Decreased but diverse activity of cortical and thalamic neurons in consciousness-impairing rodent absence seizures.

Absence seizures are brief episodes of impaired consciousness, behavioral arrest, and unresponsiveness, with yet-unknown neuronal mechanisms. Here we report that an awake female rat model recapitulates the behavioral, electroencephalographic, and cortical functional magnetic resonance imaging characteristics of human absence seizures. Neuronally, seizures feature overall decreased but rhythmic firing of neurons in cortex and thalamus. Individual cortical and thalamic neurons express one of four distinct patterns of seizure-associated activity, one of which causes a transient initial peak in overall firing at seizure onset, and another which drives sustained decreases in overall firing. 40-60 s before seizure onset there begins a decline in low frequency electroencephalographic activity, neuronal firing, and behavior, but an increase in higher frequency electroencephalography and rhythmicity of neuronal firing. Our findings demonstrate that prolonged brain state changes precede consciousness-impairing seizures, and that during seizures distinct functional groups of cortical and thalamic neurons produce an overall transient firing increase followed by a sustained firing decrease, and increased rhythmicity.

A systematic review of the effects of gut microbiota depletion on social and anxiety-related behaviours in adult rodents: Implications for translational research.

The microbiota-gut-brain axis is associated with several behaviours, including those relevant to anxiety or sociability in rodents, however, no conceptual framework has yet been available. Summary of the effects of antibiotic-mediated gut microbiota depletion on anxiety and sociability is essential to both inform further preclinical investigations and to guide translational research into human studies. The main objective is to examine the role of gut microbiota depletion on anxiety and sociability in rodents, and to consider how the findings can be translated to inform the design of research in humans. We reviewed 13 research articles, indicating significant changes in gut microbiota composition and diversity have been found in animals treated with a mix or a single antibiotic. Nonetheless, there is no consensus regarding the impact of gut microbiota depletion on anxiety-like or social behaviour. Gut microbiota depletion may be a useful strategy to examine the role of gut microbes in anxiety and sociability, but the lack of data from rigorous animal investigations precludes any definitive interpretations for a translational impact on human health.

Gut microbiome effects on neuronal excitability & activity: Implications for epilepsy.

It is now well established that the bacterial population of the gastrointestinal system, known as the gut microbiome, is capable of influencing the brain and its dependent functions. Links have been demonstrated between the microbiome and a variety of normal and pathological neural functions, including epilepsy. Many of these microbiome-brain links involve the direct or indirect modulation of the excitability and activity of individual neurons by the gut microbiome. Such links may be particularly significant when it comes to microbiome modulation of epilepsy, often considered a disorder of neuronal excitability. In this review we consider the current evidence of a relationship between the gut microbiome and the excitability or activity of neurons in the context of epilepsy. The review focuses particularly on evidence of direct, causal microbiome effects on neuronal excitability or activity, but also considers demonstrations of microbiome to host interactions that are likely to have an indirect influence. While we identify a few common themes, it is apparent that deriving general mechanistic principles of microbiome influence on these parameters in epilepsy will require considerable further study to tease out the many interacting factors, systems, and conditions.

Microbiota and sleep: awakening the gut feeling.

Various lifestyle and environmental factors are known to influence sleep. Increasingly, evidence points to a role for the microbiota in regulating brain and behaviour. This article explores how the microbiota-gut-brain axis affects sleep directly and indirectly. We summarize the possible molecular mechanisms underlying sleep-microbiome interactions and discuss how various factors interact with the gut microbiota to influence sleep. Furthermore, we present the current evidence of alterations of the microbiota-gut-brain axis in various sleep disorders and pathologies where comorbid sleep disturbances are common. Since manipulating the gut microbiota could potentially improve sleep, we outline ways in which this can be achieved.

Cortical low-frequency power correlates with behavioral impairment in animal model of focal limbic seizures.

OBJECTIVE: Impairment in consciousness is a debilitating symptom during and after seizures; however, its mechanism remains unclear. Limbic seizures have been shown to spread to arousal circuitry to result in a "network inhibition" phenomenon. However, prior animal model studies did not relate physiological network changes to behavioral responses during or following seizures. METHODS: Focal onset limbic seizures were induced while rats were performing an operant conditioned behavioral task requiring response to an auditory stimulus to quantify how and when impairment of behavioral response occurs. Correct responses were rewarded with sucrose. Cortical and hippocampal electrophysiology measured by local field potential recordings was analyzed for changes in low- and high-frequency power in relation to behavioral responsiveness during seizures. RESULTS: As seen in patients with seizures, ictal (p < .0001) and postictal (p = .0015) responsiveness was variably impaired. Analysis of cortical and hippocampal electrophysiology revealed that ictal (p = .002) and postictal (p = .009) frontal cortical low-frequency 3-6-Hz power was associated with poor behavioral performance. In contrast, the hippocampus showed increased power over a wide frequency range during seizures, and suppression postictally, neither of which were related to behavioral impairment. SIGNIFICANCE: These findings support prior human studies of temporal lobe epilepsy as well as anesthetized animal models suggesting that focal limbic seizures depress consciousness through remote network effects on the cortex, rather than through local hippocampal involvement. By identifying the cortical physiological changes associated with impaired arousal and responsiveness in focal seizures, these results may help guide future therapies to restore ictal and postictal consciousness, improving quality of life for people with epilepsy.

Thalamic Stimulation Improves Postictal Cortical Arousal and Behavior.

The postictal state following seizures is characterized by impaired consciousness and has a major negative impact on individuals with epilepsy. Previous work in disorders of consciousness including the postictal state suggests that bilateral deep brain stimulation (DBS) of the thalamic intralaminar central lateral nucleus (CL) may improve level of arousal. We tested the effects of postictal thalamic CL DBS in a rat model of secondarily generalized seizures elicited by electrical hippocampal stimulation. Thalamic CL DBS was delivered at 100 Hz during the postictal period in 21 female rats while measuring cortical electrophysiology and behavior. The postictal period was characterized by frontal cortical slow waves, like other states of depressed consciousness. In addition, rats exhibited severely impaired responses on two different behavioral tasks in the postictal state. Thalamic CL stimulation prevented postictal cortical slow wave activity but produced only modest behavioral improvement on a spontaneous licking sucrose reward task. We therefore also tested responses using a lever-press shock escape/avoidance (E/A) task. Rats achieved high success rates responding to the sound warning on the E/A task even during natural slow wave sleep but were severely impaired in the postictal state. Unlike the spontaneous licking task, thalamic CL DBS during the E/A task produced a marked improvement in behavior, with significant increases in lever-press shock avoidance with DBS compared with sham controls. These findings support the idea that DBS of subcortical arousal structures may be a novel therapeutic strategy benefitting patients with medically and surgically refractory epilepsy.SIGNIFICANCE STATEMENT The postictal state following seizures is characterized by impaired consciousness and has a major negative impact on individuals with epilepsy. For the first time, we developed two behavioral tasks and demonstrate that bilateral deep brain stimulation (DBS) of the thalamic intralaminar central lateral nucleus (CL) decreased cortical slow wave activity and improved task performance in the postictal period. Because preclinical task performance studies are crucial to explore the effectiveness and safety of DBS treatment, our work is clinically relevant as it could support and help set the foundations for a human neurostimulation trial to improve postictal responsiveness in patients with medically and surgically refractory epilepsy.

Clinical and experimental insight into pathophysiology, comorbidity and therapy of absence seizures.

Absence seizures in children and teenagers are generally considered relatively benign because of their non-convulsive nature and the large incidence of remittance in early adulthood. Recent studies, however, show that 30% of children with absence seizures are pharmaco-resistant and 60% are affected by severe neuropsychiatric comorbid conditions, including impairments in attention, cognition, memory and mood. In particular, attention deficits can be detected before the epilepsy diagnosis, may persist even when seizures are pharmacologically controlled and are aggravated by valproic acid monotherapy. New functional MRI-magnetoencephalography and functional MRI-EEG studies provide conclusive evidence that changes in blood oxygenation level-dependent signal amplitude and frequency in children with absence seizures can be detected in specific cortical networks at least 1 min before the start of a seizure, spike-wave discharges are not generalized at seizure onset and abnormal cortical network states remain during interictal periods. From a neurobiological perspective, recent electrical recordings and imaging of large neuronal ensembles with single-cell resolution in non-anaesthetized models show that, in contrast to the predominant opinion, cortical mechanisms, rather than an exclusively thalamic rhythmogenesis, are key in driving seizure ictogenesis and determining spike-wave frequency. Though synchronous ictal firing characterizes cortical and thalamic activity at the population level, individual cortico-thalamic and thalamocortical neurons are sparsely recruited to successive seizures and consecutive paroxysmal cycles within a seizure. New evidence strengthens previous findings on the essential role for basal ganglia networks in absence seizures, in particular the ictal increase in firing of substantia nigra GABAergic neurons. Thus, a key feature of thalamic ictogenesis is the powerful increase in the inhibition of thalamocortical neurons that originates at least from two sources, substantia nigra and thalamic reticular nucleus. This undoubtedly provides a major contribution to the ictal decrease in total firing and the ictal increase of T-type calcium channel-mediated burst firing of thalamocortical neurons, though the latter is not essential for seizure expression. Moreover, in some children and animal models with absence seizures, the ictal increase in thalamic inhibition is enhanced by the loss-of-function of the astrocytic GABA transporter GAT-1 that does not necessarily derive from a mutation in its gene. Together, these novel clinical and experimental findings bring about paradigm-shifting views of our understanding of absence seizures and demand careful choice of initial monotherapy and continuous neuropsychiatric evaluation of affected children. These issues are discussed here to focus future clinical and experimental research and help to identify novel therapeutic targets for treating both absence seizures and their comorbidities.

Increased amygdalar metabotropic glutamate receptor 7 mRNA in a genetic mouse model of impaired fear extinction.

RATIONALE: Post-traumatic stress disorder (PTSD) is a devastating anxiety-related disorder which develops subsequent to a severe psychologically traumatic event. Only ~ 9% of people who experience such a trauma develop PTSD. It is clear that a number of factors, including genetics, influence whether an individual will develop PTSD subsequent to a trauma. The 129S1/SvImJ (S1) inbred mouse strain displays poor fear extinction and may be useful to model this specific aspect of PTSD. The metabotropic glutamate receptor 7 (mGlu7 receptor) has previously been shown to be involved in cognitive processes and anxiety-like behaviour placing it in a key position to regulate fear extinction processes. We sought to compare mGlu7 receptor mRNA levels in the S1 strain with those in the robustly extinguishing C57BL/6J (B6) inbred strain using in situ hybridisation (ISH) in three brain regions associated with fear extinction: the amygdala, hippocampus and prefrontal cortex (PFC). RESULTS: Compared to the B6 strain, S1 mice had increased mGlu7 receptor mRNA levels in the lateral amygdala (LA) and basolateral amygdala (BLA) subdivisions. An increase was also seen in the hippocampal CA1 and CA3 subregions of S1 mice. No difference in mGlu7 receptor levels were seen in the central nucleus (CeA) of the amygdala, dentate gyrus (DG) of the hippocampus or prefrontal cortex. CONCLUSIONS: These data show altered mGlu7 receptor expression in key brain regions associated with fear extinction in two different inbred mouse strains which differ markedly in their fear extinction behaviour. Altered mGlu7 receptor levels may contribute to the deficit fear extinction processes seen in fear extinction in the S1 strain.

Cortical drive and thalamic feed-forward inhibition control thalamic output synchrony during absence seizures.

Behaviorally and pathologically relevant cortico-thalamo-cortical oscillations are driven by diverse interacting cell-intrinsic and synaptic processes. However, the mechanism that gives rise to the paroxysmal oscillations of absence seizures (ASs) remains unknown. Here we report that, during ASs in behaving animals, cortico-thalamic excitation drives thalamic firing by preferentially eliciting tonic rather than T-type Ca 2+ channel (T-channel)-dependent burst firing in thalamocortical (TC) neurons and by temporally framing thalamic output via feedforward reticular thalamic (NRT)-to-TC neuron inhibition. In TC neurons, overall ictal firing was markedly reduced and bursts rarely occurred. Moreover, blockade of T-channels in cortical and NRT neurons suppressed ASs, but such blockade in TC neurons had no effect on seizures or on ictal thalamic output synchrony. These results demonstrate ictal bidirectional cortico-thalamic communications and provide the first mechanistic understanding of cortico-thalamo-cortical network firing dynamics during ASs in behaving animals.

Seizures and Sleep in the Thalamus: Focal Limbic Seizures Show Divergent Activity Patterns in Different Thalamic Nuclei.

The thalamus plays diverse roles in cortical-subcortical brain activity patterns. Recent work suggests that focal temporal lobe seizures depress subcortical arousal systems and convert cortical activity into a pattern resembling slow-wave sleep. The potential simultaneous and paradoxical role of the thalamus in both limbic seizure propagation, and in sleep-like cortical rhythms has not been investigated. We recorded neuronal activity from the central lateral (CL), anterior (ANT), and ventral posteromedial (VPM) nuclei of the thalamus in an established female rat model of focal limbic seizures. We found that population firing of neurons in CL decreased during seizures while the cortex exhibited slow waves. In contrast, ANT showed a trend toward increased neuronal firing compatible with polyspike seizure discharges seen in the hippocampus. Meanwhile, VPM exhibited a remarkable increase in sleep spindles during focal seizures. Single-unit juxtacellular recordings from CL demonstrated reduced overall firing rates, but a switch in firing pattern from single spikes to burst firing during seizures. These findings suggest that different thalamic nuclei play very different roles in focal limbic seizures. While limbic nuclei, such as ANT, appear to participate directly in seizure propagation, arousal nuclei, such as CL, may contribute to depressed cortical function, whereas sleep spindles in relay nuclei, such as VPM, may interrupt thalamocortical information flow. These combined effects could be critical for controlling both seizure severity and impairment of consciousness. Further understanding of differential effects of seizures on different thalamocortical networks may lead to improved treatments directly targeting these modes of impaired function.SIGNIFICANCE STATEMENT Temporal lobe epilepsy has a major negative impact on quality of life. Previous work suggests that the thalamus plays a critical role in thalamocortical network modulation and subcortical arousal maintenance, but its precise seizure-associated functions are not known. We recorded neuronal activity in three different thalamic regions and found divergent activity patterns, which may respectively participate in seizure propagation, impaired level of conscious arousal, and altered relay of information to the cortex during focal limbic seizures. These very different activity patterns within the thalamus may help explain why focal temporal lobe seizures often disrupt widespread network function, and can help guide future treatments aimed at restoring normal thalamocortical network activity and cognition.

Restoring Conscious Arousal During Focal Limbic Seizures with Deep Brain Stimulation.

Impaired consciousness occurs suddenly and unpredictably in people with epilepsy, markedly worsening quality of life and increasing risk of mortality. Focal seizures with impaired consciousness are the most common form of epilepsy and are refractory to all current medical and surgical therapies in about one-sixth of cases. Restoring consciousness during and following seizures would be potentially transformative for these individuals. Here, we investigate deep brain stimulation to improve level of conscious arousal in a rat model of focal limbic seizures. We found that dual-site stimulation of the central lateral nucleus of the intralaminar thalamus (CL) and the pontine nucleus oralis (PnO) bilaterally during focal limbic seizures restored normal-appearing cortical electrophysiology and markedly improved behavioral arousal. In contrast, single-site bilateral stimulation of CL or PnO alone was insufficient to achieve the same result. These findings support the "network inhibition hypothesis" that focal limbic seizures impair consciousness through widespread inhibition of subcortical arousal. Driving subcortical arousal function would be a novel therapeutic approach to some forms of refractory epilepsy and may be compatible with devices already in use for responsive neurostimulation. Multisite deep brain stimulation of subcortical arousal structures may benefit not only patients with epilepsy but also those with other disorders of consciousness.

Optogenetic stimulation of cholinergic brainstem neurons during focal limbic seizures: Effects on cortical physiology.

Focal temporal lobe seizures often cause impaired cortical function and loss of consciousness. Recent work suggests that the mechanism for depressed cortical function during focal seizures may depend on decreased subcortical cholinergic arousal, which leads to a sleep-like state of cortical slow-wave activity. To test this hypothesis, we sought to directly activate subcortical cholinergic neurons during focal limbic seizures to determine the effects on cortical function. Here we used an optogenetic approach to selectively stimulate cholinergic brainstem neurons in the pedunculopontine tegmental nucleus during focal limbic seizures induced in a lightly anesthetized rat model. We found an increase in cortical gamma activity and a decrease in delta activity in response to cholinergic stimulation. These findings support the mechanistic role of reduced subcortical cholinergic arousal in causing cortical dysfunction during seizures. Through further work, electrical or optogenetic stimulation of subcortical arousal networks may ultimately lead to new treatments aimed at preventing cortical dysfunction during seizures.

GABAB Receptors Regulate Extrasynaptic GABAA Receptors.

Tonic inhibitory GABA(A) receptor-mediated currents are observed in numerous cell types in the CNS, including thalamocortical neurons of the ventrobasal thalamus, dentate gyrus granule cells, and cerebellar granule cells. Here we show that in rat brain slices, activation of postsynaptic GABA(B) receptors enhances the magnitude of the tonic GABA(A) current recorded in these cell types via a pathway involving G G proteins, adenylate cyclase, and cAMP-dependent protein kinase. Using a combination of pharmacology and knockout mice, we show that this pathway is independent of potassium channels or GABA transporters. Furthermore, the enhancement in tonic current is sufficient to significantly alter the excitability of thalamocortical neurons. These results demonstrate for the first time a postsynaptic crosstalk between GABA(B) and GABA(A) receptors.