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Concept: Paroxysmal extreme pain disorder

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Mutations in SCN9A have been reported in (1) congenital insensitivity to pain (CIP); (2) primary erythromelalgia; (3) paroxysmal extreme pain disorder; (4) febrile seizures and recently (5) small fibre sensory neuropathy. We sought to investigate for SCN9A mutations in a clinically well-characterised cohort of patients with CIP and erythromelalgia.

Concepts: Pain, Peripheral neuropathy, Paroxysmal extreme pain disorder

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Mutations in the SCN9A gene cause chronic pain and pain insensitivity syndromes. We aimed to study clinical, genetic, and electrophysiological features of paroxysmal extreme pain disorder (PEPD) caused by a novel SCN9A mutation.

Concepts: DNA, Genetics, Evolution, Action potential, Electrophysiology, Ion channels, Paroxysmal extreme pain disorder, Erythromelalgia

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The importance of NaV1.7 (encoded by SCN9A) in the regulation of pain sensing is exemplified by the heterogeneity of clinical phenotypes associated with its mutation. Gain-of-function mutations are typically pain-causing and have been associated with inherited erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD). IEM is usually caused by enhanced NaV1.7 channel activation, whereas mutations that alter steady-state fast inactivation often lead to PEPD. In contrast, nonfunctional mutations in SCN9A are known to underlie congenital insensitivity to pain (CIP). Although well documented, the correlation between SCN9A genotypes and clinical phenotypes is still unclear. Here we report three families with novel SCN9A mutations. In a multiaffected dominant family with IEM, we found the heterozygous change L245 V. Electrophysiological characterization showed that this mutation did not affect channel activation but instead resulted in incomplete fast inactivation and a small hyperpolarizing shift in steady-state slow inactivation, characteristics more commonly associated with PEPD. In two compound heterozygous CIP patients, we found mutations that still retained functionality of the channels, with two C-terminal mutations (W1775R and L1831X) exhibiting a depolarizing shift in channel activation. Two mutations (A1236E and L1831X) resulted in a hyperpolarizing shift in steady-state fast inactivation. To our knowledge, these are the first descriptions of mutations with some retained channel function causing CIP. This study emphasizes the complex genotype-phenotype correlations that exist for SCN9A and highlights the C-terminal cytoplasmic region of NaV1.7 as a critical region for channel function, potentially facilitating analgesic drug development studies.

Concepts: Protein, Genetics, Mutation, Natural selection, Evolution, Compound heterozygosity, Paroxysmal extreme pain disorder, Nav1.7

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Flushing disorders with involvement of the gastrointestinal tract represent a heterogeneous group of conditions. In part 1 discussed were neuroendocrine tumors (NET), mast cell activation disorders (MCAD), and hyperbasophilia. In this section are covered the remaining flushing disorders which primarily or secondarily involves the gastrointestinal tract. This includes dumping syndrome, mesenteric traction syndrome, rosacea, hyperthyroidism and thyroid storm, anaphylaxis, panic disorders, paroxysmal extreme pain disorder, and food, alcohol and medications. With the exception of paroxysmal pain disorders, panic disorders and some medication these disorders presents with dry flushing. A detailed and comprehensive family, social, medical and surgical history and recognizing the presence of other systemic symptoms are important in distinguishing the different disease that cause flushing with gastrointestinal symptoms.

Concepts: Immune system, Medicine, Digestive system, Mast cell, Gastroenterology, Abdomen, Anaphylaxis, Paroxysmal extreme pain disorder

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Once patients have failed first line therapy, there is an apparent lack of knowledge on how to proceed with choosing subsequent therapy. To choose amongst alternative agents, an understanding of pharmacology, pharmacokinetics, and available evidence in targeting various pain conditions is necessary. This article focuses on the use of the carboxamide class of voltage-gated sodium channel blockers (carbamazepine, oxcarbazepine, eslicarbazepine acetate) for adjunct pain medication management; including research updates in pharmacology, pharmacokinetics, and evidence for pain along on this therapeutic group with promising future areas of research. Although evidence for voltage-gated sodium channel blockers in chronic pain management is limited, emerging research has identified this area as promising for additional clinical trials to better guide clinical practice.

Concepts: Pharmacology, Medicine, Clinical trial, The Canon of Medicine, Pain, Drug development, Chronic pain, Paroxysmal extreme pain disorder

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The Nav1.7 channel represents a promising target for pain relief. In the recent decades, a number of Nav1.7 channel inhibitors have been developed. According to the effects on channel kinetics, these inhibitors could be divided into two major classes: reducing activation or enhancing inactivation. To date, however, only several inhibitors have moved forward into phase 2 clinical trials and most of them display a less than ideal analgesic efficacy, thus intensifying the controversy regarding if an ideal candidate should preferentially affect the activation or inactivation state. In the present study, we investigated the action mechanisms of a recently clinically confirmed inhibitor CNV1014802 using both electrophysiology and site-directed mutagenesis. We found that CNV1014802 inhibited Nav1.7 channels through stabilizing a nonconductive inactivated state. When the cells expressing Nav1.7 channels were hold at 70 mV or 120 mV, the half maximal inhibitory concentration (IC50) values (with 95% confidence limits) were 1.77 (1.20-2.33) and 71.66 (46.85-96.48) μmol/L, respectively. This drug caused dramatic hyperpolarizing shift of channel inactivation but did not affect activation. Moreover, CNV1014802 accelerated the onset of inactivation and delayed the recovery from inactivation. Notably, application of CNV1014802 (30 μmol/L) could rescue the Nav1.7 mutations expressed in CHO cells that cause paroxysmal extreme pain disorder (PEPD), thereby restoring the impaired inactivation to those of the wild-type channel. Our study demonstrates that CNV1014802 enhances the inactivation but does not reduce the activation of Nav1.7 channels, suggesting that identifying inhibitors that preferentially affect inactivation is a promising approach for developing drugs targeting Nav1.7.

Concepts: Mutation, Effectiveness, Action potential, Pain, Site-directed mutagenesis, Mutagenesis, Channel, Paroxysmal extreme pain disorder

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Gain-of-function mutations in SCN9A gene that encodes the voltage-gated sodium channel NaV1.7 have been associated with a wide spectrum of painful syndromes in humans including inherited erythromelalgia, paroxysmal extreme pain disorder and small fibre neuropathy. These mutations change the biophysical properties of NaV1.7 channels leading to hyperexcitability of dorsal root ganglion nociceptors and pain symptoms. There is a need for better understanding of how gain-of-function mutations alter the atomic structure of Nav1.7.

Concepts: Nervous system, Neuron, Action potential, Pain, Nociception, Sodium channel, Paroxysmal extreme pain disorder, Erythromelalgia

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Lacosamide is an antiseizure agent targeting voltage-dependent sodium channels. Previous experiments have suggested that lacosamide is unusual in binding selectively to the slow-inactivated state of sodium channels, in contrast to drugs like carbamazepine and phenytoin that bind tightly to fast-inactivated states. We examined the state-dependent effects of lacosamide, using heterologously-expressed human Nav1.7 sodium channels. Lacosamide induced a reversible shift in the voltage-dependence of fast inactivation studied with 100 ms prepulses, suggesting binding to fast-inactivated states. Using steady holding potentials, lacosamide block was very weak at -120 mV (3% inhibition by 100 μM lacosamide) but greatly enhanced at -80 mV (43% inhibition by 100 μM lacosamide), where there is partial fast inactivation but little or no slow inactivation. During long depolarizations, lacosamide slowly (over seconds) put channels into states that recovered availability slowly (100’s of ms) at -120 mV. This resembles enhancement of slow inactivation, but the effect was much more pronounced at -40 mV, where fast inactivation is complete but slow inactivation is not, than at 0 mV, where slow inactivation is maximal, more consistent with slow binding to fast inactivated states than selective binding to slow inactivated states. Furthermore, inhibition by lacosamide was greatly reduced by pre-treatment with 300 μM lidocaine or 300 μM carbamazepine, suggesting that lacosamide, lidocaine, and carbamazepine all bind to the same site. The results suggest that lacosamide binds to fast-inactivated states in a manner similar to other antiseizure agents but with slower kinetics of binding and unbinding.

Concepts: Action potential, Voltage-gated ion channel, Sodium, Sodium channel, Depolarization, Binding, Paroxysmal extreme pain disorder, Erythromelalgia

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Voltage-gated sodium channels (Navs) are crucial for the generation and propagation of action potentials in all excitable cells, and therefore for the function of sensory neurons as well. Preclinical research over the past 20 years identified three Nav-isoforms in sensory neurons, namely Nav1.7, Nav1.8 and Nav1.9. A specific role for the function of nociceptive neurons was postulated for each. Whereas no selective sodium channel inhibitors have been established in the clinic so far, the relevance of all three isoforms regarding the pain sensitivity in humans is currently undergoing a remarkable verification through the translation of preclinical data into clinically manifest pictures. For the last ten years, Nav1.7 has been the main focus of clinical interest, as a large number of hereditary mutants were identified. The so-called “gain-of-function” mutations of Nav1.7 cause the pain syndromes hereditary erythromelalgia and paroxysmal extreme pain disorder. In addition, several Nav1.7 mutants were shown to be associated with small-fiber neuropathies. On the contrary, “loss-of-function” Nav1.7 mutants lead to a congenital insensitivity to pain. Recently, several gain-of-function mutations in Nav1.8 and Nav1.9 have been identified in patients suffering from painful peripheral neuropathies. However, another gain-of-function Nav1.9 mutation is associated with congenital insensitivity to pain. This review offers an overview of published work on painful Nav mutations with clinical relevance, and proposes possible consequences for the therapy of different pain symptoms resulting from these findings.

Concepts: Mutation, Natural selection, Action potential, Pain, Nociceptor, Sodium channel, Peripheral neuropathy, Paroxysmal extreme pain disorder

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Mutations in the voltage-gated sodium channel Nav1.7 are linked to inherited pain syndromes such as erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD). PEPD mutations impair Nav1.7 fast inactivation and increase persistent currents. PEPD mutations also increase resurgent currents, which involve the voltage-dependent release of an open channel blocker. In contrast, IEM mutations, whenever tested, leave resurgent currents unchanged. Accordingly, the IEM deletion mutation L955 (ΔL955) fails to produce resurgent currents despite enhanced persistent currents, which have hitherto been considered a prerequisite for resurgent currents. Additionally, ΔL955 exhibits a prominent enhancement of slow inactivation (SI). We introduced mutations into Nav1.7 and Nav1.6 that either enhance or impair SI in order to investigate their effects on resurgent currents. Our results show that enhanced SI is accompanied by impaired resurgent currents, which suggests that SI may interfere with open-channel block.

Concepts: DNA, Mutation, Voltage-gated ion channel, Sodium channel, Ion channels, Genetic disorders, Paroxysmal extreme pain disorder, Erythromelalgia