Electrogenic uptake of glutamate and aspartate into glial cells isolated from the salamander (Ambystoma) retina.

Abstract

SUMMARY1. The effects of excitatory amino acids on the membrane current of isolated retinal glial cells (Muller cells) were investigated using whole-cell patch clamping.2. L-Glutamate evoked an inward current at membrane potentials between -140 and +50 mV. The current was larger at more negative potentials.3. The glutamate-evoked current was activated by external cations with relative efficacies: Na+ > Li+> K+ > Cs+, choline. It was activated by internal cations with relative efficacies K+ > Rb+ > Cs+ > choline. Chloride and divalent cations did not affect the glutamate-evoked current.4. Raising the intracellular sodium or glutamate concentrations, or raising the extracellular potassium concentration, reduced the current evoked by external glutamate. The suppressive effect of internal glutamate was larger when the internal sodium concentration was high.5. Some analogues of glutamate also evoked an inward current. Responses to Laspartate resembled those to glutamate, but for aspartate the apparent affinity was higher and the voltage dependence of the current was steeper. In the physiological potential range the current evoked by a saturating dose of aspartate was less than that evoked by a saturating dose of glutamate.6. The uptake blocker threo-3-hydroxy-DL-aspartate (30,tM) reduced the glutamate-evoked current, but also generated a current itself. Dihydrokainate (510 /LM) weakly inhibited the glutamate-evoked current without generating a current itself.7. The commonly used blockers of glutamate-gated ion channels, 2-amino-5-phosphonovalerate (APV; 100 ,M), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 /bM), and kynurenate (1 mM) had no effect on the glutamate-evoked current. 8. The voltage dependence, cation dependence and pharmacological profile of the current evoked by excitatory amino acids indicate that it is caused by activation of the high-affinity glutamate uptake carrier. This carrier appears to transport one glutamate anion into the cell, one K+ ion out of the cell, and two or more Na+ ions into the cell, on each carrier cycle. At the inner membrane surface some or all of the transported Na+ dissociates from the carrier after the transported glutamate has dissociated.MS 8441

Effects of inhibiting glutamine synthetase and blocking glutamate uptake on b-wave generation in the isolated rat retina

“…The inclusion of 0.25 mM glutamate (or glutamine) in the media fulfilled this need. Since the amplitude of the b-wave was maintained in the rat retinas incubated in media containing 0.25 mM glutamate, we believe that the level of glutamate in the synaptic regions in the outer plexiform layer is at a physiological concentration, perhaps in the range of 1-10 μM (Brew & Attwell, 1987;Barbour et al, 1991), though its concentration at this site has not been measured in this tissue. An unphysiologically high level of glutamate in the synaptic zones between photoreceptor cells and ON-bipolars would be expected to saturate the glutamate receptors on the bipolar cells in darkness and in light, thereby resulting in a decline in the amplitude of the b-wave, as is observed when the bath concentration of glutamate is raised to 2-5 mM (data not shown).…”

“…Given the high release in the dark, for continued normal function of excitatory glutamatergic synapses in the retina, it is important that the extracellular concentration of glutamate be controlled so that the receptors are not saturated. This is usually accomplished by uptake of the released glutamate into cells within the localized region bordering the synaptic region, rather than by extracellular enzymatic degradation (Hertz, 1979;Brew & Attwell, 1987;Barbour et al, 1991;Kanai et al, 1993;Eliasoff & Werblin, 1993;Rauen & Kanner, 1994;Derouiche & Rauen, 1995;Sontheimer, 1995;Yang & Wu, 1997). The cells which could participate in the uptake of synaptically released glutamate include the photoreceptor cells themselves, the horizontal cells, the bipolar cells, and the glial (Müller) cells.…”

Segregated Expression of AMPA-Type Glutamate Receptors and Glutamate Transporters Defines Distinct Astrocyte Populations in the Mouse Hippocampus

“…3C). This probably indicated intracellular accumulation of Na ϩ or glutamate, or both, and a breakdown of the respective concentration gradients over the plasma membrane (Barbour et al, 1991). In another subgroup of green fluorescent cells, morphologically resembling GluR cells, glutamate evoked neither GluR responses nor uptake currents ("silent cells"; 16% of the weakly fluorescent cells with few processes).…”

A Review of Glutamate Receptors I: Current Understanding of Their Biology

“…GluTPs are pharmacologically distinct from both iGluRs and mGluRs and L-Glu, L-Asp, and D-Asp are s u b s t r a t e s f o r t h e t r a n s p o r t e r s 1 2 9 , 1 3 0 , 1 7 5 ; G l u R agonists 129,130,175,176 and antagonists 130,177 are not. Glu uptake can be blocked by the transporter blockers dihydrokainate (DHKA) and DL-threo-β-hydroxyaspartate (HA) 130,177 .…”

“…Glu uptake can be blocked by the transporter blockers dihydrokainate (DHKA) and DL-threo-β-hydroxyaspartate (HA) 130,177 .…”