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.