view avi movie (3 MB) Mouse synapse lights up when stimulated, grows dark after stimulation stops. The graph shows the average fluorescence intensity. The nerve was stimulated during the rising phase of the graph (100 Hz for 10 seconds). How does it work? A transgenic mouse was made by Rafa Chacon. It expresses a Green Fluorescent Protein (GFP, from a jellyfish), which is attached to a mouse protein that is embedded in synaptic vesicle membranes. 1. The GFP (black) is non-fluorescent inside the vesicle, because the acidic environment (blue) quenches the fluorescence. 2. During stimulation, vesicles undergo exocytosis and spill neurotransmitter into the synapse. The GFP is no longer quenched, and so it fluoresces green when blue light shines on it. 3. When stimulation stops, the GFP is retrieved (endocytosis), and when the vesicle is acidified again, the fluorescence is quenched and the vesicle goes dark again.