Members of the Department of Neurophysiology at the University of Tokyo have identified a highly sensitive fluorescent indicator dye for calcium imaging of neural activity in vitro and in vivo. With their study, they aim to establish in what ways this indicator, Cal-520^® AM, is better suited than other dyes for neural imaging.

Mayumi Tada and her group noted that although current bolus loading of acetoxymethyl ester (AM) dervatives of calcium dyes enable simultaneous observation of the activity of hundreds of neurons, the scattering of the fluorescence signals in thicker brain tissue samples causes detection to suffer from low signal-to-noise ratio (SNR), especially in in vivo experiments. This makes it difficult to detect single action potentials (AP) amid a mass, and monitor any neurons with a low firing rate. To determine how effectively Cal-520^® imaging addressed these issues, Tada's group proceeded to use high-speed linescan imaging to analyze in vivo (neocortical neuron and cerebellar Purkinje cell) and in vitro (neocortical pyramidal cell) samples injected with Cal-520^®, and samples injected with another popular green fluorescent indicator (OBG-1) for comparison. Their findings indicated that:

Cal-520^® AM is highly sensitive, with excellent SNR to reliably detect separate APs as well as monitor the activity of neurons with very low firing rates.

Cal-520^® AM remained fluorescent within neurons for several hours after injection, proving its stability and rendering it suitable for long-duration imaging of neural activity.

Cal-520^® AM exhibited faster decay time in dendrites, where Cal-520^®'s kinetics were not affected as much by small surface-to-volume ratio.

Cal-520^® AM demonstrated higher specificity of neuron labeling.

These conclusions completely align with the fact that Cal-520^® is specifically optimized for better intracellular retention, which also helps increase its signal's SNR, sensitivity, and stability. As confirmed by Tada's group, Cal-520^® is a robust calcium indicator for in vivo and in vitro experiments, especially when imaging neurons.