Conantokin G is an NR2B-selective competitive antagonist of N-methyl-D-aspartate receptors

Conantokin G (Con G) is a 17-amino-acid peptide antagonist of N-methyl-D-as partate (NMDA) receptors isolated from the venom of the marine cone snail, Conus geographus. The mechanism of action of Con G has not been well define d; both competitive and noncompetitive interactions with the NMDA-binding s ite have been proposed. In this study the mechanism of action and subunit s electivity of Con G was examined in whole-cell voltage-clamp recordings fro m cultured neurons and in two electrode voltage-clamp recordings from Xenop us oocytes expressing recombinant NMDA receptors. Con G was a potent and se lective antagonist of NMDA-evoked currents in murine cortical neurons (IC50 = 480 nM). The slow onset of Con G block could be prevented by coapplicati on with high concentrations of NMDA or of the competitive antagonist (RS)-3 -(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid. Furthermore, in oocyt es expressing NR1a/NR2B receptors, Con G produced a rightward shift in the concentration-response curve for NMDA, providing support for a competitive interaction with the NMDA-binding site. Con G produced an apparent noncompe titive shift in the concentration-response curve for spermine potentiation of NMDA responses, but this was due to spermine-induced enhancement of Con G block. Spermine produced a similar enhancement of DL-2-amino-S-phosphopen tanoic acid block. Finally, Con G selectively blocked NMDA receptors contai ning the NR2B subunit. These results demonstrate that Con G is a subunit-sp ecific competitive antagonist of NMDA receptors. The unique subunit selecti vity profile of Con G may explain its favorable in vivo profile compared wi th nonselective NMDA antagonists.