[Time-dependent changes in the structure of the synapses of hippocampal field CA3 after their potentiation]

The time-dependent quantitative ultrastructure of synapses in the stratum lucidum of the hippocampal CA3 area was investigated after the in vitro modulation of the synaptic efficacy by short tetanus applied to the fascia dentate (a so called long-term potentiation-LTP). Two distinct phases were shown in the structural alterations accompanying LTP. Firstly, significant reactive changes in the spine geometry and the active zone shape were observed. These changes reversibly disappeared during 2-3 hours of the subsequent incubation, whereas LTP became well pronounced at this time. Secondly, a substantial increase in the postsynaptic thickness was revealed. This morphological feature was matching more closely the time course of LTP, up to the disappearance of both structure and function within 5 hours or longer as a result of destructive changes during a prolonged in vitro incubation. We conclude that the increased postsynaptic thickness may structurally correspond to LTP.