Ultrastructure of Excitatory Synaptic Contacts in Foci of Epileptiform Activity: Experiments on Intraocular Neural Transplants

Experiments were performed using a model of the synaptic mechanisms of the induction of epilepsy in nervous tissue, consisting of intraocular transplants of the septal area of the brain. Donor tissue was harvested from the brains of 17-day rat fetuses. Comparative electron microscopic and morphometric studies of excitatory synapses in transplants were performed three months after surgery to identify normal or epileptiform activity. Electrophysiological testing of transplants for the presence or absence of epileptiform activity was performed using brief electrical stimulation with single impulses. Synaptic terminals are regarded as three-component structural complexes (tripartite synapses) including not only pre- and postsynaptic components, but also astrocytes surrounding their processes. Although in comparison with the control transplants, epileptic transplants showed more multisynaptic axon boutons and dendritic spines as well as perforated postsynaptic densities (PSD), morphometric analysis did not identify any significant differences in the mean sizes of the presynaptic compartments and PSD. At the same time, synapses in transplants with epileptiform activity were significantly (1.8-fold) less surrounded by astrocyte processes. This is likely to facilitate the propagation of neurotransmitter along extracellular spaces and lead to synchronized neuron activity. It is suggested that perisynaptic astroglial processes are the primary microstructural components which react to electrical stimulation and the development of epileptiform activity.

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