Light regulation of Ca2+ in the cone photoreceptor synaptic  terminal.
Retinal cones are depolarized in darkness, keeping voltage-gated Ca2+ channels open and sustaining exocytosis of synaptic vesicles. Light hyperpolarizes the membrane potential, closing Ca2+ channels and suppressing exocytosis. Here, we quantify the Ca2+ concentration in cone terminals, with Ca2+ indicator dyes. Two-photon ratiometric imaging of fura-2 shows that global Ca2+ averages approximately 360 nM in darkness and falls to approximately 190 nM in bright light. Depolarizing cones from their light to their dark membrane potential reveals hot spots of Ca2+ that co-label with a fluorescent probe for the synaptic ribbon protein ribeye, consistent with tight localization of Ca2+ channels near ribbons. Measurements with a low-affinity Ca2+ indicator show that the local Ca2+ concentration near the ribbon exceeds 4 M in darkness. The high level of Ca2+ near the ribbon combined with previous estimates of the Ca2+ sensitivity of release leads to a predicted dark release rate that is much faster than observed, suggesting that the cone synapse operates in a maintained state of synaptic depression in darkness.
Choi
Sue-Yeon
SY
Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3200, USA.
Jackman
Skyler
S
Thoreson
Wallace B
WB
Kramer
Richard H
RH
D000557:Ambystoma; D000818:Animals; D002118:Calcium / Q000502:physiology*; D004594:Electrophysiology; D005089:Exocytosis / Q000502:physiology; D007091:Image Processing, Computer-Assisted; D066298:In Vitro Techniques; D008027:Light; D008116:Lizards; D008564:Membrane Potentials / Q000502:physiology / Q000528:radiation effects; D018613:Microscopy, Confocal; D018408:Patch-Clamp Techniques; D017729:Presynaptic Terminals / Q000502:physiology* / Q000528:radiation effects*; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology* / Q000528:radiation effects*; D009435:Synaptic Transmission / Q000502:physiology
D002118:Calcium