Description:
<jats:p>Multi‐neuronal recordings with Ca<jats:sup>2+</jats:sup> indicator dyes usually relate [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> to action potentials (APs) assuming a stereotypical dependency between the two. However, [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> affects and is affected by numerous complex mechanisms that differ from cell type to cell type, from cell compartment to cell compartment. Moreover, [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> depends on the specific way a cell is activated. Here we investigate, by combining calcium imaging and on‐cell patch clamp recordings, the relationship between APs (spiking) and somatic [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> in mitral and granule cells of the olfactory bulb in <jats:italic>Xenopus laevis</jats:italic> tadpoles. Both cell types exhibit ongoing and odour‐modulated [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> dynamics. In mitral cells, the occurrence of APs in both spontaneous and odour‐evoked situations correlates tightly to step‐like [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> increases. Moreover, odorant‐induced suppression of spontaneous firing couples to a decrease in [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub>. In contrast, granule cells show a substantial number of uncorrelated events such as increases in [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> without APs occurring or APs without any effect upon [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub>. The correlation between spiking and [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> is low, possibly due to somatic NMDAR‐mediated and subthreshold voltage‐activated Ca<jats:sup>2+</jats:sup> entries, and thus does not allow a reliable prediction of APs based on calcium imaging. Taken together, our results demonstrate that the relationship between somatic [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> and APs can be cell type specific. Taking [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> dynamics as an indicator for spiking activity is thus only reliable if the correlation has been established in the system of interest. When [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> and APs are precisely correlated, fast calcium imaging is an extremely valuable tool for determining spatiotemporal patterns of APs in neuronal population.</jats:p>