Sleep, epilepsy and thalamic reticular inhibitory neurons

Neuropathic pain impairs sleep architecture, non-rapid eye movement sleep, and reticular thalamic neuronal activity

BACKGROUND

Neuropathic pain (NP) is a chronic and debilitating condition frequently comorbid with insomnia. However, the alterations in sleep architecture under NP conditions and the mechanisms underlying both pain and sleep disturbances remain poorly understood. The reticular thalamic nucleus (RTN) plays a crucial role in non-rapid eye movement sleep (NREMS) and pain processing, but its involvement in NP-related sleep disruptions has not been fully elucidated.

METHODS

To investigate sleep-related electrophysiological changes in NP, we performed continuous 24-hour electroencephalogram/electromyogram (EEG/EMG) recordings in rats exhibiting allodynia following L5-L6 spinal nerve lesions. Additionally, we assessed the in vivo neuronal activity of the RTN in both NP and sham-operated control rats. Spectral analyses were conducted to examine alterations in sleep oscillatory dynamics. Reticular thalamic nucleus neuronal responses to nociceptive pinch stimuli were classified as increased, decreased, or unresponsive.

RESULTS

Neuropathic pain rats exhibited a significant reduction in NREMS (-20%, P < .001) and an increase in wakefulness (+ 19.13%, P < .05) compared to controls, whereas rapid eye movement sleep (REMS) remained unchanged. Sleep fragmentation was pronounced in NP animals (P < .0001), with frequent brief awakenings, particularly during the inactive/light phase. Spectral analysis revealed increased delta and theta power during both NREMS and REMS. Reticular thalamic nucleus neurons in NP rats displayed a higher basal tonic firing rate, along with increased phasic activity (number of bursts), although the percentage of spikes in bursts remained unchanged.

CONCLUSIONS

Neuropathic pain is characterized by disrupted sleep architecture, reduced NREMS, and heightened RTN neuronal firing activity with partial compensation of burst activity. Given that RTN burst activity is essential for optimal NREMS, its disruption may contribute to NP-induced sleep impairments. These findings suggest that altered EEG/EMG signals, alongside dysregulated RTN neuronal activity, may serve as potential brain markers for NP-related insomnia.

The thalamus: Structure, function, and neurotherapeutics

The complexity and expansive nature of thalamic research has led to numerous interventions for varied disease states. At the same time, this complexity along with siloed areas of study can hinder a comprehensive understanding. The goal of this paper is to give the reader a broader and more detailed perspective on the thalamus. In order to accomplish this goal, the paper begins with a summary of the function, electrophysiology, and anatomy of the normal thalamus. With this foundation, thalamic involvement in neurological diseases is discussed with a focus on epilepsy. Therapeutic interventions in the thalamus for epilepsy as well as movement disorders, psychiatric conditions and disorders of consciousness are described. Lastly limitations in the field and future models of data sharing and cooperation are explored.

The Role of Acetylcholinesterase Enzyme Inhibitor Rivastigmine on Spike-Wave Discharges, Learning-Memory, Anxiety, and TRPV1 Channel Expression in Genetic Absence Epileptic WAG/Rij Rats

In the present study, the effects of the acetylcholinesterase (AChE) enzyme inhibitor rivastigmine (RIVA) on spike-wave discharges (SWDs), memory impairment, anxiety-like behavior, and the transient receptor potential vanilloid 1 (TRPV1) gene expression were investigated in genetic absence epileptic Wistar Albino Glaxo/Rijswijk (WAG/Rij) rats. After tripolar electrodes were implanted on the WAG/Rij rats' skulls, single doses of 0.125, 0.25, 0.5, 1, and 2 mg/kg RIVA were intraperitoneally (i.p.) administered, and electrocorticogram (ECoG) recordings of SWDs were recorded for three hours before and after injections. Additionally, once significant doses were determined in acute studies, WAG/Rij rats were administered low-dose (0.5 mg/kg) and high-dose (2 mg/kg) of RIVA for 21 consecutive days and SWDs were recorded. Learning-memory abilities (Y-maze test), anxiety-like behavior (elevated plus maze test), and TRPV1 gene expression were determined and compared in 8-month-old WAG/Rij and age-matched Wistar rats. Acute RIVA administration dose-dependently reduced the total number of SWDs and was even entirely inhibited at 1 and 2 mg/kg RIVA doses. On the other hand, long-term high-dose RIVA administration increased the total number of SWDs. Long-term high-dose RIVA treatment reduced learning-memory and anxiety-like behavior in WAG/Rij rats, while only anxiety-like behavior decreased in Wistar rats. TRPV1 gene expression increased in WAG/Rij rats and decreased in Wistar rats with long-term RIVA administration. These data indicate that the sudden increase of acetylcholine (ACh) causes a significant decrease in absence seizures. In contrast, prolonged maintenance of ACh elevation causes an increase in absence seizures, probably by altering the expression of channels such as TRPV1.

Sleep and circadian rhythms modeling: From hypothalamic regulatory networks to cortical dynamics and behavior

Sleep and circadian rhythms are regulated by dynamic physiologic processes that operate across multiple spatial and temporal scales. These include, but are not limited to, genetic oscillators, clearance of waste products from the brain, dynamic interplay among brain regions, and propagation of local dynamics across the cortex. The combination of these processes, modulated by environmental cues, such as light-dark cycles and work schedules, represents a complex multiscale system that regulates sleep-wake cycles and brain dynamics. Physiology-based mathematical models have successfully explained the mechanisms underpinning dynamics at specific scales and are a useful tool to investigate interactions across multiple scales. They can help answer questions such as how do electroencephalographic (EEG) features relate to subthalamic neuron activity? Or how are local cortical dynamics regulated by the homeostatic and circadian mechanisms? In this chapter, we review two types of models that are well-positioned to consider such interactions. Part I of the chapter focuses on the subthalamic sleep regulatory networks and a model of arousal dynamics capable of predicting sleep, circadian rhythms, and cognitive outputs. Part II presents a model of corticothalamic circuits, capable of predicting spatial and temporal EEG features. We then discuss existing approaches and unsolved challenges in developing unified multiscale models.

Thalamocortical Hodology to Personalize Electrical Stimulation for Focal Epilepsy

Targeted electrical stimulation to specific thalamic regions offers a therapeutic approach for patients with refractory focal and generalized epilepsy who are not candidates for resective surgery. However, clinical outcome varies significantly, in particular for focal epilepsy, influenced by several factors, notably the precise anatomical and functional alignment between cortical regions generating epileptic discharges and the targeted thalamic stimulation sites. Here we hypothesized that targeting thalamic nuclei with precise anatomical and functional connections to epileptic cortical areas (an approach that we refer to as hodological matching) could enhance neuromodulatory effects on focal epileptic discharges. To investigate this, we examined three thalamic subnuclei (pulvinar nucleus, anterior nucleus, and ventral intermediate nucleus/ventral oral posterior nuclei) in a retrospective study involving 32 focal epilepsy patients. Specifically, we first identified hodologically organized thalamocortical fibers connecting these nuclei to individual seizure onset zones (SOZs), combining neuroimaging and electrophysiological techniques. Further, analysis of 216 spontaneous seizures revealed the critical role of matched thalamic nuclei in seizure development and termination. Importantly, electrical stimulation of hodologically-matched thalamic nuclei immediately suppressed intracortical interictal epileptiform discharges, contrasting with ineffective outcomes from stimulation of unmatched targets. Finally, we retrospectively evaluated 7 patients with a chronic hodologically-matched neurostimulation system, which led to a clinically relevant reduction in seizure frequency (median reduction 86.5%), that outstands the current clinical practice of unmatched targets (39%). Our results underscore the potential of hodological thalamic targeting to modulate epileptiform activity in specific cortical regions, highlighting the promise of precision medicine in thalamic neuromodulation for focal refractory epilepsy.

Solving the Etiology of Developmental and Epileptic Encephalopathy with Spike-Wave Activation in Sleep (D/EE-SWAS)

OBJECTIVE

To understand the etiological landscape and phenotypic differences between 2 developmental and epileptic encephalopathy (DEE) syndromes: DEE with spike-wave activation in sleep (DEE-SWAS) and epileptic encephalopathy with spike-wave activation in sleep (EE-SWAS).

METHODS

All patients fulfilled International League Against Epilepsy (ILAE) DEE-SWAS or EE-SWAS criteria with a Core cohort (n = 91) drawn from our Epilepsy Genetics research program, together with 10 etiologically solved patients referred by collaborators in the Expanded cohort (n = 101). Detailed phenotyping and analysis of molecular genetic results were performed. We compared the phenotypic features of individuals with DEE-SWAS and EE-SWAS. Brain-specific gene co-expression analysis was performed for D/EE-SWAS genes.

RESULTS

We identified the etiology in 42/91 (46%) patients in our Core cohort, including 29/44 (66%) with DEE-SWAS and 13/47 (28%) with EE-SWAS. A genetic etiology was identified in 31/91 (34%). D/EE-SWAS genes were highly co-expressed in brain, highlighting the importance of channelopathies and transcriptional regulators. Structural etiologies were found in 12/91 (13%) individuals. We identified 10 novel D/EE-SWAS genes with a range of functions: ATP1A2, CACNA1A, FOXP1, GRIN1, KCNMA1, KCNQ3, PPFIA3, PUF60, SETD1B, and ZBTB18, and 2 novel copy number variants, 17p11.2 duplication and 5q22 deletion. Although developmental regression patterns were similar in both syndromes, DEE-SWAS was associated with a longer duration of epilepsy and poorer intellectual outcome than EE-SWAS.

INTERPRETATION

DEE-SWAS and EE-SWAS have highly heterogeneous genetic and structural etiologies. Phenotypic analysis highlights valuable clinical differences between DEE-SWAS and EE-SWAS which inform clinical care and prognostic counseling. Our etiological findings pave the way for the development of precision therapies. ANN NEUROL 2024;96:932-943.

Timing Mechanisms for Circadian Seizures

For centuries, epileptic seizures have been noticed to recur with temporal regularity, suggesting that an underlying biological rhythm may play a crucial role in their timing. In this review, we propose to adopt the framework of chronobiology to study the circadian timing of seizures. We first review observations made on seizure timing in patients with epilepsy and animal models of the disorder. We then present the existing chronobiology paradigm to disentangle intertwined circadian and sleep-wake timing mechanisms. In the light of this framework, we review the existing evidence for specific timing mechanisms in specific epilepsy syndromes and highlight that current knowledge is far from sufficient. We propose that individual seizure chronotypes may result from an interplay between independent timing mechanisms. We conclude with a research agenda to help solve the urgency of ticking seizures.

Local activation of CB1 receptors by synthetic and endogenous cannabinoids dampens burst firing mode of reticular thalamic nucleus neurons in rats under ketamine anesthesia

The reticular thalamic nucleus (RTN) is a thin shell that covers the dorsal thalamus and controls the overall information flow from the thalamus to the cerebral cortex through GABAergic projections that contact thalamo-cortical neurons (TC). RTN neurons receive glutamatergic afferents fibers from neurons of the sixth layer of the cerebral cortex and from TC collaterals. The firing mode of RTN neurons facilitates the generation of sleep-wake cycles; a tonic mode or desynchronized mode occurs during wake and REM sleep and a burst-firing mode or synchronized mode is associated with deep sleep. Despite the presence of cannabinoid receptors CB1 (CB1Rs) and mRNA that encodes these receptors in RTN neurons, there are few works that have analyzed the participation of endocannabinoid-mediated transmission on the electrical activity of RTN. Here, we locally blocked or activated CB1Rs in ketamine anesthetized rats to analyze the spontaneous extracellular spiking activity of RTN neurons. Our results show the presence of a tonic endocannabinoid input, since local infusion of AM 251, an antagonist/inverse agonist, modifies RTN neurons electrical activity; furthermore, local activation of CB1Rs by anandamide or WIN 55212-2 produces heterogeneous effects in the basal spontaneous spiking activity, where the main effect is an increase in the spiking rate accompanied by a decrease in bursting activity in a dose-dependent manner; this effect is inhibited by AM 251. In addition, previous activation of GABA-A receptors suppresses the effects of CB1Rs on reticular neurons. Our results show that local activation of CB1Rs primarily diminishes the burst firing mode of RTn neurons.

Thalamic epileptic spikes disrupt sleep spindles in patients with epileptic encephalopathy

In severe epileptic encephalopathies, epileptic activity contributes to progressive cognitive dysfunction. Epileptic encephalopathies share the trait of spike-wave activation during non-REM sleep (EE-SWAS), a sleep stage dominated by sleep spindles, which are brain oscillations known to coordinate offline memory consolidation. Epileptic activity has been proposed to hijack the circuits driving these thalamocortical oscillations, thereby contributing to cognitive impairment. Using a unique dataset of simultaneous human thalamic and cortical recordings in subjects with and without EE-SWAS, we provide evidence for epileptic spike interference of thalamic sleep spindle production in patients with EE-SWAS. First, we show that epileptic spikes and sleep spindles are both predicted by slow oscillations during stage two sleep (N2), but at different phases of the slow oscillation. Next, we demonstrate that sleep-activated cortical epileptic spikes propagate to the thalamus (thalamic spike rate increases after a cortical spike, P ≈ 0). We then show that epileptic spikes in the thalamus increase the thalamic spindle refractory period (P ≈ 0). Finally, we show that in three patients with EE-SWAS, there is a downregulation of sleep spindles for 30 s after each thalamic spike (P < 0.01). These direct human thalamocortical observations support a proposed mechanism for epileptiform activity to impact cognitive function, wherein epileptic spikes inhibit thalamic sleep spindles in epileptic encephalopathy with spike and wave activation during sleep.

Clinical Utility of Sleep Recordings During Presurgical Epilepsy Evaluation With Stereo-Electroencephalography: A Systematic Review

SUMMARY

Although the role of sleep in modulating epileptic activity is well established, many epileptologists overlook the significance of considering sleep during presurgical epilepsy evaluations in cases of drug-resistant epilepsy. Here, we conducted a comprehensive literature review from January 2000 to May 2023 using the PubMed electronic database and compiled evidence to highlight the need to revise the current clinical approach. All articles were assessed for eligibility by two independent reviewers. Our aim was to shed light on the clinical value of incorporating sleep monitoring into presurgical evaluations with stereo-electroencephalography. We present the latest developments on the important bidirectional interactions between sleep and various forms of epileptic activity observed in stereo-electroencephalography recordings. Specifically, epileptic activity is modulated by different sleep stages, peaking in non-rapid eye movement sleep, while being suppressed in rapid eye movement sleep. However, this modulation can vary across different brain regions, underlining the need to account for sleep to accurately pinpoint the epileptogenic zone during presurgical assessments. Finally, we offer practical solutions, such as automated sleep scoring algorithms using stereo-electroencephalography data alone, to seamlessly integrate sleep monitoring into routine clinical practice. It is hoped that this review will provide clinicians with a readily accessible roadmap to the latest evidence concerning the clinical utility of sleep monitoring in the context of stereo-electroencephalography and aid the development of therapeutic and diagnostic strategies to improve patient surgical outcomes.

Development of reciprocal connections between the dorsal lateral geniculate nucleus and the thalamic reticular nucleus

The thalamic reticular nucleus (TRN) serves as an important node between the thalamus and neocortex, regulating thalamocortical rhythms and sensory processing in a state dependent manner. Disruptions in TRN circuitry also figures prominently in several neurodevelopmental disorders including epilepsy, autism, and attentional defects. An understanding of how and when connections between TRN and 1st order thalamic nuclei, such as the dorsal lateral geniculate nucleus (dLGN), develop is lacking. We used the mouse visual thalamus as a model system to study the organization, pattern of innervation and functional responses between TRN and the dLGN. Genetically modified mouse lines were used to visualize and target the feedforward and feedback components of these intra-thalamic circuits and to understand how peripheral input from the retina impacts their development.Retrograde tracing of thalamocortical (TC) afferents through TRN revealed that the modality-specific organization seen in the adult, is present at perinatal ages and seems impervious to the loss of peripheral input. To examine the formation and functional maturation of intrathalamic circuits between the visual sector of TRN and dLGN, we examined when projections from each nuclei arrive, and used an acute thalamic slice preparation along with optogenetic stimulation to assess the maturation of functional synaptic responses. Although thalamocortical projections passed through TRN at birth, feedforward axon collaterals determined by vGluT2 labeling, emerged during the second postnatal week, increasing in density through the third week. Optogenetic stimulation of TC axon collaterals in TRN showed infrequent, weak excitatory responses near the end of week 1. During weeks 2-4, responses became more prevalent, grew larger in amplitude and exhibited synaptic depression during repetitive stimulation. Feedback projections from visual TRN to dLGN began to innervate dLGN as early as postnatal day 2 with weak inhibitory responses emerging during week 1. During week 2-4, inhibitory responses continued to grow larger, showing synaptic depression during repetitive stimulation. During this time TRN inhibition started to suppress TC spiking, having its greatest impact by week 4-6. Using a mutant mouse that lacks retinofugal projections revealed that the absence of retinal input led to an acceleration of TRN innervation of dLGN but had little impact on the development of feedforward projections from dLGN to TRN. Together, these experiments reveal how and when intrathalamic connections emerge during early postnatal ages and provide foundational knowledge to understand the development of thalamocortical network dynamics as well as neurodevelopmental diseases that involve TRN circuitry.

Globus pallidus externus drives increase in network-wide alpha power with propofol-induced loss-of-consciousness in humans

States of consciousness are likely mediated by multiple parallel yet interacting cortico-subcortical recurrent networks. Although the mesocircuit model has implicated the pallidocortical circuit as one such network, this circuit has not been extensively evaluated to identify network-level electrophysiological changes related to loss of consciousness (LOC). We characterize changes in the mesocircuit in awake versus propofol-induced LOC in humans by directly simultaneously recording from sensorimotor cortices (S1/M1) and globus pallidus interna and externa (GPi/GPe) in 12 patients with Parkinson disease undergoing deep brain stimulator implantation. Propofol-induced LOC is associated with increases in local power up to 20 Hz in GPi, 35 Hz in GPe, and 100 Hz in S1/M1. LOC is likewise marked by increased pallidocortical alpha synchrony across all nodes, with increased alpha/low beta Granger causal (GC) flow from GPe to all other nodes. In contrast, LOC is associated with decreased network-wide beta coupling and beta GC from M1 to the rest of the network. Results implicate an important and possibly central role of GPe in mediating LOC-related increases in alpha power, supporting a significant role of the GPe in modulating cortico-subcortical circuits for consciousness. Simultaneous LOC-related suppression of beta synchrony highlights that distinct oscillatory frequencies act independently, conveying unique network activity.

GABAergic neurons of anterior thalamic reticular nucleus regulate states of consciousness in propofol- and isoflurane-mediated general anesthesia

BACKGROUND

The thalamus system plays critical roles in the regulation of reversible unconsciousness induced by general anesthetics, especially the arousal stage of general anesthesia (GA). But the function of thalamus in GA-induced loss of consciousness (LOC) is little known. The thalamic reticular nucleus (TRN) is the only GABAergic neurons-composed nucleus in the thalamus, which is composed of parvalbumin (PV) and somatostatin (SST)-expressing GABAergic neurons. The anterior sector of TRN (aTRN) is indicated to participate in the induction of anesthesia, but the roles remain unclear. This study aimed to reveal the role of the aTRN in propofol and isoflurane anesthesia.

METHODS

We first set up c-Fos straining to monitor the activity variation of aTRNPV and aTRNSST neurons during propofol and isoflurane anesthesia. Subsequently, optogenetic tools were utilized to activate aTRNPV and aTRNSST neurons to elucidate the roles of aTRNPV and aTRNSST neurons in propofol and isoflurane anesthesia. Electroencephalogram (EEG) recordings and behavioral tests were recorded and analyzed. Lastly, chemogenetic activation of the aTRNPV neurons was applied to confirm the function of the aTRN neurons in propofol and isoflurane anesthesia.

RESULTS

c-Fos straining showed that both aTRNPV and aTRNSST neurons are activated during the LOC period of propofol and isoflurane anesthesia. Optogenetic activation of aTRNPV and aTRNSST neurons promoted isoflurane induction and delayed the recovery of consciousness (ROC) after propofol and isoflurane anesthesia, meanwhile chemogenetic activation of the aTRNPV neurons displayed the similar effects. Moreover, optogenetic and chemogenetic activation of the aTRN neurons resulted in the accumulated burst suppression ratio (BSR) during propofol and isoflurane GA, although they represented different effects on the power distribution of EEG frequency.

CONCLUSION

Our findings reveal that the aTRN GABAergic neurons play a critical role in promoting the induction of propofol- and isoflurane-mediated GA.

Characterization of the neural circuitry of the auditory thalamic reticular nucleus and its potential role in salicylate-induced tinnitus

Introduction

Subjective tinnitus, the perception of sound without an external acoustic source, is often subsequent to noise-induced hearing loss or ototoxic medications. The condition is believed to result from neuroplastic alterations in the auditory centers, characterized by heightened spontaneous neural activities and increased synchrony due to an imbalance between excitation and inhibition. However, the role of the thalamic reticular nucleus (TRN), a structure composed exclusively of GABAergic neurons involved in thalamocortical oscillations, in the pathogenesis of tinnitus remains largely unexplored.

Methods

We induced tinnitus in mice using sodium salicylate and assessed tinnitus-like behaviors using the Gap Pre-Pulse Inhibition of the Acoustic Startle (GPIAS) paradigm. We utilized combined viral tracing techniques to identify the neural circuitry involved and employed immunofluorescence and confocal imaging to determine cell types and activated neurons.

Results

Salicylate-treated mice exhibited tinnitus-like behaviors. Our tracing clearly delineated the inputs and outputs of the auditory-specific TRN. We discovered that chemogenetic activation of the auditory TRN significantly reduced the salicylate-evoked rise in c-Fos expression in the auditory cortex.

Discussion

This finding posits the TRN as a potential modulatory target for tinnitus treatment. Furthermore, the mapped sensory inputs to the auditory TRN suggest possibilities for employing optogenetic or sensory stimulations to manipulate thalamocortical activities. The precise mapping of the auditory TRN-mediated neural pathways offers a promising avenue for designing targeted interventions to alleviate tinnitus symptoms.

Individualized temporal patterns dominate cortical upstate and sleep depth in driving human sleep spindle timing

Sleep spindles are critical for memory consolidation and strongly linked to neurological disease and aging. Despite their significance, the relative influences of factors like sleep depth, cortical up/down states, and spindle temporal patterns on individual spindle production remain poorly understood. Moreover, spindle temporal patterns are typically ignored in favor of an average spindle rate. Here, we analyze spindle dynamics in 1008 participants from the Multi-Ethnic Study of Atherosclerosis using a point process framework. Results reveal fingerprint-like temporal patterns, characterized by a refractory period followed by a period of increased spindle activity, which are highly individualized yet consistent night-to-night. We observe increased timing variability with age and distinct gender/age differences. Strikingly, and in contrast to the prevailing notion, individualized spindle patterns are the dominant determinant of spindle timing, accounting for over 70% of the statistical deviance explained by all of the factors we assessed, surpassing the contribution of slow oscillation (SO) phase (~14%) and sleep depth (~16%). Furthermore, we show spindle/SO coupling dynamics with sleep depth are preserved across age, with a global negative shift towards the SO rising slope. These findings offer novel mechanistic insights into spindle dynamics with direct experimental implications and applications to individualized electroencephalography biomarker identification.

Temporal association between sleep spindles and ripples in the human anterior and mediodorsal thalamus

Sleep spindles are major oscillatory components of Non-Rapid Eye Movement (NREM) sleep, reflecting hyperpolarization-rebound sequences of thalamocortical neurons. Reports suggest a link between sleep spindles and several forms of high-frequency oscillations which are considered as expressions of pathological off-line neural plasticity in the central nervous system. Here we investigated the relationship between thalamic sleep spindles and ripples in the anterior and mediodorsal nuclei (ANT and MD) of epilepsy patients. Whole-night LFP from the ANT and MD were co-registered with scalp EEG/polysomnography by using externalized leads in 15 epilepsy patients undergoing a Deep Brain Stimulation protocol. Slow (~12 Hz) and fast (~14 Hz) sleep spindles were present in the human ANT and MD and roughly, 20% of them were associated with ripples. Ripple-associated thalamic sleep spindles were characterized by longer duration and exceeded pure spindles in terms of spindle power as indicated by time-frequency analysis. Furthermore, ripple amplitude was modulated by the phase of sleep spindles within both thalamic nuclei. No signs of pathological processes were correlated with measures of ripple and spindle association, furthermore, the density of ripple-associated sleep spindles in the ANT showed a positive correlation with verbal comprehension. Our findings indicate the involvement of the human thalamus in coalescent spindle-ripple oscillations of NREM sleep.

Functionally Distinct Circuits Are Linked by Heterocellular Electrical Synapses in the Thalamic Reticular Nucleus

The thalamic reticular nucleus (TRN) inhibits sensory thalamocortical relay neurons and is a key regulator of sensory attention as well as sleep and wake states. Recent developments have identified two distinct genetic subtypes of TRN neurons, calbindin-expressing (CB) and somatostatin-expressing (SOM) neurons. These subtypes differ in localization within the TRN, electrophysiological properties, and importantly, targeting of thalamocortical relay channels. CB neurons send inhibition to and receive excitation from first-order thalamic relay nuclei, while SOM neurons send inhibition to and receive excitation from higher-order thalamic areas. These differences create distinct channels of information flow. It is unknown whether TRN neurons form electrical synapses between SOM and CB neurons and consequently bridge first-order and higher-order thalamic channels. Here, we use GFP reporter mice to label and record from CB-expressing and SOM-expressing TRN neurons. We confirm that GFP expression properly differentiates TRN subtypes based on electrophysiological differences, and we identified electrical synapses between pairs of neurons with and without common GFP expression for both CB and SOM types. That is, electrical synapses link both within and across subtypes of neurons in the TRN, forming either homocellular or heterocellular synapses. Therefore, we conclude that electrical synapses within the TRN provide a substrate for functionally linking thalamocortical first-order and higher-order channels within the TRN.

The Differing Effects of Sleep on Ictal and Interictal Network Dynamics in Drug-Resistant Epilepsy

OBJECTIVE

Sleep has important influences on focal interictal epileptiform discharges (IEDs), and the rates and spatial extent of IEDs are increased in non-rapid eye movement (NREM) sleep. In contrast, the influence of sleep on seizures is less clear, and its effects on seizure topography are poorly documented. We evaluated the influences of NREM sleep on ictal spatiotemporal dynamics and contrasted these with interictal network dynamics.

METHODS

We included patients with drug-resistant focal epilepsy who underwent continuous intracranial electroencephalography (iEEG) with depth electrodes. Patients were selected if they had 1 to 3 seizures from each vigilance state, wakefulness and NREM sleep, within a 48-hour window, and under the same antiseizure medication. A 10-minute epoch of the interictal iEEG was selected per state, and IEDs were detected automatically. A total of 25 patients (13 women; aged 32.5 ± 7.1 years) were included.

RESULTS

The seizure onset pattern, duration, spatiotemporal propagation, and latency of ictal high-frequency activity did not differ significantly between wakefulness and NREM sleep (all p > 0.05). In contrast, IED rates and spatial distribution were increased in NREM compared with wakefulness (p < 0.001, Cliff's d = 0.48 and 0.49). The spatial overlap between vigilance states was higher for seizures (57.1 ± 40.1%) than IEDs (41.7 ± 46.2%; p = 0.001, Cliff's d = 0.51).

INTERPRETATION

In contrast to its effects on IEDs, NREM sleep does not affect ictal spatiotemporal dynamics. This suggests that once the brain surpasses the seizure threshold, it will follow the underlying epileptic network irrespective of the vigilance state. These findings offer valuable insights into neural network dynamics in epilepsy and have important clinical implications for localizing seizure foci. ANN NEUROL 2023.

Preliminary evidence of a relationship between sleep spindles and treatment response in epileptic encephalopathy

OBJECTIVE

Epileptic encephalopathy with spike-wave activation in sleep (EE-SWAS) is a challenging neurodevelopmental disease characterized by abundant epileptiform spikes during non-rapid eye movement (NREM) sleep accompanied by cognitive dysfunction. The mechanism of cognitive dysfunction is unknown, but treatment with high-dose diazepam may improve symptoms. Spike rate does not predict treatment response, but spikes may disrupt sleep spindles. We hypothesized that in patients with EE-SWAS: (1) spikes and spindles would be anti-correlated, (2) high-dose diazepam would increase spindles and decrease spikes, and (3) spindle response would be greater in those with cognitive improvement.

METHODS

Consecutive EE-SWAS patients treated with high-dose diazepam that met the criteria were included. Using a validated automated spindle detector, spindle rate, duration, and percentage were computed in pre- and post-treatment NREM sleep. Spikes were quantified using a validated automated spike detector. The cognitive response was determined from a chart review.

RESULTS

Spindle rate was anti-correlated with the spike rate in the channel with the maximal spike rate (p = 0.002) and averaged across all channels (p = 0.0005). Spindle rate, duration, and percentage each increased, and spike rate decreased, after high-dose diazepam treatment (p ≤ 2e-5, all tests). Spindle rate, duration, and percentage (p ≤ 0.004, all tests) were increased in patients with cognitive improvement after treatment, but not those without. Changes in spindle rate but not changes in spike rate distinguished between groups.

INTERPRETATION

These findings confirm thalamocortical disruption in EE-SWAS, identify a mechanism through which benzodiazepines may support cognitive recovery, and introduce sleep spindles as a promising mechanistic biomarker to detect treatment response in severe epileptic encephalopathies.

Sleep spindles in primates: Modeling the effects of distinct laminar thalamocortical connectivity in core, matrix, and reticular thalamic circuits

Sleep spindles are associated with the beginning of deep sleep and memory consolidation and are disrupted in schizophrenia and autism. In primates, distinct core and matrix thalamocortical (TC) circuits regulate sleep spindle activity through communications that are filtered by the inhibitory thalamic reticular nucleus (TRN); however, little is known about typical TC network interactions and the mechanisms that are disrupted in brain disorders. We developed a primate-specific, circuit-based TC computational model with distinct core and matrix loops that can simulate sleep spindles. We implemented novel multilevel cortical and thalamic mixing, and included local thalamic inhibitory interneurons, and direct layer 5 projections of variable density to TRN and thalamus to investigate the functional consequences of different ratios of core and matrix node connectivity contribution to spindle dynamics. Our simulations showed that spindle power in primates can be modulated based on the level of cortical feedback, thalamic inhibition, and engagement of model core versus matrix, with the latter having a greater role in spindle dynamics. The study of the distinct spatial and temporal dynamics of core-, matrix-, and mix-generated sleep spindles establishes a framework to study disruption of TC circuit balance underlying deficits in sleep and attentional gating seen in autism and schizophrenia.

Pallidal GABA B receptors: involvement in cortex beta dynamics and thalamic reticular nucleus activity

The external globus pallidus (GP) firing rate synchronizes the basal ganglia-thalamus-cortex network controlling GABAergic output to different nuclei. In this context, two findings are significant: the activity and GABAergic transmission of the GP modulated by GABA B receptors and the presence of the GP-thalamic reticular nucleus (RTn) pathway, the functionality of which is unknown. The functional participation of GABA B receptors through this network in cortical dynamics is feasible because the RTn controls transmission between the thalamus and cortex. To analyze this hypothesis, we used single-unit recordings of RTn neurons and electroencephalograms of the motor cortex (MCx) before and after GP injection of the GABA B agonist baclofen and the antagonist saclofen in anesthetized rats. We found that GABA B agonists increase the spiking rate of the RTn and that this response decreases the spectral density of beta frequency bands in the MCx. Additionally, injections of GABA B antagonists decreased the firing activity of the RTn and reversed the effects in the power spectra of beta frequency bands in the MCx. Our results proved that the GP modulates cortical oscillation dynamics through the GP-RTn network via tonic modulation of RTn activity.

Preliminary evidence of a relationship between sleep spindles and treatment response in epileptic encephalopathy

Objective

Epileptic encephalopathy with spike wave activation in sleep (EE-SWAS) is a challenging neurodevelopmental disease characterized by abundant epileptiform spikes during non-rapid eye movement (NREM) sleep accompanied by cognitive dysfunction. The mechanism of cognitive dysfunction is unknown, but treatment with high-dose diazepam may improve symptoms. Spike rate does not predict treatment response, but spikes may disrupt sleep spindles. We hypothesized that in patients with EE-SWAS: 1) spikes and spindles would be anticorrelated, 2) high-dose diazepam would increase spindles and decrease spikes, and 3) spindle response would be greater in those with cognitive improvement.

Methods

Consecutive EE-SWAS patients treated with high-dose diazepam that met criteria were included. Using a validated automated spindle detector, spindle rate, duration, and percentage were computed in pre- and post-treatment NREM sleep. Spikes were quantified using a validated automated spike detector. Cognitive response was determined from chart review.

Results

Spindle rate was anticorrelated with spike rate in the channel with the maximal spike rate ( p =0.002) and averaged across all channels ( p =0.0005). Spindle rate, duration, and percentage each increased, and spike rate decreased, after high-dose diazepam treatment ( p≤ 2e-5, all tests). Spindle rate, duration, and percentage ( p ≤0.004, all tests) were increased in patients with cognitive improvement after treatment, but not those without. Changes in spike rate did not distinguish between groups.

Interpretation

These findings confirm thalamocortical disruption in EE-SWAS, identify a mechanism through which benzodiazepines may support cognitive recovery, and introduce sleep spindles as a promising mechanistic biomarker to detect treatment response in severe epileptic encephalopathies.

GluN2D subunit-containing NMDA receptors regulate reticular thalamic neuron function and seizure susceptibility

Thalamic regulation of cortical function is important for several behavioral aspects including attention and sensorimotor control. This region has also been studied for its involvement in seizure activity. Among the NMDA receptor subunits GluN2C and GluN2D are particularly enriched in several thalamic nuclei including nucleus reticularis of the thalamus (nRT). We have previously found that GluN2C deletion does not have a strong influence on the basal excitability and burst firing characteristics of reticular thalamus neurons. Here we find that GluN2D ablation leads to reduced depolarization-induced spike frequency and reduced hyperpolarization-induced rebound burst firing in nRT neurons. Furthermore, reduced inhibitory neurotransmission was observed in the ventrobasal thalamus (VB). A model with preferential downregulation of GluN2D from parvalbumin (PV)-positive neurons was generated. Conditional deletion of GluN2D from PV neurons led to a decrease in excitability and burst firing. In addition, reduced excitability and burst firing was observed in the VB neurons together with reduced inhibitory neurotransmission. Finally, young mice with GluN2D downregulation in PV neurons showed significant resistance to pentylenetetrazol-induced seizure and differences in sensitivity to isoflurane anesthesia but were normal in other behaviors. Conditional deletion of GluN2D from PV neurons also affected expression of other GluN2 subunits and GABA receptor in the nRT. Together, these results identify a unique role of GluN2D-containing receptors in the regulation of thalamic circuitry and seizure susceptibility which is relevant to mutations in GRIN2D gene found to be associated with pediatric epilepsy.

Thalamic control of sensory processing and spindles in a biophysical somatosensory thalamoreticular circuit model of wakefulness and sleep

Thalamoreticular circuitry plays a key role in arousal, attention, cognition, and sleep spindles, and is linked to several brain disorders. A detailed computational model of mouse somatosensory thalamus and thalamic reticular nucleus has been developed to capture the properties of over 14,000 neurons connected by 6 million synapses. The model recreates the biological connectivity of these neurons, and simulations of the model reproduce multiple experimental findings in different brain states. The model shows that inhibitory rebound produces frequency-selective enhancement of thalamic responses during wakefulness. We find that thalamic interactions are responsible for the characteristic waxing and waning of spindle oscillations. In addition, we find that changes in thalamic excitability control spindle frequency and their incidence. The model is made openly available to provide a new tool for studying the function and dysfunction of the thalamoreticular circuitry in various brain states.

The adjustment mechanism of the spike and wave discharges in thalamic neurons: a simulation analysis

Different from many previous theoretical studies, this paper explores the regulatory mechanism of the spike and wave discharges (SWDs) in the reticular thalamic nucleus (TRN) by a dynamic computational model. We observe that the SWDs appears in the TRN by changing the coupling weights and delays in the thalamocortical circuit. The abundant poly-spikes wave discharges is also induced when the delay increases to large enough. These discharges can be inhibited by tuning the inhibitory output from the basal ganglia to the thalamus. The mechanisms of these waves can be explained in this model together with simulation results, which are different from the mechanisms in the cortex. The TRN is an important target in treating epilepsy, and the results may be a theoretical evidence for experimental study in the future.

The effect of interictal epileptic discharges and following spindles on motor sequence learning in epilepsy patients

PURPOSE

Interictal epileptic discharges (IEDs) are known to affect cognitive function in patients with epilepsy, but the mechanism has not been elucidated. Sleep spindles appearing in synchronization with IEDs were recently demonstrated to impair memory consolidation in rat, but this has not been investigated in humans. On the other hand, the increase of sleep spindles at night after learning is positively correlated with amplified learning effects during sleep for motor sequence learning. In this study, we examined the effects of IEDs and IED-coupled spindles on motor sequence learning in patients with epilepsy, and clarified their pathological significance.

MATERIALS AND METHODS

Patients undergoing long-term video-electroencephalography (LT-VEEG) at our hospital from June 2019 to November 2021 and age-matched healthy subjects were recruited. Motor sequence learning consisting of a finger-tapping task was performed before bedtime and the next morning, and the improvement rate of performance was defined as the sleep-dependent learning effect. We searched for factors associated with the changes in learning effect observed between the periods of when antiseizure medications (ASMs) were withdrawn for LT-VEEG and when they were returned to usual doses after LT-VEEG.

RESULTS

Excluding six patients who had epileptic seizures at night after learning, nine patients and 11 healthy subjects were included in the study. In the patient group, there was no significant learning effect when ASMs were withdrawn. The changes in learning effect of the patient group during ASM withdrawal were not correlated with changes in sleep duration or IED density; however, they were significantly negatively correlated with changes in IED-coupled spindle density.

CONCLUSION

We found that the increase of IED-coupled spindles correlated with the decrease of sleep-dependent learning effects of procedural memory. Pathological IED-coupled sleep spindles could hinder memory consolidation, that is dependent on physiological sleep spindles, resulting in cognitive dysfunction in patients with epilepsy.

Spontaneous Cell Cluster Formation in Human iPSC-Derived Neuronal Spheroid Networks Influences Network Activity

Three-dimensional neuronal culture systems such as spheroids, organoids, and assembloids constitute a branch of neuronal tissue engineering that has improved our ability to model the human brain in the laboratory. However, the more elaborate the brain model, the more difficult it becomes to study functional properties such as electrical activity at the neuronal level, similar to the challenges of studying neurophysiology in vivo We describe a simple approach to generate self-assembled three-dimensional neuronal spheroid networks with defined human cell composition on microelectrode arrays. Such spheroid networks develop a highly three-dimensional morphology with cell clusters up to 60 µm in thickness and are interconnected by pronounced bundles of neuronal fibers and glial processes. We could reliably record from up to hundreds of neurons simultaneously per culture for ≤90 d. By quantifying the formation of these three-dimensional structures over time, while regularly monitoring electrical activity, we were able to establish a strong link between spheroid morphology and network activity. In particular, the formation of cell clusters accelerates formation and maturation of correlated network activity. Astrocytes both influence electrophysiological network activity as well as accelerate the transition from single cell layers to cluster formation. Higher concentrations of astrocytes also have a strong effect of modulating synchronized network activity. This approach thus represents a practical alternative to often complex and heterogeneous organoids, providing easy access to activity within a brain-like 3D environment.Significance StatementNeuronal "organoid" cultures with multiple cell types grown on elaborate three-dimensional scaffolds have become popular tools to generate brain-like properties in vitro but bring with them similar problems concerning access to physiological function as real brain tissue. Here, we developed a new approach to form simple brain-like spheroid networks from human neurons, but using the normal supporting cells of the brain, astrocytes, as the scaffold. By growing these cultures on conventional microelectrode arrays, we were able to observe development of complex patterns of electrical activity for months. Our results highlight how formation of three-dimensional structures accelerated the formation of synchronized neuronal network activity and provide a promising new simple model system for studying interactions between known human cell types in vitro.

The human thalamus orchestrates neocortical oscillations during NREM sleep

A hallmark of non-rapid eye movement sleep is the coordinated interplay of slow oscillations (SOs) and sleep spindles. Traditionally, a cortico-thalamo-cortical loop is suggested to coordinate these rhythms: neocortically-generated SOs trigger spindles in the thalamus that are projected back to neocortex. Here, we used intrathalamic recordings from human epilepsy patients to test this canonical interplay. We show that SOs in the anterior thalamus precede neocortical SOs (peak −50 ms), whereas concurrently-recorded SOs in the mediodorsal thalamus are led by neocortical SOs (peak +50 ms). Sleep spindles, detected in both thalamic nuclei, preceded their neocortical counterparts (peak −100 ms) and were initiated during early phases of thalamic SOs. Our findings indicate an active role of the anterior thalamus in organizing sleep rhythms in the neocortex and highlight the functional diversity of thalamic nuclei in humans. The thalamic coordination of sleep oscillations could have broad implications for the mechanisms underlying memory consolidation.

Slow oscillations, which are instrumental to memory consolidation, have been assumed to be solely generated in neocortex. Here, the authors show that the anterior thalamus might play a fundamental role in organizing slow oscillations in human sleep.

High-density EEG sleep correlates of cognitive and affective impairment at 12-month follow-up after COVID-19

Objective

To disentangle the pathophysiology of cognitive/affective impairment in Coronavirus Disease-2019 (COVID-19), we studied long-term cognitive and affective sequelae and sleep high-density electroencephalography (EEG) at 12-month follow-up in people with a previous hospital admission for acute COVID-19.

Methods

People discharged from an intensive care unit (ICU) and a sub-intensive ward (nonICU) between March and May 2020 were contacted between March and June 2021. Participants underwent cognitive, psychological, and sleep assessment. High-density EEG recording was acquired during a nap. Slow and fast spindles density/amplitude/frequency and source reconstruction in brain gray matter were extracted. The relationship between psychological and cognitive findings was explored with Pearson correlation.

Results

We enrolled 33 participants ( 17 nonICU) and 12 controls. We observed a lower Physical Quality of Life index, higher post-traumatic stress disorder (PTSD) score, and a worse executive function performance in nonICU participants. Higher PTSD and Beck Depression Inventory scores correlated with lower executive performance. The same group showed a reorganization of spindle cortical generators.

Conclusions

Our results show executive and psycho-affective deficits and spindle alterations in COVID-19 survivors – especially in nonICU participants – after 12 months from discharge.

Significance

These findings may be suggestive of a crucial contribution of stress experienced during hospital admission on long-term cognitive functioning.

Pretreatment electroencephalographic features in patients with childhood absence epilepsy

OBJECTIVE

To analyze the ictal and interictal electroencephalographic (EEG) features in newly diagnosed childhood absence epilepsy (CAE) and determine the association between seizure onset topography, interictal focal spike-wave discharges (FSWDs) and accompanying clinical features of absence seizures.

METHODS

The authors searched the EEG database for a definite diagnosis of CAE according to ILAE 2017 criteria. Video-EEGs of untreated pediatric patients during sleep and wakefulness were evaluated retrospectively.

RESULTS

The study included 47 patients (25 males, 22 females). Interictal FSWDs were observed in 49% of patients with CAE during wakefulness and in 85.1% during sleep (p = 0.001). Interictal FSWDs were most frequently observed in the frontal regions (awake: 34%; asleep: 74.5%), followed by the posterior temporoparietooccipital region (awake: 21.2%; asleep: 36.1%), and the centrotemporal region (awake: 6.4%; asleep: 8.5%). Eleven patients (23.4%) had polyspikes during sleep. Both bilateral symmetric and asymmetric seizure onset were noted in 32%, whereas focal seizure onset was observed in 14.9% of the patients. Absence seizures with and without motor components were seen in 72.3% and 61.7% of patients, respectively, and in 33% of patients both occurred. There were no associations between the existence of interictal FSWDs, focal/asymmetric seizure onset, and absence seizures with and/or without motor components.

CONCLUSION

Asymmetric and/or focal seizure onset, interictal FSWDs, and absence seizures with motor components are commonly observed in drug-naive CAE. This study found no association between seizure onset topography, interictal FSWDs, and semiological features of absence seizures.

Elevation of GABA levels in the globus pallidus disinhibits the thalamic reticular nucleus and desynchronized cortical beta oscillations

The external globus pallidus (GP) is a GABAergic node involved in motor control regulation and coordinates firing and synchronization in the basal ganglia-thalamic-cortical network through inputs and electrical activity. In Parkinson's disease, high GABA levels alter electrical activity in the GP and contribute to motor symptoms. Under normal conditions, GABA levels are regulated by GABA transporters (GATs). GAT type 1 (GAT-1) is highly expressed in the GP, and pharmacological blockade of GAT-1 increases the duration of currents mediated by GABA A receptors and induces tonic inhibition. The functional contribution of the pathway between the GP and the reticular thalamic nucleus (RTn) is unknown. This pathway is important since the RTn controls the flow of information between the thalamus and cortex, suggesting that it contributes to cortical dynamics. In this work, we investigated the effect of increased GABA levels on electrical activity in the RTn by obtaining single-unit extracellular recordings from anesthetized rats and on the motor cortex (MCx) by corticography. Our results show that high GABA levels increase the spontaneous activity rate of RTn neurons and desynchronize oscillations in the beta frequency band in the MCx. Our findings provide evidence that the GP exerts tonic control over RTn activity through the GP-reticular pathway and functionally contributes to cortical oscillation dynamics.

The Interaction Between Sleep and Epilepsy

PURPOSE OF REVIEW

To review the mutual interactions between sleep and epilepsy, including mechanisms of epileptogenesis, the relationship between sleep apnea and epilepsy, and potential strategies to treat seizures.

RECENT FINDINGS

Recent studies have highlighted the role of functional network systems underlying epileptiform activation in sleep in several epilepsy syndromes, including absence epilepsy, benign focal childhood epilepsy, and epileptic encephalopathy with spike-wave activation in sleep. Sleep disorders are common in epilepsy, and early recognition and treatment can improve seizure frequency and potentially reduce SUDEP risk. Additionally, epilepsy is associated with cyclical patterns, which has led to new treatment approaches including chronotherapy, seizure monitoring devices, and seizure forecasting. Adenosine kinase and orexin receptor antagonists are also promising new potential drug targets that could be used to treat seizures. Sleep and epilepsy have a bidirectional relationship that intersects with many aspects of clinical management. In this article, we identify new areas of research involving future therapeutic opportunities in the field of epilepsy.

Sleep spindles changes in people with previous COVID-19 infection

Stage 2 sleep spindles are considered useful biomarkers for the integrity of the central nervous system and for cognitive and memory skills. We investigated sleep spindles patterns in subjects after 12 months of their hospitalization in the intensive care unit (ICU) of the Padova Teaching Hospital due to COVID-19 between March and November 2020. Before the nap, participants (13 hospitalized in ICU - ICU; 9 hospitalized who received noninvasive ventilation - nonlCU; 9 age and sex-matched healthy controls - CTRL, i.e., not infected by COVID-19) underwent a cognitive and psychological as-sessment. During the nap, high-density electroencephalography (EEG) recordings were acquired. Slow (i.e., [9]-[12] Hz) and fast (i.e.,]12-16] Hz) spindles were automatically detected. Spindle density and spindle source reconstruction in brain grey matter were extracted. The psychological assessment revealed a statistical difference comparing CTRL and nonlCU in Beck Depression Inventory score and in the Physical Quality of Life index (p_value = 0.03). The cognitive assessment revealed a trend of worsening results in executive functions in COVID-19 survivors. Slow spindle density significantly decreased comparing CTRL to COVID-19 survivors (p_value= 0.001). There were statistically significant differences in EEG source-waveforms fast spindle amplitude onset among the three groups, mainly between CTRL and nonlCU. Clinical Relevance- Our results suggest that nonlCU were more susceptible to the hospitalization experience than ICU participants with a slight effect on cognitive tests. This impacted the spindle generation revealing a decreased density of slow spindles and affecting the generators of fast spindles in COVID-19 survivors especially in nonlCU.

On the Diverse Functions of Electrical Synapses

Electrical synapses are the neurophysiological product of gap junctional pores between neurons that allow bidirectional flow of current between neurons. They are expressed throughout the mammalian nervous system, including cortex, hippocampus, thalamus, retina, cerebellum, and inferior olive. Classically, the function of electrical synapses has been associated with synchrony, logically following that continuous conductance provided by gap junctions facilitates the reduction of voltage differences between coupled neurons. Indeed, electrical synapses promote synchrony at many anatomical and frequency ranges across the brain. However, a growing body of literature shows there is greater complexity to the computational function of electrical synapses. The paired membranes that embed electrical synapses act as low-pass filters, and as such, electrical synapses can preferentially transfer spike after hyperpolarizations, effectively providing spike-dependent inhibition. Other functions include driving asynchronous firing, improving signal to noise ratio, aiding in discrimination of dissimilar inputs, or dampening signals by shunting current. The diverse ways by which electrical synapses contribute to neuronal integration merits furthers study. Here we review how functions of electrical synapses vary across circuits and brain regions and depend critically on the context of the neurons and brain circuits involved. Computational modeling of electrical synapses embedded in multi-cellular models and experiments utilizing optical control and measurement of cellular activity will be essential in determining the specific roles performed by electrical synapses in varying contexts.

Research progress of the paraventricular thalamus in the regulation of sleep-wake and emotional behaviors

The paraventricular thalamus (PVT) is a major component of the midline structure of the thalamus. It is one of the nonspecific nuclei of the thalamus, which plays a great role in the regulation of cortical arousal. PVT, an important node in the central nervous system, sends widespread outputs to many brain regions and also accepts plentiful inputs from many brain regions to modulate diverse functions, including sleep-wake state, attention, memory, and pain. Recently, with the increasing prevalence of sleep disorders and mood disorders, people pay great attention to PVT, which was implicated in arousal and emotional behaviors. Therefore, the main purpose of this review is to illustrate the characteristic of PVT to provide a reference for future research.

Tonic serotonergic input increases the burst firing mode and diminishes the firing rate of reticular thalamic nucleus neurons through 5-HT1A receptors activation in anesthetized rats

The reticular thalamic nucleus (RTn) is a thin shell of GABAergic neurons that covers the dorsal thalamus that regulate the global activity of all thalamic nuclei. RTn controls the flow of information between thalamus and cerebral cortex since it receives glutamatergic information from collaterals of thalamo-cortical (TCs) and cortico-thalamic neurons. It also receives aminergic information from several brain stem nuclei, including serotonergic fibers originated in the dorsal raphe nucleus. RTn neurons express serotonergic receptors including the 5-HT1A subtype, however, the role of this receptor in the RTn electrical activity has been scarcely analyzed. In this work, we recorded in vivo the unitary spontaneous electrical activity of RTn neurons in anesthetized rats; our study aimed to obtain information about the effects of 5-HT1A receptors in RTn neurons. Local application of fluoxetine (a serotonin reuptake inhibitor) increases burst firing index accompanied by a decrease in the basal spiking rate. Local application of different doses of serotonin and 8-OH-DPAT (a specific 5-HT1A receptor agonist) causes a similar response to fluoxetine effects. Local 5-HT1A receptors blockade produces opposite effects and suppresses the effect by 8-OH-DPAT. Our findings indicate the presence of a serotonergic tonic discharge in the RTn that increases the burst firing index and simultaneously decreases the basal spiking frequency through 5-HT1A receptors activation.

GABABR Modulation of Electrical Synapses and Plasticity in the Thalamic Reticular Nucleus

Two distinct types of neuronal activity result in long-term depression (LTD) of electrical synapses, with overlapping biochemical intracellular signaling pathways that link activity to synaptic strength, in electrically coupled neurons of the thalamic reticular nucleus (TRN). Because components of both signaling pathways can also be modulated by GABA_B receptor activity, here we examined the impact of GABA_B receptor activation on the two established inductors of LTD in electrical synapses. Recording from patched pairs of coupled rat neurons in vitro, we show that GABA_B receptor inactivation itself induces a modest depression of electrical synapses and occludes LTD induction by either paired bursting or metabotropic glutamate receptor (mGluR) activation. GABA_B activation also occludes LTD from either paired bursting or mGluR activation. Together, these results indicate that afferent sources of GABA, such as those from the forebrain or substantia nigra to the reticular nucleus, gate the induction of LTD from either neuronal activity or afferent glutamatergic receptor activation. These results add to a growing body of evidence that the regulation of thalamocortical transmission and sensory attention by TRN is modulated and controlled by other brain regions. Significance: We show that electrical synapse plasticity is gated by GABA_B receptors in the thalamic reticular nucleus. This effect is a novel way for afferent GABAergic input from the basal ganglia to modulate thalamocortical relay and is a possible mediator of intra-TRN inhibitory effects.

Forced Normalization Revisited: New Concepts About a Paradoxical Phenomenon

The phenomenon of Forced Normalization (FN) was first described by Landolt in 1953, who described the disappearance of epileptiform discharges in the EEG of patients with epilepsy, concomitant with the development of psychotic symptoms. Later, Tellenbach coined the term “alternative psychosis” referring specifically to the alternation between clinical phenomena. Finally, in 1991, Wolf observed a degenerative process involved in the phenomenon, which he called “paradoxical normalization.” Initially, FN was explained through experimental models in animals and the demonstration of the kindling phenomenon, in its electrical and pharmacological subdivisions. At this stage of research on the epileptic phenomenon, repetitive electrical stimuli applied to susceptible regions of the brain (hippocampus and amygdala) were considered to explain the pathophysiological basis of temporal lobe epileptogenesis. Likewise, through pharmacological manipulation, especially of dopaminergic circuits, psychiatric comorbidities began to find their basic mechanisms. With the development of new imaging techniques (EEG/fMRI), studies in the area started to focus on the functional connectivity (FC) of different brain regions with specific neuronal networks, which govern emotions. Thus, a series of evidence was produced relating the occurrence of epileptic discharges in the limbic system and their consequent coactivation and deactivation of these resting-state networks. However, there are still many controversies regarding the basic mechanisms of network alterations related to emotional control, which will need to be studied with a more homogeneous methodology, in order to try to explain this interesting neuropsychiatric phenomenon with greater accuracy.

Diazepam induced sleep spindle increase correlates with cognitive recovery in a child with epileptic encephalopathy

BACKGROUND

Continuous spike and wave of sleep with encephalopathy (CSWS) is a rare and severe developmental electroclinical epileptic encephalopathy characterized by seizures, abundant sleep activated interictal epileptiform discharges, and cognitive regression or deceleration of expected cognitive growth. The cause of the cognitive symptoms is unknown, and efforts to link epileptiform activity to cognitive function have been unrevealing. Converging lines of evidence implicate thalamocortical circuits in these disorders. Sleep spindles are generated and propagated by the same thalamocortical circuits that can generate spikes and, in healthy sleep, support memory consolidation. As such, sleep spindle deficits may provide a physiologically relevant mechanistic biomarker for cognitive dysfunction in epileptic encephalopathies.

CASE PRESENTATION

We describe the longitudinal course of a child with CSWS with initial cognitive regression followed by dramatic cognitive improvement after treatment. Using validated automated detection algorithms, we analyzed electroencephalograms for epileptiform discharges and sleep spindles alongside contemporaneous neuropsychological evaluations over the course of the patient's disease. We found that sleep spindles increased dramatically with high-dose diazepam treatment, corresponding with marked improvements in cognitive performance. We also found that the sleep spindle rate was anticorrelated to spike rate, consistent with a competitively shared underlying thalamocortical circuitry.

CONCLUSIONS

Epileptic encephalopathies are challenging electroclinical syndromes characterized by combined seizures and a deceleration or regression in cognitive skills over childhood. This report identifies thalamocortical circuit dysfunction in a case of epileptic encephalopathy and motivates future investigations of sleep spindles as a biomarker of cognitive function and a potential therapeutic target in this challenging disease.

Optogenetic activation of the reticular nucleus of the thalamus attenuates limbic seizures via inhibition of the midline thalamus

OBJECTIVE

The nucleus reticularis of the thalamus (nRT) is most studied in epilepsy for its role in the genesis of absence seizures; much less is known regarding its role in other seizure types, including those originating in limbic structures and the temporal lobe. As it is a major source of inhibitory input to higher order thalamic nuclei, stimulation of the nRT may be an effective strategy to disrupt seizure activity that requires thalamic engagement.

METHODS

We recorded single unit activity from the nRT prior to and after infusion of bicuculline into the area tempestas. We monitored single unit activity time-locked with interictal spikes. We optogenetically activated the nRT in both the area tempestas and amygdala kindling models. We tested a role for projections from the nRT to higher order midline thalamic nuclei through the use of retrogradely trafficked viral vector.

RESULTS

Mean firing rate in the nRT was decreased after infusion of bicuculline into the area tempestas as compared to the preinfusion baseline. nRT unit firing in response to interictal spikes was heterogeneous, with an approximately equal proportion of neurons displaying (1) no change in firing, (2) increased firing, and (3) decreasing firing. Optogenetic activation of the nRT significantly suppressed seizure activity in both the area tempestas and amygdala kindling models. Optogenetic activation of contralaterally targeting projections but not ipsilaterally targeting projections from the nRT to the midline thalamus significantly suppressed seizures in the kindling model.

SIGNIFICANCE

Although the nRT is typically thought of in the context of absence seizures, our data show that it may be a viable target for other seizure types. In two models that recapitulate the seizure types seen in temporal lobe epilepsy, nRT activation suppressed both electrographic and behavioral seizures. These data suggest that the nRT should be considered more broadly in the context of epilepsy.

Short-term changes in cortical physiological arousal measured by electroencephalography during thalamic centromedian deep brain stimulation

OBJECTIVE

The intralaminar thalamus is well implicated in the processes of arousal and attention. Stimulation of the intralaminar thalamus has been used therapeutically to improve level of alertness in minimally conscious individuals and to reduce seizures in refractory epilepsy, both presumably through modulation of thalamocortical function. Little work exists that directly measures the effects of intralaminar thalamic stimulation on cortical physiological arousal in humans. Therefore, our goal was to quantify cortical physiological arousal in individuals with epilepsy receiving thalamic intralaminar deep brain stimulation.

METHODS

We recorded scalp electroencephalogram (EEG) during thalamic intralaminar centromedian (CM) nucleus stimulation in 11 patients with medically refractory epilepsy. Participants underwent stimulation at 130 Hz and 300 µs for periods of 5 min alternating with 5 min of rest while stimulus voltage was titrated upward from 1 to 5 V. EEG signal power was analyzed in different frequency ranges in relation to stimulus strength and time.

RESULTS

We found a progressive increase in broadband gamma (25-100 Hz) cortical EEG power (F = 7.64, p < .05) and decrease in alpha (8-13 Hz) power (F = 4.37, p < .05) with thalamic CM stimulation. Topographic maps showed these changes to be widely distributed across the cortical surface rather than localized to one region.

SIGNIFICANCE

Previous work has shown that broadband increases in gamma frequency power and decreases in alpha frequency power are generally associated with states of cortical activation and increased arousal/attention. Our observed changes therefore support the possible role of cortical activation and increased physiological arousal in therapeutic effects of intralaminar thalamic stimulation for improving both epilepsy and attention. Further investigations with this approach may lead to methods for determining optimal deep brain stimulation parameters to improve clinical outcome in these disorders.

Thalamocortical Spectral Transmission Relies on Balanced Input Strengths

The thalamus is a key element of sensory transmission in the brain, as it gates and selects sensory streams through a modulation of its internal activity. A preponderant role in these functions is played by its internal activity in the alpha range ([8–14] Hz), but the mechanism underlying this process is not completely understood. In particular, how do thalamocortical connections convey stimulus driven information selectively over the back-ground of thalamic internally generated activity? Here we investigate this issue with a spiking network model of feedforward connectivity between thalamus and primary sensory cortex reproducing the local field potential of both areas. We found that in a feedforward network, thalamic oscillations in the alpha range do not entrain cortical activity for two reasons: (i) alpha range oscillations are weaker in neurons projecting to the cortex, (ii) the gamma resonance dynamics of cortical networks hampers oscillations over the 10–20 Hz range thus weakening alpha range oscillations. This latter mechanism depends on the balance of the strength of thalamocortical connections toward excitatory and inhibitory neurons in the cortex. Our results highlight the relevance of corticothalamic feedback to sustain alpha range oscillations and pave the way toward an integrated understanding of the sensory streams traveling between the periphery and the cortex.

Deep Versus Lobar Intraparenchymal Hemorrhage: Seizures, Hyperexcitable Patterns, and Clinical Outcomes

OBJECTIVES

To compare electrographic seizures, hyperexcitable patterns, and clinical outcomes in lobar and deep intraparenchymal hemorrhage. Additionally, to characterize electrographic seizure and hyperexcitable pattern predictors in each group and determine seizure risk with thalamic involvement.

DESIGN

Retrospective cohort study.

SETTING

Tertiary academic medical center.

PATIENTS

Consecutive adult patients with nontraumatic intraparenchymal hemorrhage undergoing continuous electroencephalography at our center between January 2013 and December 2016.

INTERVENTIONS

Not applicable.

MEASUREMENTS AND MAIN RESULTS

Based on head CT closest to the initial continuous electroencephalography session, we classified intraparenchymal hemorrhage as isolated deep (no insular, subarachnoid, subdural extension) or lobar. Hyperexcitable patterns included the following: periodic discharges, spike-wave complexes, any rhythmic delta other than generalized. We used Fisher exact test for categorical and Mann-Whitney U test for continuous variables. Multivariable regression identified predictors of electrographic seizures, hyperexcitable patterns, and poor outcomes (score of 1-2 on Glasgow Outcome Scale) in lobar intraparenchymal hemorrhage. The cohort comprised of 128 patients, 88 lobar, and 40 deep intraparenchymal hemorrhage. Electrographic seizures occurred in 17% of lobar and 5% of deep intraparenchymal hemorrhage (p = 0.09). Hyperexcitable patterns were more frequent in the lobar group (44.3% vs 17.5%; p = 0.005). In multivariable analyses in the lobar group, lateralized rhythmic delta activity predicted electrographic seizures (odds ratio, 6.24; CI, 1.49-26.08; p = 0.012); insular involvement predicted hyperexcitable patterns (odds ratio, 4.88; CI, 1.36-17.57; p = 0.015); coma, temporal lobe involvement, intraparenchymal hemorrhage volume, and electrographic seizures predicted poor outcome. Thalamic involvement did not affect electrographic seizures or hyperexcitable patterns in either group.

CONCLUSIONS

Electrographic seizures are frequent in lobar intraparenchymal hemorrhage, occurring in one in six monitored patients, as opposed to only 5% in isolated deep intraparenchymal hemorrhage not extending to cortex/insula, subarachnoid, or subdural spaces. Patients with lobar intraparenchymal hemorrhage and lateralized rhythmic delta activity were six times as likely to have electrographic seizures, which were associated with 5.47 higher odds of a poor outcome. Coma, temporal lobe involvement, hematoma volume, and electrographic seizures predicted poor outcome in lobar intraparenchymal hemorrhage.

Shared genetic basis between genetic generalized epilepsy and background electroencephalographic oscillations

Objective

Paroxysmal epileptiform abnormalities on electroencephalography (EEG) are the hallmark of epilepsies, but it is uncertain to what extent epilepsy and background EEG oscillations share neurobiological underpinnings. Here, we aimed to assess the genetic correlation between epilepsy and background EEG oscillations.

Methods

Confounding factors, including the heterogeneous etiology of epilepsies and medication effects, hamper studies on background brain activity in people with epilepsy. To overcome this limitation, we compared genetic data from a genome‐wide association study (GWAS) on epilepsy (n = 12 803 people with epilepsy and 24 218 controls) with that from a GWAS on background EEG (n = 8425 subjects without epilepsy), in which background EEG oscillation power was quantified in four different frequency bands: alpha, beta, delta, and theta. We replicated our findings in an independent epilepsy replication dataset (n = 4851 people with epilepsy and 20 428 controls). To assess the genetic overlap between these phenotypes, we performed genetic correlation analyses using linkage disequilibrium score regression, polygenic risk scores, and Mendelian randomization analyses.

Results

Our analyses show strong genetic correlations of genetic generalized epilepsy (GGE) with background EEG oscillations, primarily in the beta frequency band. Furthermore, we show that subjects with higher beta and theta polygenic risk scores have a significantly higher risk of having generalized epilepsy. Mendelian randomization analyses suggest a causal effect of GGE genetic liability on beta oscillations.

Significance

Our results point to shared biological mechanisms underlying background EEG oscillations and the susceptibility for GGE, opening avenues to investigate the clinical utility of background EEG oscillations in the diagnostic workup of epilepsy.

Lysergic acid diethylamide differentially modulates the reticular thalamus, mediodorsal thalamus, and infralimbic prefrontal cortex: An in vivo electrophysiology study in male mice

BACKGROUND

The reticular thalamus gates thalamocortical information flow via finely tuned inhibition of thalamocortical cells in the mediodorsal thalamus. Brain imaging studies in humans show that the psychedelic lysergic acid diethylamide (LSD) modulates activity and connectivity within the cortico-striato-thalamo-cortical (CSTC) circuit, altering consciousness. However, the electrophysiological effects of LSD on the neurons in these brain areas remain elusive.

METHODS

We employed in vivo extracellular single-unit recordings in anesthetized adult male mice to investigate the dose-response effects of cumulative LSD doses (5-160 µg/kg, intraperitoneal) upon reticular thalamus GABAergic neurons, thalamocortical relay neurons of the mediodorsal thalamus, and pyramidal neurons of the infralimbic prefrontal cortex.

RESULTS

LSD decreased spontaneous firing and burst-firing activity in 50% of the recorded reticular thalamus neurons in a dose-response fashion starting at 10 µg/kg. Another population of neurons (50%) increased firing and burst-firing activity starting at 40 µg/kg. This modulation was accompanied by an increase in firing and burst-firing activity of thalamocortical neurons in the mediodorsal thalamus. On the contrary, LSD excited infralimbic prefrontal cortex pyramidal neurons only at the highest dose tested (160 µg/kg). The dopamine D2 receptor (D₂) antagonist haloperidol administered after LSD increased burst-firing activity in the reticular thalamus neurons inhibited by LSD, decreased firing and burst-firing activity in the mediodorsal thalamus, and showed a trend towards further increasing the firing activity of neurons of the infralimbic prefrontal cortex.

CONCLUSION

LSD modulates firing and burst-firing activity of reticular thalamus neurons and disinhibits mediodorsal thalamus relay neurons at least partially in a D₂-mediated fashion. These effects of LSD on thalamocortical gating could explain its consciousness-altering effects in humans.

Focal Sleep Spindle Deficits Reveal Focal Thalamocortical Dysfunction and Predict Cognitive Deficits in Sleep Activated Developmental Epilepsy

Childhood epilepsy with centrotemporal spikes (CECTS) is the most common focal epilepsy syndrome, yet the cause of this disease remains unknown. Now recognized as a mild epileptic encephalopathy, children exhibit sleep-activated focal epileptiform discharges and cognitive difficulties during the active phase of the disease. The association between the abnormal electrophysiology and sleep suggests disruption to thalamocortical circuits. Thalamocortical circuit dysfunction resulting in pathologic epileptiform activity could hinder the production of sleep spindles, a brain rhythm essential for memory processes. Despite this pathophysiologic connection, the relationship between spindles and cognitive symptoms in epileptic encephalopathies has not been previously evaluated. A significant challenge limiting such work has been the poor performance of available automated spindle detection methods in the setting of sharp activities, such as epileptic spikes. Here, we validate a robust new method to accurately measure sleep spindles in patients with epilepsy. We then apply this detector to a prospective cohort of male and female children with CECTS with combined high-density EEGs during sleep and cognitive testing at varying time points of disease. We show that: (1) children have a transient, focal deficit in spindles during the symptomatic phase of disease; (2) spindle rate anticorrelates with spike rate; and (3) spindle rate, but not spike rate, predicts performance on cognitive tasks. These findings demonstrate focal thalamocortical circuit dysfunction and provide a pathophysiological explanation for the shared seizures and cognitive symptoms in CECTS. Further, this work identifies sleep spindles as a potential treatment target of cognitive dysfunction in this common epileptic encephalopathy.SIGNIFICANCE STATEMENT Childhood epilepsy with centrotemporal spikes is the most common idiopathic focal epilepsy syndrome, characterized by self-limited focal seizures and cognitive symptoms. Here, we provide the first evidence that focal thalamocortical circuit dysfunction underlies the shared seizures and cognitive dysfunction observed. In doing so, we identify sleep spindles as a mechanistic biomarker, and potential treatment target, of cognitive dysfunction in this common developmental epilepsy and provide a novel method to reliably quantify spindles in brain recordings from patients with epilepsy.

Sleep Deprivation and Neurological Disorders

Sleep plays an important role in maintaining neuronal circuitry, signalling and helps maintain overall health and wellbeing. Sleep deprivation (SD) disturbs the circadian physiology and exerts a negative impact on brain and behavioural functions. SD impairs the cellular clearance of misfolded neurotoxin proteins like α-synuclein, amyloid-β, and tau which are involved in major neurodegenerative diseases like Alzheimer's disease and Parkinson's disease. In addition, SD is also shown to affect the glymphatic system, a glial-dependent metabolic waste clearance pathway, causing accumulation of misfolded faulty proteins in synaptic compartments resulting in cognitive decline. Also, SD affects the immunological and redox system resulting in neuroinflammation and oxidative stress. Hence, it is important to understand the molecular and biochemical alterations that are the causative factors leading to these pathophysiological effects on the neuronal system. This review is an attempt in this direction. It provides up-to-date information on the alterations in the key processes, pathways, and proteins that are negatively affected by SD and become reasons for neurological disorders over a prolonged period of time, if left unattended.