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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.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Summary: 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. [ABSTRACT FROM AUTHOR]

Arousal from sleep in response to CO2is 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 CO2is 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 CO2could 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 CO2can 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. [ABSTRACT FROM AUTHOR]