Pharmacological fractionation of tetrodotoxin-sensitive sodium currents in rat dorsal root ganglion neurons by μ-conotoxins

Abstract

Background and Purpose Adult rat dorsal root ganglion (DRG) neurons normally express transcripts for five isoforms of the α-subunit of voltage-gated sodium channels: NaV1.1, 1.6, 1.7, 1.8 and 1.9. Tetrodotoxin (TTX) readily blocks all but NaV1.8 and 1.9, and pharmacological agents that discriminate among the TTX-sensitive Na V1-isoforms are scarce. Recently, we used the activity profile of a panel of μ-conotoxins in blocking cloned rodent NaV1-isoforms expressed in Xenopus laevis oocytes to conclude that action potentials of A- and C-fibres in rat sciatic nerve were, respectively, mediated primarily by Na V1.6 and NaV1.7. Experimental Approach We used three μ-conotoxins, μ-TIIIA, μ-PIIIA and μ-SmIIIA, applied individually and in combinations, to pharmacologically differentiate the TTX-sensitive I Na of voltage-clamped neurons acutely dissociated from adult rat DRG. We examined only small and large neurons whose respective INa were >50% and >80% TTX-sensitive. Key Results In both small and large neurons, the ability of the toxins to block TTX-sensitive INa was μ-TIIIA < μ-PIIIA < μ-SmIIIA, with the latter blocking ≳90%. Comparison of the toxin-susceptibility profiles of the neuronal INa with recently acquired profiles of rat NaV1-isoforms, co-expressed with various NaVβ-subunits in X. laevis oocytes, were consistent: NaV1.1, 1.6 and 1.7 could account for all of the TTX-sensitive I Na, with NaV1.1 < NaV1.6 < Na V1.7 for small neurons and NaV1.7 < NaV1.1 < NaV1.6 for large neurons. Conclusions and Implications Combinations of μ-conotoxins can be used to determine the probable Na V1-isoforms underlying the INa in DRG neurons. Preliminary experiments with sympathetic neurons suggest that this approach is extendable to other neurons. © 2013 The Authors. British Journal of Pharmacology © 2013 The British Pharmacological Society.