Early markers of retinal degeneration in rd/rd mice

In the rd/rd mouse, the cell death of rod photoreceptors has been correlated to abnormal levels of the cyclic nucleotide cGMP within photoreceptors. Given that cGMP is required for opening of the cationic channels, there is the possibility that a high cGMP concentration would maintain these channels open, at a high energy cost for the retina. We investigated whether cation channels were maintained in an open state in the rd/rd mouse retina by determining the labeling pattern of an organic cationic probe (agmatine, AGB) which selectively enters cells through open cationic channels. The metabolic activity of the rd/rd mice was measured by assaying lactate dehydrogenase (LDH) activity in several tissues and Na+/K+ ATPase activity was measured as a function of development and degeneration of the retina. AGB neuronal labeling showed a systematic increase consistent with the known neuronal functional maturation in the normal retina. There was a significant higher AGB labeling of photoreceptors in the rd/rd mouse retina from P6 supporting the possibility of open cationic channels from an early age. There were no changes in the LDH activity of tissues that contain PDE6 or that have a similar LDH distribution as the retina. However, LDH activity was significantly higher in the rd/rd mouse retina than in those of control mice from birth to P6, and it dramatically decreased from P9 as the photoreceptors degenerated. The predominant LDH isoenzyme changes and loss after degeneration appeared to be LDH5. ATPase activity increased with age, reaching adult levels by P16. Unlike LDH activity, there was no significant difference in Na+/K+ ATPase activity between control and rd/rd mice at any age examined. We conclude that AGB is a useful marker of photoreceptors destined to degenerate. We discard the possibility of a generalized metabolic effect in the rd/rd mice. However, the elevated LDH activity present before photoreceptor differentiation indicated altered retinal metabolic activity that could not be associated with open cationic channels alone. Therefore, altered metabolic activity as indicated by LDH measurements in the retina appeared to be the earliest sensitive sign of future photoreceptor dysfunction in the rd/rd mice.