Plasticity, synaptic strength, and epilepsy: what can we learn from ultrastructural data?

Central nervous system synapses have an intrinsic plastic capacity to adapt to new conditions with rapid changes in their structure. Such activity-dependent refinement occurs during development and learning, and shares features with diseases such as epilepsy. Quantitative ultrastructural studies based on serial sectioning and reconstructions have shown various structural changes associated with synaptic strength involving both dendritic spines and postsynaptic densities (PSDs) during long-term potentiation (LTP). In this review, we focus on experimental studies that have analyzed at the ultrastructural level the consequences of LTP in rodents, and plastic changes in the hippocampus of experimental models of epilepsy and human tissue obtained during surgeries for intractable temporal lobe epilepsy (TLE). Modifications in spine morphology, increases in the proportion of synapses with perforated PSDs, and formation of multiple spine boutons arising from the same dendrite are the possible sequence of events that accompany hippocampal LTP. Structural remodeling of mossy fiber synapses and formation of aberrant synaptic contacts in the dentate gyrus are common features in experimental models of epilepsy and in human TLE. Combined electrophysiological and ultrastructural studies in kindled rats and chronic epileptic animals have indicated the occurrence of seizure- and neuron loss-induced changes in the hippocampal network. In these experiments, the synaptic contacts on granule cells are similar to those described for LTP. Such changes could be associated with enhancement of synaptic efficiency and may be important in epileptogenesis.