POSTSYNAPTIC RESPONSE KINETICS ARE CONTROLLED BY A GLUTAMATE TRANSPORTER AT CONE PHOTORECEPTORS

We evaluated the role of the sodium/glutamate transporter at the synap tic terminals of cone photoreceptors in controlling postsynaptic respo nse kinetics. The strategy was to measure the changes in horizontal ce ll 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 specificall y blocked uptake in cones without affecting uptake in Mueller cells. H orizontal cells depolarized from about -70 to -20 mV as the exogenous glutamate concentration was increased from similar to 1 to 40 mu M, so horizontal cells can serve as ''glutamate electrodes'' during the lig ht response. DHK slowed the rate of hyperpolarization of the horizonta l cells in a dose-dependent way, but didn't affect the kinetics of the cone responses. At 300 mu M DHK, the rate of the horizontal cell hype rpolarization was slowed to only 17 +/- 8.5% (mean +/- SD) of control. Translating this to changes in glutamate concentration using the slic e dose response curve as calibration in Fig. 2, DHK reduced the rate o f removal of glutamate from similar to 0.12 to 0.031 mu M/s. The volta ge dependence of uptake rate in the transporter alone was capable of m odulating glutamate concentration: we blocked vesicular released gluta mate with bathed 20 mM Mg2+ and then added 30 mu M glutamate to the ba th to reestablish a physiological glutamate concentration level at the synapse and thereby depolarize the horizontal cells. Under these cond itions, a light flash elicited a 17-mV hyperpolarization in the horizo ntal cells. When we substituted kainata, which is not transported, for glutamate, horizontal cells were depolarized but Light did not elicit any response, indicating that the transporter alone was responsible f or the removal of glutamate under these conditions. This suggests that the transporter was both voltage dependent and robust enough to modul ate glutamate concentration. The transporter must be at least as effec tive 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 r esponse kinetics.