J Physiology (1992) 86, 23-30 2.3 © Elsevier, Paris Intracellular Ca stores in neurons. Identification and functional aspects J Meldolesi, A Villa, P Podini, E Ctementi, D Zacchetti, P D'Andrea, P Lorenzon, F Grohovaz Department of Pharmacology, CNR Cytopharmacology and B Ceccarelli Centers, and Scientific Institute S Raffaele, Via Olgettina, 60, University of Milan, 20132 Milan, Italy Summary - Various aspects of the rapidly exchanging intracellular Ca 2+ stores of neurons and nerve cells are reviewed: their multiplicity, with separate sensitivity to either the second messenger, inositol 1,4,5-trisphosphate, or ryanodine-caffeine (the latter stores are probably activated via Ca2+-induced Ca 2÷ release); their control of the plasma membrane Ca 2÷ per- meability, via the activation of a peculiar type of cation channels; their ability to sustain localized heterogeneities of the [Ca2+]i that could be of physiological key-importance. Finally, the molecular composition of these stores is discussed. They are shown (by high resolution immunocytochemistry and subcellular fractionation) to express: i) a Ca 2÷ ATPase responsible for the accumulation of the cation; ii) Ca 2÷ binding protein(s) of low affinity and high capacity to keep Ca 2+ stored; and iii) a Ca 2÷ channel, activated by either one of the mechanisms mentioned above, to release Ca 2÷ to the cytosol. Results obtained in Purkinje neurons document the heterogeneity of the stores and the strategical distribution of the corresponding organelles (catciosomes; specialized portions of the ER) within the cell body, dendrites and dendritic spines. inositol 1,4,5-trisphosphate / intracellular Ca z+ ryanodlne-eaffeine Introduction Within cells, the total amount of calcium is of the same order of magnitude as in the extracellu- lar space, yet the concentration of free, ionized Ca2+ in the cytosol, [Ca2+]i, is = four orders of magnitude lower, retool/1 vs 0.1 gM. Thus, [Ca2+]i is only a minute fraction of cellular calcium, the result of an equilibrium among a number of coordinated processes occurring at the cell surface (influx through the various types of channels per- meable to Ca2+; extrusion via both the high af- 2+ + finity Ca2+ pump and the lower affinity Ca INa exchanger) as well as in the depth of both the cytoplasm and the nucleus (binding to specific molecules of appropriate affinity, especially pro- teins; uptake into, storage within and release from membrane-bound organeltes) (see fig 1; for general reviews see Carafoli, 1987; Pietrobon et al, 1990). Because of its strategic location, sur- rounded by the extracellular space and distributed in between the intracellular organelles, the cytosol can rapidly modify its [CaZ+]i in response to specific stimulations. Up until 10 years ago, the information about [Ca2+]i events was mostly in- direct, based on electrophysiology of Ca 2÷ and Ca2+-dependent currents together with biochemi- cal studies of 45Ca uptake and release from cells and subcellular fractions. Although these two ap- proaches were (and remain) very difficult to correlate, the idea that many cell types, including neurons, possess intracellular Ca2+ stores similar to the sarcoplasmic reticulum of striated muscle cells, ie capable of rapid (and regulated) ex- change, had already been documented. However, the cytology of these stores and the mechanisms of their control remained unclear until about 10 years ago when fluorescent [Ca2+]i dyes were developed, and at the same time inositol 1,4,5- trisphosphate (IP3) was recognized as a specific, Ca z÷ release second messenger generated by a variety of hormones and neurotransmitters via the activation of specific, G protein coupled receptors (see Berridge, 1989; Tsien, 1989; Tsien and Tsien, 1990). Exciting developments have followed these major advances in the field, and rapidly ex-