The reduced unsubstituted pteroate moiety is required for folate toxicity of cultured cerebellar granule neurons

The proconvulsive and putative excitotoxic properties of folates led us to study folate toxicity in cultured rat cerebellar granule neurons. Exposure of these neurons to tetrahydrofolate (THF) (EC₅₀ = 263 ± 12 μM) and dihydrofolate (EC₅₀ = 709 ± 29 μM), but not to folate, formyl-THF, methyl-THF or methotrexate, resulted in a concentration-dependent neurotoxicity. THF and dihydrofolate toxicity were characterized by prominent neuronal swelling and early loss of neurite networking and could not be blocked by glutamate receptor antagonists, L-channel calcium blockers or anticonvulsants. Cleavage of THF into glutamate and tetrahydropteroate by glutamate carboxypeptidase in the presence of 1 μM MK-801 and 5 μM 6- cyano-7-nitroquinoxaline-2,3,dione shifted the concentration-toxicity curve to the left (EC₅₀ = 47 ± 4 μM). These results suggest (1) that the glutamate moiety is not necessary for folate toxicity, (2) that the unsubstituted reduced pteroate moiety is required for folate toxicity and (3) that unsubstituted reduced pteroates are previously unrecognized neurotoxins. Because there is no endogenous glutamate carboxypeptidase activity in cerebellar granule neuron cultures and no increase of glutamate levels in the medium of cultures exposed to THF or dihydrofolate, pteroate metabolites are unlikely to mediate the THF or dihydrofolate toxicity of cultured cerebellar granule neurons.