Cytochemical localization of high-affinity Ca2(+)-ATPase activity in synaptic terminals

High-affinity Ca2(+)-ATPase activity in the optic tectum of the brain of cichlid fish was cytochemically localized using cerium ions to precipitate phosphate. Activation of the enzyme with micromolar instead of millimolar calcium concentrations (i.e., physiological cytoplasmic instead of extracellular concentrations) resulted in intracellular localization of reaction product attached to the cytoplasmic side of plasma membranes and to synaptic vesicles. The plasmalemmal enzyme activity was concentrated in synaptic regions. Synaptic vesicles in some terminals exhibited high amounts of ATPase activity, whereas others were free of reaction product. By use of electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) techniques, even small amounts of cerium-containing precipitates could be analyzed and precisely localized. The cytochemical observations are in good agreement with biochemical findings and therefore indicate that the calcium pump of neuronal plasma membranes can be successfully localized.