Veronika Souslova

The tetrodotoxin-resistant sodium channel SNS has a specialized function in pain pathways

Many damage-sensing neurons express tetrodotoxin (TTX)-resistant voltage-gated sodium channels. Here we examined the role of the sensory-neuron-specific (SNS) TTX-resistant sodium channel α subunit in nociception and pain by constructing sns-null mutant mice. These mice expressed only TTX-sensitive sodium currents on step depolarizations from normal resting potentials, showing that all slow TTX-resistant currents are encoded by the sns gene. Null mutants were viable, fertile and apparently normal, although lowered thresholds of electrical activation of C-fibers and increased current densities of TTX-sensitive channels demonstrated compensatory upregulation of TTX-sensitive currents in sensory neurons. Behavioral studies demonstrated a pronounced analgesia to noxious mechanical stimuli, small deficits in noxious thermoreception and delayed development of inflammatory hyperalgesia. These data show that SNS is involved in pain pathways and suggest that blockade of SNS expression or function may produce analgesia without side effects.

Warm-coding deficits and aberrant inflammatory pain in mice lacking P2X3 receptors

ATP activates damage-sensing neurons (nociceptors) and can evoke a sensation of pain. The ATP receptor P2X3 is selectively expressed by nociceptors and is one of seven ATP-gated, cation-selective ion channels. Here we demonstrate that ablation of the P2X3 gene results in the loss of rapidly desensitizing ATP-gated cation currents in dorsal root ganglion neurons, and that the responses of nodose ganglion neurons to ATP show altered kinetics and pharmacology resulting from the loss of expression of P2X2/3 heteromultimers. Null mutants have normal sensorimotor function. Behavioural responses to noxious mechanical and thermal stimuli are also normal, although formalin-induced pain behaviour is reduced. In contrast, deletion of the P2X3 receptor causes enhanced thermal hyperalgesia in chronic inflammation. Notably, although dorsal-horn neuronal responses to mechanical and noxious heat application are normal, P2X3-null mice are unable to code the intensity of non-noxious ‘warming’ stimuli.

A role for the TTX-resistant sodium channel Nav 1.8 in NGF-induced hyperalgesia, but not neuropathic pain

The tetrodotoxin-resistant voltage-gated sodium channel Nav 1.8 is expressed only in nociceptive sensory neurons. This channel has been proposed to contribute significantly to the sensitization of primary sensory neurons after injury. We have studied the nociceptive behaviours of mice carrying a null mutation in the Nav 1.8 gene (Nav 1.8 −/−) in models of peripheral inflammation as well as a model of neuropathic pain. The results from the present studies reveal that Nav 1.8 is a necessary mediator of NGF-induced thermal hyperalgesia but is not essential for PGE2-evoked hypersensitivity. Neuropathic pain behaviours were unchanged in Nav 1.8 −/− mice indicating that this channel is not involved in the alteration of sensory thresholds following peripheral nerve injury.

The Tetrodotoxin‐Resistant Na+ Channel Nav1.8 is Essential for the Expression of Spontaneous Activity in Damaged Sensory Axons of Mice

The tetrodotoxin‐resistant sodium channel α subunit, Nav1.8, is exclusively expressed in primary sensory neurons and is suggested to play a role in the generation of ectopic action potentials after axonal injury and thereby contribute to neuropathic pain. Here we investigated the involvement of Nav1.8 in ectopic impulse generation in damaged axons by examining spontaneous activity and mechanosensitivity in neuromas formed by section of the saphenous nerve in Nav1.8 null mice and in their wild‐type littermates. We recorded 522 identified units from 24 neuromas in vitro at two time points, 8–11 days (median 10 days) and 19–29 days (median 22 days) post‐operatively. At ≈10 days, neither genotype showed spontaneous activity, but a significantly higher proportion of fibres were mechanosensitive in wild‐type (54 %) compared to Nav1.8 null neuromas (18 %). At ≈22 days, 19 % of fibres recorded in wild‐type neuromas showed spontaneous activity, whereas only one fibre of the 238 (0.4 %) recorded in neuromas taken from null mice showed ongoing activity. In recordings at ≈22 days, a similar proportion of fibres were mechanosensitive in wild‐type and Nav1.8 null neuromas (51 and 46 %, respectively). We conclude that Nav1.8 is essential for the expression of spontaneous activity in damaged sensory axons, and may also contribute to the development of ectopic mechanosensitivity.

TRANS-SPLICING OF A VOLTAGE-GATED SODIUM CHANNEL IS REGULATED BY NERVE GROWTH FACTOR

Mammalian sensory neurons express a voltage‐gated sodium channel named SNS. Here we report the identification of an SNS transcript (SNS‐A) that contains an exact repeat of exons 12, 13 and 14 encoding a partial repeat of domain II. Because the exons 12–14 are present in single copies in genomic DNA, the SNS‐A transcript must arise by trans‐splicing. Nerve growth factor, which regulates pain thresholds, and the functional expression of voltage‐gated sodium channels increases the levels of the SNS‐A transcript several‐fold both in vivo and in vitro as measured by RNase protection methods, as well as RT‐PCR. These data demonstrate a novel regulatory role for the nerve growth factor and are the first example of trans‐splicing in the vertebrate nervous system.

Structure and distribution of a broadly expressed atypical sodium channel.

A cDNA clone isolated from a rat dorsal root ganglion library encodes a 195 kDa voltage‐gated sodium channel‐like protein (SCL‐11) with homology to the mouse (87%) and human (72%) atypical Na+ channels and rat partial clone NaG (98%). Two dominant mRNAs of 4.5 and 7 kb are expressed. The transcripts are present in lung, Schwann cells, pituitary and tissues containing smooth muscle cells. No functional channels could be detected on oocyte injection with cRNA, consistent with the absence of structural features necessary for voltage‐gated sodium channel activity.

Structure and Chromosomal Mapping of The Mouse P2X3 Gene

P2X3 is one of seven cloned ATP-gated non-selective cation channels. We have isolated a full-length mouse P2X3 gene from a phage lambda-129/Sv genomic library. The gene consists of 12 exons spanning a locus of approximately 40 kb. No significant similarities have been found between the genomic organisation of the mouse P2X3 gene and genes encoding other ion channels. The encoded mouse P2X3 protein consists of 397 amino acids and shows 99% identity with rat P2X3. Using RNase protection and primer extension assays, multiple transcription initiation sites have been mapped in the mouse P2X3 promoter to a region 162-168 bp upstream of the translation initiation codon. The P2X3 gene has been mapped to mouse chromosome 2p by fluorescence in situ hybridisation. The RAG locus-associated gene T160 is located 1.8 kb upstream of the transcription start site of mouse P2X3 gene. The promoter region of the mouse P2X3 gene lacks a conventional TATA and CCAAT consensus sites, and initiator elements. P2X3 is the first member of the P2X gene family to be completely characterised.

correction: Warm-coding deficits and aberrant inflammatory pain in mice lacking P2X3

This corrects the article DOI: 35039526