METABOTROPIC AND IONOTROPIC TRANSDUCERS OF GLUTAMATE SIGNAL INVERSELYCONTROL CYTOPLASMIC CA2- PIVOTAL ROLE OF PROTEIN-KINASE-C( CONCENTRATION AND EXCITOTOXICITY IN CULTURED CEREBELLAR GRANULE CELLS )

We investigated the functional role of metabotropic glutamate receptor s (mGluRs) in modulating glutamate-affected neuronal intracellular cal cium concentration ([Ca2+](i)) and cell viability in rat cerebellar gr anule cells. The mGluR agonist trans-1-aminocyclopentane-1,3-dicarboxy lic acid (tACPD) induced a transient increase in [Ca2+](i), which seem ed to be developmentally regulated and maximal at 4 days in vitro. In addition, tACPD significantly prevented the [Ca2+](i) rise produced by glutamate or by N-methyl-D-aspartate. The mGluR antagonists L-2-amino -3-phosphonopropionic and (+)-alpha-methyl-4-carboxyphenyl-glycine blo cked the effects of tACPD, but intrinsically, they magnified the gluta mate-mediated [Ca2+](i) elevation. The tACPD-mediated decrease in [Ca2 +](i) rise occurred under ex perimental conditions superimposable on t hose producing neuroprotection in glutamate-exposed cultures. tACPD af fected neither [Ca2+](i) elevation due to KCl nor that evoked by the c alcium ionophore A 23187. The inhibitory effect of tACPD was also unaf fected by K+ channel blockade produced by tetraethylammonium. The tACP D effects were fully mimicked by quisqualate and (RS)-3,5-dihydroxyphe nylglycine, whereas they were only partially reproduced by (2S,1'S,2'S )-2-carboxycyclopropyl-glycine. L-2-Amino-4-phosphonobutyrate was inac tive in preventing glutamate-mediated [Ca2+](i) rise and neurotoxicity . The tACPD inhibitory responses seemed to be highly sensitive to prot ein kinase C blockade by bisindolylmaleimide or staurosporine, whereas they were weakly affected by the cAMP analogue dibutyryl cAMP. The pr otein kinase C activator 4 beta-phorbol-12,13-dibutyrate reproduced mG luR-mediated inhibition of both glutamate-induced [Ca2+](i) rise and n eurotoxicity. In summary, these data suggest that activation of mGluR1 -5 subtypes reduce glutamate-mediated [Ca2+](i) rise through a mechani sm involving protein kinase C activation. Such an effect results in ne uroprotection.