Different vulnerability of rat retinal cells to methylmercury exposure

We tested the hypothesis that methylmercury chloride (MMC) caused a selective vulnerability in rat retinal cells during the intact preparation. MMC was injected subcutaneously daily at 3 different doses (0.25, 0.70 or 1.50mg/kg/day) for 30 days. The electroretinograms under dark- and light-adaptation were recorded before and at 10-day intervals during the treatment period. With the lowest dose of MMC, only the amplitude of the light-adapted (LA) 20 Hz response significantly decreased on Day 30. At the intermediate dose, amplitude reductions were observed on Day 20 for the LA 20 Hz response and dark-adapted (DA) a-wave, while reductions in the LA 2 Hz b-wave and DA b-wave were noted only on Day 30. At the highest dose, these changes occurred earlier during the course of treatment. However, the amplitude of the DA second positive oscillatory potentials and the implicit times of any response components remained unchanged at all dosages. These results suggest that the cones are more sensitive than the rods, bipolar cells and Müller cells to MMC. However, amacrine cells were found to be relatively insensitive. Therefore, each retinal cell was found to have a different vulnerability to MMC.