Zinc distribution in rat eye tissues: normal vs. streptozotocin-induced diabetes

Background. Reviewing the content and distribution of zinc in tissues from available foreign literature sources, we found that the highest concentration occurs in the cytoplasm of cells with endocrine function, smooth muscle cells, anisotropic disks of striated muscle fibers, and plasma membranes. The concentration varies depending on the functional state of the latter. Considering the complexity of the eye's structure and function, studying the functional histotopography of zinc in its membranes and tissues is of significant interest. Literature presents sporadic and isolated data on zinc content in eye tissues of humans across different age groups and in certain diseases. In cataract development, zinc levels decrease in the lens. In glaucoma, zinc levels increase in the cornea but remain unchanged in other tissues, including the vitreous humor. Aim. Our research aimed to investigate the specific histotopographic distribution of zinc in the tissues of the eyeball in normal conditions and at the early stage of experimentally induced streptozotocin-induced diabetes mellitus. Methods. The study was conducted on 15 mature, non-pedigreed, white male rats weighing 120-130 g. Experimental diabetes was induced by a single intraperitoneal injection of streptozotocin (Sigma) at a dose of 7 mg per 100 g body weight (prepared in 0.1 mol citrate buffer, pH = 4.5). The development of diabetes was monitored for two weeks by observing the increase in blood glucose levels, measured using the glucose oxidase method. The study was conducted from the second week of the experiment on animals with blood glucose levels exceeding 12.23 mmol/L. The study included two groups of animals: 1. Diabetic group (10 animals): Animals with developing diabetes (2 weeks after streptozotocin injection). 2. Control group (5 animals): Animals received injections of 0.9% saline solution for two weeks. For microscopic study, the rat eyeballs were used (sectioned at the limbal region). Tissues for histological examination were prepared using standard techniques. We studied the distribution of zinc in different tissues of the eyeball in rats with streptozotocin-induced diabetes mellitus. The content and distribution of zinc in tissue elements were investigated using a histochemical, sulfide silver method. Results. Our observations demonstrate that zinc concentrates in the retina in areas where synapses are formed between photoreceptor neurons and the second system of neurons, between the second and third systems, as well as in the terminal regions of rods and cones. It is evident that zinc plays a significant role in nerve impulse transmission in synapses and even in the photochemical process of light perception by the terminal regions of photoreceptor neurons. At the end of the second week of experimental streptozotocin-induced diabetes, a decreased zinc content was observed, indicated by the lighter staining of the aforementioned retinal bands. Our research findings indicate a specific localization of zinc in the tissues of the eye, particularly the retina, suggesting that zinc is an essential bioelement for the normal functioning of the photoreceptor process in light-sensitive neurons and the transmission of visual sensations to the dendrites of the second and third systems of retinal neurons.