Astrocytes from Human Hippocampal Epileptogenic Foci Exhibit Action Potential–Like Responses

Abstract

Summary: Purpose: We studied Na' channel expression and the ability to generate action potential (AP)-like responses in primary cultures of human astrocytes by whole cell patchclamp recording techniques.Methods: Tissue samples from 22 patients with various classifications of temporal lobe epilepsy (TLE) were plated to form separate astrocyte cultures from three regions; the hippocampus, parahippocampus, and anterolateral temporal neocortex.Results: The resting membrane potential of seizure focus astiocytes (MTLE, mesial TLE) was significantly depolarized (approximately -55 mV) as compared with cortical astrocytes (-80 mV). Hippocampal astrocytes from other substrates for TLE (MaTLE, mass-associated TLE; PTLE, paradoxical TLE) in which the hippocampus is not the seizure focus displayed resting membrane potentials similar to those of neocortical astrocytes (approximately -75 mV). Astrocytes from the seizure focus (MTLE) displayed much larger tetrodotoxin (TTX)-sensitive Na' currents with -66-fold higher Na' channel density (113.5 -c 17.41 pA/pf) than that of comparison neocortical astrocytes (1.7 r 3.7 pA/pf) or than that of the hippocampal and parahippocampal astrocytes of the MaTLE and PTLE groups. As a consequence of this higher channel density, seizure focus astrocytes were capable of generating AP-like responses. However, at the resting potential, most Na' channels are inactive and no spontaneous firing was observed. In contrast, astrocytes in the comparison neocortex from all groups and the hippocampus and parahippocampus from the MaTLE and PTLE groups could not fire AP-like responses even after large current injections.Conclusions: The function of Na' channels in these astrocytes is unclear. However, the marked differences in seizure focus astrocytes as compared with cortical and nonseizure focus hippocampal astrocytes suggest a more active role for astrocytes associated with hyperexcitable neurons at a seizures focus.

Astrocytes and Epilepsy

“…[26][27][28][29][30][31] Comparative patch-clamp studies were carried out on astrocytes in hippocampal specimens with and without significant sclerosis removed in epilepsy surgery. In a study from our laboratory, primary cultures of astrocytes established from the hippocampus (dentate gyrus and Ammon's horn) and parahippocampus (entorhinal region) of sclerotic hippocampi displayed much larger tetrodotoxin (TTX)-sensitive Na ϩ currents with ϳ66-fold higher Naϩ channel density than in astrocytes from nonsclerotic hippocampi and comparison temporal neocortex of the same patient 32 (FIG. 3).…”

“…One group of patients had hippocampal sclerosis (MTLE) whereas two others (mass associated temporal lobe epilepsy and paradoxical temporal lobe epilepsy) had no sclerosis. 32 A large proportion (ϳ60%) of astrocytes in primary cultures derived from sclerotic epileptogenic hippocampal seizure foci are capable of generating action potentiallike responses after depolarizing currents in comparison to astrocytes from nonsclerotic foci and neocortex from which such action potential-like responses could not be elicited 32 (FIG. 3).…”

“…32 Action potential like responses was obtained in approximately 60% of hippocampal and parahippocampal astrocytes from sclerotic hippocampi when depolarized by a series of current steps of increasing amplitude. 32 These observations strongly suggest the presence of NG2/GluR cells in the sclerotic hippocampus. Seifert et al 14 provide more direct evidence.…”

“…NG2-like cells have been demonstrated in animal studies to be capable of being depolarized to yield action potentials, sometimes even producing spontaneous action potentials. 79 Cells capable of being depolarized to generate actions potentials 32 and GluR1 receptors with elevated flip to flop ratios, which would facilitate and prolong depolarization 14 are demonstrated in the human hippocampus. Such cells may facilitate the generation or spread of waves of depolarization from granule cells to the subiculum without synaptic pathways between these two regions.…”

Gene Expression in Temporal Lobe Epilepsy is Consistent with Increased Release of Glutamate by Astrocytes

“…Astrocytes in sclerotic regions of the hippocampus have many unique properties, and may contribute to the hyperexcitability of the hippocampal seizure focus. The astrocytes have an increase in Na + channels on their membranes, a more depolarized resting membrane potential, and a reduction of inward rectifying potassium (Kir) channel function (12,13); have an elevated flip-to-flop mRNA ratio of GluR1 that make them more responsive to glutamate (14); have a significant down regulation of the enzyme glutamine synthetase (15); and show increased expression of the transcription factor Nuclear Factor kappa B (NFκB), which is thought to play a crucial role in many acute inflammatory reactions (16). This paper further explores the molecular organization of the sclerotic substrate in TLE and proposes the hypothesis that the molecular changes in astrocytes from sclerotic hippocampi are associated with calcium dependent glutamate release into the extracellular space, and that several important initial precipitating events in the causation of TLE may activate this mechanism.…”