Methylmercury-induced increase of intracellular Ca2+ increases spontaneous synaptic current frequency in rat cerebellar slices.

The relationship between increased intracellular calcium concentration ([Ca(2+)](i)) and changes in spontaneous synaptic current frequency caused by the neurotoxicant methylmercury (MeHg) was examined in Purkinje cells of cerebellar slices using confocal microscopy and whole-cell recording. MeHg (10-100 microM) stimulated and then suppressed completely the frequency of spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs). Current amplitude was also initially increased. The same MeHg concentrations markedly increased fluorescence of the Ca(2+) indicator Fluo-4 throughout the molecular layer as well as the granule cells. No changes in fluorescence occurred in Purkinje cell soma, although fluorescence increased in their subplasmalemmal shell. Simultaneous confocal imaging and whole-cell recording revealed that time to onset of MeHg-induced increase in fluorescence in the molecular layer correlated with that of increased sEPSC and sIPSC frequency in Purkinje cells. Pretreatment with the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) significantly suppressed the MeHg-induced increase in sIPSC frequency, further suggesting that MeHg-induced elevation of [Ca(2+)](i) is partially responsible for its early stimulatory effects on spontaneous synaptic responses. However when spontaneous synaptic currents ceased with MeHg, Fluo-4 fluorescence remained elevated. Thus synaptic transmission cessation is apparently not related to changes in [Ca(2+)](i). It may result from effects of MeHg on transmitter release or sensitivity of postsynaptic receptors. The lack of effect of MeHg on Purkinje cell somal fluorescence reinforces that they are more resistant to MeHg-induced elevations of [Ca(2+)](i) than other cells, including cerebellar granule cells.