Dentate gyrus autonomous ictal activity in the status epilepticus rat model of epilepsy.

The dentate gyrus (DG) as part of the hippocampal formation is believed to serve as a gatekeeper with strong inhibitory properties against uncontrolled propagation of neuronal activity from the entorhinal cortex and neocortical structures. In temporal lobe epilepsy, the DG becomes hyperexcitable and loses its gate function, enabling propagation of ictal activity into downstream structures such as CA3 and CA1 areas. Furthermore, the DG, apart from facilitating propagation, may also be able to autonomously generate ictal activity, but this point has remained open so far. To tackle this question, we used intrinsic optical imaging in combination with electrophysiological recordings in brain slice preparations from rats in which status epilepticus had been induced electrically several weeks prior to measurements. Upon omission of Mg++ from the artificial cerebrospinal fluid, in 15 out of 33 slices (45.4%) from 9 out of 13 epileptic animals (69.2%), spontaneous and autonomous ictal activity, mostly seizure-like events (SLE), was observed in the DG. This activity manifested independently from SLE generated in adjacent cortices and never occurred in slices from control animals. SLE generated in the DG differed from those with origin in the entorhinal or temporal cortex by longer latency to the first event after Mg++ omission (p<0.001), a higher SLE frequency (p<0.05), higher amplitude (p<0.001) and a longer SLE duration (p<0.05). We conclude that in epilepsy, the DG, in addition to facilitated gating of activity from upstream structures, can serve as an independent generator of ictal activity.

Circadian dentate gyrus excitability in a rat model of temporal lobe epilepsy.

In human mesial temporal lobe epilepsy (mTLE), seizure occurrence peaks in the late afternoon and early evening. This temporal binding of seizures has been replicated in animal models of mTLE following electrically-induced status epilepticus (SE). We hypothesized that in chronic epilepsy, alterations of circadian excitatory and inhibitory functions of the dentate gyrus (DG), which is believed to regulate the generation of limbic seizures, pathophysiologically contribute to the temporal binding of ictogenesis. We performed electrophysiological single and paired pulse measurements hourly over 24h in the DG of epileptic rats (n=8) 8 weeks after electrically induced SE. Results were compared to individual data obtained before induction of SE and to those of control animals (n=3). Pre and post SE data were analyzed in two distinct phases of the day, i.e. a high-seizure phase between 2p.m. and 10p.m. and a low-seizure phase between 10p.m. and 2p.m. In chronic epileptic animals, latency of evoked potentials was significantly reduced in the high-seizure phase (p=0.027) but not in the low-seizure phase. Compared to baseline values, paired pulse inhibition was significantly increased during the low-seizure phase (interpulse interval (IPI) 25ms, p=0.003; IPI 30ms; p<0.001) but not in the high-seizure phase. Similarly, when compared to controls, inhibition at IPI 20ms was diminished only in the high-seizure phase (p=0.027). Thus, in chronic epileptic animals, DG excitability is increased in the afternoon and early evening possibly contributing to the time of day-dependency of spontaneous seizures in this model system of mTLE. Alterations of circadian DG excitability in epileptic animals may be influenced by changes in hypothalamus-regulated superordinate functions such as excretion of endocrine hormones but further studies are needed.

Functional and morphological changes in the dentate gyrus after experimental status epilepticus.

Status epilepticus may cause long-term functional and structural consequences possibly resulting in brain dysfunctions such as chronic epilepsy. In epileptogenesis, the dentate gyrus plays a key role in regulating the excitability of highly vulnerable and potentially epileptogenic downstream structures in the hippocampus proper. One, four and eight weeks after electrically induced status epilepticus, excitability and neuronal degeneration in the rat dentate gyrus were examined with intracerebral electrodes and Fluoro Jade (FJ) staining, respectively. Half of the animals had developed chronic epilepsy by 8 weeks after status epilepticus. Sham-operated controls did not exhibit seizures, and the excitatory parameters remained unchanged. Compared to controls, 8 weeks after status epilepticus the population spike latency in the dentate gyrus was significantly reduced (p<0.05) and substantial neuronal degeneration was seen (p<0.05). In summary, status epilepticus results in functional and morphological alterations in the dentate gyrus likely contributing to epileptogenesis.

Temperature regulation is compromised in experimental limbic status epilepticus.

Temperature dysregulation is well known in generalized convulsive status epilepticus but so far has not been reported in non-convulsive forms. In order to detect possible subtle alterations, we have analyzed the capability to compensate for external cooling in an animal model of limbic status epilepticus. Rats with electrically induced self-sustaining status epilepticus (SSSE) (n=6) as well as rats without electrical stimulation (n=6) were cooled for 3 h and then rewarmed for another hour. The time course of changes in epidural temperature in animals of both groups that underwent cooling and in control rats that were not cooled and not stimulated (n=6) was compared. In animals with limbic SSSE, temperature fell continuously and was significantly lower at all time points under cooling as compared with each of the two other groups. In animals that were not stimulated, temperature under cooling fell by 1 to 2 degrees C only and was not significantly different at any time point as compared with controls. The effect of cooling was reversible in both groups. The current data indicate that temperature homeostasis in limbic status epilepticus is markedly disturbed. This finding may suggest ictal involvement of primary thermoregulatory neurons in the anterior hypothalamus probably by spread of epileptic activity from temporo-mesial structures.

Diagnosis of psychogenic nonepileptic status epilepticus in the emergency setting.

Episodes of psychogenic nonepileptic status epilepticus (PNESE) characterized by pronounced generalized motor features were compared with those of refractory generalized convulsive status epilepticus. Patients with PNESE were younger, had port systems implanted more frequently, received higher doses of benzodiazepines until seizure termination or respiratory failure, and had lower serum creatine kinase levels.

Repetitive spreading depression-like events result in cell damage in juvenile hippocampal slice cultures maintained in normoxia.

Prolonged seizures, e.g., induced by fever, experienced early in life are considered a precipitating injury for the subsequent development of temporal lobe epilepsy. During in vitro epileptiform activity, spreading depressions (SDs) have often been observed. However, their contribution to changes in the properties of juvenile neuronal tissue is unknown. We therefore used the juvenile hippocampal slice culture preparation (JHSC) maintained in normoxia (20% O(2)-5% CO(2)-75% N(2)) to assess the effect of repetitive SD-like events (SDLEs) on fast field potentials and cell damage. Repetitive SDLEs in the CA1 region could be induced in about two-thirds of the investigated JHSCs (n = 61) by repetitive electrical stimulation with 2-200 pulses. SDLEs were characterized by a transient large negative field potential shift accompanied by intracellular depolarization, ionic redistribution, slow propagation (assessed by intrinsic optical signals) and glutamate receptor antagonist sensitivity. The term "SDLE" was used because evoked fast field potentials were only incompletely suppressed and superimposed discharges occurred. With 20 +/- 1 repetitive SDLEs (interval of 10-15 min, n = 7 JHSCs), the events got longer, their amplitude of the first peak declined, while threshold for induction became reduced. Evoked fast field potentials deteriorated and cell damage (assessed by propidium iodide fluorescence) occurred, predominantly in regions CA1 and CA3. As revealed by measurements of tissue partial oxygen pressure during SDLEs repetitive transient anoxia accompanying SDLE might be critical for the observed cell damage. These results, limited so far to the slice culture preparation, suggest SDs to be harmful events in juvenile neuronal tissue in contrast to what is known about their effect on adult neuronal tissue.

A "malignant" variant of status epilepticus.

BACKGROUND: Status epilepticus (SE) frequently does not respond to common first-line anticonvulsants. In a substantial portion of patients, administration of anticonvulsant anesthetics is inevitable. Even this aggressive approach fails to terminate SE in an undefined number of cases. We have coined the term malignant SE for this most severe variant of SE. OBJECTIVE: To assess frequency, risk factors, and in-hospital outcome of malignant SE. DESIGN: Retrospective cohort study. SETTING: Neurologic intensive care unit of a large university hospital. Patients Sample of 35 episodes of SE not responding to first-line anticonvulsants in 34 patients. MAIN OUTCOME MEASURES: Predictive and prognostic features of episodes of malignant SE with persistent epileptic activity after high-dose anesthetics compared with features of the remainder of cases with refractory SE and persistent epileptic activity after failure of first-line anticonvulsants. RESULTS: Status epilepticus that could not be controlled by first-line anticonvulsants resulted in malignant SE in 20% of cases. Patients with malignant SE were significantly younger than patients with refractory SE (P = .03). Encephalitis was identified as an independent risk factor for malignant SE (P = .008). Outcome in malignant SE was poor, with significantly longer duration of seizure activity (P<.001), longer stay in the neurologic intensive care unit (P<.001) and in the hospital (P = .007), and more patients with functional dependency at discharge from the hospital (P = .04). CONCLUSIONS: Malignant SE is not rare after failure of first-line anticonvulsants. The patient at risk is typically young and suffers from encephalitis. Such patients should be treated aggressively early in the course of SE to prevent malignant SE.

Processes and components participating in the generation of intrinsic optical signal changes in vitro.

Imaging of intrinsic optical signals has become an important tool in the neurosciences. To better understand processes underlying changes in intrinsic optical signals, we studied electrical stimulation at varying strengths in hippocampal slices of adult Wistar rats. Following serial stimulation we observed an increase in light transmittance in all tested slices. During antidromic stimulation at minimum stimulation strength the increase in light transmittance was 75 +/- 8% (P < 0.05), and during orthodromic minimum stimulation 19.6 +/- 5.6% (P < 0.001) in the stratum pyramidale of the CA1-region. During orthodromic stimulation no significant difference between submaximum, maximum and supramaximum stimulation was found, indicating saturation. In contrast, submaximum antidromic stimulation yielded 56.2 +/- 12% (P < 0.05) of maximum stimulation strength, indicating recruitment. In a further set of experiments serial stimulation was carried out under glial blockade with fluoroacetate (FAC) or blockage of mitochondrial function. Amplitude and slope of the intrinsic optical signal significantly decreased in the presence of FAC (amplitude: 36 +/- 6%, P < 0.01; slope: 37 +/- 11% as compared with baseline conditions, P < 0.05). This suggests a glial participation in signal generation. Rotenone, an inhibitor of mitochondrial complex I, yielded decreased amplitudes of the intrinsic optical signal (27 +/- 7% after 40 min, P < 0.01). Our data indicate that the intrinsic optical signal change reflects type and strength of neuronal activation and point to glia and mitochondria as important participants in signal generation.

Oxygen consumption and mitochondrial membrane potential indicate developmental adaptation in energy metabolism of rat cortical neurons.

Neuronal energy needs are mainly covered via mitochondrial oxidative phosphorylation. Even if the energy supply appears identical in immature and adult brain, nevertheless quantitative differences exist. The present study focuses on the adaptations in cellular energy metabolism caused by the neuronal maturation. As main parameters of oxidative phosphorylation, cellular oxygen consumption and mitochondrial membrane potential were measured in isolated rat cortical cells using a Clark-type oxygen electrode and microfluorometric techniques. In four age groups (E18-P2, P8-P12, P16-P20, > or = P28), unstimulated neurons showed a significant age-dependent increase in basal oxygen consumption (6.1 up to 10.2 nM/min/10(7) cells). The excitatory neurotransmitter glutamate induced a further, but age- and concentration-independent, elevation of oxygen consumption to a plateau > or = 14 nM/min/10(7) cells and a complete depolarization of mitochondrial membrane in neurons > or = P8. Stimulation using K+ (5-50 mM) effected a concentration- and age-dependent increase in oxygen consumption, but a similar nearby complete depolarization of mitochondrial membrane in all tested age groups. Furthermore, uncoupling mitochondrial membrane function followed by a complete depolarization of mitochondrial membrane showed a maximal oxygen consumption (14-15 nM/min/10(7) cells) only in neurons > or = P8. These data suggest that developing and adult cortical neurons cover their increased need of energy following stimulation by an efficiency improvement of mitochondrial oxidative phosphorylation. The age-independent limited capacity of mitochondrial oxidative phosphorylation, however, causes a reduction in cellular energy disposal in mature neurons and therefore may play a critical role in the increased sensitivity of adult neurons against excitotoxicity and ischaemia.

Increased extracellular K+ concentration reduces the efficacy of N-methyl-D-aspartate receptor antagonists to block spreading depression-like depolarizations and spreading ischemia.

BACKGROUND AND PURPOSE: Spreading depression (SD)-like depolarizations may augment neuronal damage in neurovascular disorders such as stroke and traumatic brain injury. Spreading ischemia (SI), a particularly malignant variant of SD-like depolarization, is characterized by inverse coupling between the spreading depolarization wave and cerebral blood flow. SI has been implicated in particular in the pathophysiology of subarachnoid hemorrhage. Under physiological conditions, SD is blocked by N-methyl-D-aspartate receptor (NMDAR) antagonists. However, because both SD-like depolarizations and SI occur in presence of an increased extracellular K+ concentration ([K+]o), we tested whether this increase in baseline [K+]o would reduce the efficacy of NMDAR antagonists. METHODS: Cranial window preparations, laser Doppler flowmetry, and K+-sensitive/reference microelectrodes were used to record SD, SD-like depolarizations, and SI in rats in vivo; microelectrodes and intrinsic optical signal measurements were used to record SD and SD-like depolarizations in human and rat brain slices. RESULTS: In vivo, the noncompetitive NMDAR antagonist dizocilpine (MK-801) blocked SD propagation under physiological conditions, but did not block SD-like depolarizations or SI under high baseline [K+]o. Similar results were found in human and rat neocortical slices with both MK-801 and the competitive NMDAR antagonist D-2-amino-5-phosphonovaleric acid. CONCLUSIONS: Our data suggest that elevated baseline [K+]o reduces the efficacy of NMDAR antagonists on SD-like depolarizations and SI. In conditions of moderate energy depletion, as in the ischemic penumbra, or after subarachnoid hemorrhage, NMDAR inhibition may not be sufficient to block these depolarizations.

Seizure spread through the life cycle: optical imaging in combined brain slices from immature, adult, and senile rats in vitro.

The semiology of epileptic seizures changes during the lifetime. Hence, it can be assumed that age-related changes in brain plasticity influence the patterns of seizure onset, spread and propagation velocity. We employed the 4-aminopyridine model of epilepsy to study seizure-like events in vitro. Combined entorhinal cortex-hippocampus brain slices from juvenile (10-13 days), adult (2-3 months), and senile (24-27 months) rats were examined using electrophysiological recordings and imaging of intrinsic optical signals. In the juvenile group, seizure onset was multifocal in all slice regions including the hippocampus. Onset in adult animals was confined to the entorhinal cortex and to neocortical regions. In slices from senile animals, there was a preponderance of seizure onsets in the neocortex. Spread patterns were highly variable in the juvenile group and became gradually more monomorph with increasing age. Propagation velocities were highest in the adult group, with maximum values of 1.51 +/- 0.68 mm/s. In the juvenile group, they amounted to 0.97 +/- 0.39 mm/s, and to 1.18 +/- 0.42 mm/s in senile slices. The results of this study indicate that age-related changes in brain plasticity profoundly affect spread patterns, which may contribute to the clinically observed changes in seizure semiology during early childhood, adulthood and senescence.

Limbic self-sustaining status epilepticus in rats is not associated with hyperthermia.

PURPOSE: To evaluate the impact of limbic status epilepticus on temperature. METHODS: The perforant path in freely moving rats was stimulated electrically for 120 min to induce self-sustaining status epilepticus (SSSE). For 150 min after the end of stimulation, epidural temperature and electrographic and clinical seizure activity were assessed in animals with limbic and motor SSSE, as well as in animals without development of SE. RESULTS: Temperature in all animals with SSSE was elevated by 1.5+/-0.8 degrees C after the end of stimulation compared with baseline values (p<0.01). In animals with pure limbic SE, temperature decreased continuously to baseline values over the 150-min period of observation. In contrast, in animals with motor SSSE, temperature remained elevated during continuing epileptic activity and was still significantly higher 150 min after the end of stimulation compared with baseline (p<0.01). In animals that did not develop SSSE, temperature was not changed after the end of electrical stimulation and in the 150 min thereafter compared with baseline values. CONCLUSIONS: The results indicate that hyperthermia as seen in SE is the consequence of motor convulsions and not of epileptic activity itself, as seen in limbic SSSE.

Decrease in haemoglobin oxygenation during absence seizures in adult humans.

Near-infrared spectroscopy (NIRS) is a noninvasive method that allows the assessment of activation-induced cortical oxygenation changes in humans. It has been demonstrated that an increase in oxygenated and a decrease in deoxygenated haemoglobin can be expected over an area activated by functional stimulation. Likewise, an inverse oxygenation pattern has been shown to be associated with cortical deactivation. The aim of the current study was to determine the oxygenation changes that occur during absence seizures. We performed ictal NIRS simultaneously with video-EEG telemetry in three adult patients with typical absence seizures. NIRS probes were placed over the frontal cortex below the F1/F2 leads. During all absence seizures studied, pronounced changes in cerebral Hb-oxygenation were noted and there were no changes in the interval. We observed a reproducible decrease in [oxy-Hb] and an increase in [deoxy-Hb] during absence seizures indicating a reduction of cortical activity. Oxygenation changes started several seconds after the EEG-defined absence onset and outlasted the clinically defined event by 20-30 s.

Furosemide terminates limbic status epilepticus in freely moving rats.

PURPOSE: To evaluate the anticonvulsant properties of furosemide and to determine sedative side effects compared with pentobarbital and diuretic side effects compared with saline-treated controls in an experimental model of limbic status epilepticus. METHODS: Self-sustaining status epilepticus was induced in rats by continuous electrical stimulation of the perforant path. Five minutes after the end of the stimulation, animals were given 100 mg/kg furosemide, 30 mg/kg pentobarbital, or an equal amount of saline, intraperitoneally. After administration of the substance, animals were monitored clinically and electrographically for 3 h regarding status epilepticus, level of sedation, and diuresis. RESULTS: In seven of 10 animals, furosemide terminated status epilepticus after 68 +/- 26 min, whereas pentobarbital was successful in all animals after 5 +/- 0.8 min. In contrast to pentobarbital, sedation did not occur with furosemide. Weight loss after furosemide was 10.2 +/- 1.7% compared with 6.5 +/- 1.1% in animals given saline (p < 0.001). CONCLUSIONS: The results suggest that furosemide may serve as an alternative or additional agent for refractory complex partial status epilepticus in patients in whom common anesthetics are not justifiable.

Intrinsic optical imaging reveals regionally different manifestation of spreading depression in hippocampal and entorhinal structures in vitro.

The spatiotemporal features of spreading depression (SD) were analyzed in vitro by using combined hippocampal-entorhinal cortex slices. SDs were induced by microinjection of 1 M KCl in the stratum radiatum of the CA1 region of the hippocampus. Measurements of extracellular field potentials, extracellular space (ECS) volume changes and intrinsic optical signal changes were combined to study SD features in different regions of the slice. Each SD was associated with a pronounced shrinkage of the extracellular space (ECS) volume and a decrease in light transmittance. The beginning of the optical signal change occurred simultaneously with the electrographic onset as measured with extracellular microelectrodes but outlasted the dc shift for tens of seconds. The amplitude of the intrinsic optical signal change displayed marked regional variations with greatest changes of 12% in cortical regions. The signal amplitudes were considerably lower in hippocampal regions. The analysis of spread patterns revealed two types of waves: fully propagated waves spreading from CA1 all the way to the temporal neocortex and abortive waves that ceased earlier. The spread velocities displayed pronounced regional differences with highest velocities of 5.4 +/- 0.3 mm/min in the area CA3 of the hippocampal formation and lowest velocities of 2.7 +/- 0.1 mm/min in cortical regions.