α- And β-subunit composition of voltage-gated sodium channels investigated with μ-conotoxins and the recently discovered μO§-conotoxin GVIIJ

Abstract

We investigated the identities of the isoforms of the α (NaV1)- and β (NaVβ)-subunits of voltage-gated sodium channels, including those responsible for action potentials in rodent sciatic nerves. To examine α-subunits, we used seven μ-conotoxins, which target site 1 of the channel. With the use of exogenously expressed channels, we show that two of the μ-conotoxins, μ-BuIIIB and μ-SxIIIA, are 50-fold more potent in blocking NaV1.6 from mouse than that from rat. Furthermore, we observed that μ-BuIIIB and μ-SxIIIA are potent blockers of large, myelinated A-fiber compound action potentials (A-CAPs) [but not small, unmyelinated C-fiber CAPs (C-CAPs)] in the sciatic nerve of the mouse (unlike A-CAPs of the rat, previously shown to be insensitive to these toxins). To investigate β-subunits, we used two synthetic derivatives of the recently discovered μO§-conotoxin GVIIJ that define site 8 of the channel, as previously characterized with cloned rat NaV1- and NaVβ-subunits expressed in Xenopus laevis oocytes, where it was shown that μO§-GVIIJ is a potent inhibitor of several NaV1-isoforms and that coexpression of NaVβ2 or -β4 (but not NaVβ1 or -β3) totally protects against block by μO§-GVIIJ. We report here the effects of μO§-GVIIJ on 1) sodium currents of mouse NaV1.6 coexpressed with various combinations of NaVβ-subunits in oocytes; 2) A- and C-CAPs of mouse and rat sciatic nerves; and 3) sodium currents of small and large neurons dissociated from rat dorsal root ganglia. Our overall results lead us to conclude that action potentials in A-fibers of the rodent sciatic nerve are mediated primarily by NaV1.6 associated with NaVβ2 or NaVβ4.