Hypergravity Induces Rod Photoreceptor Damage in Rats

Abstract

Abstract: : Purpose: Rod photoreceptor degeneration can result from a variety of environmental and genetic factors. The microgravity environment encountered in space shuttle flight can disrupt normal retinal development and mimic earth based stimuli that induce similar retinal degenerations (Tombran–Tink & Barnstable, this meeting). We have now examined eyes from animals subjected to hypergravity and find that these also show signs of rod degeneration degeneration. Methods: Pregnant rats were maintained at 2.0 xG for approximately 14 days by centrifugation with a normal lighting schedule. Age– and weight–matched stationary controls were exposed to the same environmental conditions in the centrifuge rotunda. Eyes from the adult rats were fixed in formalin, cryoprotected, frozen, and sectioned. Sections were labeled with a variety of cell type–specific antibodies including those recognizing rod photoreceptor cell bodies, inner segments and outer segments. Results: Eyes from rats maintained at 2.0 xG showed loss of rod photoreceptors with varying degrees of severity. In the most extreme cases the cell body layer was reduced to approximately 20% of its normal thickness. The outer segments were shorter and much more disorganized although inner segments showed less change. Other cell layers across the retina showed no detectable change. Stationary control retinas showed normal photoreceptor morphology. Conclusions: This study shows that exposure to extended periods of hypergravity can lead to deleterious changes in rod photoreceptors. The results are unlikely to be due to changes in intraocular pressure since this usually affects retinal ganglion cells and not rod photoreceptors. Although these changes in gravitational forces could be transient, understanding how prolonged exposure to altered gravitational fields damage photoreceptors is important for future manned space flight to other planets. The hypergravity induced retinal degeneration model is genetically uncompromised and could be useful in studying mechanisms of photoreceptor degeneration and the effects of neuroprotective agents in the eye.

Keywords: retinal degenerations: cell biology • retinal development • blood supply