Abstract
Purpose: :
To investigate whether constant light would affect the development of melanopsin-containing retinal ganglion cells (mcRGCs).
Methods: :
Neonatal rats were raised under constant light (LL) for 20 days from birth, morphology and density of mcRGCs were analyzed. To evaluate actual density of mcRGCs, rats (LL20) were transferred to cyclic light/dark (LD) for 10 days. To exam whether the effects of constant light on mcRGCs were temporal or irreversible, rats (LL20) were transferred to cyclic light/dark for 20 and 30 days. To determine whether constant light had similar effects on mature mcRGCs, rats raised under cyclic light/dark for 20 days were transferred to constant light for 20 days then were moved to cyclic light/dark again for 10 days (LD20-LL20-LD10). The density of mcRGCs in experimental groups was compared with that of age-matched control. mcRGCs were visualized by immunohistochemistry on whole-mounted retinas. The density of mcRGCs was counted on flat-mounted retinas depending on quadrants and distance from optic disc.
Results: :
After being exposed to constant light for 20 days from birth (LL20), the density of mcRGCs decreased. To test whether the decrease of mcRGCs was caused by decrease of melanopsin expression or by decrease of melanopsin-containing retinal ganglion cells, rats (LL20) were transferred to cyclic light/dark for 10 days (LL20-LD10). Beyond our expectation, instead of decrease, the density of mcRGCs increased significantly in some parts of superior and temperal quadrants. Furthermore, the increase of mcRGCs was irreversible, the density of mcRGCs did not decrease even if these rats were raised under cyclic light/dark for 20 and 30 days (LL20-LD20, LL20-LD30), allowing enough recovery time of the melanopsin expression. However, in mature retinas, constant light could not result increase of mcRGCs’ density (LD20-LL20-LD10) compared with that of age-matched control (LD50)
Conclusions: :
Constant light suppressed melanopsin expression but increased density of mcRGCs during development. The increase of mcRGCs was irreversible and it did not decrease even if excessive cells survived. Whether the changes of mcRGCs caused by constant light would result in changes of pupillary light reflex and circadian rhythm need further invesitation.
Keywords: development • retina