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

Intrathecal sensory neuron-specific receptor agonists bovine adrenal medulla 8-22 and (Tyr6)-γ2-msh-6-12 inhibit formalin-evoked nociception and neuronal Fos-like immunoreactivity in the spinal cord of the rat

The finding that sensory neuron-specific G-protein-coupled receptor mRNA is solely expressed in small primary sensory neurons suggests involvement of the receptor in nociceptive modulation. The present study was designed to assess effects of intrathecal administration of bovine adrenal medulla 8-22 and (Tyr6)-gamma2-MSH-6-12, selective sensory neuron-specific receptor agonists, on nocifensive behaviors and expression of spinal c-Fos-like immunoreactivity evoked by intraplantar injection of 2.5% formalin in rats. The agonists were administered 10 min before (pretreatment) and/or after (post-treatment) injection of formalin. Pretreatment with bovine adrenal medulla 8-22 dose-dependently (3, 10 and 30 nmol) decreased time lifting and licking the paw mainly in the second phase. Intrathecal bovine adrenal medulla 8-22 (30 nmol) remarkably suppressed nocifensive behaviors in the first and second phases and the expression of formalin-evoked c-Fos-like immunoreactivity in laminae I-II and V-VI of the spinal dorsal horn at L4-5. Moreover, naloxone (20 microg, intrathecal) failed to antagonize the inhibitory effects of bovine adrenal medulla 8-22. Post-treatment with bovine adrenal medulla 8-22 also exerted inhibition on the second phase behaviors in a dose-dependent manner with a similar efficacy observed in pretreatment groups. Furthermore, post-treatment with (Tyr6)-gamma2-MSH-6-12 (0.5, 1.5 and 5 nmol) also suppressed formalin-evoked nocifensive behaviors in the second phase and c-Fos-like immunoreactivity in the spinal dorsal horn similar with bovine adrenal medulla 8-22. Our results suggest that sensory neuron-specific receptor may play an important role in modulation of spinal nociceptive transmission. This is the first to demonstrate that activation of sensory neuron-specific receptor produces analgesia in the persistent pain model.

cAMP and cGMP Contribute to Sensory Neuron Hyperexcitability and Hyperalgesia in Rats With Dorsal Root Ganglia Compression

Numerous studies have implicated the cAMP-protein kinase A (PKA) pathway in producing hyperexcitability of dorsal root ganglia (DRG) sensory neurons under conditions associated with pain. Evidence is presented for roles of both the cAMP-PKA and cGMP-protein kinase G (PKG) pathways in maintaining neuronal hyperexcitability and behavioral hyperalgesia in a neuropathic pain model: chronic compression of the DRG (CCD treatment). Lumbar DRGs were compressed by a steel rod inserted into the intervertebral foramen. Thermal hyperalgesia was revealed by shortened latencies of foot withdrawal to radiant heat. Intracellular recordings were obtained in vitro from lumbar ganglia after in vivo DRG compression. Activators of the cAMP-PKA pathway, 8-Br-cAMP and Sp-cAMPS, and of the cGMP-PKG pathway, 8-Br-cGMP and Sp-cGMPS, increased the hyperexcitability of DRG neurons already produced by CCD treatment, as shown by further decreases in action potential threshold and increased repetitive discharge during depolarization. The adenylate cyclase inhibitor, SQ22536, the PKA antagonist, Rp-cAMPS, the guanylate cyclase inhibitor, ODQ, and the PKG inhibitor, Rp-8-pCPT-cGMPS, reduced the hyperexcitability of CCD DRG neurons. In vivo application of PKA and PKG antagonists transiently depressed behavioral hyperalgesia induced by CCD treatment. Unexpectedly, application of these agonists and antagonists to ganglia of naïve, uninjured animals had little effect on electrophysiological properties of DRG neurons and no effect on foot withdrawal, suggesting that sensitizing actions of these pathways in the DRG are enabled by prior injury or stress. The only effect observed in uncompressed ganglia was modest depolarization of DRG neurons by PKA and PKG agonists. CCD treatment also depolarized DRG neurons, but CCD-induced depolarization was not affected by agonists or antagonists of these pathways.

Effects of extracellular δ-aminolaevulinic acid on sodium currents in acutely isolated rat hippocampal CA1 neurons

The effects of delta-aminolaevulinic acid (ALA) on voltage-gated sodium channel (VGSC) currents (I(Na)) in acutely isolated hippocampal CA1 neurons from 10- to 12-day-old Wistar rats were examined by using the whole-cell patch-clamp technique under voltage-clamp conditions. ALA from 0.01 microm to 20 microm was applied to the recorded neurons. Low concentrations of ALA (0.01-1.0 microM) increased I(Na) amplitude, whereas high concentrations of ALA (5.0-20.0 microM) decreased it. The average I(Na) amplitude reached a maximum of 117.4 +/- 3.9% (n = 9, P < 0.05) with 0.1 microM ALA, and decreased to 78.1 +/- 3.8% (n = 13, P < 0.05) with 10 microm ALA. ALA shifted the steady-state activation and inactivation curves of I(Na) in the hyperpolarizing direction with different V0.5, suggesting that ALA could depress the opening threshold of the voltage-gated sodium channel (VGSC) and thus increase the excitability of neurons through facilitating the opening of VGSC. The time course of recovery from inactivation was significantly prolonged at both low and high concentrations of ALA, whereas either low or high concentrations of ALA had no significant effect on the attenuation of I(Na) during stimulation at 5 Hz, indicating that the effect of ALA on VGSC is state-independent. Furthermore, we found that application of ascorbic acid, which blocks pro-oxidative effects in neurons, could prevent the increase of I(Na) amplitude at low concentrations of ALA. Baclofen, an agonist of GABAb receptors, induced some similar effects to ALA on VGSC, whereas bicuculline, an antagonist of GABAa receptors, could not prevent ALA-induced effects on VGSC. These results suggested that ALA regulated VGSC mainly through its pro-oxidative effects and GABAb receptor-mediated effects.

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.

Challenges in the development of novel treatment strategies for neuropathic pain

Neuropathic pain might best be considered as a collection of various pain states with a common feature, that being symptoms suggestive of dysfunction of peripheral nerves. The development of therapeutic options for the treatment of neuropathic pain is complicated significantly by several factors. Neuropathic pain may arise from widely diverse etiologies such as physical trauma, disease, infection, or chemotherapy. Symptoms indicative of neuropathic pain may also arise in individuals with no evidence of any type of nerve trauma (idiopathic). Although neuropathic pain is a substantial health care issue, it is relatively uncommon and only occurs in a small fraction (<10%) of individuals with these initiating factors. Moreover, the efficacy of treatment protocols, even against the same type of symptoms, differ depending on the underlying initiating cause of the neuropathy. Although these observations strongly suggest that there are predisposing factors that may impart susceptibility to the development of neuropathic pain, no common predisposing factors or genetic markers have been satisfactorily identified. Because of these vagaries, treatment of neuropathic pain has been based on trial and error. However, recent progress in the understanding of neurophysiologic changes that accompany peripheral nerve dysfunction indicate that regulation of ion channels that maintain membrane potentials or generate action potentials may provide an important therapeutic approach. Neuropathic pain is accompanied by increased activity of peripheral nociceptors, which is produced in part by changes in levels of specific calcium and sodium channels. The identification of sodium and/or calcium channels subtypes that are expressed almost exclusively on nociceptors may provide a way of regulating the activity of exaggerated nociceptor function without altering other sensory modalities. Thus, the selective targeting of ion channels may represent a viable therapeutic target for the management of the neuropathic pain state, regardless of etiology.

The treatment of neuropathic pain

Neuropathic pain is responsible for a significant amount of the morbidity associated with generalized and focal peripheral neuropathies. It is a consequence of alterations in neuronal function, chemistry, and structure that occur secondary to nerve injury. These manifestations of neuronal plasticity occur in the peripheral nerve, spinal cord, and brain. A variety of agents from diverse pharmacologic classes, the so-called adjuvant analgesics, have been used to treat neuropathic pain. These include antidepressants, first- and second-generation anticonvulsants, antiarrhythmic agents, topical agents, N-methyl-D-aspartate receptor antagonists, and opioid analgesics. The use of these adjuvant analgesics, either alone or in combination, should result in the alleviation of neuropathic pain in most patients. Recent advances in the understanding of pain mechanisms at multiple central nervous system levels should pave the way toward more effective treatment modalities.

The effect of carbamazepine on injury-induced ectopic discharge in the lingual nerve

Previous studies have shown that the development of ectopic activity from damaged axons following nerve injury may contribute to the aetiology of sensory disturbances, including dysaesthesia. Pharmacological manipulation of this activity could provide a method of treatment for this intractable condition. In this study we have investigated the effect of carbamazepine, an anti-convulsant, as it is known to have membrane stabilising properties. In eight anaesthetised adult ferrets the left lingual nerve was sectioned and the animals allowed to recover for 3 days. Then, in terminal experiments under general anaesthesia, the nerve was re-exposed and electrophysiological recordings were made from spontaneously active units in fine filaments dissected from the nerve proximal to the injury site. Carbamazepine in a modified cyclodextrin (hydroxypropyl-beta-cyclodextrin) was administered intravenously in increments, in order to achieve a progressively increasing systemic concentration, and serum levels were determined at the point that activity ceased. Twenty-one spontaneously active units were studied, with conduction velocities of 2.1-28.9 m s(-1) and discharge frequencies of 0.25-15.3 Hz. Spontaneous activity ceased in 13 units with a serum concentration of carbamazepine ranging from 3.5 to 8.4 mg/l, which was within the normal therapeutic range (4-12 mg/l). Four units ceased activity with carbamazepine levels above the therapeutic range (15.4-17.2 mg/ml), but the remaining four continued to discharge throughout the recording period. These data suggest that systemic carbamazepine can reduce the level of spontaneous activity initiated in some axons following lingual nerve injury.

Memory-like alterations in Aplysia axons after nerve injury or localized depolarization

Adaptive, long-term alterations of excitability have been reported in dendrites and presynaptic terminals but not along axons. Persistent enhancement of axonal excitability has been described in proximal nerve stumps at sites of nerve section in mammals, but this hyperexcitability is considered a pathological derangement important only as a cause of neuropathic pain. Identified neurons in Aplysia were used to test the hypothesis that either axonal injury or the focal depolarization that accompanies axonal injury can trigger a local decrease in action potential threshold [long-term hyperexcitability (LTH)] having memory-like properties. Nociceptive tail sensory neurons and a giant secretomotor neuron, R2, exhibited localized axonal LTH lasting 24 hr after a crush of the nerve or connective that severed the tested axons. Axons of tail sensory neurons and tail motor neurons, but not R2, displayed similar localized LTH after peripheral depolarization produced by 2 min exposure to elevated extracellular [K(+)]. Neither the induction nor expression of either form of LTH was blocked by saline containing 1% normal [Ca(2+)] during treatment or testing. However, both were prevented by local application of the protein synthesis inhibitors anisomycin or rapamycin. The features of (1) long-lasting alteration by localized depolarization, (2) restriction of alterations to intensely depolarized regions, and (3) dependence of the alterations on local, rapamycin-sensitive protein synthesis are shared with synaptic mechanisms considered important for memory formation. This commonality suggests that relatively simple, accessible axons may offer an opportunity to define fundamental plasticity mechanisms that were important in the evolution of memory.

Sodium-mediated axonal degeneration in inflammatory demyelinating disease

Axonal degeneration is a major cause of permanent neurological deficit in multiple sclerosis (MS). The mechanisms responsible for the degeneration remain unclear, but evidence suggests that a failure to maintain axonal sodium ion homeostasis may be a key step that underlies at least some of the degeneration. Sodium ions can accumulate within axons due to a series of events, including impulse activity and exposure to inflammatory factors such as nitric oxide. Recent findings have demonstrated that partial blockade of sodium channels can protect axons from nitric oxide-mediated degeneration in vitro, and from the effects of neuroinflammatory disease in vivo. This review describes some of the reasons why sodium ions might be expected to accumulate within axons in MS, and recent observations suggesting that it is possible to protect axons from degeneration in neuroinflammatory disease by partial sodium channel blockade.

Voltage-gated cation channel modulators for the treatment of stroke

Neuronal voltage-gated cation channels regulate the transmembrane flux of calcium, sodium and potassium. Neuronal ischaemia occurring during acute ischaemic stroke results in the breakdown in the normal function of these ion channels, contributing to a series of pathological events leading to cell death. A dramatic increase in the intracellular concentration of calcium during neuronal ischaemia plays a particularly important role in the neurotoxic cascade resulting in stroke-related acute neurodegeneration. One approach to provide therapeutic benefit following ischaemic stroke has been to target neuronal voltage-gated cation channels, and particularly blockers of calcium and sodium channels, for post-stroke neuroprotection. A recent development has been the identification of openers of large-conductance calcium- and voltage-dependent potassium channels (maxi-K channels), which hyperpolarize ischaemic neurons, reduce excitatory amino acid release, and reduce ischaemic calcium entry. Thus far, targeting these voltage-gated cation channels has not yet yielded significant clinical benefit. The reasons for this may involve the lack of small-molecule blockers of many neuronal members of these ion channel families and the design of preclinical stroke models, which do not adequately emulate the clinical condition and hence lack sufficient rigor to predict efficacy in human stroke. Furthermore, there may be a need for changes in clinical trial designs to optimise the selection of patients and the course of drug treatment to protect neurons during all periods of potential neuronal sensitivity to neuro-protectants. Clinical trials may also have to be powered to detect small effect sizes or be focused on patients more likely to respond to a particular therapy. The development of future solutions to these problems should result in an improved probability of success for the treatment of stroke.

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.

Synthesis and Structure−Activity Relationship Studies for Hydantoins and Analogues as Voltage-Gated Sodium Channel Ligands

We previously developed a preliminary 3-D QSAR model for the binding of 14 hydantoins to the neuronal voltage-gated sodium channel; this model was successful in designing an effective non-hydantoin ligand. To further understand structural features that result in optimum binding, here we synthesized a variety of compound classes and evaluated their binding affinities to the neuronal voltage-gated sodium channel using the [3H]-batrachotoxinin A 20-alpha-benzoate ([3H]BTX-B) binding assay. In order to understand the importance of the hydantoin ring for good sodium channel binding, related non-hydantoins such as hydroxy amides, oxazolidinediones, hydroxy acids, and amino acids were included. Two major conclusions were drawn: (1) The hydantoin ring is not critical for compounds with long alkyl side chains, but it is important for compounds with shorter side chains. (2) Relative to Khodorov's pharmacophore, which contains two hydrophobic regions, a third hydrophobic region may enhance binding to provide nanomolar inhibitors.

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.

Expression of the sodium channel beta3 subunit in injured human sensory neurons

Voltage-gated sodium channel alpha-subunits play a key role in pain pathophysiology, and are modulated by beta-subunits. We previously reported that beta1- and beta2-subunits were decreased in human sensory neurons after spinal root avulsion injury. We have now detected, by immunohistochemistry, beta3-subunits in 82% of small/medium and 67% of large diameter sensory neurons in intact human dorsal root ganglia: 54% of beta3 small/medium neurons were NGF receptor trkA negative. Unlike beta1- and beta2, beta3-immunoreactivity did not decrease after avulsion injury, and the beta3:neurofilament ratio was significantly increased in proximal injured human nerves. beta3-subunit expression may thus be regulated differently from beta1, beta2 and Nav1.8. Targeting beta3 interactions with key alpha-subunits, particularly Nav1.3 and Nav1.8, may provide novel selective analgesics.

Redox modulation of peripheral T-type Ca2+ channels in vivo: alteration of nerve injury-induced thermal hyperalgesia

We reported recently that redox agents, including the endogenous amino acid L-cysteine, modulate T-type Ca2+ currents in primary sensory neurons in vitro, and alter mechanical and thermal nociception in peripheral nociceptors in vivo in intact animals [Neuron 31 (2001) 75]. Here, we studied the effects of locally applied redox agents (L-cysteine and 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) on thermal hyperalgesia in animals with neuropathic pain due to chronic constrictive injury (CCI) of the sciatic nerve. We found that, following injection into the peripheral receptive fields, the endogenous reducing agent L-cysteine increased thermal hyperalgesia in a dose-dependent manner in rats with CCI of the sciatic nerve as well as in sham-operated rats. However, the magnitude of the increase was smaller and duration of effect was shorter in rats with CCI of the sciatic nerve compared to sham-operated animals. DTNB, an exogenous oxidizing agent, induced dose-dependent alleviation of thermal hyperalgesia in rats with CCI of the sciatic nerve and caused analgesia in sham-operated rats. DTNB completely blocked L-cysteine-induced thermal hyperalgesia in both animal groups. Mibefradil, a potent and preferential T-type Ca2+ channel blocker, abolished L-cysteine-induced increase in thermal hyperalgesia in both animal groups suggesting the involvement of T-type Ca2+ channels in peripheral nociception. These results indicate for the first time that redox modulation of T-type Ca2+ channels in rat peripheral nociceptors is operational in pain states caused by peripheral axonal injury. Since thermal hyperalgesia is a common symptom of axonal injury, locally applied oxidizing agents could be used as a novel treatment to ameliorate neuropathic pain.

Coordinated regulation of gene expression by Brn3a in developing sensory ganglia

Mice lacking the POU-domain transcription factor Brn3a exhibit marked defects in sensory axon growth and abnormal sensory apoptosis. We have determined the regulatory targets of Brn3a in the developing trigeminal ganglion using microarray analysis of Brn3a mutant mice. These results show that Brn3 mediates the coordinated expression of neurotransmitter systems, ion channels, structural components of axons and inter- and intracellular signaling systems. Loss of Brn3a also results in the ectopic expression of transcription factors normally detected in earlier developmental stages and in other areas of the nervous system. Target gene expression is normal in heterozygous mice, consistent with prior work showing that autoregulation by Brn3a results in gene dosage compensation. Detailed examination of the expression of several of these downstream genes reveals that the regulatory role of Brn3a in the trigeminal ganglion appears to be conserved in more posterior sensory ganglia but not in the CNS neurons that express this factor.

Nerve injury alters profile of receptor-mediated Ca2+ channel modulation in vagal afferent neurons of rat nodose ganglia

Although nerve injury is known to up- and down-regulate some metabotropic receptors in vagal afferent neurons of the nodose ganglia (NG), the functional significance has not been elucidated. In the present study, thus, we examined whether nerve injury affected receptor-mediated Ca2+ channel modulation in the NG neurons. In this regard, unilateral vagotomy was performed using male Sprague-Dawley rats. One week after vagotomy, Ca2+ currents were recorded using the whole-cell variant of patch-clamp technique in enzymatically dissociated NG neurons. In sham controls, norepinephrine (NE)-induced Ca2+ current inhibition was negligible. Following vagotomy, however, the NE responses were dramatically increased. This phenomenon was in accordance with up-regulation of alpha2A/B-adrenergic receptor mRNAs as quantified using real-time RT-PCR analysis. In addition, neuropeptide Y (NPY) and prostaglandin E2 responses were moderately augmented in vagotomized NG neurons. The altered NPY response appears to be caused by up-regulation of Y2 receptors negatively coupled to Ca2+ channels. In contrast, nerve injury significantly suppressed opioid (tested with DAMGO)-induced Ca2+ current inhibition with down-regulation of micro-receptors. Taken together, these results demonstrated for the first time that the profile of neurotransmitter-induced Ca2+ channel modulation is significantly altered in the NG neurons under pathophysiological state of nerve injury.

Heterologous expression and functional analysis of rat NaV1.8 (SNS) voltage-gated sodium channels in the dorsal root ganglion neuroblastoma cell line ND7–23

The voltage-gated sodium channel NaV1.8 (SNS, PN3) is thought to be a molecular correlate of the dorsal root ganglion (DRG) tetrodotoxin resistant (TTX-R) Na+ current. TTX-R/NaV1.8 is an attractive therapeutic drug target for inflammatory and neuropathic pain on the basis of its specific distribution in sensory neurones and its modulation by inflammatory mediators. However, detailed analysis of recombinant NaV1.8 has been hampered by difficulties in stably expressing the functional protein in mammalian cells. Here, we show stable expression and functional analysis of rat NaV1.8 (rNaV1.8) in the rat DRG/mouse N18Tg2 neuroblastoma hybridoma cell line ND7-23. Rat NaV1.8 Na+ currents were recorded (789 +/- 89 pA, n=62, over 20-cell passages) that qualitatively resembled DRG TTX-R in terms of gating kinetics and voltage-dependence of activation and inactivation. The local anaesthetic drug tetracaine produced tonic inhibition of rNaV1.8 (mean IC50 value 12.5 microM) and in repeated gating paradigms (2-10 Hz) also showed frequency-dependent block. There was a correlation between the ability of several analogues of the anticonvulsant/analgesic compound lamotrigine to inhibit TTX-R and rNaV1.8 (r=0.72, P<0.001). RT-PCR analysis of wild type ND7-23 cells revealed endogenous expression of the beta1 and beta3 accessory Na+ channel subunits-the possibility that the presence of these subunits assists and stabilises expression of rNaV1.8 is discussed. We conclude that the neuroblastoma ND7-23 cell line is a suitable heterologous expression system for rNaV1.8 Na+ channels in that it allows stable expression of a channel with biophysical properties that closely resemble the native TTX-R currents in DRG neurones. This reagent will prove useful in the search for pharmacological inhibitors of rNaV1.8 as novel analgesics.

In vivo sensitivity of human root dentin to air blast and scratching

BACKGROUND

The present study evaluated the prevalence of radicular sensitivity to scratching as well as the effects of a common oxalate desensitizing agent on sensitivity to air blast and scratching.

METHODS

Eighty-seven patients self-reporting dentin hypersensitivity, with at least two hypersensitive teeth, were included. Prior to any treatment, their sensitivity to air blast was recorded and rated as absent or present, and the force necessary to trigger pain when scratching was measured with a scratchometer in cN. For each patient one sensitive tooth was treated with an oxalate desensitizing agent and the other one with a placebo solution. The same measurements were carried out after treatment.

RESULTS

Following treatment with a placebo solution, 70% of the teeth remained sensitive to air blast while only 38% of the desensitizing agent-treated teeth remained sensitive to air blast (P < 0.01). The mean force required to elicit pain prior to any treatment was 44 +/- 17 cN. This force statistically increased significantly after application of the placebo (53 +/- 17 cN) (P < 0.05). After using the desensitizing agent, the force was even higher (95 +/- 24 cN) (P < 0.01). Only 8% of the teeth treated with the desensitizing agent did not respond to treatment compared to 37% of the teeth treated with the placebo solution.

CONCLUSIONS

The placebo solution had a significant effect on sensitivity to air blast and to scratching (P < 0.05). The oxalate desensitizing agent was more effective than the placebo solution at decreasing the sensitivity both to air blast and to scratching (P < 0.01). The sensitivity to air blast seems to be overestimated because, after using the desensitizing agent, 38% of the teeth remained sensitive to air blast but only 8% remained sensitive to scratching. Pulpal inflammation may be involved in those teeth that did not respond to treatment.

Dual effects of intrathecal BAM22 on nociceptive responses in acute and persistent pain--potential function of a novel receptor

Bovine adrenal medulla 22 (BAM22) peptide is one of the cleavage products of proenkephalin A. It binds with high affinity to both opioid receptors and a newly discovered receptor in vitro. This latter receptor was first named sensory neuron-specific receptor and is here named BAM peptide-activated receptor with non-opioid activity (BPAR). BPAR is uniquely distributed in small-diameter DRG neurons, most of which are associated with the IB4 class of nociceptor afferent. The present study examined the effects of intrathecal administration of BAM22 on formalin-induced nocifensive behaviors and tail-withdrawal latency in the rat. Intrathecal (i.t.) administration of BAM22 decreased nocifensive behavior scores, measured as the sum of flinching and lifting/licking, in the first and second phases of the formalin test. This decrease was partially attenuated by systemic injection of naloxone. In the presence of naloxone, i.t. BAM22 produced a dose-dependent suppression of the nocifensive behaviors observed during the formalin test. The ratio of the efficacy of BAM22 (5 nmol) in the presence of naloxone over that in the absence of naloxone was 0.65 for flinching and 0.74 for lifting/licking in the second phase. BAM22 at a dose of 5 nmol increased the tail-withdrawal latency by 193 and 119% of baseline in the absence and presence of naloxone, respectively. Systemic administration of naloxone alone enhanced the nocifensive behaviors in the second, but not in the first phase of the formalin test. Naloxone treatment did not alter the tail-withdrawal latency. These data confirm earlier in vitro data showing that BAM22 has both opioid and non-opioid biological actions. The non-opioid action of BAM22 involves inhibition of acute and persistent nociceptive behaviors at the spinal level, presumably mediated via BPAR. The name suggested for this novel receptor, its potential physiological function and its ligand are discussed. British Journal of Pharmacology (2004) 141, 423-430. doi:10.1038/sj.bjp.0705637

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.

Peripheral neuropathy pain: mechanisms and treatment

Pain is a disabling symptom for many patients with peripheral neuropathy. The mechanisms responsible for neuropathic pain are not fully understood. Theories include ectopic discharges of damaged fibers and dorsal root ganglia, sensitization of nociceptors, central sensitization of dorsal horn neurons, loss of inhibitory neurons, and sprouting of afferent fibers after nerve injury into new regions of the nervous system. Several drugs have been effective in controlled clinical trials, but not all patients respond or tolerate the available agents.

Arylacetamide kappa-opioid receptor agonists produce a tonic- and use-dependent block of tetrodotoxin-sensitive and -resistant sodium currents in colon sensory neurons

We have previously reported that U50,488 [(trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide] enantiomers contribute to visceral antinociception by a nonopioid receptor-mediated blockade of sodium currents in colon sensory neurons. The present experiments were undertaken to examine the effect of arylacetamide kappa-opioid receptor agonists (kappa-ORAs) U50,488 and EMD 61,753 [(N-methyl-N-[1S)-1-phenyl)-2-(13S))-3-hydroxypyrrolidine-1-yl)-ethyl]-2,2-diphenylacetamide HCl] on tetrodotoxin-sensitive (TTX-S) and -resistant (TTX-R) sodium currents, and the mechanism of their sodium channel-blocking actions. Whole cell patch-clamp experiments were performed on colon sensory neurons from the S1 dorsal root ganglion identified by content of retrograde tracer 1.1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine metanesulfonate. The concentration-response curves of U50,488 and EMD 61,753, for tonic inhibition of total, TTX-S, and TTX-R sodium currents were similar (EC50 values for U50,488 and EMD 61,753 were 8.4 +/- 1.69 and 1.2 +/- 1.78 microM, respectively). In contrast, the peptide kappa-ORA dynorphin was without effect in these experiments. U50,488 (10 microM) shifted the voltage dependence of steady-state inactivation curves for total, TTX-S, and TTX-R currents to more negative potentials. Inhibition was present at holding potentials of -100 to -20 mV. After the tonic block elicited by 10 microM U50,488, repetitive stimulation with 5-ms depolarizing pulses at a frequency of 3 Hz further enhanced the inhibition of total, TTX-R, and TTX-S currents by 43.8 +/- 4.9, 46.2 +/- 4.9, and 40 +/- 3.2%, respectively. These results demonstrate that arylacetamide kappa-ORAs nonselectively inhibit voltage-evoked sodium currents in a manner similar to local anesthetics, by enhancing closed-state inactivation and induction of use-dependent block.

The tetrodotoxin-resistant Na+ channel Nav1.8 is essential for the expression of spontaneous activity in damaged sensory axons of mice

The tetrodotoxin-resistant sodium channel alpha subunit, Nav1.8, is exclusively expressed in primary sensory neurons and is suggested to play a role in the generation of ectopic action potentials after axonal injury and thereby contribute to neuropathic pain. Here we investigated the involvement of Nav1.8 in ectopic impulse generation in damaged axons by examining spontaneous activity and mechanosensitivity in neuromas formed by section of the saphenous nerve in Nav1.8 null mice and in their wild-type littermates. We recorded 522 identified units from 24 neuromas in vitro at two time points, 8-11 days (median 10 days) and 19-29 days (median 22 days) post-operatively. At approximately 10 days, neither genotype showed spontaneous activity, but a significantly higher proportion of fibres were mechanosensitive in wild-type (54%) compared to Nav1.8 null neuromas (18%). At approximately 22 days, 19% of fibres recorded in wild-type neuromas showed spontaneous activity, whereas only one fibre of the 238 (0. %) recorded in neuromas taken from null mice showed ongoing activity. In recordings at approximately 22 days, a similar proportion of fibres were mechanosensitive in wild-type and Nav1.8 null neuromas (51 and 46%, respectively). We conclude that Nav1.8 is essential for the expression of spontaneous activity in damaged sensory axons, and may also contribute to the development of ectopic mechanosensitivity.

Extra spike formation in sensory neurons and the disruption of afferent spike patterning

The peculiar pseudounipolar geometry of primary sensory neurons can lead to ectopic generation of "extra spikes" in the region of the dorsal root ganglion potentially disrupting the fidelity of afferent signaling. We have used an explicit model of myelinated vertebrate sensory neurons to investigate the location and mechanism of extra spike formation, and its consequences for distortion of afferent impulse patterning. Extra spikes originate in the initial segment axon under conditions in which the soma spike becomes delayed and broadened. The broadened soma spike then re-excites membrane it has just passed over, initiating an extra spike which propagates outwards into the main conducting axon. Extra spike formation depends on cell geometry, electrical excitability, and the recent history of impulse activity. Extra spikes add to the impulse barrage traveling toward the spinal cord, but they also travel antidromically in the peripheral nerve colliding with and occluding normal orthodromic spikes. As a result there is no net increase in afferent spike number. However, extra spikes render firing more staccato by increasing the number of short and long interspike intervals in the train at the expense of intermediate intervals. There may also be more complex changes in the pattern of afferent spike trains, and hence in afferent signaling.

Analysis of pulpal reactions to restorative procedures, materials, pulp capping, and future therapies

Every year, despite the effectiveness of preventive dentistry and dental health care, 290 million fillings are placed each year in the United States; two-thirds of these involve the replacement of failed restorations. Improvements in the success of restorative treatments may be possible if caries management strategies, selection of restorative materials, and their proper use to avoid post-operative complications were investigated from a biological perspective. Consequently, this review will examine pulp injury and healing reactions to different restorative variables. The application of tissue engineering approaches to restorative dentistry will require the transplantation, replacement, or regeneration of cells, and/or stimulation of mineralized tissue formation. This might solve major dental problems, by remineralizing caries lesions, vaccinating against caries and oral diseases, and restoring injured or replacing lost teeth. However, until these therapies can be introduced clinically, the avoidance of post-operative complications with conventional therapies requires attention to numerous aspects of treatment highlighted in this review.

Proinflammatory mediators, stimulators of sensory neuron excitability via the expression of acid-sensing ion channels

Tissue acidosis is an important feature of inflammation. It is a direct cause of pain and hyperalgesia. Protons activate sensory neurons mainly through acid-sensing ion channels (ASICs) and the subsequent membrane depolarization that leads to action potential generation. We had previously shown that ASIC transcript levels were increased in inflammatory conditions in vivo. We have now found that this increase is caused by the proinflammatory mediators NGF, serotonin, interleukin-1, and bradykinin. A mixture of these mediators increases ASIC-like current amplitude on sensory neurons as well as the number of ASIC-expressing neurons and leads to a higher sensory neuron excitability. An analysis of the promoter region of the ASIC3 encoding gene, an ASIC specifically expressed in sensory neurons and associated with chest pain that accompanies cardiac ischemia, reveals that gene transcription is controlled by NGF and serotonin.

Roles of tetrodotoxin (TTX)-sensitive Na+ current, TTX-resistant Na+ current, and Ca2+ current in the action potentials of nociceptive sensory neurons

Nociceptive sensory neurons are unusual in expressing voltage-gated inward currents carried by sodium channels resistant to block by tetrodotoxin (TTX) as well as currents carried by conventional TTX-sensitive sodium channels and voltage-dependent calcium channels. To examine how currents carried by each of these helps to shape the action potential in small-diameter dorsal root ganglion cell bodies, we voltage clamped cells by using the action potential recorded from each cell as the command voltage. Using intracellular solutions of physiological ionic composition, we isolated individual components of current flowing during the action potential with the use of channel blockers (TTX for TTX-sensitive sodium currents and a mixture of calcium channel blockers for calcium currents) and ionic substitution (TTX-resistant current measured by the replacement of extracellular sodium by N-methyl-D-glucamine in the presence of TTX, with correction for altered driving force). TTX-resistant sodium channels activated quickly enough to carry the largest inward charge during the upstroke of the nociceptor action potential (approximately 58%), with TTX-sensitive sodium channels also contributing significantly ( approximately 40%), especially near threshold, and high voltage-activated calcium currents much less (approximately 2%). Action potentials had a prominent shoulder during the falling phase, characteristic of nociceptive neurons. TTX-resistant sodium channels did not inactivate completely during the action potential and carried the majority (58%) of inward current flowing during the shoulder, with high voltage-activated calcium current also contributing significantly (39%). Unlike calcium current, TTX-resistant sodium current is not accompanied by opposing calcium-activated potassium current and may provide an effective mechanism by which the duration of action potentials (and consequently calcium entry) can be regulated.

Deficits in visceral pain and referred hyperalgesia in Nav1.8 (SNS/PN3)-null mice

The tetrodotoxin-resistant sodium channel alpha subunit Nav1.8 is expressed exclusively in primary sensory neurons and is proposed to play an important role in sensitization of nociceptors. Here we compared visceral pain and referred hyperalgesia in Nav1.8-null mice and their wild-type littermates in five tests that differ in the degree to which behavior depends on spontaneous, ongoing firing in sensitized nociceptors. Nav1.8-null mice showed normal nociceptive behavior provoked by acute noxious stimulation of abdominal viscera (intracolonic saline or intraperitoneal acetylcholine). However, Nav1.8-null mutants showed weak pain and no referred hyperalgesia to intracolonic capsaicin, a model in which behavior is sustained by ongoing activity in nociceptors sensitized by the initial application. Nav1.8-null mice also showed blunted pain and hyperalgesia to intracolonic mustard oil, which sensitizes nociceptors but also provokes tissue damage. To distinguish between a possible role for Nav1.8 in ongoing activity per se and ongoing activity after sensitization in the absence of additional stimuli, we tried a visceral model of tonic noxious chemical stimulation, cyclophosphamide cystitis. Cyclophosphamide produces cystitis by gradual accumulation of toxic metabolites in the bladder. In this model, Nav1.8-null mice showed normal responses. There were no differences between null mutants and their normal littermates in tissue damage and inflammation evoked by any of the stimuli tested, suggesting that the behavioral differences are not secondary to impairment of inflammatory responses. We conclude that there is an essential role for Nav1.8 in mediating spontaneous activity in sensitized nociceptors.

An overview of toxins and genes from the venom of the Asian scorpion Buthus martensi Karsch

Among the different scorpion species, Buthus martensi Karsch (BmK), a widely distributed scorpion species in Asia, has received a lot of attention. Indeed, over the past decade, more than 70 different peptides, toxins or homologues have been isolated and more peptides are probably still to be revealed. This review is focusing on the many peptides isolated from the venom of this scorpion, their targets, their genes and their structures. The aim is to give both a 'state of the art' view of the research on BmK venom and an illustration of the complexity of this scorpion venom. In the present manuscript, we have listed the different ion channel toxins and homologues isolated from the venom of BmK, either from the literature or from databases. We have described here 51 long-chain peptides related to the Na(+) channel toxins family: 34 related to the alpha-toxin family, four related to the excitatory insect toxin family, 10 related to the depressant insect toxin, one beta-like toxin plus two peptides, BmK AS and AS1, that act on ryanodine receptors. We also listed 18 peptides related to the K(+) channel toxin family: 14 short chain toxins or homologues, two long chain K(+) toxin homologues and two putative K(+) toxin precursors. Additionally, two chlorotoxin like peptides (Bm-12 and 12 b) have been isolated in the venom of BmK. Besides these ion channels toxins, two peptides without disulfide bridges (the bradykinin-potentiating peptide BmK bpp and BmK n1) and three peptides with no known functions have also been discovered in this venom. We have also taken the opportunity of this review to update the classification of scorpion K(+) toxins () which now presents 17 subfamilies instead of the 12 described earlier. The work on the venom of BmK led to the discovery of two new subfamilies, alpha-KT x 14 and alpha-KT x 17.

Annexin II light chain regulates sensory neuron-specific sodium channel expression

The tetrodotoxin-resistant sodium channel Na(V)1.8/SNS is expressed exclusively in sensory neurons and appears to have an important role in pain pathways. Unlike other sodium channels, Na(V)1.8 is poorly expressed in cell lines even in the presence of accessory beta-subunits. Here we identify annexin II light chain (p11) as a regulatory factor that facilitates the expression of Na(V)1.8. p11 binds directly to the amino terminus of Na(V)1.8 and promotes the translocation of Na(V)1.8 to the plasma membrane, producing functional channels. The endogenous Na(V)1.8 current in sensory neurons is inhibited by antisense downregulation of p11 expression. Because direct association with p11 is required for functional expression of Na(V)1.8, disrupting this interaction may be a useful new approach to downregulating Na(V)1.8 and effecting analgesia.

Continual systemic infusion of lidocaine provides analgesia in an animal model of neuropathic pain

We examined whether continual constant-rate infusion of lidocaine would provide analgesia during the initial post-injury phase in the chronic constriction injury model of neuropathic pain. Male Sprague-Dawley rats were divided into control and ligated groups and infused with saline or lidocaine (0.15, 0.33, 0.67, and 1.3mg/kg/h) via subcutaneously implanted Alzet((R)) osmotic minipumps. Thermal withdrawal latencies were obtained prior (Day 0) and 3 days after loose sciatic ligation and pump implantation surgery. Ligated animals receiving lidocaine at 0.67 or 1.3mg/kg/h exhibited no change in withdrawal latency on Day 3 after surgery, indicating that lidocaine at these doses prevented the development of thermal hyperalgesia as a sign of neuropathic pain. In contrast, ligated animals treated with saline or lidocaine at 0.15 or 0.33mg/kg/h exhibited hyperalgesia on Day 3 after surgery, indicating that these lower doses of lidocaine failed to provide analgesia. Control animals treated with saline or any of the lidocaine doses exhibited no change in withdrawal latencies between Day 0 and Day 3. In a separate group of ligated animals, lidocaine infusion (0.67mg/kg/h) that was started 24h after sciatic ligation surgery reversed the already present thermal hyperalgesia. Average plasma lidocaine concentrations were 0.11, 0.36, and 0.45microg/ml for animals receiving 0.33, 0.67 and 1.3mg/kg/h of lidocaine, respectively. These results suggest that continual systemic infusion of lidocaine prevents or reverses the development of neuropathic pain following chronic constriction injury. These results add to the increasing body of evidence supporting the therapeutic value of preemptive and post-operative lidocaine administration for the relief of neuropathic pain.

Targeting afferent hyperexcitability for therapy of the painful bladder syndrome

The involvement of C-fiber afferent pathways in urinary frequency and pain associated with painful bladder syndrome raises the possibility of multiple targets for the treatment of this disease. Using an in vivo measurement of bladder activity as well as whole-cell patch clamp recording techniques to examine the properties of bladder afferent neurons in animal models of chronic cystitis, we have documented that tetrodotoxin-resistant sodium channels encoded by the Na(v) 1.8 (PN3/SNS) gene and nitric oxide acting via a cyclic guanosine monophosphate (cGMP)-dependent mechanism are important in modulating bladder pain responses. Thus, suppression of C-fiber afferent nerve activity by blocking specific sodium channels, elevating nitric oxide levels, or activating cGMP-dependent pathways might represent novel strategies for the treatment of symptoms in patients with painful bladder syndrome. Another treatment strategy is suppression of release or activity of proinflammatory agents that can cause normally unexcitable C-fiber afferents to become hyperactive or hyperexcitable. This approach to management of bladder pain was tested in patients with painful bladder syndrome by examining the effectiveness of the antiallergic agent suplatast tosilate (IPD-1151T), which suppresses urinary frequency in a rat model of cystitis. IPD-1151T is an immunoregulator that suppresses cytokine production in T-helper 2 cells and inhibits immunoglobulin E antibody formation and antigen-induced histamine release from mast cells. Preliminary data from an open-label clinical trial showed that 16 of 23 (70%) patients responded to treatment with IPD-1151T (300 mg/day orally for 12 months). The finding that expression of platelet-derived endothelial cell growth factor, which can activate mast cells, was lower in the bladder of responders than nonresponders indicates that bladder levels of platelet-derived endothelial cell growth factor may be a useful marker for this disease.

Calcium and calcium-activated currents in vagotomized rat primary vagal afferent neurons

Adult inferior vagal ganglion neurons (nodose ganglion neurons, NGNs) were acutely isolated 4-6 days after section of their peripheral axons (vagotomy) and examined with the whole-cell patch-clamp technique. A subset (approximately 25 %) of vagotomized NGNs displayed depolarizing after-potentials (DAPs), not present in control NGNs. DAPs were inhibited by niflumic acid (125 microM) or cadmium (100 microM), and had a reversal potential near E(Cl), indicating that they were due to Ca(2+)-activated chloride current (I(Cl(Ca))). N-type, L-type, T-/R- and other types of voltage-dependent Ca(2+) channels provided about 43, 2, 16 and 40 % of the trigger Ca(2+) for DAP generation, respectively. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was estimated using fura-2 fluorescence. Resting [Ca(2+)](i) and peak [Ca(2+)](i) elevation induced by activating Ca(2+)-induced Ca(2+) release (CICR) stores with 10 mM caffeine were not significantly different among control NGNs, vagotomized NGNs with DAPs and vagotomized NGNs without DAPs, averaging 54 +/- 7.9 (n = 19; P = 0.49) and 2022 +/- 1059 nM (n = 19; P = 0.44), respectively. Blocking CICR with 10 microM ryanodine reduced DAP amplitude by approximately 37 %. Ca(2+) influx induced by action potential waveforms was increased by over 250 % in vagotomized NGNs with DAPs (19.0 +/- 2.1 pC) compared to control NGNs (5.0 +/- 0.8 pC) or vagotomized NGNs without DAPs (7.0 +/- 0.8 pC). L-type, N-type, T-/R-type and other types of Ca(2+) influx were increased proportionately in vagotomized NGNs with DAPs. In conclusion, a subset of vagotomized NGNs have increased Ca(2+) currents and express I(Cl(Ca)). These NGNs respond electrically to increases in [Ca(2+)](i) during regeneration.

Experimental ulcers alter voltage-sensitive sodium currents in rat gastric sensory neurons

BACKGROUND & AIMS

Voltage-dependent Na+ currents are important determinants of excitability. We hypothesized that gastric inflammation alters Na+ current properties in primary sensory neurons.

METHODS

The stomach was surgically exposed in rats to inject the retrograde tracer 1.1'-dioctadecyl-3,3,3,'3-tetramethylindocarbocyanine methanesulfonate and saline (control) or 20% acetic acid (ulcer group) into the gastric wall. Nodose or thoracic dorsal root ganglia (DRG) were harvested after 7 days to culture neurons and record Na+ currents using patch clamp techniques.

RESULTS

There were no lesions in the control and 3 +/- 1 ulcers in the ulcer group. Na+ currents recovered significantly more rapidly from inactivation in nodose and DRG neurons obtained from animals in the ulcer group compared with controls. This was partially a result of an increase in the relative contribution of the tetrodotoxin-resistant to the peak sodium current. In addition, the recovery kinetics of the tetrodotoxin-sensitive current were faster. In DRG neurons, gastric inflammation shifted the voltage-dependence of activation of the tetrodotoxin-resistant current to more hyperpolarized potentials.

CONCLUSIONS

Gastric injury alters the properties of Na+ currents in gastric sensory neurons. This may enhance excitability, thereby contributing to the development of dyspeptic symptoms.

Pharmacology of airway afferent nerve activity

Afferent nerves in the airways serve to regulate breathing pattern, cough, and airway autonomic neural tone. Pharmacologic agents that influence afferent nerve activity can be subclassified into compounds that modulate activity by indirect means (e.g. bronchial smooth muscle spasmogens) and those that act directly on the nerves. Directly acting agents affect afferent nerve activity by interacting with various ion channels and receptors within the membrane of the afferent terminals. Whether by direct or indirect means, most compounds that enter the airspace will modify afferent nerve activity, and through this action alter airway physiology.

Analgesic activity of a novel use-dependent sodium channel blocker, crobenetine, in mono-arthritic rats

1. Although sodium channel blockers are effective analgesics in neuropathic pain, their effectiveness in inflammatory pain has been little studied. Sodium channels are substantially up-regulated in inflamed tissue, which suggests they play a role in maintenance of chronic inflammatory pain. We have examined the effects of sodium channel blockers on mobility, joint hyperalgesia and inflammation induced by complete Freund's adjuvant injected in one ankle joint of adult rats. The clinically effective sodium channel blocker, mexiletine, was compared with crobenetine (BIII 890 CL), a new, highly use-dependent sodium channel blocker. 2. Rats were treated for 5 days, starting on the day of induction of arthritis and were tested daily for joint hyperalgesia, hind limb posture and mobility. At post-mortem, joint stiffness and oedema were assessed. Dose response curves were constructed for each test compound (3 - 30 mg kg day(-1)). Control groups were treated with vehicle or with the non-steroidal anti-inflammatory drug, meloxicam (4 mg kg day(-1) i.p.). 3. Both sodium channel blockers produced dose dependent and significant reversal of mechanical joint hyperalgesia and impaired mobility with an ID50 of 15.5+/-1.1 mg kg day(-1) for crobenetine and 18.1+/-1.2 mg kg day(-1) for mexiletine. Neither compound affected the responses of the contralateral non-inflamed joint, nor had any effect on swelling and stiffness of the inflamed joint. 4. We conclude that sodium channel blockers are analgesic and anti-hyperalgesic in this model of arthritis. These data suggest that up regulation of sodium channel expression in primary afferent neurones may play an important role in the pain and hyperalgesia induced by joint inflammation.

Sodium currents in vagotomized primary afferent neurones of the rat

1. Nodose ganglion neurones (NGNs) become less excitable following section of the vagus nerve. To determine the role of sodium currents (I(Na)) in these changes, standard patch-clamp recording techniques were used to measure I(Na) in rat NGNs maintained in vivo for 5-6 days following vagotomy, and then in vitro for 2-9 h. 2. Total I(Na) and I(Na) density in vagotomized NGNs were similar to control values. However, steady-state I(Na) inactivation in vagotomized NGNs was shifted -9 mV relative to control values (V(1/2), -74 +/- 2 vs. -65 +/- 2 mV, P < 0.01) and I(Na) activation was shifted by -7 mV (V(1/2), -21 +/- 2 vs. -14 +/- 2 mV, P < 0.006). I(Na) recovery from inactivation was also slower in vagotomized NGNs (fast time constant, 2.8 +/- 0.4 vs. 1.6 +/- 0.3 ms, P < 0.02). 3. The fraction of I(Na) resistant to 1 microM tetrodotoxin (TTX-R) was halved in vagotomized NGNs (21 +/- 8 vs. 56 +/- 8 % of total I(Na), P < 0.05). This change from TTX-R I(Na) to TTX-sensitive (TTX-S) I(Na) may explain altered I(Na) activation, inactivation and repriming in vagotomized NGNs. 4. The contribution of alterations in I(Na) to NGN firing patterns was assessed by measuring I(Na) evoked by a series of action potential (AP) waveforms. In general, control NGNs produced large, repetitive TTX-R I(Na) while vagotomized NGNs produced smaller TTX-S I(Na) that rapidly inactivated during AP discharge. We conclude that TTX-R I(Na) is important for sustained AP discharge in NGNs, and that its diminution underlies the decreased AP discharge of vagotomized NGNs.

Neuropathic pain: what do we do with all these theories?

Only a generation ago there were few ideas as to what might cause neuropathic pain, and even fewer relevant data. In contrast, we can currently point to hundreds of distinct cellular changes that are triggered by nerve injury and that might be relevant to the emergence of pain symptomatology. The number may soon increase to thousands. It is essential, therefore, to redirect efforts towards the development of experimental strategies for testing which of these are essential parts of the pain process and which are tangential. In this paper I point out four such strategies: timing, deletion, prevention and genetic heterogeneity, and summarize how one neuropathic pain theory, the ectopic pacemaker hypothesis, holds up to scrutiny.

Excitability of human axons

The excitability of human axons can be studied reliably using the technique of threshold tracking, which allows the strength of a test stimulus to be adjusted by computer to activate a defined fraction of the maximal nerve or muscle action potential. The stimulus current that just evokes the target response is considered the "threshold" for that response. More useful than the resting threshold are other indices of axonal excitability derived from pairs of threshold measurements, such as refractoriness, supernormality, strength-duration time constant and "threshold electrotonus" (i.e. the changes in threshold produced by long-lasting depolarizing or hyperpolarizing current pulses). Each of these measurements depends on membrane potential and on other biophysical properties of the axons. Together they can provide new information about the pathophysiology underlying abnormalities in excitability in neuropathy.

Sodium channel inactivation in an animal model of acute quadriplegic myopathy

We previously demonstrated that muscle fibers become unable to fire action potentials in both patients and an animal model of acute quadriplegic myopathy (AQM). In the animal model, skeletal muscle is denervated in rats treated with high-dose corticosteroids (steroid-denervated; SD), and muscle fibers become inexcitable despite resting potentials and membrane resistances similar to those of control denervated fibers that remain excitable. We show here that unexcitability of SD fibers is due to increased inactivation of sodium channels at the resting potential of affected fibers. A hyperpolarizing shift in the voltage dependence of inactivation in combination with the depolarization of the resting potential induced by denervation results in inexcitability. Our findings suggest that paralysis in the animal model of AQM is the result of an abnormality in the voltage dependence of sodium channel inactivation.

Vincristine-induced allodynia in the rat

The aims of this study were two-fold: first, to simplify the method for creating a recently described neuropathic pain model in the rat, and second, to evaluate the effects of a number of drugs with analgesic or antihyperalgesic properties, in this model. Continuous intravenous vincristine infusion (1-100 microg kg(-1) day (-1)) for 14 days resulted in a dose dependent tactile allodynia (as measured by von Frey filaments) by 7 days at doses between 30 - 100 microg kg(-1) day (-1), with a hindlimb motor deficit observed at doses greater than 50 microg kg(-1) day (-1). No thermal hyperalgesia was observed. Systemic morphine, lidocaine, mexiletine and pregabalin (given intraperitoneally) produced significant reduction of the allodynia, while tetrodotoxin was without effect. Continuous intravenous infusion of vincristine in rats thus provides a reliable model of chemotherapy induced neuropathy which may be used in defining the mechanism and pharmacology of this clinically relevant condition.

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

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

A standing Na+ conductance in rat carotid body type I cells

Substitution of extracellular Na+ with N-methyl D-glucamine caused marked hyperpolarisation in rat isolated carotid body type I cells, suggesting the presence of a standing Na+ conductance. Choline substitution produced smaller hyperpolarisations, whilst Li+ was virtually without effect. This Na+ conductance was not blocked by amiloride, tetrodotoxin, Zn2+ or Gd3+ and did not arise from electrogenic Na-glucose co-transport, since substitution of glucose with sucrose could not mimic the effects of Na+ substitution. Hypoxia and acidosis did not modify the tonic Na+ influx. Our results suggest that Na+ influx provides a constant depolarising influence on type I cells which acts to shift membrane potential beyond that required for initiation of neurosecretion, an essential step in carotid body chemotransduction.

Subthreshold membrane oscillations underlying integer multiples firing from injured sensory neurons

Integer multiples firing (IMF), a special temporal pattern of firing, was recently observed in spontaneous discharge from injured dorsal root ganglion (DRG) neurons. To investigate the mechanism underlying IMF, the injured DRG neurons of rat were recorded intracellulary. Of 64 recorded A-neurons discharging spontaneously, eight fired spikes in the IMF pattern. Interspike interval (ISI) time series of IMF showed a structure of distinct bands on scatter map. Regular subthreshold membrane oscillations (SMOs) with relatively stable amplitude and frequency were observed on all eight IMF neurons. IMF could be induced from the neurons in periodic firing by local application of tetrodotoxin (TTX), a Na+ channel antagonist. During this process, the amplitude of SMOs varied markedly. Some SMOs were below action potential threshold so that they did not trigger spikes. Nor did some SMOs, though their amplitude were obviously beyond the threshold measured from nearby spikes. The results indicate that regular SMOs existent in injured DRG A-neurons underlie IMF, and the TTX-induced transformation of firing pattern from periodic to integer multiples may occur in two ways: decreasing the amplitude of SMOs and/or elevating action potential threshold.

Nerve injury-induced pain in the trigeminal system

This article reviews some recent findings on peripheral mechanisms related to the development of oro-facial pain after trigeminal nerve injury. Chronic injury-induced oro-facial pain is not in itself a life-threatening condition, but patients suffering from this disorder undoubtedly have a reduced quality of life. The vast majority of the work on pain mechanisms has been carried out in spinal nerve systems. Those studies have provided great insight into mechanisms of neuropathic spinal pain, and much of the data from them is obviously relevant to studies of trigeminal pain. However, it is now clear that the pathophysiology of the trigeminal nerve (a cranial nerve) is in many ways different to that found in spinal nerves. Whereas some of the changes seen in animal models of trigeminal nerve injury mimic those occurring after spinal nerve injury (e.g., the development of spontaneous activity from the damaged axons), others are different, such as the time-course of the spontaneous activity, some of the neuropeptide changes in the trigeminal ganglion, and the lack of sprouting of sympathetic terminals in the ganglion. Recent findings provide new insights that help our understanding of the etiology of chronic injury-induced oro-facial pain. Future investigations will hopefully explain how data gained from these studies relate to clinical pain experience in man and should enable the rapid development of new therapeutic regimes.

Plasticity of TTX-sensitive sodium channels PN1 and brain III in injured human nerves

Sensory neurones co-express voltage-gated sodium channels that mediate TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) currents, which may contribute to chronic pain after nerve injury. We previously demonstrated that TTX-R channels were decreased acutely in human sensory cell bodies after central axotomy, but accumulated in nerve terminals after peripheral axotomy. We have now studied the TTX-S channels PN1 and Brain III, using specific antibodies for immunohistochemistry, in dorsal root ganglia (DRG) from 10 patients with traumatic central axotomy, nerves from 16 patients with peripheral axotomy, and controls. PN1 showed temporal changes similar to the TTX-R channels in sensory cell bodies of injured DRG. In contrast, Brain III was found only in injured nerves (not control nerves, or control/central axotomy DRG). PNI and Brain III are distinct targets for novel analgesics.

Sodium channels and pain therapy

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

Mild ciguatera poisoning: Case reports with neurophysiological evaluations

Ciguatera poisoning causes mainly gastrointestinal and neurological effects of variable severity. However, symptoms of peripheral neuropathy with paresthesias and paradoxical disturbance of thermal sensation are the hallmark. Electrophysiological studies are often normal, except in severe cases. We report four people who developed mild ciguatera poisoning after barracuda ingestion. Electrophysiological studies documented normocalcemic latent tetany. These findings are consistent with ciguatoxin's mechanism of toxicity, which involves inactivation of voltage-gated Na(+) channels and eventually increases nerve membrane excitability.