Stimulation-based anticipation and control of state transitions in the epileptic brain.

We focus on the implications that the underlying neuronal dynamics might have on the possibility of anticipating seizures and designing an effective paradigm for their control. Transitions into seizures can be caused by parameter changes in the dynamic state or by interstate transitions as occur in multi-attractor systems; in the latter case, only a weak statistical prognosis of the seizure risk can be achieved. Nevertheless, we claim that by applying a suitable perturbation to the system, such as electrical stimulation, relevant features of the system's state may be detected and the risk of an impending seizure estimated. Furthermore, if these features are detected early, transitions into seizures may be blocked. On the basis of generic and realistic computer models we explore the concept of acute seizure control through state-dependent feedback stimulation. We show that in some classes of dynamic transitions, this can be achieved with a relatively limited amount of stimulation.

Development of tolerance to levetiracetam in rats with chronic epilepsy.

PURPOSE: Pharmacoresistance is a major problem in the treatment of epilepsy. We showed previously that pharmacoresistance, at least partially, is due to an up-regulation of the multidrug transporter (MDT) P-glycoprotein (P-gp): inhibition of P-gp improves seizure control in phenytoin-treated epileptic rats (poststatus epilepticus rat model for temporal lobe epilepsy). Since it has been suggested that levetiracetam (LEV) is no substrate for MDTs, we hypothesized that LEV would more adequately control seizures in this rat model. METHODS: Chronic epileptic rats were treated repeatedly with LEV (2-week interval; different dosages) via continuous infusion using osmotic minipumps, 5-6 months after electrically induced status epilepticus. The anticonvulsive effects were determined by video-EEG monitoring and the concentration of LEV was measured in plasma and brain homogenates using gas chromatography. RESULTS: LEV adequately entered the epileptic brain and dose-dependently suppressed spontaneous seizures in chronic epileptic rats for 3-4 days. Hereafter, seizure frequency increased, while LEV plasma levels did not change. Seizure behavior was less severe throughout the whole treatment. LEV did not affect seizure duration. After a withdrawal period of 2 weeks all rats initially responded again to LEV. CONCLUSIONS: The initial seizure control by LEV supports the observation that LEV is not impeded by MDTs. However, the failure to control seizures for a longer period of time indicates the development of tolerance to this drug. This poses another problem in the treatment of this kind of epilepsy. Whether tolerance may be prevented by intermittent administration of LEV should be further investigated.

Phase clustering of high frequency EEG: MEG components.

The study of phase consistency of high frequency EEG/MEG components can reveal properties of neuronal networks that are informative about their excitability state. The clue is that these properties are easier to put in evidence when the response of the neuronal networks is evoked by an adequate stimulation paradigm. The latter may be considered a probe of neuronal excitability state capable of revealing hidden information contained in the phase structure of neuronal activities. In this context the high frequency band components appear to be the most reactive signals.

Increased expression of ferritin, an iron-storage protein, in specific regions of the parahippocampal cortex of epileptic rats.

PURPOSE: Iron accumulation in the brain has been associated with neurodegenerative disorders, including epilepsy. In our previous SAGE study, we showed that ferritin, an iron-storage protein, was one of the genes (Ferritin-H) that showed overexpression before the chronic epileptic phase. In this study we used ferritin as indicator for disturbed iron homeostasis to acquire insight into whether this could play a role in the pathogenesis of temporal lobe epilepsy. METHODS: With immunocytochemistry, we studied the regional and cellular distribution of ferritin protein in an animal model for temporal lobe epilepsy in which spontaneous seizures develop a few weeks after electrically induced status epilepticus (SE). RESULTS: Increased ferritin expression was observed in regions known to be vulnerable to cell death, mainly in reactive microglial cells of epileptic rats. Ferritin expression after SE was initially high, especially throughout the hippocampus, but decreased over time. In the chronic epileptic phase, it was still upregulated in regions where extensive cell loss occurs during the early acute and latent period. Within the parahippocampal region, the most persistent ferritin overexpression was present in microglial cells in layer III of the medial entorhinal area. The upregulation was most extensive in rats that had developed a progressive form of epilepsy with frequent seizures (approximately five to 10 seizures per day). CONCLUSIONS: The fact that ferritin upregulation is still present in specific limbic regions in chronic epileptic rats, when neuronal loss is absent or minimal, suggests a role of iron in the pathogenesis and progression of epilepsy.

Physiological changes in chronic epileptic rats are prominent in superficial layers of the medial entorhinal area.

PURPOSE: We investigated whether the functional network properties of the medial entorhinal area (MEA) of the entorhinal cortex were altered in a rat model of chronic epilepsy that is characterized by extensive cell loss in MEA layer III. METHODS: Responses were evoked in the entorhinal cortex by electrical stimulation of the subiculum in anesthetized chronic epileptic rats, 2-4 months after status epilepticus, induced by systemic kainate (KA) injections. Laminar field potentials were measured using a 16-channel silicon probe that covered all six layers of the MEA; an estimate of the local transmembrane currents was made using current source density analysis. RESULTS: Double-pulse stimulation of the subiculum evoked responses in deep and superficial layers of the MEA in control and KA rats. A current sink in layer I and at the border of layer I and II that was induced by antidromic activation of MEA-II, was much more prominent in KA rats with extensive neuronal loss in MEA-III than in control rats or KA rats with minor MEA-III loss. Furthermore, KA rats that displayed MEA-III loss presented a series of oscillations induced by subicular stimulation in the beta/gamma-frequency range (20-100 Hz), which were confined to superficial layers of MEA. These oscillations were never observed in control rats or KA rats with minor MEA-III loss. CONCLUSIONS: These results indicate that the observed alterations in the superficial MEA responses to subiculum stimulation and the occurrence of beta/gamma-oscillations are related phenomena, which are a consequence of altered and impaired inhibition within these MEA layers in chronic epileptic rats.

Magnetoencephalography: an investigational tool or a routine clinical technique?

Magnetoencephalography (MEG) is a relatively novel noninvasive technique, with a much shorter history than EEG, that conveys neurophysiological information complementary to that provided by EEG, with high temporal and spatial resolution. Despite its a priori, highly competitive profile, the role of MEG in the clinical setting is still controversial. We briefly review the major obstacles MEG faces in becoming a routine clinical test and the different strategies needed to bypass them. The high cost and complexity associated with MEG equipment are powerful hindrances to wide acceptance of this relatively new technique in clinical practice. The most straightforward advantage is based on the relative facility of MEG recordings in the process of source localization, which also carries some degree of uncertainty, thus partly explaining why the development of clinical applications of MEG has been so slow. Obviously, a decrease in the cost and the elaboration of semiautomatic protocols that could reduce the complexity of the studies and favor the development of consensual strategies, as well as a major effort on the part of clinicians to identify clinical issues where MEG could be decisive, would be most welcome.

GAP-43 mRNA and protein expression in the hippocampal and parahippocampal region during the course of epileptogenesis in rats.

In order to reveal axonal rewiring in the hippocampal and parahippocampal regions after status epilepticus, we investigated the temporal evolution of growth-associated protein-43 (GAP-43) mRNA and protein expression in two rat models of mesial temporal lobe epilepsy (MTLE). Status epilepticus (SE) was induced by electrical stimulation of the angular bundle or by intraperitoneal kainic acid (KA) injections. Despite increased GAP-43 mRNA expression in dentate granule cells at 24 h after SE, GAP-43 protein expression in the inner molecular layer (IML) of the dentate gyrus decreased progressively after 24 h after SE in both models. Nevertheless robust mossy fiber sprouting (MFS) was evident in the IML of chronic epileptic rats. Remaining GAP-43 protein expression in the IML in chronic epileptic rats did not correlate with the extent of MFS, but with the number of surviving hilar neurons. In the parahippocampal region, GAP-43 mRNA expression was decreased in layer III of the medial entorhinal area (MEAIII) in parallel with extensive neuronal loss in this layer. There was a tendency of GAP-43 mRNA up-regulation in the presubiculum, a region that projects to MEAIII. With regard to this parahippocampal region, however, changes in GAP-43 mRNA expression were not followed by protein changes. The presence of the presynaptic protein GAP-43 in a neurodegenerated MEAIII indicates that fibers still project to this layer. Whether reorganization of fibers has occurred in this region after SE needs to be investigated with tools other than GAP-43.