NaN/Nav1.9: a sodium channel with unique properties

Analysis of the variation in use-dependent inactivation of high-threshold tetrodotoxin-resistant sodium currents recorded from rat sensory neurons

This study addressed variation in the use-dependent inactivation (UDI) of high-threshold tetrodotoxin-resistant Na+ currents (TTX-R currents) and action potential firing behavior among acutely isolated rat dorsal root ganglion (DRG) cells. UDI was quantified as the percent decrease in current amplitude caused by increasing the current activation rate from 0.1-1.0 Hz for 20 s. TTX-R current UDI varied from 6% to 66% among 122 DRG cells examined, suggesting the existence of two or more levels of UDI. The voltage-dependency of the TTX-R currents was consistent with Na(V)1.8, regardless of UDI. However, TTX-R currents with more UDI had a more negative voltage-dependency of inactivation, a greater tendency to enter slow inactivation, and a slower recovery rate from slow inactivation, compared with those with less UDI. TTX-R currents with more UDI ran down faster than those with less UDI. However, UDI itself changed little over time, regardless of the initial UDI level observed in a particular DRG cell. Together, these two observations suggest that individual DRG cells did not express mixtures of TTX-R channels that varied regarding UDI. TTX-R current UDI was correlated with expression of a low-threshold A-current and whole-cell capacitance, suggesting that it varied among different nociceptor types. Whole-cell inward currents (WCI-currents), recorded without channel blockers, also exhibited UDI. WCI-current UDI varied similarly to TTX-R current UDI in magnitude, and relative to whole-cell capacitance and A-current expression, suggesting that the WCI-currents were carried predominantly by TTX-R channels. DRG cells with more WCI-current UDI exhibited a greater decrease in action potential amplitude and number, and a greater increase in action potential threshold over seven ramp depolarizations, compared with DRG cells with less WCI-current UDI. Variation in UDI of Na(V)1.8 channels expressed by different nociceptor types could contribute to shaping their individual firing patterns in response to noxious stimuli.

Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

This review focuses on the role of the peripheral nervous system in cutaneous biology and disease. During the last few years, a modern concept of an interactive network between cutaneous nerves, the neuroendocrine axis, and the immune system has been established. We learned that neurocutaneous interactions influence a variety of physiological and pathophysiological functions, including cell growth, immunity, inflammation, pruritus, and wound healing. This interaction is mediated by primary afferent as well as autonomic nerves, which release neuromediators and activate specific receptors on many target cells in the skin. A dense network of sensory nerves releases neuropeptides, thereby modulating inflammation, cell growth, and the immune responses in the skin. Neurotrophic factors, in addition to regulating nerve growth, participate in many properties of skin function. The skin expresses a variety of neurohormone receptors coupled to heterotrimeric G proteins that are tightly involved in skin homeostasis and inflammation. This neurohormone-receptor interaction is modulated by endopeptidases, which are able to terminate neuropeptide-induced inflammatory or immune responses. Neuronal proteinase-activated receptors or transient receptor potential ion channels are recently described receptors that may have been important in regulating neurogenic inflammation, pain, and pruritus. Together, a close multidirectional interaction between neuromediators, high-affinity receptors, and regulatory proteases is critically involved to maintain tissue integrity and regulate inflammatory responses in the skin. A deeper understanding of cutaneous neuroimmunoendocrinology may help to develop new strategies for the treatment of several skin diseases.

Characterization of Myenteric Sensory Neurons in the Mouse Small Intestine

We recorded from myenteric AH/Dogiel type II cells, demonstrated mechanosensitive responses, and characterized their basic properties. Recordings were obtained using the mouse longitudinal muscle myenteric plexus preparation with patch-clamp and sharp intracellular electrodes. The neurons had an action potential hump and a slow afterhyperpolarization (AHP) current. The slow AHP was carried by intermediate conductance Ca2+-dependent K+-channel currents sensitive to charybdotoxin and clotrimazole. All possessed a hyperpolarization-activated current that was blocked by extracellular cesium. They also expressed a TTX-resistant Na+ current with an onset near the resting potential. Pressing on the ganglion containing the patched neuron evoked depolarizing potentials in 17/18 cells. The potentials persisted after synaptic transmission was blocked. Volleys of presynaptic electrical stimuli evoked slow excitatory postsynaptic potentials (EPSPs) in 9/11 sensory neurons, but 0/29 cells received fast EPSP input. The slow EPSP was generated by removal of a voltage-insensitive K+ current. Patch-clamp recording with a KMeSO4-containing, but not a conventional KCl-rich, intracellular solution reproduced the single-spike slow AHPs and low input resistances seen with sharp intracellular recording. Cell-attached recording of intermediate conductance potassium channels supported the conclusion that the single-spike slow AHP is an intrinsic property of intestinal AH/sensory neurons. Unitary current recordings also suggested that the slow AHP current probably does not contribute significantly to the high resting background conductance seen in these cells. The characterization of mouse myenteric sensory neurons opens the way for the study of their roles in normal and pathological physiology.

The effect of dibutyryl cAMP on tetrodotoxin-sensitive and -resistant voltage-gated sodium currents in rat dorsal root ganglion neurons and the consequences for their sensitivity to lidocaine

Tetrodotoxin-sensitive (TTXS) sodium currents in dorsal root ganglia (DRG) neurons were enhanced by DcAMP applied acutely or by pre-treatment. Pre-treatment increased peak TTXS by 28%. This compared to the increase of tetrodotoxin-resistant sodium currents (TTXR) of 123%. In both cases the increase was associated with a hyperpolarizing shift in activation potentials. Slow inactivation was slower for both TTXR and TTXS in DcAMP treated neurons but rates of recovery from inactivation were not altered. Lidocaine blocked TTX-R with an IC(50) of 0.51+/-0.15mM (n=9) which was reduced to 0.14+/-0.05mM (n=8, P<0.05) in DcAMP treated cells. The sensitivity of TTX-S currents to lidocaine was not altered by DcAMP (control EC(50)=0.89+/-0.16mM, n=9; DcAMP EC(50)=0.73+/-0.19mM, n=6). It is concluded that TTXS currents in DRG are, like TTX-R currents, enhanced by cAMP but whilst the pharmacology of TTXR channels with respect to lidocaine is altered, that to TTXS channels is not.

Sodium channel beta2 subunits regulate tetrodotoxin-sensitive sodium channels in small dorsal root ganglion neurons and modulate the response to pain

Voltage-gated sodium channel (Na(v)1) beta2 subunits modulate channel gating, assembly, and cell-surface expression in CNS neurons in vitro and in vivo. beta2 expression increases in sensory neurons after nerve injury, and development of mechanical allodynia in the spared nerve injury model is attenuated in beta2-null mice. Thus, we hypothesized that beta2 modulates electrical excitability in dorsal root ganglion (DRG) neurons in vivo. We compared sodium currents (I(Na)) in small DRG neurons from beta2+/+ and beta2-/- mice to determine the effects of beta2 on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na(v)1 in vivo. Small-fast DRG neurons acutely isolated from beta2-/- mice showed significant decreases in TTX-S I(Na) compared with beta2+/+ neurons. This decrease included a 51% reduction in maximal sodium conductance with no detectable changes in the voltage dependence of activation or inactivation. TTX-S, but not TTX-R, I(Na) activation and inactivation kinetics in these cells were slower in beta2(-/-) mice compared with controls. The selective regulation of TTX-S I(Na) was supported by reductions in transcript and protein levels of TTX-S Na(v)1s, particularly Na(v)1.7. Low-threshold mechanical sensitivity was preserved in beta2-/- mice, but they were more sensitive to noxious thermal stimuli than wild type whereas their response during the late phase of the formalin test was attenuated. Our results suggest that beta2 modulates TTX-S Na(v)1 mRNA and protein expression resulting in increased TTX-S I(Na) and increases the rates of TTX-S Na(v)1 activation and inactivation in small-fast DRG neurons in vivo. TTX-R I(Na) were not significantly modulated by beta2.

Impaired nociception and inflammatory pain sensation in mice lacking the prokineticin receptor PKR1: focus on interaction between PKR1 and the capsaicin receptor TRPV1 in pain behavior

Bv8, prokineticin-1 or EG-VEGF (endocrine gland-derived vascular endothelial growth factor), and prokineticin-2, are naturally occurring peptide agonists of two G-protein-coupled receptors (GPCRs), prokineticin receptor 1 (PKR1) and PKR2. PKRs are expressed in neurons in the CNS and peripheral nervous system and many dorsal root ganglion (DRG) cells expressing PKRs also express transient receptor potential vanilloid receptor-1 (TRPV1). Mice lacking the pkr1 gene were generated to explore the role of the PKR1 receptor in nociceptive signaling and in nociceptor sensitization. When compared with wild-type littermates, mice lacking the pkr1 gene showed impaired responsiveness to noxious heat, mechanical stimuli, capsaicin, and protons. In wild-type mice, activation of PKRs by the PKR agonist Bv8 caused hyperalgesia and sensitized to the actions of capsaicin. pkr1-null mice exhibited impaired responses to Bv8 but showed normal hyperalgesic responses to bradykinin and PGE2 (prostaglandin E2). Conversely, trpv1-null mice showed a reduced pronociceptive response to Bv8. Additionally, pkr1-null mice showed diminished thermal hyperalgesia after acute inflammation elicited by mustard oil and reduced pain behavior after chronic inflammation produced by complete Freund's adjuvant. The number of neurons that responded with a [Ca2+]i increase to Bv8 exposure was five times lower in pkr1-null DRG cultures than in wild-type cultures. Furthermore, Bv8-responsive neurons from pkr1-null mice showed a significant reduction in the [Ca2+]i response to capsaicin. These findings indicate a modulatory role of PKR1 in acute nociception and inflammatory pain and disclose a pharmacological interaction between PKR1 and TRPV1 in nociceptor activation and sensitization.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

Where is the spike generator of the cochlear nerve? Voltage-gated sodium channels in the mouse cochlea

The origin of the action potential in the cochlea has been a long-standing puzzle. Because voltage-dependent Na+ (Nav) channels are essential for action potential generation, we investigated the detailed distribution of Nav1.6 and Nav1.2 in the cochlear ganglion, cochlear nerve, and organ of Corti, including the type I and type II ganglion cells. In most type I ganglion cells, Nav1.6 was present at the first nodes flanking the myelinated bipolar cell body and at subsequent nodes of Ranvier. In the other ganglion cells, including type II, Nav1.6 clustered in the initial segments of both of the axons that flank the unmyelinated bipolar ganglion cell bodies. In the organ of Corti, Nav1.6 was localized in the short segments of the afferent axons and their sensory endings beneath each inner hair cell. Surprisingly, the outer spiral fibers and their sensory endings were well labeled beneath the outer hair cells over their entire trajectory. In contrast, Nav1.2 in the organ of Corti was localized to the unmyelinated efferent axons and their endings on the inner and outer hair cells. We present a computational model illustrating the potential role of the Nav channel distribution described here. In the deaf mutant quivering mouse, the localization of Nav1.6 was disrupted in the sensory epithelium and ganglion. Together, these results suggest that distinct Nav channels generate and regenerate action potentials at multiple sites along the cochlear ganglion cells and nerve fibers, including the afferent endings, ganglionic initial segments, and nodes of Ranvier.

Hyperalgesia and allodynia: peripheral mechanisms

Nociceptive signals are generated by peripheral sensory organs called nociceptors, which are endings of small-diameter nerve fibers responsive to the tissue environment. The myriad chemical mediators capable of activating, sensitizing, or arousing nociceptors include kinins, proinflammatory and anti-inflammatory cytokines, prostanoids, lipooxygenases, the "central immune response mediator" NF-kappaB, neurotrophins and other growth factors, neuropeptides, nitric oxide, histamine, serotonin, proteases, excitatory amino acids, adrenergic amines, and opioids. These mediators may act in combination or at a given time in the inflammatory process, producing subtle changes that result in hyperalgesia or allodynia. We will review the most extensively studied molecular and cellular mechanisms underlying these two clinical abnormalities. The role of the peripheral nervous system in progression of inflammatory joint disease to chronicity is discussed.

Relationship between sodium channel NaV1.3 expression and neuropathic pain behavior in rats

A multitude of voltage-gated sodium channel subtypes (NaV1) are expressed in primary sensory neurons where they influence excitability via their role in the generation and propagation of action potentials. Peripheral nerve injury alters the expression of several NaV1subtypes, but among these only NaV1.3 is up-regulated in dorsal root ganglia (DRG) neurons. The increased expression of NaV1.3 implicates this subtype in the development and maintenance of neuropathic pain, but its contribution to neuropathic pain behavior has not been examined. Using the spared nerve injury (SNI) model, we found that peripheral nerve lesion increased NaV1.3-like immunoreactivity (-LI) in DRG neurons and that mechanical allodynia was partially alleviated following oral administration of two NaV1 blockers, mexiletine (30 and 100 mg/kg, p.o.) and lamotrigine (30 and 100 mg/kg, p.o.). Intrathecal administration of antisense oligonucleotides (4 days) selective for NaV1.3 decreased NaV1.3 immunostaining in the DRG by 50% in the SNI model, but did not attenuate mechanical or cold allodynia. Moreover, we found that only 18% of NaV1.3 positive neurons also expressed activated transcription factor-3 (ATF3), a marker of injured neurons. We then selectively axotomized a cutaneous nerve (sural) and a muscle nerve (gastrocnemius) in order to identify if NaV1.3 up-regulation is dependent on cutaneous and/or muscle afferent activation and found that the numbers of neurons expressing NaV1.3 was proportional to the magnitude of the injury, but independent of the nature of innervation. These results suggest that NaV1.3 increases in primary sensory neurons that are not directly damaged in response to injury. Thus, although NaV1.3 is up-regulated in a subpopulation of DRG neurons after injury, reduction in the expression of NaV1.3 subtype alone is not sufficient to influence the NaV1-dependent behavioral hypersensitivity associated with nerve injury.

A novel conotoxin from Conus striatus, mu-SIIIA, selectively blocking rat tetrodotoxin-resistant sodium channels

Mu-conotoxin SIIIA, a novel blocker of tetrodotoxin-resistant (TTX-R) voltage-gated sodium channels (VGSCs) has been identified from the fish-hunting cone snail, Conus striatus. The deduced sequence consists of a 20-residue signal peptide, a 31-residue pro-peptide, and a 20-residue mature toxin with its N-terminal Gln cyclized and C-terminus amidated. Mu-SIIIA shares the common cysteine arrangement with other mu-conotoxins. Besides, it exhibits high sequence homology with mu-SmIIIA, a toxin recently characterized from C. stercusmuscarum which potently blocks the TTX-R VGSCs in frog neurons. With whole-cell recording, mu-SIIIA potently and selectively inhibits the TTX-R VGSCs of dissociated adult rat small-diameter dorsal root ganglia (DRG) neurons with a dose- and time-dependent property and irreversibly. Homology-based modeling of mu-PIIIA, SIIIA and SmIIIA implies that they share a common backbone conformation except at the N termini. The hydroxyl-proline residue only present in mu-PIIIA is absent and substituted by an Asp residue in mu-SIIIA and SmIIIA. Similarly, one crucial basic residue (Arg12 in mu-PIIIA) is replaced by serine in the latter two toxins. Such differences might endow them with the capacity to selectively inhibit TTX-S or TTX-R VGSCs. Considering that TTX-R VGSCs predominantly expressed in DRG neurons play pivotal roles in the nociceptive information transmission and that their specific antagonists are still lacking, mu-SIIIA might provide a useful tool for functional studies of these channels, and potentially be developed as an efficient pain killer.

Actions of brain-derived neurotrophic factor on spinal nociceptive transmission during inflammation in the rat

The aim of the current study was to investigate whether, and if so how, brain-derived neurotrophic factor (BDNF) acts to develop the spinal sensitization underlying inflammation-induced hyperalgesia. In spinal cord slice preparations from rats with inflammation induced by complete Freund's adjuvant (CFA), BDNF, but not nerve growth factor (NGF) or neurotrophin-3 (NT-3), acted presynaptically to increase the frequency of excitatory miniature EPSCs in substantia gelatinosa (SG) neurones of the CFA-treated, but not untreated rats, through activation of lidocaine (lignocaine)-sensitive, TTX-resistant Na+ channels. This effect was observed in the spinal cord slices of the CFA-treated rat only 2-4 days after the CFA injection. On the other hand, the number of monosynaptic Abeta afferent inputs to the SG significantly increased 1 week after the onset of the inflammation, and this increase was significantly suppressed by treatment with anti-BDNF antiserum administered 1 day before and just after the CFA injection. In addition, the treatment with anti-BDNF antiserum significantly attenuated the CFA-induced hyperalgesia and/or allodynia. These findings, taken together, suggest that BDNF, which is considered to be released from the sensitized primary afferents, increases the excitability of SG neurones through its action on the presynaptic terminals. BDNF may thereafter induce monosynaptic Abeta afferents to the SG, thereby developing hyperalgesia and/or allodynia during inflammation.

Electrophysiological characterization of the tetrodotoxin-resistant Na+ channel, Na(v)1.9, in mouse dorsal root ganglion neurons

Small dorsal root ganglion neurons express preferentially the Na+ channel isoform Na(v)1.9 that mediates a tetrodotoxin-resistant (TTX-R) Na+ current. We investigated properties of the Na+ current mediated by Na(v)1.9 (I(NaN)) using the whole-cell, patch-clamp recording technique. To isolate I(NaN) from heterogeneous TTX-R Na+ currents that also contain another type of TTX-R Na+ current mediated by Na(v)1.8, we used Na(v)1.8-null mutant mice. When F- was used as an internal anion in the patch pipette solution, both the activation and inactivation kinetics for I(NaN) shifted in the hyperpolarizing direction with time. Such a time-dependent shift of the kinetics was not observed when Cl- was used as an internal anion. Functional expression of I(NaN) declined with time after cell dissociation and recovered during culture, implying that Na(v)1.9 may be regulated dynamically by trophic factors or depend on subtle environmental factors for its survival. During whole-cell recordings, the peak amplitude of I(NaN) increased dramatically after a variable delay, as if inactive or silent channels had been "kindled". Such an unusual increase of the amplitude could be prevented by adding ATP to the pipette solution or by recording with the nystatin-perforated patch-clamp technique, suggesting that the rupture of patch membrane affected the behaviour of Na(v)1.9. These peculiar properties of I(NaN) may provide an insight into the plasticity of Na+ channels that are related to pathological functions of Na+ channels accompanying abnormal pain states.

Fibroblast growth factor 14 is an intracellular modulator of voltage-gated sodium channels

Genetic ablation of the fibroblast growth factor (Fgf) 14 gene in mice or a missense mutation in Fgf14 in humans causes ataxia and cognitive deficits. These phenotypes suggest that the neuronally expressed Fgf14 gene is essential for regulating normal neuronal activity. Here, we demonstrate that FGF14 interacts directly with multiple voltage-gated Na(+) (Nav) channel alpha subunits heterologously expressed in non-neuronal cells or natively expressed in a murine neuroblastoma cell line. Functional studies reveal that these interactions result in the potent inhibition of Nav channel currents (I(Na)) and in changes in the voltage dependence of channel activation and inactivation. Deletion of the unique amino terminus of the splice variant of Fgf14, Fgf14-1b, or expression of the splice variant Fgf14-1a modifies the modulatory effects on I(Na), suggesting an important role for the amino terminus domain of FGF14 in the regulation of Na(v) channels. To investigate the function of FGF14 in neurones, we directly expressed Fgf14 in freshly isolated primary rat hippocampal neurones. In these cells, the addition of FGF14-1a-GFP or FGF14-1b-GFP increased I(Na) density and shifted the voltage dependence of channel activation and inactivation. In fully differentiated neurones, FGF14-1a-GFP or FGF14-1b-GFP preferentially colocalized with endogenous Nav channels at the axonal initial segment, a critical region for action potential generation. Together, these findings implicate FGF14 as a unique modulator of Nav channel activity in the CNS and provide a possible mechanism to explain the neurological phenotypes observed in mice and humans with mutations in Fgf14.

Persistent sodium current is a target for cAMP-induced neuronal plasticity in a state-setting modulatory interneuron

We have identified a TTX-resistant low-threshold persistent inward sodium current in the cerebral giant cells (CGCs) of Lymnaea, an important state-setting modulatory cell type of molluscan feeding networks. This current has slow voltage-dependent activation and de-activation kinetics, ultra-slow inactivation kinetics and fast de-inactivation kinetics. It activates at approximately -90 mV, peaks at approximately -30 mV, reverses at approximately +35 mV and does not show full voltage-dependent inactivation even at positive voltage steps. Lithium-sodium replacement experiments indicate that the persistent sodium current makes a significant contribution to the CGC membrane potential. Injection of cyclic adenosine monophosphate (cAMP) into the CGC cell body produces a large increase in the persistent sodium current that lasts for several hours. cAMP injection also leads to increased bursting, a significant decrease in the resistance and a significant depolarization of the soma membrane, indicating that cAMP-dependent mechanisms induce prolonged neuronal plasticity in the CGCs. Our observations provide the first link between cAMP-mediated modulation of a TTX-resistant persistent sodium current and prolonged neuronal plasticity in an identified modulatory cell type that plays an important role in behavioral state setting.

Protein kinase C mediates up-regulation of tetrodotoxin-resistant, persistent Na+ current in rat and mouse sensory neurones

The tetrodotoxin-resistant (TTX-r) persistent Na(+) current, attributed to Na(V)1.9, was recorded in small (< 25 mum apparent diameter) dorsal root ganglion (DRG) neurones cultured from P21 rats and from adult wild-type and Na(V)1.8 null mice. In conventional whole-cell recordings intracellular GTP-gamma-S caused current up-regulation, an effect inhibited by the PKC pseudosubstrate inhibitor, PKC19-36. The current amplitude was also up-regulated by 25 microM intracellular 1-oleoyl-2-acetyl-sn-glycerol (OAG) consistent with PKC involvement. In perforated-patch recordings, phorbol 12-myristate 13-acetate (PMA) up-regulated the current, whereas membrane-permeant activators of protein kinase A (PKA) were without effect. PGE(2) did not acutely up-regulate the current. Conversely, both PGE(2) and PKA activation up-regulated the major TTX-r Na(+) current, Na(V)1.8. Extracellular ATP up-regulated the persistent current with an average apparent K(d) near 13 microM, possibly consistent with P2Y receptor activation. Numerical simulation of the up-regulation qualitatively reproduced changes in sensory neurone firing properties. The activation of PKC appears to be a necessary step in the GTP-dependent up-regulation of persistent Na(+) current.

Pharmacological properties of neuronal TTX-resistant sodium channels and the role of a critical serine pore residue

Voltage-gated sodium channels can be characterized by their sensitivity to inhibitors. Na(v)1.5 is sensitive to block by cadmium and extracellular QX-314, but relatively insensitive to tetrodotoxin and saxitoxin. Na(v)1.4 is tetrodotoxin- and saxitoxin-sensitive but resistant to cadmium and extracellular QX-314. Na(v)1.8 and Na(v)1.9 generate slowly inactivating (I(TTXr-Slow)) and persistent (I(TTXr-Per)) currents in sensory neurons that are tetrodotoxin-resistant. Tetrodotoxin sensitivity is largely determined by the identity of a single residue; tyrosine 401 in Na(v)1.4, cysteine 374 in Na(v)1.5 and serine 356 and 355 in Na(v)1.8 and Na(v)1.9. We asked whether Na(v)1.8 and Na(v)1.9 share other pharmacological properties as a result of this serine residue. I(TTXr-Slow) and I(TTXr-Per) were saxitoxin-resistant and resistant to internal QX-314. I(TTXr-Slow) was also resistant to external QX-314 and displayed a approximately fourfold higher sensitivity than I(TTXr-Per) to cadmium. The impact of the serine residue was investigated by replacing tyrosine 401 in Na(v)1.4 with serine (Y401S) or cysteine (Y401C). Both mutants were resistant to tetrodotoxin and saxitoxin. Whereas Na(v)1.4-Y401C displayed an increased sensitivity to cadmium and extracellular QX-314, the serine substitution did not alter the sensitivity of Na(v)1.4 to cadmium or QX-314. Our data indicates that while the serine residue determines the sensitivity of I(TTXr-Slow) and I(TTXr-Per) to tetrodotoxin and saxitoxin, it does not determine their insensitivity to QX-314 or their differential sensitivities to cadmium.

Inflammation-induced hyperexcitability of nociceptive gastrointestinal DRG neurones: the role of voltage-gated ion channels

Gastrointestinal (GI) inflammation modulates the intrinsic properties of nociceptive dorsal root ganglia neurones, which innervate the GI tract and these changes are important in the genesis of abdominal pain and visceral hyperalgesia neurones exhibit hyperexcitability characterized by a decreased threshold for activation and increased firing rate, and changes in voltages-gated Na(+) and K(+) channels play a major role in this plasticity. This review highlights emerging evidence that specific subsets of channels and signalling pathways are involved and their potential to provide novel selective therapeutics targets for the treatment of abdominal pain.

Low voltage-activated calcium and fast tetrodotoxin-resistant sodium currents define subtypes of cholinergic and noncholinergic neurons in rat basal forebrain

Neurons of the basal forebrain (BF) possess unique combinations of voltage-gated membrane currents. Here, we describe subtypes of rat basal forebrain neurons based on patch-clamp analysis of low-voltage activated (LVA) calcium and tetrodotoxin-resistant (TTX-R) sodium currents combined with single-cell RT-PCR analysis. Neurons were identified by mRNA expression of choline acetyltransferase (ChAT+, cholinergic) and glutamate decarboxylase (GAD67, GABAergic). Four cell types were encountered: ChAT+, GAD+, ChAT+/GAD+ and ChAT-/GAD- cells. Both ChAT+ and ChAT+/GAD+ cells (71/75) displayed LVA currents and most (34/39) expressed mRNA for LVA Ca(2+) channel subunits. Ca(v)3.2 was detected in 31/34 cholinergic neurons and Ca(v)3.1 was expressed in 6/34 cells. Three cells expressed both subunits. No single neurons showed Ca(v)3.3 mRNA expression, although BF tissue expression was observed. In young rats (2-4 mo), ChAT+/GAD+ cells displayed larger LVA current densities compared to ChAT+ neurons, while these latter neurons displayed an age-related increase in current densities. Most (29/38) noncholinergic neurons (GAD+ and ChAT-/GAD-) possessed fast TTX-R sodium currents resembling those mediated by Na(+) channel subunit Na(v)1.5. This subunit was expressed predominately in noncholinergic neurons. No cholinergic cells (0/75) displayed fast TTX-R currents. The TTX-R currents were faster and larger in GAD+ neurons compared to ChAT-/GAD- neurons. The properties of ChAT+/GAD+ neurons resemble those of ChAT+ neurons, rather than of GAD+ neurons. These results suggest novel features of subtypes of cholinergic and noncholinergic neurons within the BF that may provide new insights for understanding normal BF function.

From modulator to mediator: rapid effects of BDNF on ion channels

Neurotrophins (NTs) are [?AUTHOR] a family of structurally related, secreted proteins that regulate the survival, differentiation and maintenance of function of different populations of peripheral and central neurons.1,2 Among these, BDNF (brain-derived neurotrophic factor) has drawn considerable interest because both its synthesis and secretion are increased by physiological levels of activity, indicating a unique role of this neurotrophin in coupling neuronal activity to structural and functional properties of neuronal circuits. In addition to its classical neurotrophic effects, which are evident within hours or days and which usually result from changes in cellular gene expression, BDNF exerts acute effects on synaptic transmission and is involved in the induction of long-term potentiation. Many of these rapid effects of BDNF are mediated by its modulation of ion channel properties following TrkB-mediated activation of intracellular second messenger cascades and protein phosphorylation. However, recent reports have shown that BDNF not only acts as a modulator of ion channels, but can also directly and rapidly gate a Na(+) channel, thereby assigning BDNF the properties of a classical excitatory transmitter. Thus, BDNF, in addition its role as a potent neuromodulator, emerges as an excitatory transmitter-like substance which acutely controls resting membrane potential, neuronal excitability, synaptic transmission and participates in the induction of synaptic plasticity.

Posttranslational mechanisms of peripheral sensitization

The sensation of pain can be dramatically altered in response to injury or disease. This sensitization can occur at the level of the primary sensory neuron, and can be mediated by multiple biochemical mechanisms, including, but not limited to, changes in gene transcription, changes in translation, stability, or subcellular localization of translated proteins, and posttranslational modifications. This review focuses on posttranslational modifications to ion channels expressed in primary sensory neurons that form the machinery driving peripheral sensitization and pain hypersensitivity. Studies published to date show strong evidence for modulation of ion channels involved in transduction and transmission of nociceptive inputs coincident with biophysical and behavioral sensitization. The roles of phosphorylation and oxidation/reduction reactions of voltage-dependent sodium, potassium, and calcium channels are discussed, as well as phosphorylation-mediated modulation of sensory transduction channels.

The neurobiology of the ascidian tadpole larva: recent developments in an ancient chordate

With little more than 330 cells, two thirds within the sensory vesicle, the CNS of the tadpole larva of the ascidian Ciona intestinalis provides us with a chordate nervous system in miniature. Neurulation, neurogenesis and its genetic bases, as well as the gene expression territories of this tiny constituency of cells all follow a chordate plan, giving rise in some cases to frank structural homologies with the vertebrate brain. Recent advances are fueled by the release of the genome and EST expression databases and by the development of methods to transfect embryos by electroporation. Immediate prospects to test the function of neural genes are based on the isolation of mutants by classical genetics and insertional mutagenesis, as well as by the disruption of gene function by morpholino antisense oligo-nucleotides. Coupled with high-speed video analysis of larval swimming, optophysiological methods offer the prospect to analyze at single-cell level the function of a CNS built on a vertebrate plan.

Alteration of the second branch of the trigeminal nerve activity following inferior alveolar nerve transection in rats

After transection of the inferior alveolar nerve (IAN), the whisker pad area, which is innervated by the infraorbital nerve (ION) that was not injured, showed hypersensitivity to mechanical stimulation. Two days after IAN transection, threshold intensity for escape behavior to mechanical stimulation of the ipsilateral whisker pad area was less than 4.0 g, indicating mechanical allodynia. A total of 68 single fiber discharges were recorded from ION fibers at 3 days after IAN transection. The responses of C- and A-fibers were classified according to their conduction velocity. The C-fiber activities were not affected by IAN transection, whereas A-fiber activities were significantly enhanced by IAN transection as indicated by an increase in background activity and mechanically evoked response. Since the A-fiber responses were significantly affected by IAN transection, patch clamp recording was performed from middle to large diameter retrogradely labeled and acutely dissociated trigeminal ganglion (TRG) neurons. The I(K) (sustained) and I(A) (transient) currents were significantly smaller and hyperpolarization-activated current (I(h)) was significantly larger in TRG neurons of rats with IAN transection as compared to those of naive rats. Furthermore, current injection into TRG neurons induced high frequency spike discharges in rats with IAN transection. These data suggest that changes in K(+) current and I(h) observed in the uninjured TRG neurons reflect an increase in excitability of TRG neurons innervated by the ION after IAN transection, resulting in the development of mechano-allodynia in the area adjacent to the injured IAN innervated region.

Two TTX-resistant Na+ currents in mouse colonic dorsal root ganglia neurons and their role in colitis-induced hyperexcitability

The composition of Na+ currents in dorsal root ganglia (DRG) neurons depends on their neuronal phenotype and innervation target. Two TTX-resistant (TTX-R) Na+ currents [voltage-gated Na channels (Nav)] have been described in small DRG neurons; one with slow inactivation kinetics (Nav1.8) and the other with persistent kinetics (Nav1.9), and their modulation has been implicated in inflammatory pain. This has not been studied in neurons projecting to the colon. This study examined the relative importance of these currents in inflammation-induced changes in a mouse model of inflammatory bowel disease. Colonic sensory neurons were retrogradely labeled, and colitis was induced by instillation of trinitrobenzenesulfonic acid (TNBS) into the lumen of the distal colon. Seven to ten days later, immunohistochemical properties were characterized in controls, and whole cell recordings were obtained from small (<40 pF) labeled DRG neurons from control and TNBS animals. Most neurons exhibited both fast TTX-sensitive (TTX-S)- and slow TTX-R-inactivating Na+ currents, but persistent TTX-R currents were uncommon (<15%). Most labeled neurons were CGRP (79%), tyrosine kinase A (trkA) (84%) immunoreactive, but only a small minority bind IB4 (14%). TNBS-colitis caused ulceration, thickening of the colon and significantly increased neuronal excitability. The slow TTX-R-inactivating Na current density (Nav1.8) was significantly increased, but other Na currents were unaffected. Most small mouse colonic sensory neurons are CGRP, trkA immunoreactive, but not isolectin B4 reactive and exhibit fast TTX-S, slow TTX-R, but not persistent TTX-R Na+ currents. Colitis-induced hyperexcitability is associated with increased slow TTX-R (Nav1.8) Na+ current. Together, these findings suggest that colitis alters trkA-positive neurons to preferentially increase slow TTX-R Na+ (Nav1.8) currents.

RNA editing generates tissue-specific sodium channels with distinct gating properties

Sodium channels play an essential role in generating the action potential in eukaryotic cells, and their transcripts, especially those in insects, undergo extensive A-to-I RNA editing. The functional consequences of RNA editing of sodium channel transcripts, however, have yet to be determined. We characterized 20 splice variants of the German cockroach sodium channel gene BgNa(v). Functional analysis revealed that these variants exhibited a broad range of voltage-dependent activation and inactivation. Further analysis of two variants, BgNa(v)1-1 and BgNa(v)1-2, which activate at more depolarizing membrane potentials than other variants, showed that RNA editing events were responsible for variant-specific gating properties. Two U-to-C editing sites identified in BgNa(v)1-1 resulted in a Leu to Pro change in segment 1 of domain III (IIIS1) and a Val to Ala change in IVS4. The Leu to Pro change shifted both the voltage dependence of activation and steady-state inactivation in the depolarizing direction. Two A-to-I editing events in BgNa(v)1-2 resulted in a Lys to Arg change in IS2 and an Ile to Met change in IVS3. The Lys to Arg change shifted the voltage dependence of activation in the depolarizing direction. Moreover, these RNA editing events occurred in a tissue-specific and development-specific manner. Our findings provide direct evidence that RNA editing is an important mechanism generating tissue-/cell type-specific functional variants of sodium channels.

Phenoxyphenyl Pyridines as Novel State-Dependent, High-Potency Sodium Channel Inhibitors

In the search for more efficacious drugs to treat neuropathic pain states, a series of phenoxyphenyl pyridines was designed based on 4-(4-flurophenoxy)benzaldehyde semicarbazone. Through variation of the substituents on the pyridine ring, several potent state-dependent sodium channel inhibitors were identified. From these compounds, 23 dose dependently reversed tactile allodynia in the Chung model of neuropathic pain. Administered orally at 10 mg/kg the level of reversal was ca. 50%, comparable to the effect of carbamazepine administered orally at 100 mg/kg.

Modulation of sodium currents in rat sensory neurons by nucleotides

The effects of various nucleotides on the fast tetrodotoxin-sensitive (f-TTX-S) and the slow tetrodotoxin-resistant (s-TTX-R) sodium currents in rat dorsal root ganglion (DRG) neurons were investigated using the patch-clamp technique. Nucleoside triphosphates (NTPs; ATP, GTP, UTP and CTP) and nucleoside diphosphates (NDPs; ADP, GDP, UDP and CDP) decreased f-TTX-S current, whereas they increased s-TTX-R current, when currents were evoked by step depolarizations to 0 mV from a holding potential of -80 mV. NTPs and NDPs shifted both the conductance-voltage relationship curve and the steady-state inactivation curve in the hyperpolarizing direction in both types of sodium currents. Most of them also increased the maximum conductance of s-TTX-R current. ITP, a derivative of ribonucleotide, and dTTP, a deoxyribonucleotide, modulated both types of sodium currents similarly to NTPs and NDPs. However, nucleoside monophosphates (NMPs; AMP, GMP, UMP and CMP) and adenosine had little or no effect on either type of sodium current. Therefore, it seems that nucleotides, regardless of the kind of base, should have two or more phosphates to be able to modulate sodium currents in DRG neurons. Extracellular nucleotides with di- or tri-phosphates would influence the perception by modulating sodium currents in sensory neurons. Particularly, the increase of the maximum conductance and the hyperpolarizing shift of the conductance-voltage relationship of s-TTX-R sodium current would result in an intensified nociception.

Differential action potentials and firing patterns in injured and uninjured small dorsal root ganglion neurons after nerve injury

The profile of tetrodotoxin sensitive (TTX-S) and resistant (TTX-R) Na(+) channels and their contribution to action potentials and firing patterns were studied in isolated small dorsal root ganglion (DRG) neurons after L5/L6 spinal nerve ligation (SNL). Total TTX-R Na(+) currents and Na(v) 1.8 mRNA were reduced in injured L5 DRG neurons 14 days after SNL. In contrast, TTX-R Na(+)currents and Na(v) 1.8 mRNA were upregulated in uninjured L4 DRG neurons after SNL. Voltage-dependent inactivation of TTX-R Na(+) channels in these neurons was shifted to hyperpolarized potentials by 4 mV. Two types of neurons were identified in injured L5 DRG neurons after SNL. Type I neurons (57%) had significantly lower threshold but exhibited normal resting membrane potential (RMP) and action potential amplitude. Type II neurons (43%) had significantly smaller action potential amplitude but retained similar RMP and threshold to those from sham rats. None of the injured neurons could generate repetitive firing. In the presence of TTX, only 26% of injured neurons could generate action potentials that had smaller amplitude, higher threshold, and higher rheobase compared with sham rats. In contrast, action potentials and firing patterns in uninjured L4 DRG neurons after SNL, in the presence or absence of TTX, were not affected. These results suggest that TTX-R Na(+) channels play important roles in regulating action potentials and firing patterns in small DRG neurons and that downregulation in injured neurons and upregulation in uninjured neurons confer differential roles in shaping electrogenesis, and perhaps pain transmission, in these neurons.

Yohimbine inhibits firing activities of rat dorsal root ganglion neurons by blocking Na+ channels and vanilloid VR1 receptors

Yohimbine, an indole alkaloid, is a natural alpha(2)-adrenoceptor antagonist and is frequently used to assess the mechanism of a drug's effect on alpha-adrenoceptors. Recently, several studies showed that yohimbine exhibited analgesic effects in in vivo animal models. However, the underlying mechanism is not known. We investigated the effects of yohimbine on Na(+) channels and vanilloid VR1 receptors in dorsal root ganglion cells. We found that yohimbine inhibited tetrodotoxin-sensitive Na(+) channels (Na(V)1.2), the tetrodotoxin-resistant Na(+) channels, including both slow inactivating (Na(V)1.8) and persistent (Na(V)1.9) Na(+) channels, and capsaicin-sensitive vanilloid VR1 receptors. Action potential firing activities of dorsal root ganglion neurons evoked by current injection or capsaicin were eliminated by yohimbine. The blocking effects of yohimbine on nociceptive-related ion channels and firing activities of dorsal root ganglion neurons may underlie the ionic mechanism of yohimbine's analgesic effects observed in in vivo studies.

Voltage-gated sodium channels and pain pathways

Acute, inflammatory, and neuropathic pain can all be attenuated or abolished by local treatment with sodium channel blockers such as lidocaine. The peripheral input that drives pain perception thus depends on the presence of functional voltage-gated sodium channels. Remarkably, two voltage-gated sodium channel genes (Nav1.8 and Nav1.9) are expressed selectively in damage-sensing peripheral neurons, while a third channel (Nav1.7) is found predominantly in sensory and sympathetic neurons. An embryonic channel (Nav1.3) is also upregulated in damaged peripheral nerves and associated with increased electrical excitability in neuropathic pain states. A combination of antisense and knock-out studies support a specialized role for these sodium channels in pain pathways, and pharmacological studies with conotoxins suggest that isotype-specific antagonists should be feasible. Taken together, these data suggest that isotype-specific sodium channel blockers could be useful analgesics.

Ileitis modulates potassium and sodium currents in guinea pig dorsal root ganglia sensory neurons

Intestinal inflammation induces hyperexcitability of dorsal root ganglia sensory neurons, which has been implicated in increased pain sensation. This study examined whether alteration of sodium (Na+) and/or potassium (K+) currents underlies this hyperexcitability. Ileitis was induced in guinea pig ileum with trinitrobenzene sulphonic acid (TBNS) and dorsal root ganglion neurons innervating the site of inflammation were identified by Fast Blue or DiI fluorescence labelling. Whole cell recordings were made from acutely dissociated small-sized neurons at 7-10 days. Neurons exhibited transient A-type and sustained outward rectifier K+ currents. Compared to control, both A-type and sustained K+ current densities were significantly reduced (42 and 34%, respectively; P < 0.05) in labelled neurons from the inflamed intestine but not in non-labelled neurons. A-type current voltage dependence of inactivation was negatively shifted in labelled inflamed intestine neurons. Neurons also exhibited tetrodotoxin-sensitive and resistant Na+ currents. Tetrodotoxin-resistant sodium currents were increased by 37% in labelled neurons from the inflamed intestine compared to control (P < 0.01), whereas unlabelled neurons were unaffected. The activation and inactivation curves of these currents were unchanged by inflammation. These data suggest ileitis increases excitability of intestinal sensory neurons by modulating multiple ionic channels. The lack of effect in non-labelled neurons suggests signalling originated at the nerve terminal rather than through circulating mediators and, given that Na+ currents are enhanced whereas K+ currents are suppressed, one or more signalling pathways may be involved.

Diclofenac inhibition of sodium currents in rat dorsal root ganglion neurons

The effects of diclofenac, a nonsteroidal anti-inflammatory drug (NSAID), on the fast tetrodotoxin-sensitive (TTX-S) and the slow tetrodotoxin-resistant (TTX-R) sodium currents in rat dorsal root ganglion neurons were investigated using the whole-cell patch-clamp method. Diclofenac suppressed both sodium currents in a dose-dependent manner. The apparent dissociation constants for the diclofenac suppression of TTX-S and TTX-R sodium currents were estimated to be 14 and 97 microM, respectively, at a holding potential of -80 mV. Diclofenac had no effect on the kinetic parameters of the activation process in either type of sodium current. However, diclofenac produced shifts of the steady-state inactivation curves in the hyperpolarizing direction in both types of sodium currents in a dose-dependent manner. At sufficiently negative holding potentials, the inhibitory effects of diclofenac on both types of sodium currents were minimal. The results suggested that diclofenac might bind to sodium channels with a greater affinity when they are in the inactivated state than when they are in the resting state. Effects of other NSAIDs (acetylsalicylic acid, antipyrin, indomethacin and flufenamic acid) on sodium currents were tested. Among these, only flufenamic acid suppressed the sodium currents to a considerable extent. Thus, the chemical structure of each NSAID, not the inhibition of cyclooxygenase, seems to be an important determinant in the sodium current inhibition. The suppression of sodium currents in sensory neurons by diclofenac and flufenamic acid would contribute to their analgesic activity in addition to their inhibition of cyclooxygenase.

Distinct repriming and closed-state inactivation kinetics of Nav1.6 and Nav1.7 sodium channels in mouse spinal sensory neurons

While large, myelinated dorsal root ganglion (DRG) neurons are capable of firing at high frequencies, small unmyelinated DRG neurons typically display much lower maximum firing frequencies. However, the molecular basis for this difference has not been delineated. Because the sodium currents in large DRG neurons exhibit rapid repriming (recovery from inactivation) kinetics and the sodium currents in small DRG neurons exhibit predominantly slow repriming kinetics, it has been proposed that differences in sodium channels might contribute to the determination of repetitive firing properties in DRG neurons. A recent study demonstrated that Nav1.7 expression is negatively correlated with conduction velocity and DRG cell size, while the Nav1.6 voltage-gated sodium channel has been implicated as the predominant isoform present at nodes of Ranvier of myelinated fibres. Therefore we characterized and compared the functional properties, including repriming, of recombinant Nav1.6 and Nav1.7 channels expressed in mouse DRG neurons. Both Nav1.6 and Nav1.7 channels generated fast-activating and fast-inactivating currents. However recovery from inactivation was significantly faster (approximately 5-fold at -70 mV) for Nav1.6 currents than for Nav1.7 currents. The recovery from inactivation of Nav1.6 channels was also much faster than that of native tetrodotoxin-sensitive sodium currents recorded from small spinal sensory neurons, but similar to that of tetrodotoxin-sensitive sodium currents recorded from large spinal sensory neurons. Development of closed-state inactivation was also much faster for Nav1.6 currents than for Nav1.7 currents. Our results indicate that the firing properties of DRG neurons can be tuned by regulating expression of different sodium channel isoforms that have distinct repriming and closed-state inactivation kinetics.

The TTX-resistant sodium channel Nav1.8 (SNS/PN3): expression and correlation with membrane properties in rat nociceptive primary afferent neurons

We have examined the distribution of the sensory neuron-specific Na+ channel Nav1.8 (SNS/PN3) in nociceptive and non-nociceptive dorsal root ganglion (DRG) neurons and whether its distribution is related to neuronal membrane properties. Nav1.8-like immunoreactivity (Nav1.8-LI) was examined with an affinity purified polyclonal antiserum (SNS11) in rat DRG neurons that were classified according to sensory receptive properties and by conduction velocity (CV) as C-, Adelta- or Aalpha/beta. A significantly higher proportion of nociceptive than low threshold mechanoreceptive (LTM) neurons showed Nav1.8-LI, and nociceptive neurons had significantly more intense immunoreactivity in their somata than LTM neurons. Results showed that 89, 93 and 60% of C-, Adelta- and Aalpha/beta-fibre nociceptive units respectively and 88% of C-unresponsive units were positive. C-unresponsive units had electrical membrane properties similar to C-nociceptors and were considered to be nociceptive-type neurons. Weak positive Nav1.8-LI was also present in some LTM units including a C LTM, all Adelta LTM units (D hair), about 10% of cutaneous LTM Aalpha/beta-units, but no muscle spindle afferent units. Nav1.8-LI intensity was negatively correlated with soma size (all neurons) and with dorsal root CVs in A- but not C-fibre neurons. Nav1.8-LI intensity was positively correlated with action potential (AP) duration (both rise and fall time) in A-fibre neurons and with AP rise time only in positive C-fibre neurons. It was also positively correlated with AP overshoot in positive neurons. Thus high levels of Nav1.8 protein may contribute to the longer AP durations (especially in A-fibre neurons) and larger AP overshoots that are typical of nociceptors.

Selective expression of a persistent tetrodotoxin-resistant Na+ current and NaV1.9 subunit in myenteric sensory neurons

Voltage-gated Na(+) currents play critical roles in shaping electrogenesis in neurons. Here, we have identified a TTX-resistant Na(+) current (TTX-R I(Na)) in duodenum myenteric neurons of guinea pig and rat and have sought evidence regarding the molecular identity of the channel producing this current from the expression of Na(+) channel alpha subunits and the biophysical and pharmacological properties of TTX-R I(Na). Whole-cell patch-clamp recording from in situ neurons revealed the presence of a voltage-gated Na(+) current that was highly resistant to TTX (IC(50), approximately 200 microm) and selectively distributed in myenteric sensory neurons but not in interneurons and motor neurons. TTX-R I(Na) activated slowly in response to depolarization and exhibited a threshold for activation at -50 mV. V(1/2) values of activation and steady-state inactivation were -32 and -31 mV in the absence of fluoride, respectively, which, as predicted from the window current, generated persistent currents. TTX-R I(Na) also had prominent ultraslow inactivation, which turns off 50% of the conductance at rest (-60 mV). Substituting CsF for CsCl in the intracellular solution shifted the voltage-dependent parameters of TTX-R I(Na) leftward by approximately 20 mV. Under these conditions, TTX-R I(Na) had voltage-dependent properties similar to those reported previously for NaN/Na(V)1.9 in dorsal root ganglion neurons. Consistent with this, reverse transcription-PCR, single-cell profiling, and immunostaining experiments indicated that Na(V)1.9 transcripts and subunits, but not Na(V)1.8, were expressed in the enteric nervous system and restricted to myenteric sensory neurons. TTX-R I(Na) may play an important role in regulating subthreshold electrogenesis and boosting synaptic stimuli, thereby conferring distinct integrative properties to myenteric sensory neurons.

GTP-induced tetrodotoxin-resistant Na+ current regulates excitability in mouse and rat small diameter sensory neurones

Peripheral pain thresholds are regulated by the actions of inflammatory mediators. Some act through G-protein-coupled receptors on voltage-gated sodium channels. We have found that a low-threshold, persistent tetrodotoxin-resistant Na+ current, attributed to NaV1.9, is upregulated by GTP and its non-hydrolysable analogue GTP-gamma-S, but not by GDP. Inclusion of GTP-gamma-S (500 microM) in the internal solution led to an increase in maximal current amplitude of > 300 % within 5 min. In current clamp, upregulation of persistent current was associated with a more negative threshold for action potential induction (by 15-16 mV) assessed from a holding potential of -90 mV. This was not seen in neurones without the low-threshold current or with internal GDP (P < 0.001). In addition, persistent current upregulation depolarized neurones. At -60 mV, internal GTP-gamma-S led to the generation of spontaneous activity in initially silent neurones only when persistent current was upregulated. These findings suggest that regulation of the persistent current has important consequences for nociceptor excitability.

ATP modulation of sodium currents in rat dorsal root ganglion neurons

The modulation of tetrodotoxin-sensitive (TTX-S) and slow tetrodotoxin-resistant (TTX-R) sodium currents in rat dorsal root ganglion neurons by ATP was studied using the whole-cell patch-clamp method. The effects of ATP on two types of sodium currents were either stimulatory or inhibitory depending on the kinetic parameters tested. At a holding potential of -80 mV ATP suppressed TTX-S sodium currents when the depolarizing potential was positive to -30 mV but it increased them when the depolarizing potential was negative to -30 mV. At the same holding potential slow TTX-R sodium currents were always increased by ATP regardless of the depolarizing potential. In both types of sodium currents ATP shifted both the conductance-voltage relationship curve and the steady-state inactivation curve in the hyperpolarizing direction, and accelerated the time-dependent inactivation. ATP decreased the maximum conductance of TTX-S sodium currents but increased that of slow TTX-R sodium currents. The results suggest that ATP would decrease the excitability of neurons with TTX-S sodium channels but would increase that of neurons with slow TTX-R sodium channels. The effects of ATP on sodium currents were preserved in the presence of a G-protein inhibitor, GDP-beta-S, or purinergic antagonists, suramin and Reactive Blue-2, suggesting that purinergic receptors might not be involved in ATP modulation of sodium currents.

Na+ channel Nav1.9: in search of a gating mechanism

Voltage-gated Na(+) channels play key roles in generating and propagating action potentials. Their gating is believed to rely exclusively on changes in membrane potential. However, recent data from Blum, Kafitz and Konnerth provide direct evidence that the opening of Na(v)1.9, a member of the voltage-gated Na(+) channel family, is mediated by ligand binding rather than by voltage. This is arguably one of the most influential ideas brought to us in the history of the Na(+) channel field.

Modulation of the cardiac sodium channel Nav1.5 by fibroblast growth factor homologous factor 1B

We have previously shown that fibroblast growth factor homologous factor 1B (FHF1B), a cytosolic member of the fibroblast growth factor family, associates with the sensory neuron-specific channel Na(v)1.9 but not with the other sodium channels present in adult rat dorsal root ganglia neurons. We show in this study that FHF1B binds to the C terminus of the cardiac voltage-gated sodium channel Na(v)1.5 and modulates the properties of the channel. The N-terminal 41 amino acid residues of FHF1B are essential for binding to Na(v)1.5, and the conserved acidic rich domain (amino acids 1773-1832) in the C terminus of Na(v)1.5 is sufficient for association with this factor. Binding of the growth factor to recombinant wild type human Na(v)1.5 in human embryonic kidney 293 cells produces a significant hyperpolarizing shift in the voltage dependence of channel inactivation. An aspartic acid to glycine substitution at position 1790 of the channel, which underlies one of the LQT-3 phenotypes of cardiac arrythmias, abolishes the interaction of the Na(v)1.5 channel with FHF1B. This is the first report showing that interaction with a growth factor can modulate properties of a voltage-gated sodium channel.

Overview of the voltage-gated sodium channel family

Selective permeation of sodium ions through voltage-dependent sodium channels is fundamental to the generation of action potentials in excitable cells such as neurons. These channels are large integral membrane proteins and are encoded by at least ten genes in mammals. The different sodium channels have remarkably similar functional properties, but small changes in sodium-channel function are biologically relevant, as underscored by mutations that cause several human diseases of hyperexcitability.

The sea anemone toxins BgII and BgIII prolong the inactivation time course of the tetrodotoxin-sensitive sodium current in rat dorsal root ganglion neurons

We have characterized the effects of BgII and BgIII, two sea anemone peptides with almost identical sequences (they only differ by a single amino acid), on neuronal sodium currents using the whole-cell patch-clamp technique. Neurons of dorsal root ganglia of Wistar rats (P5-9) in primary culture (Leibovitz's L15 medium; 37 degrees C, 95% air/5% CO2) were used for this study (n = 154). These cells express two sodium current subtypes: tetrodotoxin-sensitive (TTX-S; K(i) = 0.3 nM) and tetrodotoxin-resistant (TTX-R; K(i) = 100 microM). Neither BgII nor BgIII had significant effects on TTX-R sodium current. Both BgII and BgIII produced a concentration-dependent slowing of the TTX-S sodium current inactivation (IC50 = 4.1 +/- 1.2 and 11.9 +/- 1.4 microM, respectively), with no significant effects on activation time course or current peak amplitude. For comparison, the concentration-dependent action of Anemonia sulcata toxin II (ATX-II), a well characterized anemone toxin, on the TTX-S current was also studied. ATX-II also produced a slowing of the TTX-S sodium current inactivation, with an IC50 value of 9.6 +/- 1.2 microM indicating that BgII was 2.3 times more potent than ATX-II and 2.9 times more potent than BgIII in decreasing the inactivation time constant (tau(h)) of the sodium current in dorsal root ganglion neurons. The action of BgIII was voltage-dependent, with significant effects at voltages below -10 mV. Our results suggest that BgII and BgIII affect voltage-gated sodium channels in a similar fashion to other sea anemone toxins and alpha-scorpion toxins.

Changes in Na(+) channel currents of rat dorsal root ganglion neurons following axotomy and axotomy-induced autotomy

Section of rat sciatic nerve (axotomy) increases the excitability of neurons in the L(4)-L(5) dorsal root ganglia (DRG). These changes are more pronounced in animals that exhibit a self-mutilatory behavior known as autotomy. We used whole cell recording to examine changes in the tetrodotoxin-sensitive (TTX-S) and the tetrodotoxin-resistant (TTX-R) components of sodium channel currents (I(Na)) that may contribute to axotomy-induced increases in excitability. Cells were initially divided on the basis of size into "large," "medium," and "small" groups. TTX-S I(Na) predominated in "large" cells, whereas TTX-R I(Na) predominated in some, but not all "small cells." "Small" cells were therefore subdivided into "small-slow" cells, which predominantly exhibited TTX-R I(Na) and "small fast" cells that exhibited more TTX-S I(Na). In contrast to results obtained in other laboratories, where slightly different experimental procedures were used, we found that axotomy increased TTX-R and/or TTX-S I(Na) and slowed inactivation. The effects were greatest in "small-slow" cells and least in "large" cells. The changes promoted by axotomy were expressed more clearly in animals that exhibited autotomy. Also, the presence of autotomy correlated with a shift in the properties of I(Na) in "large" rather than "small-slow," putative nociceptive cells. These trends parallel previous observations on axotomy-induced increases in excitability, spike height, and spike width that are also greatest in "small" cells and least in "large" cells. In addition, the presence of autotomy correlates with an increase in excitability of "large" rather than "small" cells. Increases in TTX-R and TTX-S I(Na) thus coincide with axotomy-induced increases in excitability and alterations in spike shape across the whole population of sensory neurons. Injury-induced changes of this type are likely associated with the onset of chronic pain in humans.

The effects of polarizing current on nerve terminal impulses recorded from polymodal and cold receptors in the guinea-pig cornea

It was reported recently that action potentials actively invade the sensory nerve terminals of corneal polymodal receptors, whereas corneal cold receptor nerve terminals are passively invaded (Brock, J.A., S. Pianova, and C. Belmonte. 2001. J. Physiol. 533:493-501). The present study investigated whether this functional difference between these two types of receptor was due to an absence of voltage-activated Na(+) conductances in cold receptor nerve terminals. To address this question, the study examined the effects of polarizing current on the configuration of nerve terminal impulses recorded extracellularly from single polymodal and cold receptors in guinea-pig cornea isolated in vitro. Polarizing currents were applied through the recording electrode. In both receptor types, hyperpolarizing current (+ve) increased the negative amplitude of nerve terminal impulses. In contrast, depolarizing current (-ve) was without effect on polymodal receptor nerve terminal impulses but increased the positive amplitude of cold receptor nerve terminal impulses. The hyperpolarization-induced increase in the negative amplitude of nerve terminal impulses represents a net increase in inward current. In both types of receptor, this increase in inward current was reduced by local application of low Na(+) solution and blocked by lidocaine (10 mM). In addition, tetrodotoxin (1 microM) slowed but did not reduce the hyperpolarization-induced increase in the negative amplitude of polymodal and cold nerve terminal impulses. The depolarization-induced increase in the positive amplitude of cold receptor nerve terminal impulses represents a net increase in outward current. This change was reduced both by lidocaine (10 mM) and the combined application of tetraethylammomium (20 mM) and 4-aminopyridine (1 mM). The interpretation is that both polymodal and cold receptor nerve terminals possess high densities of tetrodotoxin-resistant Na(+) channels. This finding suggests that in cold receptors, under normal conditions, the Na(+) conductances are rendered inactive because the nerve terminal region is relatively depolarized.