Transient increase of cytoplasmic calcium concentration in the rat hippocampus after kindling-induced seizures. An ultrastructural study with the oxalate-pyro-antimonate technique.

Kindling stimulations were applied to the Schaffer collateral/commissural fibers in the CA1 area of the dorsal rat hippocampus. In fully kindled animals the ultrastructural distribution of calcium was studied at different time intervals after an induced generalized seizure, using the oxalate-pyro-antimonate technique. Semi-quantitative analysis of the amount of precipitate revealed no change in the investigated structures analysed after 2 h or 24 h: boutons and spines of the Schaffer-collateral/pyramidal-dendrite synaptic contacts, cytoplasm and mitochondria of terminals on pyramidal cell bodies and smooth dendrites. The major change was found 15 min after a seizure, when calcium precipitate in boutons and spines of stratum radiatum was strongly increased, precipitate in somata terminals only slightly, while smooth dendrites were not affected. These results imply a seizure-related increase of the intracellular calcium concentration. The transient character suggests that the investigated cellular compartments in kindled tissue are still capable of maintaining calcium homeostasis. The observed increase in precipitate density for at least 15 min may initiate the neurochemical mechanisms leading to an enhanced seizure sensitivity in the kindling model of epilepsy.