Paroxysmal extreme pain disorder (PEPD), previously known as familial rectal pain (FRP, or OMIM 167400), is an inherited condition characterized by paroxysms of rectal, ocular, or submandibular pain with flushing. A genome-wide linkage search followed by mutational analysis of the candidate gene SCN9A, which encodes hNa(v)1.7, identified eight missense mutations in 11 families and 2 sporadic cases. Functional analysis in vitro of three of these mutant Na(v)1.7 channels revealed a reduction in fast inactivation, leading to persistent sodium current. Other mutations in SCN9A associated with more negative activation thresholds are known to cause primary erythermalgia (PE). Carbamazepine, a drug that is effective in PEPD, but not PE, showed selective block of persistent current associated with PEPD mutants, but did not affect the negative activation threshold of a PE mutant. PEPD and PE are allelic variants with distinct underlying biophysical mechanisms and represent a separate class of peripheral neuronal sodium channelopathy.

Nine voltage-gated sodium channels are expressed in complex patterns in mammalian nerve and muscle. Three channels, Nav1.7, Nav1.8, and Na v1.9, are expressed selectively in peripheral damage-sensing neurons. Because there are no selective blockers of these channels, we used gene ablation in mice to examine the function of Nav1.7 (PN1) in pain pathways. A global Nav1.7-null mutant was found to die shortly after birth. We therefore used the Cre؊loxP system to generate nociceptor-specific knockouts. Na v1.8 is only expressed in peripheral, mainly nociceptive, sensory neurons. We knocked Cre recombinase into the Na v1.8 locus to generate heterozygous mice expressing Cre recombinase in Nav1.8-positive sensory neurons. Crossing these animals with mice where Na v1.7 exons 14 and 15 were flanked by loxP sites produced nociceptor-specific knockout mice that were viable and apparently normal. These animals showed increased mechanical and thermal pain thresholds. Remarkably, all inflammatory pain responses evoked by a range of stimuli, such as formalin, carrageenan, complete Freund's adjuvant, or nerve growth factor, were reduced or abolished. A congenital pain syndrome in humans recently has been mapped to the Na v1.7 gene, SCN9A. Dominant Na v1.7 mutations lead to edema, redness, warmth, and bilateral pain in human erythermalgia patients, confirming an important role for Na v1.7 in inflammatory pain. Nociceptor-specific gene ablation should prove useful in understanding the role of other broadly expressed genes in pain pathways.

SUMMARY1. The nature, distribution and function of rectifying channels in rat spinal root myelinated axons has been assessed with selective blocking agents and a variety of intracellular and extracellular recording techniques.2. The electrotonic responses of roots poisoned with tetrodotoxin (TTX) to constant current pulses had fast (rise time << 1 ms) and slow components, which were interpreted in terms of Barrett & Barrett 's (1982) revised cable model for myelinated nerve. Depolarization evoked a rapid outward rectification (time constant, T -.0 5 ms), selectively blocked by 4-aminopyridine (4AP, 1 mM), and a slow outward rectification (,r-15 ms), selectively blocked by tetraethylammonium (TEA, 1 mM) or Ba2+ (0-5 mM). Hyperpolarization evoked an even slower inward rectification, selectively blocked by Cs+ (3 mM) but not by Ba2 .3. From the different effects of the blocking agents on the fast and slow components of electrotonus, it was deduced (a) that the inward rectification is a property of the internodal axon, (b) that the slow outward rectifier is present at the nodes, and probably the internodes as well, and (c) that the 4AP-sensitive channels have a minor nodal and a major internodal representation.4. TEA and Ba2+ reduced the accommodation of roots and fibres not poisoned with TTX to long current pulses, whereas 4AP facilitated short bursts of impulses in response to a single brief stimulus.5. TEA and Ba2+ also abolished a late hyperpolarizing after-potential (peaking at 20-80 ms), while 4AP enhanced the depolarizing after-potential in normal fibres, and abolished an early hyperpolarizing after-potential (peaking at 1-3 ms) in depolarized fibres. Corresponding to the later after-potentials were post-spike changes in excitability and conduction velocity, which were affected similarly by the blocking agents. Cs+ increased the post-tetanic depression attributable to electrogenic hyperpolarization.6. The physiological roles of the three different rectifying conductances are discussed. It is also argued that the prominent ohmic 'leak conductance', usually ascribed to the nodal axon, must arise in an extracellular pathway in series with the rectifying internodal axon.

The activity of voltage-gated sodium channels has long been linked to disorders of neuronal excitability such as epilepsy and chronic pain. Recent genetic studies have now expanded the role of sodium channels in health and disease, to include autism, migraine, multiple sclerosis, cancer as well as muscle and immune system disorders. Transgenic mouse models have proved useful in understanding the physiological role of individual sodium channels, and there has been significant progress in the development of subtype selective inhibitors of sodium channels. This review will outline the functions and roles of specific sodium channels in electrical signalling and disease, focusing on neurological aspects. We also discuss recent advances in the development of selective sodium channel inhibitors.

NaV1.8 is a voltage-gated sodium channel expressed only in a subset of sensory neurons of which more than 85% are nociceptors. In order to delete genes in nociceptive neurons, we generated heterozygous transgenic mice expressing Cre recombinase under the control of the NaV1.8 promoter. Functional Cre recombinase expression replicated precisely the expression pattern of NaV1.8. Cre expression began at embryonic day 14 in small diameter neurons in dorsal root, trigeminal and nodose ganglia, but was absent in non-neuronal or CNS tissues into adulthood. Sodium channel subtypes were normal in isolated DRG neurons. Pain behaviour in response to mechanical or thermal stimuli, and in acute, inflammatory and neuropathic pain was also normal. These data demonstrate that the heterozygous NaV1.8-Cre mouse line is a useful tool to analyse the effects of deleting floxed genes on pain behaviour.

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

Acute, inflammatory, and neuropathic pain can all be attenuated or abolished by local treatment with sodium channel blockers such as lidocaine. The peripheral input that drives pain perception thus depends on the presence of functional voltage-gated sodium channels. Remarkably, two voltage-gated sodium channel genes (Nav1.8 and Nav1.9) are expressed selectively in damage-sensing peripheral neurons, while a third channel (Nav1.7) is found predominantly in sensory and sympathetic neurons. An embryonic channel (Nav1.3) is also upregulated in damaged peripheral nerves and associated with increased electrical excitability in neuropathic pain states. A combination of antisense and knock-out studies support a specialized role for these sodium channels in pain pathways, and pharmacological studies with conotoxins suggest that isotype-specific antagonists should be feasible. Taken together, these data suggest that isotype-specific sodium channel blockers could be useful analgesics.

A central hallmark of Alzheimer's disease is the presence of extracellular amyloid plaques chiefly consisting of amyloid-β (Aβ) peptides in the brain interstitium. Aβ largely exists in two isoforms, 40 and 42 amino acids long, but a large body of evidence points to Aβ(1-42) rather than Aβ(1-40) as the cytotoxic form. One proposed mechanism by which Aβ exerts toxicity is the formation of ion channel pores that disrupt intracellular Ca homeostasis. However, previous studies using membrane mimetics have not identified any notable difference in the channel forming properties between Aβ(1-40) and Aβ(1-42). Here, we tested whether a more physiological environment, membranes excised from HEK293 cells of neuronal origin, would reveal differences in the relative channel forming ability of monomeric, oligomeric, and fibrillar forms of both Aβ(1-40) and Aβ(1-42). Aβ preparations were characterized with transmission electron microscopy and thioflavin T fluorescence. Aβ was then exposed to the extracellular face of excised membranes, and transmembrane currents were monitored using patch clamp. Our data indicated that Aβ(1-42) assemblies in oligomeric preparations form voltage-independent, non-selective ion channels. In contrast, Aβ(1-40) oligomers, fibers, and monomers did not form channels. Ion channel conductance results suggested that Aβ(1-42) oligomers, but not monomers and fibers, formed three distinct pore structures with 1.7-, 2.1-, and 2.4-nm pore diameters. Our findings demonstrate that only Aβ(1-42) contains unique structural features that facilitate membrane insertion and channel formation, now aligning ion channel formation with the differential neurotoxic effect of Aβ(1-40) and Aβ(1-42) in Alzheimer's disease.