The Orexin System: A Potential Player in the Pathophysiology of Absence Epilepsy

How is the P2X7 receptor signaling pathway involved in epileptogenesis?

Epilepsy, a condition characterized by spontaneous recurrent epileptic seizures, is among the most prevalent neurological disorders. This disorder is estimated to affect approximately 70 million people worldwide. Although antiseizure medications are considered the first-line treatments for epilepsy, most of the available antiepileptic drugs are not effective in nearly one-third of patients. This calls for the development of more effective drugs. Evidence from animal models and epilepsy patients suggests that strategies that interfere with the P2X7 receptor by binding to adenosine triphosphate (ATP) are potential treatments for this patient population. This review describes the role of the P2X7 receptor signaling pathways in epileptogenesis. We highlight the genes, purinergic signaling, Pannexin1, glutamatergic signaling, adenosine kinase, calcium signaling, and inflammatory response factors involved in the process, and conclude with a synopsis of these key connections. By unraveling the intricate interplay between P2X7 receptors and epileptogenesis, this review provides ideas for designing potent clinical therapies that will revolutionize both prevention and treatment for epileptic patients.

Involvement of orexin type-2 receptors in genetic absence epilepsy rats

Introduction

Orexin is a neuropeptide neurotransmitter that regulates the sleep/wake cycle produced by the lateral hypothalamus neurons. Recent studies have shown the involvement of orexin system in epilepsy. Limited data is available about the possible role of orexins in the pathophysiology of absence seizures. This study aims to understand the role of orexinergic signaling through the orexin-type 2 receptor (OX2R) in the pathophysiology of absence epilepsy. The pharmacological effect of a selective OX2R agonist, YNT-185 on spike-and-wave-discharges (SWDs) and the OX2R receptor protein levels in the cortex and thalamus in adult GAERS were investigated.

Methods

The effect of intracerebroventricular (ICV) (100, 300, and 600 nmol/10 μL), intrathalamic (30 and 40 nmol/500 nL), and intracortical (40 nmol/500 nL) microinjections of YNT-185 on the duration and number of spontaneous SWDs were evaluated in adult GAERS. The percentage of slow-wave sleep (SWS) and spectral characteristics of background EEG were analyzed after the ICV application of 600 nmol YNT-185. The level of OX2R expression in the somatosensory cortex and projecting thalamic nuclei of adult GAERS were examined by Western blot and compared with the non-epileptic Wistar rats.

Results

We showed that ICV administration of YNT-185 suppressed the cumulative duration of SWDs in GAERS compared to the saline-administered control group (p < 0.05). However, intrathalamic and intracortical microinjections of YNT-185 did not show a significant effect on SWDs. ICV microinjections of YNT-185 affect sleep states by increasing the percentage of SWS and showed a significant treatment effect on the 1-4 Hz delta frequency band power during the 1-2 h post-injection period where YNT-185 significantly decreased the SWDs. OXR2 protein levels were significantly reduced in the cortex and thalamus of GAERS when compared to Wistar rats.

Conclusion

This study investigated the efficacy of YNT-185 for the first time on absence epilepsy in GAERS and revealed a suppressive effect of OX2R agonist on SWDs as evidenced by the significantly reduced expression of OX2R in the cortex and thalamus. YNT-185 effect on SWDs could be attributed to its regulation of wake/sleep states. The results constitute a step toward understanding the effectiveness of orexin neuropeptides on absence seizures in GAERS and might be targeted by therapeutic intervention for absence epilepsy.

Subthalamic nucleus stimulation attenuates motor seizures via modulating the nigral orexin pathway

Background

Focal motor seizures that originate in the motor region are a considerable challenge because of the high risk of permanent motor deficits after resection. Deep brain stimulation of the subthalamic nucleus (STN-DBS) is a potential treatment for motor epilepsy that may enhance the antiepileptic actions of the substantia nigra pars reticulata (SNr). Orexin and its receptors have a relationship with both STN-DBS and epilepsy. We aimed to investigate whether and how STN inputs to the SNr regulate seizures and the role of the orexin pathway in this process.

Methods

A penicillin-induced motor epileptic model in adult male C57BL/6 J mice was established to evaluate the efficacy of STN-DBS in modulating seizure activities. Optogenetic and chemogenetic approaches were employed to regulate STN-SNr circuits. Selective orexin receptor type 1 and 2 antagonists were used to inhibit the orexin pathway.

Results

First, we found that high-frequency ipsilateral or bilateral STN-DBS was effective in reducing seizure activity in the penicillin-induced motor epilepsy model. Second, inhibition of STN excitatory neurons and STN-SNr projections alleviates seizure activities, whereas their activation amplifies seizure activities. In addition, activation of the STN-SNr circuits also reversed the protective effect of STN-DBS on motor epilepsy. Finally, we observed that STN-DBS reduced the elevated expression of orexin and its receptors in the SNr during seizures and that using a combination of selective orexin receptor antagonists also reduced seizure activity.

Conclusion

STN-DBS helps reduce motor seizure activity by inhibiting the STN-SNr circuit. Additionally, orexin receptor antagonists show potential in suppressing motor seizure activity and may be a promising therapeutic option in the future.