**5. Retinal neural and glial cell impairment**

The neural retina is transparent and largely undistinguished during clinical examination in contrast to retinal vessels. Nevertheless, the retina comprises a complex network of neurons and glia closely interconnecting with vasculature. The neurons: photoreceptors, bipolar cells, horizontal cells, amacrine cells, and retinal ganglion cells percept, integrate, and transmit visual signals into the brain. Glia comprises astrocytes, Müller cells (MCs), and microglial cells. MCs are the primary glial cells of the retina and play a pivotal role in retinal metabolism. It is now broadly acknowledged that in addition to the vascular alterations structural and functional detriment to nonvascular cells contributes to the pathogenesis of DR. Abnormalities of the neural retina have been found in experimental and human diabetes. There is evidence demonstrating an early neurodegeneration of photoreceptors in animal diabetic models [106]. Apoptosis of retinal ganglion cells has been observed as well in cases of short-term experimental diabetes and in humans with diabetes [107]. Decline of color sensitivity [108] and contrast sensitivity [109] are early signs of neural retinal malfunction that take place after only 2 years of diabetes. As glia maintenance functions of neurons and endothelium, apparently, glial reactive changes affect the function and survival both of vascular and of neuronal cells of the retina. It was detected that high glutamate levels in the retinas of diabetic animals as a consequence of MC reduced ability to convert glutamate into glutamine [110, 111]. Glutamate has been demonstrated to be toxic to neuronal cell [112, 113]. These findings suggested an early and probably persistent glutamate excitotoxicity in the retina during diabetes that courses neural degeneration. One of the early signs of retinal metabolic stress is the upregulation of glial fibrillary acidic protein (GFAP) in MCs and astocytes, which was detected in animals and in human patients with non-proliferative retinopathy [111, 114]. It was found that in activated MCs and astrocytes, VEGF expression was significantly increased [115, 116]. Glial cell proliferation is a well-recognized latest change in DR that induces epiretinal membrane formation, and fibrous tissue grows [43]. Microglia become activated early in diabetes in human and diabetic animal models [117]. It is supposed that diabetic conditions lead to an elevation of proinflammatory cytokine expression within the retina that induce microglia activation [118]. Being activated, microglia migrate to the source of inflammation and start to produce a wide range of proinflammatory cytokines, such as TNF- α, IL-6, IL-1, and IL-1, glutamate, ROS, NO, matrix metalloproteinases. All of these factors are implicated in the pathogenesis of DR, affect neuronal cell functions, and induce apoptosis [117–121]. It has been recognized that inflammation plays a pivotal role in pathophysiology of DR. Microglia, as highly sensitive to even low pathological changes in immune-effector cells in the retina, might be expected to have a significant role in the promotion and sustaining this inflammatory response.
