The Midline Thalamus: The New Kid in Town for Absence Seizure Therapy

Abstract

Attenuating Midline Thalamus Bursting to Mitigate Absence Epilepsy

Dong P, Bakhurin K, Li Y, Mikati MA, Cui J, Grill WM, Yin HH, Yang H. Attenuating midline thalamus bursting to mitigate absence epilepsy. Proc Natl Acad Sci USA. 2024 Jul 9;121(28):e2403763121. doi: 10.1073/pnas.2403763121. Epub 2024 Jul 5. PMID: 38968111; PMCID: PMC11252967.

Advancing the mechanistic understanding of absence epilepsy is crucial for developing new therapeutics, especially for patients unresponsive to current treatments. Utilizing a recently developed mouse model of absence epilepsy carrying the BK gain-of-function channelopathy D434G, here we report that attenuating the burst firing of midline thalamus (MLT) neurons effectively prevents absence seizures. We found that enhanced BK channel activity in the BK-D434G MLT neurons promotes synchronized bursting during the ictal phase of absence seizures. Modulating MLT neurons through pharmacological reagents, optogenetic stimulation, or deep brain stimulation effectively attenuates burst firing, leading to reduced absence seizure frequency and increased vigilance. Additionally, enhancing vigilance by amphetamine, a stimulant medication, or physical perturbation also effectively suppresses MLT bursting and prevents absence seizures. These findings suggest that the MLT is a promising target for clinical interventions. Our diverse approaches offer valuable insights for developing next generation therapeutics to treat absence epilepsy.

Commentary

The identification of effective therapies that prevent absence seizures remains a significant pediatric clinical challenge. Approximately 30% of patients diagnosed with childhood absence epilepsy (CAE) are pharmacoresistant to treatment with antiseizure medications such as ethosuximide or valproic acid. Independent of whether their seizures are controlled, approximately 60% of these patients exhibit significant neuropsychiatric comorbidities, including deficits in attention, cognition, memory, and mood. ¹ For many patients, the neuropsychiatric deficits persist after the seizures have been effectively controlled. ² Clinically, typical absence seizures are characterized by brief, 2.5–4 Hz generalized spike and wave discharges (SWDs) in the EEG, accompanied by a loss of consciousness. The degree of loss of consciousness can vary from patient to patient and from seizure to seizure in an individual patient. ³ As with other types of epilepsy, a better understanding of the pathophysiology underlying the development of CAE is necessary to identify new targets for therapeutic intervention to effectively treat both the seizures and the accompanying neuropsychiatric deficits.

A working model of the circuitry underlying the generation of SWDs and absence seizures involves hypersynchrony in the thalamocortical (TC) network. ⁴ Briefly, excitatory TC relay neurons are reciprocally connected to both cortical neurons and to inhibitory neurons in the thalamic reticular nucleus (TRN). TC neurons express high levels of T-type voltage-gated Ca²⁺ (T-CaV) channels, which allow for burst firing in response to hyperpolarization. TRNs also express T-CaV channels, which contribute to burst firing in these neurons, as well. Synchronized firing of TRN neurons results in synchronized inhibition of TC neurons. Inhibition of the TC neurons activates T-CaV channels, resulting in rebound, synchronized TC cell burst firing. Burst firing of TC neurons drives both the cortical neurons and TRN neurons. It is this hypersynchronized firing of all three cell types in the TC network that generates SWDs and absence seizures. While this model provides a framework for how absence seizures are generated, it fails to provide a basis for the neuropsychiatric comorbidities that accompany absence seizures, implicating the contribution of other subcortical structures. However, it highlights SWDs and burst firing as key targets for therapeutic intervention.

Based on the observation that some patients with absence epilepsy expressed a gain-of-function (GOF) BK channelopathy, ⁵ Dong et al ⁶ developed a knock-in mouse model carrying the human GOF BK-D434G variant Ca²⁺-activated large-conductance BK K⁺ channel. These animals exhibit many of the features present in patients with CAE, including frequent spontaneous absence seizures with SWDs accompanied by behavioral arrest. The seizure activity in these mice is responsive to treatment with both ethosuximide and valproic acid.

Using a multidisciplinary approach, the highlighted study identified that burst firing of midline thalamic (MLT) neurons contributes to the generation of absence seizures in homozygous BK-D434G (BK^DG/DG) mice. ⁷ Subsequent experiments used pharmacology, optogenetic stimulation, and deep-brain stimulation (DBS) to demonstrate that inhibition of burst firing in these neurons could prevent or mitigate spontaneous absence seizures and accompanying alterations in behavior. An examination of immunohistochemical expression of c-Fos in the cortical-TRN-thalamus circuit after absence seizure activity was used to identify that MLT neurons in the intermediodorsal, central median, and reuniens thalamic nuclei participated in the generation of absence seizures in BK^DG/DG mice. Surprisingly, no c-Fos staining was observed in the TRN or ventral posteromedial thalamic nuclei implicated in the generation of absence seizures in other models of absence epilepsy. ⁸ Given the inverse relationship between vigilance and absence seizure frequency, ⁹ the effect of increased vigilance on absence seizure frequency in BK^DG/DG mice was tested by placing the mice on a running wheel to increase vigilance. The forced increase in vigilance resulted in a decrease in spontaneous seizure activity, SWDs, and c-Fos expression in the MLT neurons. Treatment with the stimulant D-amphetamine sulfate (3 mg/kg, i.p.) had a similar effect on SWDs, absence seizures, and vigilance, further supporting the observation that increased vigilance can interfere with absence seizure activity.

Having identified that MLT neurons participate in the generation of absence seizures in BK^DG/DG mice, in vivo EEG recording combined with single unit recording revealed that burst firing of MLT neurons occurred at the peak of the SWDs. Whole cell current clamp recordings of MLT cells in ex vivo slices from BK^DG/DG mice revealed low threshold spikes (LTS), which contribute to burst firing in TC neurons, ¹⁰ exhibited faster repolarization, shortened action potential (AP) duration, and increased after hyperpolarization amplitude. Together, these changes likely lead to rapid deinactivation of voltage-gated sodium channels, which provides a mechanism for enhanced rebound bursting in these cells.

Several approaches were used to demonstrate that suppression of MLT bursting could prevent the generation of absence seizures in BK^DG/DG mice. Paxilline, a fungal alkaloid and potent inhibitor of BK channels, administered systemically and by local application, blocked MLT neuronal bursting. When tested in the in vitro slice, paxilline suppressed LTS-mediated bursting. Together, these results confirmed the contribution of the BK channel to the generation of burst firing and the generation of absence seizures in BK^DG/DG mice. Optogenetic stimulation of MLT neurons in vitro entrained MLT neurons to fire APs tonically and prevented burst firing. When optogenetic stimulation was tested in vivo, it had a similar effect of blocking burst firing but also suppressing absence seizure activity. Similar results were obtained with closed-loop DBS of the MLT nuclei. Closed-loop stimulation delivered in response to the detection of SWDs inhibited the absence seizures and the accompanying behavioral arrest.

In conclusion, Dong et al ⁷ performed a series of elegant experiments using the BK^DG/DG knock in mouse model of absence epilepsy to (1) identify the contribution of the MLT to the generation of absence seizures and therefore a new target for therapeutic intervention for the treatment of absence epilepsy and (2) the suppression of burst firing in MLT neurons could be the key to preventing not just SWDs and absence seizures but also the accompanying behavioral arrest. The observation that the conversion of burst firing to tonic firing suppressed both the absence seizure and the behavioral arrest is consistent with the findings in the genetic Stargazer mouse and WAGRij rat models of absence epilepsy. ¹¹ Further experiments will need to be performed to determine if the MLT contributes to the generation of SWDs and seizures in other experimental models of absence epilepsy. Since both ethosuximide and valproic acid are effective in the BK^DG/DG model, ⁶ it is unclear if targeting the MLT clinically will be effective in pharmacoresistant patients. However, since DBS of other thalamic nuclei is used clinically to treat other types of epilepsy, it is not a great leap to begin testing the efficacy of DBS delivered to MLT in patients with pharmacoresistant absence epilepsy.