Supralinear Ca21 Signaling by Cooperative and Mobile Ca21 Buffering in Purkinje Neurons

that mice deficient in calbindin-D 28K exhibit cerebellar ataxia (Airaksinen et al., 1997a) and impaired spatial Hongo, Bunkyo-ku Tokyo 113 learning (Molinari et al., 1996). However, the properties of high-affinity Ca 2ϩ buffering in vivo and its specific Japan † Department of Pharmacology and Physiology consequences for neuronal Ca 2ϩ signaling have not been elucidated. We have therefore now investigated Ca 2ϩ buffering in Purkinje cells with the use of a low-affinity (‫01ف‬ ␮M) Ca 2ϩ indicator dye (BTC) and a high-affinity (19 nM) caged Ca 2ϩ compound (DMNPE-4). Neither of these Japan compounds interfered to a substantial extent with the intrinsic Ca 2ϩ-buffering properties of these cells. We detected the existence of a large concentration of a high-Summary affinity, cooperative, and mobile Ca 2ϩ buffer, the properties of which resemble those of calbindin-D 28K. We also Endogenous high-affinity Ca 2؉ buffering and its roles demonstrated the presence of a fixed, low-affinity Ca 2ϩ were investigated in mouse cerebellar Purkinje cells buffer with a binding ratio of ‫.001ف‬ Demonstration of with the use of a low-affinity Ca 2؉ indicator and a high-the existence of two Ca 2ϩ buffers with markedly different affinity caged Ca 2؉ compound. Increases in the cyto-Ca 2ϩ binding properties allowed the diffusion of Ca 2ϩ solic Ca 2؉ concentration ([Ca 2؉ ] i) were markedly facili-to be described by a nonlinear cable equation, which tated during repetitive depolarization, resulting in the predicts two-phase Ca 2ϩ dynamics. Such dynamics may generation of steep micromolar Ca 2؉ gradients along contribute to various computational processes in neu-dendrites. Such supralinear Ca 2؉ responses were at-rons, including coincidence detection mechanisms of tributed to the saturation of a large concentration (0.36 Hebbian learning. Some of the present data were pre-mM) of a mobile, high-affinity (dissociation constant, sented in preliminary form (H. 2؉ ] i. clamped in the whole-cell mode at Ϫ70 mV and then depolarized to 0 mV for 0.5 s. Ratiometric measurement of the cytosolic free Ca 2ϩ concentration ([Ca 2ϩ ] i) with Introduction the low-affinity Ca 2ϩ indicator benzothiazole coumarin (BTC) revealed that the depolarization-induced increase Cytosolic Ca 2ϩ regulates many key processes in neu-in [Ca 2ϩ ] i was sigmoidal, or occurred in two phases; it rons, and its buffering and diffusion therefore have was slow initially and then markedly accelerated during marked effects on neuronal information processing the depolarization command (Figures 1A and 1B), with (Neher, 1998). …