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Light regulation of Ca2+ in the cone photoreceptor synaptic terminal

Published online by Cambridge University Press:  01 September 2008

SUE-YEON CHOI ,
SKYLER JACKMAN ,
WALLACE B. THORESON  and
RICHARD H. KRAMER
SUE-YEON CHOI
Affiliation:
Department of Molecular and Cell Biology, University of California, Berkeley, California
SKYLER JACKMAN
Affiliation:
Department of Physics, University of California, Berkeley, California
WALLACE B. THORESON
Affiliation:
Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, Nebraska
RICHARD H. KRAMER*
Affiliation:
Department of Molecular and Cell Biology, University of California, Berkeley, California
*
*Address correspondence to: Richard H. Kramer, Department of Molecular and Cell Biology, University of California, 121 Life Sciences Addition, Berkeley, CA 94720-3200. E-mail: rhkramer@berkeley.edu
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Abstract

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 ~360 nM in darkness and falls to ~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 μM 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.

Keywords

ConePhotoreceptorCa2+Synaptic transmissionExocytosis
Type
Brief Communications
Information
Visual Neuroscience , Volume 25 , Issue 5-6 , September 2008 , pp. 693 - 700
Copyright
Copyright © Cambridge University Press 2008

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