Properties of human glial cells associated with epileptic seizure foci
Introduction
Epileptic seizures are characterized by a transient increase of cerebral electrical activity. These episodes of neuronal hyperexcitability are accompanied by significant chemical imbalances of the extracellular fluid. Specifically, extracellular levels of Na+ and Ca2+ decrease markedly (Hablitz and Heinemann, 1987, Hablitz and Heinemann, 1994) while [Glutamate]o and [K+]o increase dramatically during seizures (Prince et al., 1973, Lux, 1974, Hablitz and Heinemann, 1987). Glial cells have been implicated in actively regulating extracellular ions (Walz, 1989) and transmitters (Kimelberg et al., 1993), and it has been speculated that epilepsy may be associated with a defect in the ability of glial cells to buffer extracellular ionic changes (Hashimoto et al., 1979, Heinemann et al., 1986, Meldrum, 1994). This inability may be reflected in the marked morphological and histochemical changes of glial cells, features often referred to as gliosis. Gliosis is a prominent feature of human epilepsy (Ransom, 1979, Beach et al., 1995) and is also present in animal models of epilepsy (Niquet et al., 1993, Niquet et al., 1994, Beach et al., 1995, Jacobs et al., 1996).
Buffering of extracellular K+ by glial cells has been particularly well studied in various in vitro and in situ models (Kuffler et al., 1966, Orkand et al., 1966, Tang et al., 1980, Gardner-Medwin et al., 1981, Heinemann and Dietzel, 1984, Kettenmann et al., 1985, Ballanyi et al., 1987, Kettenmann et al., 1987, Hoppe et al., 1991a, Hoppe et al., 1991b), and is generally believed to involve the diffusional uptake of K+ through potassium ion channels. A class of inwardly rectifying, Ba2+- and Cs+-sensitive K+ channels has been particularly implicated in this function since these channels show high open probability at the resting potential and since their conductance increases with increasing concentrations of K+ (Newman, 1993, Ransom and Sontheimer, 1995). Inwardly rectifying K+ channels are abundantly expressed by glial cells in the normal CNS, and are the most prominent channel expressed in adult hippocampal astrocytes in situ (Bordey and Sontheimer, 1997). While much has been speculated about possible impairments of glial ion homeostasis in conjunction with epilepsy, little is known about features of glial cells associated with seizure foci. Specifically it is largely unknown if astrocytes associated with seizure foci show altered expression of neurotransmitter transporters, changes in ion channel complement, or gap junction coupling. In our initial attempts to better understand whether astrocytes at epileptic seizure foci are intrinsically different from `normal' astrocytes, we studied short-term cultures of astrocytes isolated from patients with intractable epilepsy. These studies provided compelling evidence for enhanced expression of voltage-activated sodium channels (de Lanerolle et al., 1994), and enhanced gap junction coupling (de Lanerolle et al., 1994) in concert with significantly depolarized resting membrane potential. However, these changes may have arisen in conjunction with culturing these human glial cells and may not necessarily be related to the disease origin of these cells.
To get a better understanding of intrinsic changes of astrocytes that are associated with epileptic seizures, we characterized astrocytes in acute slices from seven patients with intractable epilepsy and compared their features to comparison tissue from a patient operated for other reasons. Using whole-cell patch-clamp recordings we show marked changes in the channel complement of astrocytes from epileptic patients as compared to non-seizure derived astrocytes. Most notably, we observed a 7-fold increase in Na+ channel expression in concert with a significant reduction in expression of inwardly rectifying K+ channels in some cells. These changes endow glial cells with the ability to fire slow action potentials. Whether such responses occur in vivo and whether they could contribute to electrical abnormalities during seizures is unclear. However, the marked changes in the relative expression of Na+ to K+ channels suggest that seizure-associated astrocytes may be impaired in their ability to regulate extracellular K+, and may thereby indirectly contribute to seizures.
Section snippets
Patient selection
The resected lateral temporal lobes from seven patients with intractable seizures unresponsive to medical therapy were examined. The ages of the patients ranged from 23 to 44 years. Additional details of these patients are given in Table 1. It is difficult to address epilepsy-specific features in these human tissues without adequate controls. We used parallel recordings in adult rat hippocampus (Bordey and Sontheimer, 1997) and brain tissues resected from a non-epileptic patient for control
Results
We obtained whole-cell recordings from 17 astrocytes in acute biopsy slices from seven patients surgically treated for intractable epilepsy (Table 1). Recordings were compared to astrocytes in biopsy slices of a patient operated for another neurological condition that did not present with seizures (neoplasm) or to astrocytes recorded in adult rat hippocampal brain slices. We refer to cells recorded from the non-seizure biopsy as human comparison cells, to cells from rat tissue as rat control
Discussion
We show expression of voltage-activated Na+ and K+ currents in human astrocytes in situ. A comparison of astrocytes from biopsies reveal marked differences between cells recorded from patients that were operated for intractable seizures as compared to astrocytes from a patient operated for other reasons. Most notable, we observed a 7-fold higher expression of voltage-activated Na+ channels in seizure-associated astrocytes than in comparison cells. In addition, the ratio of Na+ to K+
Acknowledgements
Supported by NIH grants RO1-NS31234 and P50-HD-32901.
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