Ca2+ Diffusion through Endoplasmic Reticulum Supports Elevated Intraterminal Ca2+ Levels Needed to Sustain Synaptic Release from Rods in Darkness

J Neurosci. 2015 Aug 12;35(32):11364-73. doi: 10.1523/JNEUROSCI.0754-15.2015.


In addition to vesicle release at synaptic ribbons, rod photoreceptors are capable of substantial slow release at non-ribbon release sites triggered by Ca(2+)-induced Ca(2+) release (CICR) from intracellular stores. To maintain CICR as rods remain depolarized in darkness, we hypothesized that Ca(2+) released into the cytoplasm from terminal endoplasmic reticulum (ER) can be replenished continuously by ions diffusing within the ER from the soma. We measured [Ca(2+)] changes in cytoplasm and ER of rods from Ambystoma tigrinum retina using various dyes. ER [Ca(2+)] 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 Ca(2+) from terminal ER, followed by a decline in somatic ER [Ca(2+)]. Local activation of ryanodine receptors in terminals with a spatially confined puff of ryanodine caused a decline in terminal ER [Ca(2+)], followed by a secondary decrease in somatic ER. Localized photolytic uncaging of Ca(2+) from o-nitrophenyl-EGTA in somatic ER caused an abrupt Ca(2+) increase in somatic ER, followed by a slower Ca(2+) increase in terminal ER. These data suggest that, during maintained depolarization, a soma-to-terminal [Ca(2+)] gradient develops within the ER that promotes diffusion of Ca(2+) ions to resupply intraterminal ER Ca(2+) stores and thus sustain CICR-mediated synaptic release. The ability of Ca(2+) to move freely through the ER may also promote bidirectional communication of Ca(2+) changes between soma and terminal.

Significance statement: Vertebrate rod and cone photoreceptors both release vesicles at synaptic ribbons, but rods also exhibit substantial slow release at non-ribbon sites triggered by Ca(2+)-induced Ca(2+) release (CICR). Blocking CICR inhibits >50% of release from rods in darkness. How do rods maintain sufficiently high [Ca(2+)] in terminal endoplasmic reticulum (ER) to support sustained CICR-driven synaptic transmission? We show that maintained depolarization creates a [Ca(2+)] gradient within the rod ER lumen that promotes soma-to-terminal diffusion of Ca(2+) to replenish intraterminal ER stores. This mechanism allows CICR-triggered synaptic release to be sustained indefinitely while rods remain depolarized in darkness. Free diffusion of Ca(2+) within the ER may also communicate synaptic Ca(2+) changes back to the soma to influence other critical cell processes.

Keywords: calcium imaging; calcium-induced calcium release; endoplasmic reticulum; retina; rod photoreceptors; synaptic terminal.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Ambystoma
  • Animals
  • Calcium / metabolism*
  • Calcium Signaling / drug effects
  • Calcium Signaling / physiology*
  • Darkness*
  • Endoplasmic Reticulum / drug effects
  • Endoplasmic Reticulum / metabolism*
  • Female
  • Male
  • Retinal Rod Photoreceptor Cells / drug effects
  • Retinal Rod Photoreceptor Cells / metabolism*
  • Ryanodine / pharmacology
  • Ryanodine Receptor Calcium Release Channel / metabolism
  • Synapses / drug effects
  • Synapses / physiology*
  • Synaptic Transmission / drug effects
  • Synaptic Transmission / physiology


  • Ryanodine Receptor Calcium Release Channel
  • Ryanodine
  • Calcium