The role of the synthetic enzyme GAD65 in the control of neuronal gamma-aminobutyric acid release

Dynamic changes in seizure state and anxiety-like behaviors during pentylenetetrazole kindling in rats

INTRODUCTION

Excessive anxiety is a mental disorder, and its treatment involves the use of benzodiazepines, a class of drugs that enhance the effects of the neurotransmitter gamma-aminobutyric acid (GABA) at the GABAA receptor. Anxiety disorders are frequent comorbidities in patients with epilepsy, and it has been speculated that anxiety disorders and epileptic seizures share common neurobiological mechanisms. However, conflicting results regarding anxiolytic and anxiogenic effects have been reported in animal models of epilepsy induced by pentylenetetrazole (PTZ) injections, and the causes of this discrepancy are unknown. We hypothesized that anxiety-like behaviors would change dynamically according to the changes in epilepsy susceptibility that occur during the PTZ kindling process. Therefore, we attempted to change anxiety-like behaviors bidirectionally depending on the number of PTZ injections.

METHODS

Adult male rats were injected with PTZ 20 times every other day, and stages of seizure onset were classified according to the Racine staging system. Anxiety-like behaviors were measured after 10 and 20 injections. The control group was injected with an equal volume of saline solution. Anxiety-like behaviors were investigated using the open-field, light/dark transition, elevated plus maze, and social interaction tests.

RESULTS

Bimodal changes in seizure stage were observed in response to PTZ kindling. The increase in the seizure stage was transiently suppressed after 10 injections, and this decrease in epileptic sensitivity disappeared after 20 injections. However, none of the rats reached a fully kindled state after 20 PTZ injections. After 10 PTZ injections, anxiety-like behaviors decreased compared with those of the control group in the open field, light/dark transition, and elevated plus-maze tests. The anxiolytic effects correlated with the seizure stage in individual rats. After 20 PTZ injections, anxiety-like behaviors returned to control levels.

CONCLUSION

PTZ kindling induced bimodal changes in the seizure stage. Anxiety-like behaviors decreased with transient decrease in epileptic sensitivity and returned to control levels with the disappearance of these states. These findings suggest a common neurobiological mechanism underlying anxiety disorders and epileptic seizures. In addition, the discrepancy in the previous studies, in which anxiety levels increase or decrease in PTZ-kindled animals, may be due to examination at different phases of the kindling process.

HDAC inhibitors as a potential therapy for chemotherapy-induced neuropathic pain

Cancer, a chronic disease characterized by uncontrolled cell development, kills millions of people globally. The WHO reported over 10 million cancer deaths in 2020. Anticancer medications destroy healthy and malignant cells. Cancer treatment induces neuropathy. Anticancer drugs cause harm to spinal cord, brain, and peripheral nerve somatosensory neurons, causing chemotherapy-induced neuropathic pain. The chemotherapy-induced mechanisms underlying neuropathic pain are not fully understood. However, neuroinflammation has been identified as one of the various pathways associated with the onset of chemotherapy-induced neuropathic pain. The neuroinflammatory processes may exhibit varying characteristics based on the specific type of anticancer treatment delivered. Neuroinflammatory characteristics have been observed in the spinal cord, where microglia and astrocytes have a significant impact on the development of chemotherapy-induced peripheral neuropathy. The patient's quality of life might be affected by sensory deprivation, loss of consciousness, paralysis, and severe disability. High cancer rates and ineffective treatments are associated with this disease. Recently, histone deacetylases have become a novel treatment target for chemotherapy-induced neuropathic pain. Chemotherapy-induced neuropathic pain may be treated with histone deacetylase inhibitors. Histone deacetylase inhibitors may be a promising therapeutic treatment for chemotherapy-induced neuropathic pain. Common chemotherapeutic drugs, mechanisms, therapeutic treatments for neuropathic pain, and histone deacetylase and its inhibitors in chemotherapy-induced neuropathic pain are covered in this paper. We propose that histone deacetylase inhibitors may treat several aspects of chemotherapy-induced neuropathic pain, and identifying these inhibitors as potentially unique treatments is crucial to the development of various chemotherapeutic combination treatments.

Melatonin improves synapse development by PI3K/Akt signaling in a mouse model of autism spectrum disorder

JOURNAL/nrgr/04.03/01300535-202407000-00043/figure1/v/2023-11-20T171125Z/r/image-tiff

Autism spectrum disorders are a group of neurodevelopmental disorders involving more than 1100 genes, including Ctnnd2 as a candidate gene. Ctnnd2 knockout mice, serving as an animal model of autism, have been demonstrated to exhibit decreased density of dendritic spines. The role of melatonin, as a neurohormone capable of effectively alleviating social interaction deficits and regulating the development of dendritic spines, in Ctnnd2 deletion-induced nerve injury remains unclear. In the present study, we discovered that the deletion of exon 2 of the Ctnnd2 gene was linked to social interaction deficits, spine loss, impaired inhibitory neurons, and suppressed phosphatidylinositol-3-kinase (PI3K)/protein kinase B (Akt) signal pathway in the prefrontal cortex. Our findings demonstrated that the long-term oral administration of melatonin for 28 days effectively alleviated the aforementioned abnormalities in Ctnnd2 gene-knockout mice. Furthermore, the administration of melatonin in the prefrontal cortex was found to improve synaptic function and activate the PI3K/Akt signal pathway in this region. The pharmacological blockade of the PI3K/Akt signal pathway with a PI3K/Akt inhibitor, wortmannin, and melatonin receptor antagonists, luzindole and 4-phenyl-2-propionamidotetralin, prevented the melatonin-induced enhancement of GABAergic synaptic function. These findings suggest that melatonin treatment can ameliorate GABAergic synaptic function by activating the PI3K/Akt signal pathway, which may contribute to the improvement of dendritic spine abnormalities in autism spectrum disorders.

Pentylenetetrazole kindling induces dynamic changes in GAD65 expression in hippocampal somatostatin interneurons

INTRODUCTION

One of the mechanisms of epileptgenesis is impairment of inhibitory neural circuits. Several studies have compared neural changes among subtypes of gamma-aminobutyric acid-related (GABAergic) neurons after acquired epileptic seizure. However, it is unclear that GABAergic neural modifications that occur during acquisition process of epileptic seizure.

METHODS

Male rats were injected with pentylenetetrazole (PTZ kindling: n = 30) or saline (control: n = 15) every other day to observe the development of epileptic seizure stages. Two time points were identified: the point at which seizures were most difficult to induce, and the point at which seizures were most easy to induce. The expression of GABAergic neuron-related proteins in the hippocampus was immunohistochemically compared among GABAergic subtypes at each of these time points.

RESULTS

Bimodal changes in seizure stages were observed in response to PTZ kindling. The increase of seizure stage was transiently suppressed after 8 or 10 injections, and then progressed again by the 16th injection. Based on these results, we defined 10 injections as a short-term injection period during which seizures are less likely to occur, and 20 injections as a long-term injection period during which continuous seizures are likely to occur. The immunohistochemical analysis showed that hippocampal glutamic acid decarboxylase 65 (GAD65) expression was increased after short-term kindling but unchanged after long-term kindling. Increased GAD65 expression was limited to somatostatin-positive (SOM+) cells among several GABAergic subtypes. By contrast, GAD, GABA, GABAAR α1, GABABR1, and VGAT cells showed no change following short- or long-term PTZ kindling.

CONCLUSION

PTZ kindling induces bimodal changes in the epileptic seizure stage. Seizure stage is transiently suppressed after short-term PTZ injection with GAD65 upregulation in SOM+ cells. The seizure stage is progressed again after long-term PTZ injection with GAD65 reduction to baseline level.

Striatal insights: a cellular and molecular perspective on repetitive behaviors in pathology

Animals often behave repetitively and predictably. These repetitive behaviors can have a component that is learned and ingrained as habits, which can be evolutionarily advantageous as they reduce cognitive load and the expenditure of attentional resources. Repetitive behaviors can also be conscious and deliberate, and may occur in the absence of habit formation, typically when they are a feature of normal development in children, or neuropsychiatric disorders. They can be considered pathological when they interfere with social relationships and daily activities. For instance, people affected by obsessive-compulsive disorder, autism spectrum disorder, Huntington's disease and Gilles de la Tourette syndrome can display a wide range of symptoms like compulsive, stereotyped and ritualistic behaviors. The striatum nucleus of the basal ganglia is proposed to act as a master regulator of these repetitive behaviors through its circuit connections with sensorimotor, associative, and limbic areas of the cortex. However, the precise mechanisms within the striatum, detailing its compartmental organization, cellular specificity, and the intricacies of its downstream connections, remain an area of active research. In this review, we summarize evidence across multiple scales, including circuit-level, cellular, and molecular dimensions, to elucidate the striatal mechanisms underpinning repetitive behaviors and offer perspectives on the implicated disorders. We consider the close relationship between behavioral output and transcriptional changes, and thereby structural and circuit alterations, including those occurring through epigenetic processes.

Sequential maturation of stimulus-specific adaptation in the mouse lemniscal auditory system

Stimulus-specific adaptation (SSA), the reduction of neural activity to a common stimulus that does not generalize to other, rare stimuli, is an essential property of our brain. Although well characterized in adults, it is still unknown how it develops during adolescence and what neuronal circuits are involved. Using in vivo electrophysiology and optogenetics in the lemniscal pathway of the mouse auditory system, we observed SSA to be stable from postnatal day 20 (P20) in the inferior colliculus, to develop until P30 in the auditory thalamus and even later in the primary auditory cortex (A1). We found this maturation process to be experience-dependent in A1 but not in thalamus and to be related to alterations in deep but not input layers of A1. We also identified corticothalamic projections to be implicated in thalamic SSA development. Together, our results reveal different circuits underlying the sequential SSA maturation and provide a unique perspective to understand predictive coding and surprise across sensory systems.

Localization and Diagnostic Specificity of Glutamic Acid Decarboxylase Transcript Alterations in the Dorsolateral Prefrontal Cortex in Schizophrenia

BACKGROUND

Working memory (WM) deficits in schizophrenia are thought to reflect altered inhibition in the dorsolateral prefrontal cortex (DLPFC). This interpretation is supported by findings of lower transcript levels of the 2 enzymes, GAD67 and GAD65, which mediate basal and activity-dependent GABA (gamma-aminobutyric acid) synthesis, respectively. However, the relative magnitude, location within the depth of the DLPFC, and specificity to the disease process of schizophrenia of alterations in GAD67 and/or GAD65 remain unclear.

METHODS

Levels of GAD67 and GAD65 messenger RNAs (mRNAs) in superficial (layers 2/superficial 3) and deep (deep layer 6/white matter) zones of the DLPFC were quantified by quantitative polymerase chain reaction in subjects with schizophrenia (n = 41), major depression (n = 42), or bipolar disorder (n = 39) and unaffected comparison (n = 43) subjects.

RESULTS

Relative to the unaffected comparison group, GAD67 and GAD65 mRNA levels in the schizophrenia group were lower (p = .039, effect size = -0.69 and p = .027, effect size = -0.72, respectively) in the superficial zone but were unaltered in the deep zone. In the major depression group, only GAD67 mRNA levels were lower and only in the superficial zone (p = .089, effect size = 0.70). No differences were detected in the bipolar disorder group. Neither GAD67 nor GAD65 mRNA alterations were explained by psychosis, mood disturbance, or common comorbid factors.

CONCLUSIONS

Alterations in markers of GABA synthesis demonstrated transcript, DLPFC zone, and diagnostic specificity. Given the dependence of WM on GABA neurotransmission in the superficial DLPFC, our findings suggest that limitations to GABA synthesis in this location contribute to WM impairments in schizophrenia, especially during demanding WM tasks, when GABA synthesis requires the activity of both GAD67 and GAD65.

A review of the auditory-gut-brain axis

Hearing loss places a substantial burden on medical resources across the world and impacts quality of life for those affected. Further, it can occur peripherally and/or centrally. With many possible causes of hearing loss, there is scope for investigating the underlying mechanisms involved. Various signaling pathways connecting gut microbes and the brain (the gut-brain axis) have been identified and well established in a variety of diseases and disorders. However, the role of these pathways in providing links to other parts of the body has not been explored in much depth. Therefore, the aim of this review is to explore potential underlying mechanisms that connect the auditory system to the gut-brain axis. Using select keywords in PubMed, and additional hand-searching in google scholar, relevant studies were identified. In this review we summarize the key players in the auditory-gut-brain axis under four subheadings: anatomical, extracellular, immune and dietary. Firstly, we identify important anatomical structures in the auditory-gut-brain axis, particularly highlighting a direct connection provided by the vagus nerve. Leading on from this we discuss several extracellular signaling pathways which might connect the ear, gut and brain. A link is established between inflammatory responses in the ear and gut microbiome-altering interventions, highlighting a contribution of the immune system. Finally, we discuss the contribution of diet to the auditory-gut-brain axis. Based on the reviewed literature, we propose numerous possible key players connecting the auditory system to the gut-brain axis. In the future, a more thorough investigation of these key players in animal models and human research may provide insight and assist in developing effective interventions for treating hearing loss.

Astrocytes regulate inhibitory neurotransmission through GABA uptake, metabolism, and recycling

Synaptic regulation of the primary inhibitory neurotransmitter γ-aminobutyric acid (GABA) is essential for brain function. Cerebral GABA homeostasis is tightly regulated through multiple mechanisms and is directly coupled to the metabolic collaboration between neurons and astrocytes. In this essay, we outline and discuss the fundamental roles of astrocytes in regulating synaptic GABA signaling. A major fraction of synaptic GABA is removed from the synapse by astrocytic uptake. Astrocytes utilize GABA as a metabolic substrate to support glutamine synthesis. The astrocyte-derived glutamine is subsequently transferred to neurons where it serves as the primary precursor of neuronal GABA synthesis. The flow of GABA and glutamine between neurons and astrocytes is collectively termed the GABA-glutamine cycle and is essential to sustain GABA synthesis and inhibitory signaling. In certain brain areas, astrocytes are even capable of synthesizing and releasing GABA to modulate inhibitory transmission. The majority of oxidative GABA metabolism in the brain takes place in astrocytes, which also leads to synthesis of the GABA-related metabolite γ-hydroxybutyric acid (GHB). The physiological roles of endogenous GHB remain unclear, but may be related to regulation of tonic inhibition and synaptic plasticity. Disrupted inhibitory signaling and dysfunctional astrocyte GABA handling are implicated in several diseases including epilepsy and Alzheimer's disease. Synaptic GABA homeostasis is under astrocytic control and astrocyte GABA uptake, metabolism, and recycling may therefore serve as relevant targets to ameliorate pathological inhibitory signaling.

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.

Closed-loop direct control of seizure focus in a rodent model of temporal lobe epilepsy via localized electric fields applied sequentially

Direct electrical stimulation of the seizure focus can achieve the early termination of epileptic oscillations. However, direct intervention of the hippocampus, the most prevalent seizure focus in temporal lobe epilepsy is thought to be not practicable due to its large size and elongated shape. Here, in a rat model, we report a sequential narrow-field stimulation method for terminating seizures, while focusing stimulus energy at the spatially extensive hippocampal structure. The effects and regional specificity of this method were demonstrated via electrophysiological and biological responses. Our proposed modality demonstrates spatiotemporal preciseness and selectiveness for modulating the pathological target region which may have potential for further investigation as a therapeutic approach.

Autoantibody Encephalitis: Presentation, Diagnosis, and Management

Autoantibody encephalitis causes distinct clinical syndromes involving alterations in mentation, abnormal movements, seizures, psychiatric symptoms, sleep disruption, spasms, and neuromyotonia. The diagnoses can be confirmed by specific antibody tests, although some antibodies may be better detected in spinal fluid and others in serum. Each disorder conveys a risk of certain tumors which may inform diagnosis and be important for treatment. Autoantibodies to receptors and other neuronal membrane proteins are generally thought to be pathogenic and result in loss of function of the targets, so understanding the pharmacology of the receptors may inform our understanding of the syndromes. Patients may be profoundly ill but the syndromes usually respond to immune therapy, although there are differences in the types of immune therapy that are thought to be most effective for the various disorders.

Choosing memory retrieval strategies: A critical role for inhibition in the dentate gyrus

Remembering the location of food is essential for survival. Rodents and humans employ mainly hippocampus-dependent spatial strategies, but when being stressed they shift to striatum-mediated stimulus-based strategies. To investigate underlying brain circuits, we tested mice with a heightened stress susceptibility due to a lack of the GABA-synthetizing enzyme GAD65 (GAD65−/− mice) in a dual solution task. Here, GAD65−/− mice preferred to locate a food reward in an open field via a proximal cue, while their wildtype littermates preferred a spatial strategy. The analysis of cFos co-activation across brain regions and of stress-induced mRNA expression changes of GAD65 pointed towards the hippocampal dorsal dentate gyrus (dDG) as a central structure for mediating stress effects on strategy choices via GAD65. Reducing the GAD65 expression locally in the dDG by a shRNA mediated knock down was sufficient to replicate the phenotype of the global GAD65 knock out and to increase dDG excitability. Using DREADD vectors to specifically interfere with dDG circuit activity during dual solution retrieval but not learning confirmed that the dDG modulates strategy choices and that a balanced excitability of this structure is necessary to establish spatial strategy preference. These data highlight the dDG as a critical hub for choosing between spatial and non-spatial foraging strategies.

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Stress reduces spatial memory preferences for locating rewards in an open field.

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GAD65 deficient mice show reduced preferences for spatial memory strategy.

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Dorsal dentate gyrus knock down of GAD65 is sufficient to reduce spatial strategies.

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Excitability in the dorsal dentate gyrus modulates retrieval strategy choices.

Inhibitory co-transmission from midbrain dopamine neurons relies on presynaptic GABA uptake

Dopamine (DA)-releasing neurons in the substantia nigra pars compacta (SNc^DA) inhibit target cells in the striatum through postsynaptic activation of γ-aminobutyric acid (GABA) receptors. However, the molecular mechanisms responsible for GABAergic signaling remain unclear, as SNc^DA neurons lack enzymes typically required to produce GABA or package it into synaptic vesicles. Here, we show that aldehyde dehydrogenase 1a1 (Aldh1a1), an enzyme proposed to function as a GABA synthetic enzyme in SNc^DA neurons, does not produce GABA for synaptic transmission. Instead, we demonstrate that SNc^DA axons obtain GABA exclusively through presynaptic uptake using the membrane GABA transporter Gat1 (encoded by Slc6a1). GABA is then packaged for vesicular release using the vesicular monoamine transporter Vmat2. Our data therefore show that presynaptic transmitter recycling can substitute for de novo GABA synthesis and that Vmat2 contributes to vesicular GABA transport, expanding the range of molecular mechanisms available to neurons to support inhibitory synaptic communication.

Melani and Tritsch demonstrate that inhibitory co-transmission from midbrain dopaminergic neurons does not depend on cell-autonomous GABA synthesis but instead on presynaptic import from the extracellular space through the membrane transporter Gat1 and that GABA loading into synaptic vesicles relies on the vesicular monoamine transporter Vmat2.

Crosstalk Between GABAergic Neurotransmission and Inflammatory Cascades in the Post-ischemic Brain: Relevance for Stroke Recovery

Adaptive plasticity processes are required involving neurons as well as non-neuronal cells to recover lost brain functions after an ischemic stroke. Recent studies show that gamma-Aminobutyric acid (GABA) has profound effects on glial and immune cell functions in addition to its inhibitory actions on neuronal circuits in the post-ischemic brain. Here, we provide an overview of how GABAergic neurotransmission changes during the first weeks after stroke and how GABA affects functions of astroglial and microglial cells as well as peripheral immune cell populations accumulating in the ischemic territory and brain regions remote to the lesion. Moreover, we will summarize recent studies providing data on the immunomodulatory actions of GABA of relevance for stroke recovery. Interestingly, the activation of GABA receptors on immune cells exerts a downregulation of detrimental anti-inflammatory cascades. Conversely, we will discuss studies addressing how specific inflammatory cascades affect GABAergic neurotransmission on the level of GABA receptor composition, GABA synthesis, and release. In particular, the chemokines CXCR4 and CX3CR1 pathways have been demonstrated to modulate receptor composition and synthesis. Together, the actual view on the interactions between GABAergic neurotransmission and inflammatory cascades points towards a specific crosstalk in the post-ischemic brain. Similar to what has been shown in experimental models, specific therapeutic modulation of GABAergic neurotransmission and inflammatory pathways may synergistically promote neuronal plasticity to enhance stroke recovery.

A Breakdown of Immune Tolerance in the Cerebellum

Cerebellar dysfunction can be associated with ataxia, dysarthria, dysmetria, nystagmus and cognitive deficits. While cerebellar dysfunction can be caused by vascular, traumatic, metabolic, genetic, inflammatory, infectious, and neoplastic events, the cerebellum is also a frequent target of autoimmune attacks. The underlying cause for this vulnerability is unclear, but it may be a result of region-specific differences in blood–brain barrier permeability, the high concentration of neurons in the cerebellum and the presence of autoantigens on Purkinje cells. An autoimmune response targeting the cerebellum—or any structure in the CNS—is typically accompanied by an influx of peripheral immune cells to the brain. Under healthy conditions, the brain is protected from the periphery by the blood–brain barrier, blood–CSF barrier, and blood–leptomeningeal barrier. Entry of immune cells to the brain for immune surveillance occurs only at the blood-CSF barrier and is strictly controlled. A breakdown in the barrier permeability allows peripheral immune cells uncontrolled access to the CNS. Often—particularly in infectious diseases—the autoimmune response develops because of molecular mimicry between the trigger and a host protein. In this review, we discuss the immune surveillance of the CNS in health and disease and also discuss specific examples of autoimmunity affecting the cerebellum.

Postpartum State, but Not Maternal Caregiving or Level of Anxiety, Increases Medial Prefrontal Cortex GAD65 and vGAT in Female Rats

Upregulation of the inhibitory neurotransmitter, GABA, is involved in many of the behavioral differences between postpartum and nulliparous female rodents. This is evidenced by studies showing that pharmacological blockade of GABAergic activity impairs maternal caregiving and postpartum affective behaviors. However, the influence of motherhood on the capacity for GABA synthesis or release in the medial prefrontal cortex (mPFC; brain region involved in many social and affective behaviors) is not well-understood. Western blotting was used to compare postpartum and nulliparous rats in protein levels of the 65-kD isoform of glutamic acid decarboxylase (GAD65; synthesizes most GABA released from terminals) and vesicular GABA transporter (vGAT; accumulates GABA into synaptic vesicles for release) in the mPFC. We found that postpartum mothers had higher GAD65 and vGAT compared to virgins, but such differences were not found between maternally sensitized and non-sensitized virgins, indicating that reproduction rather than just the display of maternal caregiving is required. To test whether GAD65 and vGAT levels in the mPFC were more specifically related to anxiety-related behavior within postpartum mothers, we selected 8 low-anxiety and 8 high-anxiety dams based on their time spent in the open arms of an elevated plus maze on postpartum day 7. There were no significant differences between the anxiety groups in either GAD65 or vGAT levels. These data further indicate that frontal cortical GABA is affected by female reproduction and more likely contributes to differences in the display of socioemotional behaviors across, but not within, female reproductive state.

Heterogeneous GAD65 Expression in Subtypes of GABAergic Neurons Across Layers of the Cerebral Cortex and Hippocampus

Gamma-aminobutyric acid (GABA), a major inhibitory transmitter in the central nervous system, is synthesized via either of two enzyme isoforms, GAD65 or GAD67. GAD65 is synthesized in the soma but functions at synaptic terminals in an activity-dependent manner, playing a distinct role in excitatory-inhibitory balance. However, the extent to which each GABAergic subtype expresses GAD65 in the resting state remains unclear. In this study, we compared GAD65 expression among six GABAergic subtypes: NPY⁺, nNOS⁺, PV⁺, SOM⁺, CR⁺, and CCK⁺. According to the results, the GABAergic subtypes were classified into two groups per region based on GAD65 expression levels: high-expression (NPY⁺ and nNOS⁺) and low-expression groups (PV⁺, SOM⁺, CR⁺, and CCK⁺) in the cerebral cortex and high-expression (NPY⁺, nNOS⁺, and CCK⁺) and low-expression groups (PV⁺, SOM⁺, and CR⁺) in the hippocampus. Moreover, these expression patterns revealed a distinct laminar distribution in the cerebral cortex and hippocampus. To investigate the extent of GAD65 transport from the soma to synaptic terminals, we examined GAD65 expression in colchicine-treated rats in which GAD65 was synthesized in the soma but not transported to terminals. We found a significant positive correlation in GAD65 expression across subtypes between colchicine-treated and control rats. In summary, each GABAergic subtype exhibits a distinct GAD65 expression pattern across layers of the cerebral cortex and hippocampus. In addition, the level of GAD65 expression in the soma can be used as a proxy for the amount of GAD65 in the cytoplasm. These findings suggest that exploration of the distinct profiles of GAD65 expression among GABAergic subtypes could clarify the roles that GABAergic subtypes play in maintaining the excitatory-inhibitory balance.

GAD67-mediated GABA Synthesis and Signaling Impinges on Directing Basket Cell Axonal Projections Toward Purkinje Cells in the Cerebellum

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the central nervous system, synthesized by two isoforms of glutamate decarboxylase (GAD): GAD65 and GAD67. GABA may act as a trophic factor during brain development, but its contribution to the development and maturation of cerebellar neural circuits is not known. To understand the roles of GABA in cerebellar organization and associated functions in motor coordination and balance, we examined GAD65 conventional knock out (KO) mice and mice in which GAD67 was eliminated in parvalbumin-expressing neurons (PV-Cre; GAD67^flox/flox mice). We found aberrant subcellular localization of the Shaker-type K channel Kv1.1 in basket cell collaterals of PV-Cre; GAD67 ^flox/flox mice and abnormal projections from basket cells to Purkinje cells in both mouse strains. We also found that altered synaptic properties of basket cell terminals to Purkinje cells in PV-Cre; GAD67^flox/flox mice. Furthermore, PV-Cre; GAD67 ^flox/flox mice exhibited abnormal motor coordination in the rotarod test. These results indicate that GABA signaling in the cerebellum is critical for establishing appropriate connections between basket cells and Purkinje cells and is associated with motor coordination in mice.

Impact of Dysfunctional Feed-Forward Inhibition on Glutamate Decarboxylase Isoforms and γ-Aminobutyric Acid Transporters

Absence seizures are associated with generalised synchronous 2.5–4 Hz spike-wave discharges causing brief and sudden alteration of awareness during childhood, which is known as childhood absence epilepsy (CAE). CAE is also associated with impaired learning, psychosocial challenges, and physical danger. Absence seizures arise from disturbances within the cortico-thalamocortical (CTC) network, including dysfunctional feed-forward inhibition (FFI); however, the precise mechanisms remain unclear. In epileptic stargazers, a genetic mouse model of CAE with chronic seizures, levels of γ-aminobutyric acid (GABA), and expression of GABA receptors are altered within the CTC network, implicating altered GABAergic transmission in absence seizures. However, the expression of GABA synthesising enzymes (GAD65 and GAD67) and GABA transporters (GAT-1 and 3) have not yet been characterised within absence seizure models. We found a specific upregulation of GAD65 in the somatosensory cortex but not the thalamus of epileptic stargazer mice. No differences were detected in GAD67 and GAT-3 levels in the thalamus or somatosensory cortex. Then, we assessed if GAD65 upregulation also occurred in Gi-DREADD mice exhibiting acute absence seizures, but we found no change in the expression profiles of GAD65/67 or GAT-3. Thus, the upregulation of GAD65 in stargazers may be a compensatory mechanism in response to long-term dysfunctional FFI and chronic absence seizures.

Altered Parvalbumin Basket Cell Terminals in the Cortical Visuospatial Working Memory Network in Schizophrenia

BACKGROUND

Visuospatial working memory (vsWM), which is commonly impaired in schizophrenia, involves information processing across the primary visual cortex, association visual cortex, posterior parietal cortex, and dorsolateral prefrontal cortex (DLPFC). Within these regions, vsWM requires inhibition from parvalbumin-expressing basket cells (PVBCs). Here, we analyzed indices of PVBC axon terminals across regions of the vsWM network in schizophrenia.

METHODS

For 20 matched pairs of subjects with schizophrenia and unaffected comparison subjects, tissue sections from the primary visual cortex, association visual cortex, posterior parietal cortex, and DLPFC were immunolabeled for PV, the 65- and 67-kDa isoforms of glutamic acid decarboxylase (GAD65 and GAD67) that synthesize GABA (gamma-aminobutyric acid), and the vesicular GABA transporter. The density of PVBC terminals and of protein levels per terminal was quantified in layer 3 of each cortical region using fluorescence confocal microscopy.

RESULTS

In comparison subjects, all measures, except for GAD65 levels, exhibited a caudal-to-rostral decline across the vsWM network. In subjects with schizophrenia, the density of detectable PVBC terminals was significantly lower in all regions except the DLPFC, whereas PVBC terminal levels of PV, GAD67, and GAD65 proteins were lower in all regions. A composite measure of inhibitory strength was lower in subjects with schizophrenia, although the magnitude of the diagnosis effect was greater in the primary visual, association visual, and posterior parietal cortices than in the DLPFC.

CONCLUSIONS

In schizophrenia, alterations in PVBC terminals across the vsWM network suggest the presence of a shared substrate for cortical dysfunction during vsWM tasks. However, regional differences in the magnitude of the disease effect on an index of PVBC inhibitory strength suggest region-specific alterations in information processing during vsWM tasks.

Molecular Basis of GABA Hypofunction in Adolescent Schizophrenia-Like Animals

Schizophrenia is a neurodevelopmental disorder that NMDA receptor (NMDAR) hypofunction appears centrally involved. Schizophrenia typically emerges in adolescence or early adulthood. Electrophysiological and several neurochemical changes have linked the GABA deficits to abnormal behaviors induced by NMDAR hypofunction. However, few studies have systematically investigated the molecular basis of GABA deficits, especially during adolescence. To address this issue, we transiently administrated MK-801 to mice on PND 10, which exhibited schizophrenia-relevant deficits in adolescence. Slice recording showed reduced GABA transmission and PVI+ hypofunction, indicating GABAergic hypofunction. Cortical proteomic evaluation combined with analysis of single cell data from the Allen Brain showed that various metabolic processes were enriched in top ranks and differentially altered in excitatory neurons, GABAergic interneurons, and glial cells. Notably, the GABA-related amino acid metabolic process was disturbed in both astrocytes and interneurons, in which we found a downregulated set of GABA-related proteins (GAD65, SYNPR, DBI, GAT3, SN1, and CPT1A). They synergistically regulate GABA synthesis, release, reuptake, and replenishment. Their downregulation indicates impaired GABA cycle and homeostasis regulated by interneuron-astrocyte communication in adolescence. Our findings on molecular basis of GABA deficits could provide potential drug targets of GABAergic rescue for early prevention and intervention.

Quantitative brain imaging analysis of neurological syndromes associated with anti-GAD antibodies

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The pattern of cortical atrophy is present throughout all anti-GAD phenotypes.

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The radiomic features correctly classify anti-GAD patients versus healthy subjects.

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The different neurological anti-GAD phenotypes should be considered as a continuum.

Neurological disorders associated with anti-glutamic acid decarboxylase (GAD) autoimmunity are rare and include a variety of neurological syndromes: stiff-person syndrome, cerebellar ataxia or limbic encephalitis. The diagnosis remains challenging due to the variety of symptoms and normal brain imaging.

The morphological MRI of 26 patients (T1-weighted and Fluid-attenuated inversion recovery (FLAIR)-weighted images) was analyzed at the initial stage of diagnosis, matched by age and sex to 26 healthy subjects. We performed a vertex-wise analysis using a generalized linear model, adjusting by age, to compare the brain cortical thickness of both populations. In addition, we used a voxel-based morphometry of cerebellum thickness obtained by CEREbellum Segmentation (CERES), as well as the hippocampus volumetry comparison using HIPpocampus subfield Segmentation (HIPS). Finally, we extracted 62 radiomics features using LifeX to assess the classification performance using a random forest model to identify an anti-GAD related MRI.

The results suggest a peculiar profile of atrophy in patients with anti-GAD, with a significant atrophy in the temporal and frontal lobes (adjusted p-value < 0.05), and a focal cerebellar atrophy of the V-lobule, independently of the anti-GAD phenotype. Finally, the MRIs from anti-GAD patients were correctly classified when compared to the control group, with an area under the curve (AUC) of 0.98.

This study suggests a particular pattern of cortical atrophy throughout all anti-GAD phenotypes. These results reinforce the notion that the different neurological anti-GAD phenotypes should be considered as a continuum due to their similar cortical thickness profiles.

Spinal GABAergic neurons are under feed-forward inhibitory control driven by A and C fibers in Gad2 td-Tomato mice

BACKGROUND

Spinal GABAergic neurons act as a critical modulator in sensory transmission like pain or itch. The monosynaptic or polysynaptic primary afferent inputs onto GABAergic neurons, along with other interneurons or projection neurons make up the direct and feed-forward inhibitory neural circuits. Previous research indicates that spinal GABAergic neurons mainly receive excitatory inputs from Aδ and C fibers. However, whether they are controlled by other inhibitory sending signals is not well understood.

METHODS

We applied a transgenic mouse line in which neurons co-expressed the GABA-synthesizing enzyme Gad65 and the enhanced red fluorescence (td-Tomato) to characterize the features of morphology and electrophysiology of GABAergic neurons. Patch-clamp whole cell recordings were used to record the evoked postsynaptic potentials of fluorescent neurons in spinal slices in response to dorsal root stimulation.

RESULTS

We demonstrated that GABAergic neurons not only received excitatory drive from peripheral Aβ, Aδ and C fibers, but also received inhibitory inputs driven by Aδ and C fibers. The evoked inhibitory postsynaptic potentials (eIPSPs) mediated by C fibers were mainly Glycinergic (66.7%) as well as GABAergic mixed with Glycinergic (33.3%), whereas the inhibition mediated by Aδ fibers was predominately both GABA and Glycine-dominant (57.1%), and the rest of which was purely Glycine-dominant (42.9%).

CONCLUSION

These results indicated that spinal GABAergic inhibitory neurons are under feedforward inhibitory control driven by primary C and Aδ fibers, suggesting that this feed-forward inhibitory pathway may play an important role in balancing the excitability of GABAergic neurons in spinal dorsal horn.

Impaired Expression of GABA Signaling Components in the Alzheimer's Disease Middle Temporal Gyrus

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter, playing a central role in the regulation of cortical excitability and the maintenance of the excitatory/inhibitory (E/I) balance. Several lines of evidence point to a remodeling of the cerebral GABAergic system in Alzheimer’s disease (AD), with past studies demonstrating alterations in GABA receptor and transporter expression, GABA synthesizing enzyme activity and focal GABA concentrations in post-mortem tissue. AD is a chronic neurodegenerative disorder with a poorly understood etiology and the temporal cortex is one of the earliest regions in the brain to be affected by AD neurodegeneration. Utilizing NanoString nCounter analysis, we demonstrate here the transcriptional downregulation of several GABA signaling components in the post-mortem human middle temporal gyrus (MTG) in AD, including the GABA_A receptor α₁, α₂, α₃, α₅, β₁, β₂, β₃, δ, γ₂, γ₃, and θ subunits and the GABA_B receptor 2 (GABA_BR2) subunit. In addition to this, we note the transcriptional upregulation of the betaine-GABA transporter (BGT1) and GABA transporter 2 (GAT2), and the downregulation of the 67 kDa isoform of glutamate decarboxylase (GAD₆₇), the primary GABA synthesizing enzyme. The functional consequences of these changes require further investigation, but such alterations may underlie disruptions to the E/I balance that are believed to contribute to cognitive decline in AD.

Neurological Syndromes Associated with Anti-GAD Antibodies

Glutamic acid decarboxylase (GAD) is an intracellular enzyme whose physiologic function is the decarboxylation of glutamate to gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter within the central nervous system. GAD antibodies (Ab) have been associated with multiple neurological syndromes, including stiff-person syndrome, cerebellar ataxia, and limbic encephalitis, which are all considered to result from reduced GABAergic transmission. The pathogenic role of GAD Ab is still debated, and some evidence suggests that GAD autoimmunity might primarily be cell-mediated. Diagnosis relies on the detection of high titers of GAD Ab in serum and/or in the detection of GAD Ab in the cerebrospinal fluid. Due to the relative rarity of these syndromes, treatment schemes and predictors of response are poorly defined, highlighting the unmet need for multicentric prospective trials in this population. Here, we reviewed the main clinical characteristics of neurological syndromes associated with GAD Ab, focusing on pathophysiologic mechanisms.

Inhibitory regulation of the prefrontal cortex following behavioral sensitization to amphetamine and/or methamphetamine psychostimulants: A review of GABAergic mechanisms

Behavioral sensitization to repeated psychostimulant administration has been proposed to reflect many of the neurochemical and behavioral changes that are characteristic of a range of disorders, including drug addiction and psychoses. While previous studies have examined the role of dopamine and glutamate neurotransmission in mediating sensitization, particularly within the prefrontal cortex (PFC), the role of inhibitory GABAergic processing of the PFC in the expression of sensitization is not well understood. Recent research, however, has proposed an emerging role of GABA synthesis, reuptake, ionotropic and metabotropic receptor regulation, and interneuronal changes following sensitization to methamphetamine and/or amphetamine within the PFC. The aim of this review, therefore, is to synthesize research findings on changes to the GABAergic network following sensitization induced by amphetamines (i.e., amphetamine and/or methamphetamine) in the PFC. In addition to providing an overview of global PFC changes, we also provide evidence of regional specific inhibitory influences on sensitized circuitry, focusing on the prelimbic and orbitofrontal cortices. We propose a neural circuit through which inhibitory PFC GABA changes mediate sensitized disease states, focusing on the interaction between the prelimbic and orbitofrontal cortices with subcortical brain structures and the mesolimbic system. Methodological considerations and avenues for future research are also discussed.

Vitamin D deficiency induces the excitation/inhibition brain imbalance and the proinflammatory shift

Vitamin D₃ is among the major neurosteroids whose role in developing and adult brain is intensively studied now. Its active form 1,25(OH)₂D₃ regulates the expression and functioning of a range of brain-specific proteins, which orchestrate the neurotransmitter turnover, neurogenesis and neuroplasticity. Despite numerous studies of the vitamin D role in normal and pathological brain function, there is little evidence on the mechanisms of alterations in excitatory and inhibitory neurotransmission under vitamin D deficiency (VDD). Using the animal model we characterized the dysfunction of excitatory and inhibitory neurotransmission under alimentary VDD. The shift between unstimulated and evoked GABA release under VDD was largely reversed after treatment of VDD, whereas the impairments in glutamatergic system were only partially recovered after 1-month vitamin D₃ supplementation. The increase of the external glutamate level and unstimulated GABA release in brain nerve terminals was associated with intensified ROS production and higher [Ca²⁺]_i in presynapse. The negative allosteric modulation of presynaptic mGlu7 receptors significantly enhanced exocytotic GABA release, which was decreased under VDD, thereby suggesting the neuroprotective effect of such modulation of inhibitory neurotransmission. Synaptic plasma membranes and cytosolic proteins contribute to the decreased stimulated release of neurotransmitter, by being the crucial components, whose functional state is impaired under VDD. The critical changes with synaptic vesicles occurred at the docking step of the process, whereas malfunctioning of synaptic cytosolic proteins impacted the fusion event foremost. The decreased amplitude of exocytosis was inherent for non-excitable cells as well, as evidenced by lower platelet degranulation. Our data suggest the presynaptic dysfunction and proinflammatory shift as the early events in the pathogenesis of VDD-associated disorders and provide evidences for the neuroprotective role of vitamin D₃.

The regulation of glutamic acid decarboxylases in GABA neurotransmission in the brain

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter that is required for the control of synaptic excitation/inhibition and neural oscillation. GABA is synthesized by glutamic acid decarboxylases (GADs) that are widely distributed and localized to axon terminals of inhibitory neurons as well as to the soma and, to a lesser extent, dendrites. The expression and activity of GADs is highly correlated with GABA levels and subsequent GABAergic neurotransmission at the inhibitory synapse. Dysregulation of GADs has been implicated in various neurological disorders including epilepsy and schizophrenia. Two isoforms of GADs, GAD67 and GAD65, are expressed from separate genes and have different regulatory processes and molecular properties. This review focuses on the recent advances in understanding the structure of GAD, its transcriptional regulation and post-transcriptional modifications in the central nervous system. This may provide insights into the pathological mechanisms underlying neurological diseases that are associated with GAD dysfunction.

Pb exposure induces an imbalance of excitatory and inhibitory synaptic transmission in cultured rat hippocampal neurons

An appropriate balance of excitatory and inhibitory synapse maintains the network stability of the central nervous system. Our recent work showed lead (Pb) exposure can inhibit synaptic transmission in cultured hippocampal neurons. However, it is not clear whether Pb exposure disrupt the balance of excitatory and inhibitory synaptic transmission. Here, primary cultured hippocampal neurons from Sprague-Dawley (SD) rats were exposed to Pb (0.2 μM, 1 μM, 5 μM, respectively) from Days in Vitro (DIV) 7 to DIV 12 for 5 days and the excitatory and inhibitory synaptic transmission was examined. Patch clamp recording results showed that distinct from exposures of 0.2 μM and 5 μM, 1 μM Pb exposure significantly increased the mIPSC frequency and decreased the mEPSC frequency, leading to a uniform inhibitory outcome. Further, the number of inhibitory presynaptic puncta was significantly increased after 1 μM Pb exposure, while the number of excitatory presynaptic terminals was decreased. In addition 1 μM Pb increased the glutamic acid decarboxylase (GAD65) expression and the surface GABA_A receptor (GABA_AR) clusters. This shift might potentiate the synthesis of GABA and enhance the surface distribution of postsynaptic GABA_AR clusters in hippocampus neurons. Together, these data showed that Pb exposure disrupted the balance of excitatory and inhibitory synaptic transmission via abnormal GABAergic neurotransmission.

Mechanism and effects of pulsatile GABA secretion from cytosolic pools in the human beta cell

Pancreatic beta cells synthesize and secrete the neurotransmitter γ-aminobutyric acid (GABA) as a paracrine and autocrine signal to help regulate hormone secretion and islet homeostasis. Islet GABA release has classically been described as a secretory vesicle-mediated event. Yet, a limitation of the hypothesized vesicular GABA release from islets is the lack of expression of a vesicular GABA transporter in beta cells. Consequentially, GABA accumulates in the cytosol. Here we provide evidence that the human beta cell effluxes GABA from a cytosolic pool in a pulsatile manner, imposing a synchronizing rhythm on pulsatile insulin secretion. The volume regulatory anion channel (VRAC), functionally encoded by LRRC8A or Swell1, is critical for pulsatile GABA secretion. GABA content in beta cells is depleted and secretion is disrupted in islets from type 1 and type 2 diabetic patients, suggesting that loss of GABA as a synchronizing signal for hormone output may correlate with diabetes pathogenesis.

Tianeptine antagonizes the reduction of PV+ and GAD67 cells number in dorsal hippocampus of socially isolated rats

Adult male rats exposed to chronic social isolation (CSIS) show depressive- and anxiety-like behaviors and reduce the numbers of parvalbumin-positive (PV+) interneurons in the dorsal hippocampus. We aimed to determine whether tianeptine (Tian), administered during the last three weeks of a six-week-social isolation (10 mg/kg/day), may reverse CSIS-induced behavioral changes and antagonize the CSIS-induced reduction in the number of PV+ interneurons. We also studied whether Tian affects the GABA-producing enzyme GAD67+ cells, in Stratum Oriens (SO), Stratum Pyramidale (SP), Stratum Radiatum (SR) and Stratum Lacunosum Moleculare (LM) of CA1-3, as well as in molecular layer-granule cell layer (ML-GCL) and Hilus (H) of the dentate gyrus (DG). CSIS-induced reduction in the number of PV+ cells was layer/subregion-specific with the greatest decrease in SO of CA2. Reduction in the number of PV+ cells was significantly higher than GAD67+ cells, indicating that PV+ cells are the main target following CSIS. Tian reversed CSIS-induced behavior phenotype and antagonized the reduction in the number of PV+ and GAD67+ cells in all subregions. In controls, Tian led to an increase in the number of PV+ and GAD67+ cells in SP of all subregions and PV+ interneurons in ML-GCL of DG, while treatment during CSIS, compared to CSIS alone, resulted with an increase of PV+ interneurons in SO and SP CA1, SP CA2/CA3 and ML-GCL DG with simultaneous increase in GAD67+ cells in all CA1, LM CA2, SO/SR/LM CA3. Data show that Tian offers protection from CSIS via modulation of the dorsal hippocampal GABAergic system.

Tropomyosin Receptor Kinase B Receptor Activation in the Locus Coeruleus Restores Impairment of Endogenous Analgesia at a Late Stage Following Nerve Injury in Rats

A rat model of neuropathic pain at 6 weeks after spinal nerve ligation (SNL6w) exhibits both mechanical hypersensitivity and impaired noxious stimuli-induced analgesia (NSIA). Repeated treatment with antidepressants can produce antihypersensitivity and restore NSIA. To examine the involvement of a brain-derived neurotrophic factor-mediated mechanism, a tropomyosin receptor kinase B (TrkB) agonist, 7,8-dihydroxyflavone (DHF), was administered to SNL6w rats (5 mg/kg/d for 5 days). Mechanical hypersensitivity was evaluated using the von Frey filament test and paw pressure test. NSIA was examined by measuring the change in the hind paw withdrawal threshold 30 minutes after painful stimulation induced by capsaicin injection into the fore paw. Changes in the concentrations of glutamate and GABA in the locus coeruleus area were measured by in vivo microdialysis. DHF treatment did not affect mechanical hypersensitivity, although it restored NSIA by reducing GABA release in response to the fore paw capsaicin injection. DHF treatment did not alter the baseline concentration of glutamate or GABA. These findings suggest that DHF treatment restored the stimuli-response activity of the locus coeruleus without affecting the tonic activity of the locus coeruleus. The brain-derived neurotrophic factor-TkB signaling is also involved in the NSIA-restoring effect of amitriptyline. PERSPECTIVE: This article demonstrates that repeated treatment with TrkB agonist, DHF, restored endogenous analgesia. Repeated amitriptyline treatment showed similar effect via TrkB-mediated mechanisms, and the effect may be independent from the effect of antihypersensitivity. This effect of TrkB activation is promising for patients with chronic pain with impaired descending inhibition.

Role of Endogenous Metabolite Alterations in Neuropsychiatric Disease

The potential role in neuropsychiatric disease risk arising from alterations and derangements of endogenous small-molecule metabolites remains understudied. Alterations of endogenous metabolite concentrations can arise in multiple ways. Marked derangements of single endogenous small-molecule metabolites are found in a large group of rare genetic human diseases termed "inborn errors of metabolism", many of which are associated with prominent neuropsychiatric symptomology. Whether such metabolites act neuroactively to directly lead to distinct neural dysfunction has been frequently hypothesized but rarely demonstrated unequivocally. Here we discuss this disease concept in the context of our recent findings demonstrating that neural dysfunction arising from accumulation of the schizophrenia-associated metabolite l-proline is due to its structural mimicry of the neurotransmitter GABA that leads to alterations in GABA-ergic short-term synaptic plasticity. For cases in which a similar direct action upon neurotransmitter binding sites is suspected, we lay out a systematic approach that can be extended to assessing the potential disruptive action of such candidate disease metabolites. To address the potentially important and broader role in neuropsychiatric disease, we also consider whether the more subtle yet more ubiquitous variations in endogenous metabolites arising from natural allelic variation may likewise contribute to disease risk but in a more complex and nuanced manner.

Stereological Estimates of Glutamatergic, GABAergic, and Cholinergic Neurons in the Pedunculopontine and Laterodorsal Tegmental Nuclei in the Rat

The pedunculopontine tegmental nucleus (PPN) and laterodorsal tegmental nucleus (LDT) are functionally associated brainstem structures implicated in behavioral state control and sensorimotor integration. The PPN is also involved in gait and posture, while the LDT plays a role in reward. Both nuclei comprise characteristic cholinergic neurons intermingled with glutamatergic and GABAergic cells whose absolute numbers in the rat have been only partly established. Here we sought to determine the complete phenotypical profile of each nucleus to investigate potential differences between them. Counts were obtained using stereological methods after the simultaneous visualization of cholinergic and either glutamatergic or GABAergic cells. The two isoforms of glutamic acid decarboxylase (GAD), GAD65 and GAD67, were separately analyzed. Dual in situ hybridization revealed coexpression of GAD65 and GAD67 mRNAs in ∼90% of GAD-positive cells in both nuclei; thus, the estimated mean numbers of (1) cholinergic, (2) glutamatergic, and (3) GABAergic cells in PPN and LDT, respectively, were (1) 3,360 and 3,650; (2) 5,910 and 5,190; and (3) 4,439 and 7,599. These data reveal significant differences between PPN and LDT in their relative phenotypical composition, which may underlie some of the functional differences observed between them. The estimation of glutamatergic cells was significantly higher in the caudal PPN, supporting the reported functional rostrocaudal segregation in this nucleus. Finally, a small subset of cholinergic neurons (8% in PPN and 5% in LDT) also expressed the glutamatergic marker Vglut2, providing anatomical evidence for a potential corelease of transmitters at specific target areas.

Peripheral GABAA receptor-mediated signaling facilitates persistent inflammatory hypersensitivity

Unlike in the central nervous system (CNS), in the adult peripheral nervous system (PNS), activation of GABA_A receptors (GABA_AR) is excitatory because of the relatively high concentration of intracellular chloride in these neurons. Indeed, exogenous GABA and muscimol, a GABA_AR agonist, exacerbate acute inflammatory hypersensitivity in rodents. However, it remains unclear whether peripheral GABA_AR and the endogenous GABA play an important role in persistent inflammatory hypersensitivity. In this study, we thus investigated how peripheral GABA_AR affects pain hypersensitivity by using the complete Freund's adjuvant (CFA)-induced persistent inflammatory pain mouse model. We found that intraplantar (i.pl.) administration of GABA_AR antagonists, picrotoxin, and 1(S),9(R)-(-)-bicuculline methiodide significantly inhibited both spontaneous nociceptive (paw licking and flinching) behavior and mechanical hypersensitivity in CFA-injected mice at day 3 (D3), but not in naïve mice. Interestingly, CFA-induced mechanical hypersensitivity was significantly reversed by anti-GABA antibody (anti-GABA, i.pl.). In addition, RT-qPCR revealed that glutamate decarboxylase Gad1 (GAD 67) and Gad2 (GAD 65) mRNA expression was also upregulated in the ipsilateral hind paw of CFA-injected mice at D3. Finally, 5α-pregnan-3α-ol-20-one (3α,5α-THP), a selective positive allosteric modulator of GABA_AR, produced mechanical hypersensitivity in naïve mice in a dose-dependent manner. Taken together, our results indicate that peripheral GABA_AR and endogenous GABA, possibly produced by the inflamed tissue, potentiate CFA-induced persistent inflammatory hypersensitivity, suggesting that they can be used as a therapeutic target for alleviating inflammatory pain.

Stereological Assessments of Neuronal Pathology in Auditory Cortex in Schizophrenia

It has long been known that auditory processing is disrupted in schizophrenia. More recently, postmortem studies have provided direct evidence that morphological alterations to neurons in auditory cortex are implicated in the pathophysiology of this illness, confirming previous predictions. Potential neural substrates for auditory impairment and gray matter loss in auditory cortex in schizophrenia have been identified, described, and are the focus of this review article. Pyramidal cell somal volume is reduced in auditory cortex, as are dendritic spine density and number in schizophrenia. Pyramidal cells are not lost in this region in schizophrenia, indicating that dendritic spine reductions reflect fewer spines per pyramidal cell, consistent with the reduced neuropil hypothesis of schizophrenia. Stereological methods have aided in the proper collection, reporting and interpretation of this data. Mechanistic studies exploring relationships between genetic risk for schizophrenia and altered dendrite morphology represent an important avenue for future research in order to further elucidate cellular pathology in auditory cortex in schizophrenia.

Cytosolic Accumulation of L-Proline Disrupts GABA-Ergic Transmission through GAD Blockade

Proline dehydrogenase (PRODH), which degrades L-proline, resides within the schizophrenia-linked 22q11.2 deletion suggesting a role in disease. Supporting this, elevated L-proline levels have been shown to increase risk for psychotic disorders. Despite the strength of data linking PRODH and L-proline to neuropsychiatric diseases, targets of disease-relevant concentrations of L-proline have not been convincingly described. Here, we show that Prodh-deficient mice with elevated CNS L-proline display specific deficits in high-frequency GABA-ergic transmission and gamma-band oscillations. We find that L-proline is a GABA-mimetic and can act at multiple GABA-ergic targets. However, at disease-relevant concentrations, GABA-mimesis is limited to competitive blockade of glutamate decarboxylase leading to reduced GABA production. Significantly, deficits in GABA-ergic transmission are reversed by enhancing net GABA production with the clinically relevant compound vigabatrin. These findings indicate that accumulation of a neuroactive metabolite can lead to molecular and synaptic dysfunction and help to understand mechanisms underlying neuropsychiatric disease.

Depolarizing, inhibitory GABA type A receptor activity regulates GABAergic synapse plasticity via ERK and BDNF signaling

γ-aminobutyric acid (GABA) begins as the key excitatory neurotransmitter in newly forming circuits, with chloride efflux from GABA type A receptors (GABA_ARs) producing membrane depolarization, which promotes calcium entry, dendritic outgrowth and synaptogenesis. As development proceeds, GABAergic signaling switches to inhibitory hyperpolarizing neurotransmission. Despite the evidence of impaired GABAergic neurotransmission in neurodevelopmental disorders, little is understood on how agonist-dependent GABA_AR activation controls the formation and plasticity of GABAergic synapses. We have identified a weakly depolarizing and inhibitory GABA_AR response in cortical neurons that occurs during the transition period from GABA_AR depolarizing excitation to hyperpolarizing inhibitory activity. We show here that treatment with the GABA_AR agonist muscimol mediates structural changes that diminish GABAergic synapse strength through postsynaptic and presynaptic plasticity via intracellular Ca²⁺ stores, ERK and BDNF/TrkB signaling. Muscimol decreases synaptic localization of surface γ2 GABA_ARs and gephyrin postsynaptic scaffold while β2/3 non-γ2 GABA_ARs accumulate in the synapse. Concurrent with this structural plasticity, muscimol treatment decreases synaptic currents while enhancing the γ2 containing benzodiazepine sensitive GABA_AR tonic current in an ERK dependent manner. We further demonstrate that GABA_AR activation leads to a decrease in presynaptic GAD65 levels via BDNF/TrkB signaling. Together these data reveal a novel mechanism for agonist induced GABAergic synapse plasticity that can occur on the timescale of minutes, contributing to rapid modification of synaptic and circuit function.

mir-500-Mediated GAD67 Downregulation Contributes to Neuropathic Pain

Neuropathic pain is a common neurobiological disease involving multifaceted maladaptations ranging from gene modulation to synaptic dysfunction, but the interactions between synaptic dysfunction and the genes that are involved in persistent pain remain elusive. In the present study, we found that neuropathic pain induced by the chemotherapeutic drug paclitaxel or L5 ventral root transection significantly impaired the function of GABAergic synapses of spinal dorsal horn neurons via the reduction of the GAD67 expression. We also found that mir-500 expression was significantly increased and involved in the modulation of GAD67 expression via targeting the specific site of Gad1 gene in the dorsal horn. In addition, knock-out of mir-500 or using mir-500 antagomir rescued the GABAergic synapses in the spinal dorsal horn neurons and attenuated the sensitized pain behavior in the rats with neuropathic pain. To our knowledge, this is the first study to investigate the function significance and the underlying molecular mechanisms of mir-500 in the process of neuropathic pain, which sheds light on the development of novel therapeutic options for neuropathic pain.

SIGNIFICANCE STATEMENT

Neuropathic pain is a common neurobiological disease involving multifaceted maladaptations ranging from gene modulation to synaptic dysfunction, but the underlying molecular mechanisms remain elusive. The present study illustrates for the first time a mir-500-mediated mechanism underlying spinal GABAergic dysfunction and sensitized pain behavior in neuropathic pain induced by the chemotherapeutic drug paclitaxel or L5 ventral root transection, which sheds light on the development of novel therapeutic options for neuropathic pain.

Neuroimaging studies of GABA in schizophrenia: a systematic review with meta-analysis

Data from animal models and from postmortem studies suggest that schizophrenia is associated with brain GABAergic dysfunction. The extent to which this is reflected in data from in vivo studies of GABA function in schizophrenia is unclear. The Medline database was searched to identify articles published until 21 October 2016. The search terms included GABA, proton magnetic resonance spectroscopy (¹H-MRS), positron emission tomography (PET), single photon emission computed tomography (SPECT), schizophrenia and psychosis. Sixteen GABA ¹H-MRS studies (538 controls, 526 patients) and seven PET/SPECT studies of GABA_A/benzodiazepine receptor (GABA_A/BZR) availability (118 controls, 113 patients) were identified. Meta-analyses of ¹H-MRS GABA in the medial prefrontal cortex (mPFC), parietal/occipital cortex (POC) and striatum did not show significant group differences (mFC: g=-0.3, 409 patients, 495 controls, 95% confidence interval (CI): -0.6 to 0.1; POC: g=-0.3, 139 patients, 111 controls, 95% CI: -0.9 to 0.3; striatum: g=-0.004, 123 patients, 95 controls, 95% CI: -0.7 to 0.7). Heterogeneity across studies was high (I²>50%), and this was not explained by subsequent moderator or meta-regression analyses. There were insufficient PET/SPECT receptor availability studies for meta-analyses, but a systematic review did not suggest replicable group differences in regional GABA_A/BZR availability. The current literature does not reveal consistent alterations in in vivo GABA neuroimaging measures in schizophrenia, as might be hypothesized from animal models and postmortem data. The analysis highlights the need for further GABA neuroimaging studies with improved methodology and addressing potential sources of heterogeneity.

Functional Differentiation of Cholecystokinin-Containing Interneurons Destined for the Cerebral Cortex

Although extensively studied postnatally, the functional differentiation of cholecystokinin (CCK)-containing interneurons en route towards the cerebral cortex during fetal development is incompletely understood. Here, we used CCKBAC/DsRed mice encoding a CCK promoter-driven red fluorescent protein to analyze the temporal dynamics of DsRed expression, neuronal identity, and positioning through high-resolution developmental neuroanatomy. Additionally, we developed a dual reporter mouse line (CCKBAC/DsRed::GAD67gfp/+) to differentiate CCK-containing interneurons from DsRed+ principal cells during prenatal development. We show that DsRed is upregulated in interneurons once they exit their proliferative niche in the ganglionic eminence and remains stably expressed throughout their long-distance migration towards the cerebrum, particularly in the hippocampus. DsRed+ interneurons, including a cohort coexpressing calretinin, accumulated at the palliosubpallial boundary by embryonic day 12.5. Pioneer DsRed+ interneurons already reached deep hippocampal layers by embryonic day 14.5 and were morphologically differentiated by birth. Furthermore, we probed migrating interneurons entering and traversing the cortical plate, as well as stationary cells in the hippocampus by patch-clamp electrophysiology to show the first signs of Na+ and K+ channel activity by embryonic day 12.5 and reliable adult-like excitability by embryonic day 18.5. Cumulatively, this study defines key positional, molecular, and biophysical properties of CCK+ interneurons in the prenatal brain.

GABAergic Mechanisms in Schizophrenia: Linking Postmortem and In Vivo Studies

Schizophrenia is a psychiatric disorder characterized by hallucinations, delusions, disorganized thinking, and impairments in cognitive functioning. Evidence from postmortem studies suggests that alterations in cortical γ-aminobutyric acid (GABAergic) neurons contribute to the clinical features of schizophrenia. In vivo measurement of brain GABA levels using magnetic resonance spectroscopy (MRS) offers the possibility to provide more insight into the relationship between problems in GABAergic neurotransmission and clinical symptoms of schizophrenia patients. This study reviews and links alterations in the GABA system in postmortem studies, animal models, and human studies in schizophrenia. Converging evidence implicates alterations in both presynaptic and postsynaptic components of GABAergic neurotransmission in schizophrenia, and GABA may thus play an important role in the pathophysiology of schizophrenia. MRS studies can provide direct insight into the GABAergic mechanisms underlying the development of schizophrenia as well as changes during its course.

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

Alterations in hypoglossal motor neurons due to GAD67 and VGAT deficiency in mice

There is an emerging body of evidence that glycinergic and GABAergic synaptic inputs onto motor neurons (MNs) help regulate the final number of MNs and axonal muscle innervation patterns. Using mutant glutamate decarboxylase 67 (GAD67) and vesicular inhibitory amino acid transporter (VGAT) deficient mice, we describe the effect that deficiencies of presynaptic GABAergic and/or glycinergic release have on the post-synaptic somato-dendritic structure of motor neurons, and the development of excitatory and inhibitory synaptic inputs to MNs. We use whole-cell patch clamp recording of synaptic currents in E18.5 hypoglossal MNs from brainstem slices, combined with dye-filling of these recorded cells with Neurobiotin™, high-resolution confocal imaging and 3-dimensional reconstructions. Hypoglossal MNs from GAD67- and VGAT-deficient mice display decreased inhibitory neurotransmission and increased excitatory synaptic inputs. These changes are associated with increased dendritic arbor length, increased complexity of dendritic branching, and increased density of spiny processes. Our results show that presynaptic release of inhibitory amino acid neurotransmitters are potent regulators of hypoglossal MN morphology and key regulators of synaptic inputs during this critical developmental time point.

l-Proline, GABA Synthesis and Gamma Oscillations in Schizophrenia

Altered inhibition from parvalbumin-containing GABA neurons is thought to contribute to impaired gamma frequency oscillations and cognitive deficits in schizophrenia. Crabtree and colleagues report that proline dehydrogenase deficits produce excessive cytosolic levels of the GABA-mimetic l-proline which impairs GABA synthesis and gamma oscillations in a manner that mimics schizophrenia.