METHYL MERCURY INCREASES INTRACELLULAR CA-2+ AND INOSITOL PHOSPHATE LEVELS IN CULTURED CEREBELLAR GRANULE NEURONS

In an effort to explain the previously observed methyl mecury (MeHg)-i nduced stimulation of protein phosphorylation in cerebellar granule ne uron cultures, the effect of MeHg on protein kinase activities in cell -free assays and on second messenger systems in cultured neurons has b een examined. Using cell-free assays for several protein kinases, no s timulation of enzyme activity was found at any concentration of MeHg t ested. After 24 h exposure, 1-5 muM MeHg was found to have no signific ant effect on neuronal cyclic AMP levels. In contrast, intracellular l evels of Ca2+ and rates of Ca-45(2+) uptake were elevated 2.2-fold and 3.6-fold, respectively, by 5 muM MeHg. These effects were not observe d with mercuric chloride, triethyllead, or lead acetate. Measurement o f inositol phosphate production in granule cell cultures revealed a se nsitive, pretoxic effect of MeHg with twofold stimulation following 30 -min exposure to 5 muM MeHg and 1.6-fold after 24-h exposure to 3 muM MeHg. Detection of inositol phosphate production after 30 min of MeHg was largely neuronspecific. These results suggest that second messenge r-mediated activation of select protein kinase enzymes may be the mech anism underlying MeHg-induced stimulation of protein phosphorylation i n cerebellar neuronal culture. In addition, these findings indicate a specific interference with neuronal signal transduction and suggest a basis for the selective neurotoxic action of this agent.