Increase in extracellular calcium in the optic tectum of fish after optic nerve transection

In this study the extracellular distribution of cytochemically generated calcium reaction product in the denervated optic tectum of a cichild fish (Oreochromis mossambicus) was investigated. The left optic nerve had been transected and the fish (5 per experimental condition) maintained for 2, 10 and 21 days. The amount of the calcium-containing precipitates was estimated using energy-filtering transmission electron microscopy (EFTEM) and image analysis. A special degeneration type of the optic terminals (neurofibrillar hypertrophy) was found which seems to be rare in other teleosts and was therefore chosen for quantification of the calcium deposits. These terminals are surrounded by astroglial processes and the calcium reaction product in the extracellular spaces between these glial processes and the terminals was measured and compared to normal optic terminals in nonoperated controls. A distinct and significant increase in the amount of calcium deposits was found 2 and 10 days after surgery which decreased to control levels after 21 days. This rise of deposits around the degenerating terminals was very local as arbitrarily selected extracellular spaces near these terminals showed values which were at the level of the nonoperated controls. Therefore, a transient and local increase in extracellular calcium precipitates was found after optic nerve transection which affected only the degenerating synapses.