Limits...
Low micromolar Ba(2+) potentiates glutamate transporter current in hippocampal astrocytes.

Afzalov R, Pryazhnikov E, Shih PY, Kondratskaya E, Zobova S, Leino S, Salminen O, Khiroug L, Semyanov A - Front Cell Neurosci (2013)

Bottom Line: A Ba(2+)-sensitive K(+) current mediated by inward rectifying potassium channels (Kir) accompanied the transporter current.Surprisingly, Ba(2+) not only suppressed the K(+) current and changed holding current (presumably, mediated by Kir) but also increased the transporter current at lower concentrations.However, Ba(2+) did not significantly increase the uptake of aspartate in cultured astrocytes, suggesting that increase in the amplitude of the transporter current does not always reflect changes in glutamate uptake.

View Article: PubMed Central - PubMed

Affiliation: Neuroscience Center, University of Helsinki Helsinki, Finland ; RIKEN Brain Science Institute Wako-shi, Japan.

ABSTRACT
Glutamate uptake, mediated by electrogenic glutamate transporters largely localized in astrocytes, is responsible for the clearance of glutamate released during excitatory synaptic transmission. Glutamate uptake also determines the availability of glutamate for extrasynaptic glutamate receptors. The efficiency of glutamate uptake is commonly estimated from the amplitude of transporter current recorded in astrocytes. We recorded currents in voltage-clamped hippocampal CA1 stratum radiatum astrocytes in rat hippocampal slices induced by electrical stimulation of the Schaffer collaterals. A Ba(2+)-sensitive K(+) current mediated by inward rectifying potassium channels (Kir) accompanied the transporter current. Surprisingly, Ba(2+) not only suppressed the K(+) current and changed holding current (presumably, mediated by Kir) but also increased the transporter current at lower concentrations. However, Ba(2+) did not significantly increase the uptake of aspartate in cultured astrocytes, suggesting that increase in the amplitude of the transporter current does not always reflect changes in glutamate uptake.

No MeSH data available.


Related in: MedlinePlus

Potentiation of transporter current is independent of Kir blockade. (A) Dose-response curve of TC (circles) and Δ Ihold (triangles) changes. (B) Transporter current (TC) elicited by local glutamate uncaging in control and in 200 μM Ba2+. The downward shift in the holding current was less pronounced in Cs+-loaded cells (CsCl). The TC amplitude increased to a similar degree in both cases. (C,D) Mean Ba2+ effect on transporter current (TC, B) and Ihold (Δ Ihold, C) for CsCl- and KCl-based intracellular solutions. (E) Transporter current centroids in control and in 20 μM Ba2+. Error bars—SEM. *P < 0.05; N.S.—non significant; paired (E) and unpaired (C,D) t-test.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3755269&req=5

Figure 3: Potentiation of transporter current is independent of Kir blockade. (A) Dose-response curve of TC (circles) and Δ Ihold (triangles) changes. (B) Transporter current (TC) elicited by local glutamate uncaging in control and in 200 μM Ba2+. The downward shift in the holding current was less pronounced in Cs+-loaded cells (CsCl). The TC amplitude increased to a similar degree in both cases. (C,D) Mean Ba2+ effect on transporter current (TC, B) and Ihold (Δ Ihold, C) for CsCl- and KCl-based intracellular solutions. (E) Transporter current centroids in control and in 20 μM Ba2+. Error bars—SEM. *P < 0.05; N.S.—non significant; paired (E) and unpaired (C,D) t-test.

Mentions: Because the Ba2+ concentration gradually increases in the tissue after adding the drug to the perfusion system, the faster effect on transporter current suggests its higher sensitivity to Ba2+ than K+ current. One possibility is that low Ba2+ concentrations increased presynaptic glutamate release, leading to a larger transporter current. To test this we replaced synaptic stimulation with local glutamate uncaging. Because no action potential or synaptic activation was involved, a K+ current did not accompany the transporter current in this case. We plotted dose-response curves for the effect of Ba2+ on the transporter current and Δ Ihold (Figure 3A). Consistent with the synaptic stimulation results, the half-maximal effective concentration (EC50) for the transporter current was 13 μM, while EC50 for Δ Ihold was 183 μM. Strikingly, low micromolar Ba2+ concentrations (1-50 μM) effectively increased the transporter current, but had no detectable effect on the holding current. This finding suggests that the Ba2+-induced potentiation of transporter current was not mediated by its effect on presynaptic glutamate release.


Low micromolar Ba(2+) potentiates glutamate transporter current in hippocampal astrocytes.

Afzalov R, Pryazhnikov E, Shih PY, Kondratskaya E, Zobova S, Leino S, Salminen O, Khiroug L, Semyanov A - Front Cell Neurosci (2013)

Potentiation of transporter current is independent of Kir blockade. (A) Dose-response curve of TC (circles) and Δ Ihold (triangles) changes. (B) Transporter current (TC) elicited by local glutamate uncaging in control and in 200 μM Ba2+. The downward shift in the holding current was less pronounced in Cs+-loaded cells (CsCl). The TC amplitude increased to a similar degree in both cases. (C,D) Mean Ba2+ effect on transporter current (TC, B) and Ihold (Δ Ihold, C) for CsCl- and KCl-based intracellular solutions. (E) Transporter current centroids in control and in 20 μM Ba2+. Error bars—SEM. *P < 0.05; N.S.—non significant; paired (E) and unpaired (C,D) t-test.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3755269&req=5

Figure 3: Potentiation of transporter current is independent of Kir blockade. (A) Dose-response curve of TC (circles) and Δ Ihold (triangles) changes. (B) Transporter current (TC) elicited by local glutamate uncaging in control and in 200 μM Ba2+. The downward shift in the holding current was less pronounced in Cs+-loaded cells (CsCl). The TC amplitude increased to a similar degree in both cases. (C,D) Mean Ba2+ effect on transporter current (TC, B) and Ihold (Δ Ihold, C) for CsCl- and KCl-based intracellular solutions. (E) Transporter current centroids in control and in 20 μM Ba2+. Error bars—SEM. *P < 0.05; N.S.—non significant; paired (E) and unpaired (C,D) t-test.
Mentions: Because the Ba2+ concentration gradually increases in the tissue after adding the drug to the perfusion system, the faster effect on transporter current suggests its higher sensitivity to Ba2+ than K+ current. One possibility is that low Ba2+ concentrations increased presynaptic glutamate release, leading to a larger transporter current. To test this we replaced synaptic stimulation with local glutamate uncaging. Because no action potential or synaptic activation was involved, a K+ current did not accompany the transporter current in this case. We plotted dose-response curves for the effect of Ba2+ on the transporter current and Δ Ihold (Figure 3A). Consistent with the synaptic stimulation results, the half-maximal effective concentration (EC50) for the transporter current was 13 μM, while EC50 for Δ Ihold was 183 μM. Strikingly, low micromolar Ba2+ concentrations (1-50 μM) effectively increased the transporter current, but had no detectable effect on the holding current. This finding suggests that the Ba2+-induced potentiation of transporter current was not mediated by its effect on presynaptic glutamate release.

Bottom Line: A Ba(2+)-sensitive K(+) current mediated by inward rectifying potassium channels (Kir) accompanied the transporter current.Surprisingly, Ba(2+) not only suppressed the K(+) current and changed holding current (presumably, mediated by Kir) but also increased the transporter current at lower concentrations.However, Ba(2+) did not significantly increase the uptake of aspartate in cultured astrocytes, suggesting that increase in the amplitude of the transporter current does not always reflect changes in glutamate uptake.

View Article: PubMed Central - PubMed

Affiliation: Neuroscience Center, University of Helsinki Helsinki, Finland ; RIKEN Brain Science Institute Wako-shi, Japan.

ABSTRACT
Glutamate uptake, mediated by electrogenic glutamate transporters largely localized in astrocytes, is responsible for the clearance of glutamate released during excitatory synaptic transmission. Glutamate uptake also determines the availability of glutamate for extrasynaptic glutamate receptors. The efficiency of glutamate uptake is commonly estimated from the amplitude of transporter current recorded in astrocytes. We recorded currents in voltage-clamped hippocampal CA1 stratum radiatum astrocytes in rat hippocampal slices induced by electrical stimulation of the Schaffer collaterals. A Ba(2+)-sensitive K(+) current mediated by inward rectifying potassium channels (Kir) accompanied the transporter current. Surprisingly, Ba(2+) not only suppressed the K(+) current and changed holding current (presumably, mediated by Kir) but also increased the transporter current at lower concentrations. However, Ba(2+) did not significantly increase the uptake of aspartate in cultured astrocytes, suggesting that increase in the amplitude of the transporter current does not always reflect changes in glutamate uptake.

No MeSH data available.


Related in: MedlinePlus