Abstracts

Rationale: Epilepsy is a devastating neurological disorder affecting 2% of the population worldwide. Mesial temporal lobe epilepsy (MTLE) is a type of epilepsy characterized by partial seizures that originate in the limbic structures (e.g. hippocampus). Granule cells of dentate gyrus innervate pyramidal cells of the CA3 region via their axons (mossy fibers) and presynaptic terminals, known as mossy fiber boutons (MFBs). In previous studies, synaptic plasticity changes have been examined in the mossy fiber pathways of chronic epileptic rats, such as MFB enlargement and mossy fiber sprouting and new synapse formation on dendritic fields of granule cells. However, there is limited data on presynaptic abnormalities during epileptogenesis, specifically in the mossy fiber-CA3 pyramidal cell synapses. We hypothesize that epileptogenesis will be associated with abnormal ultrastructural plasticity of presynaptic elements in the hippocampal mossy fiber pathway in MTLE. Methods: In this study, we used transmission electron microscopy (TEM) analysis of hippocampal tissue obtained from epileptic Sprague Dawley rats (n=7) and compared to age-matched controls (n=6). Chronic epileptic rats were obtained by the pilocarpine model of MTLE. Several ultrastructural variables of active zones in large MFBs contacting dendritic spines and shafts of pyramidal CA3 neurons were measured in TEM images using Image J (NIH) software. We determine the length of the active zones, the area of the postsynaptic density, the size of the ultrastructural correlates of the readily-releasable pool of vesicles (RRP) and recycling pool of vesicles (RP), densities for the RRP and RP regions, number and fraction of docked vesicles, number and length of synaptic perforations, synaptic vesicle areas and perimeter, clathrin-mediated endocytosis and bulk endocytosis. Data was analyzed by Kolgomorov-Smirnov comparison of distributions and Student t-test (significance level set at P<0.05). Results: Our data revealed a significant increase in the length of the active zones, increase in the of the RP and the area of bulk endocytosis in active zones of MFBs in chronically epileptic rats. Conclusions: These findings indicate an aberrant rearrangement of presynaptic machinery during epileptogenesis that may be involved in the pathogenesis of temporal lobe epilepsy.