Conantokin-G: a novel peptide antagonist to the N-methyl-D-aspartic acid (NMDA) receptor.

Conantokin-G is a 17 amino acid peptide isolated from the venom of the fish-eating snail Conus geographus which produces hyperactivity when injected into the brains of adult mice. We show that this peptide is a selective N-methyl-D-aspartate (NMDA) antagonist based on its ability to block NMDA-induced elevation of cGMP in rat cerebellar slices in vitro (IC50 = 171 nM), but not kainic acid-induced elevations. This inhibition could not be overcome by increasing the NMDA concentration, indicating non-competitive inhibition. Conantokin-G displayed no affinity for binding sites for thienylcyclohexylpiperidine, various glutamate subclasses or those for several other neurotransmitters/neuromodulators. This peptide, however, enhanced [3H]glycine binding to rat forebrain membranes but not to spinal cord membranes. The activity profile of the peptide in various assays indicates that it is a novel type of non-competitive NMDA antagonist.

L-beta-methylaminoalanine inhibits [3H]glutamate binding in the presence of bicarbonate ions.

We examined the ability of the neurotoxin, L-beta-methylaminoalanine (L-BMAA), to inhibit [3H]glutamate binding to rat brain synaptic junctions. In a tris(hydroxymethyl)aminomethane acetate buffer, L-BMAA did not affect [3H]glutamate binding (IC50 greater than 10 mM). However, in the presence of ammonium bicarbonate (20 mM) L-BMAA blocked [3H]glutamate binding with an IC50 of 1 mM. This inhibition was not caused by ammonium ion since other ammonium salts were inactive. Furthermore, identical inhibition was obtained in the presence of potassium bicarbonate. Bicarbonate ion did not alter the ability of N-methyl-D-aspartic acid to block glutamate binding. These results indicate that bicarbonate ion is required for the interaction of L-BMAA with the glutamate receptor and may account for the observation that beta-methylaminoalanine is neurotoxic in vitro only in the presence of bicarbonate.