Ingmar Bl�mcke

Mitochondria are cellular organelles crucial for energy supply and calcium homeostasis in neuronal cells, and their dysfunction causes seizure activity in some rare human epilepsies. To directly test whether mitochondrial respiratory chain enzymes are abnormal in the most common form of chronic epilepsy, temporal lobe epilepsy (TLE), living human brain specimens from 57 epileptic patients and 2 nonepileptic controls were investigated. In TLE patients with a hippocampal epileptic focus, we demonstrated a specific deficiency of complex I of the mitochondrial respiratory chain in the hippocampal CA3 region. In contrast, TLE patients with a parahippocampal epileptic focus showed reduced complex I activity only in parahippocampal tissue. Inhibitor titrations of the maximal respiration rate of intact human brain slices revealed that the observed reduction in complex I activity is sufficient to affect the adenosine triphosphate production rate. The abnormal complex I activity in the hippocampal CA3 region was paralleled by increased succinate dehydrogenase staining of neurons and marked ultrastructural abnormalities of mitochondria. Therefore, mitochondrial dysfunction is suggested to be specific for the epileptic focus and may constitute a pathomechanism contributing to altered excitability and selective neuronal vulnerability in TLE.

In addition to functionally affected neuronal signaling pathways, altered axonal, dendritic, and synaptic morphology may contribute to hippocampal hyperexcitability in chronic mesial temporal lobe epilepsies (MTLE). The sclerotic hippocampus in Ammon's horn sclerosis (AHS)-associated MTLE, which shows segmental neuronal cell loss, axonal reorganization, and astrogliosis, would appear particularly susceptible to such changes. To characterize the cellular hippocampal pathology in MTLE, we have analyzed hilar neurons in surgical hippocampus specimens from patients with MTLE. Anatomically well-preserved hippocampal specimens from patients with AHS (n = 44) and from patients with focal temporal lesions (non-AHS; n = 20) were studied using confocal laser scanning microscopy (CFLSM) and electron microscopy (EM). Hippocampal samples from three tumor patients without chronic epilepsies and autopsy samples were used as controls. Using intracellular Lucifer Yellow injection and CFLSM, spiny pyramidal, multipolar, and mossy cells as well as non-spiny multipolar neurons have been identified as major hilar cell types in controls and lesion-associated MTLE specimens. In contrast, none of the hilar neurons from AHS specimens displayed a morphology reminiscent of mossy cells. In AHS, a major portion of the pyramidal and multipolar neurons showed extensive dendritic ramification and periodic nodular swellings of dendritic shafts. EM analysis confirmed the altered cellular morphology, with an accumulation of cytoskeletal filaments and increased numbers of mitochondria as the most prominent findings. To characterize cytoskeletal alterations in hilar neurons further, immunohistochemical reactions for neurofilament proteins (NFP), microtubule-associated proteins, and tau were performed. This analysis specifically identified large and atypical hilar neurons with an accumulation of low weight NFP. Our data demonstrate striking structural alterations in hilar neurons of patients with AHS compared with controls and non-sclerotic MTLE specimens. Such changes may develop during cellular reorganization in the epileptogenic hippocampus and are likely to contribute to the pathogenesis or maintenance of temporal lobe epilepsy.