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Calcium-Activated Potassium Channels at Nodes of Ranvier Secure Axonal Spike Propagation.

Gründemann J, Clark BA - Cell Rep (2015)

Bottom Line: Functional connectivity between brain regions relies on long-range signaling by myelinated axons.This is secured by saltatory action potential propagation that depends fundamentally on sodium channel availability at nodes of Ranvier.We show that the recruitment of calcium-activated potassium channels (IK, K(Ca)3.1) by local, activity-dependent calcium (Ca(2+)) influx at nodes of Ranvier via a T-type voltage-gated Ca(2+) current provides a powerful mechanism that likely opposes depolarizing block at the nodes and is thus pivotal to securing continuous axonal spike propagation in spontaneously firing Purkinje cells.

View Article: PubMed Central - PubMed

Affiliation: Wolfson Institute for Biomedical Research, University College London, Gower Street, London WC1E 6BT, UK.

No MeSH data available.


Related in: MedlinePlus

Local, Activity-Dependent Ca2+ Influx at Nodes of Ranvier(A) Two-photon image of cerebellar Purkinje cell indicating line scan locations.(B) Ca2+ transients (top, line scans) at locations shown in (A) during current-evoked spike trains (bottom). AIS, axon initial segment. BP1 and BP2, first and second axonal branchpoint.(C) Frame-scan time series of BP1 during spike train. Green, axon morphology.(D) Spike train-evoked ΔF/F at ROIs shown in (C) (red boxes). Normalized integrated ΔF/F (ΔF/F∗s) against distance from an axonal branchpoint (bottom, n = 13 neurons).(E) Soma, AIS, and first BP ΔF/F in response to 500-ms somatic voltage steps (voltage clamp) in bath-applied TTX (0.5 μM).(F) Pooled data for max ΔF/F versus somatic command potential (soma, black; AIS, red; BP1, blue. n = 6 neurons).(G) Activity-dependent changes in ΔF/F upon somatic current injection. Baseline holding current: 0 pA.(H) Summary data for the change in ΔF/F∗s during silence and activity (n = 5 cells).(I) Ca2+ influx at the axon initial segment, first branchpoint and presynaptic boutons before and after bath application of 0 mM extracellular Ca2+ (n = 4), Agatoxin (AgaTX, n = 4, AIS = 3), and Mibefradil (Mibef, n = 9).Error bars, ±SEM.
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fig3: Local, Activity-Dependent Ca2+ Influx at Nodes of Ranvier(A) Two-photon image of cerebellar Purkinje cell indicating line scan locations.(B) Ca2+ transients (top, line scans) at locations shown in (A) during current-evoked spike trains (bottom). AIS, axon initial segment. BP1 and BP2, first and second axonal branchpoint.(C) Frame-scan time series of BP1 during spike train. Green, axon morphology.(D) Spike train-evoked ΔF/F at ROIs shown in (C) (red boxes). Normalized integrated ΔF/F (ΔF/F∗s) against distance from an axonal branchpoint (bottom, n = 13 neurons).(E) Soma, AIS, and first BP ΔF/F in response to 500-ms somatic voltage steps (voltage clamp) in bath-applied TTX (0.5 μM).(F) Pooled data for max ΔF/F versus somatic command potential (soma, black; AIS, red; BP1, blue. n = 6 neurons).(G) Activity-dependent changes in ΔF/F upon somatic current injection. Baseline holding current: 0 pA.(H) Summary data for the change in ΔF/F∗s during silence and activity (n = 5 cells).(I) Ca2+ influx at the axon initial segment, first branchpoint and presynaptic boutons before and after bath application of 0 mM extracellular Ca2+ (n = 4), Agatoxin (AgaTX, n = 4, AIS = 3), and Mibefradil (Mibef, n = 9).Error bars, ±SEM.

Mentions: Using two-photon Ca2+-imaging, we investigated whether the spatial distribution of calcium signals in PC axons might confer NoR specificity of IK recruitment. We detected prominent activity-dependent increases in intracellular Ca2+ concentration ([Ca2+]i) at NoRs (identified by their location at branchpoints) and at the AIS (see Bender and Trussell, 2009) during trains of evoked APs (Figures 3A–3C; 204 ± 21 Hz). Ca2+ signals were restricted to approximately 5 μm from the center of NoRs (Figures 3C and 3D), and there were no detectable changes in internodal [Ca2+]i (Figure 3B), consistent with the lack of effect on spike propagation of 0 mM Ca2+, 10 mM BAPTA application to the internodes (Figure S2B). [Ca2+]i at NoRs required axonal APs, were suppressed when spontaneous firing was arrested by somatic hyperpolarization (Figure 3G), and could not be driven by somatic depolarization when spikes were blocked with bath-applied TTX (Figures 3E and 3F). Increases in [Ca2+]i were spike rate dependent (Figure S3), slow, and cumulative and lead to sustained [Ca2+]i during continuous activity (Figures 3G and 3H). [Ca2+]i increase was also suppressed by removal of extracellular Ca2+ (16% ± 6% of control n = 4; Figure 3I) and bath application of mibefradil (Figures 3I and 5 μM, 43% ± 14% of control, p = 0.013) but was not sensitive to agatoxin IVa, which blocked Ca2+ transients in PC synaptic boutons as expected (Hillman et al., 1991) (Figures 3I; 9% ± 3% of control, p < 0.0001). This implies that T-type and not P-type CaVs are the primary source of Ca2+ entry at NoRs. Additionally, although prior depolarization of the soma reduced [Ca2+]i at the AIS, AP-triggered nodal Ca2+ signals were unaffected (Figures S3C and S3D), demonstrating that, in PCs, nodal [Ca2+]i is independent of somatic Vm.


Calcium-Activated Potassium Channels at Nodes of Ranvier Secure Axonal Spike Propagation.

Gründemann J, Clark BA - Cell Rep (2015)

Local, Activity-Dependent Ca2+ Influx at Nodes of Ranvier(A) Two-photon image of cerebellar Purkinje cell indicating line scan locations.(B) Ca2+ transients (top, line scans) at locations shown in (A) during current-evoked spike trains (bottom). AIS, axon initial segment. BP1 and BP2, first and second axonal branchpoint.(C) Frame-scan time series of BP1 during spike train. Green, axon morphology.(D) Spike train-evoked ΔF/F at ROIs shown in (C) (red boxes). Normalized integrated ΔF/F (ΔF/F∗s) against distance from an axonal branchpoint (bottom, n = 13 neurons).(E) Soma, AIS, and first BP ΔF/F in response to 500-ms somatic voltage steps (voltage clamp) in bath-applied TTX (0.5 μM).(F) Pooled data for max ΔF/F versus somatic command potential (soma, black; AIS, red; BP1, blue. n = 6 neurons).(G) Activity-dependent changes in ΔF/F upon somatic current injection. Baseline holding current: 0 pA.(H) Summary data for the change in ΔF/F∗s during silence and activity (n = 5 cells).(I) Ca2+ influx at the axon initial segment, first branchpoint and presynaptic boutons before and after bath application of 0 mM extracellular Ca2+ (n = 4), Agatoxin (AgaTX, n = 4, AIS = 3), and Mibefradil (Mibef, n = 9).Error bars, ±SEM.
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fig3: Local, Activity-Dependent Ca2+ Influx at Nodes of Ranvier(A) Two-photon image of cerebellar Purkinje cell indicating line scan locations.(B) Ca2+ transients (top, line scans) at locations shown in (A) during current-evoked spike trains (bottom). AIS, axon initial segment. BP1 and BP2, first and second axonal branchpoint.(C) Frame-scan time series of BP1 during spike train. Green, axon morphology.(D) Spike train-evoked ΔF/F at ROIs shown in (C) (red boxes). Normalized integrated ΔF/F (ΔF/F∗s) against distance from an axonal branchpoint (bottom, n = 13 neurons).(E) Soma, AIS, and first BP ΔF/F in response to 500-ms somatic voltage steps (voltage clamp) in bath-applied TTX (0.5 μM).(F) Pooled data for max ΔF/F versus somatic command potential (soma, black; AIS, red; BP1, blue. n = 6 neurons).(G) Activity-dependent changes in ΔF/F upon somatic current injection. Baseline holding current: 0 pA.(H) Summary data for the change in ΔF/F∗s during silence and activity (n = 5 cells).(I) Ca2+ influx at the axon initial segment, first branchpoint and presynaptic boutons before and after bath application of 0 mM extracellular Ca2+ (n = 4), Agatoxin (AgaTX, n = 4, AIS = 3), and Mibefradil (Mibef, n = 9).Error bars, ±SEM.
Mentions: Using two-photon Ca2+-imaging, we investigated whether the spatial distribution of calcium signals in PC axons might confer NoR specificity of IK recruitment. We detected prominent activity-dependent increases in intracellular Ca2+ concentration ([Ca2+]i) at NoRs (identified by their location at branchpoints) and at the AIS (see Bender and Trussell, 2009) during trains of evoked APs (Figures 3A–3C; 204 ± 21 Hz). Ca2+ signals were restricted to approximately 5 μm from the center of NoRs (Figures 3C and 3D), and there were no detectable changes in internodal [Ca2+]i (Figure 3B), consistent with the lack of effect on spike propagation of 0 mM Ca2+, 10 mM BAPTA application to the internodes (Figure S2B). [Ca2+]i at NoRs required axonal APs, were suppressed when spontaneous firing was arrested by somatic hyperpolarization (Figure 3G), and could not be driven by somatic depolarization when spikes were blocked with bath-applied TTX (Figures 3E and 3F). Increases in [Ca2+]i were spike rate dependent (Figure S3), slow, and cumulative and lead to sustained [Ca2+]i during continuous activity (Figures 3G and 3H). [Ca2+]i increase was also suppressed by removal of extracellular Ca2+ (16% ± 6% of control n = 4; Figure 3I) and bath application of mibefradil (Figures 3I and 5 μM, 43% ± 14% of control, p = 0.013) but was not sensitive to agatoxin IVa, which blocked Ca2+ transients in PC synaptic boutons as expected (Hillman et al., 1991) (Figures 3I; 9% ± 3% of control, p < 0.0001). This implies that T-type and not P-type CaVs are the primary source of Ca2+ entry at NoRs. Additionally, although prior depolarization of the soma reduced [Ca2+]i at the AIS, AP-triggered nodal Ca2+ signals were unaffected (Figures S3C and S3D), demonstrating that, in PCs, nodal [Ca2+]i is independent of somatic Vm.

Bottom Line: Functional connectivity between brain regions relies on long-range signaling by myelinated axons.This is secured by saltatory action potential propagation that depends fundamentally on sodium channel availability at nodes of Ranvier.We show that the recruitment of calcium-activated potassium channels (IK, K(Ca)3.1) by local, activity-dependent calcium (Ca(2+)) influx at nodes of Ranvier via a T-type voltage-gated Ca(2+) current provides a powerful mechanism that likely opposes depolarizing block at the nodes and is thus pivotal to securing continuous axonal spike propagation in spontaneously firing Purkinje cells.

View Article: PubMed Central - PubMed

Affiliation: Wolfson Institute for Biomedical Research, University College London, Gower Street, London WC1E 6BT, UK.

No MeSH data available.


Related in: MedlinePlus