Complex blockade of TTX-resistant Na+ currents by lidocaine and bupivacaine reduce firing frequency in DRG neurons

Carvacrol decreases neuronal excitability by inhibition of voltage-gated sodium channels

The monoterpenoid carvacrol (1) is present in many essential oils of plants and has attracted attention because of its beneficial biological activities, especially analgesic activity. However, the mechanism of action of 1 remains unknown. The present study aimed to explore the mechanisms whereby 1 produces its effects on the peripheral nervous system. Carvacrol reversibly blocked the excitability of the rat sciatic nerve in a concentration-dependent manner with an IC(50) value of 0.50 ± 0.04 mM. At 0.6 mM, 1 increased the rheobase from 3.30 ± 0.06 V to 4.16 ± 0.14 V and the chronaxy from 59.6 ± 1.22 μs to 75.0 ± 1.82 μs. Also, 1 blocked the generation of action potentials (IC(50) 0.36 ± 0.14 mM) of the intact dorsal root ganglion (DRG) neurons without altering the resting potential and input resistance. Carvacrol reduced the voltage-gated sodium current of dissociated DRG neurons (IC(50) 0.37 ± 0.05 mM). In this study it has been demonstrated that 1 blocks neuronal excitability by a direct inhibition of the voltage-gated sodium current, which suggests that this compound acts as a local anesthetic. The present findings add valuable information to help understand the mechanisms implicated in the analgesic activity of carvacrol.

Inhibition by pregnenolone sulphate, a metabolite of the neurosteroid pregnenolone, of voltage-gated sodium channels expressed in Xenopus oocytes

Neurosteroids are known as allosteric modulators of the ligand-gated ion channel superfamily. Voltage-gated sodium channels (Na(v)) play an important role in mediating excitotoxic damages. Here we report the effects of neurosteroids on the function of Na(v), using voltage-clamp techniques in Xenopus oocytes expressed with the Na(v)1.2 α subunit. Pregnenolone sulphate, but not pregnenolone, inhibited sodium currents (I(Na)) at 3 - 100 μmol/L. The suppression of I(Na) by pregnenolone sulphate was due to increased inactivation with little change in activation. These findings suggest that pregnenolone sulphate, a metabolite of pregnenolone, suppresses the function of Na(v) via increased inactivation, which may contribute to the neuroprotection.

Enhanced excitability of small dorsal root ganglion neurons in rats with bone cancer pain

Background

Primary and metastatic cancers that affect bone are frequently associated with severe and intractable pain. The mechanisms underlying the development of bone cancer pain are largely unknown. The aim of this study was to determine whether enhanced excitability of primary sensory neurons contributed to peripheral sensitization and tumor-induced hyperalgesia during cancer condition. In this study, using techniques of whole-cell patch-clamp recording associated with immunofluorescent staining, single-cell reverse-transcriptase PCR and behavioral test, we investigated whether the intrinsic membrane properties and the excitability of small-sized dorsal root ganglion (DRG) neurons altered in a rat model of bone cancer pain, and whether suppression of DRG neurons activity inhibited the bone cancer-induced pain.

Results

Our present study showed that implantation of MRMT-1 tumor cells into the tibial canal in rats produced significant mechanical and thermal hyperalgesia in the ipsilateral hind paw. Moreover, implantation of tumor cells provoked spontaneous discharges and tonic excitatory discharges evoked by a depolarizing current pulse in small-sized DRG neurons. In line with these findings, alterations in intrinsic membrane properties that reflect the enhanced neuronal excitability were observed in small DRG neurons in bone cancer rats, of which including: 1) depolarized resting membrane potential (RMP); 2) decreased input resistance (R_in); 3) a marked reduction in current threshold (CT) and voltage threshold (TP) of action potential (AP); 4) a dramatic decrease in amplitude, overshot, and duration of evoked action potentials as well as in amplitude and duration of afterhyperpolarization (AHP); and 5) a significant increase in the firing frequency of evoked action potentials. Here, the decreased AP threshold and increased firing frequency of evoked action potentials implicate the occurrence of hyperexcitability in small-sized DRG neurons in bone cancer rats. In addiotion, immunofluorescent staining and single-cell reverse-transcriptase PCR revealed that in isolated small DRG neurons, most neurons were IB4-positive, or expressed TRPV1 or CGRP, indicating that most recorded small DRG neurons were nociceptive neurons. Finally, using in vivo behavioral test, we found that blockade of DRG neurons activity by TTX inhibited the tumor-evoked mechanical allodynia and thermal hyperalgesia in bone cancer rats, implicating that the enhanced excitability of primary sensory neurons underlied the development of bone cancer pain.

Conclusions

Our present results suggest that implantation of tumor cells into the tibial canal in rats induces an enhanced excitability of small-sized DRG neurons that is probably as results of alterations in intrinsic electrogenic properties of these neurons. Therefore, alterations in intrinsic membrane properties associated with the hyperexcitability of primary sensory neurons likely contribute to the peripheral sensitization and tumor-induced hyperalgesia under cancer condition.

Mechanisms of pain transmission and pharmacologic management

BACKGROUND

Pain sensation involves multiple signaling and modulatory pathways, employing a variety of neurotransmitters and other mediators. Inhibitory and facilitatory mechanisms affect the perception of stimuli as painful or non-painful, and in addition may affect the perceived intensity of pain. Endogenous opioids are key mediators in the descending pain suppression pathways. Additionally, monoaminergic neurotransmitters such as norepinephrine, serotonin and dopamine positively or negatively modulate pain signaling, depending on receptor type and location. The various mediators involved in pain signaling provide potential targets for pharmacological interventions. Single analgesic therapies may be limited in their ability to comprehensively target these complex pain signaling pathways. Therapeutic approaches acting on multiple pain transmission pathways through different mechanisms of action provide an opportunity to maximize efficacy and tolerability in the treatment of pain.

SCOPE

This article discusses the various physiologic processes involved in pain signaling and modulation, describes the mechanisms by which various classes of analgesic agents are believed to produce their clinical effects, and explores the potential benefits of a multiple-mechanism approach to analgesia. Published articles describing the physiologic processes involved in pain signaling and modulation and the mechanisms of analgesia for different drug classes were reviewed. MEDLINE searches were conducted to identify relevant studies published through August 2009 that evaluated the efficacy and tolerability of multiple-mechanism analgesic regimens. English language-only randomized controlled trials and meta-analyses of randomized controlled trials were considered.

FINDINGS/CONCLUSION

Multiple neurotransmitters and other mediators are involved in the endogenous modulation of pain signaling, providing numerous opportunities for intervention with different classes of analgesics. Data from numerous clinical trials indicate that multiple-mechanism approaches to analgesia provide comparable or superior analgesic efficacy with lower doses of the individual agents and reduced incidence of side effects. These data support current guidelines which endorse multiple-mechanism strategies for both acute and chronic pain management.

Blockade of TTX-resistant and TTX-sensitive Na+ currents in cultured dorsal root ganglion neurons by fomocaine and the fomocaine derivative Oe 9000

Fomocaine and its new derivative Oe 9000 are local anesthetics in which the inner aromatic moiety carries a phenoxymethyl substituent and is linked to the tertiary amine by an alkylene chain, rendering these compounds considerably lipophilic and increasing their chemical and metabolic stability. Although fomocaine was used for surface anesthesia, the presumed mode of action of fomocaine and Oe 9000, the blockade of voltage-gated Na(+) currents in neurons, has not been investigated. In the present experiments we used the whole-cell mode of the patch-clamp technique and studied the effect of both drugs on voltage-gated Na(+) currents in isolated and cultured dorsal root ganglion (DRG) neurons from adult rats. Both drugs reversibly reduced slowly activating and inactivating tetrodotoxin-resistant (TTX-R) Na(+) currents as well as rapidly activating and inactivating TTX-sensitive (TTX-S) Na(+) currents at low micromolar concentrations. For the reduction of TTX-R Na(+) currents the IC(50) of fomocaine was 10.3μM, and the IC(50) for the more hydrophilic Oe 9000 was 4.5μM. These IC(50) values are more than one order of magnitude lower than the corresponding IC(50) of other local anesthetics such as lidocaine. Similar as for other local anesthetics, the effects showed a frequency dependence indicating that the compounds preferentially bind to the open and/or inactivated state of the channel. These data establish for the first time the functional suppression of TTX-R and TTX-S Na(+) currents by fomocaine and Oe 9000 in neurons. They support the further research into the use of Oe 9000 as a novel local anesthetic.

Electrical stimulation of sympathetic neurons induces autocrine/paracrine effects of NGF mediated by TrkA

Neuronal remodeling with increased sympathetic innervation density has been implicated in the pathogenesis of atrial fibrillation (AF). Recently, increased transcardiac nerve growth factor (NGF) levels were observed in a canine model of AF. Whether atrial myocytes or cardiac sympathetic neurons are the source of neurotrophins, and whether NGF is the main neurotrophic factor contributing to sympathetic nerve sprouting (SNS) in AF still remains unclear. Therefore, neonatal rat atrial myocytes were cultured under conditions of high frequency electrical field stimulation (HFES) to mimic rapid atrial depolarization. Likewise, sympathetic neurons from the superior cervical ganglia of neonatal rats were exposed to HFES to simulate the physiological effect of sympathetic stimulation. Real-time PCR, ELISA and Western blots were performed to analyze the expression pattern of NGF and neurotrophin-3 (NT-3). Baseline NGF and NT-3 content was 3-fold higher in sympathetic neurons than in atrial myocytes (relative NGF protein expression: 1+/-0.0 vs. 0.37+/-0.11, all n=5, p<0.05). HFES of sympathetic neurons induced a frequency dependent NGF and NT-3 gene and protein up-regulation (relative NGF protein expression: 0Hz=1+/-0.0 vs. 5Hz=1.13+/-0.19 vs. 50Hz=1.77+/-0.08, all n=5, 0Hz/5Hz vs. 50Hz p<0.05), with a subsequent increase of growth associated protein 43 (GAP-43) expression and morphological SNS. Moreover, HFES of sympathetic neurons increased the tyrosine kinase A (TrkA) receptor expression. HFES induced neurotrophic effects could be abolished by lidocaine, TrkA blockade or NGF neutralizing antibodies, while NT-3 neutralizing antibodies had no significant effect on SNS. In neonatal rat atrial myocytes, HFES resulted in myocyte hypertrophy accompanied by an increase in NT-3 and a decrease in NGF expression. In summary, this study provides evidence that high-rate electrical stimulation of sympathetic neurons mediates nerve sprouting by an increase in NGF expression that targets the TrkA receptor in an autocrine/paracrine manner.

Block of sensory neuronal Na+ channels by the secreolytic ambroxol is associated with an interaction with local anesthetic binding sites

Voltage-gated Na(+) channels (Na(v)) regulate the excitability of sensory neurons and are potential targets for novel analgesics. The secreolytic ambroxol reduces pain-related behavior in rodents and alleviates pain in humans. With properties resembling those of local anesthetics, ambroxol has been reported to block Na(+) currents in sensory neurons with a preference for tetrodotoxin-resistant (TTXr) Na(+) currents encoded by Na(v)1.8. However, the molecular determinants for ambroxol-induced block of Na(+) channels and a preferential block of Na(v)1.8 opposed to tetrodotoxin-sensitive (TTXs) Na(v) alpha-subunits have not been studied in detail. By means of whole-cell voltage clamp recordings, we studied the effects of ambroxol and local anesthetics on the recombinant TTXr subunit Na(v)1.8, on TTXs Na(v) alpha-subunits and on mutants of Na(v)1.4 that are insensitive to local anesthetics. Tonic and use-dependent block by ambroxol was strongly alleviated in local anesthetic-insensitive Na(v)1.4 mutants. Use-dependent block, but not tonic block was significantly stronger on Na(v)1.8 than on TTXs channels. The TTXs subunit Na(v)1.3 displayed the least degree of use-dependent block by ambroxol. The local anesthetics mepivacaine and S(-)-bupivacaine also blocked Na(v)1.8 and TTXs channels differentially. While mepivacaine displayed a preferential use-dependent block of Na(v)1.8, S(-)-bupivacaine displayed a preference for TTXs Na(+) channels. Our data show that ambroxol acts as a typical local anesthetic on Na(+) channels interacting with specific residues in the S6 segments. This property probably meditates the analgesic effect of ambroxol. Ambroxol preferentially blocks Na(v)1.8, however shares this property with established local anesthetics like mepivacaine.

Coapplication of lidocaine and the permanently charged sodium channel blocker QX-314 produces a long-lasting nociceptive blockade in rodents

BACKGROUND

Nociceptive-selective local anesthesia is produced by entry of the permanently charged lidocaine-derivative QX-314 into nociceptors when coadministered with capsaicin, a transient receptor potential vanilloid 1 (TRPV1) channel agonist. However, the pain evoked by capsaicin before establishment of the QX-314-mediated block would limit clinical utility. Because TRPV1 channels are also activated by lidocaine, the authors tested whether lidocaine can substitute for capsaicin to introduce QX-314 into nociceptors through TRPV1 channels and produce selective analgesia.

METHODS

Lidocaine (0.5% [17.5 mM], 1% [35 mM], and 2% [70 mM]) alone, QX-314 (0.2% [5.8 mM]) alone, and a combination of the two were injected subcutaneously and adjacent to the sciatic nerve in rats and mice. Mechanical and thermal responsiveness were measured, as was motor block.

RESULTS

Coapplication of 0.2% QX-314 with lidocaine prolonged the nociceptive block relative to lidocaine alone, an effect attenuated in TRPV1 knockout mice. The 0.2% QX-314 alone had no effect when injected intraplantary or perineurally, and it produced only weak short-lasting inhibition of the cutaneous trunci muscle reflex. Perisciatic nerve injection of lidocaine with QX-314 produced a differential nociceptive block much longer than the transient motor block, lasting 2 h (for 1% lidocaine) to 9 h (2% lidocaine). Triple application of lidocaine, QX-314, and capsaicin further increased the duration of the differential block.

CONCLUSIONS

Coapplication of lidocaine and its quaternary derivative QX-314 produces a long-lasting, predominantly nociceptor-selective block, likely by facilitating QX-314 entry through TRPV1 channels. Delivery of QX-314 into nociceptors by using lidocaine instead of capsaicin produces sustained regional analgesia without nocifensive behavior.

Pyramidal cells of rodent presubiculum express a tetrodotoxin-insensitive Na+ current

Presubicular neurons are activated physiologically by a specific preferred head direction. Here we show that firing in these neurones is characterized by action potentials with a large overshoot and a reduced firing frequency adaptation during repetitive firing. We found that a component of the sodium current of presubicular cells was not abolished by tetrodotoxin (TTX, 10 mum) and was activated at more depolarized voltages than TTX-sensitive currents. This inward current was completely abolished by the removal of external sodium, suggesting that sodium is the charge carrier of this TTX-insensitive (TTX-I) current. The channels responsible for the TTX-I sodium current seemed to be expressed at sites distant from the soma, giving rise to a voltage-dependent delay in current activation. The voltage required for half-maximal activation was 21 mV, and 36 mV for inactivation, which is similar to that reported for Na(V)1.8 sodium channels. However, the kinetics were considerably slower, with a time constant of current decay of 1.4 s. The current was not abolished in pyramidal cells from animals lacking either the Na(V)1.8 or the Na(V)1.9 subunit. This, possibly novel, TTX-I sodium current could contribute to the coding functions of presubicular neurons, specifically the maintained firing associated with signalling of a stable head position.

Comparative effects of halogenated inhaled anesthetics on voltage-gated Na+ channel function

BACKGROUND

Inhibition of voltage-gated Na channels (Na(v)) is implicated in the synaptic actions of volatile anesthetics. We studied the effects of the major halogenated inhaled anesthetics (halothane, isoflurane, sevoflurane, enflurane, and desflurane) on Na(v)1.4, a well-characterized pharmacological model for Na(v) effects.

METHODS

Na currents (I(Na)) from rat Na(v)1.4 alpha-subunits heterologously expressed in Chinese hamster ovary cells were analyzed by whole cell voltage-clamp electrophysiological recording.

RESULTS

Halogenated inhaled anesthetics reversibly inhibited Na(v)1.4 in a concentration- and voltage-dependent manner at clinical concentrations. At equianesthetic concentrations, peak I(Na) was inhibited with a rank order of desflurane > halothane approximately enflurane > isoflurane approximately sevoflurane from a physiologic holding potential (-80 mV). This suggests that the contribution of Na channel block to anesthesia might vary in an agent-specific manner. From a hyperpolarized holding potential that minimizes inactivation (-120 mV), peak I(Na) was inhibited with a rank order of potency for tonic inhibition of peak I(Na) of halothane > isoflurane approximately sevoflurane > enflurane > desflurane. Desflurane produced the largest negative shift in voltage-dependence of fast inactivation consistent with its more prominent voltage-dependent effects. A comparison between isoflurane and halothane showed that halothane produced greater facilitation of current decay, slowing of recovery from fast inactivation, and use-dependent block than isoflurane.

CONCLUSIONS

Five halogenated inhaled anesthetics all inhibit a voltage-gated Na channel by voltage- and use-dependent mechanisms. Agent-specific differences in efficacy for Na channel inhibition due to differential state-dependent mechanisms creates pharmacologic diversity that could underlie subtle differences in anesthetic and nonanesthetic actions.

The pharmacology of voltage-gated sodium channels in sensory neurones

Voltage-gated sodium channels (VGSCs) are vital for the normal functioning of most excitable cells. At least nine distinct functional subtypes of VGSCs are recognized, corresponding to nine genes for their pore-forming alpha-subunits. These have different developmental expression patterns, different tissue distributions in the adult and are differentially regulated at the cellular level by receptor-coupled cell signalling systems. Unsurprisingly, VGSC blockers are found to be useful as drugs in diverse clinical applications where excessive excitability of tissue leads to pathological dysfunction, e.g. epilepsy or cardiac tachyarrhythmias. The effects of most clinically useful VGSC blockers are use-dependent, i.e. their efficacy depends on channel activity. In addition, many natural toxins have been discovered that interact with VGSCs in complex ways and they have been used as experimental probes to study the structure and function of the channels and to better understand how drugs interact with the channels. Here we have attempted to summarize the properties of VGSCs in sensory neurones, discuss how they are regulated by cell signalling systems and we have considered briefly current concepts of their physiological function. We discuss in detail how drugs and toxins interact with archetypal VGSCs and where possible consider how they act on VGSCs in peripheral sensory neurones. Increasingly, drugs that block VGSCs are being used as systemic analgesic agents in chronic pain syndromes, but the full potential for VGSC blockers in this indication is yet to be realized and other applications in sensory dysfunction are also possible. Drugs targeting VGSC subtypes in sensory neurones are likely to provide novel systemic analgesics that are tissue-specific and perhaps even disease-specific, providing much-needed novel therapeutic approaches for the relief of chronic pain.

n-Alcohols inhibit voltage-gated Na+ channels expressed in Xenopus oocytes

Voltage-gated sodium channels are essential for the initiation and propagation of action potentials in excitable cells and are known as a target of local anesthetics. In addition, inhibition of sodium channels by volatile anesthetics has been proposed as a mechanism of general anesthesia. The n-alcohols produce anesthesia, and their potency increases with carbon number until a "cut-off" is reached. In this study, we examined effects of a range of n-alcohols on Na(v)1.2 subunits to determine the alcohol cut-off for this channel. We also studied the effect of a short-chain alcohol (ethanol) and a long-chain alcohol (octanol) on Na(v)1.2, Na(v)1.4, Na(v)1.6, and Na(v)1.8 subunits, and we investigated the effects of alcohol on channel kinetics. Ethanol and octanol inhibited sodium currents of all subunits, and the inhibition of the Na(v)1.2 channel by n-alcohols indicated a cut-off at nonanol. Ethanol and octanol produced open-channel block, which was more pronounced for Na(v)1.8 than for the other sodium channels. Inhibition of Na(v)1.2 was due to decreased activation and increased inactivation. These results suggest that sodium channels may have a hydrophobic binding site for n-alcohols and demonstrate the differences in the kinetic mechanisms of inhibition for n-alcohols and inhaled anesthetics.

Nav1.7 expression is increased in painful human dental pulp

BACKGROUND

Animal studies and a few human studies have shown a change in sodium channel (NaCh) expression after inflammatory lesions, and this change is implicated in the generation of pain states. We are using the extracted human tooth as a model system to study peripheral pain mechanisms and here examine the expression of the Nav1.7 NaCh isoform in normal and painful samples. Pulpal sections were labeled with antibodies against: 1) Nav1.7, N52 and PGP9.5, and 2) Nav1.7, caspr (a paranodal protein used to identify nodes of Ranvier), and myelin basic protein (MBP), and a z-series of optically-sectioned images were obtained with the confocal microscope. Nav1.7-immunofluorescence was quantified in N52/PGP9.5-identified nerve fibers with NIH ImageJ software, while Nav1.7 expression in myelinated fibers at caspr-identified nodal sites was evaluated and further characterized as either typical or atypical as based on caspr-relationships.

RESULTS

Results show a significant increase in nerve area with Nav1.7 expression within coronal and radicular fiber bundles and increased expression at typical and atypical caspr-identified nodal sites in painful samples. Painful samples also showed an augmentation of Nav1.7 within localized areas that lacked MBP, including those associated with atypical caspr-identified sites, thus identifying NaCh remodeling within demyelinating axons as the basis for a possible pulpal pain mechanism.

CONCLUSION

This study identifies the increased axonal expression and augmentation of Nav1.7 at intact and remodeling/demyelinating nodes within the painful human dental pulp where these changes may contribute to constant, increased evoked and spontaneous pain responses that characterize the pain associated with toothache.

Role of TTX-Sensitive and TTX-Resistant Sodium Channels in Aδ- and C-Fiber Conduction and Synaptic Transmission

Thin afferent axons conduct nociceptive signals from the periphery to the spinal cord. Their somata express two classes of Na+ channels, TTX-sensitive (TTX-S) and TTX-resistant (TTX-R), but their relative contribution to axonal conduction and synaptic transmission is not well understood. We studied this contribution by comparing effects of nanomolar TTX concentrations on currents associated with compound action potentials in the peripheral and central branches of Aδ- and C-fiber axons as well as on the Aδ- and C-fiber-mediated excitatory postsynaptic currents (EPSCs) in spinal dorsal horn neurons of rat. At room temperature, TTX completely blocked Aδ-fibers (IC50, 5–7 nM) in dorsal roots (central branch) and spinal, sciatic, and sural nerves (peripheral branch). The C-fiber responses were blocked by 85–89% in the peripheral branch and by 65–66% in dorsal roots (IC50, 14–33 nM) with simultaneous threefold reduction in their conduction velocity. At physiological temperature, the degree of TTX block in dorsal roots increased to 93%. The Aδ- and C-fiber-mediated EPSCs in dorsal horn neurons were also sensitive to TTX. At room temperature, 30 nM blocked completely Aδ-input and 84% of the C-fiber input, which was completely suppressed at 300 nM TTX. We conclude that in mammals, the TTX-S Na+ channels dominate conduction in all thin primary afferents. It is the only type of functional Na+ channel in Aδ-fibers. In C-fibers, the TTX-S Na+ channels determine the physiological conduction velocity and control synaptic transmission. TTX-R Na+ channels could not provide propagation of full-amplitude spikes able to trigger synaptic release in the spinal cord.

Expression of Nav1.9 Channels in Human Dental Pulp and Trigeminal Ganglion

There is a higher incidence of local anesthetic failure in endodontic patients experiencing pulpal hyperalgesia. Up-regulation of Nav1.9, a voltage-gated sodium channel isoform, might play a key role in local anesthetic failure because Nav1.9 channels increase neuronal excitability and have low sensitivity to blockade by local anesthetics. Immunocytochemistry was used to examine Nav1.9 channel expression in axons of symptomatic (painful) versus asymptomatic human dental pulp and to determine Nav1.9 expression levels in neuronal somata of the human trigeminal ganglion. Nav1.9 channel immunoreactivity on pulpal axons was significantly increased in painful teeth. Nav1.9 channels were expressed in membranes and cytoplasm of human trigeminal ganglion neurons, with the highest expression in small neuronal somata. Nav1.9 expression in the trigeminal ganglion coupled with increased expression in symptomatic pulp might contribute to hypersensitivity of inflamed pulps and local anesthetic failure. Furthermore, the present study suggests that Nav1.9 channels are potential targets for novel anesthetics.

Effects of ralfinamide, a Na+ channel blocker, on firing properties of nociceptive dorsal root ganglion neurons of adult rats

Recent studies revealed that ralfinamide, a Na(+) channel blocker, suppressed tetrodotoxin-resistant Na(+) currents in dorsal root ganglion (DRG) neurons and reduced pain reactions in animal models of inflammatory and neuropathic pain. Here, we investigated the effects of ralfinamide on Na(+) currents; firing properties and action potential (AP) parameters in capsaicin-responsive and -unresponsive DRG neurons from adult rats in the presence of TTX (0.5 microM). Ralfinamide inhibited TTX-resistant Na(+) currents in a frequency- and voltage-dependent manner. Small to medium sized neurons exhibited different firing properties during prolonged depolarizing current pulses (600 ms). One group of neurons fired multiple spikes (tonic), while another group fired four or less APs (phasic). In capsaicin-responsive tonic firing neurons, ralfinamide (25 microM) reduced the number of APs from 10.6+/-1.8 to 2.6+/-0.7 APs/600 ms, whereas in capsaicin-unresponsive tonic neurons, the drug did not significantly change firing (8.4+/-0.9 in control to 6.6+/-2.0 APs/600 ms). In capsaicin-responsive phasic neurons, substance P and 4-aminopyridine induced multiple spikes, an effect that was reversed by ralfinamide (25 microM). In addition to effects on firing, ralfinamide increased the threshold, decreased the overshoot, and increased the rate of rise of the AP. To conclude, ralfinamide suppressed afferent hyperexcitability selectively in capsaicin-responsive, presumably nociceptive neurons, but had no measurable effects on firing in CAPS-unresponsive neurons. The action of ralfinamide to selectively inhibit tonic firing in nociceptive neurons very likely contributes to the effectiveness of the drug in reducing inflammatory and neuropathic pain as well as bladder overactivity.

Lidocaine Promotes the Trafficking and Functional Expression of Nav1.8 Sodium Channels in Mammalian Cells

Nociceptive neurons of the dorsal root ganglion (DRG) express a combination of rapidly gating TTX-sensitive and slowly gating TTX-resistant Na currents, and the channels that produce these currents have been cloned. The Nav1.7 and Nav1.8 channels encode for the rapidly inactivating TTX-sensitive and slowly inactivating TTX-resistant Na currents, respectively. Although the Nav1.7 channel expresses well in cultured mammalian cell lines, attempts to express the Nav1.8 channel using similar approaches has been met with limited success. The inability to heterologously express Nav1.8 has hampered detailed characterization of the biophysical properties and pharmacology of these channels. In this study, we investigated the determinants of Nav1.8 expression in tsA201 cells, a transformed variant of HEK293 cells, using a combination of biochemistry, immunochemistry, and electrophysiology. Our data indicate that the unusually low expression levels of Nav1.8 in tsA201 cells results from a trafficking defect that traps the channel protein in the endoplasmic reticulum. Incubating the cultured cells with the local anesthetic lidocaine dramatically enhanced the cell surface expression of functional Nav1.8 channels. The biophysical properties of the heterologously expressed Nav1.8 channel are similar but not identical to those of the TTX-resistant Na current of native DRG neurons, recorded under similar conditions. Our data indicate that the lidocaine acts as a molecular chaperone that promotes efficient trafficking and increased cell surface expression of Nav1.8 channels.

Effects of capsaicin on VGSCs in TRPV1-/- mice

Two different mechanisms by which capsaicin blocks voltage-gated sodium channels (VGSCs) were found by using knockout mice for the transient receptor potential V1 (TRPV1(-/-)). Similar with cultured rat trigeminal ganglion (TG) neurons, the amplitude of tetrodotoxin-resistant (TTX-R) sodium current was reduced 85% by 1 muM capsaicin in capsaicin sensitive neurons, while only 6% was blocked in capsaicin insensitive neurons of TRPV1(+/+) mice. The selective effect of low concentration capsaicin on VGSCs was reversed in TRPV1(-/-) mice, which suggested that this effect was dependent on TRPV1 receptor. The blockage effect of high concentration capsaicin on VGSCs in TRPV1(-/-) mice was the same as that in capsaicin insensitive neurons of rats and TRPV1(+/+) mice. It is noted that non-selective effect of capsaicin on VGSCs shares many similarities with local anesthetics. That is, firstly, both blockages are concentration-dependent and revisable. Secondly, being accompanied with the reduction of amplitude, voltage-dependent inactivation curve shifts to hyperpolarizing direction without a shift of activation curve. Thirdly, use-dependent blocks are induced at high stimulus frequency.

The Effects of the Local Anesthetics Lidocaine and Procaine on Glycine and γ-Aminobutyric Acid Receptors Expressed in Xenopus Oocytes

BACKGROUND

The voltage-dependent sodium channel is the primary site of action for local anesthetics (LAs). Although systemically administered low-dose LAs have been shown to exert antihyperalgesic effects, the molecular targets responsible for these effects are not fully known and their functional effects on inhibitory neurotransmitter receptors associated with antinociception have not been sufficiently studied.

METHODS

We examined the effects of lidocaine and procaine (0.1 microM to 3 or 10 mM) on recombinant human alpha1 glycine, alpha1beta2gamma2S gamma-aminobutyric acid type A (GABA(A)), and rho1 GABA(C) receptors expressed in Xenopus laevis oocytes, using a two-electrode voltage-clamp system. We also evaluated the effects of LAs on two mutant glycine receptors, alpha1(S267C) and alpha1(S267Q), in an effort to clarify the interaction between LAs and glycine receptors.

RESULTS

Low concentrations of both lidocaine and procaine enhanced glycine receptor function, whereas high concentrations of lidocaine and procaine inhibited glycine receptor function. Lidocaine (10 microM) produced a significant leftward shift in the glycine concentration-response curve, indicating an increase in the apparent affinity for glycine. This enhancement was not altered in the mutant receptors. Both lidocaine and procaine at high concentrations inhibited GABA(A) receptor currents, whereas neither lidocaine nor procaine affected GABA(C) receptor function.

CONCLUSIONS

Lidocaine and procaine enhanced glycine receptor function at low concentrations and inhibited the functions of glycine and GABA(A) receptors at high concentrations. The mechanism of the LA-induced enhancement of glycine receptor function probably differs from that of general anesthetics. These findings may explain the pharmacological effects of LAs, such as antinociception and convulsion.

Relationship Between the Firing Frequency of Injured Peripheral Neurons and Inhibition of Firing by Sodium Channel Blockers

Animal models of neuropathic pain in which a peripheral nerve is damaged result in spontaneous activity in primary afferents that can be inhibited by intravenous administration of sodium channel blockers. Many of these compounds exhibit use-dependent block of sodium current, leading to the prediction that they should more readily inhibit neurons that fire at higher frequencies. This prediction was tested in 2 rat models of nerve injury, L5 spinal nerve section and sciatic nerve section. Sciatic nerve section produced average firing frequencies that were higher than spinal nerve section and often manifested as high-frequency bursting. Inhibition of firing by intravenous sodium channel blockers was longer lasting in this model. Within each model, higher frequency of firing did not translate into more effective block. In the spinal nerve section model, there was a robust inverse correlation between frequency and inhibition. Within the sciatic section model, only neurons that fired in rhythmic bursts were inhibited, and again, those firing at lower mean frequencies were more effectively inhibited. These results indicate that the efficacy of sodium channel blockers depends on the nature of the injury and the pattern of the resulting activity rather than simply the frequency of action potentials generated.

PERSPECTIVE

This study examines the ability of frequency-dependent sodium channel blockers to inhibit spontaneous firing of injured peripheral nerves in vivo. It outlines the conditions under which inhibition is more and less effective and will provide insight into conditions under which sodium channel blockers are likely to be therapeutically useful.

An overview of the dental pulp: its functions and responses to injury

The dental pulp is a unique tissue and its importance in the long-term prognosis of the tooth is often ignored by clinicians. It is unique in that it resides in a rigid chamber which provides strong mechanical support and protection from the microbial rich oral environment. If this rigid shell loses its structural integrity, the pulp is under the threat of the adverse stimuli from the mouth, such as caries, cracks, fractures and open restoration margins, all of which provide pathways for micro-organisms and their toxins to enter the pulp. The pulp initially responds to irritation by becoming inflamed and, if left untreated, this will progress to pulp necrosis and infection. The inflammation will also spread to the surrounding alveolar bone and cause periapical pathosis. The magnitude of pulp-related problems should not be underestimated since their most serious consequence is oral sepsis, which can be life threatening, and hence correct diagnosis and management are essential. Clinicians must have a thorough understanding of the physiological and pathological features of the dental pulp as well as the biological consequences of treatment interventions.

Context-dependent prefrontal cortex regulation of cocaine self-administration and reinstatement behaviors in rats

Evidence of stimulus attribute-specificity within the prefrontal cortex (PFC) suggests that different prefrontal subregions may contribute to cocaine addiction in functionally distinct ways. Thus, the present study examined the effects of lidocaine-induced inactivation of two distinct PFC subregions, the prelimbic (PL) or dorsal agranular insular (AId) cortices, on drug-seeking and drug-taking behaviors under cocaine maintenance and reinstatement testing conditions in rats trained to self-administer 1 mg/kg cocaine under a second-order schedule of drug delivery. Throughout maintenance and reinstatement phases, rats were exposed to conditioned light cues and contextual odor or sound cues. Results showed that PL inactivation during maintenance test sessions significantly reduced drug-seeking and drug-taking behaviors, and disrupted patterns of responding in rats exposed to light-sound, but not light-odor, cues. Moreover, lidocaine-induced inactivation of the PL significantly attenuated drug-seeking behavior during cue-induced and cocaine prime-induced reinstatement in rats exposed to light-sound cues only. In contrast, AId inactivation significantly attenuated cue-induced reinstatement of drug-seeking behavior in rats exposed to light-odor cues only. Drug-seeking and drug-taking behaviors in these rats were not disrupted during maintenance and cocaine prime-induced reinstatement testing regardless of the type of contextual cues used. Together, these data suggest that PL and AId subregions play separate yet overlapping roles in regulating cocaine addiction in rats in ways that are dependent on the presence or absence of cocaine and on the types of contextual cues present in the cocaine self-administration environment.

Use-Dependent Block by Lidocaine but Not Amitriptyline Is More Pronounced in Tetrodotoxin (TTX)-Resistant Nav1.8 Than in TTX-Sensitive Na+ Channels

The excitability of sensory neurons depends on the expression of various voltage-gated Na+ channel isoforms. The tetrodotoxin-resistant (TTXr) Na+ channel Na(v)1.8 accounts for the electroresponsiveness of nociceptive neurons and contributes to inflammatory and neuropathic pain. Na+ channel blockers are clinically employed for chronic pain management, but side effects limit their use. There is conflicting information whether their potency to block tetrodotoxin-sensitive (TTXs) and TTXr Na+ channels differs. We analyzed the action of lidocaine and amitriptyline on TTXr Na(v)1.8 heterologously expressed in ND7/23 cells in comparison with TTXs Na+ channels endogenously expressed in ND7/23 cells. TTXr Na(v)1.8 and TTXs currents were investigated under whole-cell voltage-clamp. At a holding potential of -80 mV, lidocaine was 5-fold and amitriptyline 8-fold more potent to tonically block TTXs than Na(v)1.8 currents. This was due to a higher percentage of TTXs channels residing in the inactivated, high-affinity state at this potential. Tonic block of either resting or inactivated channels by lidocaine or amitriptyline revealed little differences between TTXs and Na(v)1.8 channels. Use-dependent block by amitriptyline was similar in TTXs and Na(v)1.8 channels. Surprisingly, use-dependent block by lidocaine was more pronounced in Na(v)1.8 than in TTXs channels. This result was confirmed in dorsal root ganglion neurons and is associated with the greater tendency of Na(v)1.8 to enter a slow inactivated state. Our data suggest that lidocaine could selectively block Na(v)1.8-mediated action potential firing. It is conceivable that the expression pattern of Na+ channels in sensory neurons might influence the efficiency of Na+ channel blockers used for chronic pain management.

Tramadol, fentanyl and sufentanil but not morphine block voltage-operated sodium channels

Lidocaine-like sodium channel blocking drugs provide pain relief either by interrupting impulse conduction in neurons when applied locally in high concentrations or, when given systemically, by suppressing high-frequency ectopic discharges due to preferential drug binding to inactivated channel states. Lidocaine-like actions of opioids have frequently been demonstrated clinically. However, drug binding to resting and inactivated channel conformations has been studied systematically only in the case of meperidine. The aim of this in vitro study was to investigate the effects of four currently used opioids on heterologously expressed neuronal (NaV(1.2)) voltage-gated sodium channels. Block of sodium currents was studied at hyperpolarized holding potentials and at depolarized potentials inducing either fast- or slow-inactivation. Sufentanil, fentanyl and tramadol but not morphine reversibly suppressed sodium inward currents at high concentrations (half-maximum blocking concentrations (IC50) 49+/-4, 141+/-6 and 103+/-8 microM) when depolarizations were started from hyperpolarized holding potentials. Short depolarizations inducing fast-inactivation and long prepulses inducing slow-inactivation significantly (*p < or = 0.001) increased the blocking potency for these opioids. 15% slow inactivated channels reduced the respective IC50 values to 5+/-3, 12+/-2 and 21+/-2 microM. These results show that: (1) Sufentanil, fentanyl and tramadol block voltage-gated sodium channels with half-maximum inhibitory concentrations similar to the IC50 reported for meperidine. (2) Slow inactivation--a physiological mechanism to suppress ectopic activity in response to slow shifts in membrane potential--increases binding affinity for sufentanil, fentanyl and tramadol. (3) Morphine has no such effects.

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.

Up-regulation of acid-sensing ion channel 3 in dorsal root ganglion neurons following application of nucleus pulposus on nerve root in rats

STUDY DESIGN

Immunocytochemistry for acid-sensing ion channel 3 (ASIC3) in neurons of rat dorsal root ganglions (DRGs) from animals exposed to a model of lumbar disc herniation.

OBJECTIVE

To examine expression of ASIC3 in DRGs and the effect of a sodium channel blocker applied to the nerve root in a rat model of lumbar disc herniation.

SUMMARY OF BACKGROUND DATA

Radicular pain is a common symptom of lumbar disc herniation in human beings. A depolarizing sodium channel gated by protons during tissue acidosis, ASIC3, is specifically expressed in sensory neurons. It has been associated with cardiac ischemic and inflammatory pain. We often perform spinal nerve root block for radicular pain using a sodium channel blocker, such as lidocaine; however, it has been unclear whether the effective period of this treatment is usually longer than the expected duration of efficacy.

METHODS

For the lumbar disc herniation model, nucleus pulposus was harvested from the tail and applied to the L5 nerve root, and the nerve roots were pinched. We evaluated mechanical allodynia in sham-operated animals and a disc herniation model. Immunohistochemistry was used to examine ASIC3 expression in L5 DRGs. Finally, the effect of lidocaine on pain and ASIC3 expression in the disc herniation model was examined.

RESULTS

Animals exposed to the lumbar disc herniation model showed allodynia for 8 days, and ASIC3 immunoreactivity was up-regulated in DRG neurons. After administration of lidocaine to spinal nerve roots affected by disc herniation, ASIC3 immunoreactivity was down-regulated in DRG neurons, and the level of mechanical allodynia was significantly decreased for 8 days.

CONCLUSIONS

Our results suggest that ASIC3 in DRG neurons may play an important role in nerve root pain caused by lumbar disc herniation. Lidocaine decreased ASIC3 expression in DRG neurons and pain associated with the disc herniation model.

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.

Comparison of the effects of four Na+ channel analgesics on TTX-resistant Na+ currents in rat sensory neurons and recombinant Nav1.2 channels

Na(+) channel blockers are highly effective analgesics. Among the neuronal Na(+) channel subtypes, Nav1.8 is discussed to be of importance for certain pain states, and Nav1.8-preferring Na(+) channel blockers should be able to relief pain without causing severe effects (due to the restricted expression of this channel type). In this study, the effects of four Na(+) channel blockers on rat tetrodotoxin-resistant (TTX-r) Na(+) channels (representing mostly Nav1.8) in sensory neurons were investigated using the patch-clamp technique in the voltage-clamp configuration, and compared with those on cells heterologously expressing Nav1.2 alpha subunits. The compounds were lidocaine, mexiletine, benzocaine, and ambroxol, which are clinically used to treat pain after local or systemic administration. The four compounds inhibited resting TTX-r channels concentration-dependently, with ambroxol being the most effective (IC(50) value: 34.3 microM), and benzocaine being the weakest (IC(50) value: 1,901 microM). All compounds shifted steady-state inactivation curves to more negative values. Ambroxol blocked resting TTX-r channels more potently than Nav1.2, the opposite was the case for lidocaine, mexiletine and benzocaine. Based on the drugs' potencies found in this study, and the published information on clinically achievable plasma levels, the amount of Na(+) channel block to induce analgesia after systemic administration was estimated.

Taurine-induced modulation of voltage-sensitive Na+ channels in rat dorsal root ganglion neurons

The physiological role of taurine, an abundant free amino acid in the neural system, is still poorly understood. The aim of this study was to investigate its effect on TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) Na+ currents in enzymatically dissociated neurons from rat dorsal root ganglion (DRG) with conventional whole-cell recording manner under voltage-clamp conditions. A TTX-S Na+ current was recorded preferentially from large DRG neurons and a TTX-R Na+ current preferentially from small ones. For TTX-S Na+ channel, taurine of the concentration > or = 10 mM shifted the activation curve in the depolarizing direction and the inactivation curve in the hyperpolarizing direction. There was no change in the activation curve for TTX-R Na+ channel and the inactivation curve was shifted in the hyperpolarizing direction slightly in the presence of taurine > or = 20 mM. When the recovery kinetics was examined, the presence of taurine resulted in a slower recovery from inactivation of TTX-S currents and no change of TTX-R ones. All the effects of taurine were weakly concentration-dependent and partly recovered quite slowly after washout. Our data indicate that taurine alters the properties of Na+ currents in intact DRG neurons. These may contribute to the understanding of taurine as a natural neuroprotectant and the potential of taurine as a useful medicine for the treatment of sensory neuropathies.

Neuropathic pain: early spontaneous afferent activity is the trigger

Intractable neuropathic pain often results from nerve injury. One immediate event in damaged nerve is a sustained increase in spontaneous afferent activity, which has a well-established role in ongoing pain. Using two rat models of neuropathic pain, the CCI and SNI models, we show that local, temporary nerve blockade of this afferent activity permanently inhibits the subsequent development of both thermal hyperalgesia and mechanical allodynia. Timing is critical-the nerve blockade must last at least 3-5 days and is effective if started immediately after nerve injury, but not if started at 10 days after injury when neuropathic pain is already established. Effective nerve blockade also prevents subsequent development of spontaneous afferent activity measured electrophysiologically. Similar results were obtained in both pain models, and with two blockade methods (200mg of a depot form bupivacaine at the injury site, or perfusion of the injured nerve just proximal to the injury site with TTX). These results indicate that early spontaneous afferent fiber activity is the key trigger for the development of pain behaviors, and suggest that spontaneous activity may be required for many of the later changes in the sensory neurons, spinal cord, and brain observed in neuropathic pain models. Many pre-clinical and clinical studies of pre-emptive analgesia have used much shorter duration of blockade, or have not started immediately after the injury. Our results suggest that effective pre-emptive analgesia can be achieved only when nerve block is administered early after injury and lasts several days.

Ambroxol, a Nav1.8-preferring Na(+) channel blocker, effectively suppresses pain symptoms in animal models of chronic, neuropathic and inflammatory pain

Neuropathic pain affects many patients, and treatment today is far from being perfect. Nav1.8 Na(+) channels, which are expressed by small fibre sensory neurons, are promising targets for novel analgesics. Na(+) channel blockers used today, however, show only limited selectivity for this channel subtype, and can cause dose-limiting side effects. Recently, the secretolytic ambroxol was found to preferentially inhibit Nav1.8 channels. We used this compound as a tool to investigate whether a Nav1.8-preferring blocker can suppress symptoms of chronic, neuropathic and inflammatory pain in animal models. The drug was tested in the formalin paw model, two models of mononeuropathy, and a model of monoarthritis in rats. Ambroxol's effects were compared with those of gabapentin. Ambroxol at a dose of 1g/kg had to be administered to rats to achieve the plasma levels that are reached in clinical use (for the treatment of infant and acute respiratory distress syndrome). Ambroxol (1g/kg) was only weakly effective in models for acute pain, but effectively reduced pain symptoms in all other models; in some cases it completely reversed pain behaviour. In most cases the effects were more pronounced than those of gabapentin (at 100mg/kg). These data show that a Nav1.8-preferring Na(+) channel blocker can effectively suppress pain symptoms in a variety of models for chronic, neuropathic and inflammatory pain at plasma levels, which can be achieved in the clinic.

Contribution of the tetrodotoxin-resistant voltage-gated sodium channel NaV1.9 to sensory transmission and nociceptive behavior

The transmission of pain signals after injury or inflammation depends in part on increased excitability of primary sensory neurons. Nociceptive neurons express multiple subtypes of voltage-gated sodium channels (NaV1s), each of which possesses unique features that may influence primary afferent excitability. Here, we examined the contribution of NaV1.9 to nociceptive signaling by studying the electrophysiological and behavioral phenotypes of mice with a disruption of the SCN11A gene, which encodes NaV1.9. Our results confirm that NaV1.9 underlies the persistent tetrodotoxin-resistant current in small-diameter dorsal root ganglion neurons but suggest that this current contributes little to mechanical thermal responsiveness in the absence of injury or to mechanical hypersensitivity after nerve injury or inflammation. However, the expression of NaV1.9 contributes to the persistent thermal hypersensitivity and spontaneous pain behavior after peripheral inflammation. These results suggest that inflammatory mediators modify the function of NaV1.9 to maintain inflammation-induced hyperalgesia.

Evaluation of the rat bladder-derived relaxant factor by coaxial bioassay system

The release of bladder-derived relaxant factor in a coaxial bioassay system and the effects of reactive oxygen species were studied. After precontraction with phenylephrine (10(-6)-3x10(-6)) or 50 mM K+, acetylcholine (10(-8)-10(-3) M) induced relaxation in rat anococcygeus muscle mounted within rat bladder in a tissue bath. This relaxation was not altered by the removal of the urothelium or incubation with tetrodotoxin (10(-6) M). However, bupivacaine (10(-4) M) and lidocaine (3 x 10(-4) M) inhibited this response after raising the pH of the nutrient solution to 7.8, and oxybuprocaine (10(-4) M) exerted inhibitory effect at both physiological pH (7.4) and at pH 7.8. Exposure to electrolysis-generated reactive oxygen species or incubation with hydrogen peroxide and pyrogallol did not alter the acetylcholine response. Present results indicate that the bladder-derived relaxant factor does not behave like endothelium-derived hyperpolarizing factor, but its release may be associated with tetrodotoxin-resistant Na+ channels, which are probably in the neurons of the bladder rather than in the urothelium or detrusor muscle. Furthermore, reactive oxygen species do not interact with this relaxing factor, the exact nature and the physiological importance of which, however, remains to be established.

Synaptic potentiation induced by rTMS: effect of lidocaine infusion

Repetitive transcranial magnetic stimulation (rTMS) delivered at various intensities and frequencies excites cortical motor areas. Trains of stimuli (at 5-Hz frequency, and suprathreshold intensity) progressively increase the size of motor evoked potentials (MEPs) and the duration of the cortical silent period (CSP) in normal subjects. Because antiepileptic drugs, acting mainly on sodium channels, depress MEP facilitation during rTMS, we suggested that rTMS trains facilitate the MEP size by inducing synaptic potentiation primarily involving voltage-gated sodium channels. The aim of this study was to evaluate the effect of lidocaine-a drug that acts selectively on sodium channels-on the rTMS-induced changes in cortical excitability. We tested the changes in motor threshold, MEP size, CSP duration evoked by focal rTMS and the M-wave amplitude in healthy subjects before and after lidocaine infusion. Lidocaine abolished the normal rTMS-induced facilitation of MEPs but left the other rTMS variables and the M-wave unchanged. Our results suggest that the MEP facilitation related to rTMS-induced synaptic potentiation results from an increase in cortical excitatory interneuron excitability that involves voltage-gated sodium channels.

Block of tetrodotoxin-resistant Na+ channel pore by multivalent cations: gating modification and Na+ flow dependence

Tetrodotoxin-resistant (TTX-R) Na(+) channels are much less susceptible to external TTX but more susceptible to external Cd(2+) block than tetrodotoxin-sensitive (TTX-S) Na(+) channels. Both TTX and Cd(2+) seem to block the channel near the "DEKA" ring, which is probably part of a multi-ion single-file region adjacent to the external pore mouth and is involved in the selectivity filter of the channel. In this study we demonstrate that other multivalent transitional metal ions such as La(3+), Zn(2+), Ni(2+), Co(2+), and Mn(2+) also block the TTX-R channels in dorsal root ganglion neurons. Just like Cd(2+), the blocking effect has little intrinsic voltage dependence, but is profoundly influenced by Na(+) flow. The apparent dissociation constants of the blocking ions are always significantly smaller in inward Na(+) currents than those in outward Na(+) current, signaling exit of the blocker along with the Na(+) flow and a high internal energy barrier for "permeation" of these multivalent blocking ions through the pore. Most interestingly, the activation and especially the inactivation kinetics are slowed by the blocking ions. Moreover, the gating changes induced by the same concentration of a blocking ion are evidently different in different directions of Na(+) current flow, but can always be correlated with the extent of pore block. Further quantitative analyses indicate that the apparent slowing of channel activation is chiefly ascribable to Na(+) flow-dependent unblocking of the bound La(3+) from the open Na(+) channel, whereas channel inactivation cannot happen with any discernible speed in the La(3+)-blocked channel. Thus, the selectivity filter of Na(+) channel is probably contiguous to a single-file multi-ion region at the external pore mouth, a region itself being nonselective in terms of significant binding of different multivalent cations. This region is "open" to the external solution even if the channel is "closed" ("deactivated"), but undergoes imperative conformational changes during the gating (especially the inactivation) process of the channel.

Regional anaesthesia, local anaesthetics and the surgical stress response

Epidural anaesthesia has the potential to improve patients' outcome after major surgical procedures by reducing postoperative morbidity and duration of recovery. Possible benefits include the attenuation of cardiac complications, an earlier return of gastrointestinal function associated with an increase in patients' comfort overall, decreased incidence of pulmonary dysfunction, beneficial effects on the coagulation system and a reduction in the inflammatory response. The underlying mechanisms, however, remain unclear. Since local anaesthetics (LAs), reabsorbed from the epidural space, seem to contribute to these effects, it is not easy to differentiate between the systemic effects of LAs and the effects of neuraxial blockade by epidural anaesthesia. Thus, in patients not able or willing to receive intra- and/or postoperative epidural analgesia, systemic administration of LAs may be considered to be a new therapeutic approach for the prevention of postoperative disorders by modulation of the peri- and postoperative inflammatory.

Effects of alcohols and anesthetics on recombinant voltage-gated Na+ channels

Voltage-gated Na(+) channels (Na(+) channels) mediate the rising phase of action potentials in neurons and excitable cells. Nine subtypes of the alpha subunit (Na(v)1.1-Na(v)1.9) have been shown to form functional Na(+) channels to date. Recently, anesthetic concentrations of volatile anesthetics and ethanol were reported to inhibit Na(+) channel functions, but it is not known whether all subtypes are inhibited by anesthetics. To investigate possible subtype-specific effects of anesthetics on Na(+) channels, mRNA of Na(v)1.2, Na(v)1.4, Na(v)1.6, and Na(v)1.8 alpha subunit-encoded genes were injected individually or together with a beta subunit mRNA into Xenopus oocytes. Na(+) currents were recorded using the two-electrode voltage-clamp technique. Isoflurane, at clinically relevant concentrations, inhibited the currents produced by Na(v)1.2, Na(v)1.4, and Na(v)1.6 by approximately 10% at the holding potential of -90 mV and by approximately 30% at -60 mV, but it did not affect the Na(v)1.8-mediated current. An anesthetic fluorocyclobutane (1-chloro-1,2,2-trifluorocyclobutane) also inhibited the Na(v)1.2 channel, whereas the nonanesthetic fluorocyclobutane (1,2-dichlorohexafluorocyclobutane) had no effect. The perfluorinated heptanol [CF(3)(CF(2))(5)CH(2)OH], which produces anesthesia, inhibited the Na(v)1.2 channel like other alcohols tested (ethanol, heptanol, and CF(3)CH(2)OH), even though this compound does not affect GABA, glycine, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, or kainate receptors. In contrast, most intravenous anesthetics did not have significant effects on the Na(v)1.2 channel at clinically relevant concentrations although urethane inhibited. These results show that isoflurane inhibits the Na(+) channel functions except Na(v)1.8 in a voltage-dependent manner. These findings indicate that the Na(+) channel is a neuronal target for anesthetic action.

Role of tetrodotoxin-resistant Na+ current slow inactivation in adaptation of action potential firing in small-diameter dorsal root ganglion neurons

When acutely dissociated small-diameter dorsal root ganglion (DRG) neurons were stimulated with repeated current injections or prolonged application of capsaicin, their action potential firing quickly adapted. Because TTX-resistant (TTX-R) sodium current in these presumptive nociceptors generates a large fraction of depolarizing current during the action potential, we examined the possible role of inactivation of TTX-R sodium channels in producing adaptation. Under voltage clamp, TTX-R current elicited by short depolarizations showed strong use dependence at frequencies as low as 1 Hz, although recovery from fast inactivation was complete in approximately 10-30 msec. This use-dependent reduction was the result of the entry of TTX-R sodium channels into slow inactivated states. Slow inactivation was more effectively produced by steady depolarization than by cycling channels through open states. Slow inactivation was steeply voltage dependent, with a Boltzmann slope factor of 5 mV, a midpoint near -45 mV (5 sec conditioning pulses), and completeness of approximately 93% positive to -20 mV. The time constant for entry (approximately 200 msec) was independent of voltage from -20 mV to +60 mV, whereas recovery kinetics were moderately voltage dependent (time constant, approximately 1.5 sec at -60 mV and approximately 0.5 sec at -100 mV). Using a prerecorded current-clamp response to capsaicin as a voltage-clamp command waveform, we found that adaptation of firing occurred with a time course similar to that of development of slow inactivation. Thus, slow inactivation of the TTX-R sodium current limits the duration of small DRG cell firing in response to maintained stimuli and may contribute to cross desensitization between chemical and electrical stimuli.

Local anaesthetics block hyperpolarization-activated inward current in rat small dorsal root ganglion neurones

(1) Hyperpolarizing voltage steps evoke slowly activating inward currents in a variety of neurones and in cardiac cells. This hyperpolarization-activated inward current (I(h)) is thought to play a significant role in cell excitability, firing frequency, or in setting of the resting membrane potential in these cells. We studied the effects of lidocaine, mepivacaine, QX-314 and bupivacaine as well as its enantiomers on I(h) in the membrane of dorsal root ganglion neurones (DRG). (2) The patch-clamp technique was applied to small dorsal root ganglion neurones identified in 200 micro M thin slices of young rat DRGs. Under voltage-clamp conditions, the whole-cell I(h) current was recorded in the presence of different concentrations of the local anaesthetics. In current-clamp mode the resting membrane potential and the voltage response of DRG neurones to injected current pulses were investigated. (3) I(h) was reversibly blocked by bupivacaine, lidocaine and mepivacaine applied externally in clinically relevant concentrations. Concentration-response curves gave half-maximum inhibiting concentrations of 55, 99 and 190 micro M, respectively. Bupivacaine block of the I(h) current was not stereoselective. No significant effect was observed when QX-314 was applied to the external surface of the membrane. (4) In current-clamp experiments 60 micro M bupivacaine slightly hyperpolarized the membrane. The membrane stimulation by low-amplitude current pulses in the presence of bupivacaine showed an increase of the hyperpolarizing responses. (5) Our findings suggest an important role of the I(h)-block by local anaesthetics in the complex mechanism of drug action during epidural and spinal anaesthesia.

Blockade of voltage-operated neuronal and skeletal muscle sodium channels by S(+)- and R(-)-ketamine

Besides its general anesthetic effect, ketamine has local anesthetic-like actions. We studied the voltage- and use-dependent interaction of S(+)- and R(-)-ketamine with two different isoforms of voltage-operated sodium channels, with a special emphasis on the difference in affinity between resting and inactivated channel states. Rat brain IIa and human skeletal muscle sodium channels were heterologously expressed in human embryonic kidney 293 cells. S(+)- and R(-)-ketamine reversibly suppressed whole-cell sodium inward currents; the 50% inhibitory concentration values at -70 mV holding potential were 240 +/- 60 microM and 333 +/- 93 microM for the neuronal isoform and 59 +/- 10 microM and 181 +/- 49 microM for the skeletal muscle isoform. S(+)-ketamine was significantly more potent than R(-)-ketamine in the skeletal muscle isoform only. Ketamine had a higher affinity to inactivated than to resting channels. However, the estimated difference in affinity between inactivated and resting channels was only 8- to 10-fold, and the time course of drug equilibration between inactivated and resting channels was too fast to cause use-dependent block at 10 Hz up to a concentration of 300 microM. These results suggest that ketamine is less effective than lidocaine-like local anesthetics in stabilizing the inactivated channel state.

IMPLICATIONS

Blockade of sodium channels by ketamine shows voltage dependency, an important feature of local anesthetic action. However, ketamine is less effective than lidocaine-like local anesthetics in stabilizing the inactivated state. Because it does not elicit phasic blockade at small concentrations, its ability to reduce the firing frequency of action potentials may be small.

Kv1.1 channels of dorsal root ganglion neurons are inhibited by n-butyl-p-aminobenzoate, a promising anesthetic for the treatment of chronic pain

In this study, we investigated the effects of the local anesthetic n-butyl-p-aminobenzoate (BAB) on the delayed rectifier potassium current of cultured dorsal root ganglion (DRG) neurons using the patch-clamp technique. The majority of the K(+) current of small DRG neurons rapidly activates and slowly inactivates at depolarized voltages. BAB inhibited the whole-cell K(+) current of these neurons with an IC(50) value of 228 microM. Dendrotoxin K (DTX(K)), a specific inhibitor of Kv1.1, reduced the DRG K(+) current at +20 mV by 34%, consistent with an important contribution of channels incorporating the Kv1.1 subunit to the delayed rectifier current. To further investigate the mechanism of BAB inhibition, we examined its effect on Kv1.1 channels heterologously expressed in mammalian tsA201 cells. BAB inhibits the Kv1.1 channels with an IC(50) value of 238 microM, similar to what was observed for the native DRG current. BAB accelerates the opening and closing of Kv1.1, but does not alter the midpoint of steady-state activation. BAB seems to inhibit Kv1.1 by stabilizing closed conformations of the channel. Coexpression with the Kv beta 1 subunit induces rapid inactivation and reduces the BAB sensitivity of Kv1.1. Comparison of the heterologously expressed Kv1.1 and native DRG currents indicates that the Kv beta 1 subunit does not modulate the gating of the DTX(K)-sensitive Kv1.1 channels of DRG neurons. Inhibition of the delayed rectifier current of these neurons may contribute to the long-duration anesthesia attained during the epidural administration of BAB.

Inhibition of tetrodotoxin (TTX)-resistant and TTX-sensitive neuronal Na(+) channels by the secretolytic ambroxol

Ambroxol has a long history for the treatment of airway diseases because of its beneficial effects on surfactant synthesis and mucus-modifying properties. Some findings, however, point to an additional effect on neuronal signal transduction: ambroxol can suppress reflexes such as the cough or the corneal reflex. The airways and the cornea are innervated by C-fibers, which express voltage-gated Na(+) channels with and without sensitivity to tetrodotoxin (TTX). In this study, we performed voltage-clamp experiments to investigate whether ambroxol affects these channel types. In rat dorsal root ganglia, TTX-resistant Na(+) currents were suppressed in a concentration-dependent manner with IC(50) values of 35.2 and 22.5 microM for tonic and phasic block, respectively. Depolarizing prepulses increased the potency of ambroxol, and steady-state inhibition curves were shifted to more negative values. The inhibition was not frequency-dependent. TTX-sensitive currents were inhibited with lower potency (approximately 50% inhibition with 100 microM). Recombinant rat brain IIA channels in Chinese hamster ovary cells were blocked with IC(50) values of 111.5 and 57.6 microM for tonic and phasic block, respectively; in contrast to TTX-resistant channels the block was frequency-dependent. Thus, ambroxol indeed blocks neuronal voltage-gated Na(+) channels, and TTX-resistant channels in sensory neurons were more sensitive than TTX-sensitive. Compared with known local anesthetics (e.g., lidocaine or benzocaine), the potency for Na(+) channel block was relatively high. A recent clinical trial has further confirmed that ambroxol relieved pain and was beneficial in patients who suffered from sore throat.