In vitro and in vivo characterization of conantokin-R, a selective NMDA receptor antagonist isolated from the venom of the fish-hunting snail Conus radiatus

The purification, characterization, and synthesis of conantokin-R (Con-R), an N-methyl-D-aspartate (NMDA) receptor peptide antagonist from the venom o f Conus radiatus, are described. With the use of well defined animal seizur e models, Con-R was found to possess an anticonvulsant profile superior to that of ifenprodil and dizocilpine (MK-801). With voltage-clamp recording o f Xenopus oocytes expressing heteromeric NMDA receptors from cloned NR1 and NR2 subunit RNAs, Con-R exhibited the following order of preference for NR 2 subunits: NR2B approximate to NR2A > NR2C >> NR2D. Con-R was without effe ct on oocytes expressing the alpha-amino-3-hydroxy-5-methylisoxazole-4-prop ionic acid (AMPA) receptor subunit GluR1 or the kainate receptor subunit Gl uR6. In mouse cortical neurons voltage-clamped at -60 mV, Con-R application produced a slowly developing block of inward currents evoked by 10 mu M NM DA and 1 mu M glycine (IC50 5 350 nM). At 3 mu M, Con-R did not affect gamm a-aminobutyric acid- or kainate-evoked currents. Con-R prevented sound-indu ced tonic extension seizures in the Frings audiogenic seizure-susceptible m ice at i.c.v. doses below toxic levels. It was also effective at nontoxic d oses in CF#1 mice against tonic extension seizures induced by threshold (15 mA) and maximal (50 mA) stimulation, and it partially blocked clonic seizu res induced by s.c. pentylenetetrazol. In contrast, MK-801 and ifenprodil w ere effective only at doses approaching (audiogenic seizures) or exceeding (electrical and pentylenetetrazol seizures) those required to produce signi ficant behavioral impairment. These results indicate that the subtype selec tivity and other properties of Con-R afford a distinct advantage over the n oncompetitive NMDA antagonists MK-801 and ifenprodil. Con-R is a useful new pharmacological agent for differentiation between the anticonvulsant and t oxic effects of NMDA antagonists.