GLC-3: a novel fipronil and BIDN-sensitive, but picrotoxinin-insensitive, L-glutamate-gated chloride channel subunit from Caenorhabditis elegans

1 We report the cloning and expression of a novel Caenorhabditis elegans po lypeptide, GLC-3, with high sequence identity to previously cloned L-glutam ate-gated chloride channel subunits from nematodes and insects. 2 Expression of glc-3 cRNA in Xenopus oocytes resulted in the formation of homo-oligomeric L-glutamate-gated chloride channels with robust and rapidly desensitizing currents, an EC50 of 1.9 +/- 0.03 mM and a Hill coefficient of 1.5 +/- 0.1. GABA, glycine, histamine and NMDA all failed to activate th e GLC-3 homo-oligomer at concentrations of 1 mM. The anthelminthic, ivermec tin, directly and irreversibly activated the L-glutamate-gated channel with an EC50 of 0.4 +/- 0.02 muM. 3 The GLC-3 channels were selective for chloride ions, as shown by the shif t in the reversal potential for L-glutamate-gated currents after the reduct ion of external Cl- from 107.6 to 62.5 mM. 4 Picrotoxinin failed to inhibit L-glutamate agonist responses at concentra tions up to 1 mM. The polycyclic dinitrile, 3,3-bis-trifluoromethyl-bicyclo [2,2,1]heptane-2,2-dicarbonitrile (BIDN), completely blocked L-glutamate-in duced chloride currents recorded from oocytes expressing GLC-3 with an IC50 of 0.2 +/- 0.07 muM. The phenylpyrazole insecticide, fipronil, reversibly inhibited L-glutamate-gated currents recorded from the GLC-3 receptor with an IC50 of 11.5 +/- 0.11 muM. 5 In this study, we detail the unusual antagonist pharmacology of a new Glu Cl subunit from C. elegans. Unlike all other native and recombinant nematod e GluCl reported to date, the GLC-3 receptor is insensitive to picrotoxinin , but is sensitive to two other channel blockers, BIDN and fipronil. Furthe r study of this receptor may provide insights into the molecular basis of n on-competitive antagonism by these compounds.