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

In calcium imaging studies, Ca²⁺ transients are commonly interpreted as neuronal action potentials (APs). However, our findings demonstrate that robust optical Ca²⁺ 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 Ca²⁺ recordings. In spontaneously active neuronal culture, optical Ca²⁺ 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 Ca²⁺ and voltage imaging modalities were APs combined with plateau potentials (AP-Plateaus), resembling dendritic Ca²⁺ spikes or "UP states" in pyramidal neurons. Interestingly, even the spikeless plateaus (amplitude > 10 mV, duration > 200 ms) could generate conspicuous Ca²⁺ optical signals in neurons. Therefore, in certain circumstances, Ca²⁺ transients should not be interpreted solely as indicators of neuronal AP firing.