Figure 1. Modelling GCaMP2 responses in the synapse and axon.

(a) Geometry of the 2-D model implemented in Virtual Cell. (b) Calcium buffers, channels and pumps used in the model. Distribution of calcium channels at the active zone is shown in red while distribution of synaptic vesicles in cyan. (c) The modelled response of GCaMP2 (1 μM) to a single spike (green trace) calculated as the relative change in GCaMP2 fluorescence in an ROI over the bouton. The black trace is the spatially averaged Ca²⁺ concentration reported by cytoplasmatic furaptra, also averaged over the whole bouton. This signal was obtained by adjusting the parameters of the model to reproduce the response to a single spike reported by furaptra¹². Note the slower GCaMP2 signal compared to the Ca²⁺ transient. (d) Simulations of GCaMP2 signals at different distances from calcium channels: within 50 nm of the active zone (red); averaged over the bouton (cyan); over a 2.25 μm length of axon close to the bouton (amber), and over the next 2.25 μm length of axon (blue). (e and f) Neuronal-glial cultures of rat hippocampi expressing cytoplasmic GCaMP2 (e) or synaptic SyGCaMP2 (f). Expression of the synaptic marker mRFP-VAMP is shown in the middle and merged images to the right. The rectangular field of view is expanded in the images below. Only SyGCaMP2 visualizes all synaptic boutons marked by mRFP-VAMP. For equipment and settings see Supplementary Methods online. Scale bars = 20 μm.