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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

Time-course of Ba2+ effect on transporter current and K+ current. (A) Bath-applied 200 μ M BaCl2 produces downward shift in Ihold (black triangles), and increase in transporter current (TC, empty circles) in a single cell. (B) Summary graph of mean normalized transporter current (TC, n = 5) in the presence of Ba2+ and after 5 min of washout (wash). (C) Summary graph of normalized K+ current (n = 5) in the presence of Ba2+ and after 10 min of washout (wash) (D) Summary graph of mean Δ Ihold (n = 5) in the presence of Ba2+ and after 10 min of washout (wash). Error bars—SEM; **P < 0.01; N.S.—non significant; One-Way ANOVA post-hoc Tukey test.
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Figure 2: Time-course of Ba2+ effect on transporter current and K+ current. (A) Bath-applied 200 μ M BaCl2 produces downward shift in Ihold (black triangles), and increase in transporter current (TC, empty circles) in a single cell. (B) Summary graph of mean normalized transporter current (TC, n = 5) in the presence of Ba2+ and after 5 min of washout (wash). (C) Summary graph of normalized K+ current (n = 5) in the presence of Ba2+ and after 10 min of washout (wash) (D) Summary graph of mean Δ Ihold (n = 5) in the presence of Ba2+ and after 10 min of washout (wash). Error bars—SEM; **P < 0.01; N.S.—non significant; One-Way ANOVA post-hoc Tukey test.

Mentions: Electrical stimulation of the Schaffer collaterals elicited a biphasic inward current with fast and slow components in astrocytes voltage-clamped at −80 mV. Glutamate transporters mediated the fast current component, because the current was fully blocked by a mixture of transporter blockers comprising 100 μM TBOA, 50 μM dihydrokainate, and 100 μM threo-hydroxy-aspartate (Figure 1C). We assumed that the slow current component was mediated by Kir (Kofuji and Newman, 2004) and would be sensitive to Ba2+. Therefore, to isolate the glutamate transporter current, Ba2+ was added to the bath at the same concentration (200 μM) as previously described (D'Ambrosio et al., 2002; De Saint Jan and Westbrook, 2005). Along with the expected K+ current blockade, Ba2+ strongly increased the amplitude of the transporter current (thereafter also referred to as “potentiation of the transporter current”; Figure 1D). A similar increase in transporter current was previously reported in response to a cocktail of K+ channel blockers (including Ba2+) and attributed to decrease in membrane conductance (Ge and Duan, 2007). Indeed, Kir blockade increased input resistance (Ri, 24 ± 2 MΩ in control and 31 ± 3 MΩ in Ba2+, n = 16, P = 0.002 for difference, paired t-test), potentiation of the transporter current could be explained by increase in the membrane length constant, which is proportional to the square root of Ri. In this case, a higher membrane time constant would lead to a larger contribution of transporter currents originating in distant astrocytic processes into the current recorded in the soma, and the time-courses of the effects of Ba2+ on transporter current and on membrane conductance would correlate. We found, however, that augmentation of transporter current started earlier than the change in K+ current and in holding current at the beginning of Ba2+ application (Figure 2). Moreover, the transporter current augmentation persisted longer upon Ba2+ washout (transporter current in BaCl2: 400 ± 99% of control, n = 5; 5 min washout: 411 ± 86% of control, n = 5, One-Way ANOVA, F(2, 12) = 5.42, P = 0.021; post-hoc Tukey test, P = 0.587 for difference between BaCl2 and washout; Figure 2B) than did suppression of the K+ current (K+ current in BaCl2: 44 ± 10% of control, n = 5; 5 min washout: 175 ± 40% of control, n = 5, One-Way ANOVA, F(2, 12) = 10.71, P = 0.002; post-hoc Tukey test, P = 0.002 for difference between BaCl2 and washout; Figure 2C) and holding current (Δ Ihold in BaCl2: 347 ± 61 pA, n = 5; and 84 ± 51 pA at 5 min of washout, n = 5, One-Way ANOVA, F(2, 12) = 23.83, P < 0.001; post-hoc Tukey test, P < 0.001 for difference between BaCl2 and washout; Figure 2D), suggesting that the change in the membrane conductance did not contribute to the effect of Ba2+ on the transporter current.


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)

Time-course of Ba2+ effect on transporter current and K+ current. (A) Bath-applied 200 μ M BaCl2 produces downward shift in Ihold (black triangles), and increase in transporter current (TC, empty circles) in a single cell. (B) Summary graph of mean normalized transporter current (TC, n = 5) in the presence of Ba2+ and after 5 min of washout (wash). (C) Summary graph of normalized K+ current (n = 5) in the presence of Ba2+ and after 10 min of washout (wash) (D) Summary graph of mean Δ Ihold (n = 5) in the presence of Ba2+ and after 10 min of washout (wash). Error bars—SEM; **P < 0.01; N.S.—non significant; One-Way ANOVA post-hoc Tukey test.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 2: Time-course of Ba2+ effect on transporter current and K+ current. (A) Bath-applied 200 μ M BaCl2 produces downward shift in Ihold (black triangles), and increase in transporter current (TC, empty circles) in a single cell. (B) Summary graph of mean normalized transporter current (TC, n = 5) in the presence of Ba2+ and after 5 min of washout (wash). (C) Summary graph of normalized K+ current (n = 5) in the presence of Ba2+ and after 10 min of washout (wash) (D) Summary graph of mean Δ Ihold (n = 5) in the presence of Ba2+ and after 10 min of washout (wash). Error bars—SEM; **P < 0.01; N.S.—non significant; One-Way ANOVA post-hoc Tukey test.
Mentions: Electrical stimulation of the Schaffer collaterals elicited a biphasic inward current with fast and slow components in astrocytes voltage-clamped at −80 mV. Glutamate transporters mediated the fast current component, because the current was fully blocked by a mixture of transporter blockers comprising 100 μM TBOA, 50 μM dihydrokainate, and 100 μM threo-hydroxy-aspartate (Figure 1C). We assumed that the slow current component was mediated by Kir (Kofuji and Newman, 2004) and would be sensitive to Ba2+. Therefore, to isolate the glutamate transporter current, Ba2+ was added to the bath at the same concentration (200 μM) as previously described (D'Ambrosio et al., 2002; De Saint Jan and Westbrook, 2005). Along with the expected K+ current blockade, Ba2+ strongly increased the amplitude of the transporter current (thereafter also referred to as “potentiation of the transporter current”; Figure 1D). A similar increase in transporter current was previously reported in response to a cocktail of K+ channel blockers (including Ba2+) and attributed to decrease in membrane conductance (Ge and Duan, 2007). Indeed, Kir blockade increased input resistance (Ri, 24 ± 2 MΩ in control and 31 ± 3 MΩ in Ba2+, n = 16, P = 0.002 for difference, paired t-test), potentiation of the transporter current could be explained by increase in the membrane length constant, which is proportional to the square root of Ri. In this case, a higher membrane time constant would lead to a larger contribution of transporter currents originating in distant astrocytic processes into the current recorded in the soma, and the time-courses of the effects of Ba2+ on transporter current and on membrane conductance would correlate. We found, however, that augmentation of transporter current started earlier than the change in K+ current and in holding current at the beginning of Ba2+ application (Figure 2). Moreover, the transporter current augmentation persisted longer upon Ba2+ washout (transporter current in BaCl2: 400 ± 99% of control, n = 5; 5 min washout: 411 ± 86% of control, n = 5, One-Way ANOVA, F(2, 12) = 5.42, P = 0.021; post-hoc Tukey test, P = 0.587 for difference between BaCl2 and washout; Figure 2B) than did suppression of the K+ current (K+ current in BaCl2: 44 ± 10% of control, n = 5; 5 min washout: 175 ± 40% of control, n = 5, One-Way ANOVA, F(2, 12) = 10.71, P = 0.002; post-hoc Tukey test, P = 0.002 for difference between BaCl2 and washout; Figure 2C) and holding current (Δ Ihold in BaCl2: 347 ± 61 pA, n = 5; and 84 ± 51 pA at 5 min of washout, n = 5, One-Way ANOVA, F(2, 12) = 23.83, P < 0.001; post-hoc Tukey test, P < 0.001 for difference between BaCl2 and washout; Figure 2D), suggesting that the change in the membrane conductance did not contribute to the effect of Ba2+ on the transporter current.

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