title: Tetrodotoxin-, dihydropyridine-, and riluzole-resistant persistent inward current: novel sodium channels in rodent spinal neurons.
abstract: Recently, we reported the tetrodotoxin (TTX)- and dihydropyridine (DHP)-resistant (TDR) inward currents in neonatal mouse spinal neurons. In this study, we further characterized these currents in the presence of 1-5 μM TTX and 20-30 μM DHP (nifedipine, nimodipine, or isradipine). TDR inward currents were recorded by voltage ramp (persistent inward current, TDR-PIC) and step (TDR-I(p)) protocols. TDR-PIC and TDR-I(p) were found in 80.2% of recorded neurons (101/126) crossing laminae I to X from T12 to L6. TDR-PIC activated at -28.6 ± 13 mV with an amplitude of 80.6 ± 75 pA and time constant of 470.6 ± 240 ms (n = 75). TDR-I(p) had an amplitude of 151.2 ± 151 pA and a voltage threshold of -17.0 ± 9 mV (n = 54) with a wide range of kinetics parameters. The half-maximal activation was -21.5 ± 8 mV (-37 to -12 mV, n = 29) with a time constant of 5.2 ± 2 ms (1.2-11.2 ms, n = 19), whereas the half-maximal inactivation was -26.9 ± 9 mV (-39 to -18 mV, n = 14) with a time constant of 1.4 ± 0.4 s (0.5-2.2 s, n = 19). TDR-PIC and TDR-I(p) could be reduced by 60% in zero calcium and completely removed in zero sodium solutions, suggesting that they were mediated by sodium ions. Furthermore, the reversal potential of TDR-I(p) was estimated as 56.6 ± 3 mV (n = 10). TDR-PIC and TDR-I(p) persisted in 1-205 μM TTX, 20-100 μM DHP, 3-30 μM riluzole, 50-300 μM flufenamic acid, and 2-30 mM intracellular BAPTA. They also persisted with T-, N-, P/Q-, and R-type calcium channel blockers. In conclusion, we demonstrated novel TTX-, DHP-, and riluzole-resistant sodium channels in neonatal rodent spinal neurons. The unique pharmacological and electrophysiological properties would allow these channels to play a functional role in spinal motor system.
authors.affiliation: Department of Physiology, University of Manitoba, Winnipeg, Manitoba, Canada.
authors.forename: Yue
authors.initials: Y
authors.lastname: Dai
authors.forename: Larry M
authors.initials: LM
authors.lastname: Jordan
mesh_terms: D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D000831:Animals, Newborn; D004095:Dihydropyridines / Q000494:pharmacology*; D051379:Mice; D008822:Mice, Transgenic; D009046:Motor Neurons / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D019782:Riluzole / Q000494:pharmacology*; D015222:Sodium Channels / Q000502:physiology*; D013116:Spinal Cord / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology*
chemical_list: D004095:Dihydropyridines; D015222:Sodium Channels; D013779:Tetrodotoxin; D019782:Riluzole; C038806:1,4-dihydropyridine