Propofol modulates neural dynamics of thalamo-cortical system associated with anesthetic levels in rats

The thalamocortical system plays an important role in consciousness. How anesthesia modulates the thalamocortical interactions is not completely known.  We simultaneously recorded local field potentials(LFPs) in thalamic reticular nucleus(TRN) and ventroposteromedial thalamic nucleus(VPM), and electrocorticographic(ECoG) activities in frontal and occipital cortices in freely moving rats (n = 11). We analyzed the changes in thalamic and cortical local spectral power and connectivities, which were measured with phase-amplitude coupling (PAC), coherence and multivariate Granger causality, at the states of baseline, intravenous infusion of propofol 20, 40, 80 mg/kg/h and after recovery of righting reflex. We found that propofol-induced burst-suppression results in a synchronous decrease of spectral power in thalamus and cortex (p < 0.001 for all frequency bands). The cross-frequency PAC increased by propofol, characterized by gradually stronger 'trough-max' pattern in TRN and stronger 'peak-max' pattern in cortex. The cross-region PAC increased in the phase of TRN modulating the amplitude of cortex. The functional connectivity (FC) between TRN and cortex for α/β bands also significantly increased (p < 0.040), with increased directional connectivity from TRN to cortex under propofol anesthesia. In contrast, the corticocortical FC significantly decreased (p < 0.047), with decreased directional connectivity from frontal cortex to occipital cortex. However, the thalamothalamic functional and directional connectivities remained largely unchanged by propofol anesthesia.  The spectral powers and connectivities are differentially modulated with the changes of propofol doses, suggesting the changes in neural dynamics in thalamocortical system could be used for distinguishing different vigilance levels caused by propofol.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11571-022-09912-0.

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.

HCN channels and absence seizures

Hyperpolarization-activation cyclic nucleotide-gated (HCN) channels were for the first time implicated in absence seizures (ASs) when an abnormal Ih (the current generated by these channels) was reported in neocortical layer 5 neurons of a mouse model. Genetic studies of large cohorts of children with Childhood Absence Epilepsy (where ASs are the only clinical symptom) have identified only 3 variants in HCN1 (one of the genes that code for the 4 HCN channel isoforms, HCN1-4), with one (R590Q) mutation leading to loss-of-function. Due to the multi-faceted effects that HCN channels exert on cellular excitability and neuronal network dynamics as well as their modulation by environmental factors, it has been difficult to identify the detailed mechanism by which different HCN isoforms modulate ASs. In this review, we systematically and critically analyze evidence from established AS models and normal non-epileptic animals with area- and time-selective ablation of HCN1, HCN2 and HCN4. Notably, whereas knockout of rat HCN1 and mouse HCN2 leads to the expression of ASs, the pharmacological block of all HCN channel isoforms abolishes genetically determined ASs. These seemingly contradictory results could be reconciled by taking into account the well-known opposite effects of Ih on cellular excitability and network function. Whereas existing evidence from mouse and rat AS models indicates that pan-HCN blockers may provide a novel approach for the treatment of human ASs, the development of HCN isoform-selective drugs would greatly contribute to current research on the role for these channels in ASs generation and maintenance as well as offer new potential clinical applications.

Kainic acid induced hyperexcitability in thalamic reticular nucleus that initiates an inflammatory response through the HMGB1/TLR4 pathway

OBJECTIVE

To explore the relationship between the proinflammatory factor high-mobility group box 1 (HMGB1) and glutamatergic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the development of epilepsy.

METHODS

Thalamic reticular nucleus (TRN) slices were treated with kainic acid (KA) to simulate seizures. Action potentials and spontaneous inhibitory postsynaptic currents (sIPSCs) were recorded within TRN slices using whole-cell patch clamp techniques. The translocation of HMGB1 was detected by immunofluorescence. The HMGB1/TLR4 signaling pathway and its downstream inflammatory factors (IL-1β and NF-κB) were detected by RTPCR, Western blot and ELISA.

RESULTS

KA-evoked spikings were observed in TRN slices and blocked by perampanel. sIPSCs in the TRN were enhanced by KA and reduced by perampanel. The translocation of HMGB1 in the TRN was promoted by KA and inhibited by perampanel. The expression of the HMGB1/TLR4 signaling pathway was promoted by KA and suppressed by perampanel.

CONCLUSION

KA induced hyperexcitability activates the HMGB1/TLR4 pathway, which potentially leading to neuroinflammation in epilepsy.

Structural and functional abnormalities in thalamic neurons following neocortical focal status epilepticus

Status epilepticus (SE) is a life-threatening emergency that can result in de novo development or worsening of epilepsy. We tested the hypothesis that the aberrant cortical output during neocortical focal status epilepticus (FSE) would induce structural and functional changes in the thalamus that might contribute to hyperexcitability in the thalamocortical circuit. We induced neocortical FSE by unilateral epidural application of convulsant drugs to the somatosensory cortex of anesthetized mice of both sexes. The resulting focal EEG ictal episodes were associated with behavioral seizures consisting of contralateral focal myoclonic activity and persisted for 2-3 h. Ten and 30 days later, brains were processed for either immunohistochemistry (IHC) or in vitro slice recordings. Sections from the center of the thalamic reticular nucleus (nRT, see methods), the ventral posterolateral nucleus (VPL), and the ventral posteromedial nucleus (VPM) from the ventrobasal nucleus (VB) were used to measure density of NeuN-immunoreactive neurons, GFAP-reactive astrocytes, and colocalized areas for VGLUT1 + PSD95- and VGLUT2 + PSD95-IR, presumptive excitatory synapses of cortical and thalamic origins. Whole-cell voltage-clamp recordings were used to measure spontaneous EPSC frequency in these nuclei. We found that the nRT showed no decrease in numbers of neurons or evidence of reactive astrogliosis. In contrast, there were increases in GFAP-IR and decreased neuronal counts of NeuN positive cells in VB. Dual IHC for VGLUT1-PSD95 and VGLUT2-PSD95 in VB showed increased numbers of excitatory synapses, likely of both thalamic and cortical origins. The frequency, but not the amplitude of sEPSCs was increased in nRT and VB neurons. SIGNIFICANCE STATEMENT: Previous reports have shown that prolonged neocortical seizures can induce injury to downstream targets that might contribute to long-term consequences of FSE. Effects of FSE in thalamic structures may disrupt normal thalamo-cortical network functions and contribute to behavioral abnormalities and post-SE epileptogenesis. Our results show that a single episode of focal neocortical SE in vivo has chronic consequences including cell loss in VB nuclei and increased excitatory connectivity in intra-thalamic and cortico-thalamic networks. Additional experiments will assess the functional consequences of these alterations and approaches to mitigate cell loss and alterations in synaptic connectivity.

The thalamocortical inhibitory network controls human conscious perception

Although conscious perception is a fundamental cognitive function, its neural correlates remain unclear. It remains debatable whether thalamocortical interactions play a decisive role in conscious perception. To clarify this, we used functional magnetic resonance imaging (fMRI) where flickering red and green visual cues could be perceived either as a non-fused colour or fused colour. Here we show significantly differentiated fMRI neurodynamics only in higher-order thalamocortical regions, compared with first-order thalamocortical regions. Anticorrelated neurodynamic behaviours were observed between the visual stream network and default-mode network. Its dynamic causal modelling consistently provided compelling evidence for the involvement of higher-order thalamocortical iterative integration during conscious perception of fused colour, while inhibitory control was revealed during the non-fusion condition. Taken together with our recent magnetoencephalography study, our fMRI findings corroborate a thalamocortical inhibitory model for consciousness, where both thalamic inhibitory regulation and integrative signal iterations across higher-order thalamocortical regions are essential for conscious perception.

Alterations of sleep oscillations in Alzheimer's disease: A potential role for GABAergic neurons in the cortex, hippocampus, and thalamus

Sleep abnormalities are widely reported in patients with Alzheimer's disease (AD) and are linked to cognitive impairments. Sleep abnormalities could be potential biomarkers to detect AD since they are often observed at the preclinical stage. Moreover, sleep could be a target for early intervention to prevent or slow AD progression. Thus, here we review changes in brain oscillations observed during sleep, their connection to AD pathophysiology and the role of specific brain circuits. Slow oscillations (0.1-1 Hz), sleep spindles (8-15 Hz) and their coupling during non-REM sleep are consistently reduced in studies of patients and in AD mouse models although the timing and magnitude of these alterations depends on the pathophysiological changes and the animal model studied. Changes in delta (1-4 Hz) activity are more variable. Animal studies suggest that hippocampal sharp-wave ripples (100-250 Hz) are also affected. Reductions in REM sleep amount and slower oscillations during REM are seen in patients but less consistently in animal models. Thus, changes in a variety of sleep oscillations could impact sleep-dependent memory consolidation or restorative functions of sleep. Recent mechanistic studies suggest that alterations in the activity of GABAergic neurons in the cortex, hippocampus and thalamic reticular nucleus mediate sleep oscillatory changes in AD and represent a potential target for intervention. Longitudinal studies of the timing of AD-related sleep abnormalities with respect to pathology and dysfunction of specific neural networks are needed to identify translationally relevant biomarkers and guide early intervention strategies to prevent or delay AD progression.

Thalamocortical bistable switch as a theoretical model of fibromyalgia pathogenesis inferred from a literature survey

Fibromyalgia (FM) is an unsolved central pain processing disturbance. We aim to provide a unifying model for FM pathogenesis based on a loop network involving thalamocortical regions, i.e., the ventroposterior lateral thalamus (VPL), the somatosensory cortex (SC), and the thalamic reticular nucleus (TRN). The dynamics of the loop have been described by three differential equations having neuron mean firing rates as variables and containing Hill functions to model mutual interactions among the loop elements. A computational analysis conducted with MATLAB has shown a transition from monostability to bistability of the loop behavior for a weakening of GABAergic transmission between TRN and VPL. This involves the appearance of a high-firing-rate steady state, which becomes dominant and is assumed to represent pathogenic pain processing giving rise to chronic pain. Our model is consistent with a bulk of literature evidence, such as neuroimaging and pharmacological data collected on FM patients, and with correlations between FM and immunoendocrine conditions, such as stress, perimenopause, chronic inflammation, obesity, and chronic dizziness. The model suggests that critical targets for FM treatment are to be found among immunoendocrine pathways leading to GABA/glutamate imbalance having an impact on the thalamocortical system.

Deletion of Phospholipase C β1 in the Thalamic Reticular Nucleus Induces Absence Seizures

Absence seizures are caused by abnormal synchronized oscillations in the thalamocortical (TC) circuit, which result in widespread spike-and-wave discharges (SWDs) on electroencephalography (EEG) as well as impairment of consciousness. Thalamic reticular nucleus (TRN) and TC neurons are known to interact dynamically to generate TC circuitry oscillations during SWDs. Clinical studies have suggested the association of Plcβ1 with early-onset epilepsy, including absence seizures. However, the brain regions and circuit mechanisms related to the generation of absence seizures with Plcβ1 deficiency are unknown. In this study, we found that loss of Plcβ1 in mice caused spontaneous complex-type seizures, including convulsive and absence seizures. Importantly, TRN-specific deletion of Plcβ1 led to the development of only spontaneous SWDs, and no other types of seizures were observed. Ex vivo slice patch recording demonstrated that the number of spikes, an intrinsic TRN neuronal property, was significantly reduced in both tonic and burst firing modes in the absence of Plcβ1. We conclude that the loss of Plcβ1 in the TRN leads to decreased excitability and impairs normal inhibitory neuronal function, thereby disrupting feedforward inhibition of the TC circuitry, which is sufficient to cause hypersynchrony of the TC system and eventually leads to spontaneous absence seizures. Our study not only provides a novel mechanism for the induction of SWDs in Plcβ1-deficient patients but also offers guidance for the development of diagnostic and therapeutic tools for absence epilepsy.

Cross-Modal Interaction and Integration Through Stimulus-Specific Adaptation in the Thalamic Reticular Nucleus of Rats

Stimulus-specific adaptation (SSA), defined as a decrease in responses to a common stimulus that only partially generalizes to other rare stimuli, is a widespread phenomenon in the brain that is believed to be related to novelty detection. Although cross-modal sensory processing is also a widespread phenomenon, the interaction between the two phenomena is not well understood. In this study, the thalamic reticular nucleus (TRN), which is regarded as a hub of the attentional system that contains multi-modal neurons, was investigated. The results showed that SSA existed in an interactive oddball stimulation, which mimics stimulation changes from one modality to another. In the bimodal integration, SSA to bimodal stimulation was stronger than to visual stimulation alone but similar to auditory stimulation alone, which indicated a limited integrative effect. Collectively, the present results provide evidence for independent cross-modal processing in bimodal TRN neurons.

A thalamic bridge from sensory perception to cognition

The ability to adapt to dynamic environments requires tracking multiple signals with variable sensory salience and fluctuating behavioral relevance. This complex process requires integrative crosstalk between sensory and cognitive brain circuits. Functional interactions between cortical and thalamic regions are now considered essential for both sensory perception and cognition but a clear account of the functional link between sensory and cognitive circuits is currently lacking. This review aims to document how thalamic nuclei may effectively act as a bridge allowing to fuse perceptual and cognitive events into meaningful experiences. After highlighting key aspects of thalamocortical circuits such as the classic first-order/higher-order dichotomy, we consider the role of the thalamic reticular nucleus from directed attention to cognition. We next summarize research relying on Pavlovian learning paradigms, showing that both first-order and higher-order thalamic nuclei contribute to associative learning. Finally, we propose that modulator inputs reaching all thalamic nuclei may be critical for integrative purposes when environmental signals are computed. Altogether, the thalamus appears as the bridge linking perception, cognition and possibly affect.

Thalamus-related anomalies as candidate mechanism-based biomarkers for psychosis

Identification of reliable biomarkers of prognosis in subjects with high risk to psychosis is an essential step to improve care and treatment of this population of help-seekers. Longitudinal studies highlight some clinical criteria, cognitive deficits, patterns of gray matter alterations and profiles of blood metabolites that provide some levels of prediction regarding the conversion to psychosis. Further effort is warranted to validate these results and implement these types of approaches in clinical settings. Such biomarkers may however fall short in entangling the biological mechanisms underlying the disease progression, an essential step in the development of novel therapies. Circuit-based approaches, which map on well-identified cerebral functions, could meet these needs. Converging evidence indicates that thalamus abnormalities are central to schizophrenia pathophysiology, contributing to clinical symptoms, cognitive and sensory deficits. This review highlights the various thalamus-related anomalies reported in individuals with genetic risks and in the different phases of the disorder, from prodromal to chronic stages. Several anomalies are potent endophenotypes, while others exist in clinical high-risk subjects and worsen in those who convert to full psychosis. Aberrant functional coupling between thalamus and cortex, low glutamate content and readouts from resting EEG carry predictive values for transition to psychosis or functional outcome. In this context, thalamus-related anomalies represent a valuable entry point to tackle circuit-based alterations associated with the emergence of psychosis. This review also proposes that longitudinal surveys of neuroimaging, EEG readouts associated with circuits encompassing the mediodorsal, pulvinar in high-risk individuals could unveil biological mechanisms contributing to this psychiatric disorder.

Thalamocortical inhibitory dynamics support conscious perception

Whether thalamocortical interactions play a decisive role in conscious perception remains an open question. We presented rapid red/green color flickering stimuli, which induced the mental perception of either an illusory orange color or non-fused red and green colors. Using magnetoencephalography, we observed 6-Hz thalamic activity associated with thalamocortical inhibitory coupling only during the conscious perception of the illusory orange color. This sustained thalamic disinhibition was temporally coupled with higher visual cortical activation during the conscious perception of the orange color, providing neurophysiological evidence of the role of thalamocortical synchronization in conscious awareness of mental representation. Bayesian model comparison consistently supported the thalamocortical model in conscious perception. Taken together, experimental and theoretical evidence established the thalamocortical inhibitory network as a gateway to conscious mental representations.

A single psychotomimetic dose of ketamine decreases thalamocortical spindles and delta oscillations in the sedated rat

BACKGROUND

In patients with psychotic disorders, sleep spindles are reduced, supporting the hypothesis that the thalamus and glutamate receptors play a crucial etio-pathophysiological role, whose underlying mechanisms remain unknown. We hypothesized that a reduced function of NMDA receptors is involved in the spindle deficit observed in schizophrenia.

METHODS

An electrophysiological multisite cell-to-network exploration was used to investigate, in pentobarbital-sedated rats, the effects of a single psychotomimetic dose of the NMDA glutamate receptor antagonist ketamine in the sensorimotor and associative/cognitive thalamocortical (TC) systems.

RESULTS

Under the control condition, spontaneously-occurring spindles (intra-frequency: 10-16 waves/s) and delta-frequency (1-4 Hz) oscillations were recorded in the frontoparietal cortical EEG, in thalamic extracellular recordings, in dual juxtacellularly recorded GABAergic thalamic reticular nucleus (TRN) and glutamatergic TC neurons, and in intracellularly recorded TC neurons. The TRN cells rhythmically exhibited robust high-frequency bursts of action potentials (7 to 15 APs at 200-700 Hz). A single administration of low-dose ketamine fleetingly reduced TC spindles and delta oscillations, amplified ongoing gamma-(30-80 Hz) and higher-frequency oscillations, and switched the firing pattern of both TC and TRN neurons from a burst mode to a single AP mode. Furthermore, ketamine strengthened the gamma-frequency band TRN-TC connectivity. The antipsychotic clozapine consistently prevented the ketamine effects on spindles, delta- and gamma-/higher-frequency TC oscillations.

CONCLUSION

The present findings support the hypothesis that NMDA receptor hypofunction is involved in the reduction in sleep spindles and delta oscillations. The ketamine-induced swift conversion of ongoing TC-TRN activities may have involved at least both the ascending reticular activating system and the corticothalamic pathway.

Thalamic reticular nucleus in the thalamocortical loop

Dynamic binding of different brain areas is critical for various cognitive functions. The thalamic reticular nucleus (TRN) is a GABAergic nucleus that constrains information flow through thalamocortical loop by providing inhibitory innervation to the thalamus. In this review, I summarize anatomical and single-cell-level physiological studies of the rodent TRN. Diversity and heterogeneity of TRN neurons in terms of axonal innervation, molecular expression, and physiological characteristics are described. I also outline thalamocortical and cortico-cortical connections with emphasis on interaction with the TRN. In summary, it is proposed that functional connectivity among brain regions are modulated with gating of transthalamic information flow by the TRN.

Limbic circuit connectivity and the stress response: New insights into the mammalian nociceptin peptide system

Considerable progress has been made in understanding the neurobehavioral effects of nociceptin peptide (N/OFQ) and its NOP receptor. Recent interest has focused on its role in stress and cognition, with consideration of therapeutic potential in regard to anxiety and mood disorders. Research has interrogated the mechanisms of action of N/OFQ peptide in the context of stress-related behavior. We are interested in the endogenous role of N/OFQ and NOP receptor in terms of adaptation to chronic stress. Our research has highlighted the importance of associated limbic regions including the bed nucleus, extended amygdala, in addition to thalamic reticular nucleus as important sites for long-term adaptations in endogenous N/OFQ function in chronic stress. Such research raises interest in appreciation of extended limbic connections and novel pathways which allow us to reevaluate current understanding of stress neurocircuitry. Examination of endogenous N/OFQ-NOP receptor modulation of monoaminergic and amino acid transmitter systems in this extended limbic architecture will facilitate deeper understanding of the tonic control of behavior. Application of in vivo experimental approaches to models of abnormal neurodevelopment and heightened stress vulnerability in adulthood will enable the role of N/OFQ in complex neuropsychiatric disorders including schizophrenia and post-traumatic stress disorder to be defined.

Visual hallucinations, thalamocortical physiology and Lewy body disease: A review

One of the core diagnostic criteria for Dementia with Lewy Bodies (DLB) is the presence of visual hallucinations. The presence of hallucinations, along with fluctuations in the level of arousal and sleep disturbance, point to potential pathological mechanisms at the level of the thalamus. However, the potential role of thalamic dysfunction in DLB, particularly as it relates to the presence of formed visual hallucinations is not known. Here, we review the literature on the pathophysiology of DLB with respect to modern theories of thalamocortical function and attempt to derive an understanding of how such hallucinations arise. Based on the available literature, we propose that combined thalamic-thalamic reticular nucleus and thalamocortical pathology may explain the phenomenology of visual hallucinations in DLB. In particular, diminished α7 cholinergic activity in the thalamic reticular nucleus may critically disinhibit thalamocortical activity. Further, concentrated pathological changes within the posterior regions of the thalamus may explain the predilection for the hallucinations to be visual in nature.

Increased Thalamocortical Connectivity in Schizophrenia Correlates With Sleep Spindle Deficits: Evidence for a Common Pathophysiology

BACKGROUND

Converging evidence implicates abnormal thalamocortical interactions in the pathophysiology of schizophrenia. This evidence includes consistent findings of increased resting-state functional connectivity of the thalamus with somatosensory and motor cortex during wake and reduced spindle activity during sleep. We hypothesized that these abnormalities would be correlated, reflecting a common mechanism: reduced inhibition of thalamocortical neurons by the thalamic reticular nucleus (TRN). The TRN is the major inhibitory nucleus of the thalamus and is abnormal in schizophrenia. Reduced TRN inhibition would be expected to lead to increased and less filtered thalamic relay of sensory and motor information to the cortex during wake and reduced burst firing necessary for spindle initiation during sleep.

METHODS

Overnight polysomnography and resting-state functional connectivity magnetic resonance imaging were performed in 26 outpatients with schizophrenia and 30 demographically matched healthy individuals. We examined the relations of sleep spindle density during stage 2 non-rapid eye movement sleep with connectivity of the thalamus to the cortex during wakeful rest.

RESULTS

As in prior studies, patients with schizophrenia exhibited increased functional connectivity of the thalamus with bilateral somatosensory and motor cortex and reduced sleep spindle density. Spindle density inversely correlated with thalamocortical connectivity, including in somotosensory and motor cortex, regardless of diagnosis.

CONCLUSIONS

These findings link two biomarkers of schizophrenia-the sleep spindle density deficit and abnormally increased thalamocortical functional connectivity-and point to deficient TRN inhibition as a plausible mechanism. If TRN-mediated thalamocortical dysfunction increases risk for schizophrenia and contributes to its manifestations, understanding its mechanism could guide the development of targeted interventions.

Microstimulation-induced inhibition of thalamic reticular nucleus in non-human primates

The thalamic reticular nucleus (TRN) modulates activity in the thalamus and controls excitatory input from corticothalamic and thalamocortical glutamatergic projections. It is made up of GABAergic neurons which project topographically to the thalamus. The TRN also receives inhibitory projections from the globus pallidus and the substantia nigra pars reticulata. Photostimulation of the TRN results in local inhibition in rat slice preparations but the effects of local stimulation in vivo are not known. This study aimed to characterize stimulation-evoked responses in the TRN of non-human primates (NHPs). Microelectrodes were inserted into the TRN and neurons were stimulated at 5, 10, 15, and 20 µA using 0.5 s trains at 100 Hz and the subsequent response was recorded from the same electrode. Stimulation in surrounding nuclei and the internal capsule was used for mapping the anatomical borders of the TRN. Stimulation as low as 10 µA resulted in predominantly inhibition, recorded in both single units and background unit activity (BUA). The duration of inhibition (~ 1-3 s) increased with increasing stimulation amplitude and was significantly increased in BUA when single units were present. At 20 µA of current, 93% of the single units (41/44) and 92% of BUA sites (67/73) were inhibited. Therefore, microstimulation of the NHP TRN with low currents results in current-dependent inhibition of single units and BUA. This finding may be useful to further aid in localization of deep thalamic and subthalamic nuclei during brain surgery.

Thalamic Reticular Nucleus in Caiman crocodilus: Immunohistochemical Staining

The thalamic reticular nucleus in reptiles, Caiman crocodilus, shares a number of morphological similarities with its counterpart in mammals. In view of the immunohistochemical properties of this nucleus in mammals and the more recently identified complexity of this neuronal aggregate in Caiman, this nucleus was investigated using a number of antibodies. These results were compared with findings described for other amniotes. The following antibodies gave consistent and reproducible results: polyclonal sheep anti-parvalbumin (PV), monoclonal mouse anti-PV, and polyclonal sheep anti-glutamic acid decarboxylase (GAD). In the transverse plane, this nucleus is divided into two. In each part, a compact group of cells sits on top of the fibers of the forebrain bundle with scattered cells among these fibers. In the lateral forebrain bundle, this neuronal aggregate is represented by the dorsal peduncular nucleus and the perireticular nucleus while, in the medial forebrain bundle, these parts are the interstitial nucleus and the scattered cells in this fiber tract. The results of this study are the following. First, the thalamic reticular nucleus of Caiman contains GAD(+) and PV(+) neurons, which is similar to what has been described in other amniotes. Second, the morphology and distribution of many GAD(+) and PV(+) neurons in the dorsal peduncular and perireticular nuclei are similar and suggest that these neurons colocalize these markers. Third, neurons in the interstitial nucleus and in the medial forebrain bundle are GAD(+) and PV(+). At the caudal pole of the thalamic reticular nucleus, PV immunoreactive cells predominated and avoided the central portion of this nucleus where GAD(+) cells were preferentially located. However, GAD(+) cells were sparse when compared with PV(+) cells. This immunohistochemically different area in the caudal pole is considered to be an area separate from the thalamic reticular nucleus.

Thalamic inhibitory circuits and network activity development

Inhibitory circuits in thalamus and cortex shape the major activity patterns observed by electroencephalogram (EEG) in the adult brain. Their delayed maturation and circuit integration, relative to excitatory neurons, suggest inhibitory neuronal development could be responsible for the onset of mature thalamocortical activity. Indeed, the immature brain lacks many inhibition-dependent activity patterns, such as slow-waves, delta oscillations and sleep-spindles, and instead expresses other unique oscillatory activities in multiple species including humans. Thalamus contributes significantly to the generation of these early oscillations. Compared to the abundance of studies on the development of inhibition in cortex, however, the maturation of thalamic inhibition is poorly understood. Here we review developmental changes in the neuronal and circuit properties of the thalamic relay and its interconnected inhibitory thalamic reticular nucleus (TRN) both in vitro and in vivo, and discuss their potential contribution to early network activity and its maturation. While much is unknown, we argue that weak inhibitory function in the developing thalamus allows for amplification of thalamocortical activity that supports the generation of early oscillations. The available evidence suggests that the developmental acquisition of critical thalamic oscillations such as slow-waves and sleep-spindles is driven by maturation of the TRN. Further studies to elucidate thalamic GABAergic circuit formation in relation to thalamocortical network function would help us better understand normal as well as pathological brain development.

Leptin alters somatosensory thalamic networks by decreasing gaba release from reticular thalamic nucleus and action potential frequency at ventrobasal neurons

Leptin is an adipose-derived hormone that controls appetite and energy expenditure. Leptin receptors are expressed on extra-hypothalamic ventrobasal (VB) and reticular thalamic (RTN) nuclei from embryonic stages. Here, we studied the effects of pressure-puff, local application of leptin on both synaptic transmission and action potential properties of thalamic neurons in thalamocortical slices. We used whole-cell patch-clamp recordings of thalamocortical VB neurons from wild-type (WT) and leptin-deficient obese (ob/ob) mice. We observed differences in VB neurons action potentials and synaptic currents kinetics when comparing WT vs. ob/ob. Leptin reduced GABA release onto VB neurons throughout the activation of a JAK2-dependent pathway, without affecting excitatory glutamate transmission. We observed a rapid and reversible reduction by leptin of the number of action potentials of VB neurons via the activation of large conductance Ca²⁺-dependent potassium channels. These leptin effects were observed in thalamocortical slices from up to 5-week-old WT but not in leptin-deficient obese mice. Results described here suggest the existence of a leptin-mediated trophic modulation of thalamocortical excitability during postnatal development. These findings could contribute to a better understanding of leptin within the thalamocortical system and sleep deficits in obesity.

Deconstructing Schizophrenia: Advances in Preclinical Models for Biomarker Identification

Schizophrenia is considered to develop as a consequence of genetic and environmental factors impacting on brain neural systems and circuits during vulnerable neurodevelopmental periods, thereby resulting in symptoms in early adulthood. Understanding of the impact of schizophrenia risk factors on brain biology and behaviour can help in identifying biologically relevant pathways that are attractive for informing clinical studies and biomarker development. In this chapter, we emphasize the importance of adopting a reciprocal forward and reverse translation approach that is iteratively updated when additional new information is gained, either preclinically or clinically, for offering the greatest opportunity for discovering panels of biomarkers for the diagnosis, prognosis and treatment of schizophrenia. Importantly, biomarkers for identifying those at risk may inform early intervention strategies prior to the development of schizophrenia.Given the emerging nature of this approach in the field, this review will highlight recent research of preclinical biomarkers in schizophrenia that show the most promise for informing clinical needs with an emphasis on relevant imaging, electrophysiological, cognitive behavioural and biochemical modalities. The implementation of this reciprocal translational approach is exemplified firstly by the production and characterization of preclinical models based on the glutamate hypofunction hypothesis, genetic and environmental risk factors for schizophrenia (reverse translation), and then the recent clinical recognition of the thalamic reticular thalamus (TRN) as an important locus of brain dysfunction in schizophrenia as informed by preclinical findings (forward translation).

Corticothalamic network dysfunction and Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disease that is characterized by progressive cognitive decline and a prominent loss of hippocampal-dependent memory. Therefore, much focus has been placed on understanding the function and dysfunction of the hippocampus in AD. However, AD is also accompanied by a number of other debilitating cognitive and behavioral alterations including deficits in attention, cognitive processing, and sleep maintenance. The underlying mechanisms that give rise to impairments in such diverse behavioral domains are unknown, and identifying them would shed insight into the multifactorial nature of AD as well as reveal potential new therapeutic targets to improve overall function in AD. We present here several lines of evidence that suggest that dysregulation of the corticothalamic network may be a common denominator that contributes to the diverse cognitive and behavioral alterations in AD. First, we will review the mechanisms by which this network regulates processes that include attention, cognitive processing, learning and memory, and sleep maintenance. Then we will review how these behavioral and cognitive domains are altered in AD. We will also discuss how dysregulation of tightly regulated activity in the corticothalamic network can give rise to non-convulsive seizures and other forms of epileptiform activity that have also been documented in both AD patients and transgenic mouse models of AD. In summary, the corticothalamic network has the potential to be a master regulator of diverse cognitive and behavioral domains that are affected in AD.

Astrocytes modulate thalamic sensory processing via mGlu2 receptor activation

Astrocytes possess many of the same signalling molecules as neurons. However, the role of astrocytes in information processing, if any, is unknown. Using electrophysiological and imaging methods, we report the first evidence that astrocytes modulate neuronal sensory inhibition in the rodent thalamus.

We found that mGlu2 receptor activity reduces inhibitory transmission from the thalamic reticular nucleus to the somatosensory ventrobasal thalamus (VB): mIPSC frequencies in VB slices were reduced by the Group II mGlu receptor agonist LY354740, an effect potentiated by mGlu2 positive allosteric modulator (PAM) LY487379 co-application (30 nM LY354740: 10.0 ± 1.6% reduction; 30 nM LY354740 & 30 μM LY487379: 34.6 ± 5.2% reduction).

We then showed activation of mGlu2 receptors on astrocytes: astrocytic intracellular calcium levels were elevated by the Group II agonist, which were further potentiated upon mGlu2 PAM co-application (300 nM LY354740: ratio amplitude 0.016 ± 0.002; 300 nM LY354740 & 30 μM LY487379: ratio amplitude 0.035 ± 0.003).

We then demonstrated mGlu2-dependent astrocytic disinhibition of VB neurons in vivo: VB neuronal responses to vibrissae stimulation trains were disinhibited by the Group II agonist and the mGlu2 PAM (LY354740: 156 ± 12% of control; LY487379: 144 ± 10% of control). Presence of the glial inhibitor fluorocitrate abolished the mGlu2 PAM effect (91 ± 5% of control), suggesting the mGlu2 component to the Group II effect can be attributed to activation of mGlu2 receptors localised on astrocytic processes within the VB.

Gating of thalamocortical function via astrocyte activation represents a novel sensory processing mechanism. As this thalamocortical circuitry is important in discriminative processes, this demonstrates the importance of astrocytes in synaptic processes underlying attention and cognition.

•

Thalamic inhibition is mediated by both neuronal and astrocytic mechanisms.

•

Group II mGlu receptor (mGlu2/3) activation can modulate this thalamic inhibition.

•

Thalamic astrocytes can be activated upon mGlu2 receptor stimulation.

•

This process may enable relevant activity to be discerned from background noise.

•

Targeting astrocytic mGlu2 receptors may therefore affect attention and cognition.

Toward understanding thalamocortical dysfunction in schizophrenia through computational models of neural circuit dynamics

The thalamus is implicated in the neuropathology of schizophrenia, and multiple modalities of noninvasive neuroimaging provide converging evidence for altered thalamocortical dynamics in the disorder, such as functional connectivity and oscillatory power. However, it remains a challenge to link these neuroimaging biomarkers to underlying neural circuit mechanisms. One potential path forward is a "Computational Psychiatry" approach that leverages computational models of neural circuits to make predictions for the dynamical impact dynamical impact on specific thalamic disruptions hypothesized to occur in the pathophysiology of schizophrenia. Here we review biophysically-based computational models of neural circuit dynamics for large-scale resting-state networks which have been applied to schizophrenia, and for thalamic oscillations. As a key aspect of thalamocortical dysconnectivity in schizophrenia is its regional specificity, it is important to consider potential sources of intrinsic heterogeneity of cellular and circuit properties across cortical and thalamic structures.

Acute deep brain stimulation in the thalamic reticular nucleus protects against acute stress and modulates initial events of adult hippocampal neurogenesis

Deep brain stimulation (DBS) is used as an alternative therapeutic procedure for pharmacoresistant psychiatric disorders. Recently the thalamic reticular nucleus (TRN) gained attention due to the description of a novel pathway from the amygdala to this nucleus suggesting that may be differentially disrupted in mood disorders. The limbic system is implicated in the regulation of these disorders that are accompanied by neuroplastic changes. The hippocampus is highly plastic and shows the generation of new neurons, process affected by stress but positively regulated by antidepressant drugs. We explored the impact of applying acute DBS to the TRN (DBS-TRN) in male Wistar rats exposed to acute stress caused by the forced-swim Porsolt's test (FST) and on initial events of hippocampal neurogenesis. After the first session of forced-swim, rats were randomly subdivided in a DBS-TRN and a Sham group. Stimulated rats received 10min of DBS, thus the depressant-like behavior reflected as immobility was evaluated in the second session of forced-swim. Locomotricity was evaluated in the open field test. Cell proliferation and doublecortin-associated cells were quantified in the hippocampus of other cohorts of rats. No effects of electrode implantation were found in locomotricity. Acute DBS-TRN reduced immobility in comparison to the Sham group (p<0.001). DBS-TRN increased cell proliferation (Ki67 or BrdU-positive cells; p=0.02, p=0.02) and the number of doublecortin-cells compared to the Sham group (p<0.02). Similar effects were found in rats previously exposed to the first session of forced-swim. Our data could suggest that TRN brain region may be a promising target for DBS to treat intractable depression.

mGluR-mediated calcium signalling in the thalamic reticular nucleus

The thalamic reticular nucleus (TRN) plays a major role in modulating the transfer of information from the thalamus to the cortex. GABAergic inhibition via the TRN is differentially regulated by metabotropic glutamate receptors (mGluRs) and the effect of mGluRs on the membrane potential, on ion channels, and on the plasticity of electrical coupling of TRN neurons has been studied previously. Although mGluRs are generally known to trigger Ca(2+) transients, mGluR-mediated Ca(2+)-transients in TRN neurons have not yet been investigated. In this study, we show that mGluRs can trigger Ca(2+)-transients in TRN neurons, that these transients depend on intracellular Ca(2+)-stores, and are mediated by IP3 receptors. Ca(2+) transients caused by the group I mGluR agonist DHPG elicit a current that is sensitive to flufenamic acid and has a reversal potential around -40mV. Our results add mGluR-mediated Ca(2+)-signalling in the TRN to the state-dependent modulators of the thalamocortical system.

Anti-epileptogenic effect of high-frequency stimulation in the thalamic reticular nucleus on PTZ-induced seizures

BACKGROUND

Deep brain stimulation, specifically high-frequency stimulation (HFS), is an alternative and promising treatment for intractable epilepsies; however, the optimal targets are still unknown. The thalamic reticular nucleus (TRN) occupies a key position in the modulation of the cortico-thalamic and thalamo-cortical pathways.

OBJECTIVE

We determined the efficacy of HFS in the TRN against tonic-clonic generalized seizures (TCGS) and status epilepticus (SE), which were induced by scheduled pentylenetetrazole (PTZ) injections.

METHODS

Male Wistar rats were stereotactically implanted and assigned to three experimental groups: Control group, which received only PTZ injections; HFS-TRN group, which received HFS in the left TRN prior to PTZ injections; and HFS-Adj group, which received HFS in the left adjacent nuclei prior to PTZ injections.

RESULTS

The HFS-TRN group reported a significant increase in the latency for development of TCGS and SE compared with the HFS-Adj and Control groups (P < 0.009). The number of PTZ-doses required for SE was also significantly increased (P < 0.001). Spectral analysis revealed a significant decrease in the frequency band from 0.5 Hz to 4.5 Hz of the left motor cortex in the HFS-TRN and HFS-Adj groups, compared to the Control group. Conversely, HFS-TRN provoked a significant increase in all frequency bands in the TRN. EEG asynchrony was observed during spike-wave discharges by HFS-TRN.

CONCLUSION

These data indicate that HFS-TRN has an anti-epileptogenic effect and is able to modify seizure synchrony and interrupt abnormal EEG recruitment of thalamo-cortical and, indirectly, cortico-thalamic pathways.