Abstracts

Historically, it has always been assumed that brain tumors (BT) are a frequent cause of epileptic seizures. Seizures occur in 50% of patients with intracranial brain tumors and AED therapy is prophylactically administered to most brain tumor patients. Chronic epilepsy can be the only symptom of low grade brain tumors. There is a significant overlap between genes that are associated with genetic forms of epilepsy and alterations in tumor suppressor genes. It is however not clear if a cause-effect relationship exists between epileptogenesis and tumorigenesis. Chronic epilepsy prolongs survival of patients with low-grade gliomas that present with seizures [1]. We hypothesized that two factors associated with chronic epilepsy, abnormal electrical activity and elevated [K]out can act as anti-proliferative agents. We used normal human astrocytes, epileptic glia and C6 rat glioma cells in multi-well Petri dishes equipped with an array of stainless steel electrodes connected to a PC via an I/O board. The electrodes were connected to a pulse generator interfaced with a computer. Cells were exposed to different electrical parameters of stimulation (current intensity 7.5 [mu]A) for three to five days. Cells exposed to10 Hz stimulation grew at a rate comparable to control (p[lt]0.05). Stimulation at 25 - 100Hz caused a pronounced decrease in the number of cells as early as three days after stimulation. The effects persisted and grew larger with prolonged exposure to electric pulses. We hypothesized that decreased cell proliferation rather than cell death were responsible for the decreased number of cells in stimulated wells. We confirmed that stimulation at 50 Hz decreased cell number by a direct effect on cell cycle and not by triggering cell death by measuring incorporation of BrdU and release of adenylate kinase, markers of cell division and cellular damage respectively. Applying current intensities higher 8.5 uA caused cellular damage as reveled by a statistical significant increase of AK release. Proteomic analysis demonstrated that the decreased propensity to cell proliferation was accompanied by an increased expression of a specific member of the KIR family, GIRK2 (Kir3.2). Blockade of KIR by cesium or barium abolished the effects of electrical stimulation. However, since blockade of KIR results in depolarization, we also tested whether changes in membrane potential per se may affect proliferation. This was achieved by exposing unstimulated astrocytes to increasing concentrations of KCl; manipulations of extracellular sodium were entirely ineffective. External potassium acted as deterrent for cell division at concentrations [gt]4 mM suggesting that depolarization was not crucial. Our results suggest that 1) High frequency epileptic activity acts a deterrent to cell division; 2) This effect appears to involve potassium coundactances, specifically Kir3.2.
1. Ann. Neurol., 31 (1992) 431-436. (Supported by NIH NINDS, NHLBI. )