The effects of 2,5-hexanedione on rods and cones of the retina of albino rats.

Light microscopic morphometry has shown that albino rats exposed to 2,5-hexanedione (2,5-HD) or 2,5-hexanediol (2,5-HDol) develop degeneration of rods and cones in a 12 hr light/12 hr darkness environment. To determine whether there is selective damage to rods or cones, 7-week old, male Sprague-Dawley rats were exposed to 2,5-HD (1%) in their drinking water during equal periods of light and darkness for 5 weeks. At the end of this time the rats were paralyzed in the hind limbs. Half were then sacrificed and studied while the remaining rats were allowed to recover for 13 weeks. A control group of rats were studied in parallel under identical conditions. The outer nuclear layer in the exposed and control rats was assessed immediately following treatment. The rods were found to be strongly selectively reduced (p less than 0.01) but the cones (2% of all photoreceptor cells) were not significantly affected. Rats permitted to recover for 13 weeks and then sacrificed had lost almost all their rods (p less than 0.001) while the cones were reduced by about 50% (p less than 0.01). Most of the rats had a moderate residue of rods and cones at the ora serrata, an area protected from light energy. These findings suggest that rods are more sensitive than cones to the toxic effects of 2,5-HD in combination with light energy.