Abstracts

Rationale: Consistent findings in temporal lobe epilepsy (TLE) include pathological hippocampal neuronal loss, loss of dendritic spines, and widespread gliosis. Astroglia, which support synaptic function, are also likely affected by the loss of pyramidal neurons and dendritic spines. We therefore tested the hypothesis that increased pathology was associated with increased synaptic loss, ultrastructural changes of remaining synapses, as well as an increased level of astroglial synaptic apposition.Methods: Acute hippocampal slices were prepared from hippocampi resected from patients undergoing hippocampectomy as treatment for medically-refractory TLE with increasing degrees of hippocampal sclerosis. All patients were seizure-free at a follow-up period of at least 12 months following surgery. After preparation, slices were transferred into an incubation chamber to allow recovery of ultrastructure, and then rapidly fixed. Serial section transmission electron microscopy (ssTEM) was used to analyze series from stratum radiatum of area CA1 using unbiased three-dimensional stereological procedures.Results: Excitatory synaptic density decreased as pathology increased from non-sclerotic mild gliosis (10 syns/10µm3) to moderate (5 syns/10µm3) and severe sclerosis (1 syn/10µm3). The morphology of remaining synapses was altered, and in severe sclerosis, multi-afferented giant dendritic spines were found to comprise approximately 96% of synapses. In all stages of pathology, however, axons had approximately 2-3 docked vesicles/bouton. Interestingly, nearest neighboring synapses were significantly closer in severe sclerosis despite reduced synaptic density. Astrocytic ultrastructure changed markedly as pathology increased, and the volume fraction of astrocytic processes was greatly increased in both moderate and severe sclerosis. The surface area/volume ratio was similar in both mild and moderate pathologies, though significantly increased in severe sclerosis. We also quantified a 300% increase in the occurrence of astrocytic surface specializations reminiscent of gap junctions in moderate sclerosis. The proportion of synapses with perisynaptic astroglia was approximately 41% in mild and moderate pathologies but was significantly reduced to only 16% in severe pathology. However, in all degrees of pathology, perisynaptic astrocytic apposition was associated with increased synapse size. Conclusions: Astrocytic volume increases, as well as an increase in astrocytic surface area may indicate an attempt to increase glutamate uptake in sclerosis. However, the multi-afferented structure and proximity of synapses in severe sclerosis may increase the potential for epileptic neuronal depolarization. Increased synaptic size at apposed synapses may indicate that continued contact with astroglia could maintain the persistence of efficacious synapses without limiting the potential for intersynaptic crosstalk. This work was supported by the NIH MH2000 and NS057113 (SAK), the MCG Intramural Interdisciplinary Research Program (SAK), NIH T32 NRSA NS045543 (MW, Robert Yu, PI) and NIH NS21184, NS33574, and EB002170 (KH).