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Mechanisms, pools, and sites of spontaneous vesicle release at synapses of rod and cone photoreceptors
Author(s) -
Cork Karlene M.,
Van Hook Matthew J.,
Thoreson Wallace B.
Publication year - 2016
Publication title -
european journal of neuroscience
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.346
H-Index - 206
eISSN - 1460-9568
pISSN - 0953-816X
DOI - 10.1111/ejn.13288
Subject(s) - ribbon synapse , exocytosis , biophysics , synaptic vesicle , excitatory postsynaptic potential , vesicle , glutamate receptor , neuroscience , extracellular , chemistry , biology , microbiology and biotechnology , biochemistry , inhibitory postsynaptic potential , receptor , membrane , secretion
Photoreceptors have depolarized resting potentials that stimulate calcium‐dependent release continuously from a large vesicle pool but neurons can also release vesicles without stimulation. We characterized the Ca 2+ dependence, vesicle pools, and release sites involved in spontaneous release at photoreceptor ribbon synapses. In whole‐cell recordings from light‐adapted horizontal cells ( HC s) of tiger salamander retina, we detected miniature excitatory post‐synaptic currents (mEPSCs) when no stimulation was applied to promote exocytosis. Blocking Ca 2+ influx by lowering extracellular Ca 2+ , by application of Cd 2+ and other agents reduced the frequency of mEPSCs but did not eliminate them, indicating that mEPSCs can occur independently of Ca 2+ . We also measured release presynaptically from rods and cones by examining quantal glutamate transporter anion currents. Presynaptic quantal event frequency was reduced by Cd 2+ or by increased intracellular Ca 2+ buffering in rods, but not in cones, that were voltage clamped at −70 mV. By inhibiting the vesicle cycle with bafilomycin, we found the frequency of mEPSCs declined more rapidly than the amplitude of evoked excitatory post‐synaptic currents ( EPSC s) suggesting a possible separation between vesicle pools in evoked and spontaneous exocytosis. We mapped sites of Ca 2+ ‐independent release using total internal reflectance fluorescence ( TIRF ) microscopy to visualize fusion of individual vesicles loaded with dextran‐conjugated pH rodo. Spontaneous release in rods occurred more frequently at non‐ribbon sites than evoked release events. The function of Ca 2+ ‐independent spontaneous release at continuously active photoreceptor synapses remains unclear, but the low frequency of spontaneous quanta limits their impact on noise.