Ribbon synapses of photoreceptors and bipolar cells support a continuous cycle of exocytosis and endocytosis (Lagnado et al. 1996), which is modulated by graded changes in membrane potential generated by light. The properties of this vesicle cycle have been studied in dissociated neurons, but less is known about its operation within the retina. To investigate vesicle cycling at ribbon synapses we have made stable lines of transgenic zebrafish expressing SypHy, a genetically encoded fluorescent reporter of exocytosis and endocytosis (Granseth et al. 2006) SypHy comprises a pH-sensitive GFP fused to the second intravesicluar vesicular loop of synaptophysin, a transmembrane protein of synaptic vesicles. Using SypHy, exocytosis is reported as an increase in fluorescence and vesicle reacidification after endocytosis as a decrease. Expression of SypHy was targeted to ribbon synapses using the promoter region of ribeye, the major protein component of the ribbon. SypHy fluorescence was monitored in live zebrafish (anaesthetised by immersion in 0.016% MS222) 10-14 days post fertilisation using multiphoton microscopy, allowing the visualization of terminals through different strata of the inner plexiform layer (IPL) in the retina. We have begun by investigating the encoding visual stimuli by the synaptic output from bipolar cells. Using spatially uniform amber light delivered from darkness and lasting for 30 s, reproducible some terminals become brighter (ON) and some dimmer (OFF). These changes reached a steady state, were reversible and the responding terminals were mainly situated in their expected strata (ON/OFF) within the IPL. The amplitude of these changes is graded with light intensity. Notably, the rate of exocytosis in ON and OFF terminals varied over a similar range and was modulated by the same light intensities. These results indicate that the intensity of steady lights are encoded symmetrically by the ON and OFF channels of the IPL. Using SypHy, it is also possible to differentiate between the terminals of bipolar cells responding preferentially to a steady light (sustained) or flickering light (transient). This approach to identifying different functional classes of synaptic terminal within the IPL will complement the mapping of electrical and synaptic activity using reporters of the presynaptic calcium signal (Dreosti, Odermatt and Lagnado, accompanying abstract). Using these methods, we hope to build a picture of the functional organization of the IPL, as well as changes in synaptic function underlying alterations in retinal processing during different forms of adaptation.