Abstract Gaal, Lubor, Botond Roska, Serge A. Picaud, Samuel M. Wu, Robert Marc, and Frank S. Werblin. Postsynaptic response kinetics are controlled by a glutamate transporter at cone photoreceptors. J. Neurophysiol. 79: 190–196, 1998. We evaluated the role of the sodium/glutamate transporter at the synaptic terminals of cone photoreceptors in controlling postsynaptic response kinetics. The strategy was to measure the changes in horizontal cell response rate induced by blocking transporter uptake in cones with dihydrokainate (DHK). DHK was chosen as the uptake blocker because, as we show through autoradiographic uptake measurements, DHK specifically blocked uptake in cones without affecting uptake in Mueller cells. Horizontal cells depolarized from about −70 to −20 mV as the exogenous glutamate concentration was increased from ∼1 to 40 μM, so horizontal cells can serve as “glutamate electrodes” during the light response. DHK slowed the rate of hyperpolarization of the horizontal cells in a dose-dependent way, but didn't affect the kinetics of the cone responses. At 300 μM DHK, the rate of the horizontal cell hyperpolarization was slowed to only 17 ± 8.5% (mean ± SD) of control. Translating this to changes in glutamate concentration using the slice dose response curve as calibration in Fig. 2 , DHK reduced the rate of removal of glutamate from ∼0.12 to 0.031 μM/s. The voltage dependence of uptake rate in the transporter alone was capable of modulating glutamate concentration: we blocked vesicular released glutamate with bathed 20 mM Mg 2+ and then added 30 μM glutamate to the bath to reestablish a physiological glutamate concentration level at the synapse and thereby depolarize the horizontal cells. Under these conditions, a light flash elicited a 17-mV hyperpolarization in the horizontal cells. When we substituted kainate, which is not transported, for glutamate, horizontal cells were depolarized but light did not elicit any response, indicating that the transporter alone was responsible for the removal of glutamate under these conditions. This suggests that the transporter was both voltage dependent and robust enough to modulate glutamate concentration. The transporter must be at least as effective as diffusion in removing glutamate from the synapse because there is only a very small light response once the transporter is blocked. The transporter, via its voltage dependence on cone membrane potential, appears to contribute significantly to the control of postsynaptic response kinetics. View larger version: In this window In a new window Download as PowerPoint Slide Fig. 2. Horizontal cell calibration curve. A : isolated horizontal cells: Glutamate dose-response curve for isolated horizontal cells showed an EC 50 of 18 μM. This curve represents the dose response curve of glutamate receptors in horizontal cells assuming that there is no difference in sensitivity between intra- and extrasynaptic receptors (if they exist) on horizontal cells. Hill coefficient for the isolated cell was 2.2. Slice: dose-response curve for horizontal cells in the slice with 20 mM Mg 2+ and 500 μM DHK to block vesicular release and transporter uptake had EC 50 = 32 μM. This curve identifies the voltage response range but the true dose response curve may be shifted to the left because there exists an ambient glutamate concentration that was not blocked by Mg 2+ and there exists an ambient uptake that was not blocked by DHK. Addition of Mg 2+ also might affect the position of the curve. Hill coefficient was 1.91. Data for both curves was fit with a logistic function. B : direct effect of agents on an isolated horizontal cell. DHK (1 mM) and N -methyl- d -aspartate (NMDA; 20 μM, with 5 μM glycine and 1 μM strychnine) had no effect on isolated cells. Kainate (20 μM) evoked a current similar to 100 μM glutamate, suggesting that horizontal cells have kainate receptors. Traces represent evoked currents measured at each voltage step after the subtraction of the control recordings. Footnotes Address for reprint requests: F. S. Werblin, Dept. of Molecular and Cell Biology, Division of Neurobiology, University of California at Berkeley, Berkeley, CA 94720.
Journal of Neurophysiology – The American Physiological Society
Published: Jan 1, 1998
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