Cross Talk between Metabotropic and Ionotropic Glutamate Receptor-Mediated Signaling in Parallel Fiber-Induced Inositol 1,4,5-Trisphosphate Production in Cerebellar Purkinje Cells

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Abstract
In many excitatory glutamatergic synapses, both ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs) are closely distributed on the postsynaptic membrane. However, the functional significance of the close distribution of the two types of glutamate receptors has not been fully clarified. In this study, we examined the functional interaction between iGluR and mGluR at parallel fiber (PF)→ Purkinje cell synapses in the generation of inositol 1,4,5-trisphosphate (IP3), a key second messenger that regulates many important cellular functions. We visualized local IP3dynamics in Purkinje cells using the green fluorescent protein-tagged pleckstrin homology domain (GFP-PHD) as a fluorescent IP3probe. Purkinje cells were transduced with Sindbis virus encoding GFP-PHD and imaged with a two-photon laser scanning microscope. Translocation of GFP-PHD from the plasma membrane to the cytoplasm attributable to an increase in IP3concentration was observed on PF stimulation in fine dendrites of Purkinje cells. Surprisingly, this PF-induced IP3production was blocked not only by the group I mGluR antagonist but also by the AMPA receptor (AMPAR) antagonist. The PF-induced IP3production was blocked by either the inhibition of G-protein activation by GDP-βS or intracellular Ca2+buffering by BAPTA. These results show that IP3production is mediated cooperatively by group I mGluR and AMPAR through G-protein activation and Ca2+influx at PF→ Purkinje cell synapses, identifying the robust cross talk between iGluR and mGluR for the generation of IP3signals.