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Endogenous calcium buffering at photoreceptor synaptic terminals in salamander retina
Author(s) -
Hook Matthew J.,
Thoreson Wallace B.
Publication year - 2014
Publication title -
synapse
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.809
H-Index - 106
eISSN - 1098-2396
pISSN - 0887-4476
DOI - 10.1002/syn.21768
Subject(s) - ribbon synapse , neurotransmission , biophysics , exocytosis , retina , synaptic vesicle , retinal , synapse , calcium , active zone , neuroscience , chemistry , neurotransmitter , biology , vesicle , biochemistry , receptor , membrane , central nervous system , organic chemistry
Calcium operates by several mechanisms to regulate glutamate release at rod and cone synaptic terminals. In addition to serving as the exocytotic trigger, Ca 2+ accelerates replenishment of vesicles in cones and triggers Ca 2+ ‐induced Ca 2+ release (CICR) in rods. Ca 2+ thereby amplifies sustained exocytosis, enabling photoreceptor synapses to encode constant and changing light. A complete picture of the role of Ca 2+ in regulating synaptic transmission requires an understanding of the endogenous Ca 2+ handling mechanisms at the synapse. We therefore used the “added buffer” approach to measure the endogenous Ca 2+ binding ratio ( κ endo ) and extrusion rate constant ( γ ) in synaptic terminals of photoreceptors in retinal slices from tiger salamander. We found that κ endo was similar in both cell types—∼25 and 50 in rods and cones, respectively. Using measurements of the decay time constants of Ca 2+ transients, we found that γ was also similar, with values of ∼100 s −1 and 160 s −1 in rods and cones, respectively. The measurements of κ endo differ considerably from measurements in retinal bipolar cells, another ribbon‐bearing class of retinal neurons, but are comparable to similar measurements at other conventional synapses. The values of γ are slower than at other synapses, suggesting that Ca 2+ ions linger longer in photoreceptor terminals, supporting sustained exocytosis, CICR, and Ca 2+ ‐dependent ribbon replenishment. The mechanisms of endogenous Ca 2+ handling in photoreceptors are thus well‐suited for supporting tonic neurotransmission. Similarities between rod and cone Ca 2+ handling suggest that neither buffering nor extrusion underlie differences in synaptic transmission kinetics. Synapse 68:518–528, 2014 . © 2014 Wiley Periodicals, Inc.