Protection of photoreceptor cells in adult rats from light-induced degeneration by adaptation to bright cyclic light

Light history has been shown to affect the susceptibility of the albino rat retina to the damaging effects of constant light exposure. Retinas of anim als raised in relatively bright cyclic light are protected against light-in duced degeneration compared with dim-reared animals. These effects were obs erved in animals raised from birth in bright cyclic light and are part of a n adaptive response that protects photoreceptors from stress-induced degene ration. To determine if retinas of adult animals are capable of such adapti ve changes or flexibility by switching between different light environments which do not pathologically damage photoreceptor cells, albino rats were m aintained in less than 250 lux cyclic light for more than 3 weeks. At 12-13 weeks of age. they were placed into 800 lux cyclic light for 1 week. after which they were exposed to constant illumination of 1500 lux for 1, 3 or 7 days. Retinal function was evaluated by electroretinography and photorecep tor cell death was quantified by measuring outer nuclear layer thickness. A fter 1 week in bright cyclic light, the retinas were completely protected a gainst 1 day constant light exposure that significantly damaged retinas of animals without 800 lux cyclic light adaptation. Significant protection was also observed in 3 day constant light exposed animals: limited protection occurred after 7 days exposure. These results indicate that the retinas of adult rats adapted to bright cyclic light within certain ranges that did no t significantly damage photoreceptor cells are protected from constant ligh t challenge. This phenomenon is a post-developmental response that demonstr ates a remarkable plasticity of the retina. The mechanism(s) underlying the ability of this adaptation/flexibility in protecting photoreceptors could involve endogenous molecules that encompass many aspects of retinal cell an d molecular biology and physiology. Identification of these molecules may p rovide insight into the development of therapeutic approaches to treat reti nal degeneration. (C) 2001 Academic Press.