Pharmacological fractionation of tetrodotoxin-sensitive sodium currents in rat dorsal root ganglion neurons by μ-conotoxins.
OBJECTIVE
Adult rat dorsal root ganglion (DRG) neurons normally express transcripts for five isoforms of the α-subunit of voltage-gated sodium channels: NaV 1.1, 1.6, 1.7, 1.8 and 1.9. Tetrodotoxin (TTX) readily blocks all but NaV 1.8 and 1.9, and pharmacological agents that discriminate among the TTX-sensitive NaV 1-isoforms are scarce. Recently, we used the activity profile of a panel of μ-conotoxins in blocking cloned rodent NaV 1-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 NaV 1.6 and NaV 1.7.


METHODS
We used three μ-conotoxins, μ-TIIIA, μ-PIIIA and μ-SmIIIA, applied individually and in combinations, to pharmacologically differentiate the TTX-sensitive INa 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.


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 NaV 1-isoforms, co-expressed with various NaV β-subunits in X. laevis oocytes, were consistent: NaV 1.1, 1.6 and 1.7 could account for all of the TTX-sensitive INa , with NaV 1.1 < NaV 1.6 < NaV 1.7 for small neurons and NaV 1.7 < NaV 1.1 < NaV 1.6 for large neurons.


CONCLUSIONS
Combinations of μ-conotoxins can be used to determine the probable NaV 1-isoforms underlying the INa in DRG neurons. Preliminary experiments with sympathetic neurons suggest that this approach is extendable to other neurons.
Department of Biology, University of Utah, Salt Lake City, UT 84112, USA.
Min-Min
MM
Zhang
Michael J
MJ
Wilson
Joanna
J
Gajewiak
Jean E
JE
Rivier
Grzegorz
G
Bulaj
Baldomero M
BM
Olivera
Doju
D
Yoshikami
D000818:Animals; D020916:Conotoxins / Q000008:administration & dosage / Q000494:pharmacology*; D005260:Female; D005727:Ganglia, Spinal / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D062550:NAV1.1 Voltage-Gated Sodium Channel / Q000378:metabolism; D062557:NAV1.6 Voltage-Gated Sodium Channel / Q000378:metabolism; D062556:NAV1.7 Voltage-Gated Sodium Channel / Q000378:metabolism; D009474:Neurons / Q000187:drug effects* / Q000378:metabolism; D009865:Oocytes / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013779:Tetrodotoxin / Q000494:pharmacology*; D014982:Xenopus laevis
D020916:Conotoxins; D062550:NAV1.1 Voltage-Gated Sodium Channel; D062557:NAV1.6 Voltage-Gated Sodium Channel; D062556:NAV1.7 Voltage-Gated Sodium Channel; D013779:Tetrodotoxin