Molecular analysis of the neuropathic pain state using sns-null mutant mice

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Sensory neurons in dorsal root ganglia (DRG) express at least seven isoforms of voltage-gated sodium channel (VGSC) α-subunits. These isoforms show localized distribution within the primary sensory path-way and probably mediate multiple types of sodium currents (I_<Na>) with different kinetic properties, giving rise to fast, slow or persistent I_<Na>s. However, little is known as to which isoform is responsible for a particular I_<Na> in DRG neurons. We have recently identified two types persistent I_<Na>s : one is resistant to tetrodotoxin (TTX) and found exclusively in small DRG neurons (I_<TTX-R/persist>), and the otner is sensitive to TTX and found in medium/large DRG neurons (I_<TTX-S/persist>). Available data suggest that the former is most likely to be mediated by Na_V1.9, while the latter may be mediated by Na_V1.6. To confirm these possibilities, we performed single cell nested RT-PCR combined with concurrent patch clamp recording and investigated the correlation between mRNA … More expression and phenotype of I_<Na> in an identified DRG neuron. I_<TTX-R/persist> was recorded in isolation from small DRG neurons of Na_V1.8-null mutant mice in the presence of TTX. Most of the small neurons, which were associated with Na_V1.9 transcript, manifested I_<TTX-R/persist>, confirming that the isoform mediating I_<TTX-R/persist> is probably Na_V1.9. Unexpectedly, however, Na_V1.9 was evident also in a considerable number of medium/large neurons that were totally devoid of I_<TTX-R/persist>. These results indicate that I_<TTX-R/persist> is not solely dependent on the level of Na_V1.9 transcript. An intriguing finding was that Na_V1.9 transcript was often absent in small DRG neurons from naive mice while the same transcript was evident in most of the small DRG neurons from mutants. Expression of Na_V1.6 transcript was demonstrated in a considerable portion of the medium/large neurons from both naive and mutant mice. However, these neurons did not consistently manifest I_<TTX-S/persist> suggesting that I_<TTX-S/persist> may not be carried by Na_V1.6. Less