Genetically epileptic rats show a pronounced intermediate stage of sleep

Epileptic seizure clustering and accumulation at transition from activity to rest in GAERS rats

Knowing when seizures occur may help patients and can also provide insight into epileptogenesis mechanisms. We recorded seizures over periods of several days in the Genetic Absence Epileptic Rat from Strasbourg (GAERS) model of absence epilepsy, while we monitored behavioral activity with a combined head accelerometer (ACCEL), neck electromyogram (EMG), and electrooculogram (EOG). The three markers consistently discriminated between states of behavioral activity and rest. Both GAERS and control Wistar rats spent more time in rest (55-66%) than in activity (34-45%), yet GAERS showed prolonged continuous episodes of activity (23 vs. 18 min) and rest (34 vs. 30 min). On average, seizures lasted 13 s and were separated by 3.2 min. Isolated seizures were associated with a decrease in the power of the activity markers from steep for ACCEL to moderate for EMG and weak for EOG, with ACCEL and EMG power changes starting before seizure onset. Seizures tended to occur in bursts, with the probability of seizing significantly increasing around a seizure in a window of ±4 min. Furthermore, the seizure rate was strongly increased for several minutes when transitioning from activity to rest. These results point to mechanisms that control behavioral states as determining factors of seizure occurrence.

Dexmedetomidine, an alpha 2A receptor agonist, triggers seizures unilaterally in GAERS during the pre-epileptic phase: does the onset of spike-and-wave discharges occur in a focal manner?

Introduction

The genetic absence epilepsy rat from Strasbourg (GAERS) is a rat model for infantile absence epilepsy with spike-and-wave discharges (SWDs). This study aimed to investigate the potential of alpha 2A agonism to induce seizures during the pre-epileptic period in GAERS rats.

Methods

Stereotaxic surgery was performed on male pups and adult GAERS rats to implant recording electrodes in the frontoparietal cortices (right/left) under anesthesia (PN23-26). Following the recovery period, pup GAERS rats were subjected to electroencephalography (EEG) recordings for 2 h. Before the injections, pup epileptiform activity was examined using baseline EEG data. Dexmedetomidine was acutely administered at 0.6 mg/kg to pup GAERS rats 2-3 days after the surgery and once during the post-natal (PN) days 25-29. Epileptiform activities before injections triggered unilateral SWDs and induced sleep durations, and power spectral density was evaluated based on EEG traces.

Results

The most prominent finding of this study is that unilateral SWD-like activities were induced in 47% of the animals with the intraperitoneal dexmedetomidine injection. The baseline EEGs of pup GAERS rats had no SWDs as expected since they are in the pre-epileptic period but showed low-amplitude non-rhythmic epileptiform activity. There was no difference in the duration of epileptiform activities between the basal EEG groups and DEX-injected unilateral SWD-like-exhibiting and non-SWD-like activities groups; however, the sleep duration of the unilateral SWD-like-exhibiting group was shorter. Power spectrum density (PSD) results revealed that the 1.75-Hz power in the left hemisphere peaks significantly higher than in the right.

Discussion

As anticipated, pup GAERS rats in the pre-epileptic stage showed no SWDs. Nevertheless, they exhibited sporadic epileptiform activities. Specifically, dexmedetomidine induced SWD-like activities solely within the left hemisphere. These observations imply that absence seizures might originate unilaterally in the left cortex due to α2AAR agonism. Additional research is necessary to explore the precise cortical focal point of this activity.

The Orexin System: A Potential Player in the Pathophysiology of Absence Epilepsy

BACKGROUND

Absence epilepsy is characterized by the presence of spike-and-wave discharges (SWDs) at the EEG generated within the cortico-thalamo-cortical circuit. The molecular mechanisms involved in the pathophysiology of absence epilepsy are only partially known. WAG/Rij rats older than 2-3 months develop spontaneous SWDs, and they are sensitive to anti- absence medications. Hence, WAG/Rij rats are extensively used as a model for absence epilepsy with predictive validity.

OBJECTIVE

The aim of the study was to examine the possibility that the orexin system, which supports the wake status in experimental animals and humans, plays a role in the pathophysiology of absence seizures.

METHODS

The perspective grounds its method from recent literature along with measurements of orexin receptor type-1 (OX1) protein levels in the thalamus and somatosensory cortex of WAG/Rij rats and non-epileptic Wistar control rats at two ages (25 days and 6-7 months). OX1 protein levels were measured by immunoblotting.

RESULTS

The analysis of the current literature suggests that the orexin system might be involved in the pathophysiology of absence epilepsy and might be targeted by therapeutic intervention. Experimental data are in line with this hypothesis, showing that OX1 protein levels were reduced in the thalamus and somatosensory cortex of symptomatic WAG/Rij rats (6-7 months of age) with respect to non-epileptic controls, whereas these differences were not seen in pre-symptomatic, 25 days-old WAG/Rij rats.

CONCLUSION

This perspective might pave the way for future studies on the involvement of the orexinergic system in the pathophysiology of SWDs associated with absence epilepsy and its comorbidities.

Sleep Disturbances in Rats With Genetic Pre-disposition to Spike-Wave Epilepsy (WAG/Rij)

Wistar Albino Glaxo Rijswijk (WAG/Rij) rats are widely used in basic and pre-clinical studies as a valid genetic model of absence epilepsy. Adult WAG/Rij rats exhibit generalized 8–10-Hz spike-wave discharges (SWDs) in the electroencephalogram. SWDs are known to result from thalamocortical circuit dysfunction, and this implies an intimate relationship between slow-wave EEG activity, sleep spindles, and SWDs. The present mini review summarizes relevant research on sleep-related disturbances associated with spike-wave epilepsy in WAG/Rij rats in the domain of slow-wave sleep EEG and microarousals. It also discusses enhancement of the intermediate stage of sleep. In general, sleep EEG studies provide important information about epileptogenic processes related to spike-wave epilepsy.

Automatic wavelet-based assessment of behavioral sleep using multichannel electrocorticography in rats

PURPOSE

During the last decade, the reported prevalence of sleep-disordered breathing in adults has been rapidly increasing. Therefore, automatic methods of sleep assessment are of particular interest. In a framework of translational neuroscience, this study introduces a reliable automatic detection system of behavioral sleep in laboratory rats based on the signal recorded at the cortical surface without requiring electromyography.

METHODS

Experimental data were obtained in 16 adult male WAG/Rij rats at the age of 9 months. Electrocorticographic signals (ECoG) were recorded in freely moving rats during the entire day (22.5 ± 2.2 h). Automatic wavelet-based assessment of behavioral sleep (BS) was proposed. The performance of this wavelet-based method was validated in a group of rats with genetic predisposition to absence epilepsy (n=16) based on visual analysis of their behavior in simultaneously recorded video.

RESULTS

The accuracy of automatic sleep detection was 98% over a 24-h period. An automatic BS assessment method can be adjusted for detecting short arousals during sleep (microarousals) with various duration.

CONCLUSIONS

These findings suggest that automatic wavelet-based assessment of behavioral sleep can be used for assessment of sleep quality. Current analysis indicates a temporal relationship between microarousals, sleep, and epileptic discharges in genetically prone subjects.

Does antiepileptogenesis affect sleep in genetic epileptic rats?

Recently it was established that early long lasting treatment with the anti-absence drug ethosuximide (ETX) delays the occurrence of absences and reduces depressive-like symptoms in a genetic model for absence epilepsy, rats of the WAG/Rij strain. Here it is investigated whether anti-epileptogenesis (chronic treatments with ETX for 2 and 4 months) affects REM sleep in this model. Four groups of weaned male WAG/Rij rats were treated with ETX for 4 months, two groups for 2 months (at 2-3 and 4-5 months of age), the fourth group was untreated. Next, the rats were recorded 6 days after the last day of the treatment for 22.5 h. Non-REM sleep and REM sleep parameters and delta power were analyzed in four characteristic and representative hours of the recoding period. Four months treatment with ETX reduced the amount of REM sleep and REM sleep as percentage of total sleep time. Other sleep parameters were not affected by the treatment. Clear differences between the various hours of the light-dark phase in amounts of non-REM and REM sleep and delta power were found, in line with commonly reported circadian sleep patterns. It can be concluded that the reduction of REM sleep is unique for the early and long lasting chronic treatment. The outcomes may explain our earlier finding that a reduction of REM sleep might alleviate depressive like symptoms.

The WAG/Rij strain: a genetic animal model of absence epilepsy with comorbidity of depression [corrected]

A great number of clinical observations show a relationship between epilepsy and depression. Idiopathic generalized epilepsy, including absence epilepsy, has a genetic basis. The review provides evidence that WAG/Rij rats can be regarded as a valid genetic animal model of absence epilepsy with comorbidity of depression. WAG/Rij rats, originally developed as an animal model of human absence epilepsy, share many EEG and behavioral characteristics resembling absence epilepsy in humans, including the similarity of action of various antiepileptic drugs. Behavioral studies indicate that WAG/Rij rats exhibit depression-like symptoms: decreased investigative activity in the open field test, increased immobility in the forced swimming test, and decreased sucrose consumption and preference (anhedonia). In addition, WAG/Rij rats adopt passive strategies in stressful situations, express some cognitive disturbances (reduced long-term memory), helplessness, and submissiveness, inability to make choice and overcome obstacles, which are typical for depressed patients. Elevated anxiety is not a characteristic (specific) feature of WAG/Rij rats; it is a characteristic for only a sub-strain of WAG/Rij rats susceptible to audiogenic seizures. Interestingly, WAG/Rij rats display a hyper-response to amphetamine similar to anhedonic depressed patients. WAG/Rij rats are sensitive only to chronic, but not acute, antidepressant treatments, suggesting that WAG/Rij rats fulfill a criterion of predictive validity for a putative animal model of depression. However, more and different antidepressant drugs still await evaluation. Depression-like behavioral symptoms in WAG/Rij rats are evident at baseline conditions, not exclusively after stress. Experiments with foot-shock stress do not point towards higher stress sensitivity at both behavioral and hormonal levels. However, freezing behavior (coping deficits) and blunted response of 5HT in the frontal cortex to uncontrollable sound stress, increased c-fos expression in the terminal regions of the meso-cortico-limbic brain systems and greater DA response of the mesolimbic system to forced swim stress suggest that WAG/Rij rats are vulnerable to some, but not to all types of stressors. We propose that genetic absence epileptic WAG/Rij rats have behavioral depression-like symptoms, are vulnerable to stress and might represent a model of chronic low-grade depression (dysthymia). Both 5HT and DAergic abnormalities detected in the brain of WAG/Rij rats are involved in modulation of vulnerability to stress and provocation of behavioral depression-like symptoms. The same neurotransmitter systems modulate SWDs as well. Recent studies suggest that the occurrence and repetition of absence seizures are a precipitant of depression-like behavior. Whether the neurochemical changes are primary to depression-like behavioral alterations remains to be determined. In conclusion, the WAG/Rij rats can be considered as a genetic animal model for absence epilepsy with comorbidity of dysthymia. This model can be used to investigate etiology, pathogenic mechanisms and treatment of a psychiatric comorbidity, such as depression in absence epilepsy, to reveal putative genes contributing to comorbid depressive disorder, and to screen novel psychotropic drugs with a selective and/or complex (dual) action on both pathologies.

Neonatal sensory deprivation promotes development of absence seizures in adult rats with genetic predisposition to epilepsy

Absence epilepsy has age-related onset. In a WAG/Rij rat genetic model, absence seizures appear after puberty and they are increased with age. It is known that (1) epileptic activity in WAG/Rij rats is initiated at the perioral area in the somatosensory cortex; (2) sensory deprivation, i.e., whisker trimming during the critical period of development, could enhance excitatory activity in the somatosensory cortex. It is hypothesized that the cortex may become more excitable after neonatal vibrissae removal, and this may precipitate absence seizures in adult rats. We found that whisker trimming during the first postnatal weeks caused more rapid development of EEG seizure activity in adult WAG/Rij rats. Epileptic discharges in the trimmed rats were more numerous (vs control), showed longer duration and often appeared in desynchronized and drowsy EEG. The number of absence-like spindle-shaped EEG events (spike-wave spindles) in the whisker-trimmed rats was higher than in control, especially during the intermediate sleep state. An age-dependent increase of intermediate sleep state was found in the trimmed rats, but not in the intact animals. We discuss epigenetic factors that can modulate absence epilepsy in genetically prone subjects.

An algorithm for real-time detection of spike-wave discharges in rodents

The automatic real-time detection of spike-wave discharges (SWDs), the electroencephalographic hallmark of absence seizures, would provide a complementary tool for rapid interference with electrical deep brain stimulation in both patients and animal models. This paper describes a real-time detection algorithm for SWDs based on continuous wavelet analyses in rodents. It has been implemented in a commercially available data acquisition system and its performance experimentally verified. ECoG recordings lasting 5-8h from rats (n=8) of the WAG/Rij strain were analyzed using the real-time SWD detection system. The results indicate that the algorithm is able to detect SWDs within 1s with 100% sensitivity and with a precision of 96.6% for the number of SWDs. Similar results are achieved for 24-h ECoG recordings of two rats. The dependence of accuracy and speed of detection on program settings and attributes of ECoG are discussed. It is concluded that the wavelet based real-time detecting algorithm is well suited for automatic, real-time detection of SWDs in rodents.

A role for the preoptic sleep-promoting system in absence epilepsy

Absence epilepsy (AE) in humans and the genetic AE model in WAG/Rij rats are both associated with abnormalities in sleep architecture that suggest insufficiency of the sleep-promoting mechanisms. In this study we compared the functionality of sleep-active neuronal groups within two well-established sleep-promoting sites, the ventrolateral and median preoptic nuclei (VLPO and MnPN, respectively), in WAG/Rij and control rats. Neuronal activity was assessed using c-Fos immunoreactivity and chronic single-unit recording techniques. We found that WAG/Rij rats exhibited a lack of sleep-associated c-Fos activation of GABAergic MnPN and VLPO neurons, a lower percentage of MnPN and VLPO cells increasing discharge during sleep and reduced firing rates of MnPN sleep-active neurons, compared to non-epileptic rats. The role of sleep-promoting mechanisms in pathogenesis of absence seizures was assessed in non-epileptic rats using electrical stimulation and chemical manipulations restricted to the MnPN. We found that fractional activation of the sleep-promoting system in waking was sufficient to elicit absence-like seizures. Given that reciprocally interrelated sleep-promoting and arousal neuronal groups control thalamocortical excitability, we hypothesize that malfunctioning of sleep-promoting system results in impaired ascending control over thalamocortical rhythmogenic mechanisms during wake-sleep transitions thus favoring aberrant thalamocortical oscillations. Our findings suggest a pathological basis for AE-associated sleep abnormalities and a mechanism underlying association of absence seizures with wake-sleep transitions.

Sleep spindles and spike-wave discharges in EEG: Their generic features, similarities and distinctions disclosed with Fourier transform and continuous wavelet analysis

Epileptic activity in the form of spike-wave discharges (SWD) appears in the electroencephalogram (EEG) during absence seizures. A relationship between SWD and normal sleep spindles is often assumed. This study compares time-frequency parameters of SWD and sleep spindles as recorded in the EEG in the WAG/Rij rat model of absence epilepsy. Fast Fourier transformation and continuous wavelet transformation were used for EEG analysis. Wavelet analysis was performed in non-segmented full-length EEG. A specific wavelet-based algorithm was developed for the automatic identification of sleep spindles and SWD. None of standard wavelet templates provided precise identification of all sleep spindles and SWD in the EEG and different wavelet templates were imperative in order to accomplish this task. SWD were identified with high probability using standard Morlet wavelet, but sleep spindles were identified using two types of customized adoptive 'spindle wavelets'. It was found that (1) almost 100% of SWD (but only 50-60% of spindles) were identified using the Morlet-based wavelet transform. (2) 82-91% of sleep spindles were selected using adoptive 'spindle wavelet 1' (template's peak frequency approximately 12.2 Hz), the remaining sleep spindles with 'spindle wavelet 2' (peak frequency approximately 20-25 Hz). (3) Sleep spindles and SWD were detected by the elevation of wavelet energy in different frequencies: SWD, in 30-50 Hz band, sleep spindles, in 7-14 Hz. It is concluded that the EEG patterns of sleep spindles and SWD belong to different families of phasic EEG events with different time frequency characteristics.

Midfrequency cortico-thalamic oscillations and the sleep cycle: Genetic, time of day and age effects

WAG/Rij rats have various types of mid frequency cortico-thalamic oscillations, such as anterior and posterior sleep spindles and two types of spike-wave discharges (SWD). The generalized SWD (type I) preferentially occur at transitions from wake to sleep, type II can be found at the occipital cortex during quite wakefulness. In the present experiment sleep spindles, SWD and sleep cycle characteristics of 6-month-old WAG/Rij rats were studied and compared with those of younger WAG/Rij rats with much less SWD and age-matched control (ACI) rats. EEG recordings were made during the beginning (morning) and end (afternoon) of the light period in these four groups of rats. Quantitative characteristics of SWD, sleep spindles and the sleep cycle were determined. There were strain-related and age-dependent effects in the various cortico-thalamic oscillations, older WAG/Rij had more SWDs than younger WAG/Rij rats (both types I and II) and there were more type I SWDs at the end of the light period compared to the beginning. Large strain, age and time of day effects on the sleep cycle were found. The duration of non-REM sleep and the sleep cycle was shorter in WAG/Rij rats but only at the end of the light period and only in older WAG/Rij rats. It can be concluded that the various phasic events and the length of the sleep cycle are under genetic control, and that the sleep cycle length is also controlled by time of day, age and genetic factors. Non-REM sleep and the sleep cycle are disrupted by absence seizures but only in fragile periods when drowsiness and light slow wave sleep dominate.

Propagation of spike and wave activity to the medial prefrontal cortex and dorsal raphe nucleus of WAG/Rij rats

Although there is pharmacological evidence for the involvement of the serotonergic system in the expression of spike and wave discharges (SWDs) in experimental absence epilepsy, no direct investigation of this paroxysm in the dorsal raphe nucleus (DRN), one of the main serotonergic nuclei, has been carried out. We have now recorded the EEG simultaneously with local field potentials and unit activity in DRN from WAG/Rij rats, one of the best established models of absence epilepsy during spontaneous SWDs. We have also compared this activity to that in the thalamocortical networks, where SWDs are generated, and in the medial prefrontal cortex (mPFC), as this brain area is reciprocally connected to the DRN. We have found that SWDs propagate to the DRN with a short delay, and that the firing rate of its neurons changes during this type of paroxysm. These results provide the first direct evidence for clear alterations in the firing properties of mPFC and DRN neurons during spontaneous SWDs.

An evaluation of the use of seizure prone rats when investigating intermediate stage sleep

A body of literature is developing which identifies an additional stage of sleep in rats, cats and mice. Intermediate stage (IS) sleep is a measurable sleep stage that is maintained by the hyperpolarization of GABA(A) containing thalamocortical neurons. The present study attempts to clarify inconsistencies within the sleep spindle literature. Most notably, inconsistencies between those that study sleep spindles in the rat outside and within the context of IS sleep. Ten male taiep rats weighing from 400 g to 600 g, and 9-12 months of age, were used in this study. The animals were given a one-time, .9 mg/kg dose of the benzodiazepine clonazepam. The control group had more seizure activity (mean = 13.4) than the treatment group (mean = 5.2, t(1-18) = 8.859, p < .001), and had a lower number of sleep spindles (mean=10.3) than the treatment group (mean = 13.3, t(1-18) = -3.4, p < .001). In addition, spectral analysis of sleep spindles during IS and seizure activity revealed that sleep spindles are within the frequency band of 8-11Hz, while seizure activity is within the 4-7 Hz range. This data supports the hypotheses that sleep spindles are distinguishable from seizure activity.

Pathophysiological mechanisms of genetic absence epilepsy in the rat

Generalized non-convulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike and wave discharges (SWDs) on the electroencephalogram, that are concomitant with a behavioral arrest. Many similarities between rodent and human absence seizures support the use of genetic rodent models, in which spontaneous SWDs occur. This review summarizes data obtained on the neurophysiological and neurochemical mechanisms of absence seizures with special emphasis on the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). EEG recordings from various brain regions and lesion experiments showed that the cortex, the reticular nucleus and the relay nuclei of the thalamus play a predominant role in the development of SWDs. Neither the cortex, nor the thalamus alone can sustain SWDs, indicating that both structures are intimely involved in the genesis of SWDs. Pharmacological data confirmed that both inhibitory and excitatory neurotransmissions are involved in the genesis and control of absence seizures. Whether the generation of SWDs is the result of an excessive cortical excitability, due to an unbalance between inhibition and excitation, or excessive thalamic oscillations, due to abnormal intrinsic neuronal properties under the control of inhibitory GABAergic mechanisms, remains controversial. The thalamo-cortical activity is regulated by several monoaminergic and cholinergic projections. An alteration of the activity of these different ascending inputs may induce a temporary inadequation of the functional state between the cortex and the thalamus and thus promote SWDs. The experimental data are discussed in view of these possible pathophysiological mechanisms.

Effects of the GABAB antagonist CGP 35348 on sleep-wake states, behaviour, and spike-wave discharges in old rats

The GABAB antagonist CGP 35348 was intraperitoneally given in doses of 100, 300, and 900 mg/kg to old rats. These rats were earlier chronically provided with EEG and EMG electrodes. Sleep recordings based on visual inspection of EEG and EMG recordings were made for 3 h post injection, and spontaneous behaviour in the recording cage was additionally observed. With 100 and 300 mg/kg, the drug produced an increase in the duration of REM sleep compared to the saline-injected control group. The REM sleep latency was correspondingly reduced. Non-REM sleep and total sleep duration increased and an s-shaped dose-response relationship was found. Explorative behaviour was diminished after injections with 100 and 300 mg/kg CGP 35348. The number and duration of spike-wave discharges were reduced after all doses of CGP 35348 and during all 3 recording hours. The latter outcomes confirm the strong suppressive action of this drug on spike-wave discharges; these effects have also been reported in models of absence epilepsy. The hypnotic properties and especially the increase in REM sleep after the administration of CGP 35348 deserve attention considering the paucity of drugs which facilitate REM sleep. The discovery of drugs promoting REM sleep might have theoretical as well as clinical consequences.

The transition from slow-wave sleep to paradoxical sleep: evolving facts and concepts of the neurophysiological processes underlying the intermediate stage of sleep

Paradoxical sleep in rats, cats and mice is usually preceded and sometimes followed by a short-lasting (a few seconds) electroencephalogram (EEG) stage characterized by high-amplitude spindles in the anterior cortex and low-frequency theta rhythm in the dorsal hippocampus. The former is an index of advanced slow-wave sleep; the latter is an index of limbic activation since it occurs during active waking and paradoxical sleep. Barbiturates and benzodiazepines extend this intermediate stage at the expense of paradoxical sleep while concomitantly barbiturates suppress the pontine reticular activation characteristic of this sleep stage. During the intermediate stage, thalamocortical responsiveness and thalamic transmission level, which are controlled by brain stem activating influences, are the lowest of all sleep-waking stages. The unusual EEG pattern of this stage is otherwise only observed in the acute intercollicular-transected preparation. Therefore, forebrain structures may be functionally briefly disconnected from the brain-stem during this short-lasting stage, which could possibly account for the mental content of a similar sleep period in humans. In spite of strong evidence in favour of this forebrain deafferentiation hypothesis, other data indicate that the IS is in some way linked either to slow-wave sleep or to paradoxical sleep.

Differential effects of midazolam and zolpidem on sleep-wake states and epileptic activity in WAG/Rij rats

Hypnotic drugs are known to possess antiepileptic activity. Therefore, the effects of the benzodiazepine hypnotic midazolam (10 mg/kg) and the novel imidazopyridine hypnotic zolpidem (10 mg/kg) on sleep-wake states and on the number of spike-wave discharges were evaluated in WAG/Rij rats. Rats of this strain are considered to be a model for generalized absence epilepsy. Animals were implanted with chronic monopolar EEG electrodes and, after recovery from surgery, the EEG was recorded for 6 h during the dark period on 3 consecutive days. Sleep recordings were analyzed using Hjorth's parameters and number and duration of spike-wave discharges were visually determined. It was found that both drugs facilitated nonREM sleep at the cost of wakefulness. Both hypnotics also reduced the number and duration of spike-wave discharges. The initial decrease after midazolam, however, was followed by a rebound reflecting a poorer quality of vigilance expressed as an increase in spike-wave discharges. The strong antiabsence activity of zolpidem mimics that of midazolam and is well correlated with their equipotent hypnotic action and anticonvulsant effect in the isoniazid test.

Benzodiazepines promote the intermediate stage at the expense of paradoxical sleep in the rat

The effects of diazepam, a long half-life benzodiazepine, midazolam and triazolam, two with short half-life, on the transitional stage between deep slow wave sleep and paradoxical sleep were studied in Wistar and WAG/Rij rats. This intermediate stage is characterized by the unusual association of cortical spindles and low frequency hippocampal theta rhythm. The main result was extension of the intermediate stage at the expense of paradoxical sleep by diazepam and triazolam by influencing only the duration of the intermediate stage and both the onset and maintenance of paradoxical sleep. Midazolam increased both intermediate stage and paradoxical sleep. Several differences in the qualitative modulation of the stage characteristics and between rat strains were found. In regard to the possible peculiar physiological significance of the intermediate stage, we conclude that benzodiazepines promote a transient pharmacological cerveau isolé-like stage during sleep in rats.

Characterization of high voltage spindles and spatial memory in young, mature and aged rats

EEG was recorded from rats of three age groups, and high voltage spindles (HVS) were measured during waking immobility. Total mean spindling times in 4- (young), 10- (mature) and 22- (aged) month-old rats were 0.3 +/- 0.1, 20.4 +/- 7.4 and 33.4 +/- 14.9 s, respectively. Spatial memory was assessed in these rats using a discrimination version of the Morris water maze. Performance (as measured by number of choice errors) was compared with the extent of HVS activity by characterizing rats as "spindling" if the total average duration of HVS discharges exceeded 5 s, and "non-spindling" if these discharges averaged less than 5 s. Spindling and nonspindling rats had similar performance during training; however, on a 14-day retention trial, spindling rats had a significantly higher mean error score of 2.8 +/- 0.5 compared with 1.2 +/- 0.3 for nonspindling rats (p = 0.011). These results show that spindling activity increases in mature and aged rats, and that HVS discharges may be an electrophysiological change that parallels the progression of brain dysfunction associated with memory impairment.

Aberrant transients in the EEG of epileptic rats: a spectral analytical approach

Aberrant transients in the cortical electroencephalogram of rats of the epileptic WAG/Rij strain were studied by means of spectral analysis. The EEG of rats of this strain contains, besides normal sleep spindles, high-voltage spiky phenomena, epileptic spike-wave discharges, and deviant intermediate stage. Spectral analysis of these transient phenomena shows that some features, like their peak frequency, are alike, but that they differ in other spectral characteristics, as in the first harmonic of the peak frequency and in the domain of the high frequencies. The results provide arguments for the view that spike-wave discharges might be considered as unique aberrant phenomena, presumably related but dissimilar to the high-voltage spiky phenomena and intermediate stage. Next to this, spectral analysis was used to study the intraphenomenal dynamics of spike-wave discharges. The peak frequency was found to decrease monotonously from about 10 Hz at the beginning of the spike-wave discharge to about 8 Hz at the end. Other spike-wave discharge frequency bands showed an intraphenomenal increase followed by a decrease. These time-variant EEG dynamics in spike-wave discharges might correlate with the cognitive disturbances during absence seizures in man.

Genetic models of absence epilepsy, with emphasis on the WAG/Rij strain of rats

In this review, the main characteristics of genetic models of absence epilepsy, in particular with respect to WAG/Rij rats, are presented. Genetic models are important and relevant, since evidence exists that these models mimic spontaneously occurring human epilepsy more than models in which epilepsy is artificially induced. Genetic models can be divided into models in which seizures are elicited and into those in which epilepsy appears without any sensory stimulation. The majority of genetic models show that absence type of epilepsy; during the last few years, we and others have noticed that rats of various strains exhibit spontaneously occurring spike-wave discharges in the EEG. Among the strains highly affected is the WAG/Rij strain, which is a fully inbred strain. Individuals are homozygous and because of this property, genetic studies are meaningful. Electrophysiological studies have indicated that abnormal discharges in the cortical EEG are generalized and that the hippocampus is not involved. Parts of the thalamus, together with the thalamic reticular nucleus, apparently act as a pacemaker for the abnormal discharges. There is a circadian modulation in the number of spike-wave discharges. Discharges mainly occur during intermediate levels of vigilance such as passive wakefulness and light slow-wave sleep and at transitions of sleep states. Pharmacological studies with clinically effective antiepileptic drugs have shown a close agreement in seizure response between man and rat. Studies with new compounds have emphasized the role of the GABAergic and glutamatergic system in this type of epilepsy. Particularly striking is the role of the GABAergic system. GABA agonists enhance and GABA antagonists reduce the occurrence of spike-wave discharges, which deviates from the effects of GABAergic drugs in non-convulsive epilepsy. Even more striking is the role of the benzodiazepines, generally seen as GABA agonists; these drugs do not act as such in absence epilepsy since they reduce spike-wave discharges. Also good evidence for an involvement of other neurotransmitters such as noradrenaline, dopamine and opioid peptides exists in absence epilepsy. Genetic data obtained from the WAG/Rij model for absence epilepsy show a relatively simple pattern of inheritance with one gene determining whether an individual is epileptic or not, and with other genes regulating the number and duration of seizures. This is in good agreement with the more restricted human data. Cognitive studies have shown two important features of epilepsy in the WAG/Rij strain: modulation of the number of spike-wave discharges by mental or physical activity and on the other hand, the disruption of cognitive activity by spike-wave discharges.(ABSTRACT TRUNCATED AT 400 WORDS)

Spike-wave discharges and sleep-wake states in rats with absence epilepsy

The occurrence of spike-wave discharges was studied in relation to the daily fluctuations of vigilance level in rats. Eight rats of the WAG/Rij strain, an animal model for idiopathic generalized epilepsy of the absence type, which were equipped with cortical EEG and nuchal EMG electrodes, served as subjects. It was found that spike-wave discharges predominantly occur during light slow wave sleep and passive wakefulness. REM sleep, active wakefulness, and deep slow wave sleep are less susceptible to the occurrence of spike-wave discharges. Finally, spike-wave discharges tend to prevail in transitional states. A crucial role for the degree of stability of the level of vigilance in the genesis of absence seizures is suggested.

Absence epilepsy and the level of vigilance in rats of the WAG/Rij strain

In man, a relationship exists between sleep-wake states and absence epilepsy. During wakefulness, spike-wave discharges predominantly occur when the level of vigilance is not high, while during sleep they have a preference to occur during slow-wave sleep. During this latter type of sleep, spike-wave discharges prevail in periods where slow-wave sleep is light. In a series of experiments, the WAG/Rij rat model for absence epilepsy was characterized with respect to the relationships between the level of vigilance, sleep-wake states and the occurrence of spike-wave discharges. In the first experiment, continuous recordings were made for a period of 48 h and a clear circadian rhythm was established for the number of spike-wave discharges. A maximum appeared during the middle of the dark period of the rat, whereas a minimum was detected directly after the onset of the light period, the time period during which deep slow-wave sleep predominates. The relationship of spike-wave discharges with states of vigilance was elaborated in a second study. Spike-wave discharges were mainly found during light slow-wave sleep, during passive wakefulness and in transition phases from sleep to wakefulness. During REM sleep no spike-wave discharges were found. In the last three experiments, the level of alertness was enhanced by various procedures as photostimulation, a learning task and deprivation of REM sleep. In all cases, an increase of alertness decreased the amount of epilepsy.(ABSTRACT TRUNCATED AT 250 WORDS)