Gamma oscillations have been implicated in higher cognitive processes and might critically depend on proper mitochondrial function. Using electrophysiology, oxygen sensor microelectrode, and imaging techniques, we investigated the interactions of neuronal activity, interstitial pO 2 , and mitochondrial redox state [NAD(P)H and FAD (flavin adenine dinucleotide) fluorescence] in the CA3 subfield of organotypic hippocampal slice cultures. We find that gamma oscillations and spontaneous network activity decrease significantly at pO 2 levels that do not affect neuronal population responses as elicited by moderate electrical stimuli. Moreover, pO 2 and mitochondrial redox states are tightly coupled, and electrical stimuli reveal transient alterations of redox responses when pO 2 decreases within the normoxic range. Finally, evoked redox responses are distinct in somatic and synaptic neuronal compartments and show different sensitivity to changes in pO 2 . We conclude that the threshold of interstitial pO 2 for robust CA3 network activities and required mitochondrial function is clearly above the "critical" value, which causes spreading depression as a result of generalized energy failure. Our study highlights the importance of a functional understanding of mitochondria and their implications on activities of individual neurons and neuronal networks.

The mechanisms of seizure emergence, and the role of brief interictal epileptiform discharges (IEDs) in seizure generation are two of the most important unresolved issues in modern epilepsy research and clinical epileptology. Our study shows that the transition to seizure is not a sudden phenomenon, but a slow process characterized by the progressive loss of neuronal network resilience. From a dynamical perspective, the slow transition is governed by the principles of critical slowing, a robust natural phenomenon observable in systems characterized by transitions between contrasting dynamical regimes. In epilepsy, this complex process is modulated by the synchronous synaptic input from IEDs. IEDs are external perturbations that produce phasic changes in the slow transition process and can exert opposing effects on the dynamics of a seizuregenerating network, causing either stabilizing anti-seizure or destabilizing pro-seizure effects. We show that the multifaceted nature of IEDs is defined by the dynamical state of the seizuregenerating network at the moment of the discharge occurrence, not necessarily by the existence of distinct cellular mechanisms.

Metabolic dysfunction has been implicated in the pathogenesis of temporal lobe epilepsy (TLE), but its manifestation during neuronal activation in the ex vivo hippocampus from TLE patients has not been shown. We characterized metabolic and mitochondrial functions in acute hippocampal slices from pilocarpine-treated, chronic epileptic rats and from pharmaco-resistant TLE patients. Recordings of NAD(P)H fluorescence indicated the status of cellular energy metabolism, and simultaneous monitoring of extracellular potassium concentration ([K+]o) allowed us to control the induction of neuronal activation. In control rats, electrical stimulation elicited biphasic NAD(P)H fluorescence transients that were characterized by a brief initial 'drop' and a subsequent prolonged 'overshoot' correlating to enhanced NAD(P)+ reduction. In chronic epileptic rats, overshoots were significantly smaller in area CA1, but not in the subiculum as compared to controls. In TLE patients, who were histopathologically classified in groups with and without Ammon's horn sclerosis (AHS, non-AHS), large drops and very small overshoots of NAD(P)H transients were observed in dentate gyrus, CA3, CA1 and subiculum. Nevertheless, monitoring mitochondrial membrane potential (DeltaPsi(m)) by mitochondria-specific, voltage-sensitive dye (rhodamine-123) revealed similar mitochondrial responses during neuronal activation with glutamate and protonophore application in area CA1 of control and chronic-epileptic rats. Applying confocal laser scanning microscopy, these findings were confirmed in individual neurons of AHS tissue, indicating a negative DeltaPsi(m) and activation-dependent mitochondrial depolarization. Our data demonstrate severe metabolic dysfunction during neuronal activation in the hippocampus from chronic epileptic rats and humans, although mitochondria maintain negative DeltaPsi(m). Thus, our findings provide a cellular correlate for 'hypometabolism' as described for epilepsy patients and suggest mitochondrial enzyme defects in TLE.

SUMMARYPurpose: CO 2 has been long recognized for its anticonvulsant properties. We aimed to determine whether inhaling 5% CO 2 can be used to suppress seizures in epilepsy patients. The effect of CO 2 on cortical epileptic activity accompanying behavioral seizures was studied in rats and nonhuman primates, and based on these data, preliminary tests were carried out in humans. Methods: In freely moving rats, cortical afterdischarges paralleled by myoclonic convulsions were evoked by sensorimotor cortex stimulation. Five percent CO 2 was applied for 5 min, 3 min before stimulation. In macaque monkeys, hypercarbia was induced by hypoventilation while seizure activity was electrically or chemically evoked in the sensorimotor cortex. Seven patients with drugresistant partial epilepsy were examined with video-EEG (electroencephalography) and received 5% CO 2 in medical carbogen shortly after electrographic seizure onset.Results: In rats, 5% CO 2 strongly suppressed cortical afterdischarges, by approximately 75%, whereas responses to single-pulse stimulation were reduced by about 15% only. In macaques, increasing pCO 2 from 37 to 44-45 mm Hg (corresponding to inhalation of 5% CO 2 or less) suppressed stimulation-induced cortical afterdischarges by about 70% and single, bicucullineinduced epileptiform spikes by approximately 25%. In a pilot trial carried out in seven patients, a rapid termination of electrographic seizures was seen despite the fact that the application of 5% CO 2 was started after seizure generalization. Conclusions: Five percent CO 2 has a fast and potent anticonvulsant action. The present data suggest that medical carbogen with 5% CO 2 can be used for acute treatment to suppress seizures in epilepsy patients.