title: A novel drug binding site on voltage-gated sodium channels in rat brain.
abstract: The effectiveness of several antiepileptic, analgesic, and neuroprotective drugs is attributable to state-dependent inhibition of voltage-gated sodium channels. To help characterize their site and mode of action on sodium channels, a member of the lamotrigine family, R-(-)-2,4-diamino-6-(fluromethyl)-5-(2,3,5-trichlorophenyl)-pyrimidine (BW202W92), was radiolabeled and used as a binding ligand in rat forebrain synaptosomes. Although the level of specific [(3)H]BW202W92 binding in a standard incubation medium was relatively poor, low concentrations of tetrodotoxin (EC(50) = 2-3 nM) greatly enhanced the binding, apparently by increasing the affinity of the binding sites. Tetrodotoxin-dependent binding was stereoselective (the less active enantiomer, S-(-)-2,4-diamino-6-(fluromethyl)-5-(2,3,5-trichlorophenyl)-pyrimidine (BW203W92), was up to 30-fold less potent, depending on conditions) and was extremely sensitive to inhibition by raised K(+) concentration (IC(50) = 5.9 mM), an effect that was ascribed to changes in membrane potential. In addition, the binding was inhibited by sodium channel neurotoxins acting on sites 3 and 4, but it was resistant to batrachotoxin (site 2) and brevetoxin (site 5). Several drugs acting on sodium channels displaced tetrodotoxin-dependent [(3)H]BW202W92 binding, and most of those tested showed different affinities under depolarized (100 mM K(+)) and polarized (1 mM K(+)) conditions. In a subset of compounds for which data were available, binding affinity in depolarized synaptosomes correlated well with apparent affinity for the inactivated state of sodium channels. The [(3)H]BW202W92 binding site is novel and is likely to represent a pharmacologically important site of action of drugs on voltage-gated sodium channels in the brain.
authors.affiliation: Wolfson Institute for Biomedical Research, University College London, Gower St., London WC1E 6BT, UK.
authors.forename: Dieter R
authors.initials: DR
authors.lastname: Riddall
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Leach
authors.forename: John
authors.initials: J
authors.lastname: Garthwaite
mesh_terms: D000779:Anesthetics, Local / Q000494:pharmacology*; D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology*; D001665:Binding Sites; D001921:Brain / Q000187:drug effects / Q000378:metabolism*; D015640:Ion Channel Gating; D008297:Male; D011188:Potassium / Q000378:metabolism; D011743:Pyrimidines / Q000378:metabolism / Q000494:pharmacology; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000378:metabolism*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: C507625:2,4-diamino-6-(fluoromethyl)-5-(2,3,5-trichlorophenyl)pyrimidine; D000779:Anesthetics, Local; D000927:Anticonvulsants; D011743:Pyrimidines; D015222:Sodium Channels; D013779:Tetrodotoxin; D012964:Sodium; D011188:Potassium