• M C Kiernan, M D Baker, and H Bostock.
• Sobell Department of Neurophysiology, Institute of Neurology, Queen Square, WC1N 3BG, London, UK. M.kiernan@unsw.edu.au
• Neuroscience. 2003 Jan 1; 119 (3): 653-60.

AbstractNa(+) currents were recorded using patch-clamp techniques from small-diameter (<25 micrometers) dorsal root ganglion neurons, cultured from adult rats (>150 g). Late Na(+) currents maintained throughout long-duration voltage-clamp steps (>/=200 ms) were of two types: a low-threshold, tetrodotoxin-sensitive (TTX-s) current that was largely blocked by 200 nM TTX, and a high-threshold, TTX-resistant (TTX-r) current. TTX-s late current was found in approximately 28% (10/36) of small-diameter neurons and was recorded only in neurons exhibiting TTX-s transient current. TTX-s transient current activation/inactivation gating overlap existed over a narrow potential range, centered between -30 and -40 mV, whereas late current operated over a wider range. The kinetics associated with de-inactivation of TTX-s late current were slow (tau approximately 37 ms at -50 mV), strongly suggesting that different subpopulations of TTX-s channel generate transient and late current. High-threshold TTX-r late current was only present in neurons generating TTX-r transient current. TTX-r late current operated over the same potential range as that for TTX-r transient current activation/inactivation gating overlap, and activation/inactivation gating overlap could be measured even after 1.5-s-duration pre-pulses. We suggest that TTX-s late sodium current results from channel openings different from those generating transient current. As in large-diameter sensory neurons, TTX-s channels generating late openings may play a key role in controlling membrane excitability. In contrast, a single population of high-threshold TTX-r channels may account for both transient and late TTX-r currents.

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