The Toxic Effects of Formate in Dissociated Primary Mouse Neural Cell Cultures

Abstract

Primary dissociated mouse cerebrocortical cell cultures containing both neurons and glial cells were used as an experimental model to study the neurotoxic effects of formate, the putative toxic metabolite of methanol. Neural cells were isolated and prepared from the cerebral cortex of fetal CD-1 mice on Gestational Day 15. Mature 7- to 15-day-old monolayer cultures were exposed to formate (0 to 240 mM) for 8 hr at 37°C over a range of extracellular pH (6.0 to 7.6). Cytotoxicity was evaluated by histopathology, changes in membrane integrity (lactate dehydrogenase release, LDH; [ 14C]adenine nucleotide leakage), and mitochondrial metabolic activity [reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, MTT]. Similar quantitative estimates of cell injury were obtained by LDH release or [ 14C]adenine nucleotide leakage from prelabeled cells. Exposure of neural cells produced time- and concentration-dependent toxic responses. The concentration of formate that resulted in 50% LDH leakage after an 8-hr incubation was estimated to be 45 mM. As determined by light microscopy, formate (20 to 60 mM) was specifically neuronotoxic, primarily affecting large polygonal neurons. Higher concentrations of formate (≥120 mM) induced nonspecific cytotoxicity. MTT reduction appeared to be a more sensitive endpoint by showing significant toxic effects at 20 mM (8-hr incubation), while significant leakage of LDH occurred only at formate concentrations ≥60 mM. Total intracellular ATP concentration was significantly decreased following a 20 or 40 mM formate exposure for 8 hr. These results are consistent with the hypothesis that formate may inhibit mitochondrial function resulting in decreased intracellular ATP and formate-induced neurotoxicity.