L-homocysteate is a potent neurotoxin on cultured cortical neurons

L-Homocysteate (L-HCA) neurotoxicity was quantitatively studied in dissociated cell cultures prepared from the fetal mouse neocortex. Five minute exposure to 3 microM-1 mM L-HCA was associated with neuronal cell loss, but not glial cell loss; the extent of neuronal damage was dependent on the concentration of L-HCA, with an ED50 of approximately 40 microM. The stereoisomer D-HCA was a somewhat weaker neurotoxin than L-HCA. Ion substitution experiments suggested that L-HCA neurotoxicity can be separated into two components on the basis of differences in time course and ionic dependence: an acute, sodium-dependent 'excitotoxic' component, marked by rapid early cell swelling; and a late, calcium-dependent component, marked by delayed cell degeneration. L-HCA neurotoxicity could be attenuated by 2-amino-5-phosphonovalerate (APV), ketamine, and kynurenate, but not by L-glutamate diethyl ester or gamma-D-glutamylaminomethyl sulfonate, consistent with a predominant involvement of N-methyl-D-aspartate receptors. APV and ketamine produced different effects on the L-HCA concentration-toxicity relation, the former drug consistent with a competitive, and the latter drug consistent with a non-competitive, mechanism of antagonism.