Ca²⁺ diffusion through endoplasmic reticulum supports elevated intraterminal Ca²⁺ levels needed to sustain synaptic release from rods in darkness

In addition to vesicle release at synaptic ribbons, rod photoreceptors are capable of substantial slow release at non-ribbon release sites triggered by Ca²⁺-induced Ca²⁺ release (CICR) from intracellular stores. To maintain CICR as rods remain depolarized in darkness, we hypothesized that Ca²⁺ released into the cytoplasm from terminal endoplasmic reticulum (ER) can be replenished continuously by ions diffusing within theERfrom the soma.Wemeasured [Ca²⁺] changes in cytoplasm andERof rods from Ambystoma tigrinum retina using various dyes. ER [Ca²⁺] changes were measured by loading ER with fluo-5N and then washing dye from the cytoplasm with a dye-free patch pipette solution. Small dye molecules diffused within ER between soma and terminal showing a single continuous ER compartment. Depolarization of rods to-40 mV depleted Ca²⁺ from terminal ER, followed by a decline in somatic ER [Ca²⁺]. Local activation of ryanodine receptors in terminals with a spatially confined puff of ryanodine caused a decline in terminal ER [Ca²⁺], followed by a secondary decrease in somatic ER. Localized photolytic uncaging of Ca²⁺ from o-nitrophenyl-EGTA in somatic ER caused an abrupt Ca²⁺ increase in somatic ER, followed by a slower Ca²⁺ increase in terminal ER. These data suggest that, during maintained depolarization, a soma-to-terminal [Ca²⁺] gradient develops within the ER that promotes diffusion of Ca²⁺ ions to resupply intraterminal ER Ca²⁺ stores and thus sustain CICR-mediated synaptic release. The ability of Ca²⁺ to move freely through the ER may also promote bidirectional communication of Ca²⁺ changes between soma and terminal.