Distribution of calcium pyroantimonate precipitates in Xenotoca Mauthner cells at normal and increased functional activity.

The pyroantimonate method was used for the ultrastructural localization of calcium ions (Ca2+) in Xenotoca Mauthner cells under normal conditions and after prolonged natural stimulation. In normal state, the highest concentration of these ions was observed as compact electron-dense precipitates inside the synaptic cleft exactly at the synaptic active zones. Some amount of dotted precipitates was revealed in the synaptic boutons. In the extracellular space and in the cytoplasm the precipitates are seen mainly as single membrane-bound dots. After prolonged stimulation significant redistribution of the precipitates was observed. They were entirely absent in the presynaptic areas, became diffuse and discontinuous or disappeared completely at the synaptic active zones. On the contrary, in the cytoplasmic organelles (subsynaptic cisternae, vacuoles, smooth reticulum, mitochondria) the precipitates were aggregated into continuous dense clusters inside the membranous compartments or on their surfaces. Also, large amounts of granules, not associated with membranes, were localized inside the cytoplasm directly at the cytoskeletal elements. It is suggested that membrane subsynaptic organelles are the primary structures which sequestrate, accumulate and retain Ca2+. Thus, these elements, together with deeper elements of smooth cytoplasmic reticulum, may control the cytoplasmic activity of Ca2+ and, as a consequence, control many physiologically significant reactions of the neurons.