Detecting action potentials in neuronal populations with calcium imaging.

The study of neural circuits requires methods for simultaneously recording the activity of populations of neurons. Here, using calcium imaging of neocortical brain slices we take advantage of the ubiquitous distribution of calcium channels in neurons to develop a method to reconstruct the action potentials occurring in a population of neurons. Combining calcium imaging with whole-cell or perforated patch recordings from neurons loaded with acetoxymethyl ester or potassium salt forms of calcium indicators, we demonstrate that each action potential produces a stereotyped calcium transient in the somata of pyramidal neurons. These signals are detectable without averaging, and the signal-to-noise is sufficient to carry out a reconstruction of the spiking pattern of hundreds of neurons, up to relatively high firing frequencies. This technique could in principle be applied systematically to follow the activity of neuronal populations in vitro and in vivo.