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

We investigated the identities of the isoforms of the alpha (Na(V)1)-and beta (Na-V beta)subunits of voltage-gated sodium channels, including those responsible for action potentials in rodent sciatic nerves. To examine mu-subunits, we used seven mu-conotoxins, which target site 1 of the channel. With the use of exogenously expressed channels, we show that two of the mu-conotoxins, mu-BuIIIB and mu-SxIIIA, are 50-fold more potent in blocking Na(V)1.6 from mouse than that from rat. Furthermore, we observed that mu-BuIIIB and mu-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 mu-subunits, we used two synthetic derivatives of the recently discovered mu O -conotoxin GVIIJ that define site 8 of the channel, as previously characterized with cloned rat Na(V)1- and Na-V beta-subunits expressed in Xenopus laevis oocytes, where it was shown that mu O -GVIIJ is a potent inhibitor of several Na(V)1-isoforms and that coexpression of Na-V beta 2 or -beta 4 (but not Na-V beta 1 or -beta 3) totally protects against block by mu O -GVIIJ. We report here the effects of mu O -GVIIJ on 1) sodium currents of mouse Na(V)1.6 coexpressed with various combinations of Na-V beta -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 Na(V)1.6 associated with Na-V beta 2 or Na-V beta 4.