DISTRIBUTION OF CALCIUM PYROANTIMONATE PRECIPITATES IN XENOTOCA MAUTHNER CELLS AT NORMAL AND INCREASED FUNCTIONAL-ACTIVITY

The pyroantimonate method was used for the ultrastructural localizatio n of calcium ions (Ca2+) in Xenotoca Mauthner cells under normal condi tions and after prolonged natural stimulation. In normal state, the hi ghest concentration of these ions was observed as compact electron-den se precipitates inside the synaptic cleft exactly at the synaptic acti ve zones. Some amount of dotted precipitates was revealed in the synap tic boutons. In the extracellular space and in the cytoplasm the preci pitates are seen mainly as single membrane-bound dots. After prolonged stimulation significant redistribution of the precipitates was observ ed. They were entirely absent in the presynaptic areas, became diffuse and discontinuous or disappeared completely at the synaptic active zo nes. On the contrary, in the cytoplasmic organelles (subsynaptic ciste rnae, vacuoles, smooth reticulum, mitochondria) the precipitates were aggregated into continuous dense clusters inside the membranous compar tments or on their surfaces. Also, large amounts of granules, not asso ciated 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 an d 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 r eactions of the neurons.