| Summary: | 碩士 === 國立陽明大學 === 神經科學研究所 === 97 === Calcium-mediated excitotoxicity is believed to underlie the selective loss of dendritic inhibitory interneurons in epileptic hippocampus. In the present study, we investigated Ca2+ buffering and action potential (AP)–evoked Ca2+ signaling in the dendrites of 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 recordings 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 an added Ca2+ buffer fura-2 on AP-evoked Ca2+ transients was determined by ratiometric Ca2+ imaging. To estimate the AP-mediated Ca2+ load and endogenous Ca2+-binding ratio (�臮) in the proximal dendrite, fluorescence signals were converted into Ca2+ concentrations ([Ca2+]i) using isosbestic ratioing method and were analyzed on the basis of the ‘single compartmental model’. Resting [Ca2+]i was 43 nM and the build-up of [Ca2+]i during a single AP was up to 613 nM. Analysis of Ca2+ transients during fura-2 (150 μM) loading or under different steady-state fura-2 concentrations indicated that O-LM cells have a relatively low endogenous Ca2+ buffer capacities: the �臮 was 19-23 during fura-2 loading and ~27-58 under steady-state fura-2 concentrations, respectively. The AP-evoked Ca2+ signal decays with time constants of 128-347 ms, corresponding to extrusion rates 168-258 s-1. To further examine the spatial profile of AP-evoked dendritic Ca2+ transients, we measured somatic AP-evoked Ca2+ transients along dendrites using the Ca2+-sensitive dye OGB-1. Single APs or AP trains induced by somatic current injection reliably evoked uniform and robust Ca2+ accumulation in the dendritic regions up to 150 μm from the soma. The amplitude and decay of Ca2+ transients associated with back-propagating APs are largely independent of the distance from the soma. These results show that O-LM cells exhibit low endogenous Ca2+-binding ratios associated with slow Ca2+ extrusion that allows large, uniform and prolonged [Ca2+]i accumulations in the somato-dendritic domains. These unique features of Ca2+ dynamics may be relevant to synaptic plasticity and the selective vulnerability to excitotoxicity of O-LM cells.
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