Calmodulin promotes vesicle replenishment at photoreceptor ribbon synapses, enabling cones to transmit higher-frequency visual information.

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 Ca2+ 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 Ca2+ influx by lowering extracellular Ca2+, by application of Cd2+ and other agents reduced the frequency of mEPSCs but did not eliminate them, indicating that mEPSCs can occur independently of Ca2+. We also measured release presynaptically from rods and cones by examining quantal glutamate transporter anion currents. Presynaptic quantal event frequency was reduced by Cd2+ or by increased intracellular Ca2+ 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 EPSCs suggesting a possible separation between vesicle pools in evoked and spontaneous exocytosis. We mapped sites of Ca2+-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 Ca2+-independent spontaneous release at continuously active photoreceptor synapses remains unclear, but the low frequency of spontaneous quanta limits their impact on noise.

In addition to recycling vesicles, synaptic endocytosis restores release site competence after exocytosis. Wen et al. show that endocytosis is essential for subsequent fusion but not docking of vesicles at photoreceptor ribbon synapses and for maintaining release at modest frequencies.