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

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-D-28k in adult hippocampal slices using patch-clamp recordings and immunohistochemistry. First, we show that calbindin-D-28k freely exchanges between patch pipette and cytoplasm during whole cell patch-clamp recordings with a time constant of similar to 10 min. Substituting known concentrations of recombinant calbindin-D-28k in patch pipettes enabled us to determine the endogenous calbindin-D-28k concentration by postrecording immunohistochemistry. Using this calibration procedure, we find that mature granule cells (doublecortin -) contain similar to40 muM, and newborn granule cells (doublecortin +) contain 0 - 20 muM calbindin-D-28k. CA3 stratum radiatum interneurons and CA1 pyramidal cells enclose similar to 47 and similar to 45 muM calbindin-D-28k, respectively. Numerical simulations showed that 40 muM calbindin-D-28k is capable of tuning Ca2+ microdomains associated with action potentials at the mouth of single or clustered Ca2+ channels: calbindin-D-28k 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-D-28k equips hippocampal neurons with similar to 160 muM mobile, high-affinity Ca2+-binding sites (kappa(S) similar to 200) that slow and reduce global Ca2+ signals while they enhance the spatiotemporal fidelity of submicroscopic Ca2+ signals.