Pacemaker currents in neurons and excitable cells

Emilio Carbone

Summer Course on Experimental NeuroscienceSchool of Neuroscience Department of Neuroscience

Corso Raffaello 30, 10125 TorinoTorino, July 1 2009

Different neuronal APs

from Bean, Nature Review Neurosci. (2007)

Different neuronal APs

Voltage-gated Na+ and K+ channels account for the fast APs

fast activatingfast inactivating

slowly activatingnon inactivating

INa = gNa (Vm – ENa)

IK = gK (Vm – EK)

ENa= +63 mV

EK = -102 mV

Block of V-gated Na+ and K+ channels alter the shape of APs

Ca2+-activated K+ channels do also shape the APs

Ca2+-activated K+ channels do also shape the APs

The big K+ channel (BKCa) Activation is Ca2+- and V-dependent Micromolar affinity for Ca2+

Forms nanodomains with Ca2+ channels Responsible for fast repolarizations Is blocked by charybdotoxin, iberiotoxin, paxilline and 0.5 -1 mM TEA+

Marcantoni et al., Cell Calcium 2007

Ca2+-activated K+ channels do also shape the APs

The small K+ channel (SKCa) Activation is only Ca2+-dependent (regulated by CaM) Submicromolar affinity for Ca2+

Forms microdomains with Ca2+ channels Requires more than one voltage-gated Ca2+ channel to activate Sensitive to the cytoplasmatic Ca2+

Responsible of slow repolarization Is blocked by apamin, insensitive to TEA+ and Cs+

Voltage and Ca2+-gated ion channels contribute to shape the APs

SKCa

circulating adrenaline

vesicle

Ca2+

Ca2+

sympathetic nerve stimulation

action potential

Chromaffin cells undergo spontaneous activity at rest

Preliminary observations to identify a pacemaker channel

Marcantoni et al., Cell Calcium 2007

Marcantoni et al., Cell Calcium 2007

Pacemaker channels require peculiar gating properties

Marcantoni et al., submitted 2009Cesetti et al., J. Neurosci 2003

The L-type channels: Cav1.2, Cav1.3

Ion currents viewed through the “action potential-clamp”

The voltage comand is not a step but the action potential itself previously recorded from the same cell

This technique allows to record the ion currents during the time course of the AP

It is the most direct way to analyze the kinetics of ion channels while the excitable cell is under functional conditions

It can be applied to any type of excitable cell that fires (neuron, cardiac myocytes, neuroendocrine cells, …)

Na+, Ca2+ and K+ currents during the action potential-clamp

INa = gNa (Vm – ENa)

IK = gK (Vm – EK)

ICa = gCa (Vm – ECa)

Inward Na+ and Ca2+ currents in central neurons

from Bean, Nature Review Neurosci. (2007)

Ca2+ currents before and during the AP in chromaffin cells

L-type channels dominate the pre-spike phase with respect to the other high-threshold Ca2+ channels

The experiment suggests that L-type Ca2+ channels are potentially suitable for carrying pacemaker currents during a train of APs

L-type channels carry a pacemaker current in chromaffin cells

In Cav1.3-/- mice a high percentage of chromaffin cells are silent

Strict coupling between L and BKCa channels in central neurons

from Bean, Nature Review Neurosci. (2007)

The agonist BayK 8644 enhances the pacemaker L-type currents

from Albillos et al. EJN. (1996)

from Marcantoni et al. submitted (2009)

from Puopolo et al J Neurosci (2007)

Na+ and Ca2+ pacemaker currents in midbrain dopaminergic neurons

Pacemaker currents in dopaminergic neurons

from Puopolo et al J Neurosci (2007)

L-type channels control the pacemaker current of dopaminergic neurons

Pacemaker currents in suprachiasmatic nucleus (SCN) neurons

from Jackson et al J Neurosci (2004)

Background pacemaker currents in SCN neurons

Suggested readings:

Bean B. (2007) Nature Reviews Neuroscience 8:451

Fakler B & Adelman JP (2008) Neuron 59:873

Marcantoni et al (2009) Pflügers Archiv Europ J Physiol 457:1093

Marcantoni et al (2007) Cell Calcium 42:397

Jackson et al (2004) J. Neuroscience 24:7985

Puopolo et al (2007) J. Neuroscience 27:645