In neurons, action potentials (APs) induce transient increase in intracellular calcium concentration. Synthetic calcium indicators are widely used for monitoring neuronal activity in vitro and in vivo. However, calcium signals produced by synthetic fluorescent calcium indicators are sometimes suffered from a low signal-to-noise ratio (SNR), which precludes the detection of APs particularly in neurons with low firing rates in vivo.

Here, we present the data from comparative in vitro and in vivo analysis of two different synthetic calcium indicators for monitoring the activity of individual neurons: 1) Cal-520 : a recently developed calcium indicator dye and 2) Oregon Green BAPTA-1(OGB-1) : the most commonly used calcium indicator dye.

We demonstrate that Cal-520 is sufficiently sensitive to reliably detect single APs both in vitro and in vivo. In neocortical neurons, calcium signals were linearly correlated with the number of APs, and the SNR was > 6 for in vitro slice preparations and > 1.6 for in vivo anesthetized mice. In cerebellar Purkinje cells, dendritic calcium transients evoked by climbing fiber inputs were clearly observed in anesthetized mice with a high SNR and fast decay time. These characteristics of Cal-520 are a great advantage over those of OGB-1 for monitoring neural activity both in vitro and in vivo.