Single-photon absorptions evoke synaptic depression in the retina to extend the operational range of rod vision.

Adaptation or gain control allows sensory neurons to encode diverse stimuli using a limited range of output signals. Rod vision exemplifies a general challenge facing adaptational mechanisms-balancing the benefits of averaging to create a reliable signal for adaptation with the need to adapt rapidly and locally. The synapse between rod bipolar and AII amacrine cells dominates adaptation at low light levels. We find that adaptation occurs independently at each synapse and completes in <500 ms. This limited spatial and temporal integration suggests that the absorption of a single photon modulates gain. Indeed, responses to pairs of brief dim flashes showed directly that synaptic gain was depressed for 100-200 ms following transmission of a single-photon response. Presynaptic mechanisms mediated this synaptic depression. Thus, the division of light into discrete photons controls adaptation at this synapse, and gain varies with the irreducible statistical fluctuations in photon arrival.