Vol. 292, Issue 1, 425-432, January 2000

In Vitro and In Vivo Characterization of Conantokin-R, a Selective Nmda Receptor Antagonist Isolated from the Venom of the Fish-Hunting Snail Conus radiatus¹

H. Steve White, R. Tyler McCabe, Heather Armstrong, Sean D. Donevan, Lourdes J. Cruz , Fe C. Abogadie, Josep Torres, Jean E. Rivier, Ingo Paarmann, Michael Hollmann and Baldomero M. Olivera

Department of Pharmacology and Toxicology, University of Utah, Salt Lake City, Utah (H.S.W., H.A., S.D.D.); Cognetix Inc., Salt Lake City, Utah (R.T.M.); Department of Biology, University of Utah, Salt Lake City, Utah (L.J.C., F.C.A., B.M.O.); Marine Science Institute, University of the Philippines, Diliman, Philippines (L.J.C.); The Salk Institute, La Jolla, California (J.T., J.E.R.); and Max-Planck Institute for Experimental Medicine, Gottingen, Germany (I.P., M.H.)

The purification, characterization, and synthesis of conantokin-R (Con-R), an N-methyl-D-aspartate (NMDA) receptor peptide antagonist from the venom of Conus radiatus, are described. With the use of well defined animal seizure models, Con-R was found to possess an anticonvulsant profile superior to that of ifenprodil and dizocilpine (MK-801). With voltage-clamp recording of Xenopus oocytes expressing heteromeric NMDA receptors from cloned NR1 and NR2 subunit RNAs, Con-R exhibited the following order of preference for NR2 subunits: NR2B  NR2A > NR2C NR2D. Con-R was without effect on oocytes expressing the -amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR1 or the kainate receptor subunit GluR6. In mouse cortical neurons voltage-clamped at 60 mV, Con-R application produced a slowly developing block of inward currents evoked by 10 µM NMDA and 1 µM glycine (IC₅₀ = 350 nM). At 3 µM, Con-R did not affect -aminobutyric acid- or kainate-evoked currents. Con-R prevented sound-induced tonic extension seizures in the Frings audiogenic seizure-susceptible mice at i.c.v. doses below toxic levels. It was also effective at nontoxic doses in CF#1 mice against tonic extension seizures induced by threshold (15 mA) and maximal (50 mA) stimulation, and it partially blocked clonic seizures induced by s.c. pentylenetetrazol. In contrast, MK-801 and ifenprodil were effective only at doses approaching (audiogenic seizures) or exceeding (electrical and pentylenetetrazol seizures) those required to produce significant behavioral impairment. These results indicate that the subtype selectivity and other properties of Con-R afford a distinct advantage over the noncompetitive NMDA antagonists MK-801 and ifenprodil. Con-R is a useful new pharmacological agent for differentiation between the anticonvulsant and toxic effects of NMDA antagonists.