Calmodulin enhances ribbon replenishment and shapes filtering of synaptic transmission by cone photoreceptors

Abstract

At the first synapse in the vertebrate visual pathway, light-evoked changes in photoreceptor membrane potential alter the rate of glutamate release onto second-order retinal neurons. This process depends on the synaptic ribbon, a specialized structure found at various sensory synapses, to provide a supply of primed vesicles for release. Calcium (Ca 2+ ) accelerates the replenishment of vesicles at cone ribbon synapses, but the mechanisms underlying this acceleration and its functional implications for vision are unknown. We studied vesicle replenishment using paired whole-cell recordings of cones and postsynaptic neurons in tiger salamander retinas and found that it involves two kinetic mechanisms, the faster of which was diminished by calmodulin (CaM) inhibitors. We developed an analytical model that can be applied to both conventional and ribbon synapses and showed that vesicle resupply is limited by a simple time constant,  = 1/(Ds), where D is the vesicle diffusion coefficient,  is the vesicle diameter,  is the vesicle density, and s is the probability of vesicle attachment. The combination of electrophysiological measurements, modeling, and total internal reflection fluorescence microscopy of single synaptic vesicles suggested that CaM speeds replenishment by enhancing vesicle attachment to the ribbon. Using electroretinogram and whole-cell recordings of light responses, we found that enhanced replenishment improves the ability of cone synapses to signal darkness after brief flashes of light and enhances the amplitude of responses to higherfrequency stimuli. By accelerating the resupply of vesicles to the ribbon, CaM extends the temporal range of synaptic transmission, allowing cones to transmit higher-frequency visual information to downstream neurons. Thus, the ability of the visual system to encode time-varying stimuli is shaped by the dynamics of vesicle replenishment at photoreceptor synaptic ribbons.

Two Pools of Vesicles Associated with Synaptic Ribbons Are Molecularly Prepared for Release

“…Consistent with this interpretation, stx3pep blocked a component of neurotransmitter release in cone photoreceptors that has been shown previously to correspond to the Ca 2þ -accelerated, calmodulin-dependent recruitment of synaptic vesicles to the synaptic ribbons, followed by their subsequent fusion (Fig. 9) (18,66). Together our data indicate that new SNARE formation is critical for maintaining the supply of fusion-competent vesicles at synaptic ribbons.…”

“…9 D; stx3con: À1.225 5 0.2033 pC/s, n ¼ 14; stx3pep: À1.275 5 0.1652 pC/s, n ¼ 9; p > 0.8). In the presence of stx3con, this limiting slope increased significantly when the pulse strength was increased to À10 mV (p % 0.002, paired t-test) reflecting the Ca 2þ -dependent acceleration of vesicle replenishment to ribbons (18,66). By contrast, in the presence stx3pep, the rate of charge transfer did not increase significantly after increasing pulse strength to À10 mV (p > 0.1).…”

“…9 A). After $1 s of stimulation under control conditions, the RP of vesicles on the ribbon is depleted and then charge transfer grows at a steady rate limited by the rate at which vesicles repopulate the ribbon (16,18,66).…”

The role of PDLIM1, a PDZ‐LIM domain protein, at the ribbon synapses in the chicken retina

“…The dynamics of release and replenishment of neurotransmitter at the photoreceptor synapses could be an important factor in encoding and transmitting the timing of light responses (Snellman et al, ; Van Hook et al, ). Therefore, photoreceptor ribbon synaptic structure depends on the cytoskeleton arrangement, both at the active zone‐related with exocytosis—as well as at the endocytic zone—the periactive zone.…”

A computational framework linking synaptic adaptation to circuit behaviors in the early visual system

“…These detailed biophysical models exhibit improved biophysical interpretability, elegantly depicting the cascade of vesicle movements in presynaptic terminals. However, the design of these biophysical models requires precise knowledge beyond the reach of experimental techniques, such as vesicle resupply processes from the cytoplasm to the RRP (31)(32)(33). In addition, it's infeasible to perform theoretical analysis on these models due to their complicated kinetics, severely preventing them from being widely used as an analytical tool in adaptation research.…”