Inhibition of adenosine metabolism induces changes in post-ictal depression, respiration, and mortality in genetically epilepsy prone rats

Disruption of adenosine metabolism increases risk of seizure-induced death despite decreased seizure severity

OBJECTIVE

Respiratory arrest plays an important role in sudden unexpected death in epilepsy (SUDEP). Adenosine is of interest in SUDEP pathophysiology due to its influence on seizures and breathing. The objective of this investigation was to examine the role of adenosine in seizure severity, seizure-induced respiratory disruption, and seizure-induced death using mouse models. Understanding adenosinergic contributions to seizure cessation and seizure-induced death may provide insights into how SUDEP can be prevented while avoiding increased seizure severity.

METHODS

Our approach was to examine: (1) seizure severity and seizure-induced death after 15 mA electroshock seizures and during repeated pentylenetetrazol (PTZ) administration in wild-type mice (Adk+/+) and transgenic mice with reduced adenosine metabolism (Adk+/-); (2) the postictal hypercapnic ventilatory response (HCVR) in wild-type mice (the postictal HCVR could not be examined in Adk+/- mice due to their high mortality rate); and (3) the effects of adenosinergic drugs on seizure severity and seizure-induced death following maximal electroshock (MES).

RESULTS

Adk+/- mice were more vulnerable to seizure-induced death in the 15 mA electroshock and repeated PTZ models. Despite increased mortality, Adk+/- mice had comparable seizure severity in the PTZ model and reduced seizure severity in the 15 mA electroshock model. Breathing and HCVR were suppressed by 15 mA electroshock seizures in wild-type mice. Pharmacological inhibition of adenosine metabolism decreased MES seizure severity but did not increase mortality. A1 selective and nonselective adenosine receptor antagonists increased seizure-induced death following MES.

SIGNIFICANCE

Adenosine has opposing effects on seizure severity and seizure-induced death. On the one hand, our seizure severity data highlight the importance of adenosine in seizure suppression. On the other hand, our mortality data indicate that excessive extracellular adenosine signaling can increase the risk of seizure-induced respiratory arrest.

Effect of adenosinergic manipulations on amygdala-kindled seizures in mice: Implications for sudden unexpected death in epilepsy

OBJECTIVE

Sudden unexpected death in epilepsy (SUDEP) results in more years of potential life lost than any neurological condition with the exception of stroke. It is generally agreed that SUDEP happens due to some form of respiratory, cardiac, and electrocerebral dysfunction following a seizure; however, the mechanistic cause of these perturbations is unclear. One possible explanation lies with adenosinergic signaling. Extracellular levels of the inhibitory neuromodulator adenosine rapidly rise during seizures, a countermeasure that is necessary for seizure termination. Previous evidence has suggested that excessive adenosinergic inhibition could increase the risk of SUDEP by silencing brain areas necessary for life, such as the respiratory nuclei of the brainstem. The goal of this investigation was to further clarify the role of adenosine in seizure-induced respiratory and electrocerebral dysfunction.

METHODS

To determine the role of adenosine in postictal physiological dysregulation, we pharmacologically manipulated adenosine signaling prior to amygdala-kindled seizures in mice while recording electroencephalogram (EEG), electromyogram, and breathing using whole body plethysmography. The adenosinergic drugs used in this study included selective and nonselective adenosine receptor antagonists and inhibitors of adenosine metabolism.

RESULTS

We found that high doses of adenosine receptor antagonists caused some seizures to result in seizure-induced death; however, counterintuitively, animals in these conditions that did not experience seizure-induced death had little or no postictal generalized EEG suppression. Inhibitors of adenosine metabolism had no effect on postictal breathing but did worsen some postictal electrocerebral outcomes.

SIGNIFICANCE

The unexpected effect of high doses of adenosine antagonists on seizure-induced death observed in this study may be due to the increase in seizure severity, vasoconstriction, or phosphodiesterase inhibition caused by these drugs at high doses. These findings further clarify the role of adenosine in seizure-induced death and may have implications for the consumption of caffeine in epilepsy patients and the prevention of SUDEP.

The role of the adenosine system in epilepsy and its comorbidities

Epilepsy is one of the most serious and common chronic neurological conditions, characterised by recurrent hypersynchronous electrical activity in the brain that lead to seizures. Despite over 50 million people being affected worldwide, only ~70% of people with epilepsy have their seizures successfully controlled with current pharmacotherapy, and many experience significant psychiatric and physical comorbidities. Adenosine, a ubiquitous purine metabolite, is a potent endogenous anti-epileptic substance that can abolish seizure activity via the adenosine A1 G protein-coupled receptor. Activation of A1 receptors decreases seizure activity in animal models, including models of drug-resistant epilepsy. Recent advances have increased our understanding of epilepsy comorbidities, highlighting the potential for adenosine receptors to modulate epilepsy-associated comorbidities, including cardiovascular dysfunction, sleep and cognition. This review provides an accessible resource of the current advances in understanding the adenosine system as a therapeutic target for epilepsy and epilepsy-associated comorbidities. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

A unified hypothesis of SUDEP: Seizure-induced respiratory depression induced by adenosine may lead to SUDEP but can be prevented by autoresuscitation and other restorative respiratory response mechanisms mediated by the action of serotonin on the periaqueductal gray

Sudden unexpected death in epilepsy (SUDEP) is a major cause of death in people with epilepsy (PWE). Postictal apnea leading to cardiac arrest is the most common sequence of terminal events in witnessed cases of SUDEP, and postconvulsive central apnea has been proposed as a potential biomarker of SUDEP susceptibility. Research in SUDEP animal models has led to the serotonin and adenosine hypotheses of SUDEP. These neurotransmitters influence respiration, seizures, and lethality in animal models of SUDEP, and are implicated in human SUDEP cases. Adenosine released during seizures is proposed to be an important seizure termination mechanism. However, adenosine also depresses respiration, and this effect is mediated, in part, by inhibition of neuronal activity in subcortical structures that modulate respiration, including the periaqueductal gray (PAG). Drugs that enhance the action of adenosine increase postictal death in SUDEP models. Serotonin is also released during seizures, but enhances respiration in response to an elevated carbon dioxide level, which often occurs postictally. This effect of serotonin can potentially compensate, in part, for the adenosine-mediated respiratory depression, acting to facilitate autoresuscitation and other restorative respiratory response mechanisms. A number of drugs that enhance the action of serotonin prevent postictal death in several SUDEP models and reduce postictal respiratory depression in PWE. This effect of serotonergic drugs may be mediated, in part, by actions on brainstem sites that modulate respiration, including the PAG. Enhanced activity in the PAG increases respiration in response to hypoxia and other exigent conditions and can be activated by electrical stimulation. Thus, we propose the unifying hypothesis that seizure-induced adenosine release leads to respiratory depression. This can be reversed by serotonergic action on autoresuscitation and other restorative respiratory responses acting, in part, via the PAG. Therefore, we hypothesize that serotonergic or direct activation of this brainstem site may be a useful approach for SUDEP prevention.

Adenosine-A2A Receptor Signaling Plays a Crucial Role in Sudden Unexpected Death in Epilepsy

Adenosinergic activities are suggested to participate in SUDEP pathophysiology; this study aimed to evaluate the adenosine hypothesis of SUDEP and specifically the role of adenosine A_2A receptor (A_2AR) in the development of a SUDEP mouse model with relevant clinical features. Using a combined paradigm of intrahippocampal and intraperitoneal administration of kainic acid (KA), we developed a boosted-KA model of SUDEP in genetically modified adenosine kinase (ADK) knockdown (Adk^+/-) mice, which has reduced ADK in the brain. Seizure activity was monitored using video-EEG methods, and in vivo recording of local field potential (LFP) was used to evaluate neuronal activity within the nucleus tractus solitarius (NTS). Our boosted-KA model of SUDEP was characterized by a delayed, postictal sudden death in epileptic mice. We demonstrated a higher incidence of SUDEP in Adk^+/- mice (34.8%) vs. WTs (8.0%), and the ADK inhibitor, 5-Iodotubercidin, further increased SUDEP in Adk^+/- mice (46.7%). We revealed that the NTS level of ADK was significantly increased in epileptic WTs, but not in epileptic Adk^+/- mutants, while the A_2AR level in NTS was increased in epileptic (WT and Adk^+/-) mice vs. non-epileptic controls. The A_2AR antagonist, SCH58261, significantly reduced SUDEP events in Adk^+/- mice. LFP data showed that SCH58261 partially restored KA injection-induced suppression of gamma oscillation in the NTS of epileptic WT mice, whereas SCH58261 increased theta and beta oscillations in Adk^+/- mutants after KA injection, albeit with no change in gamma oscillations. These LFP findings suggest that SCH58261 and KA induced changes in local neuronal activities in the NTS of epileptic mice. We revealed a crucial role for NTS A_2AR in SUDEP pathophysiology suggesting A_2AR as a potential therapeutic target for SUDEP risk prevention.

Adrenergic α2 receptors are implicated in seizure-induced respiratory arrest in DBA/1 mice

AIMS

Sudden unexpected death in epilepsy (SUDEP) is a serious and underestimated public health burden. Both clinical and animal studies show that seizure-induced respiratory arrest (S-IRA) is the primary cause of death in SUDEP. Our previous studies demonstrated that atomoxetine, a norepinephrine reuptake inhibitor (NRI), suppresses S-IRA in DBA/1 mice, suggesting that noradrenergic neurotransmission modulates S-IRA. However, it remains unclear which adrenoceptors are implicated in S-IRA in DBA/1 mice.

MATERIALS AND METHODS

Naïve DBA/1 mice exhibit a low incidence of S-IRA, but after primed by acoustic stimulation, they become consistently susceptible to S-IRA. Atomoxetine, adrenoceptor agonists, antagonists or vehicle was intraperitoneally (i.p.) administered alone or in combination, and the effects of drug treatments on S-IRA incidence and seizure behaviors were examined.

KEY FINDINGS

The incidence of S-IRA in primed DBA/1 mice was significantly reduced by clonidine, an α2 adrenoceptor agonist, as compared with that of the vehicle control. However, compared with the vehicle control, S-IRA was not altered by cirazoline, an α1 agonist. Consistent with previous reports, atomoxetine reduced S-IRA in primed DBA/1 mice. The suppressing effect of atomoxetine on S-IRA was prevented by injection of an α2 adrenoceptor antagonist, yohimbine or atipamezole, but not by prazosin, an α1 antagonist. Administration of α1 or α2 antagonists alone did not promote the incidence of S-IRA in nonprimed DBA/1 mice.

SIGNIFICANCE

These data demonstrate that noradrenergic neurotransmission modulates S-IRA predominantly via α2 adrenoceptors in DBA/1 mice, indicating that selective activation of α2 adrenoceptors can potentially prevent SUDEP.

The Good, the Bad, and the Deadly: Adenosinergic Mechanisms Underlying Sudden Unexpected Death in Epilepsy

Adenosine is an inhibitory modulator of neuronal excitability. Neuronal activity results in increased adenosine release, thereby constraining excessive excitation. The exceptionally high neuronal activity of a seizure results in a surge in extracellular adenosine to concentrations many-fold higher than would be observed under normal conditions. In this review, we discuss the multifarious effects of adenosine signaling in the context of epilepsy, with emphasis on sudden unexpected death in epilepsy (SUDEP). We describe and categorize the beneficial, detrimental, and potentially deadly aspects of adenosine signaling. The good or beneficial characteristics of adenosine signaling in the context of seizures include: (1) its direct effect on seizure termination and the prevention of status epilepticus; (2) the vasodilatory effect of adenosine, potentially counteracting postictal vasoconstriction; (3) its neuroprotective effects under hypoxic conditions; and (4) its disease modifying antiepileptogenic effect. The bad or detrimental effects of adenosine signaling include: (1) its capacity to suppress breathing and contribute to peri-ictal respiratory dysfunction; (2) its contribution to postictal generalized EEG suppression (PGES); (3) the prolonged increase in extracellular adenosine following spreading depolarization waves may contribute to postictal neuronal dysfunction; (4) the excitatory effects of A2A receptor activation is thought to exacerbate seizures in some instances; and (5) its potential contributions to sleep alterations in epilepsy. Finally, the adverse effects of adenosine signaling may potentiate a deadly outcome in the form of SUDEP by suppressing breathing and arousal in the postictal period. Evidence from animal models suggests that excessive postictal adenosine signaling contributes to the pathophysiology of SUDEP. The goal of this review is to discuss the beneficial, harmful, and potentially deadly roles that adenosine plays in the context of epilepsy and to identify crucial gaps in knowledge where further investigation is necessary. By better understanding adenosine dynamics, we may gain insights into the treatment of epilepsy and the prevention of SUDEP.

Relation between coffee consumption and risk of seizure-related respiratory dysfunction in patients with drug-resistant focal epilepsy

OBJECTIVE

Caffeine is an antagonist of the adenosine pathway, which is involved in regulation of breathing. Extracellular concentrations of adenosine are increased in the immediate aftermath of a seizure. Seizure-related overstimulation of adenosine receptors might promote peri-ictal apnea. However, the relation between caffeine consumption and risk of seizure-related respiratory dysfunction in patients with drug-resistant focal epilepsy remains unknown.

METHODS

We performed a cross-sectional analysis of data collected in patients included in the SAVE study in Lyon's epilepsy monitoring unit at the Adult Epilepsy Department of the Lyon University Hospital between February 2016 and October 2018. The video-electroencephalographic recordings of 156 patients with drug-resistant focal epilepsy included in the study were reviewed to identify those with ≥1 focal seizure (FS), valid pulse oximetry (SpO₂ ) measurement, and information about usual coffee consumption. This latter was collected at inclusion using a standardized self-questionnaire and further classified into four groups: none, rare (≤3 cups/week), moderate (4 cups/week to 3 cups/day), and high (≥4 cups/day). Peri-ictal hypoxemia (PIH) was defined as SpO₂ < 90% for at least 5 s occurring during the ictal period, the post-ictal period, or both.

RESULTS

Ninety patients fulfilled inclusion criteria, and 323 seizures were analyzed. Both the level of usual coffee consumption (p = .033) and the level of antiepileptic drug withdrawal (p = .004) were independent risk factors for occurrence of PIH. In comparison with FS in patients with no coffee consumption, risk of PIH was four times lower in FS in patients with moderate consumption (odds ratio [OR] = .25, 95% confidence interval [CI] = .07-.91, p = .036) and six times lower in FS in patients with high coffee consumption (OR = .16, 95% CI = .04-.66, p = .011). However, when PIH occurred, its duration was longer in patients with moderate or high consumption than in those with no coffee consumption (p = .042).

SIGNIFICANCE

Coffee consumption may be a protective factor for seizure-related respiratory dysfunction, with a dose-dependent effect.

Sex Differences in the Epilepsies and Associated Comorbidities: Implications for Use and Development of Pharmacotherapies

The epilepsies are common neurologic disorders characterized by spontaneous recurrent seizures. Boys, girls, men, and women of all ages are affected by epilepsy and, in many cases, by associated comorbidities as well. The primary courses of treatment are pharmacological, dietary, and/or surgical, depending on several factors, including the areas of the brain affected and the severity of the epilepsy. There is a growing appreciation that sex differences in underlying brain function and in the neurobiology of epilepsy are important factors that should be accounted for in the design and development of new therapies. In this review, we discuss the current knowledge on sex differences in epilepsy and associated comorbidities, with emphasis on those aspects most informative for the development of new pharmacotherapies. Particular focus is placed on sex differences in the prevalence and presentation of various focal and generalized epilepsies; psychiatric, cognitive, and physiologic comorbidities; catamenial epilepsy in women; sex differences in brain development; the neural actions of sex and stress hormones and their metabolites; and cellular mechanisms, including brain-derived neurotrophic factor signaling and neuronal-glial interactions. Further attention placed on potential sex differences in epilepsies, comorbidities, and drug effects will enhance therapeutic options and efficacy for all patients with epilepsy. SIGNIFICANCE STATEMENT: Epilepsy is a common neurological disorder that often presents together with various comorbidities. The features of epilepsy and seizure activity as well as comorbid afflictions can vary between men and women. In this review, we discuss sex differences in types of epilepsies, associated comorbidities, pathophysiological mechanisms, and antiepileptic drug efficacy in both clinical patient populations and preclinical animal models.

Revisiting the role of neurotransmitters in epilepsy: An updated review

Epilepsy is a neurologicaldisorder characterized by persistent predisposition to recurrent seizurescaused by abnormal neuronal activity in the brain. Epileptic seizures maydevelop due to a relative imbalance of excitatory and inhibitory neurotransmitters. Expressional alterations of receptors and ion channelsactivated by neurotransmitters can lead to epilepsy pathogenesis.

AIMS

In this updated comprehensive review, we discuss the emerging implication of mutations in neurotransmitter-mediated receptors and ion channels. We aim to provide critical findings of the current literature about the role of neurotransmitters in epilepsy.

MATERIALS AND METHODS

A comprehensive literature review was conducted to identify and critically evaluate studies analyzing the possible relationship between epilepsy and neurotransmitters. The PubMed database was searched for related research articles.

KEY FINDINGS

Glutamate and gamma-aminobutyric acid (GABA) are the main neurotransmitters playing a critical role in the pathophysiology of this balance, and irreversible neuronal damage may occur as a result of abnormal changes in these molecules. Acetylcholine (ACh), the main stimulant of the autonomic nervous system, mediates signal transmission through cholinergic and nicotinic receptors. Accumulating evidence indicates that dysfunction of nicotinic ACh receptors, which are widely expressed in hippocampal and cortical neurons, may be significantly implicated in the pathogenesis of epilepsy. The dopamine-norepinephrine-epinephrine cycle activates hormonal and neuronal pathways; serotonin, norepinephrine, histamine, and melatonin can act as both hormones and neurotransmitters. Recent reports have demonstrated that nitric oxide mediates cognitive and memory-related functions via stimulating neuronal transmission.

SIGNIFICANCE

The elucidation of the role of the main mediators and receptors in epilepsy is crucial for developing new diagnostic and therapeutic approaches.

Adenosine kinase and adenosine receptors A1 R and A2A R in temporal lobe epilepsy and hippocampal sclerosis and association with risk factors for SUDEP

OBJECTIVE

The "adenosine hypothesis of SUDEP" (sudden unexpected death in epilepsy) predicts that a seizure-induced adenosine surge combined with impaired metabolic clearance can foster lethal apnea or cardiac arrest. Changes in adenosine receptor density and adenosine kinase (ADK) occur in surgical epilepsy patients. Our aim was to correlate the distribution of ADK and adenosine A_2A and A₁ receptors (A_2A R and A₁ R) in surgical tissue from patients with temporal lobe epilepsy and hippocampal sclerosis (TLE/HS) with SUDEP risk factors.

METHODS

In 75 cases, patients were stratified into high-risk (n = 16), medium-risk (n = 11) and low-risk (n = 48) categories according to the frequency of generalized seizures before surgery. Using whole-slide scanning Definiens image analysis we quantified the labeling index (LI) for ADK, A_2A R, and A₁ R in seven regions of interest: temporal cortex, temporal lobe white matter, CA1, CA4, dentate gyrus, subiculum, and amygdala and relative to glial and neuronal densities with glial fibrillary acidic protein (GFAP) and neuronal nuclear antigen (NeuN).

RESULTS

A₁ R showed predominant neuronal, A_2A R astroglial, and ADK nuclear labeling in all regions but with significant variation. Compared with the low-risk group, the high-risk group had significantly lower A_2A R LI in the temporal cortex. In HS cases with severe neuronal cell loss and gliosis predominantly in the CA1 and CA4 regions, significantly higher A₁ R was present in the amygdala in high-risk than in low-risk cases. There was no significant difference in neuronal loss or gliosis between the risk groups or differences for ADK labeling.

SIGNIFICANCE

Reduced cortical A_2A R suggests glial dysfunction and impaired adenosine modulation in response to seizures in patients at higher risk for SUDEP. Increased neuronal A₁ R in the high-risk group could contribute to periictal amygdala dysfunction in SUDEP.

Deficient post-ictal cardiorespiratory compensatory mechanisms mediated by the periaqueductal gray may lead to death in a mouse model of SUDEP

Post-ictal cardiorespiratory failure is implicated as a major cause of sudden unexpected death in epilepsy (SUDEP) in patients. The DBA/1 mouse model of SUDEP is abnormally susceptible to fatal seizure-induced cardiorespiratory failure (S-CRF) induced by convulsant drug, hyperthermia, electroshock, and acoustic stimulation. Clinical and pre-clinical studies have implicated periaqueductal gray (PAG) abnormalities in SUDEP. Recent functional neuroimaging studies observed that S-CRF resulted in selective changes in PAG neuronal activity in DBA/1 mice. The PAG plays a critical compensatory role for respiratory distress caused by numerous physiological challenges in non-epileptic individuals. These observations suggest that abnormalities in PAG-mediated cardiorespiratory modulation may contribute to S-CRF in DBA/1 mice. To evaluate this, electrical stimulation (20 Hz, 20-100 μA, 10 s) was presented in the PAG of anesthetized DBA/1 and C57BL/6 (non-epileptic) control mice, and post-stimulus changes in respiration [inter-breath interval (IBI)] and heart rate variability (HRV) were examined. The post-stimulus period was considered analogous to the post-ictal period when S-CRF occurred in previous DBA/1 mouse studies. PAG stimulation caused significant intensity-related decreases in IBI in both mouse strains. However, this effect was significantly reduced in DBA/1 vis-a-vis C57BL/6 mice. These changes began immediately following cessation of stimulation and remained significant for 10 s. This time period is critical for initiating resuscitation to successfully prevent seizure-induced death in previous DBA/1 mouse experiments. Significant post-stimulus increases in HRV were also seen at ≥60 μA in the PAG in C57BL/6 mice, which were absent in DBA/1 mice. These data along with previous neuroimaging findings suggest that compensatory cardiorespiratory modulation mediated by PAG is deficient, which may be important to the susceptibility of DBA/1 mice to S-CRF. These observations suggest that correcting this deficit pharmacologically or by electrical stimulation may help to prevent S-CRF. These findings further support the potential importance of PAG abnormalities to human SUDEP.

Nitrobenzylthioinosine mimics adenosine to attenuate the epileptiform discharge of hippocampal neurons from epileptic rats

Nitrobenzylthioinosine (NBTI), a specific inhibitor of type 1 equilibrative nucleoside transporter, could regulate the extracellular adenosine concentration and have protective roles in seizures. However, the protection mechanism of NBTI in seizures remains poorly understood. Here, the expression pattern and subcellular distribution of adenosine A1 receptor were detected by Western blot analysis and double-labeling immunofluorescence staining in Lithium Chloride-Pilocarpine induced epileptic rat model. At 24 h after pilocarpine induced rat seizures, hippocampal slices were prepared and the evoked excitatory postsynaptic currents (eEPSCs) amplitude of pyramidal neurons in hippocampus CA1 region was recorded using whole-cell patch clamp. In vivo, compared to control group, Western blotting analysis showed that the expression of adenosine A1 receptor protein was increased at 24 h and 72 h after seizure, didn't change at 0 min and 1 w, and decreased at 2 w. Double-label immunofluorescence revealed that adenosine A1 receptor was mainly expressed in the membrane and cytoplasm of neurons. In Vitro, adenosine decreased the eEPSCs amplitude of pyramidal neurons in hippocampus CA1 region, NBTI also had the same effect. Meantime, NBTI could further inhibit eEPSCs amplitude on the basis of lower concentration adenosine (50μM), and adenosine A1 receptor inhibitor DPCPX partially reversed this effect. Taken together, we confirmed that the expression of adenosine A1 receptor protein was increased in the early seizures and decreased in the late seizures. At the same time, NBTI mimics adenosine to attenuate the epileptiform discharge through adenosine A1 receptor, which might provide a novel therapeutic approach toward the control of epilepsy.

The Wistar Audiogenic Rat (WAR) strain and its contributions to epileptology and related comorbidities: History and perspectives

In the context of modeling epilepsy and neuropsychiatric comorbidities, we review the Wistar Audiogenic Rat (WAR), first introduced to the neuroscience international community more than 25years ago. The WAR strain is a genetically selected reflex model susceptible to audiogenic seizures (AS), acutely mimicking brainstem-dependent tonic-clonic seizures and chronically (by audiogenic kindling), temporal lobe epilepsy (TLE). Seminal neuroethological, electrophysiological, cellular, and molecular protocols support the WAR strain as a suitable and reliable animal model to study the complexity and emergent functions typical of epileptogenic networks. Furthermore, since epilepsy comorbidities have emerged as a hot topic in epilepsy research, we discuss the use of WARs in fields such as neuropsychiatry, memory and learning, neuroplasticity, neuroendocrinology, and cardio-respiratory autonomic regulation. Last, but not least, we propose that this strain be used in "omics" studies, as well as with the most advanced molecular and computational modeling techniques. Collectively, pioneering and recent findings reinforce the complexity associated with WAR alterations, consequent to the combination of their genetically-dependent background and seizure profile. To add to previous studies, we are currently developing more powerful behavioral, EEG, and molecular methods, combined with computational neuroscience/network modeling tools, to further increase the WAR strain's contributions to contemporary neuroscience in addition to increasing knowledge in a wide array of neuropsychiatric and other comorbidities, given shared neural networks. During the many years that the WAR strain has been studied, a constantly expanding network of multidisciplinary collaborators has generated a growing research and knowledge network. Our current and major wish is to make the WARs available internationally to share our knowledge and to facilitate the planning and execution of multi-institutional projects, eagerly needed to contribute to paradigm shifts in epileptology. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Specific subcortical structures are activated during seizure-induced death in a model of sudden unexpected death in epilepsy (SUDEP): A manganese-enhanced magnetic resonance imaging study

Sudden unexpected death in epilepsy (SUDEP) is a major concern for patients with epilepsy. In most witnessed cases of SUDEP generalized seizures and respiratory failure preceded death, and pre-mortem neuroimaging studies in SUDEP patients observed changes in specific subcortical structures. Our study examined the role of subcortical structures in the DBA/1 mouse model of SUDEP using manganese-enhanced magnetic resonance imaging (MEMRI). These mice exhibit acoustically-evoked generalized seizures leading to seizure-induced respiratory arrest (S-IRA) that results in sudden death unless resuscitation is rapidly instituted. MEMRI data in the DBA/1 mouse brain immediately after acoustically-induced S-IRA were compared to data in C57 (control) mice that were exposed to the same acoustic stimulus that did not trigger seizures. The animals were anesthetized and decapitated immediately after seizure in DBA/1 mice and after an equivalent time in control mice. Comparative T1 weighted MEMRI images were evaluated using a 14T MRI scanner and quantified. We observed significant increases in activity in DBA/1 mice as compared to controls at previously-implicated auditory (superior olivary complex) and sensorimotor-limbic [periaqueductal gray (PAG) and amygdala] networks and also in structures in the respiratory network. The activity at certain raphe nuclei was also increased, suggesting activation of serotonergic mechanisms. These data are consistent with previous findings that enhancing the action of serotonin prevents S-IRA in this SUDEP model. Increased activity in the PAG and the respiratory and raphe nuclei suggest that compensatory mechanisms for apnea may have been activated by S-IRA, but they were not sufficient to prevent death. The present findings indicate that changes induced by S-IRA in specific subcortical structures in DBA/1 mice are consistent with human SUDEP findings. Understanding the changes in brain activity during seizure-induced death in animals may lead to improved approaches directed at prevention of human SUDEP.

Alcohol withdrawal in epileptic rats - Effects on postictal depression, respiration, and death

Patients with epilepsy are at risk of sudden unexpected death in epilepsy (SUDEP). The most common series of events in witnessed cases of SUDEP is a generalized convulsive seizure followed by terminal apnea. Risk factors for SUDEP include prolonged postictal depression (PID), as well as alcohol abuse. The present study examined these issues in a genetic epilepsy model that exhibits generalized convulsive audiogenic seizures (AGSz) but rarely exhibits seizure-induced death, the genetically epilepsy-prone rats (GEPR-9s). We evaluated the effect of ethanol withdrawal (ETX) in GEPR-9s on respiration patterns, duration of PID, and the incidence of seizure-induced death. Audiogenic seizures were induced in GEPR-9s and in normal Sprague-Dawley rats, which were subjected to a 4-day binge ethanol protocol, 18-24h after the last ethanol dose. Following the tonic seizures, all GEPR-9s exhibited PID, characterized by loss of the righting reflex and respiratory distress (RD), which were absent during ETX seizures in the normal rats. During ETX, GEPR-9s exhibited significant increases in the duration of PID and RD, compared with vehicle-treated GEPR-9s. A significant increase in the incidence of death following seizure in GEPR-9s subjected to ETX was observed, compared with that in vehicle-treated GEPR-9s and normal rats subjected to ETX. Death in GEPR-9s was preceded by prolonged seizures because, in part, of the emergence of post-tonic generalized clonus. These results indicate that ETX induced significant increases in the duration of PID and RD, which contributed to the greater incidence of mortality in GEPR-9s compared with that in vehicle-treated GEPR-9s and normal rats. These experiments observed an elevated risk of sudden death associated with alcohol withdrawal in a genetic epilepsy model that had previously been identified as a risk factor in human SUDEP.

Serotonergic agents act on 5-HT3 receptors in the brain to block seizure-induced respiratory arrest in the DBA/1 mouse model of SUDEP

Drugs that enhance the action of serotonin (5-hydroxytrypamine, 5-HT), including several selective serotonin reuptake inhibitors (SSRIs), reduce susceptibility to seizure-induced respiratory arrest (S-IRA) that leads to death in the DBA/1 mouse model of sudden unexpected death in epilepsy (SUDEP). However, it is not clear if specific 5-HT receptors are important in the action of these drugs and whether the brain is the major site of action of these agents in this SUDEP model. The current study examined the actions of agents that affect the 5-HT3 receptor subtype on S-IRA and whether intracerebroventricular (ICV) microinjection of an SSRI would reduce S-IRA susceptibility in DBA/1 mice. The data indicate that systemic administration of SR 57227, a 5-HT3 agonist, was effective in blocking S-IRA in doses that did not block seizures, and the S-IRA blocking effect of the SSRI, fluoxetine, was abolished by coadministration of a 5-HT3 antagonist, ondansetron. Intracerebroventricular administration of fluoxetine in the present study was also able to block S-IRA without blocking seizures. These findings suggest that 5-HT3 receptors play an important role in the block of S-IRA by serotonergic agents, such as SSRIs, which is consistent with the abnormal expression of 5-HT3 receptors in the brainstem of DBA mice observed previously. Taken together, these data indicate that systemically administered serotonergic agents act, at least, in part, in the brain, to reduce S-IRA susceptibility in DBA/1 mice and that 5-HT3 receptors may be important to this effect.

Furthering our understanding of SUDEP: the role of animal models

Sudden and unexpected death in epilepsy (SUDEP) is the most common type of death among patients with epilepsy. Here, we address the importance of the experimental models in search of the mechanisms underlying SUDEP. Most studies have investigated the cardiovascular responses in animal models of epilepsy. However, there are few proposed SUDEP models in literature. Hypoventilation, apnea, respiratory distress, pulmonary hypertension, autonomic dysregulation and arrhythmia are common findings in epilepsy models. Impairments on adenosinergic and serotonergic systems, brainstem spreading depolarization, seizure-activation of neural substrates related to cardiorespiratory control, altered autonomic control, and mutations on sodium and potassium channels are hypothesis suggested. Overall, current research highlights the evident multifactorial nature of SUDEP, which involves acute and chronic aspects ranging from systemic to molecular alterations. Thus, we are convinced that elucidation and prevention of SUDEP can be achieved only through the interaction between basic and clinical science.