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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: This is secured by saltatory action potential propagation that depends fundamentally on sodium channel availability at nodes of Ranvier.Cerebellar Purkinje cells provide continuous input to their targets in the cerebellar nuclei, reliably transmitting axonal spikes over a wide range of rates, requiring a constantly available pool of nodal sodium channels.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: This is secured by saltatory action potential propagation that depends fundamentally on sodium channel availability at nodes of Ranvier.Cerebellar Purkinje cells provide continuous input to their targets in the cerebellar nuclei, reliably transmitting axonal spikes over a wide range of rates, requiring a constantly available pool of nodal sodium channels.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