טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentMassarweh Amir
SubjectThe Effect of Prolonged Darkness on the Susceptibility of
the Albino Rat Retina to Light Damage
DepartmentDepartment of Medicine
Supervisor Professor Emeritus Ido Perlman


Abstract

Light-induced retinal damage in albino rats is an established model for studying cellular and molecular mechanisms leading to apoptosis of photoreceptors, therefore serves as an accepted model for photoreceptor apoptosis in inherited and in acquired retinal diseases. Dark rearing is known to exacerbate the extent of photoreceptor damage following exposure to bright light compared to albino rats kept in 12/12 light/dark cycle. However, most studies were qualitative in nature and did not address the causes of this effect.

In this study, the first goal was to quantify light induced damage in dark reared albino rats (dark rats) in comparison to light-induced retinal damage in albino rats raised under 12/12 hours light/dark cycle (cyclic rats).  Rats were kept in complete darkness for 3 days, one-week or two weeks and then exposed to damaging light for different intervals; 1, 2, 4, 8, 16 hours. Cyclic rats underwent the same intervals of light exposure. Our electroretinogram (ERG) recordings revealed that even 3 days of complete darkness were sufficient to obtain notable retinal susceptibility to light damage. Retinal susceptibility to light damage became more prominent in dark rats that were kept in darkness for 1 week and became profound in dark rats that were kept in darkness for two weeks. We found that the increased susceptibility of dark rats to light damage reversed when the dark rats were returned for 1 week to a cyclic 12/12 light/dark environment. 

Our results argued against the accepted assumption, which attributed the increased susceptibility to light damage of dark rats to the increase in the content of rhodopsin, the visual pigment that is the major contributor to light-induced retinal damage. Therefore, we suggested the hypothesis that changes in the genetic profile of the photoreceptors by prolonged darkness could account for the dark rearing effect. We measured and compared the whole retinal transcriptome of dark rats to cyclic rats. The important groups of genes whose expression was changed by prolonged darkness were those coding for proteins involved in DNA repair, metabolism & energy homeostasis, protein synthesis and cell viability.

The transcriptomic analysis leads us to suggest the following theory: The outer segments of the photoreceptors, a high energy consuming cells, due to the active extrusion of sodium ions and intrusion of potassium ions by the Na/ K ATPase exchanger elongate in prolonged darkness. Therefore, the inflow of sodium ions through the cyclic GMP dependent sodium/calcium channels in the outer segment and the outflow the potassium ions in the inner segment and cell body increase, demanding higher ATP consumption by the Na/K transporter. The decreased ATP balance leads to down regulation of ATP-consuming pathways and up-regulation of ATP producing pathways. The net effect is increasing the susceptibility of the photoreceptors to external or internal insults, such as exposure to bright light. We further suggest that the effects of prolonged darkness may mimic some aspects of aging, and thus this model has the potential to become a new, easy and reversible model, to study the increased susceptibility to insults during aging.