The TTX metabolite 4,9-anhydro-TTX is a highly specific blocker of the Nav1.6 voltage-dependent sodium channel

The TTX metabolite 4,9-anhydro-TTX is a highly specific blocker of the Na-v1.6 voltage dependent sodium channel. Am J Physiol Cell Physiol 293: C783-C789, 2007. First published May 23, 2007; doi:10.1152/ajpcell.00070.2007.-The blocking efficacy of 4.9-anhydro-TTX (4.9-ah-YFX) and TTX on several isoforms of voltage-dependent sodium channels, expressed in Xenopus laevis oocytes, was tested (Na-v1.2, Na-v1.3 Na-v1.4, Na-v1.5, Na-v1.6, Na-v1.7, and Na-v1.8). Generally. TTX was 40-231 times more effective, when compared with 4.9-ah-TTX, on a given isoform. An exception was Na-v1,Na-6, where 4.9-ah-TTX in nanomole per liter concentrations sufficed to result in substantial block, indicating that 4,9-ah-TTX acts specifically at this peculiar isoform. The IC50 values for TTX/4,9-ah-TTX were as follows (in nmol/l]): 7.8 +/- 1.3/1,260 +/- 121 (Nav,,A 2.8 +/- 2.3/341. 36 (Na-v1.3), 4.5 +/- 1.0/988 +/- 62 (Na-v1.4), 1,970 +/- 565/78,500 +/- 11,600 (Na-v1.5). 3-8 +/- 1.5/7.8 +/- 2.3 (Na-v1.6), 5.5 +/- 11.4/1.270 +/- 251 (Na-v1.7), and 1.330 +/- 459/> 30,000 (Na-v1.8). Analysis of approximal half-maximal doses of both compounds revealed minor effects on voltage-dependent activation only, whereas steady-state inactivation was shifted to more negative potentials by both TTX and 4,9-ah-TTX in the case of the Na-v1.6 subunit, but not in the case of other TFX-sensitive ones. TTX shifted steady-state inactivation also to more negative potentials in case of the TTX-insensitive Na-v1.5 subunit. where it also exerted profound effects on the time course of recovery from inactivation. Isoform-specific interaction of toxins with ion channels is frequently observed in the case of proteinaceous toxins. Although the sensitivity of Na-v1.1 to 4,9-ah-TTX is not known, here we report evidence on a highly isoform-specific TTX analog that may well turn out to be an invaluable tool in research for the identification of Na-v1.6-mediated function, but also for therapeutic intervention.