Imaging neural activity with improved G-CaMPs (Tian et al. 2009). (A) The improved baseline fluorescence of G-CaMP3 compared with that of G-CaMP2. Both indicators were virally delivered to layer 2/3 cortical neurons, and images were taken 12 d post-viral injection. Scale bar, 50 µm. (B) The average fluorescence change of G-CaMP3 is greater than that of G-CaMP2 in response to trains of APs given at 83 Hz in layer 2/3 cortical neurons (left). Sensory stimulation-evoked fluorescence responses of G-CaMP3 were significantly enhanced in the Drosophila antennal lobe (middle) and in C. elegans chemosensory neurons (right). (C) G-CaMP3 is suitable for long-term imaging of behaviorally correlated activity in neuronal populations over extended periods of time. Repeated imaging of the same neuronal population (L2/3 neurons of the primary motor cortex) at 72 (left) and 120 d (right) postinfection showed remarkably constant (top) G-CaMP3 expression (scale bar, 30 µm) and (bottom) signal change (DF/F of individual cells; black lines). (Red line) Relative treadmill movement (F, forward; B, backward). Scale bars, 20 µm. (Adapted, with permission, from Tian et al. 2009.)