Astrocytic and synaptic response to kindling in hippocampal subfield CA1. II. Synaptogenesis and astrocytic process increases to in vivo kindling

Astrocytic glia are important for maintaining synaptic function during physiological activity. Recent hypotheses concerning epilepsy suggest a role for astrocytes in the control of neuronal excitability and in pathogenesis. This report provides morphological evidence that the periodic electrical stimulation used in the kindling model of epilepsy induces astrocytic hypertrophy and an increase in shaft synapse density in the CA1 region of the hippocampus. The Schaffer collateral pathway in the stratum radiatum of CA1 of five pairs of rats was kindled in vivo. Control animals received the same number of stimulations at a lower intensity and frequency. The animals were killed 24-48 h after reaching the criterion of five generalized seizures, and the brains were examined by electron microscopy. Kindling produced a 37% and 33% increase in the volume fraction of astrocytic processes in the middle and distal portions, respectively, of the s. radiatum in CA1. In the same tissue, the number (areal density) of shaft synapses was increased 25% in the s. radiatum of animals exhibiting generalized seizures. On the other hand, the areal density of degenerating synapses in both kindled and control animals was low and not significantly different. These results suggest that both synaptogenesis and hypertrophy of astrocytes contribute to an early stage of epileptogenesis when degenerative changes of the sort that might induce gliosis were not prominent in the tissue under study.