XuelianMa

• 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

• 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.

• 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...

• 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.

• 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 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 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, is characterized by very slow activation and inactivation with a large overlap centred near resting potential

• this channel contributes a sodium conductance at rest that modulates the excitability of DRG neurons

Multiple sodium channel subtypes participate in electrogenesis in small 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.

• 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

These homeostatic regulation of intrinsic neuronal excitability imply a need for an
‘ electrogenistat ’ within excitable cells

• 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
-60mv
The activation of TTX-s currents
50mv
-80mv
The activation of TTX-R slow curents
2nA
5ms
200pA
5ms

Voltage-dependence of activation
Boltzmann distribution equation:

•
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.

-60mv
TTX-S inactivation current
15mv
-50mv
-10mv
TTX-R slow inactivation current
5nA
5ms
500pA
5ms

Boltzmann function: I = I max
** P < 0.01 vs no CRD group
– V m
)/ S )

• 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.

• Inactivation kinetics

• Time course of recovery from fast inactivation
-60mv
Double pulse protocol
-10mv
Tim e course of recovery from inactivation
2nA
5ms
Tim e(m s)

I/Imax=1-exp(-t/ tau )