Influence of seasonal adaptation on the distribution of cytochemically detectable endogenous calcium in synaptic terminals of the optic tectum of crucian carp (Carassius carassius L.).

Crucian carps were seasonally adapted to high (23 degrees C) or low (4 degrees C) temperatures and synaptic regions in the optic tectum were analyzed by means of energy-filtering transmission electron microscopy (EFTEM). The amount of cytochemically detectable endogenous calcium in synaptic clefts was quantified by means of image analysis and the amount of glycogen granules in synaptic regions was assessed. In cold-adapted fish, glycogen is increased, especially in glial cells, while the calcium-containing precipitates are reduced. This is interpreted as a characteristic feature of inactive synapses, in which glycogen accumulates and in which the calcium turnover is slowed down. Short-term thermal stress did not influence the amount of calcium or glycogen.