title: Mechanisms, pools, and sites of spontaneous vesicle release at synapses of rod and cone photoreceptors.
abstract: 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 (HCs) 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 (EPSCs) 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 pHrodo. 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.
authors.affiliation: Truhlsen Eye Institute, Department of Ophthalmology & Visual Sciences, 4050 Durham Research Center, University of Nebraska Medical Center, Omaha, NE, 68198-5840, USA.|Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: Karlene M
authors.initials: KM
authors.lastname: Cork
authors.affiliation: Truhlsen Eye Institute, Department of Ophthalmology & Visual Sciences, 4050 Durham Research Center, University of Nebraska Medical Center, Omaha, NE, 68198-5840, USA.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Van Hook
authors.affiliation: Truhlsen Eye Institute, Department of Ophthalmology & Visual Sciences, 4050 Durham Research Center, University of Nebraska Medical Center, Omaha, NE, 68198-5840, USA.|Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: Wallace B
authors.initials: WB
authors.lastname: Thoreson
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D019706:Excitatory Postsynaptic Potentials; D005089:Exocytosis; D005260:Female; D018698:Glutamic Acid / Q000378:metabolism; D008297:Male; D017949:Retinal Cone Photoreceptor Cells / Q000378:metabolism* / Q000502:physiology; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism* / Q000502:physiology; D013572:Synaptic Vesicles / Q000378:metabolism*; D014562:Caudata
chemical_list: D018698:Glutamic Acid; D002118:Calcium
keywords: calcium; exocytosis; retina; ribbon synapse; spontaneous synaptic release; tiger salamander