Endogenous Ca2+ buffer concentration and Ca2+ microdomains in hippocampal neurons

Abstract

Ca2+-binding proteins are ubiquitously expressed throughout the CNS and serve as valuable immunohistochemical markers for certain types of neurons. However, the functional role of most Ca2+-binding proteins has to date remained obscure because their concentration in central neurons is not known. In this study, we investigate the intracellular concentration of the widely expressed Ca2+-binding protein calbindin-D28k in adult hippocampal slices using patch-clamp recordings and immunohistochemistry. First, we show that calbindin-D28k freely exchanges between patch pipette and cytoplasm during whole cell patch-clamp recordings with a time constant of ∼10 min. Substituting known concentrations of recombinant calbindin-D28k in patch pipettes enabled us to determine the endogenous calbindin-D28k concentration by postrecording immunohistochemistry. Using this calibration procedure, we find that mature granule cells (doublecortin-) contain ∼40 μM, and newborn granule cells (doublecortin+) contain 0-20 μM calbindin-D28k. CA3 stratum radiatum interneurons and CA1 pyramidal cells enclose -47 and -45 μM calbindin-D28k, respectively. Numerical simulations showed that 40 μM calbindin-D28k is capable of tuning Ca2+ microdomains associated with action potentials at the mouth of single or clustered Ca2+ channels: calbindin-D28k reduces the increment in free Ca2+ at a distance of 100 and 200 nm by 20 and 35%, respectively, and strongly accelerates the collapse of the Ca2+ gradient after cessation of Ca2+ influx. These data suggest that calbindin-D28k equips hippocampal neurons with ∼160 μM mobile, high-affinity Ca2+-binding sites (κS ∼200) that slow and reduce global Ca2+ signals while they enhance the spatiotemporal fidelity of submicroscopic Ca2+ signals.