Methyl bromide decreases excitability without having immediate toxic effects in rat hippocampal CA1 neurons in vitro

Methyl bromide, a disinfectant gas amply used worldwide, is neurotoxic in h umans and other mammals. To study its short-term effects on neurons, it was applied in aqueous solution to hippocampal slices of young rats (1.4 and 0 .7 mM, for 8 minutes). Extracellular field recordings and intracellular mic roelectrode recordings from CA1 pyramidal neurons showed that the neurons s tay viable for at least one hour after application of the mono-halomethane. However, a moderate, but consistent, irreversible decrease in synaptic exc itability was observed. The intracellular recordings indicate that this may be attributed to a decrease in excitatory postsynaptic potentials. No effe cts were observed at 0.7 mM methyl bromide. Bromide, in a dose-dependent, p artly reversible manner (during one hour), produced a similar decrease in e xcitability. Quantitatively, the action of bromide at 0.5 mM resembled the one seen with methyl bromide at the concentration of 1.4 mM. Since methyl b romide did not induce electrophyiologic changes consistent with evidence of neurotoxicity during one hour of observation it is concluded that it lacks immediate toxic effects on hippocampal rat neurons. Its neurotoxicity may be entirely due to metabolites or other indirect effects. The slight decrea se in excitability may be due to the effect of bromide that is set free as tissue proteins and other cell molecules are methylated. (C) 1999 Inter Pre ss, Inc.