Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC50
Two forms of action potential latency-walking in rat and mouse small diameter sensory neurones in vitro, discriminated by tetrodotoxin
R. C. Wright1, S. P. O'Rourke1, M. D. Baker1
1. Neuroscience and Trauma, Queen Mary University of London, London, United Kingdom.
Action potentials recorded in IB4+ve small (<25 μm apparent diameter) dorsal root ganglion (DRG) neurones are typically of long duration, consistent with a major contribution of the kinetically slow, tetrodotoxin-resistant (TTX-r) transient Na+ current, NaV1.8, to neurone excitability (e.g. Blair & Bean 2002; Snape et al. 2010). Action potential latency-walking elicited by low frequency stimulation (2 Hz) in these neurones is probably caused by a progressive slow inactivation of Na+ currents, and we wished to confirm whether properties of either NaV1.8 or TTX-sensitive (TTX-s) Na+ currents (or both) might explain the phenomenon. We first investigated the effects of a small molecule blocker of NaV1.8 (A-803467) that is reported to show channel sub-type selective block (Jarvis et al. 2007), and we found that A-803467 could not select convincingly between Na+ currents recorded in different small diameter rat sensory neurones in voltage-clamp (fractional block by 300 nM = 0.60 ± 0.09 and 0.49 ± 0.03 (mean ± SEM) for transient TTX-s and TTX-r Na+ currents , recorded in small diameter IB4- and IB4 +ve neurones, respectively, n = 3, 3; P = 0.34, t-test), and we did not investigate it further. However, we also found that application of TTX (250 nM) in current-clamp enabled us to discriminate two forms of latency walking in neurones with long duration action potentials. In neurones with stable resting potentials of -60 mV or more negative, one form was typified by a consistently less than 1.5 ms increase in action potential latency in response to 100 stimuli at 2 Hz (n = 11), and this was unchanged by exposure to TTX (n = 4, P = 0.24, paired t-test). However, walking was also recorded that could be considerably greater than 1.5 ms (n = 4), and the latency change in these neurones was substantially reduced by TTX, Figure 1. We conclude that at least one TTX-s Na+ channel subtype and probably NaV1.8, contribute to action potential latency walking, with the activity dependent loss of TTX-s channels most dramatically affecting latency. We suggest that varying degrees of activity dependent changes in C-fibre conduction time may reflect the differential expression of Na+ channel subtypes in axons.
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