Analysis of the variation in use-dependent inactivation of high-threshold tetrodotoxin-resistant sodium currents recorded from rat sensory neurons

This study addressed variation in the use-dependent inactivation (UDI) of high-threshold tetrodotoxin-resistant Na+ currents (TTX-R currents) and action potential firing behavior among acutely isolated rat dorsal root ganglion (DRG) cells. UDI was quantified as the percent decrease in current amplitude caused by increasing the current activation rate from 0.1-1.0 Hz for 20 s. TTX-R current UDI varied from 6% to 66% among 122 DRG cells examined, suggesting the existence of two or more levels of UDI. The voltage-dependency of the TTX-R currents was consistent with Na(V)1.8, regardless of UDI. However, TTX-R currents with more UDI had a more negative voltage-dependency of inactivation, a greater tendency to enter slow inactivation, and a slower recovery rate from slow inactivation, compared with those with less UDI. TTX-R currents with more UDI ran down faster than those with less UDI. However, UDI itself changed little over time, regardless of the initial UDI level observed in a particular DRG cell. Together, these two observations suggest that individual DRG cells did not express mixtures of TTX-R channels that varied regarding UDI. TTX-R current UDI was correlated with expression of a low-threshold A-current and whole-cell capacitance, suggesting that it varied among different nociceptor types. Whole-cell inward currents (WCI-currents), recorded without channel blockers, also exhibited UDI. WCI-current UDI varied similarly to TTX-R current UDI in magnitude, and relative to whole-cell capacitance and A-current expression, suggesting that the WCI-currents were carried predominantly by TTX-R channels. DRG cells with more WCI-current UDI exhibited a greater decrease in action potential amplitude and number, and a greater increase in action potential threshold over seven ramp depolarizations, compared with DRG cells with less WCI-current UDI. Variation in UDI of Na(V)1.8 channels expressed by different nociceptor types could contribute to shaping their individual firing patterns in response to noxious stimuli. (c) 2006 IBRO. Published by Elsevier Ltd. All rights reserved.