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RP Malchow, AJ A Molina, K Hammar, PJ S Smith; Proton Flux From Retinal Horizontal Cells is Modulated by Glutamate . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3777.
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Purpose: To examine the modulation of proton fluxes from isolated horizontal cells of the skate by glutamate, and to test the hypothesis that hydrogen ions might act as the inhibitory neurotransmitter used by horizontal cells to set up the surround portion of the receptive fields of visual neurons. Methods: Self-referencing pH-selective microelectrodes were used to measure proton fluxes from enzymatically isolated external horizontal cells obtained from the retina of the skate (Raja erinacea/R. ocellata). Results: We find a continuous outward flux of hydrogen ions from isolated skate horizontal cells. Glutamate, the presumed neurotransmitter released by vertebrate photoreceptors, induced a transient depression in outward proton flux by an average of 60%. The effect of 100 µM glutamate on proton flux was abolished by 100 µM CNQX, suggesting the involvement of ionotropic glutamate receptors in this modulation. 20 µM kainate also mimicked the effect of glutamate, and its effects were also eliminated by 100 µM CNQX. Control experiments demonstrated that glutamate, CNQX and kainate by themselves did not alter the responsiveness of the self-referencing pH-selective microelectrodes. Moreover, the glutamate induced modulation of proton flux was abolished when the cells were bathed in a solution containing nominally 0 mM calcium. Our data suggest that calcium flux through the ionotropic glutamate channels is crucial in the modulation of proton flux from horizontal cells. Conclusion: Glutamate decreases proton flux from horizontal cells via activation of ionotropic glutamate receptors, and this alteration in proton flux appears to be calcium dependent. We hypothesize that the effects of glutamate on proton flux results from the activation of a calcium pump which takes in two hydrogen ions from the extracellular fluid in exchange for each calcium ion extruded from the cell. While these findings suggest that proton fluxes may indeed play a role in modulation of retinal signals within the outer retina, our data argue against the hypothesis that protons are the agents used by horizontal cells to establish the surround portion of the receptive fields of visual neurons.
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