Nitric oxide: the synthesis and effects at the level of retina

Abstract

Introduction. Nitric oxide (NO), the smallest signaling molecule known to be produced by three major isoforms of NO synthase: neuronal (nNOS), endothelial (eNOS) and inducible (iNOS), is receiving nowadays an increased interest considering its role in retinal function and pathology. This review had the intention to summarize some aspects of NO in the retina and suggest new ideas for future research. Materials and methods. Recent retrospective studies that describe the function and implication of NO in pathogenesis of eye diseases were analyzed. Results. The nNOS and eNOS are normally expressed and the NO produced in low quantities at the level of the retina is involved in neurotransmission and in the regulation of retinal arteriolar tonicity. iNOS that is found in Muller cells and in RPE it’s not normally expressed and NO produced by it in large quantities is considered to generate inflammation of the retina and even retinal degeneration, that explains its implication in pathogenesis of hypertensive and diabetic retinopathy. NO has many physiological roles in the retina, one of it as a messenger of light-dark adaptation. It is also related to excitatory amino acid and free radical neuronal injury that occurs in the retina after ischemia or to the cell death found in such disorders as glaucoma. Recent studies have shown the implication of NO, in the etiology of ischemia and induced damage in the retina that can be a result of many pathologies or systemic diseases as diabetes and hypertension. Still the involvement of NO in the retinal blood flow in response to hypoxia is still controversial. Patients with hypertension, hypercholesterolemia, diabetes etc. showed an inability of the endothelium to generate adequate amounts of bioactive NO and to produce NO-mediated vasodilation. Conclusions. Many studies performed on NOS in the retina, show us that the roles of different NOS isoforms may be much trickier than previously realized. NO acts as a regulator of different physiological processes. NO appears to have a neurodestructive or a neuroprotective action, or both in pathological conditions such as human neurodegenerative diseases. Future studies on the actions of NO and NOS in the retina will not only give us a better understanding of some processes, but may contribute to the development of pharmacological treatments for various neurodegenerative eye diseases.