Phenotype‐dependent Ca(2+) dynamics in single boutons of various anatomically identified GABAergic interneurons in the rat hippocampus

Interneurons (INs) of the hippocampus exert versatile inhibition on pyramidal cells by silencing the network at different oscillation frequencies. Although IN discharge can phase‐lock to various rhythms in the hippocampus, under high‐frequency axon firing, the boutons may not be able to follow the fast activity. Here, we studied Ca(2+) responses to action potentials (APs) in single boutons using combined two‐photon microscopy and patch clamp electrophysiology in three types of INs: non‐fast‐spiking (NFS) neurons showing cannabinoid 1 receptor labelling and dendrite targeting, fast‐spiking partially parvalbumin‐positive cells synapsing with dendrites (DFS), and parvalbumin‐positive cells with perisomatic innervation (PFS). The increase in [Ca(2+)](i) from AP trains was substantially higher in NFS boutons than in DFS or PFS boutons. The decay of bouton Ca(2+) responses was markedly faster in DFS and PFS cells compared with NFS neurons. The bouton‐to‐bouton variability of AP‐evoked Ca(2+) transients in the same axon was surprisingly low in each cell type. Importantly, local responses were saturated after shorter trains of APs in NFS cells than in PFS cells. This feature of fast‐spiking neurons might allow them to follow higher‐frequency gamma oscillations for a longer time than NFS cells. The function of NFS boutons may better support asynchronous GABA release. In conclusion, we demonstrate several neuron‐specific Ca(2+) transients in boutons of NFS, PFS and DFS neurons, which may serve differential functions in hippocampal networks.