Suppression of phrenic nerve activity as a potential predictor of imminent sudden unexpected death in epilepsy (SUDEP)

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.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

SIGNIFICANCE

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

Vagus nerve electroneurogram-based detection of acute kainic acid induced seizures

Seizures produce autonomic symptoms, mainly sympathetic but also parasympathetic in origin. Within this context, the vagus nerve is a key player as it carries information from the different organs to the brain and vice versa. Hence, exploiting vagal neural traffic for seizure detection might be a promising tool to improve the efficacy of closed-loop Vagus Nerve Stimulation. This study developed a VENG detection algorithm that effectively detects seizures by emphasizing the loss of spontaneous rhythmicity associated with respiration in acute intrahippocampal Kainic Acid rat model. Among 20 induced seizures in six anesthetized rats, 13 were detected (sensitivity: 65%, accuracy: 92.86%), with a mean VENG-detection delay of 25.3 ± 13.5 s after EEG-based seizure onset. Despite variations in detection parameters, 7 out of 20 seizures exhibited no ictal VENG modifications and remained undetected. Statistical analysis highlighted a significant difference in Delta, Theta and Beta band evolution between detected and undetected seizures, in addition to variations in the magnitude of HR changes. Binomial logistic regression analysis confirmed that an increase in delta and theta band activity was associated with a decreased likelihood of seizure detection. This results suggest the possibility of distinct seizure spreading patterns between the two groups which may results in differential activation of the autonomic central network. Despite notable progress, limitations, particularly the absence of respiration recording, underscore areas for future exploration and refinement in closed-loop stimulation strategies for epilepsy management. This study constitutes the initial phase of a longitudinal investigation, which will subsequently involve reproducing these experiments in awake conditions with spontaneous recurrent seizures.

The role of the adenosine system in epilepsy and its comorbidities

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

The role of adenosine in alcohol-induced respiratory suppression

Alcohol-related poisoning is the foremost cause of death resulting from excessive acute alcohol consumption. Respiratory failure is crucial to the pathophysiology of fatal alcohol poisoning. Alcohol increases accumulation of extracellular adenosine. Adenosine suppresses breathing. The goal of this investigation was to test the hypothesis that adenosine signaling contributes to alcohol-induced respiratory suppression. In the first experiment, the breathing of mice was monitored following an injection of the non-selective adenosine receptor antagonist caffeine (40 mg/kg), alcohol (5 g/kg), or alcohol and caffeine combined. Caffeine reduced alcohol-induced respiratory suppression suggesting that adenosine contributes to the effects of alcohol on breathing. The second experiment utilized the same experimental design, but with the blood brain barrier impermeant non-selective adenosine receptor antagonist 8-sulfophenyltheophylline (8-SPT, 60 mg/kg) instead of caffeine. 8-SPT did not reduce alcohol-induced respiratory suppression suggesting that adenosine is contributing to alcohol-induced respiratory suppression in the central nervous system. The third and fourth experiments used the same experimental design as the first, but with the selective A1 receptor antagonist DPCPX (1 mg/kg) and the selective A2A receptor antagonist istradefylline (3.3 mg/kg). Istradefylline, but not DPCPX, reduced alcohol-induced respiratory suppression indicating an A2A receptor mediated effect. In the fifth experiment, alcohol-induced respiratory suppression was evaluated in Adk+/- mice which have impaired adenosine metabolism. Alcohol-induced respiratory suppression was exacerbated in Adk+/- mice. These findings indicate that adenosinergic signaling contributes to alcohol-induced respiratory suppression. Improving our understanding of how alcohol affects breathing may lead to better treatment strategies and better outcomes for patients with severe alcohol poisoning.

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

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

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.

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

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