Up-regulated expression of neuronal nitric oxide synthase in experimental diabetic retina

Nitric oxide (NO) can play either a neuroprotective or a neurotoxic role in diverse neurodegenerative conditions. This study investigated the differential expression of neuronal nitric oxide synthase (nNOS) in the streptozotocin-induced diabetic rat retina to clarify the involvement of NO produced from neurons in the early pathogenesis of diabetic retinopathy. A decrease in thickness of the outer retina was evident at 12 and 24 weeks after onset of diabetes. nNOS was immunolocalized in two subtypes of amacrine cells, displaced amacrine cells and in some bipolar cells in the normal retinas. The densities of each type of nNOS-expressing neuron showed no significant differences in the diabetic retinas with the exception of the bipolar cells. The numbers of nNOS bipolar cells at 12 weeks of diabetes increased threefold, showing dendritic polarity of nNOS expression. Protein levels of nNOS increased throughout the diabetic retinas reaching a peak value at 24 weeks of diabetes. Thus, diabetes up-regulates the expression of nNOS in bipolar cells, and NO from these cells may aggravate the degeneration of the outer retina in the diabetic retinas.