LL; reappearance of melanopsin immunopositive cells to near normal levels occurs 3 days after transferring animals from 24 hours of LL to DD. Conclusions: Melanopsin in the hamster retina can be dynamically regulated by light.

Keywords: ganglion cells • gene/expression • immunohistochemistry