Effects of Sequential Exposure to Multiple Concentrations of Methylmercury in the Rat Hippocampal Slice

Two experiments explored the effects of sequential exposure to multiple concentrations of methylmercury (MeHg) on rat hippocampal slice synaptic transmission and excitability in area CA1. When hippocampal slices were exposed to 0.1, 1, 10, and 100 μM MeHg chloride in successive 30-min exposures, MeHg produced an increase in excitability over baseline levels throughout the 1 μM exposure and the first 5 min of the 10 μM exposure, followed by profound suppression of excitability at the 100 μM level. When hippocampal slices were exposed to 10, 25, 50, 75, and 100 μM concentrations, MeHg produced an increase in excitability throughout most of the 10 and 25 μM exposures, followed by profound suppression of excitability at the 50 μM level of exposure. In both series of concentrations, MeHg suppressed local inhibitory systems prior to suppressing excitatory systems. In a third experiment, a single exposure of 50 μM MeHg suppressed both presynaptic and postsynaptic responses recorded in stratum radiatum with the same time course, suggesting that the observed suppressive effects of MeHg were not primarily synaptic.