Phototoxic Effects of Light the Retina and Circadian Photorecepion

Abstract

Abstract: : Purpose: Exposure to phototoxic light levels leads to nearly complete loss of retinal photoreceptors associated with a severe reduction of vision and the ERG responses. Since recent evidence has shown that both intrinsically light sensitive melanopsin containing ganglion cells as well as classical rods and cones are involved in the photic synchronisation of circadian rhythms, this study aimed to assess the degree of loss of circadian responses following photoxic light exposure. Methods: Albino Wistar rats were exposed to elevated levels of short-wavelength UV light (320-420 nm) for 5 continuous days, with pupils dilated. The degree of photoreceptor loss was examined by using PNA histochemistry and anti-opsin antibodies in retinal sections and wholemounts. Entrainment to light, light induced phase shifts and masking were studied by monitoring locomotor activity in groups of phototoxically lesioned and normal rats. Photic induction of the early gene c-fos in the suprachiasmatic nucleus (SCN) was also studied in both groups. Results: Histological examination of the retina and photoreceptor counts showed that almost all rod and cone cell bodies, inner and outer segments were absent, except a few scattered cells and 3-4 rows of cones at the retinal periphery. Despite this severe degeneration, light entrainment of activity rhythms and the amplitude of light induced Fos expression in SCN neurons were identical in phototoxically treated and normal control rats. Conclusion: The results show that severe loss of classical photoreceptors in the adult does not affect circadian responses to light. This implies that the intrinsically light sensitive ganglion cells, which contain melanopsin, may be sufficient for light entrainment in the rat. The results are relevant to the design of therapeutic strategies in patients suffering from photoreceptor loss associated with retinitis pigmentosa or other ocular pathologies.

Keywords: radiation damage: light/UV • photoreceptors • circadian rhythms