Synaptic vesicle size and shape profile in the kindling model of epileptogenesis

Excitatory synapses were studied in the dentate gyrus ipsilateral to stimulated entorhinal cortex in fully kindled rats 2 weeks after the last (3rd) stage 5 seizure. In previous studies of the same model marked redistribution of synaptic vesicles to a 'strategic position' in the vicinity of the synaptic cleft and a significant enlargement of postsynaptic element were described. In this study the size and shape of synaptic vesicles were evaluated in three zones parallel to the presynaptic membrane of the synaptic cleft. The first zone (0.1 micron) directly adjoined the active zone of presynaptic membrane, the second was from 0.1 to 0.2 micron and the third from 0.2 to 0.3 micron from the presynaptic membrane. The vesicles of both the experimental and control animals were significantly smaller in zone I than in zone II and III and, in the control animals there was a tendency of vesicles to diminish towards the active zone. Such a tendency was not observed in the experimental animals, the largest vesicles were in the middle of zone II and were significantly more rounded in comparison to the vesicles in the controls. On the contrary, the vesicles in zone I and III were more elongated in the experimental animals than in controls. Our results may be taken as a sign of active reconstruction of synaptic apparatus in relation to increased functional readiness of the synapses. Possible mechanisms as well as significance for the kindling and epileptogenesis are discussed.