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I. Fahrenfort, T. Sjoerdsma, M. Kamermans; Effects of Fast Extracellular Proton Buffering on Feedback Responses in HCs of the Goldfish . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1066.
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Purpose: In goldfish, negative feedback from HCs to cones is mediated through an ephaptic mechanism, involving both hemichannels and the glutamate-gated channels present at the tips of the HCs (Kamermans et al, 2001). It was postulated that the current flowing through these channels induced a voltage drop in the synaptic cleft thereby shifting the calcium current of the cones to more negative potentials. However, both glutamate-gated channels and hemichannels are presumably permeable to protons. If so, hyperpolarization of HCs would lead to an alkalanization of the synaptic cleft, thereby shifting the calcium current activation function to more negative potentials (Barnes et al, 1993; DeVries, 2001). Since we use a bicarbonate based Ringer's solution, these fast changes in proton concentration will not be buffered. To rule out the possibility that protons mediate negative feedback, we studied the effect of carbonic anhydrase and HEPES on the feedback mediated responses in both HC's and cones. Both substances lead to a medium with fast pH buffering capacity. Methods: Patch-clamp recordings of cones and voltage recordings of horizontal cells in isolated goldfish retina. Results: Speeding up the pH buffering of the bicarbonate Ringer's solution by including carbonic anhydrase did not affect the feedback induced responses in either cones or HCs, suggesting that protons do not mediate feedback. However, 2 mM HEPES strongly reduced the feedback-induced responses in HCs and induced a hyperpolarization of the HCs. Also the feedback-induced increase in calcium current in cones was strongly reduced, accompanied by a shift of the calcium current to more negative potentials. Conclusions: It is unlikely that negative feedback from HCs to cones is mediated by fast changes in proton concentration in the cone pedicle. However, HEPES did reduced the feedback-mediated responses. This could either be due to a direct effect on the hemichannels or an effect on the membrane potential of HCs. Our data show that the HEPES induced reduction of the feedback-mediated responses is not due to a block of the hemichannels but due to hyperpolarization of HCs and a reduction of the HCs light responses.
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