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

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: Na_V1.1, 1.6, 1.7, 1.8 and 1.9. Tetrodotoxin (TTX) readily blocks all but Na_V1.8 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 Na_V1‐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 Na_V1.7.

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 I_Na were >50% and >80% TTX‐sensitive.

Key Results

In both small and large neurons, the ability of the toxins to block TTX‐sensitive I_Na was μ‐TIIIA < μ‐PIIIA < μ‐SmIIIA, with the latter blocking ≳90%. Comparison of the toxin‐susceptibility profiles of the neuronal I_Na with recently acquired profiles of rat Na_V1‐isoforms, co‐expressed with various Na_Vβ‐subunits in X. laevis oocytes, were consistent: Na_V1.1, 1.6 and 1.7 could account for all of the TTX‐sensitive I_Na, with Na_V1.1 < Na_V1.6 < Na_V1.7 for small neurons and Na_V1.7 < Na_V1.1 < Na_V1.6 for large neurons.

Conclusions and Implications

Combinations of μ‐conotoxins can be used to determine the probable Na_V1‐isoforms underlying the I_Na in DRG neurons. Preliminary experiments with sympathetic neurons suggest that this approach is extendable to other neurons.

DOI: 10.1111/bph.12119

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