Hypothalamic neuronal loss and altered circadian rhythm of temperature in a rat model of mesial temporal lobe epilepsy

Circadian rhythm of blood pressure in patients with drug-resistant mesial temporal lobe epilepsy

OBJECTIVE

To determine whether patients with drug-resistant mesial temporal lobe epilepsy present with an alteration in the autonomic circadian regulation of blood pressure.

METHODS

A prospective case‒control study was designed, with a case group comprising patients with drug-resistant mesial temporal lobe epilepsy and a control group comprising healthy volunteers. Twenty-four-hour outpatient blood pressure monitoring was performed to assess the existence of a normal (dipping) or altered (non-dipping) circadian pattern. In addition, analytical and ultrasound parameters (carotid intima-media thickness) of vascular risk and sleep quality were evaluated.

RESULTS

Twenty-four subjects were recruited in each study group, amongst whom no demographic differences or history of vascular risk were observed. A higher percentage of participants with a non-dipping pattern was observed in the group of patients with epilepsy (62.5% vs. 12.5, p = 0.001). In the case group, significant differences were also observed in carotid intima-media thickness, with a greater probability of presenting with pathological values (p = 0.022).

CONCLUSION

The results suggest a disorder of the central autonomic control of blood pressure in patients with drug-resistant mesial temporal lobe epilepsy, with a greater probability of developing an alteration of the circadian rhythm of blood pressure. This dysfunction may be a factor involved in the increased cardiovascular risk in this population.

Circadian/multidien Molecular Oscillations and Rhythmicity of Epilepsy (MORE)

The occurrence of seizures at specific times of the day has been consistently observed for centuries in individuals with epilepsy. Electrophysiological recordings provide evidence that seizures have a higher probability of occurring at a given time during the night and day cycle in individuals with epilepsy here referred to as the seizure rush hour. Which mechanisms underlie such circadian rhythmicity of seizures? Why don't they occur every day at the same time? Which mechanisms may underlie their occurrence outside the rush hour? In this commentary, I present a hypothesis: MORE - Molecular Oscillations and Rhythmicity of Epilepsy, a conceptual framework to study and understand the mechanisms underlying the circadian rhythmicity of seizures and their probabilistic nature. The core of the hypothesis is the existence of ~24-hour oscillations of gene and protein expression throughout the body in different cells and organs. The orchestrated molecular oscillations control the rhythmicity of numerous body events, such as feeding and sleep. The concept developed here is that molecular oscillations may favor seizure genesis at preferred times, generating the condition for a seizure rush hour. However, the condition is not sufficient, as other factors are necessary for a seizure to occur. Studying these molecular oscillations may help us understand seizure genesis mechanisms and find new therapeutic targets and predictive biomarkers. The MORE hypothesis can be generalized to comorbidities and the slower multidien (week/month period) rhythmicity of seizures, a phenomenon addressed in another article in this issue of Epilepsia.

Decoding Circadian Rhythm and Epileptic Activities: Clues From Animal Studies

The relationship between circadian rhythm and epilepsy has been recognized for decades. Yet many questions underlying the complex mechanisms of their interaction remain elusive. A better understanding on this topic allows the development of accurate seizure-detection algorithm and alternative precise therapeutic strategies. Preclinical laboratory studies based on epileptic animal models, with controllable epileptogenic pathology and an array of intervention strategies, shed light on the bidirectional effects between circadian rhythm and epileptic seizures as well as their underlying mechanisms. Here, we reviewed findings on the interaction between circadian rhythm and epileptic seizures in the preclinical setting. We present the possible mechanisms at molecular, cellular and circuitry levels. We propose that future experimental designs should take into account the relationship between circadian rhythm and epilepsy as well as the underlying mechanisms in different types of animal models, which may have a translational significance as stepping stones for clinical benefits.

Thermoregulation in epilepsy

Several aspects of thermoregulation play a role in epilepsy. Circuitries involved in thermoregulation are affected by seizures and epilepsy, hyperthermia may be both cause and result of seizures, and hypothermia may prevent or abort seizures. Autonomic manifestations of seizures including thermoregulatory disturbances are common in a variety of clinical epilepsy syndromes. Experimental hyperthermia has been studied extensively, predominantly to investigate febrile seizures of childhood. In particular prolonged or complex febrile seizures have been associated with the later development of epilepsy in adulthood and the pathophysiology of how febrile seizures cause epilepsy is of tremendous interest. Febrile seizures represent an opportunity to potentially intervene early in life in susceptible individuals and affect epileptogenesis. The pathophysiologic underpinnings of how hyperthermia induces seizures and how this in turn results in epilepsy are controversial, but likely involve multiple factors. Both glutamatergic and GABAergic neurotransmission is affected, and numerous mutations in genes encoding ion channels have been identified. Cytokines such as interleukin-1β have been implicated in febrile seizures as well as susceptibility to provoked seizures later in life. Hyperthermia is a common feature of generalized convulsive status epilepticus, but may also be seen with nonconvulsive seizures, indicating involvement of thermoregulatory centers.

Chronobiology of limbic seizures: Potential mechanisms and prospects of chronotherapy for mesial temporal lobe epilepsy

Mesial Temporal Lobe Epilepsy (mTLE) characterized by progressive development of complex partial seizures originating from the hippocampus is the most prevalent and refractory type of epilepsy. One of the remarkable features of mTLE is the rhythmic pattern of occurrence of spontaneous seizures, implying a dependence on the endogenous clock system for seizure threshold. Conversely, circadian rhythms are affected by epilepsy too. Comprehending how the circadian system and seizures interact with each other is essential for understanding the pathophysiology of epilepsy as well as for developing innovative therapies that are efficacious for better seizure control. In this review, we confer how the temporal dysregulation of the circadian clock in the hippocampus combined with multiple uncoupled oscillators could lead to periodic seizure occurrences and comorbidities. Unraveling these associations with additional research would help in developing chronotherapy for mTLE, based on the chronobiology of spontaneous seizures. Notably, differential dosing of antiepileptic drugs over the circadian period and/or strategies that resynchronize biological rhythms may substantially improve the management of seizures in mTLE patients.

A companion to the preclinical common data elements for physiologic data in rodent epilepsy models. A report of the TASK3 Physiology Working Group of the ILAE/AES Joint Translational Task Force

The International League Against Epilepsy/American Epilepsy Society (ILAE/AES) Joint Translational Task Force created the TASK3 working groups to create common data elements (CDEs) for various aspects of preclinical epilepsy research studies, which could help improve standardization of experimental designs. This article concerns the parameters that can be measured to assess the physiologic condition of the animals that are used to study rodent models of epilepsy. Here we discuss CDEs for physiologic parameters measured in adult rats and mice such as general health status, temperature, cardiac and respiratory function, and blood constituents. We provide detailed CDE tables and case report forms (CRFs), and with this companion manuscript we discuss the monitoring of different aspects of physiology of the animals. The CDEs, CRFs, and companion paper are available to all researchers, and their use will benefit the harmonization and comparability of translational preclinical epilepsy research. The ultimate hope is to facilitate the development of biomarkers and new treatments for epilepsy.

Rhythms of Core Clock Genes and Spontaneous Locomotor Activity in Post-Status Epilepticus Model of Mesial Temporal Lobe Epilepsy

The interaction of Mesial Temporal Lobe Epilepsy (mTLE) with the circadian system control is apparent from an oscillatory pattern of limbic seizures, daytime's effect on seizure onset and the efficacy of antiepileptic drugs. Moreover, seizures per se can interfere with the biological rhythm output, including circadian oscillation of body temperature, locomotor activity, EEG pattern as well as the transcriptome. However, the molecular mechanisms underlying this cross-talk remain unclear. In this study, we systematically evaluated the temporal expression of seven core circadian transcripts (Bmal1, Clock, Cry1, Cry2, Per1, Per2, and Per3) and the spontaneous locomotor activity (SLA) in post-status epilepticus (SE) model of mTLE. Twenty-four hour oscillating SLA remained intact in post-SE groups although the circadian phase and the amount and intensity of activity were changed in early post-SE and epileptic phases. The acrophase of the SLA rhythm was delayed during epileptogenesis, a fragmented 24 h rhythmicity and extended active phase length appeared in the epileptic phase. The temporal expression of circadian transcripts Bmal1, Cry1, Cry2, Per1, Per2, and Per3 was also substantially altered. The oscillatory expression of Bmal1 was maintained in rats imperiled to SE, but with lower amplitude (A = 0.2) and an advanced acrophase in the epileptic phase. The diurnal rhythm of Cry1 and Cry2 was absent in the early post-SE but was recovered in the epileptic phase. Per1 and Per2 rhythmic expression were disrupted in post-SE groups while Per3 presented an arrhythmic profile in the epileptic phase, only. The expression of Clock did not display rhythmic pattern in any condition. These oscillating patterns of core clock genes may contribute to hippocampal 24 h cycling and, consequently to seizure periodicity. Furthermore, by using a pool of samples collected at 6 different Zeitgeber Times (ZT), we found that all clock transcripts were significantly dysregulated after SE induction, except Per3 and Per2. Collectively, altered SLA rhythm in early post-SE and epileptic phases implies a possible role for seizure as a nonphotic cue, which is likely linked to activation of hippocampal–accumbens pathway. On the other hand, altered temporal expression of the clock genes after SE suggests their involvement in the MTLE.

Effects of acute physical exercise in the light phase of sleep in rats with temporal lobe epilepsy

OBJECTIVE

Our aim was to investigate the influence of an acute exercise session on sleep pattern in rats with temporal lobe epilepsy (TLE).

METHODS

Twenty male Wistar rats were randomly assigned to 4 groups: control (C); acute exercise (EX); epilepsy (E) and epilepsy acute exercise (EEX). Two sleep electrocorticography recordings were performed during the light phase [baseline and day 2 (after the acute physical exercise session)]. After baseline recording, the exercise groups (EX and EEX) were submitted to an exercise session on a motor-driven treadmill at 12m/min for 30min. Twelve hours later, the rats were submitted to the second sleep recording.

RESULTS

At baseline, the E group showed a higher wakefulness and a lower Total sleep time, Slow Wave Sleep and REM sleep compared with the C group. After acute exercise, there was an increase in Total sleep time and Slow Wave Sleep and a decrease of wakefulness in EEX (+11.10%, +20.29% and -11.25%, respectively) and EX (+5.20%, +11.60% and -8.12%, respectively) groups.

CONCLUSION

These findings suggest that acute physical exercise positively impacts the sleep pattern of rats with TLE, inducing a more consolidated sleep.

Phase shift in the 24-hour rhythm of hippocampal EEG spiking activity in a rat model of temporal lobe epilepsy

For over a century epileptic seizures have been known to cluster at specific times of the day. Recent studies have suggested that the circadian regulatory system may become permanently altered in epilepsy, but little is known about how this affects neural activity and the daily pattern of seizures. To investigate, we tracked long-term changes in the rate of spontaneous hippocampal EEG spikes (SPKs) in a rat model of temporal lobe epilepsy. In healthy animals, SPKs oscillated with near 24-h period; however, after injury by status epilepticus, a persistent phase shift of ∼12 h emerged in animals that later went on to develop chronic spontaneous seizures. Additional measurements showed that global 24-h rhythms, including core body temperature and theta state transitions, did not phase shift. Instead, we hypothesized that locally impaired circadian input to the hippocampus might be responsible for the SPK phase shift. This was investigated with a biophysical computer model in which we showed that subtle changes in the relative strengths of circadian input could produce a phase shift in hippocampal neural activity. MRI provided evidence that the medial septum, a putative circadian relay center for the hippocampus, exhibits signs of damage and therefore could contribute to local circadian impairment. Our results suggest that balanced circadian input is critical to maintaining natural circadian phase in the hippocampus and that damage to circadian relay centers, such as the medial septum, may disrupt this balance. We conclude by discussing how abnormal circadian regulation may contribute to the daily rhythms of epileptic seizures and related cognitive dysfunction.

Sleep and epilepsy: exploring an intriguing relationship with a translational approach

The relationship between sleep and epilepsy has been well established. There is a high prevalence of sleep disturbances in epilepsy, which are associated with a decreased quality of life of individuals with epilepsy. In view of this fact, preclinical research is necessary to address many gaps in knowledge. For instance, it is well known that sleep deprivation can trigger seizures; however, this is a complex pathophysiological event. In this context, there are many valuable animal models of epilepsy that reproduce clinical symptoms and can be used. Investigations using animal models that simulate clinical epilepsy are imperative. Furthermore, preclinical studies that reveal mechanisms related to sleep-epilepsy interactions are very important. Results of such studies can, in turn, improve the understanding of epilepsy itself and can be useful in developing new antiepileptic drugs and preventive measures to control seizures. Preclinical research should be performed using a translational framework with experimental designs that can lead to advances in the quality of life of individuals with epilepsy. In view of the fact that more than 50 million of people are affected by epilepsy around the world, understanding the relationship between sleep and epilepsy is imperative.

Thalamus lesions in chronic and acute seizure disorders

INTRODUCTION

Transient signal changes in the pulvinar have been described following status epilepticus. However, we observed persistent thalamus changes after seizures. The purpose of this study was to characterize thalamus changes in patients with seizure disorders and to correlate imaging findings with clinical features.

METHODS

We searched among 5,500 magnetic resonance imaging (MRI) exams performed in patients with seizures and identified 43 patients. The MRI scans of these patients were reviewed and correlated with clinical data.

RESULTS

We identified four patterns of thalamus lesions: (a) fluid attenuated inversion recovery-hyperintense pulvinar lesions (20 patients), as known from status epilepticus. Ten patients in this group had a status epilepticus. Among the remaining patients, three had frequent seizures and seven had sporadic seizures. Twelve patients had follow-up exams for a median of 11 months. The lesions had persisted in 11/12 cases in the last available exam and were reversible in one case only. In seven cases, cone-shaped thalamus atrophy resulted, (b) linear defects in the medial and anterior thalamus (five patients), accompanied by atrophy of the mamillary body and the fornix in patients with chronic epilepsy, (c) extensive bilateral thalamus lesions in two patients with a syndrome caused by mutation in the mitochondrial polymerase gamma, and (d) other thalamus lesions not associated with the seizure disorder (16 patients).

CONCLUSION

The spectrum of thalamus lesions in patients with seizure disorders is wider than previously reported. Postictal pulvinar lesions can persist and may result in thalamic atrophy. Linear defects in the anterior thalamus are associated with limbic system atrophy.

The sleep-wake cycle in adult rats following pilocarpine-induced temporal lobe epilepsy

The relationship between sleep and epilepsy is both complex and clinically significant. Temporal lobe epilepsy (TLE) influences sleep architecture, while sleep plays an important role in facilitating and/or inhibiting possible epileptic seizures. The pilocarpine experimental model reproduces several features of human temporal lobe epilepsy and is one of the most widely used models in basic research. The aim of the present study was to characterize, behaviorally and electrophysiologically, the phases of sleep-wake cycles (SWC) in male rats with pilocarpine-induced epilepsy. Epileptic rats presented spikes in all phases of the SWC as well as atypical cortical synchronization during attentive wakefulness and paradoxical sleep. The architecture of the sleep-wake phases was altered in epileptic rats, as was the integrity of the SWC. Because our findings reproduce many relevant features observed in patients with epilepsy, this model is suitable to study sleep dysfunction in epilepsy.

A rat model of epilepsy in women: a tool to study physiological interactions between endocrine systems and seizures

Epilepsy in women is influenced by endocrine status and antiepileptic drugs, but without an animal model, the effects of endocrine variables and antiepileptic drugs cannot be easily dissociated from the influence of epilepsy itself. Animal models have had limited utility because experimentally induced seizures typically result in reproductive failure. This study was conducted to develop an improved animal model. The muscarinic convulsant pilocarpine was used to elicit status epilepticus (SE) in adult female Sprague Dawley rats. The selective estrogen receptor modulator raloxifene was administered 30 min before pilocarpine. An anticonvulsant barbiturate, pentobarbital, was injected 5-10 min after the onset of SE and at least once thereafter to minimize acute convulsions. Mortality, morbidity, estrous cyclicity, and the ultimate success of the procedure (i.e. induction of recurrent, spontaneous seizures) were monitored. The combination of raloxifene and pentobarbital led to significantly improved estrous cyclicity compared with previous methods. Animals treated with raloxifene and pentobarbital became epileptic, as defined by the recurrence of spontaneous convulsions in the weeks after SE. The results of this study provide an improved animal model to examine the interactions between seizures and ovarian hormone secretion. The results also suggest that treatment of SE with raloxifene may benefit women with SE.

The circadian rhythm and its interaction with human epilepsy: a review of literature

Knowledge on the interaction between circadian rhythm and human epilepsy is relatively poor, although if it exists, this interaction may be of value for better knowledge of pathophysiology and for timing of diagnostic procedures and therapy. It appears that human seizure occurrence may have 24-h rhythmicity, depending on the origin. These findings are endorsed by animal studies. Rats placed in constant darkness showed spontaneous limbic seizures occurring in an endogenously mediated circadian pattern. More studies are available on the influence of epilepsy on circadian rhythms. Significant differences in chronotypes between patients with different epilepsy syndromes have been found and numerous studies have described influences of epilepsy and seizures on sleep. In contrast, knowledge on (core) body temperature and clock genes in patients is minimal. Reduced heart rate variability and changed hormone levels, which are under the influence of the biological clock, have been observed in people with epilepsy. In short, large gaps in the knowledge about the interaction of circadian rhythm and human epilepsy still remain. Proposals for studies in this borderline area between the biological clock and epilepsy will be discussed.

A single generalized seizure alters the amplitude, but not phase, of the circadian activity rhythm of the hamster

People with epilepsy exhibit high rates of sleep disturbances. In many cases, these sleep disruptions appear to be related to the occurrence of the seizures themselves. Changes in sleep structure may reflect underlying changes in the circadian clock, as circadian rhythms of locomotor activity, body temperature, and hormone release are disrupted following a seizure. The present study was designed to determine if a single generalized seizure could alter the phase and waveform of the circadian rhythm of wheel-running behavior in the Syrian hamster. Animals were housed in constant darkness, and were administered either a sham treatment or a maximal electroconvulsive shock at one of three time-points: 6 h before activity onset, 1 h after activity onset, or 6 h after activity onset. Seizures at all of these phases did not significantly affect the phase of the circadian activity rhythm. The circadian locomotor activity levels were significantly attenuated following seizures at all three phases. This attenuation was prominent over the 24 h following the seizure, and was also evident over the three post-seizure days. These data suggest that while seizures do not affect phase, they may alter the amplitude of the circadian clock. Because the amplitude of the circadian clock affects sleep quality, these findings suggest one mechanism by which persistent seizures may decrease the quality of sleep in patients with epilepsy.

Consciousness and epilepsy: why are complex-partial seizures complex?

Why do complex-partial seizures in temporal lobe epilepsy (TLE) cause a loss of consciousness? Abnormal function of the medial temporal lobe is expected to cause memory loss, but it is unclear why profoundly impaired consciousness is so common in temporal lobe seizures. Recent exciting advances in behavioral, electrophysiological, and neuroimaging techniques spanning both human patients and animal models may allow new insights into this old question. While behavioral automatisms are often associated with diminished consciousness during temporal lobe seizures, impaired consciousness without ictal motor activity has also been described. Some have argued that electrographic lateralization of seizure activity to the left temporal lobe is most likely to cause impaired consciousness, but the evidence remains equivocal. Other data correlates ictal consciousness in TLE with bilateral temporal lobe involvement of seizure spiking. Nevertheless, it remains unclear why bilateral temporal seizures should impair responsiveness. Recent evidence has shown that impaired consciousness during temporal lobe seizures is correlated with large-amplitude slow EEG activity and neuroimaging signal decreases in the frontal and parietal association cortices. This abnormal decreased function in the neocortex contrasts with fast polyspike activity and elevated cerebral blood flow in limbic and other subcortical structures ictally. Our laboratory has thus proposed the "network inhibition hypothesis," in which seizure activity propagates to subcortical regions necessary for cortical activation, allowing the cortex to descend into an inhibited state of unconsciousness during complex-partial temporal lobe seizures. Supporting this hypothesis, recent rat studies during partial limbic seizures have shown that behavioral arrest is associated with frontal cortical slow waves, decreased neuronal firing, and hypometabolism. Animal studies further demonstrate that cortical deactivation and behavioral changes depend on seizure spread to subcortical structures including the lateral septum. Understanding the contributions of network inhibition to impaired consciousness in TLE is an important goal, as recurrent limbic seizures often result in cortical dysfunction during and between epileptic events that adversely affects patients' quality of life.

Altered pharmacology and GABA-A receptor subunit expression in dorsal midline thalamic neurons in limbic epilepsy

The mediodorsal (MD) and paraventricular (PV) thalamic nuclei play a significant role in limbic epilepsy, and previous reports have shown changes in GABA-A receptor (GABAAR) mediated synaptic function. In this study, we examined changes in the pharmacology of GABAergic drugs and the expression of the GABAAR subunits in the MD and PV neurons in epilepsy. We observed nucleus specific changes in the sensitivity of sIPSCs to zolpidem and phenobarbital in MD and PV neurons from epileptic animals. In contrast, the magnitude of change in electrically evoked response (eIPSC) to zolpidem and phenobarbital were uniformly diminished in both MD and PV neurons in epilepsy. Immunohistochemical studies revealed that in epilepsy, there was a reduction in GAD65 expression and NeuN positive neurons in the MD neurons. Also, there was a decrease in immunoreactivity of the alpha1 and beta2/3 subunit of GABAARs, but not the gamma2 of the GABAAR in both MD and PV in epilepsy. These findings demonstrate significant alterations in the pharmacology of GABA and GABAARs in a key region for seizure generation, which may have implications for the physiology and pharmacology of limbic epilepsy.

Sleep and epilepsy: what we know, don't know, and need to know

Long-term video-EEG and, more recently, video-polysomnography, have provided the means to confirm and expand on the interconnections between sleep and epilepsy. Some of these relationships have become firmly established. When one of the authors (N.F.S.) presented part of this paper at a symposium on the Future of Sleep in Neurology at an American Clinical Neurophysiology Society annual meeting in 2004, the purpose was to summarize what we know, don't know, and need to know about the effects of sleep on epilepsy and epilepsy on sleep. Here we seek to summarize some of the more firmly established relationships between sleep and epilepsy and identify intriguing associations that require further elucidation.

Spontaneous epileptic rats show changes in sleep architecture and hypothalamic pathology

PURPOSE

The goal of the present study was to investigate the relationship between sleep, hypothalamic pathology, and seizures in spontaneous epileptic rats.

METHODS

Rats were implanted with radiotelemetry transmitters for measuring electrocorticogram (ECoG) and stimulation electrodes in the hippocampus. Epileptogenesis was triggered by 2 h of electical stimulation-induced self-sustained status epilepticus (SSSE). After SSSE, ECoGs were monitored over a 15-week period for the occurrence of interictal high-amplitude low-frequency (HALF) acitvity and spontaneous reoccurring seizures (SRSs).

RESULTS

Spontaneous epileptic rats showed clinical features of temporal lobe epilepsy (TLE), such as spontaneous seizures, interictal activity and neuronal cell loss in the dorsomedial hypothalamus, a region important for normal sleep regulation. Interestingly, epileptic rats showed disturbances in sleep architecture, with a high percentage of the seizures occurring during sleep.

CONCLUSIONS

Therefore we conclude that a close association exists between epileptiform activity and alterations in sleep architecture that may be related to hypothalamic pathology.

Temporal lobe seizures alter the amplitude and timing of rat behavioral rhythms

Daily rhythms of spontaneous locomotor activity (SLA) in rats were studied before and after an episode of pilocarpine-induced convulsive status epilepticus (SE). A pronounced increase in activity levels during both the light and dark phases was found 1 week after SE as compared with baseline SLA and controls administered saline. Rats with bilateral lesions of the nucleus accumbens (shell) did not exhibit any significant change in SLA 1 week after SE compared with controls. We suggest that during the first week after SE the increase in SLA was induced by abnormal neuronal activity in the hippocampus driving a descending limbic-motor pathway via the nucleus accumbens. EEG recordings revealed high-amplitude interictal spikes in hippocampal CA1. During subsequent weeks, SLA rhythms of nonlesioned chronic epileptic rats remained elevated but progressively normalized over a period of 12 weeks. Although both chronic epileptic and control groups displayed near-24-hour activity patterns under light-dark conditions, significant delays (>4 hour) in acrophase were observed after spontaneous seizures had developed. The phase delay was positively correlated with seizure history and likely the result of postictal hyperactivity associated with seizures during the normal rest period. Despite these changes, cell density in the suprachiasmatic nucleus (SCN) and intergeniculate leaflet (IGL) did not differ significantly between epileptic and control groups. In the absence of damage to brain areas directly involved with the regulation of behavioral rhythms, chronic seizure activity presumably alters the timing of activity patterns through a nonphotic mechanism, perhaps involving activation of the SCN or IGL during limbic seizures.

The mechanisms of action of PPARs

The peroxisome proliferator-activated receptors (PPARs) are a group of three nuclear receptor isoforms, PPAR gamma, PPAR alpha, and PPAR delta, encoded by different genes. PPARs are ligand-regulated transcription factors that control gene expression by binding to specific response elements (PPREs) within promoters. PPARs bind as heterodimers with a retinoid X receptor and, upon binding agonist, interact with cofactors such that the rate of transcription initiation is increased. The PPARs play a critical physiological role as lipid sensors and regulators of lipid metabolism. Fatty acids and eicosanoids have been identified as natural ligands for the PPARs. More potent synthetic PPAR ligands, including the fibrates and thiazolidinediones, have proven effective in the treatment of dyslipidemia and diabetes. Use of such ligands has allowed researchers to unveil many potential roles for the PPARs in pathological states including atherosclerosis, inflammation, cancer, infertility, and demyelination. Here, we present the current state of knowledge regarding the molecular mechanisms of PPAR action and the involvement of the PPARs in the etiology and treatment of several chronic diseases.

Seizures induce phase shifts of rat circadian rhythms

RATIONALE

Epileptic seizures may alter neuroendocrinological cycles. Light pulses induce phase shifts in circadian rhythms. Using hippocampal-kindled rats to ensure maximal clinical expression, we determined if seizures likewise induce phase shifts.

METHODS

We monitored the circadian rhythm of temperature (CRT) with intraperitoneal radiotelemetry in rats (n=21) isolated from time cues and light for 3-week trials. Seizures were triggered with hippocampal electrical stimulation at different circadian phases. Optimized, least-error phase shifts were calculated from preictal and postictal CRTs. Induced seizures were referenced to CRT (t(max)=00:00, 24-h circadian cycle).

RESULTS

Phase shifts (individual responses=57) differed across the circadian cycle. Rather than forming a clear phase-response curve, phase shifts were especially variable between 00:00 and 06:00 h.

CONCLUSIONS

This study demonstrates that electrically-induced seizures induce advances and delays in CRT in a phase-dependent fashion but in a pattern different from typical light-induced phase shifts. Disorders of circadian regulation may contribute to some of the altered endogenous cycles associated with epilepsy.

Circadian rhythms: interactions with seizures and epilepsy

Circadian rhythms are endogenously-mediated 24 h cycles of behavioral or physiological activity. The interactions among the mammalian circadian clock, acute seizures, and chronic epilepsy are not well-characterized. Evidence suggests that seizures are susceptible to circadian modulation, and that this modulation varies with epilepsy syndrome and location of seizure foci. The circadian timing system and secondary circadian cycles of hormone secretion, sleep and wakefulness, and recurrent environmental factors are discussed as potential systems that effect spontaneous seizure recurrence. Experimental designs should take into account time-of-day effects on seizure threshold and occurrence. Further work is required to determine what mechanisms account for daily variation in seizure susceptibility.

Dual epileptic foci in a single patient express distinct temporal patterns dependent on limbic versus nonlimbic brain location

How timing information is transferred from the suprachiasmatic nucleus to other regions of the brain to mediate activity, either physiological or pathological, is largely unclear. A patient with medically refractory epilepsy and a well-documented, long-term seizure diary provided a unique means to demonstrate how susceptibility to chronobiological modulation varies with brain region. Evaluation for epilepsy surgery disclosed two independent epileptic foci, one limbic and the other nonlimbic. Seizures from both foci occurred periodically with a dominant period of 24 hours but were out of phase with each other. Temporal lobe seizures occurred maximally in the light portion of the daily light-dark cycle, and parietal lobe seizures occurred nocturnally and out of phase with limbic seizures. These data suggest that neuronal excitation and inhibition, depending on the anatomical system involved in epilepsy, may be differently affected by circadian modulation.

Effects of circadian regulation and rest-activity state on spontaneous seizures in a rat model of limbic epilepsy

PURPOSE

Circadian regulation via the suprachiasmatic nuclei and rest-activity state may influence expression of limbic seizures.

METHODS

Male rats (n = 14) were made epileptic by electrical stimulation of the hippocampus, causing limbic status epilepticus and subsequent seizures. We monitored seizures with intrahippocampal electrodes in 12-12-h light/dark (LD) cycles and in continuous dark (DD). We used radiotelemetry monitoring of activity to measure state and body temperature to determine circadian phase. Cosinor analysis and chi2 tests determined whether seizures occurred rhythmically when plotted by phase. State was defined as inactive or active in 10-min epochs based on whether activity count was below or above a cut-off value validated from video observation.

RESULTS

In LD, the peak seizure occurrence was 14:59 h after circadian temperature peak (95% confidence limit, 13:37-16:19). Phasic seizure occurrence persisted in DD for 14:05 (12:31-15:38), p < 0.0001, against uniform mean distribution. In LD, 14,787 epochs contained 1, 268 seizures; seizures preferentially occurred during inactive epochs (965 observed, 878 expected in proportion to the overall distribution of inactive versus active epochs; p < 0.001). In DD, 20, 664 epochs contained 1,609 seizures; seizures had no preferential occurrence by state (999 observed, 1,025 expected; p = 0.16).

CONCLUSIONS

Limbic seizures occurred with an endogenous circadian rhythm. Seizures preferentially struck during inactivity during entrainment to the light-dark cycle.