Endothelial sodium channel activation promotes cardiac stiffness and diastolic dysfunction in Western diet fed female mice

OBJECTIVE

Obesity is associated with myocardial fibrosis and impaired diastolic relaxation, abnormalities that are especially prevalent in women. Normal coronary vascular endothelial function is integral in mediating diastolic relaxation, and recent work suggests increased activation of the endothelial cell (EC) mineralocorticoid receptor (ECMR) is associated with impaired diastolic relaxation. As the endothelial Na⁺ channel (EnNaC) is a downstream target of the ECMR, we sought to determine whether EC-specific deletion of the critical alpha subunit, αEnNaC, would prevent diet induced-impairment of diastolic relaxation in female mice.

METHODS AND MATERIALS

Female αEnNaC KO mice and littermate controls were fed a Western diet (WD) high in fat (46%), fructose corn syrup (17.5%) and sucrose (17.5%) for 12-16 weeks. Measurements were conducted for in vivo cardiac function, in vitro cardiomyocyte stiffness and EnNaC activity in primary cultured ECs. Additional biochemical studies examined indicators of oxidative stress, including aspects of antioxidant Nrf2 signaling, in cardiac tissue.

RESULTS

Deletion of αEnNaC in female mice fed a WD significantly attenuated WD mediated impairment in diastolic relaxation. Improved cardiac relaxation was accompanied by decreased EnNaC-mediated Na⁺ currents in ECs and reduced myocardial oxidative stress. Further, deletion of αEnNaC prevented WD-mediated increases in isolated cardiomyocyte stiffness.

CONCLUSION

Collectively, these findings support the notion that WD feeding in female mice promotes activation of EnNaC in the vasculature leading to increased cardiomyocyte stiffness and diastolic dysfunction.

Secondary neurotransmitter deficiencies in epilepsy caused by voltage-gated sodium channelopathies: A potential treatment target?

We describe neurotransmitter abnormalities in two patients with drug-resistant epilepsy resulting from deleterious de novo mutations in sodium channel genes. Whole exome sequencing identified a de novo SCN2A splice-site mutation (c.2379+1G>A, p.Glu717Gly.fs*30) resulting in deletion of exon 14, in a 10-year old male with early onset global developmental delay, intermittent ataxia, autism, hypotonia, epileptic encephalopathy and cerebral/cerebellar atrophy. In the cerebrospinal fluid both homovanillic acid and 5-hydroxyindoleacetic acid were significantly decreased; extensive biochemical and genetic investigations ruled out primary neurotransmitter deficiencies and other known inborn errors of metabolism. In an 8-year old female with an early onset intractable epileptic encephalopathy, developmental regression, and progressive cerebellar atrophy, a previously unreported de novo missense mutation was identified in SCN8A (c.5615G>A; p.Arg1872Gln), affecting a highly conserved residue located in the C-terminal of the Nav1.6 protein. Aside from decreased homovanillic acid and 5-hydroxyindoleacetic acid, 5-methyltetrahydrofolate was also found to be low. We hypothesize that these channelopathies cause abnormal synaptic mono-amine metabolite secretion/uptake via impaired vesicular release and imbalance in electrochemical ion gradients, which in turn aggravate the seizures. Treatment with oral 5-hydroxytryptophan, l-Dopa/Carbidopa, and a dopa agonist resulted in mild improvement of seizure control in the male case, most likely via dopamine and serotonin receptor activated signal transduction and modulation of glutamatergic, GABA-ergic and glycinergic neurotransmission. Neurotransmitter analysis in other sodium channelopathy patients will help validate our findings, potentially yielding novel treatment opportunities.

Voltage-gated Na+ channel β1B: a secreted cell adhesion molecule involved in human epilepsy

Scn1b-null mice have a severe neurological and cardiac phenotype. Human mutations in SCN1B result in epilepsy and cardiac arrhythmia. SCN1B is expressed as two developmentally regulated splice variants, β1 and β1B, that are each expressed in brain and heart in rodents and humans. Here, we studied the structure and function of β1B and investigated a novel human SCN1B epilepsy-related mutation (p.G257R) unique to β1B. We show that wild-type β1B is not a transmembrane protein, but a soluble protein expressed predominantly during embryonic development that promotes neurite outgrowth. Association of β1B with voltage-gated Na+ channels Na(v)1.1 or Na(v)1.3 is not detectable by immunoprecipitation and β1B does not affect Na(v)1.3 cell surface expression as measured by [(3)H]saxitoxin binding. However, β1B coexpression results in subtle alteration of Na(v)1.3 currents in transfected cells, suggesting that β1B may modulate Na+ current in brain. Similar to the previously characterized p.R125C mutation, p.G257R results in intracellular retention of β1B, generating a functional null allele. In contrast, two other SCN1B mutations associated with epilepsy, p.C121W and p.R85H, are expressed at the cell surface. We propose that β1B p.G257R may contribute to epilepsy through a mechanism that includes intracellular retention resulting in aberrant neuronal pathfinding.

A nonproton ligand sensor in the acid-sensing ion channel

Acid-sensing ion channels (ASICs) have long been considered as extracellular proton (H(+))-gated cation channels, and peripheral ASIC3 channels seem to be a natural sensor of acidic pain. Here, we report the identification of a nonproton sensor on ASIC3. We show first that 2-guanidine-4-methylquinazoline (GMQ) causes persistent ASIC3 channel activation at the normal pH. Using GMQ as a probe and combining mutagenesis and covalent modification analysis, we then uncovered a ligand sensor lined by residues around E423 and E79 of the extracellular "palm" domain of the ASIC3 channel that is crucial for activation by nonproton activators. Furthermore, we show that GMQ activates sensory neurons and causes pain-related behaviors in an ASIC3-dependent manner, indicating the functional significance of ASIC activation by nonproton ligands. Thus, natural ligands beyond protons may activate ASICs under physiological and pathological conditions through the nonproton ligand sensor, serving for channel activation independent of abrupt and marked acidosis.

Differences in sino-atrial and atrio-ventricular function with age and sex attributable to the Scn5a+/- mutation in a murine cardiac model

AIM

To investigate the interacting effects of age and sex on electrocardiographic (ECG) features of Scn5a(+/-) mice modelling Brugada syndrome.

METHODS

Recordings were performed on anaesthetized wild-type (WT) and Scn5a(+/-) mice and differences attributable to these risk factors statistically stratified.

RESULTS

Scn5a(+/-) exerted sex-dependent effects upon sino-atrial function that only became apparent with age. RR intervals were greater in old male than in old female Scn5a(+/-). Atrio-ventricular (AV) conduction was slower in young female mice, whether WT and Scn5a(+/-), than the corresponding young male WT and Scn5a(+/-). However, PR intervals lengthened with age in male but not in female Scn5a(+/-) giving the greatest PR intervals in old male Scn5a(+/-) compared with either old male WT or young male Scn5a(+/-) mice. In contrast, PR intervals were similar in old female Scn5a(+/-) and in old female WT. QTc was prolonged in Scn5a(+/-) compared with WT, and female Scn5a(+/-) compared with female WT. Age-dependent alterations in durations of ventricular repolarization relative to WT affected male but not female Scn5a(+/-). Thus, T-wave durations were greater in old male Scn5a(+/-) compared with old male WT, but indistinguishable between old female Scn5a(+/-) and old female WT. Finally, analysis for combined interactions of genotype, age and sex demonstrated no effects on P wave and QRS durations and QTc intervals.

CONCLUSION

We demonstrate for the first time that age, sex and genotype exert both independent and interacting ECG effects. The latter suggest alterations in cardiac pacemaker function, atrio-ventricular conduction and ventricular repolarization greatest in ageing male Scn5a(+/-).

Functional reciprocity between Na+ channel Nav1.6 and beta1 subunits in the coordinated regulation of excitability and neurite outgrowth

Voltage-gated Na(+) channel (VGSC) beta1 subunits regulate cell-cell adhesion and channel activity in vitro. We previously showed that beta1 promotes neurite outgrowth in cerebellar granule neurons (CGNs) via homophilic cell adhesion, fyn kinase, and contactin. Here we demonstrate that beta1-mediated neurite outgrowth requires Na(+) current (I(Na)) mediated by Na(v)1.6. In addition, beta1 is required for high-frequency action potential firing. Transient I(Na) is unchanged in Scn1b (beta1) null CGNs; however, the resurgent I(Na), thought to underlie high-frequency firing in Na(v)1.6-expressing cerebellar neurons, is reduced. The proportion of axon initial segments (AIS) expressing Na(v)1.6 is reduced in Scn1b null cerebellar neurons. In place of Na(v)1.6 at the AIS, we observed an increase in Na(v)1.1, whereas Na(v)1.2 was unchanged. This indicates that beta1 is required for normal localization of Na(v)1.6 at the AIS during the postnatal developmental switch to Na(v)1.6-mediated high-frequency firing. In agreement with this, beta1 is normally expressed with alpha subunits at the AIS of P14 CGNs. We propose reciprocity of function between beta1 and Na(v)1.6 such that beta1-mediated neurite outgrowth requires Na(v)1.6-mediated I(Na), and Na(v)1.6 localization and consequent high-frequency firing require beta1. We conclude that VGSC subunits function in macromolecular signaling complexes regulating both neuronal excitability and migration during cerebellar development.

Exploring the role of nociceptor-specific sodium channels in pain transmission using Nav1.8 and Nav1.9 knockout mice

Two voltage gated sodium channels, Na(v)1.8 and Na(v)1.9, are exclusively expressed in primary sensory neurons and are suggested to play a role in different pain conditions, including chronic inflammatory and neuropathic pain states. Since no selective pharmacological tools are available, we investigated the involvement of Na(v)1.8 and Na(v)1.9 in pain transmission by the phenotypic characterization of Na(v)1.8 and Na(v)1.9 knockout mice and their wild-type littermates in models of acute nociception, peripheral inflammation and neuropathic pain. The present study provides evidence for a modulatory role of Na(v)1.9, and to a lesser extent Na(v)1.8 in the development of cold, but not mechanical allodynia in neuropathic pain conditions. Moreover, our results also indicate that Na(v)1.9 signaling might be involved in visceral pain. In contrast, the presumed critical role of these two sodium channel subtypes to inflammatory pain hypersensitivity seem, according to our results, to be limited and temporarily.

Acquired temperature-sensitive paralysis as a biomarker of declining neuronal function in aging Drosophila

General locomotor activity decreases with normal aging in animals and could be partially explained by decreases in neuronal function. Voltage-gated Na(+) channels are essential in initiating and propagating rapid electrical impulses underlying normal locomotor activity and behavior in animals. Isolation of mutations conferring temperature-sensitive (ts) paralysis has been an extremely powerful paradigm for identifying genes involved in neuronal functions, such as membrane excitability and synaptic transmission. For instance, decreased expression of wild-type Na(+) channels in flies harboring the no-action-potential (nap) mutant allele (mle(napts)) confers rapid and reversible ts paralysis, because of failure of action potential propagation. Here, we report that aging wild-type Drosophila gradually develops an acquired susceptibility to ts paralysis that is indistinguishable from that seen in young ts paralytic mle(napts) mutants. Moreover, we show that this general age-dependent susceptibility is also present in mle(napts) flies, although the effects are shifted to lower temperature regimes. The mle(napts) flies also exhibit decreased lifespan and increased frailty. Paralysis and decreased lifespan of mle(napts) flies were partially rescued by increasing the dosage of para, the structural gene for the major action potential Na(+) channel in central nervous system of Drosophila. Lastly, we show a dramatic scaling of ts paralysis susceptibility with chronological age in short-lived and long-lived mutant flies, further demonstrating that this age-dependent risk is independent of genetic background. Thus, decreased neural transmission, a hallmark of which is ts paralysis, is a biomarker of aging.

Acid sensing ion channels 2 and 3 are required for inhibition of visceral nociceptors by benzamil

The Deg/ENaC family of ion channels, including ASIC1, 2 and 3, are candidate mechanotransducers in visceral and somatic sensory neurons, although each channel may play a different role in different sensory pathways. Here we determined which distinct populations of visceral sensory neurons are sensitive to the non-selective Deg/ENaC blocker benzamil, and which ASIC channels are targets for benzamil by studying its actions in knockout mice. Single afferent fiber recordings were made in vitro from mouse high threshold colonic thoracolumbar splanchnic afferents and low threshold gastroesophageal vagal afferents. mRNA expression of ASIC subtypes was compared between colonic and gastroesophageal afferents by quantitative RT-PCR of transcripts following laser capture microdissection of retrogradely labeled cell bodies. Mechanosensitivity of colonic afferents was potently reduced by benzamil (10(-6)-3 x 10(-4)M), whereas gastroesophageal afferents were marginally inhibited. Inhibition of colonic afferent mechanosensitivity by benzamil was markedly diminished in ASIC2-/- and ASIC3-/- mice, but unchanged in ASIC1a-/-. Therefore ASIC2 and 3 are targets for benzamil to inhibit colonic afferent mechanosensitivity. Conversely, gastroesophageal afferents are less sensitive to benzamil, and its action depends less on ASIC expression. mRNA for ASIC3 showed higher and ASIC1a showed lower relative expression in colonic afferents from thoracolumbar dorsal root ganglia than in gastric afferents from nodose (vagal) ganglia. These data indicate that ASICs on colonic afferents present distinct pharmacological targets for visceral pain.

Targeting ASIC1a reduces innate fear and alters neuronal activity in the fear circuit

BACKGROUND

The molecular mechanisms underlying innate fear are poorly understood. Previous studies indicated that the acid sensing ion channel ASIC1a influences fear behavior in conditioning paradigms. However, these differences may have resulted from an ASIC1a effect on learning, memory, or the expression of fear.

METHODS

To test the hypothesis that ASIC1a influences the expression of fear or anxiety independent of classical conditioning, we examined the effects of disrupting the mouse ASIC1a gene on unconditioned fear in the open field test, unconditioned acoustic startle, and fear evoked by the predator odor trimethylthiazoline (TMT). In addition, we tested the effects of acutely inhibiting ASIC1a with PcTx, an ASIC1a antagonist in tarantula venom. Our immunohistochemistry suggested ASIC1a is expressed in the bed nucleus of the stria terminalis, medial amygdala, and periaqueductal gray, which are thought to play important roles in the generation and expression of innate fear. Therefore, we also tested whether ASIC1a disruption altered c-fos expression in these structures following TMT exposure.

RESULTS

We found that the loss of ASIC1a reduced fear in the open field test, reduced acoustic startle, and inhibited the fear response to TMT. Similarly, intracerebroventricular administration of PcTx reduced TMT-evoked freezing in ASIC1a(+/+) mice but not ASIC1a(-/-) mice. In addition, loss of ASIC1a altered TMT-evoked c-fos expression in the medial amydala and dorsal periaqueductal gray.

CONCLUSIONS

These findings suggest that ASIC1a modulates activity in the circuits underlying innate fear. Furthermore, the data indicate that targeting the ASIC1a gene or acutely inhibiting ASIC1a suppresses fear and anxiety independent of conditioning.

The voltage-gated sodium channel Scn8a is a genetic modifier of severe myoclonic epilepsy of infancy

The mammalian genome contains four voltage-gated sodium channel genes that are primarily expressed in the central nervous system: SCN1A, SCN2A, SCN3A and SCN8A. Mutations in SCN1A and SCN2A are responsible for several dominant idiopathic epilepsy disorders, including generalized epilepsy with febrile seizures plus (GEFS+) and severe myoclonic epilepsy of infancy (SMEI). Mutations in SCN8A are associated with cognitive deficits and neuropsychiatric illness in humans and movement disorders in mice; however, a role for SCN8A (Na(v)1.6) in epilepsy has not been investigated. To determine the relationship between Na(v)1.6 dysfunction and seizure susceptibility, we examined the thresholds of two Scn8a mouse mutants, Scn8a(med) and Scn8a(med-jo), to flurothyl- and kainic acid (KA)-induced seizures. Both mutants were more seizure resistant than wild-type littermates, suggesting that altered Na(v)1.6 function reduces neuronal excitability. To determine whether impaired Na(v)1.6 function could ameliorate seizure severity in a mouse model of SMEI, we generated Scn1a(+/-); Scn8a(med-jo/+) double heterozygous mice. Unlike Scn1a(+/-) mice that are more susceptible to flurothyl-induced seizures, Scn1a(+/-); Scn8a(med-jo/+) mice displayed thresholds that were comparable to wild-type littermates. The Scn8a(med-jo) allele was also able to rescue the premature lethality of Scn1a(+/-) mice and extend the lifespan of Scn1a(-/-) mutants. These results demonstrate that genetic interactions can alter seizure severity and support the hypothesis that genetic modifiers contribute to the clinical variability observed in SMEI and GEFS+.

Sodium channel Scn1b null mice exhibit prolonged QT and RR intervals

In neurons, voltage-gated sodium channel beta subunits regulate the expression levels, subcellular localization, and electrophysiological properties of sodium channel alpha subunits. However, the contribution of beta subunits to sodium channel function in heart is poorly understood. We examined the role of beta1 in cardiac excitability using Scn1b null mice. Compared to wildtype mice, electrocardiograms recorded from Scn1b null mice displayed longer RR intervals and extended QT(c) intervals, both before and after autonomic block. In acutely dissociated ventricular myocytes, loss of beta1 expression resulted in a approximately 1.6-fold increase in both peak and persistent sodium current while channel gating and kinetics were unaffected. Na(v)1.5 expression increased in null myocytes approximately 1.3-fold. Action potential recordings in acutely dissociated ventricular myocytes showed slowed repolarization, supporting the extended QT(c) interval. Immunostaining of individual myocytes or ventricular sections revealed no discernable alterations in the localization of sodium channel alpha or beta subunits, ankyrin(B), ankyrin(G), N-cadherin, or connexin-43. Together, these results suggest that beta1 is critical for normal cardiac excitability and loss of beta1 may be associated with a long QT phenotype.

Paroxysmal extreme pain disorder (previously familial rectal pain syndrome)

OBJECTIVE

To describe the clinical phenotype of paroxysmal extreme pain disorder (previously called familial rectal pain syndrome), an autosomal dominant condition recently shown to be a sodium channelopathy involving SCN9A.

METHODS

An international consortium of clinicians, scientists, and affected families was formed. Clinical details of all accessible families worldwide were collected, including age at onset, features of attacks, problems between attacks, investigational results, treatments tried, and evolution over time. A validated pain questionnaire was completed by 14 affected individuals.

RESULTS

Seventy-seven individuals from 15 families were identified. The onset of the disorder is in the neonatal period or infancy and persists throughout life. Autonomic manifestations predominate initially, with skin flushing in all and harlequin color change and tonic attacks in most. Dramatic syncopes with bradycardia and sometimes asystole are common. Later, the disorder is characterized by attacks of excruciating deep burning pain often in the rectal, ocular, or jaw areas, but also diffuse. Attacks are triggered by factors such as defecation, cold wind, eating, and emotion. Carbamazepine is effective in almost all who try it, but the response is often incomplete.

CONCLUSIONS

Paroxysmal extreme pain disorder is a highly distinctive sodium channelopathy with incompletely carbamazepine-sensitive bouts of pain and sympathetic nervous system dysfunction. It is most likely to be misdiagnosed as epilepsy and, particularly in infancy, as hyperekplexia and reflex anoxic seizures.

Nociceptors of dorsal root ganglion express proton-sensing G-protein-coupled receptors

One major goal in pain research is to identify novel pain targets. Tissue injury, inflammation, and ischemia are usually accompanied by local tissue acidosis, the degree of associated pain or discomfort well correlated with the magnitude of acidification. Proton-sensing ion channels, transient receptor potential/vanilloid receptor subtype 1, and acid-sensing ion channel 3 are involved in acidosis-linked pain. However, whether recently identified proton-sensing G-protein-coupled receptors (GPCRs) also have some contributions is unclear. Proton-sensing GPCRs, including OGR1, GPR4, G2A, and TDAG8, are fully activated at pH 6.4-6.8 in vitro. To understand whether the proton-sensing GPCRs are expressed in nociceptors, we cloned the four mouse genes and examined their tissue distribution and localization in pain-relevant loci, the dorsal root ganglion (DRG). The OGR1 family members were widely expressed in neuronal and non-neuronal tissues. Their transcripts were expressed in the DRG, and most (75-82%) were present in small-diameter neurons responsible for nociception. Approximately 31-40% of total DRG neurons expressed at least two proton-sensing GPCRs. We have also demonstrated that gene expression of proton-sensing GPCRs is changed in ASIC3 knockout mice. Our finding suggests that proton-sensing GPCRs could have some roles in nociception or in compensation of loss of ASIC3 gene.

A tarantula peptide against pain via ASIC1a channels and opioid mechanisms

Psalmotoxin 1, a peptide extracted from the South American tarantula Psalmopoeus cambridgei, has very potent analgesic properties against thermal, mechanical, chemical, inflammatory and neuropathic pain in rodents. It exerts its action by blocking acid-sensing ion channel 1a, and this blockade results in an activation of the endogenous enkephalin pathway. The analgesic properties of the peptide are suppressed by antagonists of the mu and delta-opioid receptors and are lost in Penk1-/- mice.

Sodium currents in mesencephalic trigeminal neurons from Nav1.6 null mice

Previous studies using pharmacological methods suggest that subthreshold sodium currents are critical for rhythmical burst generation in mesencephalic trigeminal neurons (Mes V). In this study, we characterized transient (I(NaT)), persistent (I(N)(aP)), and resurgent (I(res)) sodium currents in Na(v)1.6-null mice (med mouse, Na(v)1.6(-/-)) lacking expression of the sodium channel gene Scn8a. We found that peak transient, persistent, and resurgent sodium currents from med (Na(v)1.6(-/-)) mice were reduced by 18, 39, and 76% relative to their wild-type (Na(v)1.6(+/+)) littermates, respectively. Current clamp recordings indicated that, in response to sinusoidal constant amplitude current (ZAP function), all neurons exhibited membrane resonance. However, Mes V neurons from med mice had reduced peak amplitudes in the impedance-frequency relationship (resonant Q-value) and attenuated subthreshold oscillations despite the similar passive membrane properties compared with wild-type littermates. The spike frequency-current relationship exhibited reduced instantaneous discharge frequencies and spike block at low stimulus currents and seldom showed maintained spike discharge throughout the stimulus in the majority of med neurons compared with wild-type neurons. Importantly, med neurons never exhibited maintained stimulus-induced rhythmical burst discharge unlike those of wild-type littermates. The data showed that subthreshold sodium currents are critical determinants of Mes V electrogenesis and burst generation and suggest a role for resurgent sodium currents in control of spike discharge.

Glial Nax Channels Control Lactate Signaling to Neurons for Brain [Na+] Sensing

Sodium (Na) homeostasis is crucial for life, and Na levels in body fluids are constantly monitored in the brain. The subfornical organ (SFO) is the center of the sensing responsible for the control of salt-intake behavior, where Na(x) channels are expressed in specific glial cells as the Na-level sensor. Here, we show direct interaction between Na(x) channels and alpha subunits of Na(+)/K(+)-ATPase, which brings about Na-dependent activation of the metabolic state of the glial cells. The metabolic enhancement leading to extensive lactate production was observed in the SFO of wild-type mice, but not of the Na(x)-knockout mice. Furthermore, lactate, as well as Na, stimulated the activity of GABAergic neurons in the SFO. These results suggest that the information on a physiological increase of the Na level in body fluids sensed by Na(x) in glial cells is transmitted to neurons by lactate as a mediator to regulate neural activities of the SFO.

ASIC3 in muscle mediates mechanical, but not heat, hyperalgesia associated with muscle inflammation

Peripheral initiators of muscle pain are virtually unknown, but likely key to development of chronic pain after muscle insult. The current study tested the hypothesis that ASIC3 in muscle is necessary for development of cutaneous mechanical, but not heat, hyperalgesia induced by muscle inflammation. Using mechanical and heat stimuli, we assessed behavioral responses in ASIC3-/- and ASIC3+/+ mice after induction of carrageenan muscle inflammation. ASIC3-/- mice did not develop cutaneous mechanical hyperalgesia after muscle inflammation when compared to ASIC3+/+ mice; heat hyperalgesia developed similarly between groups. We then tested if the phenotype could be rescued in ASIC3-/- mice by using a recombinant herpes virus vector to express ASIC3 in skin (where testing occurred) or muscle (where inflammation occurred). Infection of mouse DRG neurons with ASIC3-encoding virus resulted in functional expression of ASICs. Injection of ASIC3-encoding virus into muscle or skin of ASIC3-/- mice resulted in ASIC3 mRNA in DRG and protein expression in DRG and the peripheral injection site. Injection of ASIC3-encoding virus into muscle, but not skin, resulted in development of mechanical hyperalgesia similar to that observed in ASIC3+/+ mice. Thus, ASIC3 in primary afferent fibers innervating muscle is critical to development of hyperalgesia that results from muscle insult.

Nerve injury induces robust allodynia and ectopic discharges in Nav1.3 null mutant mice

Changes in sodium channel activity and neuronal hyperexcitability contribute to neuropathic pain, a major clinical problem. There is strong evidence that the re-expression of the embryonic voltage-gated sodium channel subunit Na_v1.3 underlies neuronal hyperexcitability and neuropathic pain.

Here we show that acute and inflammatory pain behaviour is unchanged in global Na_v1.3 mutant mice. Surprisingly, neuropathic pain also developed normally in the Na_v1.3 mutant mouse. To rule out any genetic compensation mechanisms that may have masked the phenotype, we investigated neuropathic pain in two conditional Na_v1.3 mutant mouse lines. We used Na_v1.8-Cre mice to delete Nav1.3 in nociceptors at E14 and NFH-Cre mice to delete Na_v1.3 throughout the nervous system postnatally. Again normal levels of neuropathic pain developed after nerve injury in both lines. Furthermore, ectopic discharges from damaged nerves were unaffected by the absence of Na_v1.3 in global knock-out mice. Our data demonstrate that Na_v1.3 is neither necessary nor sufficient for the development of nerve-injury related pain.

Physiological maturation of photoreceptors depends on the voltage-gated sodium channel NaV1.6 (Scn8a)

Voltage-gated sodium channels (VGSCs) ensure the saltatory propagation of action potentials along axons by acting as signal amplifiers at the nodes of Ranvier. In the retina, activity mediated by VGSCs is important for the refinement of the retinotectal map. Here, we conducted a full-field electroretinogram (ERG) study on mice null for the sodium channel NaV1.6. Interestingly, the light-activated hyperpolarization of photoreceptor cells (the a-wave) and the major "downstream" components of the ERG, the b-wave and the oscillatory potentials, are markedly reduced and delayed in these mice. The functional deficit was not associated with any morphological abnormality. We demonstrate that Scn8a is expressed in the ganglion and inner nuclear layers and at low levels in the outer nuclear layer beginning shortly before the observed ERG deficit. Together, our data reveal a previously unappreciated role for VGSCs in the physiological maturation of photoreceptors.

The mechanosensitivity of mouse colon afferent fibers and their sensitization by inflammatory mediators require transient receptor potential vanilloid 1 and acid-sensing ion channel 3

Mechanical hypersensitivity of the colon underlies in part the chronic abdominal pain experienced by patients with irritable bowel syndrome, yet the molecules that confer mechanosensitivity to colon sensory neurons and their contribution to visceral pain are unknown. We tested the hypothesis that transient receptor potential vanilloid 1 (TRPV1) and acid-sensing ion channel 3 (ASIC3) are peripheral mechanosensors in colon afferent neuronal fibers that mediate visceral nociceptive behavior in mice. Visceral nociception, modeled by the visceromotor response to colorectal distension, and colon afferent fiber mechanosensitivity were assessed in control (C57BL/6) mice and two congenic knock-out mouse strains with deletions of either TRPV1 or ASIC3. Phasic colon distension (15-60 mmHg) produced graded behavioral responses in all three mouse strains. However, both TRPV1 and ASIC3 knock-out mice were significantly less sensitive to distension, with an average response magnitude only 58 and 50% of controls, respectively. The behavioral deficits observed in both strains of knock-out mice were associated with a significant and selective reduction in afferent fiber sensitivity to circumferential stretch of the colon, an effect that was mimicked in control preparations by pretreatment with capsazepine, a TRPV1 antagonist, but not amiloride, a nonselective ASIC antagonist (both 500 microM). In addition, whereas stretch-evoked afferent fiber responses were enhanced by chemical inflammatory mediators in control mice, this effect was differentially impaired in both knock-out mouse strains. These results demonstrate a peripheral mechanosensory role for TRPV1 and ASIC3 in the mouse colon that contributes to nociceptive behavior and possibly peripheral sensitization during tissue insult.

Upregulation of the voltage-gated sodium channel beta2 subunit in neuropathic pain models: characterization of expression in injured and non-injured primary sensory neurons

The development of abnormal primary sensory neuron excitability and neuropathic pain symptoms after peripheral nerve injury is associated with altered expression of voltage-gated sodium channels (VGSCs) and a modification of sodium currents. To investigate whether the beta2 subunit of VGSCs participates in the generation of neuropathic pain, we used the spared nerve injury (SNI) model in rats to examine beta2 subunit expression in selectively injured (tibial and common peroneal nerves) and uninjured (sural nerve) afferents. Three days after SNI, immunohistochemistry and Western blot analysis reveal an increase in the beta2 subunit in both the cell body and peripheral axons of injured neurons. The increase persists for >4 weeks, although beta2 subunit mRNA measured by real-time reverse transcription-PCR and in situ hybridization remains unchanged. Although injured neurons show the most marked upregulation,beta2 subunit expression is also increased in neighboring non-injured neurons and a similar pattern of changes appears in the spinal nerve ligation model of neuropathic pain. That increased beta2 subunit expression in sensory neurons after nerve injury is functionally significant, as demonstrated by our finding that the development of mechanical allodynia-like behavior in the SNI model is attenuated in beta2 subunit null mutant mice. Through its role in regulating the density of mature VGSC complexes in the plasma membrane and modulating channel gating, the beta2 subunit may play a key role in the development of ectopic activity in injured and non-injured sensory afferents and, thereby, neuropathic pain.

Robustness of burst firing in dissociated purkinje neurons with acute or long-term reductions in sodium conductance

Cerebellar Purkinje neurons often generate all-or-none burst firing in response to depolarizing stimuli. Voltage-clamp experiments using action potential waveforms show that burst firing depends on small net inward currents that flow after spikes and reflect the net balance between multiple large currents. Given this, burst firing is surprisingly robust in the face of changes in the magnitude of the underlying currents from cell to cell. We explored the basis of this robustness by examining the effects of reducing the sodium current, the major contributor to the postspike inward current. Burst firing persisted in concentrations of tetrodotoxin that produced half-block of sodium current. This robustness of bursting reflects an acute feedback mechanism whereby waveform changes from the reduced sodium current (reduced spike height and a hyperpolarizing shift in postspike voltage) cause compensatory decreases in postspike potassium currents. In particular, reduced spike height reduces calcium entry and subsequent calcium-activated potassium current, and the hyperpolarizing shift in postspike voltage speeds deactivation of Kv3-like potassium channels. Other experiments examined bursting in Na(v)1.6-/- mice, in which sodium current density is reduced in the long term. Under these circumstances, there was upregulation of both T-type and P-type calcium current and a change in the balance of calcium current and calcium-activated potassium current such that their net influence shifted from being inhibitory during bursts in wild-type neurons to excitatory during bursts from Na(v)1.6-/- mutant neurons. Thus, Purkinje neurons have both acute and long-term feedback mechanisms that serve to maintain burst firing when voltage-dependent sodium conductance is reduced.

Inactivation of sodium channel Scn8A (Nav1.6) in purkinje neurons impairs learning in Morris Water Maze and delay but not trace eyeblink classical conditioning

To examine the isolated effects of altered currents in cerebellar Purkinje neurons, the authors used Scn8a-super(flox/flox), Purkinje cell protein-CRE (Pcp-CRE) mice in which Exon 1 of Scn8a is deleted only in Purkinje neurons. Twenty male Purkinje Scn8a knockout (PKJ Scn8a KO) mice and 20 male littermates were tested on the Morris water maze (MWM). Subsequently, half were tested in 500-ms delay and half were tested in 500-ms trace eyeblink conditioning. PKJ Scn8a KO mice were impaired in delay conditioning and MWM but not in trace conditioning. These results provide additional support for the necessary participation of cerebellar cortex in normal acquisition of delay eyeblink conditioning and MWM and raise questions about the role, if any, of cerebellar cortex in trace eyeblink conditioning.

Lessons from Mouse Mutants of Epithelial Sodium Channel and Its Regulatory Proteins

The use of gene-modified mouse models allows the experimental in vivo analysis of specific gene defects at the level of target cells. With respect to the epithelial sodium channel and some of its regulatory proteins, gene-modified models that control gene defects in a time- and tissue-dependent conditional or constitutive manner have been generated. The combination of molecular and physiologic approaches in these mouse models increases the understanding of the complex regulation and the cell signaling cascades involved in Na(+) transport in target cells and may ultimately provide new insights into the pathophysiology of renal Na(+) retention and BP regulation. This review summarizes and discusses the gene-targeting approaches that have been applied to the epithelial sodium channel and its regulatory proteins.

Impaired impulse propagation in Scn5a-knockout mice: combined contribution of excitability, connexin expression, and tissue architecture in relation to aging

BACKGROUND

The SCN5A sodium channel is a major determinant for cardiac impulse propagation. We used epicardial mapping of the atria, ventricles, and septae to investigate conduction velocity (CV) in Scn5a heterozygous young and old mice.

METHODS AND RESULTS

Mice were divided into 4 groups: (1) young (3 to 4 months) wild-type littermates (WT); (2) young heterozygous Scn5a-knockout mice (HZ); (3) old (12 to 17 months) WT; and (4) old HZ. In young HZ hearts, CV in the right but not the left ventricle was reduced in agreement with a rightward rotation in the QRS axes; fibrosis was virtually absent in both ventricles, and the pattern of connexin43 (Cx43) expression was similar to that of WT mice. In old WT animals, the right ventricle transversal CV was slightly reduced and was associated with interstitial fibrosis. In old HZ hearts, right and left ventricle CVs were severely reduced both in the transversal and longitudinal direction; multiple areas of severe reactive fibrosis invaded the myocardium, accompanied by markedly altered Cx43 expression. The right and left bundle-branch CVs were comparable to those of WT animals. The atria showed only mild fibrosis, with heterogeneously disturbed Cx40 and Cx43 expression.

CONCLUSIONS

A 50% reduction in Scn5a expression alone or age-related interstitial fibrosis only slightly affects conduction. In aged HZ mice, reduced Scn5a expression is accompanied by the presence of reactive fibrosis and disarrangement of gap junctions, which results in profound conduction impairment.

Acid-sensing ion channel 2 contributes a major component to acid-evoked excitatory responses in spiral ganglion neurons and plays a role in noise susceptibility of mice

Ion channels in the degenerin-epithelial sodium channel (DEG-ENaC) family perform diverse functions, including mechanosensation. Here we explored the role of the vertebrate DEG-ENaC protein, acid-sensing ion channel 2 (ASIC2), in auditory transduction. Contributions of ASIC2 to hearing were examined by comparing hearing threshold and noise sensitivity of wild-type and ASIC2 null mice. ASIC2 null mice showed no significant hearing loss, indicating that the ASIC2 was not directly involved in the mechanotransduction of the mammalian cochlea. However, we found that (1) ASIC2 was present in the spiral ganglion (SG) neurons in the adult cochlea and that externally applied protons induced amiloride-sensitive sodium currents and action potentials in SG neurons in vitro, (2) proton-induced responses were greatly reduced in SG neurons obtained from ASIC2 null mice, indicating that activations of ASIC2 contributed a major portion of the proton-induced excitatory response in SG neurons, and (3) ASIC2 null mice were considerably more resistant to noise-induced temporary, but not permanent, threshold shifts. Together, these data suggest that ASIC2 contributes to suprathreshold functions of the cochlea. The presence of ASIC2 in SG neurons could provide sensors to directly convert local acidosis to excitatory responses, therefore offering a cellular mechanism linking hearing losses caused by many enigmatic causes (e.g., ischemia or inflammation of the inner ear) to excitotoxicity.

Sinus node dysfunction following targeted disruption of the murine cardiac sodium channel gene Scn5a

We have examined sino-atrial node (SAN) function in hearts from adult mice with heterozygous targeted disruption of the Scn5a gene to clarify the role of Scn5a-encoded cardiac Na+ channels in normal SAN function and the mechanism(s) by which reduced Na+ channel function might cause sinus node dysfunction. Scn5a+/- mice showed depressed heart rates and occasional sino-atrial (SA) block. Their isolated peripheral SAN pacemaker cells showed a reduced Na+ channel expression and slowed intrinsic pacemaker rates. Wild-type (WT) and Scn5a+/- SAN preparations exhibited similar activation patterns but with significantly slower SA conduction and frequent sino-atrial conduction block in Scn5a+/- SAN preparations. Furthermore, isolated WT and Scn5a+/- SAN cells demonstrated differing correlations between cycle length, maximum upstroke velocity and action potential amplitude, and cell size. Small myocytes showed similar, but large myocytes reduced pacemaker rates, implicating the larger peripheral SAN cells in the reduced pacemaker rate that was observed in Scn5a+/- myocytes. These findings were successfully reproduced in a model that implicated i(Na) directly in action potential propagation through the SAN and from SAN to atria, and in modifying heart rate through a coupling of SAN and atrial cells. Functional alterations in the SAN following heterozygous-targeted disruption of Scn5a thus closely resemble those observed in clinical sinus node dysfunction. The findings accordingly provide a basis for understanding of the role of cardiac-type Na+ channels in normal SAN function and the pathophysiology of sinus node dysfunction and suggest new potential targets for its clinical management.

New mutations of SCN4A cause a potassium-sensitive normokalemic periodic paralysis

BACKGROUND

Periodic paralysis is classified into hypokalemic (hypoPP) and hyperkalemic (hyperPP) periodic paralysis according to variations of blood potassium levels during attacks.

OBJECTIVE

To describe new mutations in the muscle sodium channel gene SCN4A that cause periodic paralysis.

METHODS

A thorough clinical, electrophysiologic, and molecular study was performed of four unrelated families who presented with periodic paralysis.

RESULTS

The nine affected members had episodes of muscle weakness reminiscent of both hyperPP and hypoPP. A provocative test with potassium chloride was positive in two patients. However, repeated and carefully performed tests of blood potassium levels during attacks resulted in normal potassium levels. Remarkably, two patients experienced hypokalemic episodes of paralysis related to peculiar provocative factors (corticosteroids and thyrotoxicosis). Similarly to hyperPP, electromyography in nine patients revealed increased compound muscle action potentials after short exercise and a delayed decline during rest after long exercise as well as myotonic discharges in one patient. With use of molecular genetic analysis of the gene SCN4A, three new mutations were found affecting codon 675. They resulted in an amino acid substitution of a highly conserved arginine (R) to either a glycine (G), a glutamine (Q), or a tryptophan (W). Interestingly, hypoPP is caused by both mutations affecting nearby codons as well as the change of an arginine into another amino acid.

CONCLUSION

A potassium-sensitive and normokalemic type of periodic paralysis caused by new SCN4A mutations at codon 675 is reported.

Value of electrocardiographic parameters and ajmaline test in the diagnosis of Brugada syndrome caused by SCN5A mutations

BACKGROUND

The Brugada syndrome is an arrhythmogenic disease caused in part by mutations in the cardiac sodium channel gene, SCN5A. The electrocardiographic pattern characteristic of the syndrome is dynamic and is often absent in affected individuals. Sodium channel blockers are effective in unmasking carriers of the disease. However, the value of the test remains controversial.

METHODS AND RESULTS

We studied 147 individuals representing 4 large families with SCN5A mutations. Of these, 104 were determined to be at possible risk for Brugada syndrome and underwent both electrocardiographic and genetic evaluation. Twenty-four individuals displayed an ECG diagnostic of Brugada syndrome at baseline. Of the remaining, 71 received intravenous ajmaline. Of the 35 genetic carriers who received ajmaline, 28 had a positive test and 7 a negative ajmaline test. The sensitivity, specificity, and positive and negative predictive values of the drug challenge were 80% (28:35), 94.4% (34:36), 93.3% (28:30), and 82.9% (34:41), respectively. Penetrance of the disease phenotype increased from 32.7% to 78.6% with the use of sodium channel blockers. In the absence of ST-segment elevation under baseline conditions, a prolonged P-R interval, but not incomplete right bundle-branch block or early repolarization patterns, indicates a high probability of an SCN5A mutation carrier.

CONCLUSIONS

In families with Brugada syndrome, the data suggest that ajmaline testing is valuable in the diagnosis of SCN5A carriers. In the absence of ST-segment elevation at baseline, family members with first-degree atrioventricular block should be suspected of carrying the mutation. An ajmaline test is often the key to making the proper diagnosis in these patients.

Defective respiratory amiloride-sensitive sodium transport predisposes to pulmonary oedema and delays its resolution in mice

Pulmonary oedema results from an imbalance between the forces driving fluid into the airspace and the biological mechanisms for its removal. In mice lacking the alpha-subunit of the amiloride-sensitive sodium channel (alphaENaC(-/-)), impaired sodium transport-mediated lung liquid clearance at birth results in neonatal death. Transgenic expression of alphaENaC driven by a cytomegalovirus (CMV) promoter (alphaENaC(-/-)Tg+) rescues the lethal pulmonary phenotype, but only partially restores respiratory sodium transport in vitro. To test whether this may also be true in vivo, and to assess the functional consequences of this defect on experimental pulmonary oedema, we measured respiratory transepithelial potential difference (PD) and alveolar fluid clearance (AFC), and quantified pulmonary oedema during experimental acute lung injury in these mice. Both respiratory PD and AFC were roughly 50% lower (P < 0.01) in alphaENaC(-/-)Tg+ than in control mice. This impairment was associated with a significantly larger increase of the wet/dry lung weight ratio in alphaENaC(-/-)Tg+ than in control mice, both after exposure to hyperoxia and thiourea. Moreover, the rate of resolution of thiourea-induced pulmonary oedema was more than three times slower (P < 0.001) in alphaENaC(-/-)Tg+ mice. alphaENaC(-/-)Tg+ mice represent the first model of a constitutively impaired respiratory transepithelial sodium transport, and provide direct evidence that this impairment facilitates pulmonary oedema in conscious freely moving animals. These data in mice strengthen indirect evidence provided by clinical studies, suggesting that defective respiratory transepithelial sodium transport may also facilitate pulmonary oedema in humans.

Sodium Currents in Subthalamic Nucleus Neurons From Nav1.6-Null Mice

In some central neurons, including cerebellar Purkinje neurons and subthalamic nucleus (STN) neurons, TTX-sensitive sodium channels show unusual gating behavior whereby some channels open transiently during recovery from inactivation. This “resurgent” sodium current is effectively activated immediately after action potential-like waveforms. Earlier work using Purkinje neurons suggested that the great majority of resurgent current originates from Nav1.6 sodium channels. Here we used a mouse mutant lacking Nav1.6 to explore the contribution of these channels to resurgent, transient, and persistent components of TTX-sensitive sodium current in STN neurons. The resurgent current of STN neurons from Nav1.6−/− mice was reduced by 63% relative to wild-type littermates, a less dramatic reduction than that observed in Purkinje neurons recorded under identical conditions. The transient and persistent currents of Nav1.6−/− STN neurons were reduced by ∼40 and 55%, respectively. The resurgent current present in Nav1.6−/− null STN neurons was similar in voltage dependence to that in wild-type STN and Purkinje neurons, differing only in having somewhat slower decay kinetics. These results show that sodium channels other than Nav1.6 can make resurgent sodium current much like that from Nav1.6 channels.

Recurrent Third-Trimester Fetal Loss and Maternal Mosaicism for Long-QT Syndrome

Background— The importance of germ-line mosaicism in genetic disease is probably underestimated, even though recent studies indicate that it may be involved in 10% to 20% of apparently de novo cases of several dominantly inherited genetic diseases.

Methods and Results— We describe here a case of repeated germ-line transmission of a severe form of long-QT syndrome (LQTS) from an asymptomatic mother with mosaicism for a mutation in the cardiac sodium channel, SCN5A . A male infant was diagnosed with ventricular arrhythmias and cardiac decompensation in utero at 28 weeks and with LQTS after birth, ultimately requiring cardiac transplantation for control of ventricular tachycardia. The mother had no ECG abnormalities, but her only previous pregnancy had ended in stillbirth with evidence of cardiac decompensation at 7 months’ gestation. A third pregnancy also ended in stillbirth at 7 months, again with nonimmune fetal hydrops. The surviving infant was found to have a heterozygous mutation in SCN5A ( R1623Q ), previously reported as a de novo mutation causing neonatal ventricular arrhythmia and LQTS. Initial studies of the mother detected no genetic abnormality, but a sensitive restriction enzyme-based assay identified a small (8% to 10%) percentage of cells harboring the mutation in her blood, skin, and buccal mucosa. Cord blood from the third fetus also harbored the mutant allele, suggesting that all 3 cases of late-term fetal distress resulted from germ-line transfer of the LQTS-associated mutation.

Conclusions— Recurrent late-term fetal loss or sudden infant death can result from unsuspected parental mosaicism for LQTS-associated mutations, with important implications for genetic counseling.

[Congenital myasthenic syndromes: phenotypic expression and pathophysiological characterisation]

Congenital Myasthenic Syndromes (CMS) are a heterogeneous group of diseases caused by genetic defects affecting neuromuscular transmission. The twenty five past Years saw major advances in identifying different types of CMS due to abnormal presynaptic, synaptic, and postsynaptic proteins. CMS diagnosis requires two steps: 1) positive diagnosis supported by myasthenic signs beginning in neonatal period, efficacy of anticholinesterase medications, positive family history, negative tests for anti-acetylcholine receptor (AChR) antibodies, electromyographic studies (decremental response at low frequency, repetitive CMAP after one single stimulation); 2) pathophysiological characterisation of CMS implying specific studies: light and electron microscopic analysis of endplate (EP) morphology, estimation of the number of AChR per EP, acetylcholinesterase (AChE) expression, molecular genetic analysis. Most CMS are postsynaptic due to mutations in the AChR subunits genes that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh resulting respectively in Slow Channel Syndrome (characterized by a autosomal dominant transmission, repetitive CMAP, refractoriness to anticholinesterase medication) and fast channel, recessively transmitted. AChR deficiency without kinetic abnormalities is caused by recessive mutations in AChR genes (mostly epsilon subunit) or by primary rapsyn deficiency, a post synaptic protein involved in AChR concentration. Recently, mutations in SCN4A sodium channel have been reported in one patient. AChE deficiency is identified on the following data: recessive transmission, presence of repetitive CMAP, refractoriness to cholinesterase inhibitors, slow pupillary response to light and absent expression of the enzyme at EP. This synaptic CMS is caused by mutations in the collagenic tail subunit (ColQ) that anchors the catalytic subunits in the synaptic basal lamina. The most frequent presynaptic CMS is caused by mutations of choline acetyltransferase. Several CMS are still not characterized. Many EP molecules are potential etiological candidates. In these unidentified cases, other methods of investigations are required: linkage analysis, when sufficient number of informative relatives are available, microelectrophysiological studies performed in intercostal or anconeus muscles. Prognosis of CMS, depending on severity and evolution of symptoms, is difficult to assess, and it cannot not be simply derived from mutation identification. Most patients respond favourably to anticholinesterase medications or to 3,4 DAP which is effective not only in presynaptic but also in postsynaptic CMS. Specific therapies for slow channel CMS are quinidine and fluoxetine that normalize the prolonged opening episodes. Clinical benefits derived from the full characterisation of each case include genetic counselling and specific therapy.

[Molecular and genetic aspects of the myotonic conditions]

AIM

The aim is to review the molecular and genetic aspects of the dystrophic and no dystrophic myotonias.

BACKGROUND

Myotonic diseases are hereditary conditions of the skeletal muscle, classified in two groups depending on the symptoms. In the first group are the myotonic dystrophies, with the myotonic dystrophies type 1 and 2. In the second group are the channelopathies, characterized for the affected function of the ion channels. Myotonic dystrophy type 1, a neurodegenerative, progressive and disabling disease is caused by an expansion of the CTG trinucleotide, its size shows a positive correlation with the severity and negative with age of onset. There are enough insights to think that the gain of function of the mutant ARN is the pathophysiological mechanism occurring on this disease. Myotonic dystrophy type 2, less severe than type 1, is caused by an expansion of the CCTG tetranucleotide, its pathophysiological mechanism is similar to that one proposed for the type 1. In the second group we can find the chloride channelopathies, with autosomal dominant or recessive inheritance, caused by one of the 60 different mutations on the chloride channel gene; and the sodium channelopathies, group of three clinically overlapping diseases, with dominant heredity caused by one of the 25 different mutations on the sodium channel gene.

CONCLUSIONS

These diseases are highly clinically variable, and even though their genetic base is known, it is necessary too much research in order to understand their pathophisiology and the phenotype genotype relationships.

Delay in right ventricular activation contributes to Brugada syndrome

BACKGROUND

Although Brugada syndrome revolves around reduced net depolarizing force, the electrophysiological mechanisms of its defining features (right precordial ST-segment elevation and ventricular tachyarrhythmias) remain unresolved. Two proposed mechanisms are (1) right ventricular (RV) conduction delay and (2) selective and significant RV subepicardial action potential shortening. Both mechanisms must cause disparate contractile changes: delay in RV contraction and reduction of contractile force, respectively. We aimed to establish the electrophysiological mechanism of Brugada syndrome by studying the timing and force of RV contraction.

METHODS AND RESULTS

Using tissue Doppler echocardiography, we studied how these contractile variables change on induction of the characteristic ST-segment changes of Brugada syndrome by flecainide challenge. Accordingly, we studied patients in whom flecainide induced these changes (inducible) and those in whom these changes were not induced (control). We found that (1) the occurrence of a positive response (coved-type ST elevation) after flecainide coincides with delay in the onset of contraction between the RV and left ventricle (LV); (2) the extent of contraction delay between RV and LV correlates with the magnitude of ST elevation; and (3) RV ejection time (duration of RV ejection phase) shortens as the Brugada ECG pattern emerges.

CONCLUSIONS

These results indicate that both proposed mechanisms of Brugada syndrome may be operative.

Chronic hyperalgesia induced by repeated acid injections in muscle is abolished by the loss of ASIC3, but not ASIC1

Clinically, chronic pain and hyperalgesia induced by muscle injury are disabling and difficult to treat. Cellular and molecular mechanisms underlying chronic muscle-induced hyperalgesia are not well understood. For this reason, we developed an animal model where repeated injections of acidic saline into one gastrocnemius muscle produce bilateral, long-lasting mechanical hypersensitivity of the paw (i.e. hyperalgesia) without associated tissue damage. Since acid sensing ion channels (ASICs) are found on primary afferent fibers and respond to decreases in pH, we tested the hypothesis that ASICs on primary afferent fibers innervating muscle are critical to development of hyperalgesia and central sensitization in response to repeated intramuscular acid. Dorsal root ganglion neurons innervating muscle express ASIC3 and respond to acidic pH with fast, transient inward and sustained currents that resemble those of ASICs. Mechanical hyperalgesia produced by repeated intramuscular acid injections is prevented by prior treatment of the muscle with the non-selective ASIC antagonist, amiloride, suggesting ASICs might be involved. ASIC3 knockouts do not develop mechanical hyperalgesia to repeated intramuscular acid injection when compared to wildtype littermates. In contrast, ASIC1 knockouts develop hyperalgesia similar to their wildtype littermates. Extracellular recordings of spinal wide dynamic range (WDR) neurons from wildtype mice show an expansion of the receptive field to include the contralateral paw, an increased response to von Frey filaments applied to the paw both ipsilaterally and contralaterally, and increased response to noxious pinch contralaterally after the second intramuscular acid injection. These changes in WDR neurons do not occur in ASIC3 knockouts. Thus, activation of ASIC3s on muscle afferents is required for development of mechanical hyperalgesia and central sensitization that normally occurs in response to repeated intramuscular acid. Therefore, interfering with ASIC3 might be of benefit in treatment or prevention of chronic hyperalgesia.

Motor disturbances in mice with deficiency of the sodium channel gene Scn8a show features of human dystonia

The med(J) mouse with twisting movements related to deficiency of the sodium channel Scn8a has been proposed as a model of kinesiogenic dystonia. This prompted us to examine the phenotype of these mice in more detail. By cortical electroencephalographic (EEG) recordings, we could not detect any changes, demonstrating that the motor disturbances are not epileptic in nature, an important similarity to human dystonia. The significantly decreased body weight of med(J) mice was related to reduced food intake. Observations in the open field and by video recordings revealed that the mice exhibit sustained abnormal postures and movements of limbs, trunk and tail not only during locomotor activity but also at rest. With the exception of the head tremor, the other motor impairments were persistent rather than paroxysmal. When several neurological reflexes were tested, alterations were restricted to the posture and righting reflexes. Results of the wire hang test confirmed the greatly reduced muscle strength in the med(J) mouse. In agreement with different types of human dystonia, biperiden, haloperidol and diazepam moderately reduced the severity of motor disturbances in med(J) mice. In view of the sodium channel deficiency in med(J) mice, the beneficial effects of the sodium channel blocker phenytoin was an unexpected finding. By immunohistochemical examinations, the density of nigral dopaminergic neurons was found to be unaltered, substantiating the absence of pathomorphological abnormalities within the brain of med(J) mice shown by previous studies. With the exception of muscle weakness, many of the features of the med(J) mouse are similar to human idiopathic dystonia.

Congenital Myasthenic Syndromes: Multiple Molecular Targets at the Neuromuscular Junction

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or ACh resynthesis. Defects in ACh resynthesis have now been traced to mutations in choline acetyltransferase. A synaptic CMS is caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent the tail subunit from associating with catalytic subunits or from becoming inserted into the synaptic basal lamina. Most postsynaptic CMS are caused by mutations in subunits of the acetylcholine receptor (AChR) that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most low-expressor mutations reside in the AChR epsilon subunit and are partially compensated by residual expression of the fetal-type gamma subunit. In a subset of CMS patients, endplate AChR deficiency is caused by mutations in rapsyn, a molecule that plays a critical role in concentrating AChR in the postsynaptic membrane.

Acid-sensing ion channel 1 is localized in brain regions with high synaptic density and contributes to fear conditioning

The acid-sensing ion channel, ASIC1, contributes to synaptic plasticity in the hippocampus and to hippocampus-dependent spatial memory. To explore the role of ASIC1 in brain, we examined the distribution of ASIC1 protein. Surprisingly, although ASIC1 was present in the hippocampal circuit, it was much more abundant in several areas outside the hippocampus. ASIC1 was enriched in areas with strong excitatory synaptic input such as the glomerulus of the olfactory bulb, whisker barrel cortex, cingulate cortex, striatum, nucleus accumbens, amygdala, and cerebellar cortex. Because ASIC1 levels were particularly high in the amygdala, we focused further on this area. We found that extracellular acidosis elicited a greater current density in amygdala neurons than hippocampal neurons and that disrupting the ASIC1 gene eliminated H+-evoked currents in the amygdala. We also tested the effect of ASIC1 on amygdala-dependent behavior; ASIC1-null mice displayed deficits in cue and context fear conditioning, yet baseline fear on the elevated plus maze was intact. These studies suggest that ASIC1 is distributed to regions supporting high levels of synaptic plasticity and contributes to the neural mechanisms of fear conditioning.

The contribution of resurgent sodium current to high-frequency firing in Purkinje neurons: an experimental and modeling study

Purkinje neurons generate high-frequency action potentials and express voltage-gated, tetrodotoxin-sensitive sodium channels with distinctive kinetics. Their sodium currents activate and inactivate during depolarization, as well as reactivate during repolarization from positive potentials, producing a "resurgent" current. This reopening of channels not only generates inward current after each action potential, but also permits rapid recovery from inactivation, leading to the hypothesis that resurgent current may facilitate high-frequency firing. Mutant med mice are ataxic and lack expression of the Scn8a gene, which encodes the NaV1.6 protein. In med Purkinje cells, transient sodium current inactivates more rapidly than in wild-type cells, and resurgent current is nearly abolished. To investigate how NaV1.6-specific kinetics influence firing patterns, we recorded action potentials of Purkinje neurons isolated from wild-type and med mice. We also recorded non-sodium currents from Purkinje cells of both genotypes to test whether the Scn8a mutation induced changes in other ion channels. Last, we modeled action potential firing by simulating eight currents directly recorded from Purkinje cells in both wild-type and med mice. Regular, high-frequency firing was slowed in med Purkinje neurons. In addition to disrupted sodium currents, med neurons had small but significant changes in potassium and leak currents. Simulations indicated that these modified non-sodium currents could not account for the reduced excitability of med cells but instead slightly facilitated spiking. The loss of NaV1.6-specific kinetics, however, slowed simulated spontaneous activity. Together, the data suggest that across a range of conditions, sodium currents with a resurgent component promote and accelerate firing.