The TTX metabolite 4,9-anhydro-TTX is a highly specific blocker of the Na(v1.6) voltage-dependent sodium channel.
The blocking efficacy of 4,9-anhydro-TTX (4,9-ah-TTX) 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(v1.6), 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 IC(50) values for TTX/4,9-ah-TTX were as follows (in nmol/l): 7.8 +/- 1.3/1,260 +/- 121 (Na(v1.2)), 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 +/- 1.4/1,270 +/- 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 TTX-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.
Rosker
Christian
C
Institute for Biophysics, Center for Physiological Medicine, Medical University of Graz, Harrachgasse 21/4, Graz, Austria.
Lohberger
Birgit
B
Hofer
Doris
D
Steinecker
Bibiane
B
Quasthoff
Stefan
S
Schreibmayer
Wolfgang
W
D000818:Animals; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D008845:Microinjections; D009124:Muscle Proteins / Q000378:metabolism; D062551:NAV1.2 Voltage-Gated Sodium Channel; D062552:NAV1.3 Voltage-Gated Sodium Channel; D062554:NAV1.5 Voltage-Gated Sodium Channel; D062557:NAV1.6 Voltage-Gated Sodium Channel; D062556:NAV1.7 Voltage-Gated Sodium Channel; D062559:NAV1.8 Voltage-Gated Sodium Channel; D009419:Nerve Tissue Proteins / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D009865:Oocytes; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D026941:Sodium Channel Blockers / Q000378:metabolism / Q000494:pharmacology*; D015222:Sodium Channels / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D013779:Tetrodotoxin / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D014982:Xenopus laevis
D009124:Muscle Proteins; D062551:NAV1.2 Voltage-Gated Sodium Channel; D062552:NAV1.3 Voltage-Gated Sodium Channel; D062554:NAV1.5 Voltage-Gated Sodium Channel; D062557:NAV1.6 Voltage-Gated Sodium Channel; D062556:NAV1.7 Voltage-Gated Sodium Channel; D062559:NAV1.8 Voltage-Gated Sodium Channel; D009419:Nerve Tissue Proteins; C568317:SCN10A protein, human; C568251:SCN2A protein, human; C568320:SCN5A protein, human; C486866:SCN9A protein, human; C568318:Scn10a protein, mouse; C568319:Scn10a protein, rat; C568253:Scn2A protein, rat; C000654500:Scn2a protein, mouse; C478638:Scn3a protein, rat; C466180:Scn4a protein, rat; C568321:Scn5a protein, mouse; C568322:Scn5a protein, rat; C491046:Scn8a protein, mouse; D026941:Sodium Channel Blockers; D015222:Sodium Channels; C047739:anhydrotetrodotoxin; D013779:Tetrodotoxin; D012964:Sodium