From Physiology to Pathology of Cortico-Thalamo-Cortical Oscillations: Astroglia as a Target for Further Research

Abstract

The electrographic hallmark of childhood absence epilepsy (CAE) and other idiopathic forms of epilepsy are 2.5-4 Hz spike and wave discharges (SWDs) originating from abnormal electrical oscillations of the cortico-thalamo-cortical network. SWDs are generally associated with sudden and brief non-convulsive epileptic events mostly generating impairment of consciousness and correlating with attention and learning as well as cognitive deficits. To date, SWDs are known to arise from locally restricted imbalances of excitation and inhibition in the deep layers of the primary somatosensory cortex. SWDs propagate to the mostly GABAergic nucleus reticularis thalami (NRT) and the somatosensory thalamic nuclei that project back to the cortex, leading to the typical generalized spike and wave oscillations. Given their shared anatomical basis, SWDs have been originally considered the pathological transition of 11-16 Hz bursts of neural oscillatory activity (the so-called sleep spindles) occurring during Non-Rapid Eye Movement (NREM) sleep, but more recent research revealed fundamental functional differences between sleep spindles and SWDs, suggesting the latter could be more closely related to the slow (<1 Hz) oscillations alternating active (Up) and silent (Down) cortical activity and concomitantly occurring during NREM. Indeed, several lines of evidence support the fact that SWDs impair sleep architecture as well as sleep/wake cycles and sleep pressure, which, in turn, affect seizure circadian frequency and distribution. Given the accumulating evidence on the role of astroglia in the field of epilepsy in the modulation of excitation and inhibition in the brain as well as on the development of aberrant synchronous network activity, we aim at pointing at putative contributions of astrocytes to the physiology of slow-wave sleep and to the pathology of SWDs. Particularly, we will address the astroglial functions known to be involved in the control of network excitability and synchronicity and so far mainly addressed in the context of convulsive seizures, namely (i) interstitial fluid homeostasis, (ii) K⁺ clearance and neurotransmitter uptake from the extracellular space and the synaptic cleft, (iii) gap junction mechanical and functional coupling as well as hemichannel function, (iv) gliotransmission, (v) astroglial Ca²⁺ signaling and downstream effectors, (vi) reactive astrogliosis and cytokine release.

Keywords: NREM; astrocytes; cortico-thalamo-cortical oscillations; network plasticity; sleep; sleep/wake cycle; spike and wave discharges.

Figure 1

Anatomical and electrophysiological characterization of absence seizures. (A) Human electroencephalographical recording (EEG) displaying typical 3 Hz spike-and-wave discharges (SWDs). (B) 3 Hz SWDs associated with Childhood Absence Epilepsy (top trace, 8-year-old boy) and 8 Hz SWDs recorded in an adult WAG/Rij rat (bottom trace). (C) Blood Oxygenation Level Dependent (BOLD) functional Magnetic Resonance Imaging (fMRI) changes associated with absence seizures before (FP, frontal polar; CG, cingulate; LO, lateral occipital; PC, precuneus; LP, lateral parietal cortex) and after (LF, lateral frontal; LT, lateral temporal cortex) seizure onset (top) and brain network engagement analysis around SWDs events (bottom). (D) Anatomical organization of the cortico-thalamo-cortical network in C57BL/6N mice. Myelin Basic Protein (MBP) immunostaining indicates the axonal fiber tracts connecting the network (own data; mouse monoclonal MBP antibody, BioLegend, AB_2564741, Cat. No. 808401, 1:500). Cortico-thalamic excitatory neurons from cortical layers V and VI (red) project to both NRT and thalamus; thalamo-cortical excitatory neurons (blue) project back to cortical layer IV and NRT; NRT GABAergic neurons (green) inhibit the thalamic nuclei. I-VI, cortical layers; GP, globus pallidus; Hc, hippocampus; ic, internal capsule; NRT, nucleus reticularis thalami; SSp-ctx, primary somatosensory cortex; Str, striatum; VP, ventral posterior thalamic nuclei. (E) Glutamatergic thalamo-cortical neurons (TC) as well as GABAergic NRT neurons display T-type Ca²⁺ channel-mediated burst firing during SWDs (left, ex vivo recording from ferret thalamic slices). (F) Spike-time raster plots of two representative NRT neurons (NRT₁, top trace; NRT₂, bottom trace) and 10 TC neurons with time-matched EEG in GAERS rats. The overall TC activity decreases during SWDs and only a small portion of TC neurons fire synchronously. (G) 2-Photon laser scanning microscopy of neuronal cortical Ca²⁺ activity in stargazer mice during absence seizures. Heatmap of neuronal Ca²⁺ activity shows that only a subpopulation of neurons displays ictal synchronous firing. Modified from (A), (19); (B), (20, 21); (C), (22, 23); (E), (24, 25); (F), (25); (G), (26).

Figure 2

Astroglial homeostatic control of the extracellular space has opposite effects on epileptogenesis. The astroglial network is responsible for extracellular K⁺ uptake by means of inward rectifying K⁺ channels (K_ir), Na⁺/K⁺ pump and Na⁺/K⁺/2Cl⁻ transporter (NKCC). K⁺ clearance is coupled with water uptake through the water channel aquaporin-4 (AQP4) and possibly via yet unknown additional pathways. The excitatory amino acid transporters EAAT1 and EAAT2 are responsible for glutamate uptake. Astroglial connexins Cx43 and Cx30 enable gap-junction (GJ) coupling responsible for spatial ionic and metabolic buffering. Connexin hemichannels as well as pannexin-1 channels (Panx1) mediate glutamate and ATP release in the extracellular space possibly activating astroglial metabotropic glutamate receptors (mGluRs) and purinergic P2X and P2Y receptors (P2Rs), respectively. This, in term, induces intracellular Ca²⁺ increases in the neighboring astroglia. The figure summarizes the pro- and anti-epileptic roles of the mechanisms described above and points to the putative targets of valproic acid and the GJ blockers carbenoxolone, anandamide, and oleamide in this scenario.

Figure 3

Regional specific imbalance of E/I at the basis of absence seizures. The main components of the cortico-thalamo-cortical network (SSp-ctx, primary somatosensory cortex; NRT, nucleus reticularis thalami; thalamic nuclei) display regional specific shifts toward either excitation or inhibition associated with absence seizures. The figure summarizes the pro- (+) and anti- (-) epileptic effects of specific alterations of the regional E/I balance in the pathological phenotype of absence seizures. A1R, adenosine receptor type 1; EAAT1/2, excitatory amino acid transporters 1/2; CB1, cannabinoid receptor type 1; GABA, γ-aminobutyric acid; GABA_BR, metabotropic GABA_B receptor; GAT-1, GABA transporter 1; mGluR1/5, metabotropic glutamate receptors 1/5; TC, thalamo-cortical; TCA, tricarboxylic acid cycle; Y2R, neuropeptide Y receptor type 2.

Figure 4

Electrophysiological and cellular bases of sleep and SWD relationship. (A) Representative human electroencephalographical wave recordings during wakefulness (AW), REM sleep and different NREM sleep stages (N1, passive wakefulness or light sleep; N2, light slow-wave sleep; N3, deep slow-wave sleep). (B) Sleep spindle typically occurring during stage N2 of NREM sleep with respective magnification and comparison with SWDs. (C) Depth cortical EEG recording displaying cortical slow wave oscillations (upper traces) and time-matched intracellular recordings from cortical, thalamocortical, and NRT neurons with typical burst firing activity. (D) Schematic representation of the cellular and electrical components of cortico-thalamo-cortical oscillations. (E) Human hypnogram displaying two typical sleep cycles characterized by the succession of the NREM sleep stages followed by one episode of REM sleep (left) and schematic representation of the critical vigilance level (hatched area) promoting SWD occurrence during transitions between NREM and wakefulness, between NREM stages and from (but not to) REM sleep (right). Modified from (A), (271); (B), (272), (C), (273); (D), (274); (E), (271, 275).

Figure 5

Astroglial role in sleep/wake cycle and sleep architecture. Astrocytes contribute to sleep homeostasis in terms of both sleep/wake cycle regulation and sleep architecture and dynamics. The astroglial regulation of sleep relies on their control of extracellular K⁺ concentration, interstitial fluid exchanges and spatial buffering through gap junctions. Moreover, astroglia support and influence neural activity by means of their neurometabolic coupling to neurons, intracellular Ca²⁺ oscillations, gliotransmission, and release of, among other, cell adhesion molecules, cytokines and nitric oxide. AQP4, aquaporin-4; ATP, adenosine triphosphate; GJ, gap junction; IL-1β, interleukin-1β; K_ir, inward rectifying K⁺ channels; NO, nitric oxide; (N)REM, (non-) rapid eye movement; SW, slow wave; TNFα, tumor necrosis factor α.