Effects of sequential exposure to multiple concentrations of methylmercuryin the rat hippocampal slice

Two experiments explored the effects of sequential exposure to multiple con centrations of methylmercury (MeHg) on rat hippocampal slice synaptic trans mission and excitability in area CA1. When hippocampal slices mere exposed to 0.1, 1, 10, and 100 muM MeHg chloride in successive 30-min exposures, Me Hg produced an increase in excitability over baseline levels throughout the 1 muM exposure and the first 5 min of the 10 muM exposure, followed by pro found suppression of excitability at the 100 muM level. When hippocampal sl ices were exposed to 10, 25, 50, 75, and 100 muM concentrations, MeHg produ ced an increase in excitability throughout most of the 10 and 25 muM exposu res, followed by profound suppression of excitability at the 50 muM level o f exposure. In both series of concentrations, MeHg suppressed local inhibit ory systems prior to suppressing excitatory systems. In a third experiment, a single exposure of 50 muM MeHg suppressed both presynaptic and postsynap tic responses recorded in stratum radiatum with the same time course, sugge sting that the observed suppressive effects of MeHg were not primarily syna ptic. (C) 2000 Academic Press.