Zinc in the retina

Experimental evidence exists to suggest that zinc can have positive and neg ative effects on the physiology of cells depending on the 'local' concentra tion, localisation (extracellular vs. intracellular) and/or state (bound vs , free). The retina contains particularly high amounts of zinc suggesting a pivotal role in the tissue. There is also suggestive evidence that zinc de ficiency in humans may result in abnormal dark adaptation and/or age-relate d macular degeneration. The purpose of this article is to provide an overvi ew of various proposed functions for zinc, particularly in the retina. Endo genous chelatable zinc in the retina is localised mainly to the photorecept ors and retinal pigment epithelial cells. Moreover, the zinc localisation i n the photoreceptors varies in dark and light, suggesting a role for zinc i n a light-regulated process. Some zinc is also located to other areas of th e retina but clearly defined zinc-enriched neurones could not be identified as has been shown to occur in certain areas of the brain. Neurones post-sy naptic to zinc-enriched neurones in the brain hale been suggested to be par ticularly vulnerable in ischaemia. The role of zinc in retinal ischaemia ha s been investigated to determine how it is involved in the process. It woul d appear that when zinc is administered in low concentrations it generally has a positive effect on an insulted retina as in ischaemia. However, highe r concentrations of zinc exacerbates the influence of the insult and also a cts as a toxin. Use of zinc supplements in diet must, therefore, be taken w ith caution. (C) 2001 Elsevier Science Ltd. All rights reserved.