The mission of this project is to bring new clarity to the importance of NMDA receptors in the retina, with a special emphasis on how their contribution to retinal function is controlled by the synthesis and release of endogenous D-serine. It is now apparent that D-serine, an amino acid whose functional visibility was off the radar screen just a few years ago, is synthesized by glial cells of the retina and released and regulated in such a way that astrocytes and Muller cells have some control over the excitability of retinal ganglion cells. It is well understood that the neurotransmitter glutamate cannot open the ion channel of NMDA receptors without the presence of D-serine which serves as a coagonist or perhaps a co-neurotransmitter. One of the most pressing problems in retina and brain research is to understand the role of D-serine and NMDA receptors for three important reasons. First the NMDA receptors are unique among glutamate receptors because of their relationship to excitotoxicity, created by their permeability to calcium and induced by excessive glutamate stimulation. This property of NMDA receptors can be additive to the ganglion cell risk factors found in conditions such as glaucoma, ischemia, injury or diabetic retinopathy. A second health-related matter of importance comes from the modern treatment of schizophrenia, where thousands of patients are being treated with high doses of oral D-serine to activate NMDA receptors which are thought to be hypoactive and possibly the cause of the disease. Since D-serine crosses the blood-brain barrier to achieve its effects, we can safely assume that it also crosses the blood-retinal barrier and, based on our own work, the addition of exogenous D-serine into the retina can enhance NMDA receptor activity, particularly since no high affinity removal system for D-serine is present in the retina, as it is in the brain;this could render cells in the retina more at risk than those in the brain. Yet a third important factor in D-serine study relates to the mounting evidence that D-serine is critical for development in the retina and brain and our more recent studies suggest that abnormally elevated D-serine levels can contribute. The studies proposed in this application will provide new information about how the intrinsic control mechanisms of D-serine synthesis, storage and release serve to regulate the excitability of retinal ganglion cells through modulation of NMDA receptor availability. We propose to study three different strains of mice, each of which has a genetic defect of the regulatory mechanisms of NMDA receptor coagonist function, including a deficiency in glycine transport, a deficiency in the degradation of D-serine (DAAO) and the absence of the synthetic machinery for D-serine synthesis (serine racemase). Our studies will include electrophysiological methods, studies of the mechanisms of D-serine release through chemical detection and immunohistochemical and EM methods to evaluate whether the mechanisms related to D-serine synthesis and NMDA receptor distribution are changed due to an interruption in the normal coagonist functions of this novel amino acid.
Our knowledge of the role that NMDA receptors play in retinal function and their regulation by D-serine has critical implications for issues of public health. In the retina, D-serine and its actions on NMDA receptors make an important contribution to the light sensitivity of ganglion cells but may also be involved in exacerbating degenerative phenomena of ganglion cells, such as those associated with glaucoma, ischemia, injury or disease states like diabetic retinopathy. In addition, understanding NMDA receptors and D-serine regulation has implications for patients with schizophrenia, some of whom take high doses of D-serine to enhance NMDA receptor function related to their disease;these patients may also be at higher than normal risk for retinal damage through excessive activation of retinal NMDA receptors by therapeutic doses of D-serine which readily cross the blood-retinal barrier and have access to the retina, which lacks a high affinity transport system to remove D-serine, something that is present in the brain.
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