Visual input drives diverse ER calcium signals in neurons <i>in vivo</i>

The endoplasmic reticulum (ER) has long been thought to shape calcium signals in neurons, but stimulus-driven ER calcium fluctuations have not been directly measured <i>in vivo</i>. To measure neuronal ER calcium signals <i>in vivo</i>, we paired visual stimulus presentation with two photon imaging of ER and cytosolic calcium reporters in four different cell types in the <i>Drosophila</i> visual system. We found that visual input elicits diverse ER calcium signals, with the ER acting as a calcium sink or source depending on the cell type, subcellular compartment (dendrite versus axon), and type of visual stimulus. ER calcium signals were not simply a reflection of cytosolic signals, indicating that the ER, rather than acting as a passive calcium buffer, actively processes calcium signals in neurons in a context-specific fashion. Thus, ER-based signal processing may contribute to functional diversity across neuronal cell types, thereby enhancing the computational capacity of neural circuits.