Dorsal root ganglion sensory neurons associated with C-fibres, many of which are activated by tissue-damage, express an unusual voltage-gated sodium channel that is resistant to tetrodotoxin. We report here that we have identified a 1,957 amino-acid sodium channel in these cells that shows 65% identity with the rat cardiac tetrodotoxin-insensitive sodium channel, and is not expressed in other peripheral and central neurons, glia or non-neuronal tissues. In situ hybridization shows that the channel is expressed only by small-diameter sensory neurons in neonatal and adult dorsal root and trigeminal ganglia. The channel is resistant to tetrodotoxin when expressed in Xenopus oocytes. The electrophysiological and pharmacological properties of the expressed channel are similar to those described for the small-diameter sensory neuron tetrodotoxin-resistant sodium channels. As some noxious input into the spinal cord is resistant to tetrodotoxin, block of expression or function of such a C-fibre-restricted sodium channel may have a selective analgesic effect.

ATP is known to depolarize sensory neurons, and may play a role in nociceptor activation when released from damaged tissue. Here we report the molecular cloning and characterization of a new member of the P2X receptor family, P2X3, expressed by these cells. The channel transcript was present in a subset of rat dorsal-root-ganglion sensory neurons, some of which express nociceptor-associated markers; it was absent in other tissues that were tested, including sympathetic, enteric and central nervous system neurons. Moreover, when expressed in Xenopus oocytes, the channel showed an ATP-dependent cation flux. P2X3 is the only ligand-gated channel known to be expressed exclusively by a subset of sensory neurons. The remarkable selectivity of expression of the channel coupled with its sensory neuron-like pharmacology suggests that this channel may transduce ATP-evoked nociceptor activation.

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 alpha 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.

TTX-resistant (TTX-R) sodium currents are preferentially expressed in small C-type dorsal root ganglion (DRG) neurons, which include nociceptive neurons. Two mRNAs that are predicted to encode TTX-R sodium channels, SNS and NaN, are preferentially expressed in C-type DRG cells. To determine whether there are multiple TTX-R currents in these cells, we used patch-clamp recordings to study sodium currents in SNS-null mice and found a novel persistent voltage-dependent sodium current in small DRG neurons of both SNS-null and wild-type mice. Like SNS currents, this current is highly resistant to TTX (Ki = 39+/-9 microM). In contrast to SNS currents, the threshold for activation of this current is near 70 mV, the midpoint of steady-state inactivation is -44 +/- 1 mV, and the time constant for inactivation is 43+/-4 msec at 20 mV. The presence of this current in SNS-null and wild-type mice demonstrates that a distinct sodium channel isoform, which we suggest to be NaN, underlies this persistent TTX-R current. Importantly, the hyperpolarized voltage-dependence of this current, the substantial overlap of its activation and steady-state inactivation curves and its persistent nature suggest that this current is active near resting potential, where it may play an important role in regulating excitability of primary sensory neurons.

Proton-gated channels expressed by sensory neurons are of particular interest because low pH causes pain. Two proton-gated channels, acid-sensing ionic channel (ASIC) and dorsal root ASIC (DRASIC), that are members of the amiloride-sensitive ENaC͞Degenerin family are known to be expressed by sensory neurons. Here, we describe the cloning and characterization of an ASIC splice variant, ASIC-␤, which contains a unique N-terminal 172 aa, as well as unique 5 and 3 untranslated sequences. ASIC-␤, unlike ASIC and DRASIC, is found only in a subset of small and large diameter sensory neurons and is absent from sympathetic neurons or the central nervous system. The patterns of expression of ASIC and ASIC-␤ transcripts in rat dorsal root ganglion neurons are distinct. When expressed in COS-7 cells, ASIC-␤ forms a functional channel with electrophysiological properties distinct from ASIC and DRASIC. The pH dependency and sensitivity to amiloride of ASIC-␤ is similar to that described for ASIC, but unlike ASIC, the channel is not permeable to calcium, nor are ASIC-␤-mediated currents inhibited by extracellular calcium. The unique distribution of ASIC-␤ suggests that it may play a specialized role in sensory neuron function.