title: Picrotoxin blockade of invertebrate glutamate-gated chloride channels: subunit dependence and evidence for binding within the pore.
abstract: Glutamate-gated chloride channels have been described in nematodes, insects, crustaceans, and mollusks. Subunits from the nematode and insect channels have been cloned and are phylogenetically related to the GABA and glycine ligand-gated chloride channels. Ligand-gated chloride channels are blocked with variable potency by the nonselective blocker picrotoxin. The first two subunits of the glutamate-gated chloride channel family, GluClalpha and GluClbeta, were cloned from the free living nematode Caenorhabditis elegans. In this study, we analyze the blockade of these novel channels by picrotoxin. In vitro synthesized GluClalpha and GluClbeta RNAs were injected individually or coinjected into Xenopus oocytes. The EC50 values for picrotoxin block of homomeric GluClalpha and GluClbeta were 59 microM and 77 nM, respectively. Picrotoxin block of homomeric GluClbeta channels was promoted during activation of membrane current with glutamate. In addition, recovery from picrotoxin block was faster during current activation by glutamate. A chimeric channel between the N-terminal extracellular domain of GluClalpha and the C-terminal membrane-spanning domain of GluClbeta localized the higher affinity picrotoxin binding site to the membrane-spanning domains of GluClbeta. A point mutation within the M2 membrane-spanning domain of GluClbeta reduced picrotoxin sensitivity >10,000-fold. We conclude that picrotoxin blocks GluCl channels by binding to a site accessible when the channel is open.
authors.affiliation: Department of Cell Biochemistry and Physiology, Merck Research Laboratories, Rahway, New Jersey, USA.
authors.forename: A
authors.initials: A
authors.lastname: Etter
authors.forename: D F
authors.initials: DF
authors.lastname: Cully
authors.forename: K K
authors.initials: KK
authors.lastname: Liu
authors.forename: B
authors.initials: B
authors.lastname: Reiss
authors.forename: D K
authors.initials: DK
authors.lastname: Vassilatis
authors.forename: J M
authors.initials: JM
authors.lastname: Schaeffer
authors.forename: J P
authors.initials: JP
authors.lastname: Arena
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000969:Antinematodal Agents / Q000494:pharmacology; D001665:Binding Sites / Q000502:physiology; D017173:Caenorhabditis elegans; D018118:Chloride Channels / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D004347:Drug Interactions; D004351:Drug Resistance; D004594:Electrophysiology; D018756:GABA Antagonists / Q000494:pharmacology*; D018698:Glutamic Acid / Q000494:pharmacology*; D015640:Ion Channel Gating / Q000187:drug effects*; D007559:Ivermectin / Q000494:pharmacology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008969:Molecular Sequence Data; D009865:Oocytes / Q000502:physiology; D010852:Picrotoxin / Q000494:pharmacology*; D017354:Point Mutation; D017434:Protein Structure, Tertiary; D014981:Xenopus
chemical_list: D000969:Antinematodal Agents; D018118:Chloride Channels; D018756:GABA Antagonists; D018698:Glutamic Acid; D007559:Ivermectin; D010852:Picrotoxin