Effects of age, sex, and sertraline administration on seizure-induced respiratory arrest in the DBA/1 mouse model of sudden unexpected death in epilepsy (SUDEP)

Sudden Unexpected Death in Epilepsy: Central Respiratory Chemoreception

Sudden unexpected death in epilepsy (SUDEP) is a critical concern for individuals suffering from epilepsy, with respiratory dysfunction playing a significant role in its pathology. Fatal seizures are often characterized by central apnea and hypercapnia (elevated CO2 levels), indicating a failure in ventilatory control. Research has shown that both human epilepsy patients and animal models exhibit a reduced hypercapnic ventilatory response in the interictal (non-seizure) period, suggesting an impaired ability to regulate breathing in response to high CO2 levels. This review examines the role of central chemoreceptors-specifically the retrotrapezoid nucleus, raphe nuclei, nucleus tractus solitarius, locus coeruleus, and hypothalamus in this pathology. These structures are critical for sensing CO2 and maintaining respiratory homeostasis. Emerging evidence also implicates neuropeptidergic pathways within these chemoreceptive regions in SUDEP. Neuropeptides like galanin, pituitary adenylate cyclase-activating peptide (PACAP), orexin, somatostatin, and bombesin-like peptides may modulate chemosensitivity and respiratory function, potentially exacerbating respiratory failure during seizures. Understanding the mechanisms linking central chemoreception, respiratory control, and neuropeptidergic signaling is essential to developing targeted interventions to reduce the risk of SUDEP in epilepsy patients.

Sex-specific differences in mortality and neurocardiac interactions in the Kv1.1 knockout mouse model of sudden unexpected death in epilepsy (SUDEP)

Sudden unexpected death in epilepsy (SUDEP) is a devastating complication of epilepsy with possible sex-specific risk factors, although the exact relationship between sex and SUDEP remains unclear. To investigate this, we studied Kcna1 knockout (Kcna1-/-) mice, which lack voltage-gated Kv1.1 channel subunits and are widely used as a SUDEP model that mirrors key features in humans. To assess sex differences, we first performed survival analysis, EEG-ECG recordings, seizure threshold testing and retrospective analysis of previous intracardiac pacing data. We then applied a novel modelling approach across organs (organomics) to uncover potential sex-specific differences in brain-heart communication. Our findings revealed female Kcna1-/- mice have significantly longer lifespans than males, suggesting lower SUDEP rates. Although no sex differences were found in seizure frequency, duration, burden, susceptibility or interictal heart rate variability, females showed a higher incidence of bradycardia during spontaneous seizures than males, as well as resistance to inducible ventricular tachyarrhythmias in response to programmed electrical stimulation. Two captured SUDEP events, one per sex, displayed similar patterns of ictal bradycardia in both sexes, progressing to postictal cardiorespiratory failure. Going beyond traditional seizure and cardiac metrics, organomics analysis revealed that seizures affect brain-heart communication differently between sexes. Females exhibited more effective resetting of brain-heart interactions postictally than males. This finding may contribute to the lower SUDEP risk in females and underscores the complex interplay between sex, cardiac function and brain-heart communication in determining SUDEP susceptibility. Furthermore, seizure-resetting measures could represent a promising class of biomarkers for SUDEP risk stratification. KEY POINTS: Female Kcna1-/- mice live longer than males, suggesting lower sudden unexpected death in epilepsy (SUDEP) rates. There are no sex differences in seizure metrics or interictal heart rate variability. Females show more bradycardia during seizures and are resistant to inducible ventricular tachyarrhythmias. Seizures affect brain-heart communication differently between the sexes. Seizures in females reset brain-heart interactions more effectively postictally, potentially lowering SUDEP risk.

Diaphragm relaxation causes seizure-related apnoeas in chronic and acute seizure models in rats

Ictal central apnoea is a feature of focal temporal seizures. It is implicated as a risk factor for sudden unexpected death in epilepsy (SUDEP). Here we study seizure-related apnoeas in two different models of experimental seizures, one chronic and one acute, in adult genetically-unmodified rats, to determine mechanisms of seizure-related apnoeas. Under general anaesthesia rats receive sensors for nasal temperature, hippocampal and/or neocortical potentials, and ECG or EMG for subsequent tethered video-telemetry. Tetanus neurotoxin (TeNT), injected into hippocampus during surgery, induces a chronic epileptic focus. Other implanted rats receive intraperitoneal pentylenetetrazol (PTZ) to evoke acute seizures. In chronically epileptic rats, convulsive seizures cause apnoeas (9.9 ± 5.3 s; 331 of 730 convulsive seizures in 15 rats), associated with bradyarrhythmias. Absence of EEG and ECG biomarkers exclude obstructive apnoeas. All eight TeNT-rats with diaphragm EMG have apnoeas with no evidence of obstruction, and have apnoea EMGs significantly closer to expiratory relaxation than inspiratory contraction during pre-apnoeic respiration, which we term "atonic diaphragm". Consistent with atonic diaphragm is that the pre-apnoeic nasal airflow is expiration, as it is in human ictal central apnoea. Two cases of rat sudden death occur. One, with telemetry to the end, reveals a lethal apnoea, the other only has video during the final days, which reveals cessation of breathing shortly after the last clonic epileptic movement. Telemetry following acute systemic PTZ reveals repeated seizures and seizure-related apnoeas, culminating in lethal apnoeas; ictal apnoeas are central - in 8 of 35 cases diaphragms initially contract tonically for 8.5 ± 15.0 s before relaxing, in the 27 remaining cases diaphragms are atonic throughout apnoeas. All terminal apnoeas are atonic. Differences in types of apnoea due to systemic PTZ in rats (mainly atonic) and mice (tonic) are likely species-specific. Certain genetic mouse models have apnoeas caused by tonic contraction, potentially due to expression of epileptogenic mutations throughout the brain, including in respiratory centres, in contrast with acquired focal epilepsies. We conclude that ictal apnoeas in the rat TeNT model result from atonic diaphragms. Relaxed diaphragms could be particularly helpful for therapeutic stimulation of the diaphragm to help restore respiration.

Challenges and future directions of SUDEP models

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death among patients with epilepsy, causing a global public health burden. The underlying mechanisms of SUDEP remain elusive, and effective prevention or treatment strategies require further investigation. A major challenge in current SUDEP research is the lack of an ideal model that maximally mimics the human condition. Animal models are important for revealing the potential pathogenesis of SUDEP and preventing its occurrence; however, they have potential limitations due to species differences that prevent them from precisely replicating the intricate physiological and pathological processes of human disease. This Review provides a comprehensive overview of several available SUDEP animal models, highlighting their pros and cons. More importantly, we further propose the establishment of an ideal model based on brain-computer interfaces and artificial intelligence, hoping to offer new insights into potential advancements in SUDEP research. In doing so, we hope to provide valuable information for SUDEP researchers, offer new insights into the pathogenesis of SUDEP and open new avenues for the development of strategies to prevent SUDEP.

Protocol for modulating the noradrenergic pathway from locus coeruleus to heart to prevent sudden unexpected death in epilepsy in mouse models

The locus coeruleus (LC) and noradrenergic neurotransmission are involved in the regulation of sudden unexpected death in epilepsy (SUDEP). Here, we present a protocol for modulating the noradrenergic pathway from LC to heart to prevent SUDEP in acoustic and pentylenetetrazole-induced DBA/1 mouse models of SUDEP. We describe steps for constructing SUDEP models, calcium signal recording, and electrocardiogram monitoring. We then detail measurement of tyrosine hydroxylase content and activity, β1 and p-β1-AR content, and destruction of LCNE neurons. For complete details on the use and execution of this protocol, please refer to Lian et al.1.

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.

Sudden unexpected death in epilepsy

PURPOSE OF REVIEW

Sudden unexpected death in epilepsy (SUDEP) is a leading cause of death in patients with epilepsy. This review highlights the recent literature regarding epidemiology on a global scale, putative mechanisms and thoughts towards intervention and prevention.

RECENT FINDINGS

Recently, numerous population-based studies have examined the incidence of SUDEP in many countries. Remarkably, incidence is quite consistent across these studies, and is commensurate with the recent estimates of about 1.2 per 1000 patient years. These studies further continue to support that incidence is similar across the ages and that comparable factors portend heightened risk for SUDEP. Fervent research in patients and animal studies continues to hone the understanding of potential mechanisms for SUDEP, especially those regarding seizure-induced respiratory dysregulation. Many of these studies and others have begun to lay out a path towards identification of improved treatment and prevention means. However, continued efforts are needed to educate medical professionals about SUDEP risk and the need to disclose this to patients.

SUMMARY

SUDEP is a devastating potential outcome of epilepsy. More is continually learned about risk and mechanisms from clinical and preclinical studies. This knowledge can hopefully be leveraged into preventive measures in the near future.

Protocol for modulation of the serotonergic DR-PBC neural circuit to prevent SUDEP in the acoustic and PTZ-induced DBA/1 mouse models of SUDEP

The dorsal raphe nucleus (DR) and the pre-Bötzinger complex (PBC) may play an important role in regulating seizure-induced respiratory arrest (S-IRA), the main contributor to sudden unexpected death in epilepsy. Here, we describe pharmacological, optogenetic, and retrograde labeling approaches to specifically modulate the DR to PBC serotonergic pathway. We detail steps for implanting optical fibers and viral infusion into DR and PBC regions and optogenetic techniques for exploring the role of 5-hydroxytryptophan (5-HT) neural circuit of DR-PBC in S-IRA. For complete details on the use and execution of this protocol, please refer to Ma et al. (2022).1.

Gene expression profile for different susceptibilities to sound stimulation: a comparative study on brainstems between two inbred laboratory mouse strains

BACKGROUND

DBA/1 mice have a higher susceptibility to generalized audiogenic seizures (AGSz) and seizure-induced respiratory arrest (S-IRA) than C57/BL6 mice. The gene expression profile might be potentially related to this difference. This study aimed to investigate the susceptibility difference in AGSz and S-IRA between DBA/1 and C57BL/6 mice by profiling long noncoding RNAs (lncRNAs) and mRNA expression.

METHODS

We compared lncRNAs and mRNAs from the brainstem of the two strains with Arraystar Mouse lncRNA Microarray V3.0 (Arraystar, Rockville, MD). Gene Ontology (GO) and pathway analyses were performed to determine the potentially related biological functions and pathways based on differentially expressed mRNAs. qRT-PCR was carried out to validate the results.

RESULTS

A total of 897 lncRNAs and 438 mRNAs were differentially expressed (fold change ≥2, P < 0.05), of which 192 lncRNAs were upregulated and 705 lncRNAs were downregulated. A total of 138 mRNAs were upregulated, and 300 mRNAs were downregulated. In terms of specific mRNAs, Htr5b, Gabra2, Hspa1b and Gfra1 may be related to AGSz or S-IRA. Additionally, lncRNA Neat1 may participate in the difference in susceptibility. GO and pathway analyses suggested that TGF-β signaling, metabolic process and MHC protein complex could be involved in these differences. Coexpression analysis identified 9 differentially expressed antisense lncRNAs and 115 long intergenic noncoding RNAs (lincRNAs), and 2010012P19Rik and its adjacent RNA Tnfsf12-Tnfsf13 may have participated in S-IRA by regulating sympathetic neuron function. The results of the qRT-PCR of five selected lncRNAs (AK038711, Gm11762, 1500004A13Rik, AA388235 and Neat1) and four selected mRNAs (Hspa1b, Htr5b, Gabra2 and Gfra1) were consistent with those obtained by microarray.

CONCLUSION

We concluded that TGF-β signaling and metabolic process may contribute to the differential sensitivity to AGSz and S-IRA. Among mRNAs, Htr5b, Gabra2, Hspa1b and Gfra1 could potentially influence the susceptibility. LncRNA Neat1 and 2010012P19Rik may also contribute to the different response to sound stimulation. Further studies should be carried out to explore the underlying functions and mechanisms of differentially expressed RNAs.

Sudden unexpected death in epilepsy

PURPOSE OF REVIEW

Sudden unexpected death in epilepsy (SUDEP) is a major contributor to premature mortality in people with epilepsy. This review provides an update on recent findings on the epidemiology of SUDEP, clinical risk factors and potential mechanisms.

RECENT FINDINGS

The overall risk rate of SUDEP is approximately 1 per 1000 patients per year in the general epilepsy population and that children and older adults have a similar incidence. Generalized convulsive seizures (GCS), perhaps through their effects on brainstem cardiopulmonary networks, can cause significant postictal respiratory and autonomic dysfunction though other mechanisms likely exist as well. Work in animal models of SUDEP has identified multiple neurotransmitter systems, which may be future targets for pharmacological intervention. There are also chronic functional and structural changes in autonomic function in patients who subsequently die from SUDEP suggesting that some SUDEP risk is dynamic. Modifiable risks for SUDEP include GCS seizure frequency, medication adherence and nighttime supervision.

SUMMARY

Current knowledge of SUDEP risk factors has identified multiple targets for SUDEP prevention today as we await more specific therapeutic targets that are emerging from translational research studies.

A High-Tryptophan Diet Reduces Seizure-Induced Respiratory Arrest and Alters the Gut Microbiota in DBA/1 Mice

Background and Aims: Central 5-hydroxytryptamine (5-HT) defects are responsible for the occurrence of sudden unexpected death in epilepsy (SUDEP). The DBA/1 mouse is an animal model of SUDEP since the mouse exhibits audiogenic seizure-induced respiratory arrest (S-IRA). The synthesis of central 5-HT is closely related to the gut microbiota. Moreover, emerging studies suggest a possible role for the microbiota in mitigating seizure likelihood. Based on this, we aimed to explore the effect of a high-tryptophan diet (HTD) on SUDEP as well as the synthesis and metabolism of central 5-HT. Furthermore, we investigated the involvement of the gut microbiota in this process.

Methods: All DBA/1 mice were subjected to acoustic stimulation to induce seizures. Only those mice that exhibited S-IRA were randomly assigned to the normal diet (ND) group (n = 39) or HTD group (n = 53). After 1 month of dietary intervention, (1) S-IRA rates were evaluated, (2) the concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the plasma and brain were determined by ultra-high-pressure liquid chromatography, and (3) the fecal flora biodiversity and species composition were analyzed by 16S rDNA microbiota profiling.

Results: The S-IRA rate in DBA/1 mice was significantly reduced in the HTD group compared with that in the control group. HTD increased the levels of 5-HT and 5-HIAA in both the telencephalon and midbrain. HTD significantly elevated the species richness and diversity of the gut microbiota. Moreover, there was a significant difference in the gut microbiota composition between the two groups, and the intestinal flora was dominated by Proteobacteria and Actinobacteria after HTD.

Conclusions: HTD is efficient in lowering S-IRA rates and elevating the central 5-HT level in DBA/1 mice. The gut microbiota was altered after HTD intervention. The significant increase in Proteobacteria and Actinobacteria may be related to the SUDEP-protective effect of HTD. Our findings shed light on a candidate choice of dietary prevention for SUDEP.

Genistein, a Natural Isoflavone, Alleviates Seizure-Induced Respiratory Arrest in DBA/1 Mice

Objective: Sudden unexpected death in epilepsy (SUDEP) is a fatal event that ranks second in years of potential life lost among neurological disorders. Seizure-induced respiratory arrest (S-IRA) is the primary instigator leading to death in many SUDEP cases. However, there are currently no effective preventive strategies against S-IRA other than the seizure control. Therefore, it is critical to develop new avenues to prevent SUDEP by investigating the pharmacological interventions of S-IRA. In the present study, we examined the effect of genistein, an isoflavone found in various dietary vegetables, on the incidence of S-IRA in DBA/1 mice.

Methods: DBA/1 mice exhibited generalized seizures and S-IRA when subjected to acoustic stimulation. Genistein was intraperitoneally administered alone or in combination with an adrenoceptor antagonist and a serotonin (5-HT) receptor antagonist, respectively. The effects of drug treatments on S-IRA incidence and seizure behaviors were examined.

Results: The incidence of S-IRA in DBA/1 mice was significantly reduced 2 h after injection of genistein at 1–90 mg/kg as compared with that in the vehicle control. Genistein could block S-IRA without interfering with any component of seizures, especially at relatively lower dosages. The S-IRA-suppressing effect of genistein was reversed by an α2 adrenoceptor antagonist but was not altered by an α1 antagonist. The inhibitory effect of genistein on S-IRA was not affected by a 5-HT₃ or 5-HT_2A receptor antagonist.

Significance: Our data show that genistein reduces S-IRA incidence and can specifically block S-IRA in DBA/1 mice. Its suppressing effect on S-IRA is dependent on activating α2 adrenoceptors. Our study suggests that genistein, a dietary supplement, is potentially useful to prevent SUDEP in at-risk patients.

Serotonin 5-HT4 receptors play a critical role in the action of fenfluramine to block seizure-induced sudden death in a mouse model of SUDEP

RATIONALE

Our previous study showed that the recently approved anticonvulsant drug, fenfluramine, which enhances the release of serotonin (5-hydroxytryptamine, 5-HT) in the brain, prevents seizure-induced respiratory arrest (S-IRA) in the DBA/1 mouse model of sudden unexpected death in epilepsy (SUDEP). The present study examined the role of 5-HT receptor subtypes in mediating the effect of this agent by combined administration of fenfluramine with selective 5-HT receptor antagonists prior to seizure in DBA/1 mice.

METHODS

Fenfluramine (15 mg/kg, i.p.) was administered to primed DBA/1 mice, and audiogenic seizure (Sz) was induced 16 h later. Thirty min prior to Sz induction a selective antagonist acting on 5-HT_1A, 5-HT₂, 5-HT₃ 5-HT₄, 5-HT_5A, 5-HT₆ or 5-HT₇ receptors at a sub-toxic dose was administered, and changes in seizure-induced behaviors were evaluated. Follow-up studies examined the effect of administration of a 5-HT₄ receptor agonist, BIMU 8, as well as the effect of co-administration of ineffective doses of fenfluramine and BIMU-8 on Sz behaviors.

RESULTS

The 5-HT₄ antagonist (GR125487) was the only 5-HT receptor antagonist that was able to reverse the action of fenfluramine to block Sz and S-IRA. Treatment with the 5-HT₄ receptor agonist (BIMU-8), or co-administration of ineffective doses of BIMU-8 and fenfluramine significantly reduced the incidence of S-IRA and tonic Sz in DBA/1 mice. The antagonists for 5-HT₃, 5-HT_5A 5-HT₆, and 5-HT₇ receptors did not significantly affect the action of fenfluramine. However, the 5-HT_1A and the 5-HT₂ antagonists enhanced the anticonvulsant effects of fenfluramine.

CONCLUSIONS

These findings suggest that the action of fenfluramine to prevent seizure-induced sudden death in DBA/1 mice is mediated primarily by activation of 5-HT₄ receptors. These studies are the first to indicate the therapeutic potential of 5-HT₄ receptor agonists either alone or in combination with fenfluramine for preventing SUDEP. Enhancement of the anticonvulsant effect of fenfluramine by 5-HT_1A and 5-HT₂ antagonists may involve presynaptic actions of these antagonists. Thus, the Sz and S-IRA blocking actions of fenfluramine involve complex interactions with several 5-HT receptor subtypes. These data also provide further support for the serotonin hypothesis of SUDEP.

The effect of time-of-day and circadian phase on vulnerability to seizure-induced death in two mouse models

KEY POINTS

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of premature death in patients with refractory epilepsy. SUDEP typically occurs during the night, although the reason for this is unclear. We found that, in normally entrained mice, time-of-day alters vulnerability to seizure-induced death. We found that, in free-running mice, circadian phase alters the vulnerability to seizure-induced death. These findings suggest that circadian rhythmicity may be responsible for the increased night-time prevalence of SUDEP ABSTRACT: Sudden unexpected death in epilepsy (SUDEP) is the leading cause of epilepsy-related death. SUDEP typically occurs during the night following a seizure. Many aspects of mammalian physiology are regulated by circadian rhythms in ways that might make seizures occuring during the night more dangerous. Using two mouse models of seizure-induced death, we demonstrate that time-of-day and circadian rhythms alter vulnerability to seizure-induced death. We exposed normally entrained DBA/1 mice to a potentially seizure-inducing acoustic stimulus at different times of day and compared the characteristics and outcomes of the seizures. Time-of-day did not alter the probability of a seizure but it did alter the probability of seizure-induced death. To determine whether circadian rhythms alter vulnerability to seizure-induced death, we induced maximal electroshock seizures in free-running C57BL/6J mice at different circadian time points at the same time as measuring breathing via whole body plethysmography. Circadian phase did not affect seizure severity but it did alter postictal respiratory outcomes and the probability of seizure-induced death. By contrast to our expectations, in entrained and free-running mice, vulnerability to seizure-induced death was greatest during the night and subjective night, respectively. These findings suggest that circadian rhythmicity may be responsible for the increased night-time prevalence of SUDEP and that the underlying mechanism is phase conserved between nocturnal and diurnal mammals. All of the seizures in the present study were induced during wakefulness, indicating that the effect of time point on vulnerability to seizure-induced death was not the result of sleep. Understanding why SUDEP occurs more frequently during the night may inform future preventative countermeasures.

Central deficiency of norepinephrine synthesis and norepinephrinergic neurotransmission contributes to seizure-induced respiratory arrest

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of mortality in patients with intractable epilepsy. However, the pathogenesis of SUDEP seems to be poorly understood. Our previous findings showed that the incidence of seizure-induced respiratory arrest (S-IRA) was markedly reduced by atomoxetine in a murine SUDEP model. Because the central norepinephrine α-1 receptor (NEα-1R) plays a vital role in regulating respiratory function, we hypothesized that the suppression of S-IRA by atomoxetine was mediated by NE/NEα-1R interactions that can be reversed by NEα-1R antagonism. We examined whether atomoxetine-mediated suppression of S-IRA evoked by either acoustic stimulation or pentylenetetrazole (PTZ) in DBA/1 mice can be reversed by intraperitoneal (IP) and intracerebroventricular (ICV) administration of prazosin, a selective antagonist of NEα-1R. The content and activity of tyrosine hydroxylase (TH), a rate-limiting enzyme for NE synthesis, in the lower brainstem was measured by ELISA. Electroencephalograms (EEG) were obtained from using the PTZ-evoked SUDEP model. In our models, atomoxetine-mediated suppression of S-IRA evoked by either acoustic stimulation or PTZ was significantly reversed by low doses of IP and ICV prazosin. Neither repetitive acoustic stimulation nor S-IRA reduced TH levels in lower brainstem. However, the enzyme activity of TH levels in lower brainstem was significantly increased by mechanical ventilation with DBA/1 mice, which makes the dying DBA/1 mice suffering from S-IRA and SUDEP recover. EEG data showed that although the protective effect of atomoxetine was reversed by prazosin, neither drug suppressed EEG activity. These data suggest that deficient synthesis of NE and norepinephrinergic neurotransmission contributed to S-IRA and that the NEα-1R is a potential therapeutic target for the prevention of SUDEP.

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.

Serotonin and sudden unexpected death in epilepsy

Epilepsy is a highly prevalent disease characterized by recurrent, spontaneous seizures. Approximately one-third of epilepsy patients will not achieve seizure freedom with medical management and become refractory to conventional treatments. These patients are at greatest risk for sudden unexpected death in epilepsy (SUDEP). The exact etiology of SUDEP is unknown, but a combination of respiratory, cardiac, neuronal electrographic dysfunction, and arousal impairment is thought to underlie SUDEP. Serotonin (5-HT) is involved in regulation of breathing, sleep/wake states, arousal, and seizure modulation and has been implicated in the pathophysiology of SUDEP. This review explores the current state of understanding of the relationship between 5-HT, epilepsy, and respiratory and autonomic control processes relevant to SUDEP in epilepsy patients and in animal models.

Audiogenic seizure as a model of sudden death in epilepsy: A comparative study between four inbred mouse strains from early life to adulthood

OBJECTIVE

Mouse models of sudden unexpected death in epileptic patients (SUDEP) using audiogenic seizures (AGS) are valuable because death can occur following a sound-induced seizure in the absence of any pharmacologic or electric component. However, only a few strains of mice are AGS prone, and the vast majority of studies involve DBA/2 or DBA/1 inbred strains. With the goal of characterizing the variation of AGS susceptibility with age, and of offering a larger panel of mice available for AGS studies, we performed a comparative study of the variability in AGS responses.

METHODS

The variation of AGS with age was determined in two classically used inbred strains of mice, DBA/2 and DBA/1, and two additional strains, BALB/c and 129/SvTer. As AGS-stimulated tonic seizures can be lethal or nonlethal, even in the same inbred strain, in a second experiment, we addressed whether there is an innate capacity to reproduce the same response after a tonic AGS, referred to as "determinism," in the DBA/2J, DBA/1J, and 129/SvTer mouse strains.

RESULTS

Results show that the 129/SvTer mouse is a more versatile model of SUDEP due to its wider age range of susceptibility compared to the DBA/2J and DBA/1J mouse strains. In addition, we show that determinism is not consistently evident in DBA/2J and 129/SvTer strains after AGS. Hence, one cannot be certain that a lethal AGS will always be lethal in successive testing after resuscitation and vice versa in these two mouse strains.

SIGNIFICANCE

These studies highlight the phenotypic variability of AGS in different mouse strains, show the value of an additional mouse strain, 129/SvTer, for studies using AGS, and thus provide valuable information for future studies of AGS and SUDEP.

Effect of monoamine reuptake inhibition and α1 blockade on respiratory arrest and death following electroshock-induced seizures in mice

OBJECTIVE

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in patients with refractory epilepsy. Although the mechanisms for SUDEP are incompletely understood, seizure-induced respiratory arrest (S-IRA) has been strongly and consistently implicated. A body of evidence indicates that serotonin (5-HT), a modulator of breathing, plays a critical role in SUDEP. Because the 5-HT and norepinephrine (NE) systems interact in many biologic processes and NE is known to modulate breathing and seizures, we hypothesized that NE may play a role in S-IRA and SUDEP.

METHODS

We examined the effects of pharmacologic manipulation of 5-HT and NE on S-IRA and death following maximal electroshock (MES)-induced seizures in adult wild-type (WT) mice, genetically 5-HT neuron-deficient (Lmx1bf/f/p ) mice, and chemically NE neuron-deficient mice. Mice were treated with pharmacologic agents targeting the serotonergic and noradrenergic systems and subjected to seizure induction via MES while breathing was measured via whole-body plethysmography.

RESULTS

S-IRA and death was reduced in WT mice with NE reuptake inhibitors (NRIs), reboxetine and atomoxetine, selective serotonin reuptake inhibitors (SSRIs), fluoxetine and citalopram, and the dual 5-HT/NE reuptake inhibitor (SNRI), duloxetine. S-IRA and death was also reduced in Lmx1bf/f/p mice with reboxetine and fluoxetine. The protective effects of the reuptake inhibitors were prevented by the α1 antagonist, prazosin. Citalopram did not reduce S-IRA and death in NE neuron-deficient mice.

SIGNIFICANCE

These data suggest that 5-HT and NE critically interact in the modulation of breathing following a seizure and potentially inform preventive strategies for SUDEP.

Fenfluramine, a serotonin-releasing drug, prevents seizure-induced respiratory arrest and is anticonvulsant in the DBA/1 mouse model of SUDEP

OBJECTIVE

Prevention of sudden unexpected death in epilepsy (SUDEP) is a critical goal for epilepsy therapy. The DBA/1 mouse model of SUDEP exhibits an elevated susceptibility to seizure-induced death in response to electroconvulsive shock, hyperthermia, convulsant drug, and acoustic stimulation. The serotonin hypothesis of SUDEP is based on findings that treatments which modify serotonergic function significantly alter susceptibility to seizure-induced sudden death in several epilepsy models, including DBA/1 mice. Serotonergic abnormalities have also recently been observed in human SUDEP. Fenfluramine is a drug that enhances serotonin release in the brain. Recent studies have found that the addition of fenfluramine improved seizure control in patients with Dravet syndrome, which has a high incidence of SUDEP. Therefore, we investigated the effects of fenfluramine on seizures and seizure-induced respiratory arrest (S-IRA) in DBA/1 mice.

METHODS

The dose and time course of the effects of fenfluramine (i.p.) on audiogenic seizures (Sz) induced by an electric bell in DBA/1 mice were determined. Videos of Sz-induced behaviors were recorded for analysis. Statistical significance (P < 0.05) was evaluated using the chi-square test.

RESULTS

Sixteen hours after administration of 15 mg/kg of fenfluramine, a high incidence of selective block of S-IRA susceptibility (P < 0.001) occurred in DBA/1 mice without blocking any convulsive behavior. Thirty minutes after 20-40 mg/kg of fenfluramine, significant reductions of seizure incidence and severity, as well as S-IRA susceptibility occurred, which were long-lasting (≥48 hours). The median effective dose (ED₅₀ ) of fenfluramine for significantly reducing Sz at 30 minutes was 21 mg/kg.

SIGNIFICANCE

This study presents the first evidence for the effectiveness of fenfluramine in reducing seizure incidence, severity, and S-IRA susceptibility in a mammalian SUDEP model. The ability of fenfluramine to block S-IRA selectively suggests the potential usefulness of fenfluramine in prophylaxis of SUDEP. These results further confirm and extend the serotonin hypothesis of SUDEP.

Dead in the Night: Sleep-Wake and Time-Of-Day Influences on Sudden Unexpected Death in Epilepsy

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of epilepsy-related death in patients with refractory epilepsy. Convergent lines of evidence suggest that SUDEP occurs due to seizure induced perturbation of respiratory, cardiac, and electrocerebral function as well as potential predisposing factors. It is consistently observed that SUDEP happens more during the night and the early hours of the morning. The aim of this review is to discuss evidence from patient cases, clinical studies, and animal research which is pertinent to the nocturnality of SUDEP. There are a number of factors which might contribute to the nighttime predilection of SUDEP. These factors fall into four categories: influences of (1) being unwitnessed, (2) lying prone in bed, (3) sleep-wake state, and (4) circadian rhythms. During the night, seizures are more likely to be unwitnessed; therefore, it is less likely that another person would be able to administer a lifesaving intervention. Patients are more likely to be prone on a bed following a nocturnal seizure. Being prone in the accouterments of a bed during the postictal period might impair breathing and increase SUDEP risk. Sleep typically happens at night and seizures which emerge from sleep might be more dangerous. Lastly, there are circadian changes to physiology during the night which might facilitate SUDEP. These possible explanations for the nocturnality of SUDEP are not mutually exclusive. The increased rate of SUDEP during the night is likely multifactorial involving both situational factors, such as being without a witness and prone, and physiological changes due to the influence of sleep and circadian rhythms. Understanding the causal elements in the nocturnality of SUDEP may be critical to the development of effective preventive countermeasures.

Optogenetic activation of 5-HT neurons in the dorsal raphe suppresses seizure-induced respiratory arrest and produces anticonvulsant effect in the DBA/1 mouse SUDEP model

Sudden unexpected death in epilepsy (SUDEP) is a devastating epilepsy complication. Seizure-induced respiratory arrest (S-IRA) occurs in many witnessed SUDEP patients and animal models as an initiating event leading to death. Thus, understanding the mechanisms underlying S-IRA will advance the development of preventive strategies against SUDEP. Serotonin (5-HT) is an important modulator for many vital functions, including respiration and arousal, and a deficiency of 5-HT signaling is strongly implicated in S-IRA in animal models, including the DBA/1 mouse. However, the brain structures that contribute to S-IRA remain elusive. We hypothesized that the dorsal raphe (DR), which sends 5-HT projections to the forebrain, is implicated in S-IRA. The present study used optogenetics in the DBA/1 mouse model of SUDEP to selectively activate 5-HT neurons in the DR. Photostimulation of DR 5-HT neurons significantly and reversibly reduced the incidence of S-IRA evoked by acoustic stimulation. Activation of 5-HT neurons in the DR suppressed tonic seizures in most DBA/1 mice without altering the seizure latency and duration of wild running and clonic seizures evoked by acoustic stimulation. This suppressant effect of photostimulation on S-IRA is independent of seizure models, as optogenetic stimulation of DR also reduced S-IRA induced by pentylenetetrazole, a proconvulsant widely used to model human generalized seizures. The S-IRA-suppressing effect of photostimulation was increased by 5-hydroxytryptophan, a chemical precursor for 5-HT synthesis, and was reversed by ondansetron, a specific 5-HT3 receptor antagonist, indicating that reduction of S-IRA by photostimulation of the DR is specifically mediated by enhanced 5-HT neurotransmission. Our findings suggest that deficits in 5-HT neurotransmission in the DR are implicated in S-IRA in DBA/1 mice, and that targeted intervention in the DR is potentially useful for prevention of SUDEP.

Atomoxetine, a norepinephrine reuptake inhibitor, reduces seizure-induced respiratory arrest

Sudden unexpected death in epilepsy (SUDEP) is a devastating epilepsy complication, and no effective preventive strategies are currently available for this fatal disorder. Clinical and animal studies of SUDEP demonstrate that seizure-induced respiratory arrest (S-IRA) is the primary event leading to death after generalized seizures in many cases. Enhancing brain levels of serotonin reduces S-IRA in animal models relevant to SUDEP, including the DBA/1 mouse. Given that serotonin in the brain plays an important role in modulating respiration and arousal, these findings suggest that deficits in respiration and/or arousal may contribute to S-IRA. It is well known that norepinephrine is an important neurotransmitter that modulates respiration and arousal in the brain as well. Therefore, we hypothesized that enhancing noradrenergic neurotransmission suppresses S-IRA. To test this hypothesis, we examined the effect of atomoxetine, a norepinephrine reuptake inhibitor (NRI), on S-IRA evoked by either acoustic stimulation or pentylenetetrazole in DBA/1 mice. We report the original observation that atomoxetine specifically suppresses S-IRA without altering the susceptibility to seizures evoked by acoustic stimulation, and atomoxetine also reduces S-IRA evoked by pentylenetetrazole in DBA/1 mice. Our data suggest that the noradrenergic signaling is importantly involved in S-IRA, and that atomoxetine, a medication widely used to treat attention deficit hyperactivity disorder (ADHD), is potentially useful to prevent SUDEP.

Abnormalities of serotonergic neurotransmission in animal models of SUDEP

Sudden unexpected death in epilepsy (SUDEP) is a devastating event, and both DBA/1 and DBA/2 mice have been shown to be relevant animal models for studying SUDEP. DBA mice exhibit seizure-induced respiratory arrest (S-IRA), leading to cardiac arrest and subsequent sudden death after generalized audiogenic seizures (AGSs). This sequence of terminal events is also observed in the majority of witnessed human SUDEP cases. Several pathophysiological mechanisms, including respiratory/cardiac dysfunction, have been proposed to contribute to human SUDEP. Several (but not all) selective serotonin (5-HT) reuptake inhibitors (SSRIs), including fluoxetine, can reversibly block S-IRA, and abnormal expression of 5-HT receptors is found in the brainstem of DBA mice. DBA mice, which do not initially show S-IRA, exhibit S-IRA after treatment with a nonselective 5-HT antagonist. These studies suggest that abnormalities of 5-HT neurotransmission are involved in the pathogenesis of S-IRA in DBA mice. Serotonergic (5-HT) transmission plays an important role in normal respiration, and DBA mice exhibiting S-IRA can be resuscitated using a rodent ventilator. It is important and interesting to know if fluoxetine blocks S-IRA in DBA mice by enhancing respiratory ventilation. To test this, the effects of breathing stimulants, doxapram, and 5,6,7,8-tetrahydropyrido[4,3-d]pyrimidine (PK-THPP) were compared with the effects of fluoxetine on S-IRA in DBA/1 mice. Although fluoxetine reduces the incidence of S-IRA in DBA/1 mice, as reported previously, the same dose of fluoxetine fails to enhance baseline respiratory ventilation in the absence of AGSs. Doxapram and PK-THPP augment the baseline ventilation in DBA/1 mice. However, these breathing stimulants are ineffective in preventing S-IRA in DBA/1 mice. These data suggest that fluoxetine blocks S-IRA in DBA/1 mice by cellular/molecular mechanisms other than enhancement of basal ventilation. Future research directions are also discussed. 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.

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.

5-Hydroxytryptophan, a precursor for serotonin synthesis, reduces seizure-induced respiratory arrest

OBJECTIVE

The DBA/1 mouse is a relevant animal model of sudden unexpected death in epilepsy (SUDEP), as it exhibits seizure-induced respiratory arrest (S-IRA) evoked by acoustic stimulation, followed by cardiac arrhythmia and death. Defects in serotonergic neurotransmission may contribute to S-IRA. The tryptophan hydroxylase-2 (TPH2) enzyme converts L-tryptophan to 5-hydroxytryptophan (5-HTP), a precursor for central nervous system (CNS) serotonin (5-HT) synthesis; and DBA/1 mice have a polymorphism that decreases TPH2 activity. We, therefore, hypothesized that supplementation with 5-HTP may bypass TPH2 and suppress S-IRA in DBA/1 mice.

METHODS

TPH2 expression was examined by Western blot in the brainstem of DBA/1 and C57BL/6J mice both with and without acoustic stimulation. Changes in breathing and cardiac electrical activity in DBA/1 and C57BL/6J mice that incurred sudden death during generalized seizures evoked by pentylenetetrazole (PTZ) were studied by plethysmography and electrocardiography. The effect of 5-HTP administration on seizure-induced mortality evoked by acoustic stimulation or by PTZ was investigated in DBA/1 mice.

RESULTS

Repetitive acoustic stimulation resulted in reduced TPH2 protein in the brainstem of DBA/1 mice as compared with C57BL/6J mice. S-IRA evoked by acoustic stimulation in DBA/1 mice was significantly reduced by 5-HTP. Following S-IRA, cardiac electrical activity could be detected for minutes before terminal asystole and death in both DBA/1 and C57BL/6J mice after PTZ treatment. The incidence of S-IRA by PTZ administration was greater in DBA/1 than in C57BL/6J mice, and administration of 5-HTP also significantly reduced S-IRA by PTZ in DBA/1 mice.

SIGNIFICANCE

Our data suggest that S-IRA is the primary event leading to death incurred in most DBA/1 and some C57BL/6J mice during PTZ-evoked seizures. Suppression of S-IRA by 5-HTP suggests that 5-HT transmission contributes to the pathophysiology of S-IRA, and that 5-HTP, an over-the-counter supplement available for human consumption, may be clinically useful in preventing SUDEP.

Sudden unexpected death in epilepsy genetics: Molecular diagnostics and prevention

Epidemiologic studies clearly document the public health burden of sudden unexpected death in epilepsy (SUDEP). Clinical and experimental studies have uncovered dynamic cardiorespiratory dysfunction, both interictally and at the time of sudden death due to epilepsy. Genetic analyses in humans and in model systems have facilitated our current molecular understanding of SUDEP. Many discoveries have been informed by progress in the field of sudden cardiac death and sudden infant death syndrome. It is becoming apparent that SUDEP genomic complexity parallels that of sudden cardiac death, and that there is a pauci1ty of analytically useful postmortem material. Because many challenges remain, future progress in SUDEP research, molecular diagnostics, and prevention rests in international, collaborative, and transdisciplinary dialogue in human and experimental translational research of sudden death.

Cerebrospinal Fluid Levels of Monoamine Metabolites in the Epileptic Baboon

The baboon represents a natural model for genetic generalized epilepsy and sudden unexpected death in epilepsy (SUDEP). In this retrospective study, cerebrospinal fluid (CSF) monoamine metabolites and scalp electroencephalography (EEG) were evaluated in 263 baboons of a pedigreed colony. CSF monoamine abnormalities have been linked to reduced seizure thresholds, behavioral abnormalities and SUDEP in various animal models of epilepsy. The levels of 3-hydroxy-4-methoxyphenylglycol, 5-hydroxyindolacetic acid and homovanillic acid in CSF samples drawn from the cisterna magna were analyzed using high-performance liquid chromatography. These levels were compared between baboons with seizures (SZ), craniofacial trauma (CFT) and asymptomatic, control (CTL) baboons, between baboons with abnormal and normal EEG studies. We hypothesized that the CSF levels of major monoaminergic metabolites (i.e., dopamine, serotonin and norepinephrine) associate with the baboons' electroclinical status and thus can be used as clinical biomarkers applicable to seizures/epilepsy. However, despite apparent differences in metabolite levels between the groups, usually lower in SZ and CFT baboons and in baboons with abnormal EEG studies, we did not find any statistically significant differences using a logistic regression analysis. Significant correlations between the metabolite levels, especially between 5-HIAA and HVA, were preserved in all electroclinical groups. While we were not able to demonstrate significant differences in monoamine metabolites in relation to seizures or EEG markers of epilepsy, we cannot exclude the monoaminergic system as a potential source of pathogenesis in epilepsy and SUDEP. A prospective study evaluating serial CSF monoamine levels in baboons with recently witnessed seizures, and evaluation of abnormal expression and function of monoaminergic receptors and transporters within epilepsy-related brain regions, may impact the electroclinical status.

Genome-wide Polygenic Burden of Rare Deleterious Variants in Sudden Unexpected Death in Epilepsy

Sudden unexpected death in epilepsy (SUDEP) represents the most severe degree of the spectrum of epilepsy severity and is the commonest cause of epilepsy-related premature mortality. The precise pathophysiology and the genetic architecture of SUDEP remain elusive. Aiming to elucidate the genetic basis of SUDEP, we analysed rare, protein-changing variants from whole-exome sequences of 18 people who died of SUDEP, 87 living people with epilepsy and 1479 non-epilepsy disease controls. Association analysis revealed a significantly increased genome-wide polygenic burden per individual in the SUDEP cohort when compared to epilepsy (P = 5.7 × 10(- 3)) and non-epilepsy disease controls (P = 1.2 × 10(- 3)). The polygenic burden was driven both by the number of variants per individual, and over-representation of variants likely to be deleterious in the SUDEP cohort. As determined by this study, more than a thousand genes contribute to the observed polygenic burden within the framework of this study. Subsequent gene-based association analysis revealed five possible candidate genes significantly associated with SUDEP or epilepsy, but no one single gene emerges as common to the SUDEP cases. Our findings provide further evidence for a genetic susceptibility to SUDEP, and suggest an extensive polygenic contribution to SUDEP causation. Thus, an overall increased burden of deleterious variants in a highly polygenic background might be important in rendering a given individual more susceptible to SUDEP. Our findings suggest that exome sequencing in people with epilepsy might eventually contribute to generating SUDEP risk estimates, promoting stratified medicine in epilepsy, with the eventual aim of reducing an individual patient's risk of SUDEP.

Mechanisms of sudden unexplained death in epilepsy

PURPOSE OF REVIEW

Human and experimental research has identified cardioautonomic and respiratory dysfunction as a frequent accompaniment in human and animal model events of sudden unexpected death in epilepsy (SUDEP). This review aims to provide an overview of the scientific evidence behind the currently accepted risk factors and working hypotheses regarding SUDEP pathophysiology.

RECENT FINDINGS

Epidemiological analysis of public health burden of SUDEP has shown that it rates second only to stroke in the years of potential life lost. Clinical and experimental studies uncovered the dynamic cardiorespiratory dysfunction interictally and imminently to SUDEP, and model systems have facilitated discoveries in SUDEP mechanistic understanding and application of pilot therapeutic interventions. Pilot molecular profiling of human SUDEP has uncovered complex genomic structure in the candidate gene network.

SUMMARY

Extensive clinical and experimental work has established a rationale for the conceptual thinking about SUDEP mechanisms. The application of the global molecular profiling will be invaluable in unraveling the individually unique genomic complexities and interactions that underlie the physiological signature of each patient. At the same time, sophisticated model systems will be critical in the iterative translation of human genetics, physiology, pharmacological interventions, and in testing preventive interventions.

Fluoxetine prevents respiratory arrest without enhancing ventilation in DBA/1 mice

Sudden unexpected death in epilepsy (SUDEP) is a fatal epileptic event. DBA/1 mice are a relevant animal model for the study of SUDEP, as these mice exhibit seizure-induced respiratory arrest (S-IRA) leading to death, which has been observed in patients with witnessed SUDEP. Fluoxetine, a selective serotonin (5-hydroxytryptamine or 5-HT) reuptake inhibitor (SSRI), reduces S-IRA in DBA/1 mice. Given that DBA/1 mice with S-IRA can be resuscitated using a ventilator, we hypothesized that breathing stimulants can prevent S-IRA and that fluoxetine prevents S-IRA by enhancing ventilation in these mice. Spontaneous respiratory function in anesthetized or awake DBA/1 mice was examined using noninvasive plethysmography before and after administering fluoxetine or breathing stimulants, doxapram, and 5,6,7,8-tetrahydropyrido[4,3-d]pyrimidine (PK-THPP). The effects of these drugs on S-IRA in DBA/1 mice were tested. As reported previously, systemic administration of fluoxetine reduced S-IRA in awake DBA/1 mice, but fluoxetine in anesthetized and awake DBA/1 mice did not increase basal ventilation or the ventilatory response to 7% CO2. Both doxapram and PK-THPP increased ventilation in room air and in air+7% CO2 in anesthetized DBA/1 mice. However, neither of the breathing stimulants reduced the incidence of S-IRA. Our studies confirm that fluoxetine reduces S-IRA in DBA/1 mice without enhancing basal ventilation in the absence of seizures. Although breathing stimulants increased ventilation in the absence of seizures, they were ineffective in reducing S-IRA, indicating that drug-induced increases in ventilation are insufficient to compensate for S-IRA in DBA/1 mice.

Serotonin neurones have anti-convulsant effects and reduce seizure-induced mortality

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in patients with refractory epilepsy. Defects in central control of breathing are important contributors to the pathophysiology of SUDEP, and serotonin (5-HT) system dysfunction may be involved. Here we examined the effect of 5-HT neurone elimination or 5-HT reduction on seizure risk and seizure-induced mortality. Adult Lmx1b(f/f/p) mice, which lack >99% of 5-HT neurones in the CNS, and littermate controls (Lmx1b(f/f)) were subjected to acute seizure induction by maximal electroshock (MES) or pilocarpine, variably including electroencephalography, electrocardiography, plethysmography, mechanical ventilation or pharmacological therapy. Lmx1b(f/f/p) mice had a lower seizure threshold and increased seizure-induced mortality. Breathing ceased during most seizures without recovery, whereas cardiac activity persisted for up to 9 min before terminal arrest. The mortality rate of mice of both genotypes was reduced by mechanical ventilation during the seizure or 5-HT2A receptor agonist pretreatment. The selective serotonin reuptake inhibitor citalopram reduced mortality of Lmx1b(f/f) but not of Lmx1b(f/f/p) mice. In C57BL/6N mice, reduction of 5-HT synthesis with para-chlorophenylalanine increased MES-induced seizure severity but not mortality. We conclude that 5-HT neurones raise seizure threshold and decrease seizure-related mortality. Death ensued from respiratory failure, followed by terminal asystole. Given that SUDEP often occurs in association with generalised seizures, some mechanisms causing death in our model might be shared with those leading to SUDEP. This model may help determine the relationship between seizures, 5-HT system dysfunction, breathing and death, which may lead to novel ways to prevent SUDEP.

The consequences of refractory epilepsy and its treatment

Seizures in some 30% to 40% of patients with epilepsy fail to respond to antiepileptic drugs or other treatments. While much has been made of the risks of new drug therapies, not enough attention has been given to the risks of uncontrolled and progressive epilepsy. This critical review summarizes known risks associated with refractory epilepsy, provides practical clinical recommendations, and indicates areas for future research. Eight international epilepsy experts from Europe, the United States, and South America met on May 4, 2013, to present, review, and discuss relevant concepts, data, and literature on the consequences of refractory epilepsy. While patients with refractory epilepsy represent the minority of the population with epilepsy, they require the overwhelming majority of time, effort, and focus from treating physicians. They also represent the greatest economic and psychosocial burdens. Diagnostic procedures and medical/surgical treatments are not without risks. Overlooked, however, is that these risks are usually smaller than the risks of long-term, uncontrolled seizures. Refractory epilepsy may be progressive, carrying risks of structural damage to the brain and nervous system, comorbidities (osteoporosis, fractures), and increased mortality (from suicide, accidents, sudden unexpected death in epilepsy, pneumonia, vascular disease), as well as psychological (depression, anxiety), educational, social (stigma, driving), and vocational consequences. Adding to this burden is neuropsychiatric impairment caused by underlying epileptogenic processes ("essential comorbidities"), which appears to be independent of the effects of ongoing seizures themselves. Tolerating persistent seizures or chronic medicinal adverse effects has risks and consequences that often outweigh risks of seemingly "more aggressive" treatments. Future research should focus not only on controlling seizures but also on preventing these consequences.

Serotonin and sudden death: differential effects of serotonergic drugs on seizure-induced respiratory arrest in DBA/1 mice

In the DBA/1 mouse model of sudden unexpected death in epilepsy (SUDEP), administration of a selective serotonin (5-HT) reuptake inhibitor (SSRI), fluvoxamine, completely suppressed seizure-induced respiratory arrest (S-IRA) at 30 min after administration (i.p.) in a dose-related manner without blocking audiogenic seizures (AGSz), but another SSRI, paroxetine, reduced S-IRA but with a delayed (24 h) onset and significant toxicity. A serotonin-norepinephrine reuptake inhibitor, venlafaxine, reduced S-IRA incidence, but higher doses were ineffective. A selective 5-HT7 agonist, AS-19, was totally ineffective in reducing S-IRA. In developing DBA/1 mice that had not previously experienced AGSz, administration of a nonselective 5-HT antagonist, cyproheptadine, induced a significantly greater incidence of S-IRA than that of saline. This study confirms that certain drugs that enhance the activation of 5-HT receptors are able to prevent S-IRA, but not all serotonergic drugs are equally effective, which may be relevant to the potential use of these drugs for SUDEP prevention. Serotonergic antagonists may be problematic in patients with epilepsy.