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.

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.

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.

Dorsal Raphe Serotonin Neurons Mediate CO2-Induced Arousal from Sleep.

Arousal from sleep in response to CO2 is a critical protective phenomenon. Dysregulation of CO2-induced arousal contributes to morbidity and mortality from prevalent diseases, such as obstructive sleep apnea and sudden infant death syndrome. Despite the critical nature of this protective reflex, the precise mechanism for CO2-induced arousal is unknown. Because CO2 is a major regulator of breathing, prevailing theories suggest that activation of respiratory chemo- and mechano-sensors is required for CO2-induced arousal. However, populations of neurons that are not involved in the regulation of breathing are also chemosensitive. Among these are serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) that comprise a component of the ascending arousal system. We hypothesized that direct stimulation of these neurons with CO2 could cause arousal from sleep independently of enhancing breathing. Dialysis of CO2-rich acidified solution into DRN, but not medullary raphe responsible for modulating breathing, caused arousal from sleep. Arousal was lost in mice with a genetic absence of 5-HT neurons, and with acute pharmacological or optogenetic inactivation of DRN 5-HT neurons. Here we demonstrate that CO2 can cause arousal from sleep directly, without requiring enhancement of breathing, and that chemosensitive 5-HT neurons in the DRN critically mediate this arousal. Better understanding mechanisms underlying this protective reflex may lead to interventions to reduce disease-associated morbidity and mortality.SIGNIFICANCE STATEMENT Although CO2-induced arousal is critical to a number of diseases, the specific mechanism is not well understood. We previously demonstrated that serotonin (5-HT) neurons are important for CO2-induced arousal, as mice without 5-HT neurons do not arouse to CO2 Many have interpreted this to mean that medullary 5-HT neurons that regulate breathing are important in this arousal mechanism. Here we found that direct application of CO2-rich aCSF to the dorsal raphe nucleus, but not the medullary raphe, causes arousal from sleep, and that this arousal was lost with genetic ablation or acute inhibition of 5-HT neurons. We propose that 5-HT neurons in the dorsal raphe nucleus can be activated directly by CO2 to cause arousal independently of respiratory activation.

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.

Impaired Serotonergic Brainstem Function during and after Seizures.

Impaired breathing, cardiac function, and arousal during and after seizures are important causes of morbidity and mortality. Previous work suggests that these changes are associated with depressed brainstem function in the ictal and post-ictal periods. Lower brainstem serotonergic systems are postulated to play an important role in cardiorespiratory changes during and after seizures, whereas upper brainstem serotonergic and other systems regulate arousal. However, direct demonstration of seizure-associated neuronal activity changes in brainstem serotonergic regions has been lacking. Here, we performed multiunit and single-unit recordings from medullary raphe and midbrain dorsal raphe nuclei in an established rat seizure model while measuring changes in breathing rate and depth as well as heart rate. Serotonergic neurons were identified by immunohistochemistry. Respiratory rate, tidal volume, and minute ventilation were all significantly decreased during and after seizures in this model. We found that population firing of neurons in the medullary and midbrain raphe on multiunit recordings was significantly decreased during the ictal and post-ictal periods. Single-unit recordings from identified serotonergic neurons in the medullary raphe revealed highly consistently decreased firing during and after seizures. In contrast, firing of midbrain raphe serotonergic neurons was more variable, with a mixture of increases and decreases. The markedly suppressed firing of medullary serotonergic neurons supports their possible role in simultaneously impaired cardiorespiratory function in seizures. Decreased arousal likely arises from depressed population activity of several neuronal pools in the upper brainstem and forebrain. These findings have important implications for preventing morbidity and mortality in people living with epilepsy. SIGNIFICANCE STATEMENT: Seizures often cause impaired breathing, cardiac dysfunction, and loss of consciousness. The brainstem and, specifically, brainstem serotonin neurons are thought to play an important role in controlling breathing, cardiac function, and arousal. We used an established rat seizure model to study the overall neuronal activity in the brainstem as well as firing of specific serotonin neurons while measuring cardiorespiratory function. Our results demonstrated overall decreases in brainstem neuronal activity and marked downregulation of lower brainstem serotonin neuronal firing in association with decreased breathing and heart rate during and after seizures. These findings point the way toward new treatments to augment brainstem function and serotonin, aiming to prevent seizure complications and reduce morbidity and mortality in people living with epilepsy.

Time-of-day influences on respiratory sequelae following maximal electroshock-induced seizures in mice.

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in refractory epilepsy patients. Although specific mechanisms underlying SUDEP are not well understood, evidence suggests most SUDEP occurs due to seizure-induced respiratory arrest. SUDEP also tends to happen at night. Although this may be due to circumstances in which humans find themselves at night, such as being alone without supervision or sleeping prone, or to independent influences of sleep state, there are a number of reasons why the night (i.e., circadian influences) could be an independent risk factor for SUDEP. We explored this possibility. Adult male WT mice were instrumented for EEG, EMG, and EKG recording and subjected to maximal electroshock (MES) seizures during wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep during the nighttime/dark phase. These data were compared with data collected following seizures induced during the daytime/light phase. Seizures induced during the nighttime were similar in severity and duration to those induced during the daytime; however, seizures induced during the nighttime were associated with a lesser degree of respiratory dysregulation and postictal EEG suppression. Seizures induced during REM sleep during the nighttime were universally fatal, as is seen when seizures are induced during REM during the daytime. Taken together, these data implicate a role for time of day in influencing the physiological consequences of seizures that may contribute to seizure-induced death.NEW & NOTEWORTHY Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in patients with refractory epilepsy. SUDEP frequently occurs during the night, which has been attributed to an effect of sleep. We have shown that sleep state does indeed influence survival following a seizure. That SUDEP occurs during the night could also implicate a circadian influence. In this study we found that time of day independently affects the physiological consequences of seizures.

Influence of vigilance state on physiological consequences of seizures and seizure-induced death in mice.

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in patients with refractory epilepsy. SUDEP occurs more commonly during nighttime sleep. The details of why SUDEP occurs at night are not well understood. Understanding why SUDEP occurs at night during sleep might help to better understand why SUDEP occurs at all and hasten development of preventive strategies. Here we aimed to understand circumstances causing seizures that occur during sleep to result in death. Groups of 12 adult male mice were instrumented for EEG, EMG, and EKG recording and subjected to seizure induction via maximal electroshock (MES) during wakefulness, nonrapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep. Seizure inductions were performed with concomitant EEG, EMG, and EKG recording and breathing assessment via whole body plethysmography. Seizures induced via MES during sleep were associated with more profound respiratory suppression and were more likely to result in death. Despite REM sleep being a time when seizures do not typically occur spontaneously, when seizures were forced to occur during REM sleep, they were invariably fatal in this model. An examination of baseline breathing revealed that mice that died following a seizure had increased baseline respiratory rate variability compared with those that did not die. These data demonstrate that sleep, especially REM sleep, can be a dangerous time for a seizure to occur. These data also demonstrate that there may be baseline respiratory abnormalities that can predict which individuals have higher risk for seizure-induced death.

5-HT2A receptor activation is necessary for CO2-induced arousal.

Hypercapnia-induced arousal from sleep is an important protective mechanism pertinent to a number of diseases. Most notably among these are the sudden infant death syndrome, obstructive sleep apnea and sudden unexpected death in epilepsy. Serotonin (5-HT) plays a significant role in hypercapnia-induced arousal. The mechanism of 5-HT's role in this protective response is unknown. Here we sought to identify the specific 5-HT receptor subtype(s) involved in this response. Wild-type mice were pretreated with antagonists against 5-HT receptor subtypes, as well as antagonists against adrenergic, cholinergic, histaminergic, dopaminergic, and orexinergic receptors before challenge with inspired CO2 or hypoxia. Antagonists of 5-HT(2A) receptors dose-dependently blocked CO2-induced arousal. The 5-HT(2C) receptor antagonist, RS-102221, and the 5-HT1A receptor agonist, 8-OH-DPAT, attenuated but did not completely block CO2-induced arousal. Blockade of non-5-HT receptors did not affect CO2-induced arousal. None of these drugs had any effect on hypoxia-induced arousal. 5-HT2 receptor agonists were given to mice in which 5-HT neurons had been genetically eliminated during embryonic life (Lmx1b(f/f/p)) and which are known to lack CO2-induced arousal. Application of agonists to 5-HT(2A), but not 5-HT(2C), receptors, dose-dependently restored CO2-induced arousal in these mice. These data identify the 5-HT(2A) receptor as an important mediator of CO2-induced arousal and suggest that, while 5-HT neurons can be independently activated to drive CO2-induced arousal, in the absence of 5-HT neurons and endogenous 5-HT, 5-HT receptor activation can act in a permissive fashion to facilitate CO2-induced arousal via another as yet unidentified chemosensor system.

Optogenetic stimulation of cholinergic brainstem neurons during focal limbic seizures: Effects on cortical physiology.

Focal temporal lobe seizures often cause impaired cortical function and loss of consciousness. Recent work suggests that the mechanism for depressed cortical function during focal seizures may depend on decreased subcortical cholinergic arousal, which leads to a sleep-like state of cortical slow-wave activity. To test this hypothesis, we sought to directly activate subcortical cholinergic neurons during focal limbic seizures to determine the effects on cortical function. Here we used an optogenetic approach to selectively stimulate cholinergic brainstem neurons in the pedunculopontine tegmental nucleus during focal limbic seizures induced in a lightly anesthetized rat model. We found an increase in cortical gamma activity and a decrease in delta activity in response to cholinergic stimulation. These findings support the mechanistic role of reduced subcortical cholinergic arousal in causing cortical dysfunction during seizures. Through further work, electrical or optogenetic stimulation of subcortical arousal networks may ultimately lead to new treatments aimed at preventing cortical dysfunction during seizures.

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.

Hamman and Boerhaave syndromes - diagnostic dilemmas in a patient presenting with hyperemesis gravidarum: a case report.

INTRODUCTION: Hyperemesis gravidarum describes persistent vomiting leading to fluid and electrolyte imbalance. It is the commonest reason for admission in the first half of pregnancy. We describe a case of Hamman syndrome secondary to hyperemesis gravidarum. We also discuss Boerhaave syndrome: a particularly rare condition with only a handful of cases being described in the literature. CASE PRESENTATION: A 17 year old admitted with hyperemesis gravidarum was diagnosed with Hamman syndrome after complaining of chest pain due to the presence of subcutaneous emphysema and pneumomediastinum on chest radiograph. She was treated conservatively for potential ruptured oesophagus but then self-discharged against medical advice. CONCLUSION: Subcutaneous emphysema is an alarming finding in any pregnancy and should be treated in a timely and cautious manner. This case report adds weight to the previous literature advocating a conservative versus surgical approach to the management of a woman with Hamman syndrome secondary to hyperemesis gravidarum.

Peri-ictal activation of dorsomedial dorsal raphe serotonin neurons reduces mortality associated with maximal electroshock seizures.

Over one-third of patients with epilepsy will develop refractory epilepsy and continue to experience seizures despite medical treatment. These patients are at the greatest risk for sudden unexpected death in epilepsy. The precise mechanisms underlying sudden unexpected death in epilepsy are unknown, but cardiorespiratory dysfunction and arousal impairment have been implicated. Substantial circumstantial evidence suggests serotonin is relevant to sudden unexpected death in epilepsy as it modulates sleep/wake regulation, breathing and arousal. The dorsal raphe nucleus is a major serotonergic center and a component of the ascending arousal system. Seizures disrupt the firing of dorsal raphe neurons, which may contribute to reduced responsiveness. However, the relevance of the dorsal raphe nucleus and its subnuclei to sudden unexpected death in epilepsy remains unclear. The dorsomedial dorsal raphe may be a salient target due to its role in stress and its connections with structures implicated in sudden unexpected death in epilepsy. We hypothesized that optogenetic activation of dorsomedial dorsal raphe serotonin neurons in TPH2-ChR2-YFP (n = 26) mice and wild-type (n = 27) littermates before induction of a maximal electroshock seizure would reduce mortality. In this study, pre-seizure activation of dorsal raphe nucleus serotonin neurons reduced mortality in TPH2-ChR2-YFP mice with implants aimed at the dorsomedial dorsal raphe. These results implicate the dorsomedial dorsal raphe in this novel circuit influencing seizure-induced mortality. It is our hope that these results and future experiments will define circuit mechanisms that could ultimately reduce sudden unexpected death in epilepsy.

Central serotonin neurons are required for arousal to CO2.

There is a long-standing controversy about the role of serotonin in sleep/wake control, with competing theories that it either promotes sleep or causes arousal. Here, we show that there is a marked increase in wakefulness when all serotonin neurons are genetically deleted in mice hemizygous for ePet1-Cre and homozygous for floxed Lmx1b (Lmx1b(f/f/p)). However, this only occurs at cool ambient temperatures and can be explained by a thermoregulatory defect that leads to an increase in motor activity to generate heat. Because some serotonin neurons are stimulated by CO(2), and serotonin activates thalamocortical networks, we hypothesized that serotonin neurons cause arousal in response to hypercapnia. We found that Lmx1b(f/f/p) mice completely lacked any arousal response to inhalation of 10% CO(2) (with 21% O(2) in balance N(2)) but had normal arousal responses to hypoxia, sound, and air puff. We propose that serotonin neurons mediate the potentially life-saving arousal response to hypercapnia. Impairment of this response may contribute to sudden unexpected death in epilepsy, sudden infant death syndrome, and sleep apnea.

Investigations into hydrogen sulfide-induced suppression of neuronal activity in vivo and calcium dysregulation in vitro.

Acute exposure to high concentrations of hydrogen sulfide (H2S) leads to sudden death and, if survived, lingering neurological disorders. Clinical signs include seizures, loss of consciousness, and dyspnea. The proximate mechanisms underlying H2S-induced acute toxicity and death have not been clearly elucidated. We investigated electrocerebral, cardiac and respiratory activity during H2S exposure using electroencephalogram (EEG), electrocardiogram (EKG) and plethysmography. H2S suppressed electrocerebral activity and disrupted breathing. Cardiac activity was comparatively less affected. To test whether Ca2+ dysregulation contributes to H2S-induced EEG suppression, we developed an in vitro real-time rapid throughput assay measuring patterns of spontaneous synchronized Ca2+ oscillations in cultured primary cortical neuronal networks loaded with the indicator Fluo-4 using the fluorescent imaging plate reader (FLIPR-Tetra®). Sulfide >5 ppm dysregulated synchronous calcium oscillation (SCO) patterns in a dose-dependent manner. Inhibitors of NMDA and AMPA receptors magnified H2S-induced SCO suppression. Inhibitors of L-type voltage gated Ca2+ channels and transient receptor potential channels prevented H2S-induced SCO suppression. Inhibitors of T-type voltage gated Ca2+ channels, ryanodine receptors, and sodium channels had no measurable influence on H2S-induced SCO suppression. Exposures to > 5 ppm sulfide also suppressed neuronal electrical activity in primary cortical neurons measured by multi-electrode array (MEA), an effect alleviated by pretreatment with the nonselective transient receptor potential channel inhibitor, 2-APB. 2-APB also reduced primary cortical neuronal cell death from sulfide exposure. These results improve our understanding of the role of different Ca2+ channels in acute H2S-induced neurotoxicity and identify transient receptor potential channel modulators as novel structures with potential therapeutic benefits.

Acute hydrogen sulfide-induced neurochemical and morphological changes in the brainstem.

Hydrogen sulfide (H2S) is a toxin affecting the cardiovascular, respiratory, and central nervous systems. Acute H2S exposure is associated with a high rate of mortality and morbidity. The precise pathophysiology of H2S-induced death is a controversial topic; however, inhibition of the respiratory center in the brainstem is commonly cited as a cause of death. There is a knowledge gap on toxicity and toxic mechanisms of acute H2S poisoning on the brainstem, a brain region responsible for regulating many reflective and vital functions. Serotonin (5-HT), dopamine (DA), and γ-aminobutyric acid (GABA) play a role in maintaining a normal stable respiratory rhythmicity. We hypothesized that the inhibitory respiratory effects of H2S poisoning are mediated by 5-HT in the respiratory center of the brainstem. Male C57BL/6 mice were exposed once to an LCt50 concentration of H2S (1000 ppm). Batches of surviving mice were euthanized at 5 min, 2 h, 12 h, 24 h, 72 h, and on day 7 post-exposure. Pulmonary function, vigilance state, and mortality were monitored during exposure. The brainstem was analyzed for DA, 3,4-dehydroxyphenyl acetic acid (DOPAC), 5-HT, 5-hydroxyindoleatic acid (5-HIAA), norepinephrine (NE), GABA, glutamate, and glycine using HPLC. Enzymatic activities of monoamine oxidases (MAO) were also measured in the brainstem using commercial kits. Neurodegeneration was assessed using immunohistochemistry and magnetic resonance imaging. Results showed that DA and DOPAC were significantly increased at 5 min post H2S exposure. However, by 2 h DA returned to normal. Activities of MAO were significantly increased at 5 min and 2 h post-exposure. In contrast, NE was significantly decreased at 5 min and 2 h post-exposure. Glutamate was overly sensitive to H2S-induced toxicity manifesting a time-dependent concentration reduction throughout the 7 day duration of the study. Remarkably, there were no changes in 5-HT, 5-HIAA, glycine, or GABA concentrations. Cytochrome c oxidase activity was inhibited but recovered by 24 h. Neurodegeneration was observed starting at 72 h post H2S exposure in select brainstem regions. We conclude that acute H2S exposure causes differential effects on brainstem neurotransmitters. H2S also induces neurodegeneration and biochemical changes in the brainstem. Additional work is needed to fully understand the implications of both the short- and long-term effects of acute H2S poisoning on vital functions regulated by the brainstem.

Selective Serotonin Reuptake Inhibitors and 5-HT2 Receptor Agonists Have Distinct, Sleep-state Dependent Effects on Postictal Breathing in Amygdala Kindled Mice.

Seizures can cause profound breathing disruptions. Seizures arising from sleep cause greater breathing impairment than those emerging from wakefulness and more often result in sudden unexpected death in epilepsy (SUDEP). The neurotransmitter serotonin (5-HT) plays a major role in respiration and sleep-wake regulation. 5-HT modulates seizure susceptibility and severity and is dysregulated by seizures. Thus, the impact of seizures on breathing dysregulation may be due to impaired 5-HT neurotransmission. We examined whether pharmacologically increasing 5-HT neurotransmission prior to seizures improves postictal breathing and how sleep-state during seizure induction contributes to these effects. We assessed breathing with whole-body plethysmography in 84 amygdala-kindled mice pre-treated with selective serotonin reuptake inhibitors (SSRI) or 5-HT2 receptor agonists. SSRIs and 5-HT2 agonists increased postictal breathing frequency (fR), tidal volume (VT), and minute ventilation (VE) at different timepoints following seizures induced during wakefulness. These effects were not observed following seizures induced during NREM sleep. SSRIs suppressed ictal and postictal apnea regardless of sleep state. The SSRI citalopram and the 5-HT2 agonists TCB-2 and MK-212 decreased breathing variability following wake-occurring seizures at different postictal timepoints. Only MK-212 decreased breathing variability when seizures were induced during NREM sleep. The 5-HT2A antagonist MDL-11939 reduced the effect of citalopram on fR, VT, and VE, and enhanced its effect on breathing variability in the initial period following a seizure. These results suggest that 5-HT mechanisms that are dependent on or independent from the 5-HT2 family of receptors impact breathing on different timescales during the recovery of eupnea, and that certain serotonergic treatments may be less effective at facilitating postictal breathing following seizures emerging from sleep.

Chronobiology of epilepsy and sudden unexpected death in epilepsy.

Epilepsy is a neurological disease characterized by spontaneous, unprovoked seizures. Various insults render the brain hyperexcitable and susceptible to seizure. Despite there being dozens of preventative anti-seizure medications available, these drugs fail to control seizures in nearly 1 in 3 patients with epilepsy. Over the last century, a large body of evidence has demonstrated that internal and external rhythms can modify seizure phenotypes. Physiologically relevant rhythms with shorter periodic rhythms, such as endogenous circadian rhythms and sleep-state, as well as rhythms with longer periodicity, including multidien rhythms and menses, influence the timing of seizures through poorly understood mechanisms. The purpose of this review is to discuss the findings from both human and animal studies that consider the effect of such biologically relevant rhythms on epilepsy and seizure-associated death. Patients with medically refractory epilepsy are at increased risk of sudden unexpected death in epilepsy (SUDEP). The role that some of these rhythms play in the nocturnal susceptibility to SUDEP will also be discussed. While the involvement of some of these rhythms in epilepsy has been known for over a century, applying the rhythmic nature of such phenomenon to epilepsy management, particularly in mitigating the risk of SUDEP, has been underutilized. As our understanding of the physiological influence on such rhythmic phenomenon improves, and as technology for chronic intracranial epileptiform monitoring becomes more widespread, smaller and less invasive, novel seizure-prediction technologies and time-dependent chronotherapeutic seizure management strategies can be realized.

The role of sleep state and time of day in modulating breathing in epilepsy: implications for sudden unexpected death in epilepsy.

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death among patients with refractory epilepsy. While the exact etiology of SUDEP is unknown, mounting evidence implicates respiratory dysfunction as a precipitating factor in cases of seizure-induced death. Dysregulation of breathing can occur in epilepsy patients during and after seizures as well as interictally, with many epilepsy patients exhibiting sleep-disordered breathing (SDB), such as obstructive sleep apnea (OSA). The majority of SUDEP cases occur during the night, with the victim found prone in or near a bed. As breathing is modulated in both a time-of-day and sleep state-dependent manner, it is relevant to examine the added burden of nocturnal seizures on respiratory function. This review explores the current state of understanding of the relationship between respiratory function, sleep state and time of day, and epilepsy. We highlight sleep as a particularly vulnerable period for individuals with epilepsy and press that this topic warrants further investigation in order to develop therapeutic interventions to mitigate the risk of SUDEP.

The serotonin axis: Shared mechanisms in seizures, depression, and SUDEP.

There is a growing appreciation that patients with seizures are also affected by a number of comorbid conditions, including an increase in prevalence of depression (Kanner, 2009), sleep apnea (Chihorek et al., 2007), and sudden death (Ryvlin et al., 2006; Tomson et al., 2008). The mechanisms responsible for these associations are unclear. Herein we discuss the possibility that underlying pathology in the serotonin (5-HT) system of patients with epilepsy lowers the threshold for seizures, while also increasing the risk of depression and sudden death. We propose that postictal dysfunction of 5-HT neurons causes depression of breathing and arousal in some epilepsy patients, and this can lead to sudden unexpected death in epilepsy (SUDEP). We further draw parallels between SUDEP and sudden infant death syndrome (SIDS), which may share pathophysiologic mechanisms, and which have both been linked to defects in the 5-HT system.