Abstract
Purpose: :
Retinal degenerations, including Retinitis Pigmentosa (RP) and vitamin A deficiencies, exhibit progressive death of rod and cone photoreceptors with subsequent loss of vision. Typical symptoms are night blindness, decreases in visual field and eventually blindness. In some types of RP as well as vitamin A deficiencies, photoreceptor death is due to constitutive activation of the phototransduction pathway, but the biochemical events underlying apoptosis are presently unknown. To answer this question, we are studying the molecular mechanisms involved in retinal degeneration in Wistar rats kept in moderate light (200 lux) to activate phototransduction continuously for different periods of time.
Methods: :
Wistar rats were exposed to constant illumination with cool white fluorescent light (LL) of 200 lux for 1 to 10 days and compared with controls kept in the dark (DD) or exposed to a regular 12:12 h (LD) cycle. One eye from each rat was used for inmunohitochemical and quantitative morphometric analysis and the other for biochemical assays (western blot, Flow Cytometry Caspasa-3 assay).
Results: :
Histological analysis showed a significant reduction of the outer nuclear layer (ONL) after 4 days of LL as compared with LD or DD controls. Retinal analysis by flow cytometry showed apoptosis in light-exposed rats. Moreover there was a progressive collapse of the outer segments (OS) and inner localization of rhodopsin immunoreactivity in the photoreceptor somas during LL exposure. The study of activated caspasa-3 by western blot and enzymatic kit assay demonstrated a caspase 3-independent mechanism.
Conclusions: :
These findings strongly indicate that low LL exposure induced retinal damage involving an apoptotic, caspase-3 independent mechanism as compared with LD controls, with a significant reduction of the photoreceptor cell layer and mislocalization of rhodopsin after 4 days of treatment. The understanding of the molecular mechanism leading to the low light intensity damage in albino rats can provide valuable insights for the first time about signal photoreceptor death through the activation of downstream signal phototransduction processes.
Keywords: retina: neurochemistry • retinal degenerations: cell biology • apoptosis/cell death