Plateau depolarizations in spontaneously active neurons detected by calcium or voltage imaging.

In calcium imaging studies, Ca transients are commonly interpreted as neuronal action potentials (APs). However, our findings demonstrate that robust optical Ca transients primarily stem from complex "AP-Plateaus", while simple APs lacking underlying depolarization envelopes produce much weaker photonic signatures. Under challenging in vivo conditions, these "AP-Plateaus" are likely to surpass noise levels, thus dominating the Ca recordings. In spontaneously active neuronal culture, optical Ca transients (OGB1-AM, GCaMP6f) exhibited approximately tenfold greater amplitude and twofold longer half-width compared to optical voltage transients (ArcLightD). The amplitude of the ArcLightD signal exhibited a strong correlation with the duration of the underlying membrane depolarization, and a weaker correlation with the presence of a fast sodium AP. Specifically, ArcLightD exhibited robust responsiveness to the slow "foot" but not the fast "trunk" of the neuronal AP. Particularly potent stimulators of optical signals in both Ca and voltage imaging modalities were APs combined with plateau potentials (AP-Plateaus), resembling dendritic Ca spikes or "UP states" in pyramidal neurons. Interestingly, even the spikeless plateaus (amplitude > 10 mV, duration > 200 ms) could generate conspicuous Ca optical signals in neurons. Therefore, in certain circumstances, Ca transients should not be interpreted solely as indicators of neuronal AP firing.