Intracellular Ca2+ stores in neurons. Identification and functional aspects

Various aspects of the rapidly exchanging intracellular Ca²⁺ 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 Ca²⁺ release); their control of the plasma membrane Ca²⁺ permeability, 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 Ca²⁺ ATPase responsible for the accumulation of the cation; ii) Ca²⁺ binding protein(s) of low affinity and high capacity to keep Ca²⁺ stored; and iii) a Ca²⁺ channel, activated by either one of the mechanisms mentioned above, to release Ca²⁺ to the cytosol. Results obtained in Purkinje neurons document the heterogeneity of the stores and the strategical distribution of the corresponding organelles (calciosomes; specialized portions of the ER) within the cell body, dendrites and dendritic spines.