Slow closed-state inactivation: a novel mechanism underlying ramp currents in cells expressing the hNE/PN1 sodium channel

Noninactivating, tetrodotoxin-sensitive Na+ conductance in peripheral axons

A noninactivating, persistent sodium current has been demonstrated previously in dorsal root ganglia neurons and in rat optic nerve. We report here that Na(+) channel blockade with tetrodotoxin (TTX) in isolated dorsal and ventral roots elicits membrane hyperpolarization, suggesting the presence of a persistent Na(+) current in peripheral axons. We used a modified sucrose-gap chamber to monitor resting and action potentials and observed a hyperpolarizing shift in the nerve potential of rat dorsal and ventral roots by TTX. The block of transient inward Na(+) currents was confirmed by the abolition of compound action potentials (CAPs). Moreover, depolarization of nerve roots by elevating extracellular K(+) concentrations to 40 mM eliminated CAPs but did not significantly alter TTX-induced hyperpolarizations, indicating that the persistent Na(+) currents in nerve roots are not voltage-dependent. Tetrodotoxin-sensitive persistent inward Na(+) currents are present in both dorsal and ventral root axons at rest and may contribute to axonal excitability.

Sensory and electrophysiological properties of guinea-pig sensory neurones expressing Nav 1.7 (PN1) Na+ channel alpha subunit protein

The TTX-sensitive Na(v)1.7 (PN1) Na(+) channel alpha subunit protein is expressed mainly in small dorsal root ganglion (DRG) neurones. This study examines immunocytochemically whether it is expressed exclusively or preferentially in nociceptive primary afferent DRG neurones, and determines the electrophysiological properties of neurones that express it. Intracellular somatic action potentials (APs) evoked by dorsal root stimulation were recorded in L6/S1 DRG neurones at 30 +/- 2 degrees C in vivo in deeply anaesthetised young guinea-pigs. Each neurone was classified, from its dorsal root conduction velocity (CV) as a C-, Adelta- or Aalpha/beta-fibre unit and from its response to mechanical and thermal stimuli, as a nociceptive, low threshold mechanoreceptive (LTM) or unresponsive unit. Fluorescent dye was injected into the soma and Na(v)1.7-like immunoreactivity (Na(v)1.7-LI) was examined on sections of dye-injected neurones. All C-, 90 % of Adelta- and 40 % of Aalpha/beta-fibre units, including both nociceptive and LTM units, showed Na(v)1.7-LI. Positive units included 1/1 C-LTM, 6/6 C-nociceptive, 4/4 C-unresponsive (possible silent nociceptive) units, 5/6 Adelta-LTM (D hair), 13/14 Adelta-nociceptive, 2/9 Aalpha/beta-nociceptive, 10/18 Aalpha/beta-LTM cutaneous and 0/9 Aalpha/beta-muscle spindle afferent units. Overall, a higher proportion of nociceptive than of LTM neurones was positive, and the median relative staining intensity was greater in nociceptive than LTM units. Na(v)1.7-LI intensity was clearly positively correlated with AP duration and (less strongly) negatively correlated with CV and soma size. Since nociceptive units tend overall to have longer duration APs, slower CVs and smaller somata, these correlations may be related to the generally greater expression of Na(v)1.7 in nociceptive units.

Characterization and developmental changes of Na+ currents of petrosal neurons with projections to the carotid body

Carotid body chemoreceptors transduce a decrease in arterial oxygen tension into an increase in spiking activity on the sinus nerve, and this response increases with postnatal age over the first week or two of life. Previous work from our laboratory has suggested a major role of axonal Na(+) channels in the initiation of afferent spiking activity. Using RT-PCR of the petrosal ganglia we identified Na(+) channel TTX-S isoforms Na(v)1.1, Na(v)1.6, and Na(v)1.7 and the TTX-resistant (TTX-R) isoforms Na(v)1.8 and Na(v)1.9 at high levels. Electrophysiologic recordings (at 3 ages: 3 days, 9 days, 18-20 days) of neurons that project to the carotid body exhibited predominantly fast-inactivating sodium currents, with a bimodal recovery from inactivation at -80 mV (fast component approximately 8 ms; slow component approximately 90 ms). Developmental age had little effect with no change in peak current density (approximately 1.4 nA/pF) and was associated with a slight, but significant increase in the speed of recovery from inactivation at -140 and -120 mV but not at other potentials. Assuming that the same Na(+) channel complement is present at the nerve terminal as at the soma, the association of a sensory modality (chemoreception) with a relatively uniform Na(+) channel profile suggests that the rapid kinetics of TTX-S channels may be essential for some aspects of chemoreceptor function beyond mediating simple axonal conduction.

Sodium channel Na(v)1.6 is expressed along nonmyelinated axons and it contributes to conduction

Nodes of Ranvier in myelinated fibers exhibit a complex architecture in which specific molecules organize in distinct nodal, paranodal and juxtaparanodal domains to support saltatory conduction. The clustering of sodium channel Na(v)1.6 within the nodal membrane has led to its identification as the major nodal sodium channel in myelinated axons. In contrast, much less is known about the molecular architecture of nonmyelinated fibers. In the present study, Na(v)1.6 is shown to be a significant component of nonmyelinated PNS axons. In DRG C-fibers, Na(v)1.6 is distributed continuously from terminal receptor fields in the skin to the dorsal root entry zone in the spinal cord. Na(v)1.6 is also present in the nerve endings of corneal C-fibers. Analysis of compound action potential recordings from wildtype and med mice, which lack Na(v)1.6, indicates that Na(v)1.6 plays a functional role in nonmyelinated fibers where it contributes to action potential conduction. These observations indicate that Na(v)1.6 functions not only in saltatory conduction in myelinated axons but also in continuous conduction in nonmyelinated axons.

GDNF and NGF reverse changes in repriming of TTX-sensitive Na(+) currents following axotomy of dorsal root ganglion neurons

Uninjured C-type rat dorsal root ganglion (DRG) neurons predominantly express slowly inactivating TTX-resistant (TTX-R) and slowly repriming TTX-sensitive (TTX-S) Na(+) currents. After peripheral axotomy, TTX-R current density is reduced and rapidly repriming TTX-S currents emerge and predominate. The change in TTX-S repriming kinetics is paralleled by an increase in the level of transcripts and protein for the Na(v)1.3 sodium channel alpha-subunit, which is known to exhibit rapid repriming. Changes in Na(+) current profile and kinetics in DRG neurons may substantially alter neuronal excitability and could contribute to some states of chronic pain associated with injury of sensory neurons. In the present study, we asked whether glial-derived neurotrophic factor (GDNF) and nerve growth factor (NGF), which have been shown to prevent some axotomy-induced changes such as the loss of TTX-R Na(+) current expression in DRG neurons, can ameliorate the axotomy-induced change in TTX-S Na(+) current repriming kinetics. We show that intrathecally administered GDNF and NGF, delivered individually, can partially reverse the effect of axotomy on the repriming kinetics of TTX-S Na(+) currents. When GDNF and NGF were co-administered, the repriming kinetics were fully rescued. We observed parallel effects of GDNF and NGF on the Na(v)1.3 sodium channel transcript levels in axotomized DRG. Both GDNF and NGF were able to partially reverse the axotomy-induced increase in Na(v)1.3 mRNA, with GDNF plus NGF producing the largest effect. Our data indicate that both GDNF and NGF can partially reverse an important effect of axotomy on the electrogenic properties of sensory neurons and that their effect is additive.

Critical molecular determinants of voltage-gated sodium channel sensitivity to mu-conotoxins GIIIA/B

GIIIA/B mu-conotoxins block the rat skeletal muscle sodium channel (rNa(v)1.4) with high affinity by binding to specific residues in the pore. However, human Na(v)1.4 (hNa(v)1.4) channels, which are resistant to block by GIIIA/B, have these same pore residues. We used chimera constructs, site-directed mutagenesis, and electrophysiological techniques to investigate which residues determine GIIIA/B selectivity. Exchange of serine 729 in the D2/S5-S6 linker of rat Na(v)1.4 with leucine (S729L), the corresponding residue in hNa(v)1.4, reduces the sensitivity of rNa(v)1.4 by approximately 20-fold and largely accounts for the differential sensitivity of rNa(v)1.4 and hNa(v)1.4 to both GIIIA and GIIIB. To determine whether D2/S5-S6 linker residues might contribute to the resistance of neuronal channels to GIIIA/B, we exchanged residues in this linker that differed between rNa(v)1.4 and neuronal channels. Substitution of aspargine 732 with lysine (N732K), the corresponding residue in rNa(v)1.1a and rNa(v)1.7, reduced the GIIIB sensitivity of rNa(v)1.4 by approximately 20-fold. The N732K substitution, however, only reduced GIIIA sensitivity of rNa(v)1.4 by approximately 4-fold, demonstrating that GIIIA and GIIIB have distinct interactions with the D2/S5-S6 linker. Our data indicate that naturally occurring variants in the extra-pore region of the D2/S5-S6 linker contribute to the isoform-specific sensitivity of sodium channels to GIIIA/B. Because S729 and N732 are not part of the high-affinity binding site for mu-conotoxins, these extra-pore residues probably influence the accessibility of the toxin to the binding site within the pore and/or the stability of the toxin-channel complex. Our results should aid the development of toxins that block specific neuronal sodium channel isoforms.

Nav1.3 sodium channels: rapid repriming and slow closed-state inactivation display quantitative differences after expression in a mammalian cell line and in spinal sensory neurons

Although rat brain Nav1.3 voltage-gated sodium channels have been expressed and studied in Xenopus oocytes, these channels have not been studied after their expression in mammalian cells. We characterized the properties of the rat brain Nav1.3 sodium channels expressed in human embryonic kidney (HEK) 293 cells. Nav1.3 channels generated fast-activating and fast-inactivating currents. Recovery from inactivation was relatively rapid at negative potentials (<-80 mV) but was slow at more positive potentials. Development of closed-state inactivation was slow, and, as predicted on this basis, Nav1.3 channels generated large ramp currents in response to slow depolarizations. Coexpression of beta3 subunits had small but significant effects on the kinetic and voltage-dependent properties of Nav1.3 currents in HEK 293 cells, but coexpression of beta1 and beta2 subunits had little or no effect on Nav1.3 properties. Nav1.3 channels, mutated to be tetrodotoxin-resistant (TTX-R), were expressed in SNS-null dorsal root ganglion (DRG) neurons via biolistics and were compared with the same construct expressed in HEK 293 cells. The voltage dependence of steady-state inactivation was approximately 7 mV more depolarized in SNS-null DRG neurons, demonstrating the importance of background cell type in determining physiological properties. Moreover, consistent with the idea that cellular factors can modulate the properties of Nav1.3, the repriming kinetics were twofold faster in the neurons than in the HEK 293 cells. The rapid repriming of Nav1.3 suggests that it contributes to the acceleration of repriming of TTX-sensitive (TTX-S) sodium currents that are seen after peripheral axotomy of DRG neurons. The relatively rapid recovery from inactivation and the slow closed-state inactivation kinetics of Nav1.3 channels suggest that neurons expressing Nav1.3 may exhibit a reduced threshold and/or a relatively high frequency of firing.

Developmental expression of the TTX-resistant voltage-gated sodium channels Nav1.8 (SNS) and Nav1.9 (SNS2) in primary sensory neurons

The development of neuronal excitability involves the coordinated expression of different voltage-gated ion channels. We have characterized the expression of two sensory neuron-specific tetrodotoxin-resistant sodium channel alpha subunits, Na(v)1. (SNS/PN3) and Na(v)1.9 (SNS2/NaN), in developing rat lumbar dorsal root ganglia (DRGs). Expression of both Na(v)1.8 and Na(v)1.9 increases with age, beginning at embryonic day (E) 15 and E17, respectively, and reaching adult levels by postnatal day 7. Their distribution is restricted mainly to those subpopulations of primary sensory neurons in developing and adult DRGs that give rise to unmyelinated C-fibers (neurofilament 200 negative). Na(v)1.8 is expressed in a higher proportion of neuronal profiles than Na(v)1.9 at all stages during development, as in the adult. At E17, almost all Na(v)1.8-expressing neurons also express the high-affinity NGF receptor TrkA, and only a small proportion bind to IB4, a marker for c-ret-expressing (glial-derived neurotrophic factor-responsive) neurons. Because IB4 binding neurons differentiate from TrkA neurons in the postnatal period, the proportion of Na(v)1.8 cells that bind to IB4 increases, in parallel with a decrease in the proportion of Na(v)1.8-TrkA co-expressing cells. In contrast, an equal number of Na(v)1.9 cells bind IB4 and TrkA in embryonic life. The differential expression of Na(v)1.8 and Na(v)1.9 in late embryonic development, with their distinctive kinetic properties, may contribute to the development of spontaneous and stimulus-evoked excitability in small diameter primary sensory neurons in the perinatal period and the activity-dependent changes in differentiation they produce.

Contribution of Na(v)1.8 sodium channels to action potential electrogenesis in DRG neurons

C-type dorsal root ganglion (DRG) neurons can generate tetrodotoxin-resistant (TTX-R) sodium-dependent action potentials. However, multiple sodium channels are expressed in these neurons, and the molecular identity of the TTX-R sodium channels that contribute to action potential production in these neurons has not been established. In this study, we used current-clamp recordings to compare action potential electrogenesis in Na(v)1.8 (+/+) and (-/-) small DRG neurons maintained for 2-8 h in vitro to examine the role of sodium channel Na(v)1.8 (alpha-SNS) in action potential electrogenesis. Although there was no significant difference in resting membrane potential, input resistance, current threshold, or voltage threshold in Na(v)1.8 (+/+) and (-/-) DRG neurons, there were significant differences in action potential electrogenesis. Most Na(v)1.8 (+/+) neurons generate all-or-none action potentials, whereas most of Na(v)1.8 (-/-) neurons produce smaller graded responses. The peak of the response was significantly reduced in Na(v)1.8 (-/-) neurons [31.5 +/- 2.2 (SE) mV] compared with Na(v)1.8 (+/+) neurons (55.0 +/- 4.3 mV). The maximum rise slope was 84.7 +/- 11.2 mV/ms in Na(v)1.8 (+/+) neurons, significantly faster than in Na(v)1.8 (-/-) neurons where it was 47.2 +/- 1.3 mV/ms. Calculations based on the action potential overshoot in Na(v)1.8 (+/+) and (-/-) neurons, following blockade of Ca(2+) currents, indicate that Na(v)1.8 contributes a substantial fraction (80-90%) of the inward membrane current that flows during the rising phase of the action potential. We found that fast TTX-sensitive Na(+) channels can produce all-or-none action potentials in some Na(v)1.8 (-/-) neurons but, presumably as a result of steady-state inactivation of these channels, electrogenesis in Na(v)1.8 (-/-) neurons is more sensitive to membrane depolarization than in Na(v)1.8 (+/+) neurons, and, in the absence of Na(v)1.8, is attenuated with even modest depolarization. These observations indicate that Na(v)1.8 contributes substantially to action potential electrogenesis in C-type DRG neurons.

Expression profiles of voltage-gated Na(+) channel alpha-subunit genes in rat and human prostate cancer cell lines

BACKGROUND

Voltage-gated Na(+) channel (VGSC) activity has been implicated in prostate cancer (PC) metastasis. Although VGSCs can occur as multiple-subunit assemblies, the alpha-subunits (VGSCalphas) alone can encode functional channels. The VGSCalpha gene(s) responsible for the functional VGSCalpha expression in strongly metastatic PC cell lines is not known.

METHODS

Two reverse transcription-PCR (RT-PCR) methods, degenerate primer screening and a novel semi quantitative PCR (SQT-PCR) technique, were used. These methods enabled a detailed qualitative and quantitative investigation of VGSCalpha mRNA expression in rat (MAT-LyLu/AT-2) and human (PC-3/LNCaP) PC cells of markedly different metastatic potential.

RESULTS

Expression of eight different VGSCalpha genes (SCN1A-4A, SCN7A-9A, and SCN11A) was determined in the PC cell lines. Most were expressed as multiple splice variants. SQT-PCR results were consistent with a basal level of VGSCalpha mRNA expression occurring in weakly metastatic (AT-2/LNCaP) cells, and this being greatly elevated in cells of stronger metastatic potential (MAT-LyLu/PC-3), primarily due to the elevated expression of the SCN9A gene (also termed PN1/hNe-Na).

CONCLUSIONS

(1) Several VGSCalpha genes and their splice variants are expressed similarly in both rat and human PC cell lines. (2) Expression levels are much higher in the strongly metastatic (MAT-LyLu/PC-3) cells. (3) Levels of SCN9A mRNA specifically are predominant in MAT-LyLu and PC-3 cells; thus, SCN9A is highly likely to be the main source of the functional VGSC detected.

Beta1 adducin gene expression in DRG is developmentally regulated and is upregulated by glial-derived neurotrophic factor and nerve growth factor

Differential display technique has proven to be effective in identifying differentially regulated genes under a variety of experimental conditions. We identified beta1 adducin as a target in primary rat dorsal root ganglia (DRG) cultures that is upregulated by exposure to nerve growth factor (NGF) and glial-derived neurotrophic factor (GDNF). We used real-time reverse-transcription polymerase chain reaction (RT-PCR) for quantitative measurement of beta1 adducin gene expression both in DRG cultures and in vivo. Significant increase in beta1 adducin expression level was observed in DRG cultures treated with either GDNF or NGF, compared to untreated cultures. The expression of beta1 adducin in rat tissues was highest in the brain and high in the cerebellum, superior cervical ganglion and DRG tissues. By contrast, low expression levels of beta1 adducin are detected in sciatic nerve and in non-neural tissues. Our study also showed that expression of beta1 adducin gene is developmentally regulated in rat DRG and trigeminal ganglia, with a peak around P0 and significant attenuation by P21. The level of expression of beta1 adducin in adult rat DRG and trigeminal ganglia may be maintained by the action of neurotrophic factors that are produced in innervated targets like skin and muscle.

Sodium currents of large (Abeta-type) adult cutaneous afferent dorsal root ganglion neurons display rapid recovery from inactivation before and after axotomy

Voltage-dependent Na-currents were studied, using whole cell voltage clamp, in acutely dissociated, large (mostly Abeta-fiber type) cutaneous afferent dorsal root ganglia neurons (L(4) and L(5)) from the adult rat. Cells were dissociated 14-17 days after axotomy. Control and axotomized neurons were identified via the retrograde marker hydroxy-stilbamide (fluorogold) which was injected into the lateral and plantar region of the skin of the foot and were studied using whole cell patch clamp techniques within 12-20 h of dissociation and plating. Cells were dissociated 14-17 days after injury. Both control and axotomized neurons displayed complex Na-currents composed of components with distinct kinetic and pharmacological properties. The large (48-50 microm diameter) control cutaneous afferent neurons, many of which likely give rise to myelinated Abeta-fibers, exhibited Na-currents with both slow and fast inactivating kinetics. The fast inactivating current in large cutaneous afferent dorsal root ganglion neurons was tetrodotoxin-sensitive and recovered from inactivation approximately four-fold faster at -60 mV (P<0.001) and approximately six-fold faster at -70 mV (P<0.001) than the tetrodotoxin-sensitive current in small (<30 microm diameter) neurons. Further, while the tetrodotoxin-sensitive currents in smaller dorsal root ganglion neurons (mainly C-fiber type) reprime approximately four-fold faster following peripheral axotomy, repriming of the fast inactivating current in larger cutaneous afferent neurons was not significantly altered following axotomy. However, while 77% of control large neurons were observed to express the slower inactivating, tetrodotoxin-resistant current, only 45% of these large neurons did after axotomy. These results indicate that large adult cutaneous afferent dorsal root ganglion neurons (Abeta-type) express tetrodotoxin-sensitive Na-currents, which have much faster repriming than Na-currents in small (C-type) neurons, both before, and after axotomy. Like small neurons, the majority of large neurons downregulate the tetrodotoxin-resistant current following sciatic nerve section.

Sodium channels and their genes: dynamic expression in the normal nervous system, dysregulation in disease states(1)

Although classical neurophysiological doctrine rested on the concept of the sodium channel, it is now clear that there are nearly a dozen sodium channel genes, each encoding a molecularly distinct channel. Different repertoires of channels endow different types of neurons with distinct transduction and encoding properties. Sodium channel expression is highly dynamic, exhibiting plasticity at both the transcriptional and post-transcriptional levels. In some types of neurons within the normal nervous system, e.g. hypothalamic magnocellular neurosecretory neurons, changes in sodium channel gene expression occur in association with the transition from a quiescent to a bursting state; these changes are accompanied by the insertion of a different set of sodium channel subtypes in the cell membrane, a form of molecular plasticity which results in altered electrogenic properties. Dysregulation of sodium channel genes has been observed in a number of disease states. For example, transection of the peripheral axons of spinal sensory neurons triggers down-regulation of some sodium channel genes, and up-regulation of other sodium channel genes; the resultant changes in sodium channel expression contribute to hyperexcitability that can lead to chronic pain. There is also evidence, in experimental models of demyelination and in post-mortem tissue from patients with multiple sclerosis, for dysregulation of sodium channel gene expression in the cell bodies of some neurons whose axons have been demyelinated, suggesting that an acquired channelopathy may contribute to the pathophysiology of demyelinating diseases such as multiple sclerosis. The dynamic nature of sodium channel gene expression makes it a complex topic for investigation, but it also introduces therapeutic opportunities, since subtype-specific sodium channel modulating drugs may soon be available.

Sodium channels and pain therapy

Researchers have characterized changes in the nervous system that occur in response to tissue injury in order to identify possible targets for novel therapeutic interventions for the treatment of pain. That blockers of voltage-gated sodium channels (VGSCs) are clinically effective for the treatment of pain associated with certain types of tissue injury suggests that these channels constitute such a target. Furthermore, there are changes in biophysical properties, expression, and/or distribution of VGSCs in subpopulations of primary afferent and central nervous system neurons in response to injury that are consistent with a role for VGSCs in the generation and maintenance of pain. Injury-induced changes in four unique VGSCs have been described. However, each of these channels appears to contribute to pain associated with different forms of injury in different ways.

Expression of sodium channel SNS/PN3 and ankyrin(G) mRNAs in the trigeminal ganglion after inferior alveolar nerve injury in the rat

The inferior alveolar nerve is a sensory branch of the trigeminal nerve that is frequently damaged, and such nerve injuries can give rise to persistent paraesthesia and dysaesthesia. The mechanisms behind neuropathic pain following nerve injury is poorly understood. However, remodeling of voltage-gated sodium channels in the neuronal membrane has been proposed as one possible mechanism behind injury-induced ectopic hyperexcitability. The TTX-resistant sodium channel SNS/PN3 has been implicated in the development of neuropathic pain after spinal nerve injury. We here study the effect of chronic axotomy of the inferior alveolar nerve on the expression of SNS/PN3 mRNA in trigeminal sensory neurons. The organization of sodium channels in the neuronal membrane is maintained by binding to ankyrin, which help link the sodium channel to the membrane skeleton. Ankyrin(G), which colocalizes with sodium channels in the initial segments and nodes of Ranvier, and is necessary for normal neuronal sodium channel function, could be essential in the reorganization of the axonal membrane after nerve injury. For this reason, we here study the expression of ankyrin(G) in the trigeminal ganglion and the localization of ankyrin(G) protein in the inferior alveolar nerve after injury. We show that SNS/PN3 mRNA is down-regulated in small-sized trigeminal ganglion neurons following inferior alveolar nerve injury but that, in contrast to the persistent loss of SNS/PN3 mRNA seen in dorsal root ganglion neurons following sciatic nerve injury, the levels of SNS/PN3 mRNA appear to normalize within a few weeks. We further show that the expression of ankyrin(G) mRNA also is downregulated after nerve lesion and that these changes persist for at least 13 weeks. This decrease in the ankyrin(G) mRNA expression could play a role in the reorganization of sodium channels within the damaged nerve. The changes in the levels of SNS/PN3 mRNA in the trigeminal ganglion, which follow the time course for hyperexcitability of trigeminal ganglion neurons after inferior alveolar nerve injury, may contribute to the inappropriate firing associated with sensory dysfunction in the orofacial region.

alpha-SNS produces the slow TTX-resistant sodium current in large cutaneous afferent DRG neurons

In this study, we used sensory neuron specific (SNS) sodium channel gene knockout (-/-) mice to ask whether SNS sodium channel produces the slow Na(+) current ("slow") in large (>40 microm diam) cutaneous afferent dorsal root ganglion (DRG) neurons. SNS wild-type (+/+) mice were used as controls. Retrograde Fluoro-Gold labeling permitted the definitive identification of cutaneous afferent neurons. Prepulse inactivation was used to separate the fast and slow Na(+) currents. Fifty-two percent of the large cutaneous afferent neurons isolated from SNS (+/+) mice expressed only fast-inactivating Na(+) currents ("fast"), and 48% expressed both fast and slow Na(+) currents. The fast and slow current densities were 0.90 +/- 0.12 and 0.39 +/- 0.16 nA/pF, respectively. Fast Na(+) currents were blocked completely by 300 nM tetrodotoxin (TTX), while slow Na(+) currents were resistant to 300 nM TTX, confirming that the slow Na(+) currents observed in large cutaneous DRG neurons are TTX-resistant (TTX-R). Slow Na(+) currents could not be detected in large cutaneous afferent neurons from SNS (-/-) mice; these cells expressed only fast Na(+) current, and it was blocked by 300 nM TTX. The fast Na(+) current density in SNS (-/-) neurons was 1.47 +/- 0. 14 nA/pF, approximately 60% higher than the current density observed in SNS (+/+) mice (P < 0.02). A low-voltage-activated TTX-R Na(+) current ("persistent") observed in small C-type neurons is not present in large cutaneous afferent neurons from either SNS (+/+) or SNS (-/-) mice. These results show that the slow TTX-R Na(+) current in large cutaneous afferent DRG is produced by the SNS sodium channel.

Dopamine D1/D5 receptor activation modulates a persistent sodium current in rat prefrontal cortical neurons in vitro

The effects of dopamine (DA) on a persistent Na(+) current (I(NaP)) in layer V-VI prefrontal cortical (PFC) pyramidal cells were studied using whole cell voltage-clamp recordings in rat PFC slices. After blocking K(+) and Ca (2+) currents, a tetrodotoxin-sensitive I(NaP) was activated by slow depolarizing voltage ramps or voltage steps. DA modulated the I(NaP) in a voltage-dependent manner: increased amplitude of I(NaP) at potentials more negative than -40 mV, but decreased at more positive potentials. DA also slowed the inactivation process of I(NaP). The D1/D5 dopamine receptor agonists SKF 38393, SKF 81297, and dihydrexidine (3-10 microM), but not the dopamine D2/D3 receptor agonist qiunpirole (1-20 microM), mimicked the effects of DA on I(NaP). Modulation of I(NaP) by D1/D5 agonists was blocked by the D1/D5 antagonist SCH23390. Bath application of specific protein kinase C inhibitor, chelerhythrine, or inclusion of the specific protein kinase C inhibiting peptide([19-36]) in the recording pipette, but not protein kinase A inhibiting peptide([5-24]), blocked the effect of D1/D5 agonists on I(NaP). In current-clamp recordings, D1/D5 receptors activation enhanced the excitability of cortical pyramidal cells. Application of the D1/D5 agonist SKF 81297 induced a long-lasting decrease in the first spike latency in response to depolarizing current ramp. This was associated with a shift in the start of nonlinearity in the slope resistance to more negative membrane potentials. We proposed that this effect is due to a D1/D5 agonist-induced leftward shift in the activation of I(NaP). This enables DA to facilitate the firing of PFC neurons in response to depolarizing inputs.

The neuron as a dynamic electrogenic machine: modulation of sodium-channel expression as a basis for functional plasticity in neurons

Neurons signal each other via regenerative electrical impulses (action potentials) and thus can be thought of as electrogenic machines. Voltage-gated sodium channels produce the depolarizations necessary for action potential activity in most neurons and, in this respect, lie close to the heart of the electrogenic machinery. Although classical neurophysiological doctrine accorded 'the' sodium channel a crucial role in electrogenesis, it is now clear that nearly a dozen genes encode distinct sodium channels with different molecular structures and functional properties, and the majority of these channels are expressed within the mammalian nervous system. The transcription of these sodium-channel genes, and the deployment of the channels that they encode, can change significantly within neurons following various injuries. Moreover, the transcription of these genes and the deployment of various types of sodium channels within neurons of the normal nervous system can change markedly as neurons respond to changing milieus or physiological inputs. As a result of these changes in sodium-channel expression, the membranes of neurons may be retuned so as to alter their transductive and/or encoding properties. Neurons within the normal and injured nervous system can thus function as dynamic electrogenic machines with electroresponsive properties that change not only in response to pathological insults, but also in response to shifting functional needs.

A possible role for nerve growth factor in the augmentation of sodium channels in models of chronic pain

Inflammation induces an upregulation of sodium channels in sensory neurons. This most likely occurs as a result of the retrograde transport of cytochemical mediators released during the inflammatory response. The purpose of this study was to determine the effect of the subcutaneous administration of one such mediator, nerve growth factor (NGF), on the production of sodium channels in neurons of the rat dorsal root ganglion. For this, hindpaw withdrawal from either a thermal or mechanical stimulus was measured in rats at selected intervals for up to 2 weeks following injections of NGF. Sodium channel augmentation was then examined in dorsal root ganglia using site-specific, anti-sodium channel antibodies. Both thermal and mechanical allodynia was observed between 3 and 12 h post-injection. The hyperalgesic response returned to baseline by approximately 24 h post-injection. Sodium channel labeling was found to increase dramatically in the small neurons of the associated dorsal root ganglia beginning at 23 h, reached maximum intensity by 1 week, and persisted for up to 3 months post-injection. Pre-blocking NGF with anti-NGF prevented the NGF-induced decrease in paw withdrawal latencies and significantly reduced the intensity of sodium channel labeling. The results indicate that NGF is an important mediator both in the development of acute hyperalgesia and in the stimulation of sodium channel production in dorsal root ganglia during inflammation.

Biophysical properties and slow voltage-dependent inactivation of a sustained sodium current in entorhinal cortex layer-II principal neurons: a whole-cell and single-channel study

The functional and biophysical properties of a sustained, or "persistent," Na(+) current (I(NaP)) responsible for the generation of subthreshold oscillatory activity in entorhinal cortex layer-II principal neurons (the "stellate cells") were investigated with whole-cell, patch-clamp experiments. Both acutely dissociated cells and slices derived from adult rat entorhinal cortex were used. I(NaP), activated by either slow voltage ramps or long-lasting depolarizing pulses, was prominent in both isolated and, especially, in situ neurons. The analysis of the gating properties of the transient Na(+) current (I(NaT)) in the same neurons revealed that the resulting time-independent "window" current (I(NaTW)) had both amplitude and voltage dependence not compatible with those of the observed I(NaP), thus implying the existence of an alternative mechanism of persistent Na(+)-current generation. The tetrodotoxin-sensitive Na(+) currents evoked by slow voltage ramps decreased in amplitude with decreasing ramp slopes, thus suggesting that a time-dependent inactivation was taking place during ramp depolarizations. When ramps were preceded by increasingly positive, long-lasting voltage prepulses, I(NaP) was progressively, and eventually completely, inactivated. The V(1/2) of I(NaP) steady state inactivation was approximately -49 mV. The time dependence of the development of the inactivation was also studied by varying the duration of the inactivating prepulse: time constants ranging from approximately 6.8 to approximately 2.6 s, depending on the voltage level, were revealed. Moreover, the activation and inactivation properties of I(NaP) were such as to generate, within a relatively broad membrane-voltage range, a really persistent window current (I(NaPW)). Significantly, I(NaPW) was maximal at about the same voltage level at which subthreshold oscillations are expressed by the stellate cells. Indeed, at -50 mV, the I(NaPW) was shown to contribute to >80% of the persistent Na(+) current that sustains the subthreshold oscillations, whereas only the remaining part can be attributed to a classical Hodgkin-Huxley I(NaTW). Finally, the single-channel bases of I(NaP) slow inactivation and I(NaPW) generation were investigated in cell-attached experiments. Both phenomena were found to be underlain by repetitive, relatively prolonged late channel openings that appeared to undergo inactivation in a nearly irreversible manner at high depolarization levels (-10 mV), but not at more negative potentials (-40 mV).

Sodium channels, excitability of primary sensory neurons, and the molecular basis of pain

Following nerve injury, primary sensory neurons (dorsal root ganglion [DRG] neurons, trigeminal neurons) exhibit a variety of electrophysiological abnormalities, including increased baseline sensitivity and/or hyperexcitability, which can lead to abnormal burst activity that underlies pain, but the molecular basis for these changes has not been fully understood. Over the past several years, it has become clear that nearly a dozen distinct sodium channels are encoded by different genes and that at least six of these (including at least three distinct DRG- and trigeminal neuron-specific sodium channels) are expressed in primary sensory neurons. The deployment of different types of sodium channels in different types of DRG neurons endows them with different physiological properties. Dramatic changes in sodium channel expression, including downregulation of the SNS/PN3 and NaN sodium channel genes and upregulation of previously silent type III sodium channel gene, occur in DRG neurons following axonal transection. These changes in sodium channel gene expression are accompanied by a reduction in tetrodotoxin (TTX)-resistant sodium currents and by the emergence of a TTX-sensitive sodium current which recovers from inactivation (reprimes) four times more rapidly than the channels in normal DRG neurons. These changes in sodium channel expression poise DRG neurons to fire spontaneously or at inappropriately high frequencies. Changes in sodium channel gene expression also occur in experimental models of inflammatory pain. These observations indicate that abnormal sodium channel expression can contribute to the molecular pathophysiology of pain. They further suggest that selective blockade of particular subtypes of sodium channels may provide new, pharmacological approaches to treatment of disease involving hyperexcitability of primary sensory neurons.

High conductance sustained single-channel activity responsible for the low-threshold persistent Na(+) current in entorhinal cortex neurons

Stellate cells from entorhinal cortex (EC) layer II express both a transient Na(+) current (I(Na)) and a low-threshold persistent Na(+) current (I(NaP)) that helps to generate intrinsic theta-like oscillatory activity. We have used single-channel patch-clamp recording to investigate the Na(+) channels responsible for I(NaP) in EC stellate cells. Macropatch (more than six channels) recordings showed high levels of transient Na(+) channel activity, consisting of brief openings near the beginning of depolarizing pulses, and lower levels of persistent Na(+) channel activity, characterized by prolonged openings throughout 500 msec long depolarizations. The persistent activity contributed a noninactivating component to averaged macropatch recordings that was comparable with whole-cell I(NaP) in both voltage dependence of activation (10 mV negative to the transient current) and amplitude (1% of the transient current at -20 mV). In 14 oligochannel (less than six channels) patches, the ratio of transient to persistent channel activity varied from patch to patch, with 10 patches exhibiting exclusively transient openings and one patch showing exclusively persistent openings. In two patches containing only a single persistent channel, prolonged openings were observed in >50% of test depolarizations. Moreover, persistent openings had a significantly higher single-channel conductance (19.7 pS) than transient openings (15.6 pS). We conclude that this stable high-conductance persistent channel activity is responsible for I(NaP) in EC stellate cells. This persistent channel behavior is more enduring and has a higher conductance than the infrequent and short-lived transitions to persistent gating modes that have been described previously in brain neurons.

Mechanisms underlying spontaneous oscillation and rhythmic firing in rat subthalamic neurons

Subthalamic neurons drive basal ganglia output neurons in resting animals and relay cortical and thalamic activity to the same output neurons during movement. The first objective of this study was to determine the mechanisms underlying the spontaneous activity of subthalamic neurons in vitro and to gain insight into their resting discharge in vivo. The second objective was to determine the response of subthalamic neurons to depolarizing current injection and how intrinsic properties may shape their response to cortical and thalamic inputs during movement. Cell-attached and whole-cell recordings were made from subthalamic neurons in brain slices prepared from 3- to 4-week-old rats. The slow, rhythmic discharge of subthalamic neurons was resistant to blockade of excitatory synaptic transmission indicating that intrinsic currents underlie their spontaneous discharge. A persistent sodium current was the source of current during the depolarizing phase of the oscillation. A powerful afterhyperpolarization following each action potential was sufficient to terminate the depolarization. A long duration component of the spike afterhyperpolarization determined the period of the oscillation and was generated by an apamin-sensitive calcium-activated potassium current. Calcium entry responsible for that current was associated with action potentials. Subthalamic neurons exhibited a sigmoidal frequency-current relationship with the steeper portion starting at approximately 30-40 Hz. This property makes subthalamic neurons more sensitive to input at high firing rates associated with movement than at low rates associated with rest. We propose that the subthreshold persistent sodium current overcomes calcium activated potassium current which accumulates during high frequency firing and underlies the enhanced sensitivity to current >30 Hz.

Coding sequence, genomic organization, and conserved chromosomal localization of the mouse gene Scn11a encoding the sodium channel NaN

Previous studies have shown that sodium channel alpha-subunit NaN is preferentially expressed in small-diameter sensory neurons of dorsal root ganglia and trigeminal ganglia. These neurons include high-threshold nociceptors that are involved in transduction of pain associated with tissue and nerve injury. In this study, we show that mouse NaN is a 1765-amino-acid peptide that is predicted to produce a current that is resistant to tetrodotoxin (TTX-R). Mouse and rat NaN are 80 and 89% identical at the nucleotide and amino acid levels, respectively. The Scn11a gene encoding this cDNA is organized into 24 exons. Unlike some alpha-subunits, Scn11a does not have an alternative exon 5 in domain I. Introns of the U2 and U12 spliceosome types are present at conserved positions relative to other members of this family. Scn11a is located on mouse chromosome 9, close to the two other TTX-R sodium channel genes, Scn5a and Scn10a. The human gene, SCN11A, was mapped to the conserved linkage group on chromosome 3p21-p24, close to human SCN5A and SCN10A. The colocalization of the three sodium channel genes supports a common lineage of the TTX-R sodium channels.

The molecular pathophysiology of pain: abnormal expression of sodium channel genes and its contributions to hyperexcitability of primary sensory neurons

Although hyperexcitability and/or increased baseline sensitivity of primary sensory neurons following nerve injury can lead to abnormal burst activity associated with pain, the molecular mechanisms that contribute to it are not fully understood. Early studies demonstrated that, following axonal injury, neurons can display changes in excitability suggesting increased sodium channel expression. Consistent with this, abnormal accumulations of sodium channels have been observed at the tips of injured axons. But we now know that nearly a dozen distinct sodium channels are encoded by different genes, raising the question, what types of sodium channels underlie hyperexcitability of primary sensory neurons following injury? My laboratory has used molecular, electrophysiological, and pharmacological techniques to answer this question. Our studies have demonstrated that multiple sodium channels, with distinct physiological properties, are expressed within small dorsal root ganglion (DRG) neurons, which include nociceptive cells. Several DRG and trigeminal neuron-specific sodium channels have now been cloned and sequenced. There is a dramatic change in sodium channel expression in DRG neurons, with down-regulation of the SNS/PN3 and NaN sodium channel genes and up-regulation of previously silent Type III sodium channel gene, following injury to the axons of these cells. These changes in sodium channel gene expression can produce electrophysiological changes in DRG neurons which poise them to fire spontaneously or at inappropriate high frequencies. We have also observed changes in sodium channel gene expression in experimental models of inflammatory pain. The dynamic nature of sodium channel gene expression in DRG neurons, and the changes which occur in sodium channel and sodium current expression in these cells following axonal injury and in inflammatory pain models, suggest that abnormal expression of sodium channels contributes to the molecular pathophysiology of pain.

The pH dependence of late sodium current in large sensory neurons

The effects of altering extracellular pH on late Na+ currents were investigated in large dorsal root ganglion neurons from rats (100-300 g), using patch-clamp techniques. The late current amplitude was steeply dependent upon pH over a range which included normal physiological values: raising the pH from 7.3 to 8.3 approximately doubled the amplitude. Whole-cell late currents 60 ms after depolarization to - 30 mV were blocked with an apparent pKa of 6.96. The pH-dependent changes in current amplitude could not be accounted for by the effects of altered surface charge. In recordings of unitary Na+ currents from outside-out membrane patches, acidification promoted channel opening to a reduced conductance level, near one-half of its maximal value. Acidification to pH < 6.0 also changed the kinetics of the current recruited with the lowest threshold from non-inactivating to inactivating, with the elimination of late openings. We conclude that lowering pH from an initial alkaline or neutral value blocks late Na+ current by reducing the number of contributing channels while also reducing the single channel conductance. The pH dependence of late Na+ current helps to explain clinically relevant changes in neuronal excitability in response to small (i.e. < 1 unit) perturbations in extracellular pH.

Sodium channels and pain

Although it is well established that hyperexcitability and/or increased baseline sensitivity of primary sensory neurons can lead to abnormal burst activity associated with pain, the underlying molecular mechanisms are not fully understood. Early studies demonstrated that, after injury to their axons, neurons can display changes in excitability, suggesting increased sodium channel expression, and, in fact, abnormal sodium channel accumulation has been observed at the tips of injured axons. We have used an ensemble of molecular, electrophysiological, and pharmacological techniques to ask: what types of sodium channels underlie hyperexcitability of primary sensory neurons after injury? Our studies demonstrate that multiple sodium channels, with distinct electrophysiological properties, are encoded by distinct mRNAs within small dorsal root ganglion (DRG) neurons, which include nociceptive cells. Moreover, several DRG neuron-specific sodium channels now have been cloned and sequenced. After injury to the axons of DRG neurons, there is a dramatic change in sodium channel expression in these cells, with down-regulation of some sodium channel genes and up-regulation of another, previously silent sodium channel gene. This plasticity in sodium channel gene expression is accompanied by electrophysiological changes that poise these cells to fire spontaneously or at inappropriate high frequencies. Changes in sodium channel gene expression also are observed in experimental models of inflammatory pain. Thus, sodium channel expression in DRG neurons is dynamic, changing significantly after injury. Sodium channels within primary sensory neurons may play an important role in the pathophysiology of pain.

Molecular and functional remodeling of electrogenic membrane of hypothalamic neurons in response to changes in their input

Neurons respond to stimuli by integrating generator and synaptic potentials and generating action potentials. However, whether the underlying electrogenic machinery within neurons itself changes, in response to alterations in input, is not known. To determine whether there are changes in Na+ channel expression and function within neurons in response to altered input, we exposed magnocellular neurosecretory cells (MNCs) in the rat supraoptic nucleus to different osmotic milieus by salt-loading and studied Na+ channel mRNA and protein, and Na+ currents, in these cells. In situ hybridization demonstrated significantly increased mRNA levels for alpha-II, Na6, beta1 and beta2 Na+ channel subunits, and immunohistochemistry/immunoblotting showed increased Na+ channel protein after salt-loading. Using patch-clamp recordings to examine the deployment of functional Na+ channels in the membranes of MNCs, we observed an increase in the amplitude of the transient Na+ current after salt-loading and an even greater increase in amplitude and density of the persistent Na+ current evoked at subthreshold potentials by slow ramp depolarizations. These results demonstrate that MNCs respond to salt-loading by selectively synthesizing additional, functional Na+ channel subtypes whose deployment in the membrane changes its electrogenic properties. Thus, neurons may respond to changes in their input not only by producing different patterns of electrical activity, but also by remodeling the electrogenic machinery that underlies this activity.

In vivo NGF deprivation reduces SNS expression and TTX-R sodium currents in IB4-negative DRG neurons

Recent evidence suggests that changes in sodium channel expression and localization may be involved in some pathological pain syndromes. SNS, a tetrodotoxin-resistant (TTX-R) sodium channel, is preferentially expressed in small dorsal root ganglion (DRG) neurons, many of which are nociceptive. TTX-R sodium currents and SNS mRNA expression have been shown to be modulated by nerve growth factor (NGF) in vitro and in vivo. To determine whether SNS expression and TTX-R currents in DRG neurons are affected by reduced levels of systemic NGF, we immunized adult rats with NGF, which causes thermal hypoalgesia in rats with high antibody titers to NGF. DRG neurons cultured from rats with high antibody titers to NGF, which do not bind the isolectin IB4 (IB4(-)) but do express TrkA, were studied with whole cell patch-clamp and in situ hybridization. Mean TTX-R sodium current density was decreased from 504 +/- 77 pA/pF to 307 +/- 61 pA/pF in control versus NGF-deprived neurons, respectively. In comparison, the mean TTX-sensitive sodium current density was not significantly different between control and NGF-deprived neurons. Quantification of SNS mRNA hybridization signal showed a significant decrease in the signal in NGF-deprived neurons compared with the control neurons. The data suggest that NGF has a major role in the maintenance of steady-state levels of TTX-R sodium currents and SNS mRNA in IB4(-) DRG neurons in adult rats in vivo.