INTERACTIONS OF GLUTAMATE RECEPTOR AGONISTS COUPLED TO CHANGES IN INTRACELLULAR CA2+ IN RAT CEREBELLAR GRANULE CELLS IN PRIMARY CULTURE

Changes in cytosolic free Ca2+ concentrations in response to glutamate receptor agonists and their interactions were studied in rat cerebellar granule cells grown on cover-slips. The intracellular Ca2+ as measured with fura-2 increased by applying kainate (KA), quisqualate (QU), and N-methyl-D-aspartate (NMDA). The effect of KA could not be blocked by the NMDA receptor blocker 2-amino-5-phosphonovaleric acid (AP5). The KA- and QU-induced increase in intracellular free Ca2+ was also observed in a Na+-free medium, indicating that this response is not secondarily due to the depolarization. The effect of 10-mu-M QU on the KA-induced changes in cytosolic free Ca2+ was additive only at low KA concentrations, but QU at 0.1 mM totally blocked the response to KA. In the presence of 10-mu-M KA, the dose-response curve of QU became biphasic, whereas with 50-mu-M KA, a reduction of the response was seen around 1-100-mu-M QU. The effect of NMDA on the QU-induced response was additive only at low QU concentrations. It is proposed that rat cerebellar granule cells in primary culture express separate receptor-channel complexes for NMDA, QU, and KA, but interactions between agonists for these receptor sites exist. Thus, QU when present at intermediate concentrations seems to interact with the KA type of receptor, causing its desensitization. At high QU concentrations, an interaction of QU with the NMDA receptor site is apparent.