Tonically active GABAA receptors: modulating gain and maintaining the tone

A therapeutic small molecule enhances γ-oscillations and improves cognition/memory in Alzheimer's disease model mice

Brain rhythms provide the timing for recruitment of brain activity required for linking together neuronal ensembles engaged in specific tasks. The γ-oscillations (30 to 120 Hz) orchestrate neuronal circuits underlying cognitive processes and working memory. These oscillations are reduced in numerous neurological and psychiatric disorders, including early cognitive decline in Alzheimer's disease (AD). Here, we report on a potent brain-permeable small molecule, DDL-920 that increases γ-oscillations and improves cognition/memory in a mouse model of AD, thus showing promise as a class of therapeutics for AD. We employed anatomical, in vitro and in vivo electrophysiological, and behavioral methods to examine the effects of our lead therapeutic candidate small molecule. As a novel in central nervous system pharmacotherapy, our lead molecule acts as a potent, efficacious, and selective negative allosteric modulator of the γ-aminobutyric acid type A receptors most likely assembled from α1β2δ subunits. These receptors, identified through anatomical and pharmacological means, underlie the tonic inhibition of parvalbumin (PV) expressing interneurons (PV+INs) critically involved in the generation of γ-oscillations. When orally administered twice daily for 2 wk, DDL-920 restored the cognitive/memory impairments of 3- to 4-mo-old AD model mice as measured by their performance in the Barnes maze. Our approach is unique as it is meant to enhance cognitive performance and working memory in a state-dependent manner by engaging and amplifying the brain's endogenous γ-oscillations through enhancing the function of PV+INs.

Parkinsonian Tremor as Unstable Feedback in a Physiologically Consistent Control Framework

Parkinson's disease (PD) is characterized by decreased dopamine in the basal ganglia that causes excessive tonic inhibition of the thalamus. This excessive inhibition seems to explain inhibitory motor symptoms in PD, but the source of tremor remains unclear. This paper investigates how neural inhibition may change the closed-loop characteristics of the human motor control system to determine how this established pathophysiology could produce tremor. The rate-coding model of neural signals suggests increased inhibition decreases signal amplitude, which could create a mismatch between the closed-loop dynamics and the internal models that overcome proprioceptive feedback delays. This paper aims to identify a candidate model structure with decreased-amplitude-induced tremor in PD that also agrees with previously recorded movements of healthy and cerebellar patients. The optimal feedback control theory of human motor control forms the basis of the model. Key additional elements include gating of undesired movements via the basal ganglia-thalamus-motor cortex circuit and the treatment of the efferent copy of the control input as a measurement in the state estimator. Simulations confirm the model's ability to capture tremor in PD and also demonstrate how disease progression could affect tremor and other motor symptoms, providing insight into the existence of tremor and non-tremor phenotypes. Altogether, the physiological underpinnings of the model structure and the agreement of model predictions with clinical observations provides support for the hypothesis that unstable feedback produces parkinsonian tremor. Consequently, these results also support the associated framework for the neuroanatomy of human motor control.

Crosstalk between GABAA receptors in astrocytes and neurons triggered by general anesthetic drugs

General anesthetic drugs cause cognitive deficits that persist after the drugs have been eliminated. Astrocytes may contribute to such cognition-impairing effects through the release of one or more paracrine factors that increase a tonic inhibitory conductance generated by extrasynaptic γ-aminobutyric acid type A (GABAA) receptors in hippocampal neurons. The mechanisms underlying this astrocyte-to-neuron crosstalk remain unknown. Interestingly, astrocytes express anesthetic-sensitive GABAA receptors. Here, we tested the hypothesis that anesthetic drugs activate astrocytic GABAA receptors to initiate crosstalk leading to a persistent increase in extrasynaptic GABAA receptor function in neurons. We also investigated the signaling pathways in neurons and aimed to identify the paracrine factors released from astrocytes. Astrocytes and neurons from mice were grown in primary cell cultures and studied using in vitro electrophysiological and biochemical assays. We discovered that the commonly used anesthetics etomidate (injectable) and sevoflurane (inhaled) stimulated astrocytic GABAA receptors, which in turn promoted the release paracrine factors, that increased the tonic current in neurons via a p38 MAPK-dependent signaling pathway. The increase in tonic current was mimicked by exogenous IL-1β and abolished by blocking IL-1 receptors; however, unexpectedly, IL-1β and other cytokines were not detected in astrocyte-conditioned media. In summary, we have identified a novel form of crosstalk between GABAA receptors in astrocytes and neurons that engages a p38 MAPK-dependent pathway. Brief commentary BACKGROUND: Many older patients experience cognitive deficits after surgery. Anesthetic drugs may be a contributing factor as they cause a sustained increase in the function of "memory blocking" extrasynaptic GABAA receptors in neurons. Interestingly, astrocytes are required for this increase; however, the mechanisms underlying the astrocyte-to-neuron crosstalk remain unknown. TRANSLATIONAL SIGNIFICANCE: We discovered that commonly used general anesthetic drugs stimulate GABAA receptors in astrocytes, which in turn release paracrine factors that trigger a persistent increase in extrasynaptic GABAA receptor function in neurons via p38 MAPK. This novel form of crosstalk may contribute to persistent cognitive deficits after general anesthesia and surgery.

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.

Neurosteroids and their potential as a safer class of general anesthetics

Neurosteroids (NS) are a class of steroids that are synthesized within the central nervous system (CNS). Various NS can either enhance or inhibit CNS excitability and they play important biological roles in brain development, brain function and as mediators of mood. One class of NS, 3α-hydroxy-pregnane steroids such as allopregnanolone (AlloP) or pregnanolone (Preg), inhibits neuronal excitability; these endogenous NS and their analogues have been therapeutically applied as anti-depressants, anti-epileptics and general anesthetics. While NS have many favorable properties as anesthetics (e.g. rapid onset, rapid recovery, minimal cardiorespiratory depression, neuroprotection), they are not currently in clinical use, largely due to problems with formulation. Recent advances in understanding NS mechanisms of action and improved formulations have rekindled interest in development of NS as sedatives and anesthetics. In this review, the synthesis of NS, and their mechanism of action will be reviewed with specific emphasis on their binding sites and actions on γ-aminobutyric acid type A (GABAA) receptors. The potential advantages of NS analogues as sedative and anesthetic agents will be discussed.

A combinatory genetic strategy for targeting neurogliaform neurons in the mouse basolateral amygdala

The mouse basolateral amygdala (BLA) contains various GABAergic interneuron subpopulations, which have distinctive roles in the neuronal microcircuit controlling numerous behavioral functions. In mice, roughly 15% of the BLA GABAergic interneurons express neuropeptide Y (NPY), a reasonably characteristic marker for neurogliaform cells (NGFCs) in cortical-like brain structures. However, genetically labeled putative NPY-expressing interneurons in the BLA yield a mixture of interneuron subtypes besides NGFCs. Thus, selective molecular markers are lacking for genetically accessing NGFCs in the BLA. Here, we validated the NGFC-specific labeling with a molecular marker, neuron-derived neurotrophic factor (NDNF), in the mouse BLA, as such specificity has been demonstrated in the neocortex and hippocampus. We characterized genetically defined NDNF-expressing (NDNF+) GABAergic interneurons in the mouse BLA by combining the Ndnf-IRES2-dgCre-D transgenic mouse line with viral labeling, immunohistochemical staining, and in vitro electrophysiology. We found that BLA NDNF+ GABAergic cells mainly expressed NGFC neurochemical markers NPY and reelin (Reln) and exhibited small round soma and dense axonal arborization. Whole-cell patch clamp recordings indicated that most NDNF+ interneurons showed late spiking and moderate firing adaptation. Moreover, ∼81% of BLA NDNF+ cells generated retroaxonal action potential after current injections or optogenetic stimulations, frequently developing into persistent barrage firing. Optogenetic activation of the BLA NDNF+ cell population yielded both GABAA- and GABAB receptor-mediated currents onto BLA pyramidal neurons (PNs). We demonstrate a combinatory strategy combining the NDNF-cre mouse line with viral transfection to specifically target adult mouse BLA NGFCs and further explore their functional and behavioral roles.

A therapeutic small molecule lead enhances γ-oscillations and improves cognition/memory in Alzheimer's disease model mice

Brain rhythms provide the timing and concurrence of brain activity required for linking together neuronal ensembles engaged in specific tasks. In particular, the γ-oscillations (30-120 Hz) orchestrate neuronal circuits underlying cognitive processes and working memory. These oscillations are reduced in numerous neurological and psychiatric disorders, including early cognitive decline in Alzheimer's disease (AD). Here we report on a potent brain permeable small molecule, DDL-920 that increases γ-oscillations and improves cognition/memory in a mouse model of AD, thus showing promise as a new class of therapeutics for AD. As a first in CNS pharmacotherapy, our lead candidate acts as a potent, efficacious, and selective negative allosteric modulator (NAM) of the γ-aminobutyric acid type A receptors (GABA A Rs) assembled from α1β2δ subunits. We identified these receptors through anatomical and pharmacological means to mediate the tonic inhibition of parvalbumin (PV) expressing interneurons (PV+INs) critically involved in the generation of γ-oscillations. Our approach is unique as it is meant to enhance cognitive performance and working memory in a state-dependent manner by engaging and amplifying the brain's endogenous γ-oscillations through enhancing the function of PV+INs.

Factors affecting the GABAergic synapse function in Alzheimer's disease: Focus on microRNAs

Alzheimer's disease (AD) is a progressive neurological disease characterized by the loss of cognitive function, confusion, and memory deficit. Accumulation of abnormal proteins, amyloid beta (Aß), and phosphorylated Tau (p-tau) forms plaques and tangles that deteriorate synapse function, resulting in neurodegeneration and cognitive decline in AD. The human brain is composed of different types of neurons and/or synapses that are functionally defective in AD. The GABAergic synapse, the most abundant inhibitory neuron in the human brain was found to be dysfunctional in AD and contributes to disrupting neurological function. This study explored the types of GABA receptors associated with neurological dysfunction and various biological and environmental factors that cause GABAergic neuron dysfunction in AD, such as Aβ, p-tau, aging, sex, astrocytes, microglia, APOE, mental disorder, diet, physical activity, and sleep. Furthermore, we explored the role of microRNAs (miRNAs) in the regulation of GABAergic synapse function in neurological disorders and AD states. We also discuss the molecular mechanisms underlying GABAergic synapse dysfunction with a focus on miR-27b, miR-30a, miR-190a/b, miR-33, miR-51, miR-129-5p, miR-376-3p, miR-376c, miR-30b and miR-502-3p. The purpose of our article is to highlight the recent research on miRNAs affecting the regulation of GABAergic synapse function and factors that contribute to the progression of AD.

Divergent Population-Specific Effects of Chronic Ethanol Exposure on Excitability and Inhibitory Transmission in Male and Female Rat Central Amygdala

The central nucleus of the amygdala (CeA) is implicated in alcohol use disorder (AUD) and AUD-associated plasticity. The CeA is a primarily GABAergic nucleus that is subdivided into lateral and medial compartments with genetically diverse subpopulations. GABAA receptors are heteromeric pentamers with subunits conferring distinct physiological characteristics. GABAA receptor signaling in the CeA has been implicated in ethanol-associated plasticity; however, population-specific changes in inhibitory signaling and subunit expression remain unclear. Here, we combined electrophysiology with single-cell gene expression analysis of population markers and GABAA receptor subunits to examine population-specific changes in inhibitory control in male and female rats following chronic ethanol exposure. We found that chronic ethanol exposure and withdrawal produced global changes in GABAA receptor subunit expression at the transcript and protein levels, increased excitability in female CeA neurons, and increased inhibitory synaptic transmission in male CeA neurons. When we examined CeA neurons at the single-cell level we found heterogenous populations, as previously reported. We observed ethanol-induced increases in excitability only in somatostatin neurons in the CeA of females, decreases in excitability only in the protein kinase C delta (PKCd) population in males, and ethanol-induced increases in inhibitory transmission in male PKCd and calbindin 2-expressing CeA neurons. There were no population-specific differences in GABAA receptor (Gabr) subunits in males but reduced GabrA5 expression in female somatostatin neurons. Collectively, these findings suggest that defined CeA populations display differential ethanol sensitivity in males and females, which may play a role in sex differences in vulnerability to AUD or expression of AUD pathology.SIGNIFICANCE STATEMENT The CeA is involved in the effects of ethanol in the brain; however, the population-specific changes in CeA activity remain unclear. We used recordings of CeA neuronal activity and single-cell gene expression to examine population-specific changes in inhibitory control in male and female rats following chronic ethanol exposure and found sex- and population-specific effects of chronic ethanol exposure and withdrawal. Specifically, female CeA neurons displayed increased excitability in the somatostatin CeA population, whereas male CeA neurons displayed increased inhibitory control in both PKCd and calbindin populations and decreased excitability in the PKCd population. These findings identify CeA populations that display differential sensitivity to ethanol exposure, which may contribute to sex differences in vulnerability to alcohol use disorder.

An alpha 5-GABAa receptor positive allosteric modulator attenuates social and cognitive deficits without changing dopamine system hyperactivity in an animal model for autism

Autism Spectrum Disorders (ASD) are characterized by core behavioral symptoms in the domains of sociability, language/communication, and repetitive or stereotyped behaviors. Deficits in the prefrontal and hippocampal excitatory/inhibitory balance due to a functional loss of GABAergic interneurons are proposed to underlie these symptoms. Increasing the postsynaptic effects of GABA with compounds that selectively modulate GABAergic receptors could be a potential target for treating ASD symptoms. In addition, deficits in GABAergic interneurons have been linked to dopamine (DA) system dysregulation, and, despite conflicting evidence, abnormalities in the DA system activity may underly some ASD symptoms. Here, we investigated whether the positive allosteric modulator of α5-containing GABA A receptors (α5-GABA A Rs) SH-053-2'F-R-CH3 (10 mg/kg) attenuates behavioral abnormalities in a rat model for autism based on in utero VPA exposure. We also evaluated if animals exposed to VPA in utero present changes in the ventral tegmental area (VTA) DA system activity using in vivo electrophysiology and if SH-053-2'F-R-CH3 could attenuate these changes. In utero VPA exposure caused male and female rats to present increased repetitive behavior (self-grooming) in early adolescence and deficits in social interaction in adulthood. Male, but not female VPA rats, also presented deficits in recognition memory as adults. SH-053-2'F-R-CH3 attenuated the impairments in sociability and cognitive function in male VPA-exposed rats without attenuating the decreased social interaction in females. Male and female adult VPA-exposed rats also showed an increased VTA DA neuron population activity, which was not changed by SH-053-2'F-R-CH3. Despite sex differences, our findings indicate α5-GABA A Rs positive allosteric modulators may effectively attenuate some core ASD symptoms.

Neurodevelopment and early pharmacological interventions in Fragile X Syndrome

Fragile X Syndrome (FXS) is a neurodevelopmental disorder and the leading monogenic cause of autism and intellectual disability. For years, several efforts have been made to develop an effective therapeutic approach to phenotypically rescue patients from the disorder, with some even advancing to late phases of clinical trials. Unfortunately, none of these attempts have completely succeeded, bringing urgency to further expand and refocus research on FXS therapeutics. FXS arises at early stages of postnatal development due to the mutation and transcriptional silencing of the Fragile X Messenger Ribonucleoprotein 1 gene (FMR1) and consequent loss of the Fragile X Messenger Ribonucleoprotein (FMRP) expression. Importantly, FMRP expression is critical for the normal adult nervous system function, particularly during specific windows of embryogenic and early postnatal development. Cellular proliferation, migration, morphology, axonal guidance, synapse formation, and in general, neuronal network establishment and maturation are abnormally regulated in FXS, underlying the cognitive and behavioral phenotypes of the disorder. In this review, we highlight the relevance of therapeutically intervening during critical time points of development, such as early postnatal periods in infants and young children and discuss past and current clinical trials in FXS and their potential to specifically target those periods. We also discuss potential benefits, limitations, and disadvantages of these pharmacological tools based on preclinical and clinical research.

Amygdala activity and amygdala-hippocampus connectivity: Metabolic diseases, dementia, and neuropsychiatric issues

With rapid aging of the population worldwide, the number of people with dementia is dramatically increasing. Some studies have emphasized that metabolic syndrome, which includes obesity and diabetes, leads to increased risks of dementia and cognitive decline. Factors such as insulin resistance, hyperglycemia, high blood pressure, dyslipidemia, and central obesity in metabolic syndrome are associated with synaptic failure, neuroinflammation, and imbalanced neurotransmitter levels, leading to the progression of dementia. Due to the positive correlation between diabetes and dementia, some studies have called it "type 3 diabetes". Recently, the number of patients with cognitive decline due to metabolic imbalances has considerably increased. In addition, recent studies have reported that neuropsychiatric issues such as anxiety, depressive behavior, and impaired attention are common factors in patients with metabolic disease and those with dementia. In the central nervous system (CNS), the amygdala is a central region that regulates emotional memory, mood disorders, anxiety, attention, and cognitive function. The connectivity of the amygdala with other brain regions, such as the hippocampus, and the activity of the amygdala contribute to diverse neuropathological and neuropsychiatric issues. Thus, this review summarizes the significant consequences of the critical roles of amygdala connectivity in both metabolic syndromes and dementia. Further studies on amygdala function in metabolic imbalance-related dementia are needed to treat neuropsychiatric problems in patients with this type of dementia.

Excitatory GluN1/GluN3A glycine receptors (eGlyRs) in brain signaling

GluN3A is a glycine-binding subunit belonging to the NMDA receptor (NMDAR) family that can assemble with GluN1 subunits to form unconventional NMDARs insensitive to glutamate and activated by glycine only. The existence of such excitatory glycine receptors (eGlyRs) in the central nervous system (CNS) has long remained elusive. Recently, eGlyRs have been identified in specific brain regions, where they represent a novel neuronal signaling modality by which extracellular glycine tunes neuronal excitability, circuit function, and behavior. In this review, we summarize the emerging knowledge regarding these underappreciated receptors. The existence of eGlyRs reshapes current understanding of NMDAR diversity and of glycinergic signaling, previously thought to be primarily inhibitory. Given that GluN3A expression is concentrated in brain regions regulating emotional responses, eGlyRs are potential new targets of therapeutic interest in neuropsychiatry.

Adjusting the brakes to adjust neuronal activity: Adenosinergic modulation of GABAergic transmission

About 50 years elapsed from the publication of the first full paper on the neuromodulatory action of adenosine at a 'simple' synapse model, the neuromuscular junction (Ginsborg and Hirst, 1972). In that study adenosine was used as a tool to increase cyclic AMP and for the great surprise, it decreased rather than increased neurotransmitter release, and for a further surprise, its action was prevented by theophylline, at the time only known as inhibitor of phosphodiesterases. These intriguing observations opened the curiosity for immediate studies relating the action of adenine nucleotides, known to be released together with neurotransmitters, to that of adenosine (Ribeiro and Walker, 1973, 1975). Our understanding on the ways adenosine uses to modulate synapses, circuits, and brain activity, vastly expanded since then. However, except for A_2A receptors, whose actions upon GABAergic neurons of the striatum are well known, most of the attention given to the neuromodulatory action of adenosine has been focusing upon excitatory synapses. Evidence is growing that GABAergic transmission is also a target for adenosinergic neuromodulation through A₁ and A_2A receptors. Some o these actions have specific time windows during brain development, and others are selective for specific GABAergic neurons. Both tonic and phasic GABAergic transmission can be affected, and either neurons or astrocytes can be targeted. In some cases, those effects result from a concerted action with other neuromodulators. Implications of these actions in the control of neuronal function/dysfunction will be the focus of this review.

Insulin decreases epileptiform activity in rat layer 5/6 prefrontal cortex in vitro

Accumulating evidence indicates that insulin-mediated signaling in the brain may play important roles in regulating neuronal function. Alterations to insulin signaling are associated with the development of neurological disorders including Alzheimer's disease and Parkinson's disease. Also, hyperglycemia and insulin resistance have been associated with seizure activity and brain injury. In recent work, we found that insulin increased inhibitory GABA_A -mediated tonic currents in the prefrontal cortex (PFC). In this work, we used local field potential recordings and calcium imaging to investigate the effect of insulin on seizure-like activity in PFC slices. Seizure-like events (SLEs) were induced by perfusing the slices with magnesium-free artificial cerebrospinal fluid (ACSF) containing the proconvulsive compound 4-aminopyridine (4-AP). We found that insulin decreased the frequency, amplitude, and duration of SLEs as well as the synchronic activity of PFC neurons evoked by 4-AP. These insulin effects were mediated by the PI3K/Akt signaling pathway and mimicked by gaboxadol (THIP), a δ GABA_A receptor agonist. The effect of insulin on the number of SLEs was partially blocked by L-655,708, an inverse agonist with high selectivity for GABA_A receptors containing the α5 subunit. Our results suggest that insulin reduces neuronal excitability by an increase of GABAergic tonic currents. The physiological relevance of these findings is discussed.

Sex Differences in the Impact of Electronic Nicotine Vapor on Corticotropin-Releasing Factor Receptor 1 Neurons in the Mouse Ventral Tegmental Area

Nicotine engages dopamine neurons in the ventral tegmental area (VTA) to encode reward and drive the development of nicotine addiction, however how nicotine alters a stress associated VTA population remains unclear. Here, we used male and female CRF1-GFP mice and nicotine vapor exposure to examine the effects of nicotine in VTA corticotropin-releasing factor receptor 1 (CRF1) neurons. We use immunohistochemistry and electrophysiology to examine neuronal activity, excitability, and inhibitory signaling. We found that VTA CRF1 neurons are mainly dopaminergic and project to the nucleus accumbens (NAc; VTA-NAcCRF1 neurons). VTA-NAcCRF1 neurons show greater phasic inhibition in naive females and greater focal nicotine-induced increases in firing in naive males. Following acute nicotine vapor exposure, phasic inhibition was not altered, but focal nicotine-induced tonic inhibition was enhanced in females and diminished in males. Acute nicotine vapor exposure did not affect firing in VTA-NAcCRF1 neurons, but females showed lower baseline firing and higher focal nicotine-induced firing. Activity (cFos) was increased in the CRF1 dopaminergic VTA population in both sexes, but with greater increases in females. Following chronic nicotine vapor exposure, both sexes displayed reduced basal phasic inhibition and the sex difference in tonic inhibition following acute vapor exposure was no longer observed. Additionally, activity of the CRF1 dopaminergic VTA population was no longer elevated in either sex. These findings reveal sex-dependent and exposure-dependent changes in mesolimbic VTA-NAc CRF1 neuronal activity, inhibitory signaling, and nicotine sensitivity following nicotine vapor exposure. These changes potentially contribute to nicotine-dependent behaviors and the intersection between stress, anxiety, and addiction.SIGNIFICANCE STATEMENT Nicotine is known to engage reward systems in the brain historically centering the neurotransmitter dopamine however, how nicotine impacts other neurons in the reward pathway is less clear. The current study investigates the impact of acute and chronic electronic nicotine vapor exposure in a genetically-defined cell population containing the stress receptor corticotropin-releasing factor 1 (CRF1) that is located in the reward circuitry. This study employs functional measures of neuronal activity and identifies important sex differences in nicotine's effects across time and exposure.

White Matter Microstructure Is Associated with Serum Neuroactive Steroids and Psychological Functioning

Military service members are at increased risk for mental health issues, and comorbidity with mild traumatic brain injury (mTBI) is common. Largely overlapping symptoms between conditions suggest a shared pathophysiology. The present work investigates the associations among white matter microstructure, psychological functioning, and serum neuroactive steroids that are part of the stress-response system. Diffusion-weighted brain imaging was acquired from 163 participants (with and without military affiliation) and free-water-corrected fractional anisotropy (FAT) was extracted. Associations between serum neurosteroid levels of allopregnanolone (ALLO) and pregnenolone (PREGNE), psychological functioning, and whole-brain white matter microstructure were assessed using regression models. Moderation models tested the effect of mTBI and comorbid post-traumatic stress disorder (PTSD) and mTBI on these associations. ALLO is associated with whole-brain white matter FAT (β = 0.24, t = 3.05, p = 0.006). This association is significantly modulated by PTSD+mTBI comorbidity (β = 0.00, t = 2.50, p = 0.027), although an mTBI diagnosis alone did not significantly impact this association (p = 0.088). There was no significant association between PREGNE and FAT (p = 0.380). Importantly, lower FAT is associated with poor psychological functioning (β = -0.19, t = -2.35, p = 0.020). This study provides novel insight into a potential common pathophysiological mechanism of neurosteroid dysregulation underlying the high risk for mental health issues in military service members. Further, comorbidity of PTSD and mTBI may bring the compensatory effects of the brain's stress response to their limit. Future research is needed to investigate whether neurosteroid regulation may be a promising tool for restoring brain health and improving psychological functioning.

Correlation of age at seizure onset with GABAA receptor subunit and chloride Co-transporter configuration in Focal cortical dysplasia (FCD)

Focal cortical dysplasia (FCD) represents a group of malformations of cortical development, which are speculated to be related to early developmental defects in the cerebral cortex. According to dysmature cerebral development hypothesis of FCD altered GABAA receptor function is known to contribute to abnormal neuronal network. Here, we studied the possible association between age at seizure onset in FCD with the subunit configuration of GABAA receptors in resected brain specimens obtained from patients with FCD. We observed a significantly higher ratio of α4/α1 subunit-containing GABAA receptors in patients with early onset (EO) FCD as compared to those with late onset (LO) FCD as is seen during the course of development where α4-containing GABAA receptors expression is high as compared to α1-containing GABAA receptors expression. Likewise, the influx to efflux chloride co-transporter expression of NKCC1/KCC2 was also increased in patients with EO FCD as seen during brain development. In addition, we observed that the ratio of GABA/Glutamate neurotransmitters was lower in patients with EO FCD as compared to that in patients with LO FCD. Our findings suggest altered configuration of GABAA receptors in FCD which could be contributing to aberrant depolarizing GABAergic activity. In particular, we observed a correlation of age at seizure onset in FCD with subunit configuration of GABAA receptors, levels of NKCC1/KCC2 and the ratio of GABA/Glutamate neurotransmitters such that the patients with EO FCD exhibited a more critically modulated GABAergic network.

The GABA and GABA-Receptor System in Inflammation, Anti-Tumor Immune Responses, and COVID-19

GABA and GABA_A-receptors (GABA_A-Rs) play major roles in neurodevelopment and neurotransmission in the central nervous system (CNS). There has been a growing appreciation that GABA_A-Rs are also present on most immune cells. Studies in the fields of autoimmune disease, cancer, parasitology, and virology have observed that GABA-R ligands have anti-inflammatory actions on T cells and antigen-presenting cells (APCs), while also enhancing regulatory T cell (Treg) responses and shifting APCs toward anti-inflammatory phenotypes. These actions have enabled GABA_A-R ligands to ameliorate autoimmune diseases, such as type 1 diabetes (T1D), multiple sclerosis (MS), and rheumatoid arthritis, as well as type 2 diabetes (T2D)-associated inflammation in preclinical models. Conversely, antagonism of GABA_A-R activity promotes the pro-inflammatory responses of T cells and APCs, enhancing anti-tumor responses and reducing tumor burden in models of solid tumors. Lung epithelial cells also express GABA-Rs, whose activation helps maintain fluid homeostasis and promote recovery from injury. The ability of GABA_A-R agonists to limit both excessive immune responses and lung epithelial cell injury may underlie recent findings that GABA_A-R agonists reduce the severity of disease in mice infected with highly lethal coronaviruses (SARS-CoV-2 and MHV-1). These observations suggest that GABA_A-R agonists may provide off-the-shelf therapies for COVID-19 caused by new SARS-CoV-2 variants, as well as novel beta-coronaviruses, which evade vaccine-induced immune responses and antiviral medications. We review these findings and further advance the notions that (1) immune cells possess GABA_A-Rs to limit inflammation in the CNS, and (2) this natural "braking system" on inflammatory responses may be pharmacologically engaged to slow the progression of autoimmune diseases, reduce the severity of COVID-19, and perhaps limit neuroinflammation associated with long COVID.

Impaired neuronal integrity in traumatic brain injury detected by 123I-iomazenil single photon emission computed tomography and MRI

This study was aiming at investigating the extent of neuronal damage in cases of traumatic brain injury (TBI) with diffuse axonal injury (DAI) using ¹²³I-iomazenil(IMZ) SPECT and MRI. We compared the findings in 31 patients with TBI without any major focal brain lesions and 25 age-matched normal controls. Subjects underwent ¹²³I-IMZ SPECT and MRI, and also assessment by cognitive function tests. The partial volume effect of ¹²³I-IMZ SPECT was corrected using MRI. In the patients with TBI, decreased spatial concentration of ¹²³I-IMZ binding was detected in the medial frontal/orbitofrontal cortex, posterior cingulate gyrus, cuneus, precuneus, and superior region of the cerebellum. ROC analysis of ¹²³I-IMZ SPECT for the detection of neuronal injury showed a high diagnostic ability of ¹²³I-IMZ binding density for TBI in these areas. The decreased ¹²³I-IMZ uptake density in the cuneus and precuneus was associated with cognitive decline after the injury. In the patients with TBI, brain atrophy was detected in the frontal lobe, anterior temporal and parietal cortex, corpus callosum, and posterior part of the cerebellum. Evaluation of the neuronal integrity by ¹²³I-IMZ SPECT and MRI provides important information for the diagnosis and pathological interpretation in cases of TBI with DAI.

Cys-loop receptors on cannabinoids: All high?

Endocannabinoids (eCBS) are endogenously derived lipid signaling molecules that serve as tissue hormones and interact with multiple targets, mostly within the endocannabinoid system (ECS). The ECS is a highly conserved regulatory system involved in homeostatic regulation, organ formation, and immunomodulation of chordates. The term “cannabinoid” evolved from the distinctive class of plant compounds found in Cannabis sativa, an ancient herb, due to their action on CB1 and CB2 receptors. CB1/2 receptors are the primary targets for eCBs, but their effects are not limited to the ECS. Due to the high interest and extensive research on the ECS, knowledge on its constituents and physiological role is substantial and still growing. Crosstalk and multiple targeting of molecules are common features of endogenous and plant compounds. Cannabimimetic molecules can be divided according to their origin, natural or synthetic, including phytocannabinoids (pCB’s) or synthetic cannabinoids (sCB’s). The endocannabinoid system (ECS) consists of receptors, transporters, enzymes, and signaling molecules. In this review, we focus on the effects of cannabinoids on Cys-loop receptors. Cys-loop receptors belong to the class of membrane-bound pentameric ligand gated ion channels, each family comprising multiple subunits. Mammalians possess GABA type A receptors (GABAAR), glycine receptors (GlyR), serotonin receptors type 3 (5-HT3R), and nicotinic acetylcholine receptors (nAChR). Several studies have shown different modulatory effects of CBs on multiple members of the Cys-loop receptor family. We highlight the existing knowledge, especially on subunits and protein domains with conserved binding sites for CBs and their possible pharmacological and physiological role in epilepsy and in chronic pain. We further discuss the potential for cannabinoids as first line treatments in epilepsy, chronic pain and other neuropsychiatric conditions, indicated by their polypharmacology and therapeutic profile.

Glutamate and GABAA receptor crosstalk mediates homeostatic regulation of neuronal excitation in the mammalian brain

Maintaining a proper balance between the glutamate receptor-mediated neuronal excitation and the A type of GABA receptor (GABA_AR) mediated inhibition is essential for brain functioning; and its imbalance contributes to the pathogenesis of many brain disorders including neurodegenerative diseases and mental illnesses. Here we identify a novel glutamate-GABA_AR interaction mediated by a direct glutamate binding of the GABA_AR. In HEK293 cells overexpressing recombinant GABA_ARs, glutamate and its analog ligands, while producing no current on their own, potentiate GABA-evoked currents. This potentiation is mediated by a direct binding at a novel glutamate binding pocket located at the α⁺/β⁻ subunit interface of the GABA_AR. Moreover, the potentiation does not require the presence of a γ subunit, and in fact, the presence of γ subunit significantly reduces the potency of the glutamate potentiation. In addition, the glutamate-mediated allosteric potentiation occurs on native GABA_ARs in rat neurons maintained in culture, as evidenced by the potentiation of GABA_AR-mediated inhibitory postsynaptic currents and tonic currents. Most importantly, we found that genetic impairment of this glutamate potentiation in knock-in mice resulted in phenotypes of increased neuronal excitability, including decreased thresholds to noxious stimuli and increased seizure susceptibility. These results demonstrate a novel cross-talk between excitatory transmitter glutamate and inhibitory GABA_AR. Such a rapid and short feedback loop between the two principal excitatory and inhibitory neurotransmission systems may play a critical homeostatic role in fine-tuning the excitation-inhibition balance (E/I balance), thereby maintaining neuronal excitability in the mammalian brain under both physiological and pathological conditions.

Effect of cortical extracellular GABA on motor response

To elucidate how the flattening of sensory tuning due to a deficit in tonic inhibition slows motor responses, we simulated a neural network model in which a sensory cortical network ([Formula: see text]) and a motor cortical network ([Formula: see text]) are reciprocally connected, and the [Formula: see text] projects to spinal motoneurons (Mns). The [Formula: see text] was presented with a feature stimulus and the reaction time of Mns was measured. The flattening of sensory tuning in [Formula: see text] caused by decreasing the concentration of gamma-aminobutyric acid (GABA) in extracellular space resulted in a decrease in the stimulus-sensitive [Formula: see text] pyramidal cell activity while increasing the stimulus-insensitive [Formula: see text] pyramidal cell activity, thereby prolonging the reaction time of Mns to the applied feature stimulus. We suggest that a reduction in extracellular GABA concentration in sensory cortex may interfere with selective activation in motor cortex, leading to slowing the activation of spinal motoneurons and therefore to slowing motor responses.

Pleiotropic endophenotypic and phenotype effects of GABAergic neurosteroid synthesis deficiency in posttraumatic stress disorder

PTSD is associated with deficits in synthesis of progesterone metabolites such as allopregnanolone and pregnanolone that potently facilitate gamma-amino-butyric acid (GABA) effects at GABA_A receptors. These neurosteroids modulate neuronal firing rate, regional brain connectivity, and activation of amygdala-mediated autonomic nervous system, hypothalamic-pituitary-adrenal axis, and behavioral reactions to unconditioned and conditioned threat. They also play critical roles in learning and memory processes such as extinction and extinction retention and inhibit toll-like receptor activation of intracellular pro-inflammatory pathways. Deficient synthesis of these neurosteroids thus may contribute to individually variable PTSD clinical phenotypes encompassing symptom severity, capacity for PTSD recovery, and vulnerability to common PTSD-comorbidities such as major depression, chronic pain, alcohol and nicotine dependence, cardiovascular disease, metabolic syndrome, reproductive disorders, and autoimmune conditions.

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

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

Targeting the Cation-Chloride Co-Transporter NKCC1 to Re-Establish GABAergic Inhibition and an Appropriate Excitatory/Inhibitory Balance in Selective Neuronal Circuits: A Novel Approach for the Treatment of Alzheimer's Disease

GABA, the main inhibitory neurotransmitter in the adult brain, depolarizes and excites immature neurons because of an initially higher intracellular chloride concentration [Cl^-]i due to the delayed expression of the chloride exporter KCC2 at birth. Depolarization-induced calcium rise via NMDA receptors and voltage-dependent calcium channels is instrumental in shaping neuronal circuits and in controlling the excitatory (E)/inhibitory (I) balance in selective brain areas. An E/I imbalance accounts for cognitive impairment observed in several neuropsychiatric disorders. The aim of this review is to summarize recent data on the mechanisms by which alterations of GABAergic signaling alter the E/I balance in cortical and hippocampal neurons in Alzheimer's disease (AD) and the role of cation-chloride co-transporters in this process. In particular, we discuss the NGF and AD relationship and how mice engineered to express recombinant neutralizing anti-NGF antibodies (AD11 mice), which develop a neurodegenerative pathology reminiscent of that observed in AD patients, exhibit a depolarizing action of GABA due to KCC2 impairment. Treating AD and other forms of dementia with bumetanide, a selective KCC2 antagonist, contributes to re-establishing a proper E/I balance in selective brain areas, leading to amelioration of AD symptoms and the slowing down of disease progression.

Zolpidem Profoundly Augments Spared Tonic GABAAR Signaling in Dentate Granule Cells Ipsilateral to Controlled Cortical Impact Brain Injury in Mice

Type A GABA receptors (GABA_ARs) are pentameric combinations of protein subunits that give rise to tonic (I_TonicGABA) and phasic (i.e., synaptic; I_SynapticGABA) forms of inhibitory GABA_AR signaling in the central nervous system. Remodeling and regulation of GABA_AR protein subunits are implicated in a wide variety of healthy and injury-dependent states, including epilepsy. The present study undertook a detailed analysis of GABA_AR signaling using whole-cell patch clamp recordings from mouse dentate granule cells (DGCs) in coronal slices containing dorsal hippocampus at 1–2 or 8–13 weeks after a focal, controlled cortical impact (CCI) or sham brain injury. Zolpidem, a benzodiazepine-like positive modulator of GABA_ARs, was used to test for changes in GABA_AR signaling of DGCs due to its selectivity for α₁ subunit-containing GABA_ARs. Electric charge transfer and statistical percent change were analyzed in order to directly compare tonic and phasic GABA_AR signaling and to account for zolpidem’s ability to modify multiple parameters of GABA_AR kinetics. We observed that baseline I_TonicGABA is preserved at both time-points tested in DGCs ipsilateral to injury (Ipsi-DGCs) compared to DGCs contralateral to injury (Contra-DGCs) or after sham injury (Sham-DGCs). Interestingly, application of zolpidem resulted in modulation of I_TonicGABA across groups, with Ipsi-DGCs exhibiting the greatest responsiveness to zolpidem. We also report that the combination of CCI and acute application of zolpidem profoundly augments the proportion of GABA_AR charge transfer mediated by tonic vs. synaptic currents at both time-points tested, whereas gene expression of GABA_AR α₁, α₂, α₃, and γ₂ subunits is unchanged at 8–13 weeks post-injury. Overall, this work highlights the shift toward elevated influence of tonic inhibition in Ipsi-DGCs, the impact of zolpidem on all components of inhibitory control of DGCs, and the sustained nature of these changes in inhibitory tone after CCI injury.

Activation of LH GABAergic inputs counteracts fasting-induced changes in tVTA/RMTG neurons

KEY POINTS

While dopamine neuronal activity changes with motivational state, it is unknown if fasting influences tVTA/RMTg GABAergic neurons, a major inhibitory input to VTA dopamine neurons. In unfasted mice, there were sex differences in inhibitory synaptic transmission onto tVTA/RMTg GABAergic neurons. Activation of LH GABAergic neurons decreases firing of tVTA/RMTg GABAergic neurons through a monosynaptic input. An acute fast decreased the excitability of tVTA/RMTg GABAergic neurons. An acute fast decreases inhibitory synaptic transmission of the LH GABA input to tVTA/RMTg GABAergic neurons in both male and female mice.

ABSTRACT

Dopamine neurons in the ventral tegmental area (VTA) are strongly innervated by GABAergic neurons in the 'tail of the VTA' (tVTA), also known as the rostralmedial tegmental nucleus (RMTg). Disinhibition of dopamine neurons through firing of the GABAergic neurons projecting from the lateral hypothalamus (LH) leads to reward seeking and consumption through dopamine release in the nucleus accumbens. VTA dopamine neurons respond to changes in motivational state, yet less is known of whether tVTA/RMTg GABAergic neurons or the LH GABAergic neurons that project to them are also affected by changes in motivational state, such as fasting. An acute 16 h overnight fast decreased the excitability of tVTA/RMTg GABAergic neurons of male and female mice. In addition, fasting decreased synaptic strength at LH GABA to tVTA/RMTg GABAergic synapses, indicated by reduced amplitude of optically evoked currents, decreased readily releasable pool (RRP) size and replenishment. Optical stimulation of LH GABA terminals suppressed evoked action potentials of tVTA/RMTg GABAergic neurons in unfasted mice, but this effect decreased following fasting. Furthermore, during fasting, LH GABA inputs to tVTA/RMTg neurons maintained functional connectivity during depolarization, as depolarization block was reduced following fasting. Taken together, inhibitory synaptic transmission from LH GABA inputs onto tVTA/RMTg GABAergic neurons decreases following fasting, however ability to functionally inhibit tVTA/RMTg GABAergic neurons is preserved, allowing for possible disinhibition of dopamine neurons and subsequent foraging. Abstract figure legend  The inhibitory synaptic input is represented by the downward arrows. Following fasting, there was a decrease in inhibitory synaptic strength in both males and females. The action potentials represent the excitability, which also decreases in both males and females following fasting. Because both the LH GABA input and excitability of tVTA/RMTg GABA neurons have reduced activity following fasting, we predict that disinhibition of dopamine neurons with stimulation of LH inputs is preserved. This article is protected by copyright. All rights reserved.

Activity-Dependent Modulation of Tonic GABA Currents by Endocannabinoids in Hirudo verbana

Endocannabinoids are lipid neuromodulators that are synthesized on demand and primarily signal in a retrograde manner to elicit depression of excitatory and inhibitory synapses. Despite the considerable interest in their potential analgesic effects, there is evidence that endocannabinoids can have both pro-nociceptive and anti-nociceptive effects. The mechanisms contributing to the opposing effects of endocannabinoids in nociception need to be better understood before cannabinoid-based therapies can be effectively utilized to treat pain. Using the medicinal leech, Hirudo verbana, this work investigates whether endocannabinoids modulate tonic inhibition onto non-nociceptive afferents. In voltage clamp recordings, we analyzed changes in the tonic inhibition in pressure-sensitive (P) cells following pre-treatment with endocannabinoids, 2-arachidonoylglycerol (2-AG) or anandamide (AEA). We also tested whether high frequency stimulation (HFS) of nociceptive (N) cells could also modulate tonic inhibition. Both endocannabinoid application and N cell HFS depressed tonic inhibition in the P cell. Depression of tonic inhibition by N cell HFS was blocked by SB 366791 (a TRPV1 inhibitor). SB 366791 also prevented 2-AG-and AEA-induced depression of tonic inhibition. HFS-induced depression was not blocked by tetrahydrolipstatin (THL), which prevents 2-AG synthesis, nor AM 251 (a CB1 receptor inverse agonist). These results illustrate a novel activity-dependent modulation of tonic GABA currents that is mediated by endocannabinoid signaling and is likely to play an important role in sensitization of non-nociceptive afferent pathways.

The Influence of AA29504 on GABAA Receptor Ligand Binding Properties and Its Implications on Subtype Selectivity

The unique pharmacological properties of δ-containing γ-aminobutyric acid type A receptors (δ-GABAARs) make them an attractive target for selective and persistent modulation of neuronal excitability. However, the availability of selective modulators targeting δ-GABAARs remains limited. AA29504 ([2-amino-4-(2,4,6-trimethylbenzylamino)-phenyl]-carbamic acid ethyl ester), an analog of K+ channel opener retigabine, acts as an agonist and a positive allosteric modulator (Ago-PAM) of δ-GABAARs. Based on electrophysiological studies using recombinant receptors, AA29504 was found to be a more potent and effective agonist in δ-GABAARs than in γ2-GABAARs. In comparison, AA29504 positively modulated the activity of recombinant δ-GABAARs more effectively than γ2-GABAARs, with no significant differences in potency. The impact of AA29504's efficacy- and potency-associated GABAAR subtype selectivity on radioligand binding properties remain unexplored. Using [3H]4'-ethynyl-4-n-propylbicycloorthobenzoate ([3H]EBOB) binding assay, we found no difference in the modulatory potency of AA29504 on GABA- and THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol)-induced responses between native forebrain GABAARs of wild type and δ knock-out mice. In recombinant receptors expressed in HEK293 cells, AA29504 showed higher efficacy on δ- than γ2-GABAARs in the GABA-independent displacement of [3H]EBOB binding. Interestingly, AA29504 showed a concentration-dependent stimulation of [3H]muscimol binding to γ2-GABAARs, which was absent in δ-GABAARs. This was explained by AA29504 shifting the low-affinity γ2-GABAAR towards a higher affinity desensitized state, thereby rising new sites capable of binding GABAAR agonists with low nanomolar affinity. Hence, the potential of AA29504 to act as a desensitization-modifying allosteric modulator of γ2-GABAARs deserves further investigation for its promising influence on shaping efficacy, duration and plasticity of GABAAR synaptic responses.

GABAA and NMDA receptor density alterations and their behavioral correlates in the gestational methylazoxymethanol acetate model for schizophrenia

Hippocampal hyperactivity driven by GABAergic interneuron deficits and NMDA receptor hypofunction is associated with the hyperdopaminergic state often observed in schizophrenia. Furthermore, previous research in the methylazoxymethanol acetate (MAM) rat model has demonstrated that repeated peripubertal diazepam administration can prevent the emergence of adult hippocampal hyperactivity, dopamine-system hyperactivity, and associated psychosis-relevant behaviors. Here, we sought to characterize hippocampal GABAA and NMDA receptors in MAM-treated rats and to elucidate the receptor mechanisms underlying the promising effects of peripubertal diazepam exposure. Quantitative receptor autoradiography was used to measure receptor density in the dorsal hippocampus CA1, ventral hippocampus CA1, and ventral subiculum. Specifically, [3H]-Ro15-4513 was used to quantify the density of α5GABAA receptors (α5GABAAR), [3H]-flumazenil to quantify α1-3;5GABAAR, and [3H]-MK801 to quantify NMDA receptors. MAM rats exhibited anxiety and schizophrenia-relevant behaviors as measured by elevated plus maze and amphetamine-induced hyperlocomotion (AIH), although diazepam only partially rescued these behaviors. α5GABAAR density was reduced in MAM-treated rats in all hippocampal sub-regions, and negatively correlated with AIH. Ventral hippocampus CA1 α5GABAAR density was positively correlated with anxiety-like behavior. Dorsal hippocampus CA1 NMDA receptor density was increased in MAM-treated rats, and positively correlated with AIH. [3H]-flumazenil revealed no significant effects. Finally, we found no significant effect of diazepam treatment on receptor densities, potentially related to the only partial rescue of schizophrenia-relevant phenotypes. Overall, our findings provide first evidence of α5GABAAR and NMDA receptor abnormalities in the MAM model, suggesting that more selective pharmacological agents may become a novel therapeutic mechanism in schizophrenia.

Evaluation of diazepam as a drug treatment for water hemlock (Cicuta species) poisoning in Spanish goats

Water hemlocks (Cicuta spp.) are toxic members of the Apiaceae plant family. The best drug treatment for the convulsions associated with acute water hemlock poisoning in livestock and humans has not been determined experimentally. This work compared the therapeutic actions of benzodiazepines (diazepam) and barbiturates (phenobarbital) on water hemlock poisoning in a goat model. C. maculata tubers were orally dosed to goats. Experimental groups consisted of; control saline; 20 mg/kg phenobarbital; 1.0 mg/kg diazepam; 10 mg/kg diazepam; and 1.0 mg/kg diazepam administered as needed to moderate convulsions by intravenous (i.v.) infusion. Diazepam provided nearly instant control of convulsions. Clinical signs of poisoning were completely controlled for the duration of the experiment in the goats that received the 10 mg/kg diazepam dose. These results suggest that diazepam is effective at managing the clinical signs of water hemlock poisoning in goats. We speculate that diazepam can be used as a potential treatment for water hemlock poisoning in other livestock species and humans.

Hippocampal α5-GABAA Receptors Modulate Dopamine Neuron Activity in the Rat Ventral Tegmental Area

Background

Aberrant dopamine neuron activity is attributable to hyperactivity in hippocampal subfields driving a pathological increase in dopamine neuron activity, which is positively correlated with psychosis in humans. Evidence indicates that hippocampal hyperactivity is due to loss of intrinsic GABAergic (gamma-aminobutyric acidergic) inhibition. We have previously demonstrated that hippocampal GABAergic neurotransmission can be modulated by targeting α5-GABA_A receptors, which are preferentially expressed in hippocampal regions. Positive and negative allosteric modulators of α5-GABA_A receptors (α5-PAMs and α5-NAMs) elicit effects on hippocampal-dependent behaviors. We posited that the selective manipulation of hippocampal inhibition, using α5-PAMs or α5-NAMs, would modulate dopamine activity in control rats. Further, α5-PAMs would reverse aberrant dopamine neuron activity in a rodent model with schizophrenia-related pathophysiologies (methylazoxymethanol acetate [MAM] model).

Methods

We performed in vivo extracellular recordings of ventral tegmental area dopamine neurons in anesthetized rats to compare the effects of two novel, selective α5-PAMs (GL-II-73, MP-III-022), a nonselective α-PAM (midazolam), and two selective α5-NAMs (L-655,708, TB 21007) in control and MAM-treated male Sprague Dawley rats (n = 5-9).

Results

Systemic or intracranial administration of selective α5-GABA_A receptor modulators regulated dopamine activity. Specifically, both α5-NAMs increased dopamine neuron activity in control rats, whereas GL-II-73, MP-III-022, and L-655,708 attenuated aberrant dopamine neuron activity in MAM-treated rats, an effect mediated by the ventral hippocampus.

Conclusions

This study demonstrated that α5-GABA_A receptor modulation can regulate dopamine neuron activity under control or abnormal activity, providing additional evidence that α5-PAMs and α5-NAMs may have therapeutic applications in psychosis and other psychiatric diseases where aberrant hippocampal activity is present.

Long-Term Imaging Reveals a Circadian Rhythm of Intracellular Chloride in Neurons of the Suprachiasmatic Nucleus

Both inhibitory and excitatory GABA transmission exist in the mature suprachiasmatic nucleus (SCN), the master pacemaker of circadian physiology. Whether GABA is inhibitory or excitatory depends on the intracellular chloride concentration ([Cl^-]_i). Here, using the genetically encoded ratiometric probe Cl-Sensor, we investigated [Cl^-]_i in AVP and VIP-expressing SCN neurons for several days in culture. The chloride ratio (R_Cl) demonstrated circadian rhythmicity in AVP + neurons and VIP + neurons, but was not detected in GFAP + astrocytes. R_Cl peaked between ZT 7 and ZT 8 in both AVP + and VIP + neurons. R_Cl rhythmicity was not dependent on the activity of several transmembrane chloride carriers, action potential generation, or the L-type voltage-gated calcium channels, but was sensitive to GABA antagonists. We conclude that [Cl^-]_i is under circadian regulation in both AVP + and VIP + neurons.

Astrocyte Role in Temporal Lobe Epilepsy and Development of Mossy Fiber Sprouting

Epilepsy affects approximately 50 million people worldwide, with 60% of adult epilepsies presenting an onset of focal origin. The most common focal epilepsy is temporal lobe epilepsy (TLE). The role of astrocytes in the presentation and development of TLE has been increasingly studied and discussed within the literature. The most common histopathological diagnosis of TLE is hippocampal sclerosis. Hippocampal sclerosis is characterized by neuronal cell loss within the Cornu ammonis and reactive astrogliosis. In some cases, mossy fiber sprouting may be observed. Mossy fiber sprouting has been controversial in its contribution to epileptogenesis in TLE patients, and the mechanisms surrounding the phenomenon have yet to be elucidated. Several studies have reported that mossy fiber sprouting has an almost certain co-existence with reactive astrogliosis within the hippocampus under epileptic conditions. Astrocytes are known to play an important role in the survival and axonal outgrowth of central and peripheral nervous system neurons, pointing to a potential role of astrocytes in TLE and associated cellular alterations. Herein, we review the recent developments surrounding the role of astrocytes in the pathogenic process of TLE and mossy fiber sprouting, with a focus on proposed signaling pathways and cellular mechanisms, histological observations, and clinical correlations in human patients.

Post-acute delivery of α5-GABAA antagonist, S 44819, improves functional recovery in juvenile rats following stroke

Hypo-excitability was reported in the peri-infarct tissue following stroke, an effect counteracted by a blockage of α5-GABAA receptors in adult rodents. Our present study aims to evaluate the effect of a selective α5-GABAA receptor antagonist, S 44819, in stroke in juvenile animals. We have set up and characterized an original model of transient ischemic stroke in 28 day-old Sprague-Dawley rats (45-min occlusion of the middle cerebral artery by intraluminal suture). In this model, S 44819 (1, 3 and 10 mg/kg, b.i.d) was orally administered from day 3 to day 16 after stroke onset. Sensorimotor recovery was assessed on day 1, day 9 and day 16 after stroke onset. Results show that rats treated with S 44819 at the doses of 3 and 10 mg/kg displayed a significant improvement of the neurological deficits (neuroscore) on day 9 and day 16, when compared with animals treated with vehicle. Grip-test data analysis reveals that rats treated with S 44819 at the dose of 3 mg/kg displayed a better recovery on day 9 and day 16. These results are in agreement with those previously observed in adult rats, demonstrating that targeting α5-GABAA receptors improves neurological recovery after stroke in juvenile rats.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Modelling the functional roles of synaptic and extra-synaptic γ-aminobutyric acid receptor dynamics in circadian timekeeping

In the suprachiasmatic nucleus (SCN), γ-aminobutyric acid (GABA) is a primary neurotransmitter. GABA can signal through two types of GABAA receptor subunits, often referred to as synaptic GABAA (gamma subunit) and extra-synaptic GABAA (delta subunit). To test the functional roles of these distinct GABAA in regulating circadian rhythms, we developed a multicellular SCN model where we could separately compare the effects of manipulating GABA neurotransmitter or receptor dynamics. Our model predicted that blocking GABA signalling modestly increased synchrony among circadian cells, consistent with published SCN pharmacology. Conversely, the model predicted that lowering GABAA receptor density reduced firing rate, circadian cell fraction, amplitude and synchrony among individual neurons. When we tested these predictions, we found that the knockdown of delta GABAA reduced the amplitude and synchrony of clock gene expression among cells in SCN explants. The model further predicted that increasing gamma GABAA densities could enhance synchrony, as opposed to increasing delta GABAA densities. Overall, our model reveals how blocking GABAA receptors can modestly increase synchrony, while increasing the relative density of gamma over delta subunits can dramatically increase synchrony. We hypothesize that increased gamma GABAA density in the winter could underlie the tighter phase relationships among SCN cells.

Gabra6100Q allele Sprague-Dawley rats have a higher sensitivity to hypnosis induced by isoflurane and ethanol than the wild type rats

BACKGROUND

The neurobiological mechanisms underlying how general anesthetics render a patient's unconsciousness (hypnosis) remains elusive. The role of the cerebellum in hypnosis induced by general anesthetics is unknown. Gabra6^100Q allele Sprague-Dawley (SD) rats have a naturally occurring single nucleotide polymorphism in the GABA_A receptor α6 subunit gene that is expressed exclusively in cerebellum granule cells.

METHODS

We examined the loss of righting reflex (LORR) induced by isoflurane, and ethanol in Gabra6^100Q rats compared with those in wild type (WT) SD rats. We also examined the change of c-Fos expression induced by isoflurane exposure in cerebellum granule cells of both mutant and WT rats.

RESULTS

Gabra6^100Q rats are more sensitive than WT rats to the LORR induced by isoflurane and ethanol. Moreover, isoflurane exposure induced a greater reduction in c-Fos expression in cerebellum granule cells of Gabra6^100Q rats than WT rats.

CONCLUSIONS

Based on these data, we speculate that cerebellum may be involved in the hypnosis induced by some general anesthetics and thus may represent a novel target of general anesthetics.

Diurnal properties of tonic and synaptic GABAA receptor-mediated currents in suprachiasmatic nucleus neurons

Synaptic and extrasynaptic GABA_A receptor (GABA_AR)-mediated neurotransmission is a critical component of the suprachiasmatic nucleus (SCN) neuronal network. However, the properties of the GABA_A tonic current (I_tonic) and its origin remain unexplored. Spontaneous GABA_A postsynaptic currents (sGPSCs) and I_tonic were recorded from SCN neurons with the whole cell voltage-clamp technique at different times of the day. GABA_AR antagonists (bicuculline, gabazine, and picrotoxin) inhibited sGPSC and induced an outward shift of the holding current, which defined the I_tonic amplitude. The sGPSC frequency, synaptic charge transfer, and I_tonic amplitude all demonstrated significant diurnal rhythms, with peaks in the middle of the day [zeitgeber time (ZT)7-8] and nadirs at night (ZT19-20). The I_tonic amplitude increased proportionally with the sGPSC frequency and synaptic charge transfer during the day and required action potential-mediated GABA release, which was confirmed by TTX application. The activation of presynaptic GABA_B receptors by baclofen did not significantly alter the I_tonic of neurons with low-frequency sGPSC. The equilibrium potential (Eq) for I_tonic was similar to the Eq for chloride and GABA_A receptor-activated currents. I_tonic showed outward rectification at membrane potentials over the range of -70 to -10 mV and then was linear at voltages greater than -10 mV. GABA_AR containing α4-, α5-, and δ-subunits were expressed in SCN, and their contribution to I_tonic was confirmed by application of the GABA_AR agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) and the GABA_AR inverse agonist 11,12,13,13a-tetrahydro-7-methoxy-9-oxo-9H-imidazo[1,5-a]pyrrolo[2,1-c][1,4]benzodiazepine-1-carboxylic acid ethyl ester (L655,708). Thus, the I_tonic was mediated by extrasynaptic GABA_ARs activated predominantly by GABA diffusing out of GABAergic synapses.NEW & NOTEWORTHY A tonic current (I_tonic) mediated by GABA_A receptors (GABA_ARs) containing α4-, α5- and δ-subunits was observed in the suprachiasmatic nucleus. The I_tonic amplitude strongly depended on the action potential-mediated synaptic release of GABA. The equilibrium potential for I_tonic corresponds to that for GABA_A currents. The frequency of GABA_A postsynaptic currents and I_tonic amplitude increased during the day, with peak in the middle of the day, and then gradually declined with a nadir at night, thus showing a diurnal rhythm.

Identification of novel target molecules of l-menthol

The present study used a binding assay to identify novel target biomolecules of l-menthol ([−]-menthol) that promote mouse ambulation. Among 88 different ligands to specific biomolecules examined, 0.1 mM l-menthol inhibited the binding of 13 ligands with relatively high inhibition rates. The assays showed that l-menthol acts on calcium channels, sodium channels, γ-aminobutyric acid type A (GABA_A) receptor, GABA transporter, dopamine transporter, dopamine D4 receptor, adenosine A2a receptor, α2A-adrenergic receptor, histamine H2 receptor, bombesin receptor, angiotensin AT1 receptor, vasopressin V2 receptor, and leukotriene B4 receptor over a similar concentration range. The inhibition constant (K_i) for l-menthol inhibition of binding of [³H]-WIN35,428 to the human recombinant dopamine transporter was 6.15 × 10⁻⁴ mol/L. The K_i for l-menthol inhibition of binding of [³H]-ethynylbicycloorthobenzoate (EBOB), a ligand of GABA_A receptor picrotoxin site, was 2.88 × 10⁻⁴ mol/L. These results should aid future research by providing clues for investigating the mechanisms underlying l-menthol activities, including the ambulation-promoting effect. The present results suggest that the dopamine transporter, adenosine A2a receptor, dopamine D4 receptor, α2A-adrenergic receptor, and GABA_A receptor are promising candidate molecules that are involved in the mechanisms underlying the psychostimulant-like effect of l-menthol.

Cerebellum-Specific Deletion of the GABAA Receptor δ Subunit Leads to Sex-Specific Disruption of Behavior

Granule cells (GCs) of the cerebellar input layer express high-affinity δ GABA_A subunit-containing GABA_A receptors (δGABA_ARs) that respond to ambient GABA levels and context-dependent neuromodulators like steroids. We find that GC-specific deletion of δGABA_A (cerebellar [cb] δ knockout [KO]) decreases tonic inhibition, makes GCs hyperexcitable, and in turn, leads to differential activation of cb output regions as well as many cortical and subcortical brain areas involved in cognition, anxiety-like behaviors, and the stress response. Cb δ KO mice display deficits in many behaviors, but motor function is normal. Strikingly, δGABA_A deletion alters maternal behavior as well as spontaneous, stress-related, and social behaviors specifically in females. Our findings establish that δGABA_ARs enable the cerebellum to control diverse behaviors not previously associated with the cerebellum in a sex-dependent manner. These insights may contribute to a better understanding of the mechanisms that underlie behavioral abnormalities in psychiatric and neurodevelopmental disorders that display a gender bias.

Rudolph et al. show that deletion of the neuromodulator and hormone-sensitive δGABA_A receptor subunit from cerebellar granule cells results in anxiety-like behaviors and female-specific deficits in social behavior and maternal care. δGABA_A deletion is associated with hyperexcitability of the cerebellar input layer and altered activation of many stress-related brain regions.

Beyond Dopamine Receptor Antagonism: New Targets for Schizophrenia Treatment and Prevention

Treatment of schizophrenia (SCZ) historically relies on the use of antipsychotic drugs to treat psychosis, with all of the currently available antipsychotics acting through the antagonism of dopamine D2 receptors. Although antipsychotics reduce psychotic symptoms in many patients, they induce numerous undesirable effects and are not effective against negative and cognitive symptoms. These highlight the need to develop new drugs to treat SCZ. An advanced understanding of the circuitry of SCZ has pointed to pathological origins in the excitation/inhibition balance in regions such as the hippocampus, and restoring function in this region, particularly as a means to compensate for parvalbumin (PV) interneuron loss and resultant hippocampal hyperactivity, may be a more efficacious approach to relieve a broad range of SCZ symptoms. Other targets, such as cholinergic receptors and the trace amine-associated receptor 1 (TAAR1), have also shown some promise for the treatment of SCZ. Importantly, assessing efficacy of novel compounds must take into consideration treatment history of the patient, as preclinical studies suggest prior antipsychotic treatment may interfere with the efficacy of these novel agents. However, while novel therapeutic targets may be more effective in treating SCZ, a more effective approach would be to prevent the transition to SCZ in susceptible individuals. A focus on stress, which has been shown to be a predisposing factor in risk for SCZ, is a possible avenue that has shown promise in preclinical studies. Therefore, therapeutic approaches based on our current understanding of the circuitry of SCZ and its etiology are likely to enable development of more effective therapeutic interventions for this complex disorder.

Excitatory-inhibitory tone shapes decision strategies in a hierarchical neural network model of multi-attribute choice

We are constantly faced with decisions between alternatives defined by multiple attributes, necessitating an evaluation and integration of different information sources. Time-varying signals in multiple brain areas are implicated in decision-making; but we lack a rigorous biophysical description of how basic circuit properties, such as excitatory-inhibitory (E/I) tone and cascading nonlinearities, shape attribute processing and choice behavior. Furthermore, how such properties govern choice performance under varying levels of environmental uncertainty is unknown. We investigated two-attribute, two-alternative decision-making in a dynamical, cascading nonlinear neural network with three layers: an input layer encoding choice alternative attribute values; an intermediate layer of modules processing separate attributes; and a final layer producing the decision. Depending on intermediate layer E/I tone, the network displays distinct regimes characterized by linear (I), convex (II) or concave (III) choice indifference curves. In regimes I and II, each option's attribute information is additively integrated. In regime III, time-varying nonlinear operations amplify the separation between offer distributions by selectively attending to the attribute with the larger differences in input values. At low environmental uncertainty, a linear combination most consistently selects higher valued alternatives. However, at high environmental uncertainty, regime III is more likely than a linear operation to select alternatives with higher value. Furthermore, there are conditions where readout from the intermediate layer could be experimentally indistinguishable from the final layer. Finally, these principles are used to examine multi-attribute decisions in systems with reduced inhibitory tone, leading to predictions of different choice patterns and overall performance between those with restrictions on inhibitory tone and neurotypicals.

Tonic GABAergic inhibition, via GABAA receptors containing αβƐ subunits, regulates excitability of ventral tegmental area dopamine neurons

The activity of midbrain dopamine neurons is strongly regulated by fast synaptic inhibitory γ‐Aminobutyric acid (GABA)ergic inputs. There is growing evidence in other brain regions that low concentrations of ambient GABA can persistently activate certain subtypes of GABA_A receptor to generate a tonic current. However, evidence for a tonic GABAergic current in midbrain dopamine neurons is limited. To address this, we conducted whole‐cell recordings from ventral tegmental area (VTA) dopamine neurons in brain slices from mice. We found that application of GABA_A receptor antagonists decreased the holding current, indicating the presence of a tonic GABAergic input. Global increases in GABA release, induced by either a nitric oxide donor or inhibition of GABA uptake, further increased this tonic current. Importantly, prolonged inhibition of the firing activity of local GABAergic neurons abolished the tonic current. A combination of pharmacology and immunohistochemistry experiments suggested that, unlike common examples of tonic inhibition, this current may be mediated by a relatively unusual combination of α4βƐ subunits. Lastly, we found that the tonic current reduced excitability in dopamine neurons suggesting a subtractive effect on firing activity.

The Role of the Central Amygdala in Alcohol Dependence

Alcohol dependence is a chronically relapsing disorder characterized by compulsive drug-seeking and drug-taking, loss of control in limiting intake, and the emergence of a withdrawal syndrome in the absence of the drug. Accumulating evidence suggests an important role for synaptic transmission in the central nucleus of the amygdala (CeA) in mediating alcohol-related behaviors and neuroadaptive mechanisms associated with alcohol dependence. Acute alcohol facilitates γ-aminobutyric acid (GABA)ergic transmission in the CeA via both pre- and postsynaptic mechanisms, and chronic alcohol increases baseline GABAergic transmission. Acute alcohol inhibits glutamatergic transmission via effects at N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the CeA, whereas chronic alcohol up-regulates NMDA receptor (NMDAR)-mediated transmission. Pro- (e.g., corticotropin-releasing factor [CRF]) and antistress (e.g., nociceptin/orphanin FQ, oxytocin) neuropeptides affect alcohol- and anxiety-related behaviors, and also alter the alcohol-induced effects on CeA neurotransmission. Alcohol dependence produces plasticity in these neuropeptide systems, reflecting a recruitment of those systems during the transition to alcohol dependence.

Dentate Granule Cells Recruited in the Home Environment Display Distinctive Properties

The dentate granule cells (DGCs) play a crucial role in learning and memory. Many studies have described the role and physiological properties of these sparsely active neurons using different behavioral contexts. However, the morpho-functional features of DGCs recruited in mice maintained in their home cage (without training), considered as a baseline condition, have not yet been established. Using fosGFP transgenic mice, we observed ex vivo that DGCs recruited in animals maintained in the home cage condition are mature neurons that display a longer dendritic tree and lower excitability compared with non-activated cells. The higher GABA_A receptor-mediated shunting inhibition contributes to the lower excitability of DGCs activated in the home environment by shifting the input resistance towards lower values. Remarkably, that shunting inhibition is neither observed in non-activated DGCs nor in DGCs activated during training in virtual reality. In short, our results suggest that strong shunting inhibition and reduced excitability could constitute a distinctive neural signature of mature DGCs recruited in the context of the home environment.