The toxic effect of L-cysteine on the rat retina. A morphological and biochemical study.

Subcutaneous injection of L-cysteine into 4-day-old rats (1.2 mg/gm body weight) leads to permanent retinal dystrophy. When the eyes were examined 4 to 6 weeks later, there was a reduction of ganglion cells to less than 50% of the controls. The inner plexiform layer was reduced to about 60%, and the inner nuclear layer was reduced with approximately one cell layer. The photoreceptors were unaffected. The same layers were reduced in retinas from L-glutamate-treated animals which were examined for comparison. Electron microscopy showed that no single class of neurons was entirely eliminated by the cysteine treatment, and no specific changes were observed in the remaining neurons. The retinal lesion induced by L-cysteine was not accompanied by any reduction in glutamate decarboxylase activity, choline acetyltransferase activity, or in the high-affinity uptake of glutamate, which are markers for some of the retinal neurotransmitters. This is in striking contrast to the retinal lesion induced by L-glutamate, which is followed by a severe reduction in these biochemical parameters. Eyes enucleated4 hr after injection of L-cysteine showed widespread necrosis of neurons in the inner nuclear layer and of ganglion cells. In conclusion, L-cysteine, like L-glutainate, given systemically to newborn rats, affects neurons in the inner layers of retina. L-Cysteine, however, apparently affects other populations of cells than L-glutamate.