GABAA receptor-mediated tonic inhibition in thalamic neurons

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.

Extrasynaptic δGABAA receptors mediate resistance to migraine-like phenotype in rats

BACKGROUND

GABA, a key inhibitory neurotransmitter, has synaptic and extrasynaptic receptors on the postsynaptic neuron. Background GABA, which spills over from the synaptic cleft, acts on extrasynaptic delta subunit containing GABAA receptors. The role of extrasynaptic GABAergic input in migraine is unknown. We investigated the susceptibility to valid migraine-provoking substances with clinically relevant behavioral readouts in Genetic Absence Epilepsy of Rats Strasbourg (GAERS), in which the GABAergic tonus was altered. Subsequently, we screened relevant GABAergic mechanisms in Wistar rats by pharmacological means to identify the mechanisms.

METHODS

Wistar and GAERS rats were administered nitroglycerin (10 mg/kg) or levcromakalim (1 mg/kg). Mechanical allodynia and photophobia were assessed using von Frey monofilaments and a dark-light box. Effects of GAT-1 blocker tiagabine (5 mg/kg), GABAB receptor agonist baclofen (2 mg/kg), synaptic GABAA receptor agonist diazepam (1 mg/kg), extrasynaptic GABAA receptor agonists gaboxadol (4 mg/kg), and muscimol (0.75 mg/kg), T-type calcium channel blocker ethosuximide (100 mg/kg) or synaptic GABAA receptor antagonist flumazenil (15 mg/kg) on levcromakalim-induced migraine phenotype were screened.

RESULTS

Unlike Wistar rats, GAERS exhibited no reduction in mechanical pain thresholds or light aversion following nitroglycerin or levcromakalim injection. Ethosuximide did not reverse the resistant phenotype in GAERS, excluding the role of T-type calcium channel dysfunction in this phenomenon. Tiagabine prevented levcromakalim-induced mechanical allodynia in Wistar rats, suggesting a key role in enhanced GABA spillover. Baclofen did not alleviate mechanical allodynia. Diazepam failed to mitigate levcromakalim-induced migraine phenotype. Additionally, the resistant phenotype in GAERS was not affected by flumazenil. Extrasynaptic GABAA receptor agonists gaboxadol and muscimol inhibited periorbital allodynia in Wistar rats.

CONCLUSION

Our study introduced a rat strain resistant to migraine-provoking agents and signified a critical involvement of extrasynaptic δGABAergic receptors. Extrasynaptic δ GABAA receptors, by mediating constant background inhibition on the excitability of neurons, stand as a novel drug target with a therapeutic potential in migraine.

Cortical Auditory Evoked Potential Indices of Impaired Sensory Gating in People With Chronic Tinnitus

OBJECTIVES

The primary aim of this study was to evaluate whether there is cortical auditory evoked potential (CAEP) evidence of impaired sensory gating in individuals with tinnitus. On the basis of the proposed mechanism of tinnitus generation, including a thalamocortical inhibitory deficit, it was hypothesized that individuals with tinnitus would lack the normal inhibitory effect on the second CAEP response in a paired-click sensory gating paradigm, resulting in larger sensory gating ratios in individuals with tinnitus relative to age-, sex-, and hearing-matched controls. Further, this study assessed the relative predictive influence of tinnitus presence versus other related individual characteristics (hearing loss, age, noise exposure history, and speech perception in noise) on sensory gating.

DESIGN

A paired-click CAEP paradigm was used to measure sensory gating outcomes in an independent group's experimental design. Adults who perceived chronic unilateral or bilateral tinnitus were matched with control group counterparts without tinnitus by age, hearing, and sex (n = 18; 10 females, eight males in each group). Amplitude, area, and latency sensory gating ratios were determined for measured P1, N1, and P2 responses evoked by the first and second click in the paradigm and compared between groups by independent t tests. The relative influence of tinnitus (presence/absence), age (in years), noise exposure history (subjective self-report), hearing loss (pure-tone audiometric thresholds), and speech perception in noise (signal to noise ratio-50) on sensory gating was determined based on the proportional reduction in error associated with each variable using multiple regression.

RESULTS

A significantly larger was identified in the tinnitus group relative to the control group, consistent with the hypothesis of poorer sensory gating and poorer thalamocortical inhibition in individuals with chronic tinnitus. On the basis of the proportional reduction in error, the influence of tinnitus presence better predicted compared with other related individual characteristics (age, noise exposure history, hearing loss, and speech perception in noise).

CONCLUSIONS

Results consistent with poorer sensory gating, including a larger , were found for the tinnitus group compared with the controls. This finding supported a thalamocortical inhibitory deficit in the tinnitus group and suggests that individuals with tinnitus may have poorer sensory gating. However, the tinnitus group did differ from controls in meaningful ways including having worse pure-tone thresholds in the extended high-frequency region, lower high-frequency distortion product otoacoustic emissions, and poorer speech perception in noise. Although tinnitus best predicted sensory gating outcomes, the specific effects of tinnitus presence versus absence and other individual characteristics on sensory gating cannot be completely separated.

The type of inhibition provided by thalamic interneurons alters the input selectivity of thalamocortical neurons

A fundamental problem in neuroscience is how neurons select for their many inputs. A common assumption is that a neuron's selectivity is largely explained by differences in excitatory synaptic input weightings. Here we describe another solution to this important problem. We show that within the first order visual thalamus, the type of inhibition provided by thalamic interneurons has the potential to alter the input selectivity of thalamocortical neurons. To do this, we developed conductance injection protocols to compare how different types of synchronous and asynchronous GABA release influence thalamocortical excitability in response to realistic patterns of retinal ganglion cell input. We show that the asynchronous GABA release associated with tonic inhibition is particularly efficient at maintaining information content, ensuring that thalamocortical neurons can distinguish between their inputs. We propose a model where alterations in GABA release properties results in rapid changes in input selectivity without requiring structural changes in the network.

A role for δ subunit-containing GABA A receptors on parvalbumin positive neurons in maintaining electrocortical signatures of sleep states

GABA A receptors containing δ subunits have been shown to mediate tonic/slow inhibition in the CNS. These receptors are typically found extrasynaptically and are activated by relatively low levels of ambient GABA in the extracellular space. In the mouse neocortex, δ subunits are expressed on the surface of some pyramidal cells as well as on parvalbumin positive (PV+) interneurons. An important function of PV+ interneurons is the organization of coordinated network activity that can be measured by EEG; however, it remains unclear what role tonic/slow inhibitory control of PV+ neurons may play in shaping oscillatory activity. After confirming a loss of functional δ mediated tonic currents in PV cells in cortical slices from mice lacking Gabrd in PV+ neurons (PV δcKO), we performed EEG recordings to survey network activity across wake and sleep states. PV δcKO mice showed altered spectral content of EEG during NREM and REM sleep that was a result of increased oscillatory activity in NREM and the emergence of transient high amplitude bursts of theta frequency activity during REM. Viral reintroduction of Gabrd to PV+ interneurons in PV δcKO mice rescued REM EEG phenotypes, supporting an important role for δ subunit mediated inhibition of PV+ interneurons for maintaining normal REM cortical oscillations.

Significance statement

The impact on cortical EEG of inhibition on PV+ neurons was studied by deleting a GABA A receptor subunit selectively from these neurons. We discovered unexpected changes at low frequencies during sleep that were rescued by viral reintroduction.

Enhancing GABAergic Tonic Inhibition Reduces Seizure-Like Activity in the Neonatal Mouse Hippocampus and Neocortex

Approximately one-third of neonatal seizures do not respond to first-line anticonvulsants, including phenobarbital, which enhances phasic inhibition. Whether enhancing tonic inhibition decreases seizure-like activity in the neonate when GABA is mainly depolarizing at this age is unknown. We evaluated if increasing tonic inhibition using THIP [4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol, gaboxadol], a δ-subunit-selective GABAA receptor agonist, decreases seizure-like activity in neonatal C57BL/6J mice (postnatal day P5-8, both sexes) using acute brain slices. Whole-cell patch-clamp recordings showed that THIP enhanced GABAergic tonic inhibitory conductances in layer V neocortical and CA1 pyramidal neurons and increased their rheobase without altering sEPSC characteristics. Two-photon calcium imaging demonstrated that enhancing the activity of extrasynaptic GABAARs decreased neuronal firing in both brain regions. In the 4-aminopyridine and the low-Mg2+ model of pharmacoresistant seizures, THIP reduced epileptiform activity in the neocortex and CA1 hippocampal region of neonatal and adult brain slices in a dose-dependent manner. We conclude that neocortical layer V and CA1 pyramidal neurons have tonic inhibitory conductances, and when enhanced, they reduce neuronal firing and decrease seizure-like activity. Therefore, augmenting tonic inhibition could be a viable approach for treating neonatal seizures.

In vivo spontaneous activity and coital-evoked inhibition of mouse accessory olfactory bulb output neurons

Little is known about estrous effects on brain microcircuits. We examined the accessory olfactory bulb (AOB) in vivo, in anesthetized naturally cycling females, as model microcircuit receiving coital somatosensory information. Whole-cell recordings demonstrate that output neurons are relatively hyperpolarized in estrus and unexpectedly fire high frequency bursts of action potentials. To mimic coitus, a calibrated artificial vagino-cervical stimulation (aVCS) protocol was devised. aVCS evoked stimulus-locked local field responses in the interneuron layer independent of estrous stage. The response is sensitive to α1-adrenergic receptor blockade, as expected since aVCS increases norepinephrine release in AOB. Intriguingly, only in estrus does aVCS inhibit AOB spike output. Estrus-specific output reduction coincides with prolonged aVCS activation of inhibitory interneurons. Accordingly, in estrus the AOB microcircuit sets the stage for coital stimulation to inhibit the output neurons, possibly via high frequency bursting-dependent enhancement of reciprocal synapse efficacy between inter- and output neurons.

Functional and structural synaptic remodeling mechanisms underlying somatotopic organization and reorganization in the thalamus

The somatosensory system organizes the topographic representation of body maps, termed somatotopy, at all levels of an ascending hierarchy. Postnatal maturation of somatotopy establishes optimal somatosensation, whereas deafferentation in adults reorganizes somatotopy, which underlies pathological somatosensation, such as phantom pain and complex regional pain syndrome. Here, we focus on the mouse whisker somatosensory thalamus to study how sensory experience shapes the fine topography of afferent connectivity during the critical period and what mechanisms remodel it and drive a large-scale somatotopic reorganization after peripheral nerve injury. We will review our findings that, following peripheral nerve injury in adults, lemniscal afferent synapses onto thalamic neurons are remodeled back to immature configuration, as if the critical period reopens. The remodeling process is initiated with local activation of microglia in the brainstem somatosensory nucleus downstream to injured nerves and heterosynaptically controlled by input from GABAergic and cortical neurons to thalamic neurons. These fruits of thalamic studies complement well-studied cortical mechanisms of somatotopic organization and reorganization and unveil potential intervention points in treating pathological somatosensation.

GABA tone regulation and its cognitive functions in the brain

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter released at GABAergic synapses, mediating fast-acting phasic inhibition. Emerging lines of evidence unequivocally indicate that a small amount of extracellular GABA - GABA tone - exists in the brain and induces a tonic GABA current that controls neuronal activity on a slow timescale relative to that of phasic inhibition. Surprisingly, studies indicate that glial cells that synthesize GABA, such as astrocytes, release GABA through non-vesicular mechanisms, such as channel-mediated release, and thereby act as the source of GABA tone in the brain. In this Review, we first provide an overview of major advances in our understanding of the cell-specific molecular and cellular mechanisms of GABA synthesis, release and clearance that regulate GABA tone in various brain regions. We next examine the diverse ways in which the tonic GABA current regulates synaptic transmission and synaptic plasticity through extrasynaptic GABAA-receptor-mediated mechanisms. Last, we discuss the physiological mechanisms through which tonic inhibition modulates cognitive function on a slow timescale. In this Review, we emphasize that the cognitive functions of tonic GABA current extend beyond mere inhibition, laying a foundation for future research on the physiological and pathophysiological roles of GABA tone regulation in normal and abnormal psychiatric conditions.

The GABAA receptor modulator zolpidem augments hippocampal-prefrontal coupling during non-REM sleep

Benzodiazepines and 'Z-drugs' (including zolpidem and zopiclone) are GABAA receptor (GABAAR) positive modulators commonly prescribed as hypnotics to treat insomnia and/or anxiety. However, alongside sedation, augmenting GABAAR function may also alter coordinated neuronal activity during sleep, thereby influencing sleep-dependent processes including memory consolidation. We used simultaneous recordings of neural population activity from the medial prelimbic cortex (PrL) and CA1 of the dorsal hippocampus (dCA1) of naturally sleeping rats to detail the effects of zolpidem on network activity during the cardinal oscillations of non-REM sleep. For comparison, we also characterized the effects of diazepam and 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP/gaboxadol), which acts predominantly at extra-synaptic GABAARs. Zolpidem and THIP significantly increased the amplitudes of slow-waves, which were attenuated by diazepam. Zolpidem increased hippocampal ripple density whereas diazepam decreased both ripple density and intrinsic frequency. While none of the drugs affected thalamocortical spindles in isolation, zolpidem augmented the temporal coordination between slow-waves and spindles. At the cellular level, analyses of spiking activity from 523 PrL and 579 dCA1 neurons revealed that zolpidem significantly enhanced synchronized pauses in cortical firing during slow-wave down states, while increasing correlated activity within and between dCA1 and PrL populations. Of the drugs compared here, zolpidem was unique in augmenting coordinated activity within and between hippocampus and neocortex during non-REM sleep. Zolpidem's enhancement of hippocampal-prefrontal coupling may reflect the cellular basis of its potential to modulate offline memory processing.

Developmental Inhibitory Changes in the Primary Somatosensory Cortex of the Stargazer Mouse Model of Absence Epilepsy

Childhood absence epilepsy seizures arise in the cortico-thalamocortical network due to multiple cellular and molecular mechanisms, which are still under investigation. Understanding the precise mechanisms is imperative given that treatment fails in ~30% of patients while adverse neurological sequelae remain common. Impaired GABAergic neurotransmission is commonly reported in research models investigating these mechanisms. Recently, we reported a region-specific reduction in the whole-tissue and synaptic GABA_A receptor (GABA_AR) α1 subunit and an increase in whole-tissue GAD65 in the primary somatosensory cortex (SoCx) of the adult epileptic stargazer mouse compared with its non-epileptic (NE) littermate. The current study investigated whether these changes occurred prior to the onset of seizures on postnatal days (PN) 17-18, suggesting a causative role. Synaptic and cytosolic fractions were biochemically isolated from primary SoCx lysates followed by semiquantitative Western blot analyses for GABA_AR α1 and GAD65. We found no significant changes in synaptic GABA_AR α1 and cytosolic GAD65 in the primary SoCx of the stargazer mice at the critical developmental stages of PN 7-9, 13-15, and 17-18. This indicates that altered levels of GABA_AR α1 and GAD65 in adult mice do not directly contribute to the initial onset of absence seizures but are a later consequence of seizure activity.

Altered GABAA Receptor Expression in the Primary Somatosensory Cortex of a Mouse Model of Genetic Absence Epilepsy

Absence seizures are hyperexcitations within the cortico-thalamocortical (CTC) network, however the underlying causative mechanisms at the cellular and molecular level are still being elucidated and appear to be multifactorial. Dysfunctional feed-forward inhibition (FFI) is implicated as one cause of absence seizures. Previously, we reported altered excitation onto parvalbumin-positive (PV⁺) interneurons in the CTC network of the stargazer mouse model of absence epilepsy. In addition, downstream changes in GABAergic neurotransmission have also been identified in this model. Our current study assessed whether dysfunctional FFI affects GABA_A receptor (GABA_AR) subunit expression in the stargazer primary somatosensory cortex (SoCx). Global tissue expression of GABA_AR subunits α1, α3, α4, α5, β2, β3, γ2 and δ were assessed using Western blotting (WB), while biochemically isolated subcellular fractions were assessed for the α and δ subunits. We found significant reductions in tissue and synaptic expression of GABA_AR α1, 18% and 12.2%, respectively. However, immunogold-cytochemistry electron microscopy (ICC-EM), conducted to assess GABA_AR α1 specifically at synapses between PV⁺ interneurons and their targets, showed no significant difference. These data demonstrate a loss of phasic GABA_AR α1, indicating altered GABAergic inhibition which, coupled with dysfunctional FFI, could be one mechanism contributing to the generation or maintenance of absence seizures.

The fate of interneurons, GABAA receptor sub-types and perineuronal nets in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurological disease, which is associated with gradual memory loss and correlated with synaptic hyperactivity and abnormal oscillatory rhythmic brain activity that precedes phenotypic alterations and is partly responsible for the spread of the disease pathology. Synaptic hyperactivity is thought to be because of alteration in the homeostasis of phasic and tonic synaptic inhibition, which is orchestrated by the GABA_A inhibitory system, encompassing subclasses of interneurons and GABA_A receptors, which play a vital role in cognitive functions, including learning and memory. Furthermore, the extracellular matrix, the perineuronal nets (PNNs) which often go unnoticed in considerations of AD pathology, encapsulate the inhibitory cells and neurites in critical brain regions and have recently come under the light for their crucial role in synaptic stabilisation and excitatory-inhibitory balance and when disrupted, serve as a potential trigger for AD-associated synaptic imbalance. Therefore, in this review, we summarise the current understanding of the selective vulnerability of distinct interneuron subtypes, their synaptic and extrasynaptic GABA_A R subtypes as well as the changes in PNNs in AD, detailing their contribution to the mechanisms of disease development. We aim to highlight how seemingly unique malfunction in each component of the interneuronal GABA inhibitory system can be tied together to result in critical circuit dysfunction, leading to the irreversible symptomatic damage observed in AD.

Altered Behavioral Responses Show GABA Sensitivity in Muscleblind-Like 2-Deficient Mice: Implications for CNS Symptoms in Myotonic Dystrophy

Considerable evidence from mouse models and human postmortem brain suggests loss of Muscleblind-like protein 2 (MBNL2) function in brain is a major driver of CNS symptoms in Myotonic dystrophy type 1 (DM1). Increased hypersomnia, fatigue, and surgical complications associated with general anesthesia suggest possible sensitivity to GABAergic inhibition in DM1. To test the hypothesis that MBNL2 depletion leads to behavioral sensitivity to GABA_A receptor (GABA_A-R) modulation, Mbnl2 knock-out (KO) and wild-type (WT) littermates were treated with the anesthetic sevoflurane, the benzodiazepine diazepam, the imidazopyridine zolpidem, and the benzodiazepine rescue agent, flumazenil (Ro 15-1788), and assessed for various behavioral metrics. Mbnl2 KO mice exhibited delayed recovery following sevoflurane, delayed emergence and recovery from zolpidem, and enhanced sleep time at baseline that was modulated by flumazenil. A significantly higher proportion of Mbnl2 KO mice also loss their righting reflex [loss of righting reflex (LORR)] from a standard diazepam dose. We further examined whether MBNL2 depletion affects total GABA_A-R mRNA subunit levels and validated RNA-sequencing data of mis-spliced Gabrg2, whose isoform ratios are known to regulate GABA sensitivity and associated behaviors. While no other GABA_A-R subunit mRNA levels tested were altered in Mbnl2 KO mouse prefrontal cortex, Gabrg2S/L mRNA ratio levels were significantly altered. Taken together, our findings indicate that loss of MBNL2 function affects GABAergic function in a mouse model of myotonic dystrophy (DM1).

Enhancing GAT-3 in thalamic astrocytes promotes resilience to brain injury in rodents

Inflammatory processes induced by brain injury are important for recovery; however, when uncontrolled, inflammation can be deleterious, likely explaining why most anti-inflammatory treatments have failed to improve neurological outcomes after brain injury in clinical trials. In the thalamus, chronic activation of glial cells, a proxy of inflammation, has been suggested as an indicator of increased seizure risk and cognitive deficits that develop after cortical injury. Furthermore, lesions in the thalamus, more than other brain regions, have been reported in patients with viral infections associated with neurological deficits, such as SARS-CoV-2. However, the extent to which thalamic inflammation is a driver or by-product of neurological deficits remains unknown. Here, we found that thalamic inflammation in mice was sufficient to phenocopy the cellular and circuit hyperexcitability, enhanced seizure risk, and disruptions in cortical rhythms that develop after cortical injury. In our model, down-regulation of the GABA transporter GAT-3 in thalamic astrocytes mediated this neurological dysfunction. In addition, GAT-3 was decreased in regions of thalamic reactive astrocytes in mouse models of cortical injury. Enhancing GAT-3 in thalamic astrocytes prevented seizure risk, restored cortical states, and was protective against severe chemoconvulsant-induced seizures and mortality in a mouse model of traumatic brain injury, emphasizing the potential of therapeutically targeting this pathway. Together, our results identified a potential therapeutic target for reducing negative outcomes after brain injury.

Circuits and components of delta wave regulation

Sleep is vital and the deepest stages of sleep occur within Non-rapid-eye-movement sleep (NREM), defined by high electroencephalographic power in the delta (~0.5-4Hz) wave frequency range. Delta waves are thought to facilitate a myriad of physical and mental health functions. This review aims to comprehensively cover the historical and recent advances in the understanding of the mechanisms orchestrating NREM delta waves. We discuss a complete neurocircuit - focusing on one leg of the circuit at a time - and delve deeply into the molecular mechanistic components that contribute to NREM delta wave regulation. We also discuss the relatively localized nature in which these mechanisms have been defined, and how likely they might generalize across distinct sensory and higher order modalities in the brain.

Distinct Firing Activities of the Hypothalamic Arcuate Nucleus Neurons to Appetite Hormones

The hypothalamic arcuate nucleus (Arc) is a central unit that controls the appetite through the integration of metabolic, hormonal, and neuronal afferent inputs. Agouti-related protein (AgRP), proopiomelanocortin (POMC), and dopaminergic neurons in the Arc differentially regulate feeding behaviors in response to hunger, satiety, and appetite, respectively. At the time of writing, the anatomical and electrophysiological characterization of these three neurons has not yet been intensively explored. Here, we interrogated the overall characterization of AgRP, POMC, and dopaminergic neurons using genetic mouse models, immunohistochemistry, and whole-cell patch recordings. We identified the distinct geographical location and intrinsic properties of each neuron in the Arc with the transgenic lines labelled with cell-specific reporter proteins. Moreover, AgRP, POMC, and dopaminergic neurons had different firing activities to ghrelin and leptin treatments. Ghrelin led to the increased firing rate of dopaminergic and AgRP neurons, and the decreased firing rate of POMC. In sharp contrast, leptin resulted in the decreased firing rate of AgRP neurons and the increased firing rate of POMC neurons, while it did not change the firing rate of dopaminergic neurons in Arc. These findings demonstrate the anatomical and physiological uniqueness of three hypothalamic Arc neurons to appetite control.

Gaboxadol in Fragile X Syndrome: A 12-Week Randomized, Double-Blind, Parallel-Group, Phase 2a Study

Background: Fragile X syndrome (FXS), the most common single-gene cause of intellectual disability and autism spectrum disorder (ASD), is caused by a >200-trinucleotide repeat expansion in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene. Individuals with FXS can present with a range of neurobehavioral impairments including, but not limited to: cognitive, language, and adaptive deficits; ASD; anxiety; social withdrawal and avoidance; and aggression. Decreased expression of the γ-aminobutyric acid type A (GABA_A) receptor δ subunit and deficient GABAergic tonic inhibition could be associated with symptoms of FXS. Gaboxadol (OV101) is a δ-subunit-selective, extrasynaptic GABA_A receptor agonist that enhances GABAergic tonic inhibition, providing the rationale for assessment of OV101 as a potential targeted treatment of FXS. No drug is approved in the United States for the treatment of FXS. Methods: This 12-weeks, randomized (1:1:1), double-blind, parallel-group, phase 2a study was designed to assess the safety, tolerability, efficacy, and optimal daily dose of OV101 5 mg [once (QD), twice (BID), or three-times daily (TID)] when administered for 12 weeks to adolescent and adult men with FXS. Safety was the primary study objective, with key assessments including treatment-emergent adverse events (TEAEs), treatment-related adverse events leading to study discontinuation, and serious adverse events (SAEs). The secondary study objective was to evaluate the effect of OV101 on a variety of problem behaviors. Results: A total of 23 participants with FXS (13 adolescents, 10 adults) with moderate-to-severe neurobehavioral phenotypes (Full Scale Intelligence Quotient, 41.5 ± 3.29; ASD, 82.6%) were randomized to OV101 5 mg QD (n = 8), 5 mg BID (n = 8), or 5 mg TID (n = 7) for 12 weeks. OV101 was well tolerated across all 3 treatment regimens. The most common TEAEs were upper respiratory tract infection (n = 4), headache (n = 3), diarrhea (n = 2), and irritability (n = 2). No SAEs were reported. Improvements from baseline to end-of-treatment were observed on several efficacy endpoints, and 60% of participants were identified as treatment responders based on Clinical Global Impressions-Improvement. Conclusions: Overall, OV101 was safe and well tolerated. Efficacy results demonstrate an initial signal for OV101 in individuals with FXS. These results need to be confirmed in a larger, randomized, placebo-controlled study with optimal outcomes and in the most appropriate age group. Clinical Trial Registration: www.ClinicalTrials.gov, identifier: NCT03697161.

Structural determinants and regulation of spontaneous activity in GABAA receptors

GABAA receptors are vital for controlling neuronal excitability and can display significant levels of constitutive activity that contributes to tonic inhibition. However, the mechanisms underlying spontaneity are poorly understood. Here we demonstrate a strict requirement for β3 subunit incorporation into receptors for spontaneous gating, facilitated by α4, α6 and δ subunits. The crucial molecular determinant involves four amino acids (GKER) in the β3 subunit's extracellular domain, which interacts with adjacent receptor subunits to promote transition to activated, open channel conformations. Spontaneous activity is further regulated by β3 subunit phosphorylation and by allosteric modulators including neurosteroids and benzodiazepines. Promoting spontaneous activity reduced neuronal excitability, indicating that spontaneous currents will alter neural network activity. This study demonstrates how regional diversity in GABAA receptor isoform, protein kinase activity, and neurosteroid levels, can impact on tonic inhibition through the modulation of spontaneous GABAA receptor gating.

Age dependent effects of Retigabine on absence seizure in WAG/Rij rats; an experimental study

Retigabine (RTG, Ezogabine, DC23129) is the first neuronal potassium channel opener in the treatment of epilepsy and exerts its effects through the activation of neuronal KCNQ2/3 potassium channels; in higher doses, it acts also on sodium and voltage-gated calcium channels. The aim of this study was to investigate possible age-dependent therapeutic effects of RTG on spike-and-wave discharges (SWD) in an animal model of absence epilepsy using WAG/Rij rats. In this study, 6- and 12-month-old WAG/Rij rats were used. For both age categories, three sub-groups that consisted of one control group (n=7) by the administration of 20% DMSO (control) and two study groups by the administration of 5 mg/kg (n=7) and 15 mg/kg RTG (n=7) were designed. EEG electrodes were placed onto the skull of anaesthetized animals; and baseline EEG was recorded for one hour after a recovery period from surgery. Then, the pre-determined two distinct doses of RTG and 20% DMSO were administered as a solvent via intraperitoneal injections, and EEG was recorded for 3 hours. After injection, both doses of RTG increased the total SWD number and duration of SWD in the first and second hours in 12-month-old rats. These parameters were elevated compared to 6-month-old rats. Age-dependent effects of RTG were observed in SWD activity. Pro-epileptic effects in middle-aged WAG/Rij rats were demonstrated in both RTG doses. Differences in the distribution of KCNQ2/3 channels and switch of GABAergic system from inhibitory to excitatory with age might contribute to increased SWD activity in middle-aged rats.

Optical Stimulation of Thalamic Spindle Circuitry Sustains Electroencephalogram Patterns of General Anesthesia but not Duration of Loss of Consciousness

Alterations in thalamic GABAergic signaling are implicated in mediating the rise in 12-30 Hz electroencephalogram (EEG) activity that signals anesthetic-induced loss-of-consciousness with GABA_A receptor-targeting general anesthetics. A number of modeling studies have identified that anesthetic-induced alterations in thalamocortico-corticothalamic signaling in the same network that generates sleep spindles would be sufficient to elicit this key EEG signature of anesthetic hypnosis with general anesthetic agents. Accordingly, we hypothesize that targeted stimulation of this thalamic GABAergic circuitry into a sleep-spindle mode of activity would promote the general anesthetic effects of etomidate. We recorded EEG activity and loss-of-righting reflex in transgenic mice expressing channel rhodopsin-2 on GABAergic neurons (ChR2-VGAT, n = 8) and control, wild-type mice (C57BL/6J, n = 8). On two consecutive days mice were randomly assigned to receive spindle-rhythm stimulation via an optical probe targeting the left reticular thalamic nucleus or no stimulation. After an initial 30-minute recording, mice were administered etomidate (12 mg/kg, intraperitoneal) and recorded for 90 min with or without optical stimulation. Etomidate elicited an increase in 12-30 Hz EEG power in wild-type and ChR2-VGAT mice for 20 min following administration (p < 0.001). Optical spindle-rhythm stimulation prolonged the increase in 12-30 Hz activity in ChR2-VGAT mice only (p = 0.023). Spindle-rhythm stimulation also increased the incidence and duration of sleep spindle-like oscillations in ChR2-VGAT mice only (all p ≤ 0.001). Despite the maintained anesthetic-like changes in EEG activity, optical spindle-rhythm stimulation was not associated with changes in the time to and duration of the loss-of-righting reflex, a behavioral endpoint of etomidate-induced general anesthesia in rodents.

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

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.

A rare case of Holmes tremor in a worker with occupational carbon monoxide poisoning

Parkinsonism and encephalopathy are frequently seen in patients who survive carbon monoxide (CO) poisoning. Neurological findings associated with CO poisoning can emerge immediately after cessation of exposure or following a brief period of pseudo-recovery. When present, the tremor associated with CO poisoning is typical of the postural/intention type. Here, we report on a rare case of toxic encephalopathy with a dominant-hand Holmes-type tremor, characterized by resting, as well as postural and kinetic/intentional components, in a previously healthy 53-year-old man exposed to CO while actively engaged in the process of performing a physically demanding skilled labor task. The unique neuropathological and functional changes that give rise to Holmes-type tremor and how this relates to the selective vulnerability of the inhibitory indirect pathway of the basal ganglia to glutamatergic excitotoxicity mediated by tissue hypoxia are discussed.

The effect of GABAergic neurotransmission on the seizure-related activity of the laterodorsal thalamic nuclei and the somatosensory cortex in a genetic model of absence epilepsy

The present study aimed to investigate the alterations of the GABAergic system in the laterodorsal nucleus (LDN) of the thalamus and the somatosensory cortex (SC) in an experimental model of absence seizure. The effects of pharmacological manipulation of both GABA_A and GABA_B receptor subunits in the LDN on the generation of spike-wave discharges (SWD) were evaluated. The experiments were carried out in four groups of both WAG/Rij and Wistar rats with 2 and 6 months of age. The expressions of various GABA receptor subunits were studied in the LDN and SC. Furthermore, recordings of unit activity from the LDN and electrocorticography were simultaneously monitored before, during, and after the application of GABA_A and GABA_B antagonists in the LDN. The generation of SWD in the older WAG/Rij rats was associated with significant alterations in the expression of GABA_ARα1, GABA_ARβ3, and GABA_BR2 subunits in the LDN as well as GABA_ARα1, GABA_ARβ3, GABA_ARγ2, and GABA_BR2 subunits in the SC. Furthermore, the occurrence of SWD was associated with a significant reduction of gene expression of GABAARα1 and increase of GABAARβ3 in the LDN as well as reduction of GABAARα1, GABAARβ3, GABAARγ2, and GABABR2 in the SC. The microionthophoretic application of the GABA_A antagonist bicuculline resulted in a significant increase in the population firing rate of LDN neurons as well as the mean number and duration of SWD. The application of the GABA_B antagonist CGP35348 significantly increased the population firing rate of LDN neurons but decreased the mean number of SWD. Our data indicate the regulatory effect of the GABAergic system of the LDN and SC in absence seizures.

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.

Nonlinearities between inhibition and T-type calcium channel activity bidirectionally regulate thalamic oscillations

Absence seizures result from 3 to 5 Hz generalized thalamocortical oscillations that depend on highly regulated inhibitory neurotransmission in the thalamus. Efficient reuptake of the inhibitory neurotransmitter GABA is essential, and reuptake failure worsens human seizures. Here, we show that blocking GABA transporters (GATs) in acute rat brain slices containing key parts of the thalamocortical seizure network modulates epileptiform activity. As expected, we found that blocking either GAT1 or GAT3 prolonged oscillations. However, blocking both GATs unexpectedly suppressed oscillations. Integrating experimental observations into single-neuron and network-level computational models shows how a non-linear dependence of T-type calcium channel gating on GABAB receptor activity regulates network oscillations. Receptor activity that is either too brief or too protracted fails to sufficiently open T-type channels necessary for sustaining oscillations. Only within a narrow range does prolonging GABAB receptor activity promote channel opening and intensify oscillations. These results have implications for therapeutics that modulate inhibition kinetics.

Pharmacological activation of mGlu5 receptors with the positive allosteric modulator VU0360172, modulates thalamic GABAergic transmission

Previous studies have shown that injection of the mGlu5 receptor positive allosteric modulator (PAM) VU0360172 into either the thalamus or somatosensory cortex markedly reduces the frequency of spike-and-wave discharges (SWDs) in the WAG/Rij model of absence epilepsy. Here we have investigated the effects of VU0360172 on GABA transport in the thalamus and somatosensory cortex, as possible modes of action underlying the suppression of SWDs. Systemic VU0360172 injections increase GABA uptake in thalamic synaptosomes from epileptic WAG/Rij rats. Consistent with this observation, VU0360172 could also enhance thalamic GAT-1 protein expression, depending on the dosing regimen. This increase in GAT-1 expression was also observed in the thalamus from non-epileptic rats (presymptomatic WAG/Rij and Wistar) and appeared to occur selectively in neurons. The tonic GABA_A receptor current present in ventrobasal thalamocortical neurons was significantly reduced by VU0360172 consistent with changes in GAT-1 and GABA uptake. The in vivo effects of VU0360172 (reduction in tonic GABA current and increase in GAT-1 expression) could be reproduced in vitro by treating thalamic slices with VU0360172 for at least 1 h and appeared to be dependent on the activation of PLC. Thus, the effects of VU0360172 do not require an intact thalamocortical circuit. In the somatosensory cortex, VU0360172 reduced GABA uptake but did not cause significant changes in GAT-1 protein levels. These findings reveal a novel mechanism of regulation mediated by mGlu5 receptors, which could underlie the powerful anti-absence effect of mGlu5 receptor enhancers in animal models.

An Astrocytic Influence on Impaired Tonic Inhibition in Hippocampal CA1 Pyramidal Neurons in a Mouse Model of Rett Syndrome

Rett syndrome (RTT) is a severe neurodevelopmental disease caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene. Although altered interneuron development and function are clearly demonstrated in RTT mice, a particular mode of inhibition, tonic inhibition, has not been carefully examined. We report here that tonic inhibition is significantly reduced in pyramidal neurons in the CA1 region of the hippocampus in mice where Mecp2 is deleted either in all cells or specifically in astrocytes. Since no change is detected in the level of GABA receptors, such a reduction in tonic inhibition is likely a result of decreased ambient GABA level in the extracellular space. Consistent with this explanation, we observed increased expression of a GABA transporter, GABA transporter 3 (GAT3), in the hippocampus of the Mecp2 KO mice, as well as a corresponding increase of GAT3 current in hippocampal astrocytes. These phenotypes are relevant to RTT because pharmacological blockage of GAT3 can normalize tonic inhibition and intrinsic excitability in CA1 pyramidal neurons, and rescue the phenotype of increased network excitability in acute hippocampal slices from the Mecp2 KO mice. Finally, chronic administration of a GAT3 antagonist improved a composite symptom score and extended lifespan in the Mecp2 KO mice. Only male mice were used in this study. These results not only advance our understanding of RTT etiology by defining a new neuronal phenotype and revealing how it can be influenced by astrocytic alterations, but also reveal potential targets for intervention.SIGNIFICANCE STATEMENT Our study reports a novel phenotype of reduced tonic inhibition in hippocampal CA1 pyramidal neurons in the Rett syndrome mice, reveal a potential mechanism of increased GABA transporter expression/activity in the neighboring astrocytes, describe a disease-relevant consequence in hyperexcitability, and provide preliminary evidence that targeting this phenotype may slow down disease progression in Rett syndrome mice. These results help our understanding of the disease etiology and identify a new therapeutic target for treating Rett syndrome.

Mechanisms of GABAergic and cholinergic neurotransmission in auditory thalamus: Impact of aging

Age-related hearing loss is a complex disorder affecting a majority of the elderly population. As people age, speech understanding becomes a challenge especially in complex acoustic settings and negatively impacts the ability to accurately analyze the auditory scene. This is in part due to an inability to focus auditory attention on a particular stimulus source while simultaneously filtering out other sound stimuli. The present review examines the impact of aging on two neurotransmitter systems involved in accurate temporal processing and auditory gating in auditory thalamus (medial geniculate body; MGB), a critical brain region involved in the coding and filtering of auditory information. The inhibitory neurotransmitter GABA and its synaptic receptors (GABA_ARs) are key to maintaining accurate temporal coding of complex sounds, such as speech, throughout the central auditory system. In the MGB, synaptic and extrasynaptic GABA_ARs mediate fast phasic and slow tonic inhibition respectively, which in turn regulate MGB neuron excitability, firing modes, and engage thalamocortical oscillations that shape coding and gating of acoustic content. Acoustic coding properties of MGB neurons are further modulated through activation of tegmental cholinergic afferents that project to MGB to potentially modulate attention and help to disambiguate difficult to understand or novel sounds. Acetylcholine is released onto MGB neurons and presynaptic terminals in MGB activating neuronal nicotinic and muscarinic acetylcholine receptors (nAChRs, mAChRs) at a subset of MGB afferents to optimize top-down and bottom-up information flow. Both GABAergic and cholinergic neurotransmission is significantly altered with aging and this review will detail how age-related changes in these circuits within the MGB may impact coding of acoustic stimuli.

Increased Sensitivity of Mice Lacking Extrasynaptic δ-Containing GABAA Receptors to Histamine Receptor 3 Antagonists

Histamine/gamma-aminobutyric acid (GABA) neurons of posterior hypothalamus send wide projections to many brain areas and participate in stabilizing the wake state. Recent research has suggested that GABA released from the histamine/GABA neurons acts on extrasynaptic GABA_A receptors and balances the excitatory effect of histamine. In the current study, we show the presence of vesicular GABA transporter mRNA in a majority of quantified hypothalamic histaminergic neurons, which suggest vesicular release of GABA. As histamine/GABA neurons form conventional synapses infrequently, it is possible that GABA released from these neurons diffuses to target areas by volume transmission and acts on extrasynaptic GABA receptors. To investigate this hypothesis, mice lacking extrasynaptic GABA_A receptor δ subunit (Gabrd KO) were used. A pharmacological approach was employed to activate histamine/GABA neurons and induce histamine and presumably, GABA, release. Control and Gabrd KO mice were treated with histamine receptor 3 (Hrh3) inverse agonists ciproxifan and pitolisant, which block Hrh3 autoreceptors on histamine/GABA neurons and histamine-dependently promote wakefulness. Low doses of ciproxifan (1 mg/kg) and pitolisant (5 mg/kg) reduced locomotion in Gabrd KO, but not in WT mice. EEG recording showed that Gabrd KO mice were also more sensitive to the wake-promoting effect of ciproxifan (3 mg/kg) than control mice. Low frequency delta waves, associated with NREM sleep, were significantly suppressed in Gabrd KO mice compared with the WT group. Ciproxifan-induced wakefulness was blocked by histamine synthesis inhibitor α-fluoromethylhistidine (αFMH). The findings indicate that both histamine and GABA, released from histamine/GABA neurons, are involved in regulation of brain arousal states and δ-containing subunit GABA_A receptors are involved in mediating GABA response.

Nucleus-specific modulation of phasic and tonic inhibition by endogenous neurosteroidogenesis in the murine thalamus

Neurosteroids are potent allosteric modulators of GABA_A receptors (GABA_A Rs). Although the effects of exogenous neurosteroids on GABA_A R function are well documented, less is known about effects of neurosteroids produced by local endogenous biosynthesis. The neurosteroidogenic enzymes 5α-reductase and 3α-hydroxysteroid dehydrogenase are expressed in two nuclei of somatosensory thalamus, the thalamic reticular nucleus (nRT) and ventrobasal nucleus (VB). Here, the effects of acute blockade of neurosteroidogenesis by the 5α-reductase inhibitor finasteride on phasic and tonic GABA_A R-mediated currents were examined in nRT and VB of mice. In nRT, finasteride altered the decay and amplitude, but not the frequency, of phasic currents, with no effect on tonic inhibition. In VB neurons, by contrast, finasteride reduced both the size and frequency of phasic currents, and also reduced the degree of tonic inhibition. These studies thus provide novel evidence for endogenous modulation of GABA_A R function by 5α-reduced neurosteroids in the mature thalamus.

Sleep Spindles: Mechanisms and Functions

Sleep spindles are burstlike signals in the electroencephalogram (EEG) of the sleeping mammalian brain and electrical surface correlates of neuronal oscillations in thalamus. As one of the most inheritable sleep EEG signatures, sleep spindles probably reflect the strength and malleability of thalamocortical circuits that underlie individual cognitive profiles. We review the characteristics, organization, regulation, and origins of sleep spindles and their implication in non-rapid-eye-movement sleep (NREMS) and its functions, focusing on human and rodent. Spatially, sleep spindle-related neuronal activity appears on scales ranging from small thalamic circuits to functional cortical areas, and generates a cortical state favoring intracortical plasticity while limiting cortical output. Temporally, sleep spindles are discrete events, part of a continuous power band, and elements grouped on an infraslow time scale over which NREMS alternates between continuity and fragility. We synthesize diverse and seemingly unlinked functions of sleep spindles for sleep architecture, sensory processing, synaptic plasticity, memory formation, and cognitive abilities into a unifying sleep spindle concept, according to which sleep spindles 1) generate neural conditions of large-scale functional connectivity and plasticity that outlast their appearance as discrete EEG events, 2) appear preferentially in thalamic circuits engaged in learning and attention-based experience during wakefulness, and 3) enable a selective reactivation and routing of wake-instated neuronal traces between brain areas such as hippocampus and cortex. Their fine spatiotemporal organization reflects NREMS as a physiological state coordinated over brain and body and may indicate, if not anticipate and ultimately differentiate, pathologies in sleep and neurodevelopmental, -degenerative, and -psychiatric conditions.

Cyfip1 Haploinsufficiency Does Not Alter GABAA Receptor δ-Subunit Expression and Tonic Inhibition in Dentate Gyrus PV+ Interneurons and Granule Cells

Copy number variation (CNV) at chromosomal region 15q11.2 is linked to increased risk of neurodevelopmental disorders including autism and schizophrenia. A significant gene at this locus is cytoplasmic fragile X mental retardation protein (FMRP) interacting protein 1 (CYFIP1). CYFIP1 protein interacts with FMRP, whose monogenic absence causes fragile X syndrome (FXS). Fmrp knock-out has been shown to reduce tonic GABAergic inhibition by interacting with the δ-subunit of the GABAA receptor (GABAAR). Using in situ hybridization (ISH), qPCR, Western blotting techniques, and patch clamp electrophysiology in brain slices from a Cyfip1 haploinsufficient mouse, we examined δ-subunit mediated tonic inhibition in the dentate gyrus (DG). In wild-type (WT) mice, DG granule cells (DGGCs) responded to the δ-subunit-selective agonist THIP with significantly increased tonic currents. In heterozygous mice, no significant difference was observed in THIP-evoked currents in DGGCs. Phasic GABAergic inhibition in DGGC was also unaltered with no difference in properties of spontaneous IPSCs (sIPSCs). Additionally, we demonstrate that DG granule cell layer (GCL) parvalbumin-positive interneurons (PV+-INs) have functional δ-subunit-mediated tonic GABAergic currents which, unlike DGGC, are also modulated by the α1-selective drug zolpidem. Similar to DGGC, both IPSCs and THIP-evoked currents in PV+-INs were not different between Cyfip1 heterozygous and WT mice. Supporting our electrophysiological data, we found no significant change in hippocampal δ-subunit mRNA expression or protein level and no change in α1/α4-subunit mRNA expression. Thus, Cyfip1 haploinsufficiency, mimicking human 15q11.2 microdeletion syndrome, does not alter hippocampal phasic or tonic GABAergic inhibition, substantially differing from the Fmrp knock-out mouse model.

The Functional Role of Spontaneously Opening GABAA Receptors in Neural Transmission

Ionotropic type of γ-aminobutyric acid receptors (GABA_ARs) produce two forms of inhibitory signaling: phasic inhibition generated by rapid efflux of neurotransmitter GABA into the synaptic cleft with subsequent binding to GABA_ARs, and tonic inhibition generated by persistent activation of extrasynaptic and/or perisynaptic GABA_ARs by GABA continuously present in the extracellular space. It is widely accepted that phasic and tonic GABAergic inhibition is mediated by receptor groups of distinct subunit composition and modulated by different cytoplasmic mechanisms. Recently, however, it has been demonstrated that spontaneously opening GABA_ARs (s-GABA_ARs), which do not need GABA binding to enter an active state, make a significant input into tonic inhibitory signaling. Due to GABA-independent action mode, s-GABA_ARs promise new safer options for therapy of neural disorders (such as epilepsy) devoid of side effects connected to abnormal fluctuations of GABA concentration in the brain. However, despite the potentially important role of s-GABA_ARs in neural signaling, they still remain out of focus of neuroscience studies, to a large extent due to technical difficulties in their experimental research. Here, we summarize present data on s-GABA_ARs functional properties and experimental approaches that allow isolation of s-GABA_ARs effects from those of conventional (GABA-dependent) GABA_ARs.

Selective deficiencies in descending inhibitory modulation in neuropathic rats: implications for enhancing noradrenergic tone

Supplemental Digital Content is Available in the Text.

Descending noradrenergic pathways modulate spontaneous but not evoked thalamic neuronal hyperexcitability in neuropathic pain states. Spinal clonidine inhibits evoked and spontaneous firing, whereas reboxetine selectively inhibits evoked firing.

Pontine noradrenergic neurones form part of a descending inhibitory system that influences spinal nociceptive processing. Weak or absent descending inhibition is a common feature of chronic pain patients. We examined the extent to which the descending noradrenergic system is tonically active, how control of spinal neuronal excitability is integrated into thalamic relays within sensory-discriminative projection pathways, and how this inhibitory control is altered after nerve injury. In vivo electrophysiology was performed in anaesthetised spinal nerve–ligated (SNL) and sham-operated rats to record from wide dynamic range neurones in the ventral posterolateral thalamus (VPL). In sham rats, spinal block of α₂-adrenoceptors with atipamezole resulted in enhanced stimulus-evoked and spontaneous firing in the VPL, and produced conditioned place avoidance. However, in SNL rats, these conditioned avoidance behaviours were absent. Furthermore, inhibitory control of evoked neuronal responses was lost, but spinal atipamezole markedly increased spontaneous firing. Augmenting spinal noradrenergic tone in neuropathic rats with reboxetine, a selective noradrenergic reuptake inhibitor, modestly reinstated inhibitory control of evoked responses in the VPL but had no effect on spontaneous firing. By contrast, clonidine, an α₂ agonist, inhibited both evoked and spontaneous firing, and exhibited increased potency in SNL rats compared with sham controls. These data suggest descending noradrenergic inhibitory pathways are tonically active in sham rats. Moreover, in neuropathic states, descending inhibitory control is diminished, but not completely absent, and distinguishes between spontaneous and evoked neuronal activity. These observations may have implications for how analgesics targeting the noradrenergic system provide relief.

Potential for Drug Repositioning of Midazolam for Dentin Regeneration

Drug repositioning promises the advantages of reducing costs and expediting approval schedules. An induction of the anesthetic and sedative drug; midazolam (MDZ), regulates inhibitory neurotransmitters in the vertebrate nervous system. In this study we show the potential for drug repositioning of MDZ for dentin regeneration. A porcine dental pulp-derived cell line (PPU-7) that we established was cultured in MDZ-only, the combination of MDZ with bone morphogenetic protein 2, and the combination of MDZ with transforming growth factor-beta 1. The differentiation of PPU-7 into odontoblasts was investigated at the cell biological and genetic level. Mineralized nodules formed in PPU-7 were characterized at the protein and crystal engineering levels. The MDZ-only treatment enhanced the alkaline phosphatase activity and mRNA levels of odontoblast differentiation marker genes, and precipitated nodule formation containing a dentin-specific protein (dentin phosphoprotein). The nodules consisted of randomly oriented hydroxyapatite nanorods and nanoparticles. The morphology, orientation, and chemical composition of the hydroxyapatite crystals were similar to those of hydroxyapatite that had transformed from amorphous calcium phosphate nanoparticles, as well as the hydroxyapatite in human molar dentin. Our investigation showed that a combination of MDZ and PPU-7 cells possesses high potential of drug repositioning for dentin regeneration.

Identification of transthyretin as a novel interacting partner for the δ subunit of GABAA receptors

GABA_A receptors (GABA_A-Rs) play critical roles in brain development and synchronization of neural network activity. While synaptic GABA_A-Rs can exert rapid inhibition, the extrasynaptic GABA_A-Rs can tonically inhibit neuronal activity due to constant activation by ambient GABA. The δ subunit-containing GABA_A-Rs are expressed abundantly in the cerebellum, hippocampus and thalamus to mediate the major tonic inhibition in the brain. While electrophysiological and pharmacological properties of the δ-GABA_A-Rs have been well characterized, the molecular interacting partners of the δ-GABA_A-Rs are not clearly defined. Here, using a yeast two-hybrid screening assay, we identified transthyretin (TTR) as a novel regulatory molecule for the δ-GABA_A-Rs. Knockdown of TTR in cultured cerebellar granule neurons significantly decreased the δ receptor expression; whereas overexpressing TTR in cortical neurons increased the δ receptor expression. Electrophysiological analysis confirmed that knockdown or overexpression of TTR in cultured neurons resulted in a corresponding decrease or increase of tonic currents. Furthermore, in vivo analysis of TTR-/- mice revealed a significant decrease of the surface expression of the δ-GABA_A-Rs in cerebellar granule neurons. Together, our studies identified TTR as a novel regulator of the δ-GABA_A-Rs.

Neuropathy following spinal nerve injury shares features with the irritable nociceptor phenotype: A back-translational study of oxcarbazepine

BACKGROUND

The term 'irritable nociceptor' was coined to describe neuropathic patients characterized by evoked hypersensitivity and preservation of primary afferent fibres. Oxcarbazepine is largely ineffectual in an overall patient population, but has clear efficacy in a subgroup with the irritable nociceptor profile. We examine whether neuropathy in rats induced by spinal nerve injury shares overlapping pharmacological sensitivity with the irritable nociceptor phenotype using drugs that target sodium channels.

METHODS

In vivo electrophysiology was performed in anaesthetized spinal nerve ligated (SNL) and sham-operated rats to record from wide dynamic range (WDR) neurones in the ventral posterolateral thalamus (VPL) and dorsal horn.

RESULTS

In neuropathic rats, spontaneous activity in the VPL was substantially attenuated by spinal lidocaine, an effect that was absent in sham rats. The former measure was in part dependent on ongoing peripheral activity as intraplantar lidocaine also reduced aberrant spontaneous thalamic firing. Systemic oxcarbazepine had no effect on wind-up of dorsal horn neurones in sham and SNL rats. However, in SNL rats, oxcarbazepine markedly inhibited punctate mechanical-, dynamic brush- and cold-evoked neuronal responses in the VPL and dorsal horn, with minimal effects on heat-evoked responses. In addition, oxcarbazepine inhibited spontaneous activity in the VPL. Intraplantar injection of the active metabolite licarbazepine replicated the effects of systemic oxcarbazepine, supporting a peripheral locus of action.

CONCLUSIONS

We provide evidence that ongoing activity in primary afferent fibres drives spontaneous thalamic firing after spinal nerve injury and that oxcarbazepine through a peripheral mechanism exhibits modality-selective inhibitory effects on sensory neuronal processing.

SIGNIFICANCE

The inhibitory effects of lidocaine and oxcarbazepine in this rat model of neuropathy resemble the clinical observations in the irritable nociceptor patient subgroup and support a mechanism-based rationale for bench-to-bedside translation when screening novel drugs.

GABA beyond the synapse: defining the subtype-specific pharmacodynamics of non-synaptic GABAA receptors

KEY POINTS

Physiologically relevant combinations of recombinant GABAA receptor (GABAR) subunits were expressed in HEK293 cells. Using whole-cell voltage clamp and rapid drug application, we measured the GABAR-subtype-specific properties to convey either synaptic or extrasynaptic signalling in a range of physiological contexts. α4βδ GABARs are optimally tuned to submicromolar tonic GABA and transient surges of micromolar GABA concentrations. α5β2γ2l GABARs are better suited to higher tonic GABA levels, but also convey robust responses to brief synaptic and perisynaptic GABA fluctuations. α1β2/3δ GABARs function well at prolonged, micromolar (>2 μm) GABA levels, but not to low tonic (<1 μm GABA) or synaptic/transient GABAergic signalling. These results help illuminate the context- and isoform-specific modes of GABAergic signalling in the brain.

ABSTRACT

GABAA receptors (GABARs) mediate a remarkable diversity of signalling modalities in vivo. Yet most published work characterizing responses to GABA has focused on the properties needed to convey fast, phasic synaptic inhibition. We therefore aimed to characterize the most prevalent (α4βδ, α5β3γ2L) and least prevalent (α1β2δ) non-synaptic GABAR currents, using whole-cell voltage clamp recordings of recombinant GABAR expressed in HEK293 cells and drug application protocols to recapitulate the GABA concentration profiles occurring during both fast synaptic and slow extrasynaptic signalling. We found that α4βδ GABARs were very sensitive to submicromolar GABA, with a rank order potency of α4β2δ ≥ α4β1δ ≈ α4β3δ GABARs. In comparison, the GABA EC50 was up to 20 times higher for α1β2γ2L GABARs, with α1β2δ and α5β3γ2L GABARs having intermediate GABA potency. Both α4βδ and α5β3γ2L GABAR currents exhibited slow, but substantial, desensitization as well as prolonged rates of deactivation. These GABAR current properties defined distinct 'dynamic ranges' of responsiveness to changing GABA for α4β2δ (0.1-1 μm), α5β3γ2L (0.5-7 μm) and α1β2γ2L (0.6-9 μm) GABARs. Finally, α1β2δ GABARs were notable for their relative lack of desensitization and extremely quick deactivation. In summary, our results help delineate the roles that specific GABARs may play in mediating non-synaptic GABA signals. Since ambient GABA levels may be altered during development as well as by drugs and disease states, these findings may help future efforts to understand disrupted inhibition underlying a variety of neurological illnesses, such as epilepsy.

Chemogenetic Isolation Reveals Synaptic Contribution of δ GABAA Receptors in Mouse Dentate Granule Neurons

Two major GABA_A receptor classes mediate ionotropic GABA signaling, those containing a δ subunit and those with a γ2 subunit. The classical viewpoint equates γ2-containing receptors with IPSCs and δ-containing receptors with tonic inhibition because of differences in receptor localization, but significant questions remain because the populations cannot be pharmacologically separated. We removed this barrier using gene editing to confer a point mutation on the δ subunit in mice, rendering receptors containing the subunit picrotoxin resistant. By pharmacologically isolating δ-containing receptors, our results demonstrate their contribution to IPSCs in dentate granule neurons and weaker contributions to thalamocortical IPSCs. Despite documented extrasynaptic localization, we found that receptor localization does not preclude participation in isolated IPSCs, including mIPSCs. Further, phasic inhibition from δ subunit-containing receptors strongly inhibited summation of EPSPs, whereas tonic activity had little impact. In addition to any role that δ-containing receptors may play in canonical tonic inhibition, our results highlight a previously underestimated contribution of δ-containing receptors to phasic inhibition.SIGNIFICANCE STATEMENT GABA_A receptors play key roles in transient and tonic inhibition. The prevailing view suggests that synaptic γ2-containing GABA_A receptors drive phasic inhibition, whereas extrasynaptic δ-containing receptors mediate tonic inhibition. To re-evaluate the impact of δ receptors, we took a chemogenetic approach that offers a sensitive method to probe the synaptic contribution of δ-containing receptors. Our results reveal that localization does not strongly limit the contribution of δ receptors to IPSCs and that δ receptors make an unanticipated robust contribution to phasic inhibition.

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.

CaV3.1 isoform of T-type calcium channels supports excitability of rat and mouse ventral tegmental area neurons

Recent data have implicated voltage-gated calcium channels in the regulation of the excitability of neurons within the mesolimbic reward system. While the attention of most research has centered on high voltage L-type calcium channel activity, the presence and role of the low voltage-gated T-type calcium channel (T-channels) has not been well explored. Hence, we investigated T-channel properties in the neurons of the ventral tegmental area (VTA) utilizing wild-type (WT) rats and mice, Ca_V3.1 knock-out (KO) mice, and TH-eGFP knock-in (KI) rats in acute horizontal brain slices of adolescent animals. In voltage-clamp experiments, we first assessed T-channel activity in WT rats with characteristic properties of voltage-dependent activation and inactivation, as well as characteristic crisscrossing patterns of macroscopic current kinetics. T-current kinetics were similar in WT mice and WT rats but T-currents were abolished in Ca_V3.1 KO mice. In ensuing current-clamp experiments, we observed the presence of hyperpolarization-induced rebound burst firing in a subset of neurons in WT rats, as well as dopaminergic and non-dopaminergic neurons in TH-eGFP KI rats. Following the application of a pan-selective T-channel blocker TTA-P2, rebound bursting was significantly inhibited in all tested cells. In a behavioral assessment, the acute locomotor increase induced by a MK-801 (Dizocilpine) injection in WT mice was abolished in Ca_V3.1 KO mice, suggesting a tangible role for 3.1 T-type channels in drug response. We conclude that pharmacological targeting of Ca_V3.1 isoform of T-channels may be a novel approach for the treatment of disorders of mesolimbic reward system.

Developmental changes of GABA immunoreactivity in cortico-thalamic networks of an absence seizure model

Absence seizures (ASs) are associated with abnormalities in gamma-aminobutyric acid (GABA) neurotransmission in the thalamus and the cortex. In the present study, we used light microscopy GABA immunocytochemistry to quantify the GABA-immunoreactive (GABA-IR) neurons and neuropil in the thalamic ventral basal (VB) nucleus, the nucleus reticularis thalami (NRT), the dorsal lateral geniculate (dLGN), the primary motor cortex (M1) and perioral region of the somatosensory cortex (S1po) of genetic absence epilepsy rats from Strasbourg (GAERS). We used both the relative non-epileptic control (NEC) and normal Wistar rats as control strains, and investigated GABA immunostaining at postnatal day 15 (P15), P25, and P90. The main findings were i) an increase in GABA-IR neuropil in the VB at P25 and P90 in GAERS but not in NEC and Wistar rats; ii) an increase in NRT GABA-IR neurons in GAERS and NEC, but not Wistar, rats at both P25 and P90; and iii) an increase in GABA-IR neuron density in S1po of GAERS at P25 and P90 and in Wistar at P90. These results indicate that the increased GABAergic innervation in the VB at P25 most likely contributes to the enhanced tonic inhibition observed in GAERS prior to AS onset, whereas the lack of any anatomo-morphological GABAergic differences in GAERS S1po suggests that functional more than structural abnormalities underlie the origin of cortical paroxysms in S1po of this AS model.

This article is part of the “Special Issue Dedicated to Norman G. Bowery”.

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GABA-IR profiles increase in P25 to P90 VB neuropil in GAERS but not in NEC and Wistar rats.

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NRT GABA-IR neurons increase in P25 and P90 GAERS and NEC, but not in Wistar rats.

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GABA-IR neuron density increases in S1po of GAERS at P25 and P90 and in Wistar at P90.