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Degeneration of retinal neuronal processes and pigment epithelium in the early stage of the streptozotocin‐diabetic rats

Early pathological and electro‐physiological changes of the retina in the streptozotocin (STZ)‐diabetic rats were investigated through optical and electron microscopy in two strains and electro‐retinography in one strain. In Sprague–Dawley (SD) rats 1 month after the onset of diabetes, the thickness of the inner plexiform layer (IPL) and photoreceptor segment layer (PSL) was significantly reduced by 9.9% and 18.9%, respectively (P < 0.01, P < 0.05). In Brown–Norway (BN) rats STZ‐diabetic for 1 month, the thickness of the IPL was also significantly reduced by 15.7% (P < 0.05). Cytochemical study using peanut agglutinin (PNA), a lectin binding selectively to the cone photoreceptor‐associated domains of the inter‐photoreceptor matrix, revealed a marked reduction in intensity, number and length of the PNA‐binding cone photoreceptors. Electron microscopy showed deepened hollows in the basal infoldings of the retinal pigment epithelium (RPE) of STZ‐rats diabetic for 1 month and large concavities into the cytoplasm in STZ‐rats diabetic for 6 months. Blood vessels in the retina and choroid were unremarkable. Single‐flash electro‐retinogram revealed a reduction in the amplitudes of a‐ and b‐waves of electro‐retinogram (ERG) of 1 month STZ BN rats (P < 0.05). These findings indicate that the degeneration of rods/cones in the PSL and RPE are the most prominent pathological alteration sites in the early stage of diabetic rats.