The diverse role of the raphe 5-HTergic systems in epilepsy

SUDEP in inherited metabolic epilepsies

Inherited metabolic epilepsies (IMEs) have an increased susceptibility for early mortality, including sudden unexpected death in epilepsy (SUDEP), as they often manifest with frequent drug-resistant seizures, including bilateral tonic-clonic and nocturnal seizures. The metabolic defects inherent to their etiology predispose affected individuals to an additional risk of mortality as they can lead to brain injury, which can induce and be exacerbated by seizures, and to systemic conditions that decrease seizure-cessation and auto-resuscitation processes. The increased risk for SUDEP in IMEs mandates that special emphasis should be given to identifying them early and attempting to achieve seizure control by managing acute metabolic decompensations, anti-seizure medications, and targeted therapies when available. Providing education and support to individuals with IMEs and their families about SUDEP risk factors and prevention strategies is imperative. Despite the increased risk for SUDEP in IMEs, this topic remains understudied. As a proceeding of the 2024 Partners Against Mortality in Epilepsy (PAME) meeting and aiming to increase awareness, this Review describes the pathophysiological and clinical elements related to the heightened risk of SUDEP in IMEs and provides the perspective of a parent of a child with an IME who died from SUDEP. Calling for action, future research on epilepsy-related mortality in IMEs is required. Investigating this field may also yield insights into the general pathomechanisms of SUDEP.

Effects of aminooxyacetic acid on learning and memory function and neurochemical changes in chronic alcoholism

OBJECTIVE

This study aimed to investigate the effect of aminooxyacetic acid (AOAA) on cognitive function, particularly learning and memory, in a rat model of chronic alcoholism. Additionally, the study explored changes in cystathionine β-synthase (CBS), hydrogen sulfide (H₂S), and serotonin (5-HT) levels in the prefrontal cortex to understand the potential neurochemical mechanisms involved.

METHODS

Sixty-four male SD rats were randomly divided into four groups, with 16 rats in each: Con, Con + AOAA, Model, and Model + AOAA. The Model group received a 6 % ethanol solution for 28 days. From day 14, the Model + AOAA group was treated with daily intraperitoneal injections of AOAA (5 mg/kg) for 14 consecutive days. Cognitive function was assessed using the Morris water maze, mitochondrial function was evaluated through ATPase activity, and H₂S levels were measured. CBS and 5-HT levels in the prefrontal cortex were analyzed by immunohistochemistry.

RESULTS

Compared to the control groups, rats in the Model group exhibited significant impairments in learning and memory, increased CBS expression, elevated H₂S levels, and decreased 5-HT release. AOAA treatment improved memory performance, reduced CBS expression and H₂S levels, and increased 5-HT release, although these measures did not fully return to baseline. No significant differences were observed between the two control groups.

CONCLUSION

AOAA may alleviate cognitive deficits associated with chronic alcoholism by inhibiting CBS expression, reducing H₂S levels, and enhancing 5-HT release in the prefrontal cortex. These findings suggest AOAA as a potential therapeutic strategy for alcohol-induced cognitive impairments.

Improved green and red GRAB sensors for monitoring spatiotemporal serotonin release in vivo

The serotonergic system plays important roles in both physiological and pathological processes, and is a therapeutic target for many psychiatric disorders. Although several genetically encoded GFP-based serotonin (5-HT) sensors were recently developed, their sensitivities and spectral profiles are relatively limited. To overcome these limitations, we optimized green fluorescent G-protein-coupled receptor (GPCR)-activation-based 5-HT (GRAB5-HT) sensors and developed a red fluorescent GRAB5-HT sensor. These sensors exhibit excellent cell surface trafficking and high specificity, sensitivity and spatiotemporal resolution, making them suitable for monitoring 5-HT dynamics in vivo. Besides recording subcortical 5-HT release in freely moving mice, we observed both uniform and gradient 5-HT release in the mouse dorsal cortex with mesoscopic imaging. Finally, we performed dual-color imaging and observed seizure-induced waves of 5-HT release throughout the cortex following calcium and endocannabinoid waves. In summary, these 5-HT sensors can offer valuable insights regarding the serotonergic system in both health and disease.

Projection-defined median raphe Pet+ subpopulations are diversely implicated in seizure

The raphe nuclei, the primary resource of forebrain 5-HT, play an important but heterogeneous role in regulating subcortical excitabilities. Fundamental circuit organizations of different median raphe (MR) subsystems are far from completely understood. In the present study, using cell-specific viral tracing, Ca2+ fiber photometry and epilepsy model, we map out the forebrain efferent and afferent of different MR Pet+ subpopulations and their divergent roles in epilepsy. We found that PetMR neurons send both collateral and parallel innervations to different downstream regions through different subpopulations. Notably, CA3-projecting PetMR subpopulations are largely distinct from habenula (Hb)-projecting PetMR subpopulations in anatomical distribution and topological organization, while majority of the CA3-projecting PetMR subpopulations are overlapped with the medial septum (MS)-projecting PetMR subpopulations. Further, using Ca2+ fiber photometry, we monitor activities of PetMR neurons in hippocampal-kindling seizure, a classical epilepsy model with pathological mechanisms caused by excitation-inhibition imbalance. We found that soma activities of PetMR neurons are heterogeneous during different periods of generalized seizures. These divergent activities are contributed by different projection-defined PetMR subpopulations, manifesting as increased activities in CA3 but decreased activity in Hb resulting from their upstream differences. Together, our findings provide a novel framework of MR subsystems showing that projection-defined MR Pet+ subpopulations are topologically heterogenous with divergent circuit connections and are diversely implicated in seizures. This may help in the understanding of heterogeneous nature of MR 5-HTergic subsystems and the paradox roles of 5-HTergic systems in epilepsy.

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.