Estimating intracellular Ca2+ concentrations and buffering in a dendritic inhibitory hippocampal interneuron

Calcium is known to regulate several phenomena like neuronal excitability and plasticity. Interestingly, the spatiotemporal profile of dendritic calcium depends on several processes, specific to each neuronal type. In this study, we investigated Ca2+ buffering and action potential (AP)-evoked Ca2+ signaling in the dendrites of anatomically identified oriens lacunosum-moleculare (O-LM) cells, a major type of dendrite-targeting interneurons in the hippocampal CA1 region, using a combination of whole-cell patch-clamp recording and fast Ca2+ imaging in acute rat brain slices. Cells were loaded with fluorescent Ca2+ indicators fura-2 or Oregon Green BAPTA-1 (OGB-1) via patch-clamping electrode, and the effect of fura-2 on AP-evoked dendritic Ca2+ transients was determined by ratiometric Ca2+ imaging. To estimate intracellular Ca2+ concentrations ([Ca2+]i) and endogenous Ca2+-binding ratio (κs) in the proximal dendrite, fluorescence signals were converted into [Ca2+]i using the ratioing method and were analyzed on the basis of the “single compartment model.” Resting [Ca2+]i was 22±5 nM and the build-up of [Ca2+]i during a single AP was up to 656±226 nM. Analysis of Ca2+ transients revealed that O-LM cells have a relatively low endogenous Ca2+-binding ratio (κs): the κs was 20±8 estimated during fura-2 loading and 27 estimated under steady-state fura-2 concentrations, respectively. To further examine the spatial profile of dendritic Ca2+ transients, we measured somatic AP-evoked Ca2+ transients beyond proximal dendrites using OGB-1. Dendritic Ca2+ transients evoked by single APs or AP trains are not limited to regions close to the soma. The amplitude and decay of [Ca2+]i associated with backpropagating APs are relatively independent of the distance from the soma. In sum, O-LM cells exhibit low endogenous Ca2+-binding ratios and relatively distance-independent Ca2+ dynamics in the dendrites. These special features of Ca2+ signaling in O-LM cells may have important functional implications for both normal and pathological conditions.