title: Contribution of Na(v)1.8 sodium channels to action potential electrogenesis in DRG neurons.
abstract: C-type dorsal root ganglion (DRG) neurons can generate tetrodotoxin-resistant (TTX-R) sodium-dependent action potentials. However, multiple sodium channels are expressed in these neurons, and the molecular identity of the TTX-R sodium channels that contribute to action potential production in these neurons has not been established. In this study, we used current-clamp recordings to compare action potential electrogenesis in Na(v)1.8 (+/+) and (-/-) small DRG neurons maintained for 2-8 h in vitro to examine the role of sodium channel Na(v)1.8 (alpha-SNS) in action potential electrogenesis. Although there was no significant difference in resting membrane potential, input resistance, current threshold, or voltage threshold in Na(v)1.8 (+/+) and (-/-) DRG neurons, there were significant differences in action potential electrogenesis. Most Na(v)1.8 (+/+) neurons generate all-or-none action potentials, whereas most of Na(v)1.8 (-/-) neurons produce smaller graded responses. The peak of the response was significantly reduced in Na(v)1.8 (-/-) neurons [31.5 +/- 2.2 (SE) mV] compared with Na(v)1.8 (+/+) neurons (55.0 +/- 4.3 mV). The maximum rise slope was 84.7 +/- 11.2 mV/ms in Na(v)1.8 (+/+) neurons, significantly faster than in Na(v)1.8 (-/-) neurons where it was 47.2 +/- 1.3 mV/ms. Calculations based on the action potential overshoot in Na(v)1.8 (+/+) and (-/-) neurons, following blockade of Ca(2+) currents, indicate that Na(v)1.8 contributes a substantial fraction (80-90%) of the inward membrane current that flows during the rising phase of the action potential. We found that fast TTX-sensitive Na(+) channels can produce all-or-none action potentials in some Na(v)1.8 (-/-) neurons but, presumably as a result of steady-state inactivation of these channels, electrogenesis in Na(v)1.8 (-/-) neurons is more sensitive to membrane depolarization than in Na(v)1.8 (+/+) neurons, and, in the absence of Na(v)1.8, is attenuated with even modest depolarization. These observations indicate that Na(v)1.8 contributes substantially to action potential electrogenesis in C-type DRG neurons.
authors.affiliation: Department of Neurology and Paralyzed Veterans of America/Eastern Paralyzed Veterans Association Neuroscience Research Center, Yale University School of Medicine, New Haven 06510, USA.
authors.forename: M
authors.initials: M
authors.lastname: Renganathan
authors.forename: T R
authors.initials: TR
authors.lastname: Cummins
authors.forename: S G
authors.initials: SG
authors.lastname: Waxman
mesh_terms: D000200:Action Potentials / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D015220:Calcium Channels / Q000502:physiology; D004594:Electrophysiology; D005260:Female; D005727:Ganglia, Spinal / Q000166:cytology*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008817:Mice, Mutant Strains; D062559:NAV1.8 Voltage-Gated Sodium Channel; D009475:Neurons, Afferent / Q000502:physiology*; D009479:Neuropeptides / Q000502:physiology*; D015222:Sodium Channels / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D015220:Calcium Channels; D062559:NAV1.8 Voltage-Gated Sodium Channel; D009479:Neuropeptides; C568318:Scn10a protein, mouse; D015222:Sodium Channels; D013779:Tetrodotoxin