The mammalian rod synaptic ribbon is essential for Cav channel facilitation and ultrafast fusion of the readily releasable pool of vesicles
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 rods 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 {tau} < 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.