Depletion of nitric oxide synthase-containing neurons in the diabetic retina: reversal by aminoguanidine

A close association of neuronal nitric oxide synthase-immunoreactive (nNOS- IR) neurons with the retinal vasculature has been reported and it is propos ed that activation of these neurons could be the mechanism by which retinal blood flow and metabolism are linked. Further, advanced glycation end prod ucts (AGEs) have previously been shown to be increased in the diabetic reti na and aminoguanidine (AG), an inhibitor of advanced glycation, has been sh own to attenuate the development of AGE accumulation as well as the progres sion of experimental diabetic retinopathy. This study examined the effects of short (1 and 3 weeks) and long term (32 weeks) diabetes on nNOS-containi ng neurons of the retina using NADPH diaphorase (NADPHd) histochemistry. In addition, the effect of aminoguanidine (an inhibitor of advanced glycation and NOS) and N-G-nitro-L-arginine methyl ester (L-NAME) (a non-selective N OS inhibitor) on retinal nNOS-containing neurons was examined in short and long term control and diabetic rats. In a separate study, the effect of 2,3 diamino-phenazine (NN0028) (an inhibitor of advanced glycation, but not NO S) was examined in short term (3 weeks) diabetic rats. The number of NADPHd -positive neurons per retina was reduced after one week of diabetes and rem ained decreased in long term diabetic rats, an effect not observed in diabe tic rats rendered euglycaemic by intensified insulin treatment. Treatment o f diabetic animals with aminoguanidine or NN0028 prevented the depletion in the nNOS-containing neuron number, an effect not reproduced by L-NAME. The se studies suggest that the action of AG in restoring the number of nNOS-co ntaining retinal neurons is mediated by the inhibition of AGE formation. Th e depletion of nNOS-containing neurons may contribute to alterations in the autoregulation of blood flow which occurs in diabetes.