Single-spike detection in vitro and in vivo with a genetic Ca2+ sensor

Measurement of population activity with single-action-potential, single-neuron resolution is pivotal for understanding information representation and processing in the brain and how the brain's responses are altered by experience. Genetically encoded indicators of neuronal activity allow long-term, cell type-specific expression. Fluorescent Ca2+ indicator proteins (FCIPs), a main class of reporters of neural activity, initially suffered, in particular, from an inability to report single action potentials in vivo. Although suboptimal Ca2+-binding dynamics and Ca2+-induced fluorescence changes in FCIPs are important factors, low levels of expression also seem to play a role. Here we report that delivering D3cpv, an improved fluorescent resonance energy transfer-based FCIP, using a recombinant adeno-associated virus results in expression sufficient to detect the Ca2+ transients that accompany single action potentials. In upper-layer cortical neurons, we were able to detect transients associated with single action potentials firing at rates of <1 Hz, with high reliability, from in vivo recordings in living mice.