Postsynaptic response kinetics are controlled by a glutamate transporter at cone photoreceptors.

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 approximately 1 to 40 microM, 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 microM 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 approximately 0.12 to 0.031 microM/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 Mg2+ and then added 30 microM 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.

Noise analysis of the glutamate-activated current in photoreceptors.

The glutamate-activated current in photoreceptors has been attributed both to a sodium/glutamate transporter and to a glutamate-activated chloride channel. We have further studied the glutamate-activated current in single, isolated photoreceptors from the tiger salamander using noise analysis on whole-cell patch-clamp recordings. In cones, the current is generated by chloride channels with a single-channel conductance of 0.7 pS and an open lifetime of 2.4 ms. The number of channels per cell is in the range of 10,000-20,000. Activation of the channels requires the presence of both glutamate and sodium. The single-channel conductance and the open lifetime of the channel are independent of the external concentration of glutamate and sodium. External glutamate and sodium affect only the opening rate of the channels. D,L-Threo-3-hydroxyaspartate (THA), a glutamate-transport blocker, is shown to be a partial agonist for the channel. The single-channel conductance is the same regardless of whether glutamate or THA is the ligand, but the open lifetime of the channel is only 0.8 ms with THA as ligand. The glutamate-activated current in rods has a similar single-channel conductance (0.74 pS) and open lifetime (3 ms). We propose a kinetic model, consistent with these results, to explain how a transporter can simultaneously act both as a sodium/glutamate-gated chloride channel and a glutamate/sodium cotransporter.

Glutamate-gated chloride channel with glutamate-transporter-like properties in cone photoreceptors of the tiger salamander.

1. Using the patch-clamp technique, we investigated whether the glutamate-elicited current in mechanically isolated cone photoreceptors from the salamander retina is generated by a Cl- channel or a glutamate transporter. 2. The current reversed near the equilibrium potential for Cl-, was decreased by three Cl- channel blockers, 5-nitro-2-(3-phenyl-propylamino) benzoic acid, 4,4'-diisothiocyanostilbene-2,2'-disulfonate, and diphenylamine 2,2'-dicarboxylic acid, and was eliminated when gluconate was substituted for both internal and external Cl-, features consistent with the current being mediated by a Cl- channel. 3. The single-channel conductance of the Cl- channel was estimated by noise analysis of the glutamate-elicited current fluctuations to be 0.7 pS with an open time of 2 ms. 4. The magnitude of the current was dependent on both internal and external Na+ and K+, features consistent with the current being related to the activation of a glutamate transporter. Yet changes in their concentrations did not affect the reversal potential of the current. 5. Taken together with earlier reports on this current showing that it has a glutamate-transporter-like pharmacology, our results suggest that the glutamate-elicited current is carried by a Cl- channel but gated by a glutamate receptor whose pharmacology and ionic requirement resemble those previously described for glutamate transporters.

GABA transporters and GABAC-like receptors on catfish cone- but not rod-driven horizontal cells.

The effects of GABA and related agents were studied in solitary rod- and cone-driven horizontal cells, acutely isolated from the catfish retina using enzymatic and mechanical treatment. Both types of horizontal cells, which normally receive glutamatergic input from photoreceptors, responded to pressure ejection of the glutamate analog kainate (50 microM) with an inward current of 300-800 pA when voltage-clamped at -70 mV using the whole-cell patch-clamp technique. But pressure ejection of GABA (500 microM) elicited an inward current only in cone-driven horizontal cells. This current, ranging between 100 and 400 pA, consisted of two components: (1) a GABA receptor-gated chloride current that reversed near the chloride equilibrium potential and was blocked by bath application of picrotoxin (100-500 microM), and (2) a GABA transporter-mediated current that was picrotoxin resistant but was blocked by NO-711 (1 microM) and cis-4-hydroxynipecotic acid (250 microM), two potent GABA transporter blockers. The GABA transporter current could also be eliminated when sodium was replaced by either choline or lithium in the bathing medium. The picrotoxin-sensitive receptor-gated current could not be elicited by the GABAB receptor agonist baclofen, nor could it be blocked by the potent GABAB receptor antagonist 2-hydroxysaclofen. The picrotoxin-sensitive current could be divided into two components based on their sensitivity to the specific GABAA receptor antagonist bicuculline methiodide. The bicuculline-sensitive component was found only in some cells, whereas the bicuculline-resistant, picrotoxin-sensitive component was found in all cells tested. The bicuculline-resistant current was insensitive to pentobarbital, an allosteric modulator of GABAA receptor. To confirm the effectiveness of the specific batch of bicuculline methiodide and pentobarbital, we tested both drugs in ganglion cells in the salamander retinal slice preparation, where the GABA-elicited current is almost exclusively mediated by GABAA receptors. Bicuculline methiodide almost completely blocked, while pentobarbital significantly enhanced, the GABA current recorded in ganglion cells. Thus, in catfish cone horizontal cells the bicuculline-resistant GABA receptor current is most likely mediated by the GABAC receptor based on the above pharmacological profile. The relative effectiveness of GABA, muscimol, trans- and cis-4-aminocrotonic acid (TACA and CACA) was determined at this GABAC receptor site after cells were bathed in choline Ringer to eliminate the transporter current and in the presence of 100 microM bicuculline methiodide to block GABAA receptor current.(ABSTRACT TRUNCATED AT 400 WORDS)