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Ivan Anastassov, Richard Chappell; Modulation of Glutamate Release by Endogenous Zinc at Photoreceptor Terminals. Invest. Ophthalmol. Vis. Sci. 2012;53(14):768.
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To understand the modulatory action of zinc on vesicular release of glutamate at photoreceptor terminals.
Eyes from dark-adapted skates (Raja erinacea) were enucleated under red light, the cornea and lens removed, and vitreous drained. The resulting "eyecup" preparation, placed on a reference electrode, was superfused with oxygenated Ringer. An extracellular agar electrode positioned in the vitreous recorded ERG responses to illumination. A sharp electrode (R~60-90MOhms) was advanced remotely in steps of 0.2 to 2 microns until a horizontal cell was impaled. Endogenous zinc released from photoreceptor terminals was removed by perfusion of a membrane-impermeable zinc chelator (histidine 20-200uM) and horizontal cell responses to varying light intensities monitored concomitant with ERG responses. In related studies, zinc was continuously chelated (24-48hrs) by intraocular injections of histidine, TPEN and other chelating agents into the dark-adapted eye of the living skate. The collected tissue was examined for signs of glutamate excitotoxicity.
Application of histidine (20-200uM) to the "eye-cup" preparation resulted in an increased light response of horizontal cells across all light intensities tested. A concomitant increase in the ERG b-wave was also generally observed. Chelation of zinc via intraocular injections of histidine, TPEN, or EDTA showed histological damage to inner retinal layers similar to that caused by excitotoxic agents like kainate. Further examination of the tissue revealed that cells in the inner layers undergo some apoptosis and, to a large extent, necrosis as a result of zinc chelation in vivo.
Photoreceptor release of ionic zinc with glutamate and zinc’s action on presynaptic calcium channels has been shown. Here we show that chelation of endogenous zinc results in an increase of glutamate release, as evidenced by an increase in responses of the postsynaptic horizontal cells. In vivo zinc chelation likely removes inhibitory actions of zinc on photoreceptor glutamate release, leading to downstream excitotoxic damage to the inner retina. These observations suggest an important modulatory and neuroprotective role of ionic zinc at the photoreceptor terminal
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