β-cyano-L-alanine toxicity: Evidence for the involvement of an excitotoxic mechanism

The legume Vicia sativa (common vetch) harbors the neurotoxic nonprotein amino acid β-cyano-L-alanine (BCLA) and its γ-glutamyl derivative. BCLA elicits hyperexcitability, convulsions, and rigidity in chicks and rats after oral or intraperitoneal administration, but the mechanism of its action is unknown. The effect of different concentrations of BCLA (0.075-10.0 mM) has been investigated in an organotypic tissue culture system. BCLA concentrations of 0.075 and 0.60 mM had no effect, even up to 6 hr. No changes were observed in cultures treated with 1 mM BCLA for 4 hr. BCLA (2.0-10.0 mM) induces concentration-dependent changes in the explants. The explants display neuronal vacuolation, chromatin clumping, and dense shrunken cells, a pathological response generally seen with excitotoxin. MK-801 (35 μM), which blocks the open ion channel associated with the N-methyl-D-aspartate (NMDA) class of glutamate receptors, attenuates the neurotoxic property of BCLA, while the non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (10-20 μM), provides no significant protection. Treatment of isolated mouse brain mitochondria with up to 5 mM BCLA had no inhibitory effect on the activity of NADH-dehydrogenase (complex 1) or cytochrome c oxidase (complex IV), a cyanide-sensitive enzyme. These results suggest that the neurotoxicity of BCLA (or derivative) is mediated directly or indirectly through NMDA receptors.