Abstract
Purpose :
Vision under starlight requires rod photoreceptors to transduce and transmit single photon responses to the visual system. This remarkable sensitivity depends on a small voltage change reliably reducing the rate of glutamate release such that it can be robustly detected by post-synaptic rod bipolar cells. Our goal was to identify how small voltage changes at the rod terminal alter the statistics of vesicle release and how these changes support the transmission of single-photon responses.
Methods :
We characterized voltage-dependent properties of glutamate release in mouse rods by recording presynaptic glutamate transporter anion currents (IA(glu)) in ex vivo retinas.
Results :
Spontaneous vesicle release events at -70 mV were univesicular and occurred at random intervals (i.e., Poisson). However, when rods were voltage-clamped at their normal membrane potential in darkness of −40 mV, release occurred in coordinated multiquantal release events of ~17 vesicles. The rate of these multiquantal events was ~2 Hz and the interevent intervals were 2-to-3-fold more regular than predicted by Poisson statistics. These multiquantal events involved vesicles in the readily releasable pool and were triggered by opening of nearby, ribbon-associated Ca2+ channels. Applying a voltage waveform to mimic single photon voltage responses reduced the likelihood of multivesicular bursts nearly to zero with a rebound increase in release at stimulus offset. Simulations of release dynamics indicate that this regularly timed form of multivesicular release promotes the transmission of single photon responses to post-synaptic rod bipolar cells.
Conclusions :
Regularly timed multivesicular release from rods provides an efficient means of encoding single photon responses, requiring fewer total vesicles to be released per-second than vesicle release governed by Poisson statistics.
This is a 2021 ARVO Annual Meeting abstract.