Cell type-specific relationships between spiking and [Ca2+]i in neurons of the Xenopus tadpole olfactory bulb

Multi-neuronal recordings with Ca2+ indicator dyes usually relate [Ca2+](i) to action potentials (APs) assuming a stereotypical dependency between the two. However, [Ca2+](i) affects and is affected by numerous complex mechanisms that differ from cell type to cell type, from cell compartment to cell compartment. Moreover, [Ca2+](i) 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 [Ca2+](i) in mitral and granule cells of the olfactory bulb in Xenopus laevis tadpoles. Both cell types exhibit ongoing and odour-modulated [Ca2+](i) dynamics. In mitral cells, the occurrence of APs in both spontaneous and odour-evoked situations correlates tightly to step-like [Ca2+](i) increases. Moreover, odorant-induced suppression of spontaneous firing couples to a decrease in [Ca2+](i). In contrast, granule cells show a substantial number of uncorrelated events such as increases in [Ca2+](i) without APs occurring or APs without any effect upon [Ca2+](i). The correlation between spiking and [Ca2+](i) is low, possibly due to somatic NMDAR-mediated and subthreshold voltage-activated Ca2+ 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 [Ca2+](i) and APs can be cell type specific. Taking [Ca2+](i) dynamics as an indicator for spiking activity is thus only reliable if the correlation has been established in the system of interest. When [Ca2+](i) and APs are precisely correlated, fast calcium imaging is an extremely valuable tool for determining spatiotemporal patterns of APs in neuronal population.