Research ReportThe mode of death of epilepsy-induced “dark” neurons is neither necrosis nor apoptosis: An electron-microscopic study
Introduction
Ever since the publication of the relevant pioneering paper (Söderfeldt et al., 1983), the “dark” neurons induced by epilepsy have been unanimously believed (Thom et al., 2008) to die through the necrotic pathway. The same has been stated (Auer et al., 2008) for the “dark” neurons induced by ischemia (Smith et al., 1984) or hypoglycemia (Auer et al., 1985). This assumption was based on light- and electron-microscopic observations suggesting that, from necrotic, excitotoxic or contused brain areas, the “dark” neurons produced by these noxae are removed via a necrotic-like sequence of morphological changes.
In contrast, our recent electron-microscopic observations suggested that the mode of death of traumatic (Csordás et al., 2003), electric (Csordás et al., 2003), hypoglycemic (Gallyas et al., 2005) and ischemic (Kövesdi et al., 2007) “dark” neurons is neither necrosis nor apoptosis. This assumption was based on electron-microscopic observations proving that a number of “dark” neurons are produced by these noxae even in non-necrotic, non-excitotoxic or non-contused (apparently normal) tissue areas, from which they are removed via an apoptotic-like sequence of morphological changes.
In the present paper, we investigate whether or not this assumption also applies to “dark” neurons induced by epilepsy. Since there are biochemical differences between the pathological circumstances at issue (Auer and Siesjö, 1988, Liou et al., 2003), it is not evident that the epilepsy-produced “dark” neurons are removed from apparently normal (non-excitotoxic) brain areas via the apoptotic-like sequence of ultrastructural changes, which could support the non-necrotic and non-apoptotic nature of their death.
For the induction of epilepsy, a 4-aminopyridine paradigm was used which produces a relatively large number of “dark” neurons in an apparently normal cortical area not far from a large excitotoxic area (Baracskay et al., in press).
Section snippets
Results
The observations presented below are confined to the morphological features pertinent to the nature and fate of “dark” neurons induced by placing a 4-aminopyridine crystal on the exposed cortical surface of the rat (Baracskay et al., in press). Other aspects of epilepsy-induced morphological brain damage have been demonstrated and discussed appropriately in previous papers (Söderfeldt et al., 1983, Ingvar et al., 1988, Covolan and Mello, 2000, Pena and Tapia, 2000, Baracskay et al., in press).
Formation of “dark” neurons
In neuropathology, at least three types of “dark” neurons are generally accepted: reversible, irreversible and artifactous (Graeber et al., 2002). In connection with in-vivo or post-mortem head injuries (Csordás et al., 2003, Gallyas et al., 2004), in-vivo or post-mortem electric shocks (Csordás et al., 2003, Kellermayer et al., 2006), hypoglycemia (Gallyas et al., 2005) and ischemia (Kövesdi et al., 2007), we have demonstrated that the process of formation of “dark” neurons induced by these
Animal experiments
A total of 18 randomly elected male Sprague–Dawley rats weighing between 300 and 500 g were anesthetized with a 1.5:98.5 v/v mixture of Halothane and air. A hole of 1.5 mm in diameter was drilled into the exposed calvaria, above the right brain cortex, 6.2 mm caudal to the bregma and 2.5 mm lateral from the midline. In each rat, the dura mater was carefully removed and a 0.5 mg/kg 4-aminopyridine crystal was placed onto the cortical surface for 40 min. Thereafter, the crystal was washed out
Acknowledgments
The authors thank Andok Csabáné, Nyirádi József and Dr. Nádor Andrásné for their valuable help in the light-microscopic, electron-microscopic and photographic work, respectively. This study was supported by Hungarian research grants ETT-176/2006 and Regional Center of Excellence DNT RET and CellKom RET.
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