Inherited erythromelalgia/erythermalgia (IEM) is a neuropathy characterized by pain and redness of the extremities that is triggered by warmth. IEM has been associated with missense mutations of the voltage-gated sodium channel Na v 1.7, which is preferentially expressed in most nociceptive dorsal root ganglia (DRGs) and sympathetic ganglion neurons. Several mutations occur in cytoplasmic linkers of Na v 1.7, with only two mutations in segment 4 (S4) and S6 of domain I. We report here a simplex case with an alanine 863 substitution by proline (A863P) in S5 of domain II of Na v 1.7. The functional effect of A863P was investigated by voltage-clamp analysis in human embryonic kidney 293 cells and by current-clamp analysis to determine the effects of A863P on firing properties of small DRG neurons. Activation of mutant channels was shifted by Ϫ8 mV, whereas steady-state fast inactivation was shifted by ϩ10 mV, compared with wild-type (WT) channels. There was a marked decrease in the rate of deactivation of mutant channels, and currents elicited by slow ramp depolarizations were 12 times larger than for WT. These results suggested that A863P could render DRG neurons hyperexcitable. We tested this hypothesis by studying properties of rat DRG neurons transfected with either A863P or WT channels. A863P depolarized resting potential of DRG neurons by ϩ6 mV compared with WT channels, reduced the threshold for triggering single action potentials to 63% of that for WT channels, and increased firing frequency of neurons when stimulated with suprathreshold stimuli. Thus, A863P mutant channels produce hyperexcitability in DRG neurons, which contributes to the pathophysiology of IEM.

Sodium channel Na(V)1.7, encoded by the SCN9A gene, is preferentially expressed in nociceptive primary sensory neurons, where it amplifies small depolarizations. In studies on a family with inherited erythromelalgia associated with Na(V)1.7 gain-of-function mutation A863P, we identified a nonsynonymous single-nucleotide polymorphism within SCN9A in the affected proband and several unaffected family members; this polymorphism (c. 3448C&T, Single Nucleotide Polymorphisms database rs6746030, which produces the amino acid substitution R1150W in human Na(V)1.7 [hNa(V)1.7]) is present in 1.1 to 12.7% of control chromosomes, depending on ethnicity. In this study, we examined the effect of the R1150W substitution on function of the hNa(V)1.7 channel, and on the firing of dorsal root ganglion (DRG) neurons in which this channel is normally expressed. We show that this polymorphism depolarizes activation (7.9-11mV in different assays). Current-clamp analysis shows that the 1150W allele depolarizes (6mV) resting membrane potential and increases ( approximately 2-fold) the firing frequency in response to depolarization in DRG neurons in which it is present. Our results suggest that polymorphisms in the Na(V)1.7 channel may influence susceptibility to pain.

MuO-conotoxin MrVIB is a blocker of voltage-gated sodium channels, including TTX-sensitive and -resistant subtypes. A comprehensive characterization of this peptide has been hampered by the lack of sufficient synthetic material. Here, we describe the successful chemical synthesis and oxidative folding of MrVIB that has made an investigation of the pharmacological properties and therapeutic potential of the peptide feasible. We show for the first time that synthetic MrVIB blocks rat NaV1.8 sodium channels and has potent and long-lasting local anesthetic effects when tested in two pain assays in rats. Furthermore, MrVIB can block propagation of action potentials in A- and C-fibers in sciatic nerve as well as skeletal muscle in isolated preparations from rat. Our work provides the first example of analgesia produced by a conotoxin that blocks sodium channels. The emerging diversity of antinociceptive mechanisms targeted by different classes of conotoxins is discussed.

The anticonvulsant felbamate blocks N-methyl-D-asparate (NMDA) receptors but fails to exhibit the neurobehavioral toxicity characteristic of other NMDA receptor antagonists. To investigate the possibility that felbamate's favorable toxicity profile could be related to NMDA receptor subtype selectivity, we examined the specificity of felbamate block of recombinant NMDA receptors composed of the NR1a subunit and various NR2 subunits. Felbamate produced a rapid, concentration-dependent block of currents evoked by 50 microM NMDA and 10 microM glycine in human embryonic kidney 293 cells expressing the rat NR1a subunit, and either the NR2A, NR2B or NR2C subunits; the IC50 values for block were 2.6, 0.52 and 2.4 mM, respectively (holding potential, - 60 mV). The Hill coefficient values were < 1 and, in kinetic analyses, onset and recovery from block were well fit by double exponential functions, indicating binding to more than one blocking site on the NMDA receptor channel complex. The higher affinity of felbamate block of NMDA receptors containing the NR2B subunit could be accounted for by more rapid association and slower dissociation from these sites. We conclude that felbamate exhibits modest selectivity for NMDA receptors composed of NR1a/NR2B subunits. This selectivity could, in part, account for the more favorable clinical profile of felbamate in comparison with NMDA receptor antagonists that do not show subunit selectivity.