RATE OF QUANTAL TRANSMITTER RELEASE AT THE MAMMALIAN ROD SYNAPSE

Under scotopic conditions, the mammalian rod encodes either one photon or none within its integration time. Consequently the signal presente d to its synaptic terminal is binary. The synapse has a single active zone that releases neurotransmitter quanta tonically in darkness and p auses briefly in response to a rhodopsin isomerization by a photon. We asked: what minimum tonic rate would allow the postsynaptic bipolar c ell to distinguish this pause from an extra-long interval between quan ta due to the stochastic timing of release? The answer required a mode l of the circuit that included the rod convergence onto the bipolar ce ll and the bipolar cell's signal-to-noise ratio. Calculations from the model suggest that tonic release must be at least 40 quanta/s. This t onic rate is much higher than at conventional synapses where reliabili ty is achieved by employing multiple active zones. The rod's synaptic mechanism makes efficient use of space, which in the retina is at a pr emium.