Abstract
Rod photoreceptors (PRs) use ribbon synapses to transmit visual information. To signal 'no light detected' they release glutamate continually to activate post-synaptic receptors. When light is detected glutamate release pauses. How a rod's individual ribbon enables this process was studied here by recording evoked changes in whole-cell membrane capacitance from wild-type and ribbonless (Ribeye-ko) mice. Wild-type rods filled with high (10 mM) or low (0.5 mM) concentrations of the Ca2+-buffer EGTA created a readily releasable pool (RRP) of 87 synaptic vesicles (SVs) that emptied as a single kinetic phase with a τ<0.4 ms. The lower concentration of EGTA accelerated Cav channel opening and facilitated release kinetics. In contrast, ribbonless rods created a much smaller RRP of 22 SVs, and they lacked Cav channel facilitation; however, Ca2+ channel-release coupling remained tight. These release deficits caused a sharp attenuation of rod-driven scotopic light responses. We conclude that the synaptic ribbon facilitates Ca2+-influx and establishes a large RRP of SVs.
Highlights
Animals use their sensory systems to interact with and navigate through their environment, and sensory maps are created for this purpose
The majority of rod somata reside in the outer nuclear layer (ONL) and send a spindly axon to their singular presynaptic terminal in the outer plexiform layer (OPL), which contains an individual synaptic ribbon (Fig. 1A and B)
The minority of rod somata that lack an axon are positioned in the OPL, and they contain the synaptic ribbon within the soma compartment (Fig. 1A)
Summary
Animals use their sensory systems to interact with and navigate through their environment, and sensory maps are created for this purpose. This is especially true for vision, where perception of a real-world scene invariably asserts the location of objects in space. The building blocks for visual percepts originate from a visual field that is often in motion, and can vary greatly in luminance. Vertebrates have evolved complex processes that stabilize the eyes on the visual field (Straka and Baker, 2013), focus images on the back of the eye, and transform light of varying intensities into neural signals (Rivlin-Etzion et al, 2018). The outer most layer is a dense lawn of photoreceptors
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