PACAP promotes neuron survival in early experimental diabetic retinopathy

Metabolic changes induced by diabetes lead to a multifactorial progressive disease of the retina with an extremely complex pathogenesis. One of the mechanisms of retinal cell death in diabetes is via apoptosis. Our previous results show that pituitary adenylate cyclase activating polypeptide (PACAP) attenuates the morphological and neurochemical changes in a rat model of diabetic retinopathy. The aim of this study was to investigate the mechanisms of this protective effect. Retinas of streptozotocin-induced diabetic rats were analyzed using apoptosis detection combined with immunolabeling. Western blot was used to measure levels of pro- and anti-apoptotic pathways. Intraocular PACAP injection markedly attenuated diabetic retinal injury: increased levels of the anti-apoptotic p-Akt, p-ERK1, p-ERK2, PKC, Bcl-2, while decreased levels of the pro-apoptotic p-p38MAPK and activated caspases (8, 3, 12) were detected. The number of apoptotic cells increased in all nuclear layers of diabetic retinas, but significantly decreased after PACAP treatment. Our results clearly demonstrate that the protective effects of PACAP are mediated, at least partly, by attenuating apoptosis, including also that of the dopaminergic amacrine cells. Inhibition of apoptosis is one of the PACAP-induced pathways with therapeutic potential in early experimental diabetic retinopathy.