Electrophysiological characterization of the tetrodotoxin-resistant Na+ channel, Nav1.9, in mouse dorsal root ganglion neurons
Small dorsal root ganglion neurons express preferentially the Na+ channel isoform NaV1.9 that mediates a tetrodotoxin-resistant (TTX-R) Na+ current. We investigated properties of the Na+ current mediated by NaV1.9 (INaN) using the whole-cell, patch-clamp recording technique. To isolate INaN from heterogeneous TTX-R Na+ currents that also contain another type of TTX-R Na+ current mediated by NaV1.8, we used NaV1.8-null mutant mice. When F− was used as an internal anion in the patch pipette solution, both the activation and inactivation kinetics for INaN shifted in the hyperpolarizing direction with time. Such a time-dependent shift of the kinetics was not observed when Cl− was used as an internal anion. Functional expression of INaN declined with time after cell dissociation and recovered during culture, implying that NaV1.9 may be regulated dynamically by trophic factors or depend on subtle environmental factors for its survival. During whole-cell recordings, the peak amplitude of INaN increased dramatically after a variable delay, as if inactive or silent channels had been “kindled”. Such an unusual increase of the amplitude could be prevented by adding ATP to the pipette solution or by recording with the nystatin-perforated patch-clamp technique, suggesting that the rupture of patch membrane affected the behaviour of NaV1.9. These peculiar properties of INaN may provide an insight into the plasticity of Na+ channels that are related to pathological functions of Na+ channels accompanying abnormal pain states.
KeywordsNa channel Dorsal root ganglion neuron Tetrodotoxin Patch-clamp Gating
We thank Drs. Y. Ohishi, J. Kakimura (Hiroshima University) for their excellent suggestions and technical help with this research. This study was supported in part by the Grants-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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