Neuromodulators adjust sensory processing and synapses are key control sites for such plasticity. Less clear is how neuromodulation alters the amount of information that synapses transmit through a circuit. To understand the operation of the “vesicle code” the experimenter needs to isolate signals from individual active zones with a resolution of single vesicles while observing or controlling the incoming signal. This has not been possible using electrophysiology but can now be achieved by multiphoton imaging of the glutamate reporter iGluSnFR in the retina of larval zebrafish. This approach reveals that the visual message transmitted from bipolar cells does not use a simple binary code but is instead composed of a number of symbols, composed of one, two, three or more vesicles released as one event. Here we demonstrate that this strategy of coding by amplitude as well as rate is under diurnal control, contributing to a four-fold variation in the Shannon information transmitted at individual active zones. Dopamine contributes to this increase in information transfer by adjusting at least four synaptic properties; the number of vesicles released by a stimulus, spontaneous synaptic noise, the variability of stimulus-driven responses and the balance between univesicular and multivesicular release. By increasing the probability of multivesicular events with larger information content, dopamine also increases the efficiency of transmission quantified as bits per vesicle. The relative contributions of these various mechanisms differs between ON and OFF pathways: reduced variability of synaptic responses and increased emphasis on MVR increased information transfer through the ON pathway without an increase in synaptic gain. This study provides a quantitative understanding of the different mechanisms by which neuromodulators alter the flow of sensory information and highlights the importance of experimental measurements of the statistics of the vesicle code for understanding how information is transferred between neurons.