The mammalian rod synaptic ribbon is essential for Ca_vchannel facilitation and ultrafast fusion of the readily releasable pool of vesicles

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, and 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) rods. Wild type rods created a readily releasable pool (RRP) of 92 synaptic vesicles (SVs) that emptied as a single kinetic phase with a τ < 0.4 msec. Lowering intracellular Ca2+-buffering accelerated Cav channel opening and facilitated release kinetics, but RRP size was unaltered. In contrast, ribbonless rods created an RRP of 24 SVs, and lacked Cav channel facilitation; however, Ca2+ channel-release coupling remained tight. The release deficits caused a sharp attenuation of rod-driven light responses measured from RIBEYE-ko mice. We conclude that the synaptic ribbon facilitates Ca2+-influx and establishes a large RRP of SVs.

Resolving the molecular architecture of the photoreceptor active zone by MINFLUX nanoscopy

“…Physiological measurements of SV fusion from mouse rods have indicated an RRP of approximately 90 SVs emptied within 1 ms, roughly consistent with the notion of 40 release sites to exist on either side of the ribbon. In addition, exocytosis of these SVs was largely insensitive to the intracellular addition of the slowly binding Ca 2+ chelator EGTA (10 mM) 27 suggesting that the distance of the SV release site to the triggering CaV1.4 Ca 2+ channel(s) is less than 30 nm 57 .…”

“…Approximately 60 -80 SVs are wedged between the ribbon's base and the membrane 25,26 and are likely release-ready, since a similar number of SV fuses within 1 ms upon strong depolarization 27 . Here we established a protocol for immobilizing rod spherules and used MINFLUX nanoscopy to reveal the molecular AZ topography.…”