Activation of Endoplasmic Reticulum-Localized Metabotropic Glutamate Receptor 5 (mGlu₅) Triggers Calcium Release Distinct from Cell Surface Counterparts in Striatal Neurons

Metabotropic glutamate receptor 5 (mGlu₅) plays a fundamental role in synaptic plasticity, potentially serving as a therapeutic target for various neurodevelopmental and psychiatric disorders. Previously, we have shown that mGlu₅ can also signal from intracellular membranes in the cortex, hippocampus, and striatum. Using cytoplasmic Ca²⁺ indicators, we showed that activated cell surface mGlu₅ induced a transient Ca²⁺ increase, whereas the activation of intracellular mGlu₅ mediated a sustained Ca²⁺ elevation in striatal neurons. Here, we used the newly designed ER-targeted Ca²⁺ sensor, ER-GCaMP6-150, as a robust, specific approach to directly monitor mGlu₅-mediated changes in ER Ca²⁺ itself. Using this sensor, we found that the activation of cell surface mGlu₅ led to small declines in ER Ca²⁺, whereas the activation of ER-localized mGlu₅ resulted in rapid, more pronounced changes. The latter could be blocked by the Gq inhibitor FR9000359, the PLC inhibitor U73122, as well as IP₃ and ryanodine receptor blockers. These data demonstrate that like cell surface and nuclear mGlu₅, ER-localized receptors play a pivotal role in generating and shaping intracellular Ca²⁺ signals.