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Voltage-gated
Sodium Channels
XuelianMa
Introduction
• In1952，Hodgkin and Huxley demonstrated the role of
sodium channels in action potential electrogenesis and
predicted many of the properties of these channels
• Sodium channels play central roles in electrogenesis in
almost all types of neurons
Molecular structure
• A large α subunits of 260 kDa
and smaller βsubunits of 30–40
kDa
• The α subunit
• is sufficient for expression of
functional sodium channels
• The β subunits
• modulates the kinetics and voltage
dependence of sodium channel
activation and inactivation
• modulates localization of sodium
channel.
The β subunits
• four NaVβ subunits in total
• β1 and β3 are associated noncovalently with α subunits
• whereas β2 and β4 form
disulfide bonds with α subunits
• sodium channel properties are
modulated in a cell-type
specific manner
G616R variant of NaV1.7
• native neuronal background
Inherited Erythromelalgia(红斑肢痛症)
• L858H NaV1.7 mutation
• produce hyperexcitability within DRG
neurons and hypoexcitability within
sympathetic ganglion neurons.
the selective expression of NaV1.8 within DRG neurons, and its
absence within sympathetic ganglion neurons
Figure 6. Excitability of
sympathetic ganglion
neurons is reduced by
erythromelalgia NaV1.7
mutation L858H but can be
rescued by coexpression of
NaV1.8 A, suprathreshold
responses recorded from
representative superior
cervical ganglion (SCG)
neurons transf...
Two important principles
• The first is that the effects of ion channel mutations on
neuronal function are not necessarily unidirectional or
predictable on the basis of changes in channel function
per se;
A single ion channel mutation can have divergent
functional effects in different types of neurons.
• The second is that cell background and specifically the
precise make-up of the electrogenisome can shape the
functional effects of an ion channel mutation.
Nav channel neuronal distribution
• Sodium channel α subunits are expressed in
different excitable tissues (Table1; Goldin, 2001).
• NaV1.1, 1.2, 1.3 and 1.6 are the primary sodium
channels in the central nervous system.
• NaV1.7,1.8 and 1.9 are the primary sodium channels in
the peripheral nervous system.
• NaV1.4 is the primary sodium channel in skeletal
muscle,
• NaV1.5 is primary in heart.
The roles of different sodium
channels in nociception
Nav1.3
• Nav1.3 voltage-gated sodium channels
have been shown to be expressed at
increased levels within axotomized
dorsal root ganglion (DRG) neurons and
within injured axons within neuromas
and have been implicated in
neuropathic pain.
• The more hyperpolarized component of ramp current from
Nav1.3 is more likely to be involved in altering threshold.
• The more depolarized second component of ramp current
may, in contrast, play a role in inter spike interval
pacemaking when neurons or their axons are depolarized
after injury.
NaV1.7
• NaV1.7 activates in response to small slow depolarizations
close to resting potential so as to produce its own
depolarization
• The ability of NaV1.7 to boost subthreshold stimuli increases
the probability of neurons reaching their threshold for firing
action potentials.
• NaV1.7 is considered to be a threshold channel
Nav1.8
• Nav1.8 is relatively resistant to
inactivation by depolarization (Fig. 2A)and
recovers rapidly from inactivation.
• NaV1.8 thus produces repetitive firing in
depolarized DRG neurons
• Nav1.8 producing most of the inward
current underlying the action potential
upstroke during repetitive firing
Nav1.7 and Nav1.8 function in tandem,
with Nav1.7 amplifying small depolarizations
to bring the cell to threshold, and Nav1.8
producing most of the inward current
underlying the action potential upstroke
during repetitive firing .
Nav1.9
• NaV1.9, is characterized by very slow activation
and inactivation with a large overlap centred
near resting potential
Nav1.9
•this channel contributes a sodium
conductance at rest that modulates the
excitability of DRG neurons
NaV1.7(and NaV1.6 and/or NaV1.9 in
some cells) brings the neuron toward
threshold (dashed line),
NaV1.8 is largely responsible for the
overshooting action potential with
minor contributions of NaV1.1, NaV1.6
and NaV1.7 to the action potential
upstroke.
Multiple sodium channel
subtypes participate in
electrogenesis in small DRG
neurons
• In 2005 ,Swensen & Bean
• Purkinje neurons from NaV1.6−/−
mice in which sodium current
density is reduced in the long term,
where an upregulation of calcium
channels maintains excitability at
close to its normal level .
• The firing properties of most neurons, are
usually maintained within a circumscribed
range.
• A result of homeostatic regulation of ion
channel expression, post translational
modification, and/or interaction with
binding partners or modulators.
Changes in expression of channel ‘B’ can compensate for
changes in expression of channel ‘A’ to maintain excitability
within a particular range
‘Electrogenistat’
These homeostatic regulation of
intrinsic neuronal excitability
imply a need for an
‘electrogenistat’ within excitable
cells
Data Evaluation and
Analysis
a
current versus voltage
(I-V) curve
current versus voltage
(I-V) curve
• A number of important and useful parameters can be
readily derived from this plots, including:
•
•
•
•
reversal potential ,
voltage-dependence (rectification),
activation threshold,
as well as overall quality of voltage-clamp
• G = I/(Vm – ENa),
• where G is conductance,
• I is peak inward current,
• Vm is the membrane potential step used to
elicit the response
• ENa is the sodium reversal potential
Voltage-dependence of activation
50mv
-60mv
-80mv
The activation of TTX-s currents
The activation of TTX-R slow curents
200pA
2nA
5ms
5ms
Voltage-dependence of activation
Boltzmann distribution equation:
GNa = GNa;max/{1+ exp[ (V
V1/2
1/2 - Vm)/SS ]}
• Boltzmann distribution equation:
GNa = GNa;max/{1+ exp[ (V1/2 - Vm)/S]}
GNa is the voltage-dependent sodium conductance,
GNa,max is the maximal sodium conductance,
• V1 ⁄ 2 is the potential at which activation is halfmaximal,
• Vm is the membrane potential
• k is the slope.
The kinetics of activation
Fast inactivation process
15mv
-10mv
-50mv
-60mv
TTX-S inactivation current
TTX-R slow inactivation current
5nA
500pA
5ms
5ms
Boltzmann function: I = Imax / (1 + exp( V1/2 – Vm)/ S )
** P < 0.01 vs no CRD group
• INa = INa;max/{1+ exp[ (V1/2 Vm)/k]}
• where INa,max is the peak sodium current elicited after
the most hyperpolarized prepulse,
• Vm is the preconditioning pulse potential,
• V1 ⁄ 2 is the half maximal sodium current
• k is the slope factor.
Steady state inactivation
• Inactivation kinetics
The kinetics of recovery
from inactivation
• Time course of recovery from fast
inactivation
Double pulse protocol
-10mv
-60mv
Time course of recovery from inactivation
Current(pA)
2nA
5ms
Time(ms)
I/Imax=1-exp(-t/tau)