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

Synthetic ciguatoxins selectively activate Nav1.8-derived chimeric sodium channels expressed in HEK293 cells

The synthetic ciguatoxin CTX3C has been shown to activate tetrodotoxin (TTX)-sensitive sodium channels (Na(v)1.2, Na(v)1.4, and Na(v)1.5) by accelerating activation kinetics and shifting the activation curve toward hyperpolarization (Yamaoka, K., Inoue, M., Miyahara, H., Miyazaki, K., and Hirama, M. (2004) Br. J. Pharmacol. 142, 879-889). In this study, we further explored the effects of CTX3C on the TTX-resistant sodium channel Na(v)1.8. TTX-resistant channels have been shown to be involved in transducing pain and related sensations (Akopian, A. N., Sivilotti, L., and Wood, J. N. (1996) Nature 379, 257-262). Thus, we hypothesized that ciguatoxin-induced activation of the Na(v)1.8 current would account for the neurological symptoms of ciguatera poisoning. We found that 0.1 mum CTX3C preferentially affected the activation process of the Na(v)1.8 channel compared with those of the Na(v)1.2 and Na(v)1.4 channels. Importantly, without stimulation, 0.1 mum CTX3C induced a large leakage current (I (L)). The conductance of the I (L) calculated relative to the maximum conductance (G (max)) was 10 times larger than that of Na(v)1.2 or Na(v)1.4. To determine the molecular domain of Na(v)1.8 responsible for conferring higher sensitivity to CTX3C, we made two chimeric constructs from Na(v)1.4 and Na(v)1.8. Chimeras containing the N-terminal half of Na(v)1.8 exhibited a large response similar to wild-type Na(v)1.8, indicating that the region conferring high sensitivity to ciguatoxin action is located in the D1 or D2 domains.

When pain after surgery doesn't go away…

Chronic post-surgical pain is a common, under-recognized and important clinical problem which affects millions of patients worldwide. It results from a series of neuroplastic changes associated most commonly with peripheral nerve injury at the time of surgery. Predisposing factors include the type of surgery, pre-operative and acute post-operative pain intensity, and probably psychological (e.g. pain-catastrophizing) and genetic factors [e.g. GCH1 (GTP cyclohydrolase 1) haplotype]. Preventive measures which are currently available include selection of a minimally invasive surgical technique and an aggressive multimodal perioperative analgesic regimen. Very promising therapeutic agents which target the sensitization process are currently in development.

A rat model of chronic postinflammatory visceral pain induced by deoxycholic acid

BACKGROUND & AIMS

Chronic visceral hyperalgesia is considered an important pathophysiologic symptom in irritable bowel syndrome (IBS); previous gastrointestinal inflammation is a potent etiologic factor for developing IBS. Although there are several animal models of adult visceral hypersensitivity after neonatal perturbation or acute colonic irritation/inflammation, current models of postinflammatory chronic visceral hyperalgesia are unsatisfactory. The aim of this study was to establish a model of chronic visceral hyperalgesia after colonic inflammation in the rat.

METHODS

Deoxycholic acid (DCA) was instilled into the rat colon daily for 3 days and animals were tested for up to 4 weeks.

RESULTS

DCA induced mild, transient colonic inflammation within 3 days that resolved within 3 weeks. An exaggerated visceromotor response, referred pain to mechanical stimulation, increased spinal Fos expression, and colonic afferent and dorsal horn neuron activity were apparent by 1 week and persisted for at least 4 weeks, indicating chronic dorsal horn hyperexcitability and visceral hyperalgesia. There was no spontaneous pain, based on open field behavior. There was a significant increase in opioid-receptor activity.

CONCLUSIONS

DCA induces mild, transient colitis, resulting in persistent visceral hyperalgesia and referred pain in rats, modeling some aspects of postinflammatory IBS.

Peripheral mechanisms I: plasticity of peripheral pathways

Cough plays a vital role in protecting the lower airways from inhaled irritants, pollutants, and infectious agents. The cough reflex exhibits remarkable plasticity, such that in the context of infectious or inflammatory respiratory diseases such as asthma, chronic bronchitis, and idiopathic pulmonary fibrosis the cough reflex can become dysregulated, leading to a chronic cough. A chronic, nonproductive (dry) cough can rob sufferers of quality of life. Plasticity of the cough reflex likely involves multiple intersecting pathways within the airways, the peripheral nerves that supply them, and the central nervous system. While further studies are needed to determine the presence and relevance of many of these specific pathways in cough associated with chronic respiratory disease, the last decade has yielded unprecedented insight into the molecular identity of the ion channels and associated proteins that initiate and conduct action potentials in the primary sensory nerves involved in reflexes such as cough. We now know, for instance, that members of the transient receptor potential superfamily of nonselective cation channels function as transducers that convert specific external stimuli into neuronal activation. We also know that certain Na+ and K+ channels play specialized roles in regulating action potential discharge in irritant-sensing afferent nerves. In this chapter, we summarize the available information regarding factors that may modulate afferent neuron function acutely, via posttranslational modifications and over the longer term through neurotrophin-dependent alterations of the transcriptional programs of adult sensory neurons.

Transient receptor potential ankyrin-1 participates in visceral hyperalgesia following experimental colitis

Transient receptor potential ankyrin-1 (TRPA1) is an important receptor that contributes to inflammatory pain. However, previous studies were mainly concerned with its function in somatic hyperalgesia while few referred to visceral, especially colonic inflammatory hyperalgesia. The present study was aimed to investigate the role of TRPA1 in visceral hyperalgesia after trinitrobenzene sulfonic acid (TNBS)-induced colitis. Results indicate that TNBS induced a significant increase in visceral sensitivity to colonic distension and chemical irritation accompanied by up-regulation of TRPA1 in colonic afferent dorsal root ganglia (DRG). Intrathecal administration of TRPA1 antisense (AS) oligodeoxynucleotide (ODN) reduced the TRPA1 expression in DRG as well as suppressed the colitis-induced hyperalgesia to nociceptive colonic distension and intracolonic allyl isothiocyanate (AITC). Meanwhile the TRPA1 antisense ODN had no effect on transient receptor potential vanilloid-1 (TRPV1) expression, which was proposed to highly co-express with TRPA1, and no effect on the response to TRPV1 agonist, capsaicin. These data suggest an apparent role of TRPA1 in visceral hyperalgesia following colitis that might provide a novel therapeutic target for the relief of pain.

Urinary bladder function and somatic sensitivity in vasoactive intestinal polypeptide (VIP)-/- mice

Vasoactive intestinal polypeptide (VIP) is an immunomodulatory neuropeptide widely distributed in neural pathways that regulate micturition. VIP is also an endogenous anti-inflammatory agent that has been suggested for the development of therapies for inflammatory disorders. In the present study, we examined urinary bladder function and hindpaw and pelvic sensitivity in VIP(-/-) and littermate wildtype (WT) controls. We demonstrated increased bladder mass and fewer but larger urine spots on filter paper in VIP(-/-) mice. Using cystometry in conscious, unrestrained mice, VIP(-/-) mice exhibited increased void volumes and shorter intercontraction intervals with continuous intravesical infusion of saline. No differences in transepithelial resistance or water permeability were demonstrated between VIP(-/-) and WT mice; however, an increase in urea permeability was demonstrated in VIP(-/-) mice. With the induction of bladder inflammation by acute administration of cyclophosphamide, an exaggerated or prolonged bladder hyperreflexia and hindpaw and pelvic sensitivity were demonstrated in VIP(-/-) mice. The changes in bladder hyperreflexia and somatic sensitivity in VIP(-/-) mice may reflect increased expression of neurotrophins and/or proinflammatory cytokines in the urinary bladder. Thus, these changes may further regulate the neural control of micturition.

Mast cell-derived histamine mediates cystitis pain

Background

Mast cells trigger inflammation that is associated with local pain, but the mechanisms mediating pain are unclear. Interstitial cystitis (IC) is a bladder disease that causes debilitating pelvic pain of unknown origin and without consistent inflammation, but IC symptoms correlate with elevated bladder lamina propria mast cell counts. We hypothesized that mast cells mediate pelvic pain directly and examined pain behavior using a murine model that recapitulates key aspects of IC.

Methods and Findings

Infection of mice with pseudorabies virus (PRV) induces a neurogenic cystitis associated with lamina propria mast cell accumulation dependent upon tumor necrosis factor alpha (TNF), TNF-mediated bladder barrier dysfunction, and pelvic pain behavior, but the molecular basis for pelvic pain is unknown. In this study, both PRV-induced pelvic pain and bladder pathophysiology were abrogated in mast cell-deficient mice but were restored by reconstitution with wild type bone marrow. Pelvic pain developed normally in TNF- and TNF receptor-deficient mice, while bladder pathophysiology was abrogated. Conversely, genetic or pharmacologic disruption of histamine receptor H1R or H2R attenuated pelvic pain without altering pathophysiology.

Conclusions

These data demonstrate that mast cells promote cystitis pain and bladder pathophysiology through the separable actions of histamine and TNF, respectively. Therefore, pain is independent of pathology and inflammation, and histamine receptors represent direct therapeutic targets for pain in IC and other chronic pain conditions.

Identification of the sensory neuron specific regulatory region for the mouse gene encoding the voltage-gated sodium channel NaV1.8

Voltage-gated sodium channels (VGSC) are critical membrane components that participate in the electrical activity of excitable cells. The type one VGSC family includes the tetrodotoxin insensitive sodium channel, Na(V)1.8, encoded by the Scn10a gene. Na(V)1.8 expression is restricted to small and medium diameter nociceptive sensory neurons of the dorsal root ganglia and cranial sensory ganglia. To understand the stringent transcriptional regulation of the Scn10a gene, the sensory neuron specific promoter was functionally identified. While identifying the mRNA 5'-end, alternative splicing within the 5'-UTR was observed to create heterogeneity in the RNA transcript. Four kilobases of upstream genomic DNA was cloned and the presence of tissue specific promoter activity was tested by microinjection and adenoviral infection of fluorescent protein reporter constructs into primary mouse and rat neurons, and cell lines. The region contained many putative transcription factor-binding sites and strong homology with the predicted rat ortholog. Homology to the predicted human ortholog was limited to the proximal end and several conserved cis elements were noted. Two regulatory modules were identified by microinjection of reporter constructs into dorsal root ganglia and superior cervical ganglia neurons: a neuron specific proximal promoter region between -1.6 and -0.2 kb of the transcription start site cluster, and a distal sensory neuron switch region beyond -1.6 kb that restricted fluorescent protein expression to a subset of primary sensory neurons.

Ionic channel function in action potential generation: current perspective

Over 50 years ago, Hodgkin and Huxley laid down the foundations of our current understanding of ionic channels. An impressive progress has been made during the following years that culminated in the revelation of the details of potassium channel structure. Nevertheless, even today, we cannot separate well currents recorded in central mammalian neurons. Many modern concepts about the function of sodium and potassium currents are based on experiments performed in nonmammalian cells. The recent recognition of the fast delayed rectifier current indicates that we need to reevaluate the biophysical role of sodium and potassium currents. This review will consider high quality voltage clamp data obtained from the soma of central mammalian neurons in the view of our current knowledge about proteins forming ionic channels. Fast sodium currents and three types of outward potassium currents, the delayed rectifier, the subthreshold A-type, and the D-type potassium currents, are discussed here. An updated current classification with biophysical role of each current subtype is provided. This review shows that details of kinetics of both sodium and outward potassium currents differ significantly from the classical descriptions and these differences may be of functional significance.

Experimental autoimmune prostatitis induces chronic pelvic pain

Pain is the hallmark of patients with chronic prostatitis (CP) and chronic pelvic pain syndrome (CPPS). Despite numerous hypotheses, the etiology and pathogenesis remain unknown. To better understand CP/CPPS, we used a murine experimental autoimmune prostatitis model to examine the development, localization, and modulation of pelvic pain. Pelvic pain was detected 5 days after antigen instillation and was sustained beyond 30 days, indicating the development of chronic pain. The pain was attenuated by lidocaine treatment into the prostate, but not into the bladder or the colon, suggesting that pain originated from the prostate. Experimental autoimmune prostatitis histopathology was confined to the prostate with focal periglandular inflammatory infiltrates in the ventral, dorsolateral, and anterior lobes of the mouse prostate. Inflammation and pelvic pain were positively correlated and increased with time. Morphologically, the dorsolateral prostate alone showed significantly increased neuronal fiber distribution, as evidenced by increased protein gene product 9.5 expression. Pelvic pain was attenuated by treatment with the neuromodulator gabapentin, suggesting spinal and/or supraspinal contribution to chronic pain. These results provide the basis for identifying mechanisms that regulate pelvic pain and the testing of therapeutic agents that block pain development in CP/CPPS.

Lack of colonic-inflammation-induced acute visceral hypersensitivity to colorectal distension in Na(v)1.9 knockout mice

Tetrodotoxin-resistant voltage-gated sodium channels subtype 9 (Na(v)1.9) are expressed in small-diameter dorsal root ganglion neurons and have been involved in persistent somatic hyperalgesic responses associated with inflammation. We assessed the role of Na(v)1.9 channels on acute colonic inflammation-induced visceral hypersensitivity in conscious mice, using Na(v)1.9 knockout (KO) mice. Colorectal distension (CRD)-induced visceral pain was assessed in conscious wild-type and Na(v)1.9 KO mice (C57Bl/6 background). The mechanical activity of the abdominal muscles during isobaric colorectal distension was used as a measure of visceral pain. Acute colonic inflammation was induced by intracolonic administration of the toll-like receptor (TLR) 7 activator, R-848 (40mug/animal). CRD was performed 5h later, thereafter animals were euthanized and the colonic content of inflammatory mediators assessed. Normal pain responses were similar in Na(v)1.9 KO and wild-type mice. In wild-type mice, R-848 administration increased the response to phasic CRD by 62% compared with vehicle-treated animals (vehicle: 0.16+/-0.04, R-848: 0.26+/-0.03, n=6-7, P<0.05). However, in Na(v)1.9 KO mice, intracolonic R-848 did not affect the response to CRD (0.11+/-0.02, n=7) compared to animals treated with vehicle (0.17+/-0.03, n=5; P>0.05). After R-848 administration, the colonic content of pro-inflammatory cytokines was increased in similar proportion in wild type and Na(v)1.9 KO mice, suggesting the presence of a similar acute inflammatory reaction in both groups of animals. These results suggest that Na(v)1.9 channels do not significantly contribute to normal visceral pain responses to acute colonic mechanical stimulation but may be important for the development of inflammation-related acute visceral hyperalgesic responses.

Mutations in sodium-channel gene SCN9A cause a spectrum of human genetic pain disorders

The voltage-gated sodium-channel type IX alpha subunit, known as Na(v)1.7 and encoded by the gene SCN9A, is located in peripheral neurons and plays an important role in action potential production in these cells. Recent genetic studies have identified Na(v)1.7 dysfunction in three different human pain disorders. Gain-of-function missense mutations in Na(v)1.7 have been shown to cause primary erythermalgia and paroxysmal extreme pain disorder, while nonsense mutations in Na(v)1.7 result in loss of Na(v)1.7 function and a condition known as channelopathy-associated insensitivity to pain, a rare disorder in which affected individuals are unable to feel physical pain. This review highlights these recent developments and discusses the critical role of Na(v)1.7 in pain sensation in humans.

The effect of deletion of the orphan G - protein coupled receptor (GPCR) gene MrgE on pain-like behaviours in mice

BACKGROUND

The orphan GPCR MrgE is one of an extended family of GPCRs that are expressed in dorsal root ganglia (DRG). Based on these expression patterns it has been suggested that GPCRs like MrgE may play a role in nociception however, to date, no direct supporting evidence has emerged. We generated mutant mice lacking MrgE and examined the effects of deletion of this gene in three pain behavioural models. The effect of MrgE gene deletion on expression of Mrgs and genes involved in sensory neurone function was also investigated.

RESULTS

The absence of MrgE had no effect on the development of pain responses to a noxious chemical stimulus or an acute thermal stimulus. However, in contrast, the development but not the maintenance of neuropathic pain was affected by deletion of MrgE. The expression of Mrg genes was not significantly affected in the MrgE knockout (KO) mice with the sole exception of MrgF. In addition, the expression of 77 of 84 genes involved in sensory neuron development and function was also unaffected by deletion of MrgE. Of the 7 genes affected by MrgE deletion, 4 have previously been implicated in nociception.

CONCLUSION

The data suggests that MrgE may play a role in selective pain behavioural responses in mice.

Discovery and biological evaluation of 5-aryl-2-furfuramides, potent and selective blockers of the Nav1.8 sodium channel with efficacy in models of neuropathic and inflammatory pain

Nav1.8 (also known as PN3) is a tetrodotoxin-resistant (TTx-r) voltage-gated sodium channel (VGSC) that is highly expressed on small diameter sensory neurons and has been implicated in the pathophysiology of inflammatory and neuropathic pain. Recent studies using an Nav1.8 antisense oligonucleotide in an animal model of chronic pain indicated that selective blockade of Nav1.8 was analgesic and could provide effective analgesia with a reduction in the adverse events associated with nonselective VGSC blocking therapeutic agents. Herein, we describe the preparation and characterization of a series of 5-substituted 2-furfuramides, which are potent, voltage-dependent blockers (IC50 < 10 nM) of the human Nav1.8 channel. Selected derivatives, such as 7 and 27, also blocked TTx-r sodium currents in rat dorsal root ganglia (DRG) neurons with comparable potency and displayed >100-fold selectivity versus human sodium (Nav1.2, Nav1.5, Nav1.7) and human ether-a-go-go (hERG) channels. Following systemic administration, compounds 7 and 27 dose-dependently reduced neuropathic and inflammatory pain in experimental rodent models.

Organ cross talk modulates pelvic pain

Interstitial cystitis (IC) is a chronic bladder inflammatory disease of unknown etiology that is often regarded as a neurogenic cystitis. IC is associated with urothelial lesions, voiding dysfunction, and pain in the pelvic/perineal area, and diet can exacerbate IC symptoms. In this study, we used a murine neurogenic cystitis model to investigate the development of pelvic pain behavior. Neurogenic cystitis was induced by the injection of Bartha's strain of pseudorabies virus (PRV) into the abductor caudalis dorsalis tail base muscle of female C57BL/6J mice. Infectious PRV virions were isolated only from the spinal cord, confirming the centrally mediated nature of this neurogenic cystitis model. Pelvic pain was assessed using von Frey filament stimulation to the pelvic region, and mice infected with PRV developed progressive pelvic pain. Pelvic pain was alleviated by 2% lidocaine instillation into either the bladder or the colon but not following lidocaine instillation into the uterus. The bladders of PRV-infected mice showed markers of inflammation and increased vascular permeability compared with controls. In contrast, colon histology was normal and vascular permeability was unchanged, suggesting that development of pelvic pain was due only to bladder inflammation. Bladder-induced pelvic pain was also exacerbated by colonic administration of a subthreshold dose of capsaicin. These data indicate organ cross talk in pelvic pain and modulation of pain responses by visceral inputs distinct from the inflamed site. Furthermore, these data suggest a mechanism by which dietary modification benefits pelvic pain symptoms.

Gene therapy applications for the treatment of neuropathic pain

Neuropathic pain is notoriously difficult to treat; currently available pharmaceutical drugs result in moderate analgesia in approximately a third of patients. As our understanding of the biological processes involved in the establishment and maintenance of neuropathic pain increases, so does the development of novel treatment options. Significant advancements have been made in the past few years in gene transfer, a very powerful potential therapy that can be used to directly target affected areas of the neuraxis or body tissues involved in neuropathic pain. Candidate gene products include directly analgesic proteins as well as proteins that interfere with pain-associated biochemical changes in nerve or other tissues underlying the disease process.

Pharmacological validation of a model of cystitis pain in the mouse

Interstitial cystitis (IC) is a chronic pelvic-perineal pain syndrome of unknown etiology that mainly targets the lower urinary tract. Pain is the most prominent feature of IC and current therapies provide limited relief. Novel treatment options for IC could be identified if more predictive animal models were available. A rat model based on administration of cyclophosphamide (CP) mimics the symptoms of IC and has been well characterized. However, experiments in mice have not consistently reported both the spontaneous and evoked pain behaviors. The current series of studies demonstrate that CP (200-400mg, i.p.) increased both spontaneous and evoked pain behaviors in mice. Additionally, clinically relevant compounds: morphine (1-10mg/kg), ketorolac (1-5.6mg/kg) and duloxetine (3-30mg/kg) all significantly reversed pain behaviors. In contrast, gabapentin (56mg/kg) had no effect. Thus, CP-induced cystitis in mice may be used to evaluate novel therapeutics for the treatment of pain due to interstitial cystitis.

A-803467, a potent and selective Nav1.8 sodium channel blocker, attenuates neuropathic and inflammatory pain in the rat

Activation of tetrodotoxin-resistant sodium channels contributes to action potential electrogenesis in neurons. Antisense oligonucleotide studies directed against Na(v)1.8 have shown that this channel contributes to experimental inflammatory and neuropathic pain. We report here the discovery of A-803467, a sodium channel blocker that potently blocks tetrodotoxin-resistant currents (IC(50) = 140 nM) and the generation of spontaneous and electrically evoked action potentials in vitro in rat dorsal root ganglion neurons. In recombinant cell lines, A-803467 potently blocked human Na(v)1.8 (IC(50) = 8 nM) and was >100-fold selective vs. human Na(v)1.2, Na(v)1.3, Na(v)1.5, and Na(v)1.7 (IC(50) values >or=1 microM). A-803467 (20 mg/kg, i.v.) blocked mechanically evoked firing of wide dynamic range neurons in the rat spinal dorsal horn. A-803467 also dose-dependently reduced mechanical allodynia in a variety of rat pain models including: spinal nerve ligation (ED(50) = 47 mg/kg, i.p.), sciatic nerve injury (ED(50) = 85 mg/kg, i.p.), capsaicin-induced secondary mechanical allodynia (ED(50) approximately 100 mg/kg, i.p.), and thermal hyperalgesia after intraplantar complete Freund's adjuvant injection (ED(50) = 41 mg/kg, i.p.). A-803467 was inactive against formalin-induced nociception and acute thermal and postoperative pain. These data demonstrate that acute and selective pharmacological blockade of Na(v)1.8 sodium channels in vivo produces significant antinociception in animal models of neuropathic and inflammatory pain.

Expression and localization of the Nav1.9 sodium channel in enteric neurons and in trigeminal sensory endings: implication for intestinal reflex function and orofacial pain

The Nav1.9 sodium channel is expressed in nociceptive DRG neurons where it contributes to spontaneous pain behavior after peripheral inflammation. Here, we used a newly developed antibody to investigate the distribution of Nav1.9 in rat and mouse trigeminal ganglion (TG) nerve endings and in enteric nervous system (ENS). In TGs, Nav1.9 was expressed in the soma of small- and medium-sized, peripherin-positive neurons. Nav1.9 was present along trigeminal afferent fibers and at terminals in lip skin and dental pulp. In the ENS, Nav1.9 was detected within the soma and proximal axons of sensory, Dogiel type II, myenteric and submucosal neurons. Immunological data were correlated with the detection of persistent TTX-resistant Na(+) currents sharing similar properties in DRG, TG and myenteric neurons. Collectively, our data support a potential role of Nav1.9 in the transmission of trigeminal pain and the regulation of intestinal reflexes. Nav1.9 might therefore constitute a molecular target for therapeutic treatments of orofacial pain and gastrointestinal syndromes.

Blocking sodium channels to treat neuropathic pain

Neuropathic pain remains a large unmet medical need. A number of therapeutic options exist, but efficacy and tolerability are less than satisfactory. Based on animal models and limited data from human patients, the pain and hypersensitivity that characterize neuropathic pain are associated with spontaneous discharges of normally quiescent nociceptors. Sodium channel blockers inhibit this spontaneous activity, reverse nerve injury-induced pain behavior in animals and alleviate neuropathic pain in humans. Several sodium channel subtypes are expressed primarily in sensory neurons and may contribute to the efficacy of sodium channel blockers. In this report, the authors review the current understanding of the role of sodium channels and of specific sodium channel subtypes in neuropathic pain signaling.

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

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

Convergence of sensory pathways in the development of somatic and visceral hypersensitivity

Sensory neurons innervating different tissues converge onto second-order neurons in the spinal cord. We examined whether inflammation or transient overexpression of nerve growth factor (NGF) in one tissue triggers hypersensitivity in referral sites. Thresholds to mechanical and thermal stimulation of the hindpaw, visceromotor responses to colorectal distension, and cystometrograms were performed in appropriate controls and mice with experimentally induced cystitis, inflammation of the hindpaw or front paw, or injection of viral vectors encoding NGF or green fluorescent protein (GFP). Cystitis and NGF but not GFP overexpression in the bladder triggered bladder hyperactivity associated with mechanical and thermal hypersensitivity in cutaneous referral sites and enhanced responses to colorectal distension. Hindpaw inflammation and injection of the NGF- but not GFP-encoding viral vector or front paw inflammation induced mechanical and thermal hyperalgesia in the affected hindpaw and increased responses to colorectal distension without altering the micturition reflex. In conclusion, sensitization of sensory pathways by inflammation or NGF contributes to the development of hypersensitivity in neighboring organs and cutaneous referral sites and provides a potential mechanism underlying the coexistence of pain syndromes in patients with functional diseases.

Carbamazepine interacts with a slow inactivation state of NaV1.8-like sodium channels

Carbamazepine was tested on high-threshold TTX-resistant Na+ currents (TTX-R-currents), evoked from acutely isolated rat dorsal root ganglion (DRG) cells. Under control conditions, the TTX-R-currents recorded from different DRG cells varied greatly regarding use-dependent inactivation (TTX-R-current UDI), measured as the percent decrease in current amplitude induced by changing the current activation rate from 0.1 Hz to 1.0 Hz. Also, when TTX-R-currents were evoked at 0.1 Hz from a holding potential (hp) of -60 mV, a larger fraction of TTX-R-channels resided tonically in a slow inactivation state in DRG cells with more TTX-R-current UDI versus those with less TTX-R-current UDI. The block of TTX-R-currents evoked from hp -60 mV by 100-microM carbamazepine and the EC50 for carbamazepine block was positively correlated with TTX-R-current UDI. The slope factors estimated for the concentration-response curves averaged 0.68, suggesting the presence of low and high affinity sites. Fitting the data with a two-site binding isotherm gave estimates of 30 microM and 760 microM for the EC50s of the high and low affinity sites, respectively. The fraction of the total fit attributed to the high affinity site was positively correlated with TTX-R-current UDI. Carbamazepine increased the fast and slow time constants for recovery from inactivation and the fraction of the fit attributed to the slow time constant. These data suggest that carbamazepine interacts with a slow inactivation state of TTX-R-channels. This particular mechanism might be exploited in future research aimed at developing pain medications that selectively block Na(V)1.8 channels or Na+ channels in general.

Differential responses of rostral subnucleus caudalis and upper cervical dorsal horn neurons to mechanical and chemical stimulation of the parotid gland in rats

Blockage of the salivary duct can produce pain and inflammation from the build up of saliva in the parotid gland. The processing of parotid inflammation-induced pain, however, is poorly understood. The purpose of this study was to clarify the functional involvement of the trigeminal subnucleus interpolaris/caudalis transition region (Vi/Vc) and upper cervical spinal cord (C1/C2) in processing nociceptive input relevant to parotitis. The effect of capsaicin-induced parotitis was examined on a total of 37 nociceptive neurons isolated from the Vi/Vc (n = 23) and C1/C2 (n = 14) regions. Eight of 23 Vi/Vc neurons responded to mechanical distention of the parotid gland, whereas no C1/C2 neurons responded to the parotid distention. Receptive field characteristics in all neurons were examined following capsaicin injections into the parotid gland. Mechanical and cold responses increased significantly in C1/C2 but not Vi/Vc neurons following capsaicin. Receptive field sizes also increased in C1/C2 but not Vi/Vc neurons. At the Vi/Vc transition region, pinch-evoked activity increased in neurons receiving convergent inputs from the parotid gland and facial skin when compared to non-convergent neurons. The present data indicate that the hyperalgesia and referred pain associated with parotitis may result from sensitization of C1/C2, but not Vi/Vc nociceptive neurons.

Dissecting the role of sodium currents in visceral sensory neurons in a model of chronic hyperexcitability using Nav1.8 and Nav1.9 null mice

Tetrodotoxin-resistant (TTX-R) sodium currents have been proposed to underlie sensory neuronal hyperexcitability in acute inflammatory models, but their role in chronic models is unknown. Since no pharmacological tools to separate TTX-R currents are available, this study employs Na(v)1.8 and Na(v)1.9 null mice to evaluate these currents roles in a chronic hyperexcitability model after the resolution of an inflammatory insult. Transient jejunitis was induced by infection with Nippostrongylus brasiliensis (Nb) in Na(v)1.9 and Na(v)1.8 null, wild-type and naïve mice. Retrogradely labelled dorsal root ganglia (DRG) neurons were harvested on day 20-24 post-infection for patch clamp recording. Rheobase and action potential (AP) parameters were recorded as measures of excitability, and Na(v)1.9 and Na(v)1.8 currents were recorded. DRG neuronal excitability was significantly increased in post-infected mice compared to sham animals, despite the absence of ongoing inflammation (sham = 1.9 +/- 0.3, infected = 3.6 +/- 0.7 APs at 2x rheobase, P = 0.02). Hyperexcitability was associated with a significantly increased amplitude of TTX-R currents. Hyperexcitability was maintained in Na(v)1.9(-/-) mice, but hyperexcitability was absent and APs were blunted in Na(v)1.8(-/-) mice. This study identifies a critical role for Na(v)1.8 in chronic post-infectious visceral hyperexcitability, with no contribution from Na(v)1.9. Nb infection-induced hyperexcitability is not observed in Na(v)1.8(-/-) mice, but is still present in Na(v)1.9(-/-) mice. It is not clear whether hyperexcitability is due to a change in the function of Na(v)1.8 channels or a change in the number of Na(v)1.8 channels.

MCP-1 enhances excitability of nociceptive neurons in chronically compressed dorsal root ganglia

Previous experimental results from our laboratory demonstrated that monocyte chemoattractant protein-1 (MCP-1) depolarizes or increases the excitability of nociceptive neurons in the intact dorsal root ganglion (DRG) after a chronic compression of the DRG (CCD), an injury that upregulates neuronal expression of both MCP-1 and mRNA for its receptor CCR2. We presently explore the ionic mechanisms underlying the excitatory effects of MCP-1. MCP-1 (100 nM) was applied, after CCD, to acutely dissociated small DRG neurons with nociceptive properties. Under current clamp, the proportion of neurons depolarized was similar to that previously observed for CCD-treated neurons in the intact ganglion, although the magnitude of depolarization was greater. MCP-1 induced a decrease in rheobase by 44 +/- 10% and some cells became spontaneously active at resting potential. Action potential width at a voltage equal to 10% of the peak height was increased from 4.94 +/- 0.23 to 5.90 +/- 0.47 ms. In voltage clamp, MCP-1 induced an inward current in 27 of 50 neurons held at -60 mV, which increased with concentration over the range of 3 to 300 nM (EC(50) = 45 nM). The MCP-1-induced current was not voltage dependent and had an estimated reversal potential of -27 mV. In addition, MCP-1 inhibited a voltage-dependent, noninactivating outward current, presumably a delayed rectifier type K(+) conductance. We conclude that MCP-1 enhances excitability in CCD neurons by, at least, two mechanisms: 1) activation of a nonvoltage-dependent depolarizing current with characteristics similar to a nonselective cation conductance and 2) inhibition of a voltage-dependent outward current.

Pharmacology of 2-[4-(4-Chloro-2-fluorophenoxy)phenyl]-pyrimidine-4-carboxamide: A Potent, Broad-Spectrum State-Dependent Sodium Channel Blocker for Treating Pain States

Voltage-gated Na(+) channels may play important roles in establishing pathological neuronal hyperexcitability associated with chronic pain in humans. Na(+) channel blockers, such as carbamazepine (CBZ) and lamotrigine (LTG), are efficacious in treating neuropathic pain; however, their therapeutic utility is compromised by central nervous system side effects. We reasoned that it may be possible to gain superior control over pain states and, in particular, a better therapeutic index, by designing broad-spectrum Na(+) channel blockers with higher potency, faster onset kinetics, and greater levels of state dependence than existing drugs. 2-[4-(4-Chloro-2-fluorophenoxy)phenyl]-pyrimidine-4-carboxamide (PPPA) is a novel structural analog of the state-dependent Na(+) channel blocker V102862 [4-(4-fluorophenoxy)benzaldehyde semicarbazone]. Tested on recombinant rat Na(v)1.2 channels and native Na(+) currents in cultured rat dorsal root ganglion neurons, PPPA was approximately 1000 times more potent, had 2000-fold faster binding kinetics, and > or =10-fold higher levels of state dependence than CBZ and LTG. Tested in rat pain models against mechanical endpoints, PPPA had minimal effective doses of 1 to 3 mg/kg p.o. in partial sciatic nerve ligation, Freund's complete adjuvant, and postincisional pain. In all cases, efficacy was similar to clinically relevant comparators. Importantly, PPPA did not produce motor deficits in the accelerating Rotarod assay of ataxia at doses up to 30 mg/kg p.o., indicating a therapeutic index >10, which was superior to CBZ and LTG. Our experiments suggest that high-potency, broad-spectrum, state-dependent Na(+) channel blockers will have clinical utility for treating neuropathic, inflammatory, and postsurgical pain. Optimizing the biophysical parameters of broad-spectrum voltage-gated Na(+) channel blockers may lead to improved pain therapeutics.

A high-capacity membrane potential FRET-based assay for NaV1.8 channels

Clinical treatment of neuropathic pain can be achieved with a number of different drugs, some of which interact with all members of the voltage-gated sodium channel (NaV1) family. However, block of central nervous system and cardiac NaV1 channels can cause dose-limiting side effects, preventing many patients from achieving adequate pain relief. Expression of the tetrodotoxin-resistant NaV1.8 subtype is restricted to small-diameter sensory neurons, and several lines of evidence indicate a role for NaV1.8 in pain processing. Given these features, NaV1.8 subtype-selective blockers are predicted to be efficacious in the treatment of neuropathic pain and to be associated with fewer adverse effects than currently available therapies. To facilitate the identification of NaV1.8-specific inhibitors, we stably expressed the human NaV1.8 channel together with the auxiliary human beta1 subunit (NaV beta1) in human embryonic kidney 293 cells. Heterologously expressed human NaV1.8/NaV beta1 channels display biophysical properties that are similar to those of tetrodotoxin-resistant channels present in mouse dorsal root ganglion neurons. A membrane potential, fluorescence resonance energy transfer-based functional assay on a fluorometric imaging plate reader (FLIPR-Tetra, Molecular Devices, Sunnyvale, CA) platform has been established. This highcapacity assay is sensitive to known state-dependent NaV1 modulators and can be used to identify novel and selective NaV1.8 inhibitors.

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.

Involvement of the TTX-resistant sodium channel Nav 1.8 in inflammatory and neuropathic, but not post-operative, pain states

Antisense (AS) oligodeoxynucleotides (ODNs) targeting the Nav 1.8 sodium channel have been reported to decrease inflammatory hyperalgesia and L5/L6 spinal nerve ligation-induced mechanical allodynia in rats. The present studies were conducted to further characterize Nav 1.8 AS antinociceptive profile in rats to better understand the role of Nav 1.8 in different pain states. Consistent with earlier reports, chronic intrathecal Nav 1.8 AS, but not mismatch (MM), ODN decreased TTX-resistant sodium current density (by 60.5+/-10.2% relative to MM; p<0.05) in neurons from L4 to L5 dorsal root ganglia and significantly attenuated mechanical allodynia following intraplantar complete Freund's adjuvant. In addition, 10 days following chronic constriction injury of the sciatic nerve, Nav 1.8 AS, but not MM, ODN also attenuated mechanical allodynia (54.3+/-8.2% effect, p<0.05 vs. MM) 2 days after initiation of ODN treatment. The anti-allodynic effects remained for the duration of the AS treatment, and CCI rats returned to an allodynic state 4 days after discontinuing AS. In contrast, Nav 1.8 AS ODN failed to reduce mechanical allodynia in the vincristine chemotherapy-induced neuropathic pain model or a skin-incision model of post-operative pain. Finally, Nav 1.8 AS, but not MM, ODN treatment produced a small but significant attenuation of acute noxious mechanical sensitivity in naïve animals (17.6+/-6.2% effect, p<0.05 vs. MM). These data demonstrate a greater involvement of Nav 1.8 in frank nerve injury and inflammatory pain as compared to acute, post-operative or chemotherapy-induced neuropathic pain states.

Experimental colitis in mice and sensitization of converging visceral and somatic afferent pathways

Chronic pain syndromes affecting different organs often coexist. We hypothesized that sensitization of one afferent pathway may affect converging input from other areas of the body. We induced colitis in mice with 2,4,6-trinitrobenzenesulfonic acid (TNBS); control animals were treated with equal volumes of vehicle (50% ethanol) only. Visceromotor responses to graded colorectal distension, cystometrograms, and response thresholds to mechanical and thermal stimulation of both hind paws were determined on days 7 and 14. Inflammation of colon and bladder was assessed with validated histological markers and scores. TNBS caused significant colitis on day 7 that resolved by day 14; there was no evidence of bladder inflammation. There was a significant hypersensitivity to colorectal distension on day 7, which returned to normal on day 14. This was associated with bladder overactivity, as demonstrated by early onset of micturition and more frequent micturition on day 7 after TNBS administration. Colitis also significantly altered responses to mechanical and thermal stimulation of both hind paws on day 7 but not day 14. We conclude that cross talk between afferent visceral and somatic pathways may contribute to the coexistence of pain syndromes.

A quantitative analysis of the sensory and sympathetic innervation of the mouse pancreas

Pain from pancreatitis or pancreatic cancer can be both chronic and severe although little is known about the mechanisms that generate and maintain this pain. To define the peripheral sensory and sympathetic fibers involved in transmitting and modulating pancreatic pain, immunohistochemistry and confocal microscopy were used to examine the sensory and sympathetic innervation of the head, body and tail of the normal mouse pancreas. Myelinated sensory fibers were labeled with an antibody raised against 200 kD neurofilament H (clone RT97), thinly myelinated and unmyelinated peptidergic sensory fibers were labeled with antibodies raised against calcitonin gene-related peptide (CGRP) and post-ganglionic sympathetic fibers were labeled with an antibody raised against tyrosine hydroxylase (TH). RT97, CGRP, and TH immunoreactive fibers were present in parenchyma of the head, body and tail of the pancreas with the relative density of both RT97 and CGRP expressing fibers being head>body>tail, whereas for TH, a relatively even distribution was observed. In all three regions of the pancreas, RT97 fibers were associated mainly with large blood vessels, the CGRP fibers were associated with the large- and medium-sized blood vessels and the TH were associated with the large- and medium-sized blood vessels as well as capillaries. In addition to this extensive set of sensory and sympathetic nerve fibers that terminate in the pancreas, there were large bundles of en passant nerve fibers in the dorsal region of the pancreas that expressed RT97 or CGRP and were associated with the superior mesenteric plexus. These data suggest the pancreas receives a significant sensory and sympathetic innervation. Understanding the factors and disease states that sensitize and/or directly excite the nerve fibers that terminate in the pancreas as well as those that are en passant may aid in the development of therapies that more effectively modulate the pain that frequently accompanies diseases of the pancreas, such as pancreatitis and pancreatic cancer.

Analysis of human Nav1.8 expressed in SH-SY5Y neuroblastoma cells

The tetrodotoxin-resistant voltage-gated sodium channel alpha-subunit Nav1.8 is expressed in nociceptors and has been implicated in chronic pain. Difficulties of heterologous expression have so far precluded analysis of the pharmacological properties of human Nav1.8. To address this we have introduced human Nav1.8 in neuroblastoma SH-SY5Y cells. Voltage-clamp analysis showed that human Nav1.8 generated an inward tetrodotoxin-resistant sodium current with an activating threshold around -50 mV, half maximal activation at -11+/-3 mV and a reversal potential of 67+/-4 mV. These properties closely match those of the endogenous rat tetrodotoxin-resistant sodium current in dorsal root ganglia suggesting that the expressed human channel is in a near physiological conformation. Human Nav1.8 was resistant to tetrodotoxin and activated by the pyrethroid toxin deltamethrin. Both voltage-activated and deltamethrin-activated human Nav1.8 were inhibited by the sodium channel blockers BIII 890 CL, NW-1029, and mexiletine. Inhibition of Nav1.8 by these compounds may underlie their known analgesic effects in animal models.

Block of peripheral nerve sodium channels selectively inhibits features of neuropathic pain in rats

Several sodium channel blockers are used clinically to treat neuropathic pain. However, many patients fail to achieve adequate pain relief from these highly brain-penetrant drugs because of dose-limiting central nervous system side effects. Here, we describe the functional properties of trans-N-{[2'-(aminosulfonyl)biphenyl-4-yl]methyl}-N-methyl-N'-[4-(trifluoromethoxy)benzyl]cyclopentane-1,2-dicarboxamide (CDA54), a peripherally acting sodium channel blocker. In whole-cell electrophysiological assays, CDA54 blocked the inactivated states of hNa(V)1.7 and hNa(V)1.8, two channels of the peripheral nervous system implicated in nociceptive transmission, with affinities of 0.25 and 0.18 microM, respectively. CDA54 displayed similar affinities for the tetrodotoxin-resistant Na+ current in small-diameter mouse dorsal root ganglion neurons. Peripheral nerve injury causes spontaneous electrical activity in normally silent sensory neurons. CDA54 inhibited these injury-induced spontaneous action potentials at concentrations 10-fold lower than those required to block normal A- and C-fiber conduction. Consistent with the selective inhibition of injury-induced firing, CDA54 (10 mg/kg p.o.) significantly reduced behavioral signs of neuropathic pain in two nerve injury models, whereas the same dose of CDA54 did not affect acute nociception or motor coordination. In anesthetized dogs, CDA54, at plasma concentrations of 6.7 microM, had no effect on cardiac electrophysiological parameters including conduction. Thus, the peripheral nerve sodium channel blocker CDA54 selectively inhibits sensory nerve signaling associated with neuropathic pain.

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

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

CAP-1A is a novel linker that binds clathrin and the voltage-gated sodium channel Na(v)1.8

The voltage-gated sodium channel Na(v)1.8 produces a tetrodotoxin-resistant current and plays a key role in nociception. Annexin II/p11 binds to Na(v)1.8 and facilitates insertion of the channel within the cell membrane. However, the mechanisms responsible for removal of specific channels from the cell membrane have not been studied. We have identified a novel protein, clathrin-associated protein-1A (CAP-1A), which contains distinct domains that bind Na(v)1.8 and clathrin. CAP-1A is abundantly expressed in DRG neurons and colocalizes with Na(v)1.8 and can form a multiprotein complex with Na(v)1.8 and clathrin. Coexpression of CAP-1A and Na(v)1.8 in DRG neurons reduces Na(v)1.8 current density by approximately 50% without affecting the endogenous or recombinant tetrodotoxin-sensitive currents. This effect of CAP-1A is blocked by bafilomycin A1 treatment of transfected DRG neurons. CAP-1A thus is the first example of an adapter protein that links clathrin and a sodium channel and may regulate Na(v)1.8 channel density at the cell surface.

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.

Developmental neurotoxicity of pyrethroid insecticides: critical review and future research needs

Pyrethroid insecticides have been used for more than 40 years and account for 25% of the worldwide insecticide market. Although their acute neurotoxicity to adults has been well characterized, information regarding the potential developmental neurotoxicity of this class of compounds is limited. There is a large age dependence to the acute toxicity of pyrethroids in which neonatal rats are at least an order of magnitude more sensitive than adults to two pyrethroids. There is no information on age-dependent toxicity for most pyrethroids. In the present review we examine the scientific data related to potential for age-dependent and developmental neurotoxicity of pyrethroids. As a basis for understanding this neurotoxicity, we discuss the heterogeneity and ontogeny of voltage-sensitive sodium channels, a primary neuronal target of pyrethroids. We also summarize 22 studies of the developmental neurotoxicity of pyrethroids and review the strengths and limitations of these studies. These studies examined numerous end points, with changes in motor activity and muscarinic acetylcholine receptor density the most common. Many of the developmental neurotoxicity studies suffer from inadequate study design, problematic statistical analyses, use of formulated products, and/or inadequate controls. These factors confound interpretation of results. To better understand the potential for developmental exposure to pyrethroids to cause neurotoxicity, additional, well-designed and well-executed developmental neurotoxicity studies are needed. These studies should employ state-of-the-science methods to promote a greater understanding of the mode of action of pyrethroids in the developing nervous system.

Biologic poisons for pain

Pain therapies from natural sources date back thousands of years to the use of plant and animal extracts for a variety of painful conditions and injuries. We certainly are all familiar with modern uses of plant-derived analgesic compounds such as opium derivatives from papaverum somniferum and salicylates from willow bark (Salix species). Local anesthetics were isolated from coca leaves in the late 1800s. Sarapin, derived from carnivorous pitcher plants, has been injected for regional analgesia in human and veterinary medicine, but efficacy is controversial. Biologic organisms can play important roles in developing an understanding of pain mechanisms, either from isolation of compounds that are analgesic or of compounds that produce pain, hyperalgesia, and allodynia.

Potentiation of sural nerve Abeta action potential after neurogenic inflammation

Inflammatory mediators modulate voltage-gated sodium channels through protein kinase-mediated pathways. However, it is not clear whether neurogenic inflammation may also alter the properties of distantly located channels along axon shafts supplying the inflamed dermatome. In this study, localized inflammation was induced via intradermal injection of capsaicin within the receptive field of the sural nerve, and compound action potentials (CAP) evoked by sural nerve stimulation were recorded from the sciatic nerve proximally. The area measured under the A beta CAP increased significantly within 5 min after capsaicin injection. Distal injection of lidocaine at the ankle division of the sural nerve prior to capsaicin injection reversed this increase. In addition, application of a lipophilic protein kinase inhibitor H7 (100 microM) through a perfusion chamber placed on the sciatic nerve also reversed this increase. Our results suggest that during neurogenic inflammation, action potential activity is increased, triggering activation of protein kinases that may rapidly alter membrane conductance to potentiate action potential propagation along peripheral nerves.

Functional role of the C-terminus of voltage-gated sodium channel Nav1.8

Sodium channel Na(v)1.8 requires stronger depolarization than other sodium channels for activation and inactivation. The contribution of Na(v)1.8 C-terminus to this property was investigated by producing Na(v)1.8 and Na(v)1.4 chimeras and expressing them in ND7/23 cells. Current densities of the chimeras were significantly different than in parental channels, and the voltage-dependence of activation was depolarized in Na(v)1.4/1.8C compared to Na(v)1.4. Analysis of steady-state inactivation showed that only Na(v)1.8 and Na(v)1.4/1.8C currents demonstrate a non-inactivated fraction. Thus, the C-terminus of Na(v)1.8 contributes to regulation of channel density at the cell surface, modulates channel gating, and regulates the generation of sustained current.

Group II metabotropic glutamate receptors inhibit cAMP-dependent protein kinase-mediated enhancemednt of tetrodotoxin-resistant sodium currents in mouse dorsal root ganglion neurons

Tetrodotoxin (TTX)-resistant sodium currents are important in nociception and nociceptive sensitization, which is partially due to their cAMP/protein kinase A (PKA)-mediated enhancement. Here we studied the effects of group II mGluR activation on TTX-resistant sodium currents in cultured mouse dorsal root ganglion (DRG) neurons. Activation of adenylyl cyclase with forskolin caused an increase in the amplitude of TTX-R currents and a leftward shift of the activation curve. When neurons were treated with ammonium pyrrolidinedithiocarbamate (APDC), a selective group II mGluR agonist, both the forskolin-induced increase in current amplitude and the shift of activation curve were blocked. LY341495, a group II mGluR antagonist, prevented these inhibitory effects of APDC. Our results suggest that group II mGluRs can negatively regulate TTX-R sodium currents in mouse DRG neurons.