GABAA receptor-mediated neurotransmission: Not so simple after all

Anxiogenic-like effects of coumarin, possibly through the GABAkine interaction pathway: Animal studies with in silico approaches

BACKGROUND

Anxiety disorder is the most common mental illness and a major contributor to impairment. Thus, there is an urgent need to find novel lead compounds to mitigate anxiety. It is widely recognized that the neurobiology of anxiety-related behavior involves GABAergic systems.

OBJECTIVES

This research aimed to examine the anxiogenic action of coumarin (CMN), a natural benzopyrone derived from plants, and determine its underlying mechanism through in vivo and in silico investigations.

METHODS

This was accomplished by using a variety of behavioral procedures, including open field, swing, hole cross, and light-dark tests, on male and female Swiss albino mice that had been orally administered three experimental doses of CMN (1, 2, and 4 mg/kg). The CMN group was also examined with the GABAA receptor agonist diazepam (DZP, 2 mg/kg) and flumazenil antagonist (FLU, 0.1 mg/kg). Furthermore, CMN and standards were subjected to a molecular docking analysis to determine their binding affinities for the GABAA receptor subunits (α1, α4, β2, γ2, and δ). Several software programs were used to visualize the ligand-receptor interaction and analyze the pharmacokinetic profile.

RESULTS

Compared to typical treatments, our results show that CMN (1 mg/kg) significantly (p < 0.05) increases the locomotor activity of animals. Furthermore, CMN exerted the highest binding affinity (-6.5 kcal/mol) with the GABA-α1 receptor compared to conventional DZP. Along with FLU, CMN displayed several hydrophobic and hydrogen bonds with GABAA receptor subunits. The pharmacokinetic and drug-like properties of CMN are also remarkable. In animal studies, CMN worked synergistically with FLU to provide anxiogenic-like effects.

CONCLUSION

We conclude that, based on in vivo and in silico data, CMN, alone or in combination with FLU, may be employed in future neurological clinical studies. However, further research is needed to confirm this behavioral activity and elucidate the possible mechanism of action.

The Role of Astrocyte-Neuron Lactate Shuttle in Neuropathic Orofacial Pain

BACKGROUND

Inhibition of astrocytic energy metabolism alleviates neuropathic pain.

OBJECTIVES

To explore whether astrocyte-neuron lactate shuttle (ANLS) played any role in neuropathic orofacial pain.

METHODS

Rats with partial transection of the right infraorbital nerve (p-IONX) or sham operation were intrathecally injected with acetazolamide (a carbonic anhydrase inhibitor), bithionol (a soluble adenylyl cyclase inhibitor), α-cyano-4-hydroxycinnamic acid [α-CHCA, a monocarboxylate transporter (MCT) inhibitor] or vehicle once a day from postoperative day 1-14. The facial mechanical thresholds were tested on preoperative day 1 and 2 and postoperative days 1, 3, 5, 7, 10 and 14, expression of glucose transporters (GLUTs) and MCTs in the trigeminal subnucleus caudalis (Vc) were examined on the postoperative day 3 and neuronal activities in the Vc were examined in the p-IONX rats on postoperative days 3-5.

RESULTS

Compared with the sham group, the mechanical thresholds in the p-IONX group were significantly reduced at postoperative days 1-7, and the number of astrocytes expressing GLUT1 and MCT1/4, and neurons expressing MCT2 was significantly increased on postoperative day 3. In the p-IONX groups, neurons in the Vc were sensitised, and acetazolamide, bithionol and α-CHCA reversed the central sensitisation, significantly increased the mechanical thresholds at postoperative days 1-7 and decreased the number of astrocytes expressing GLUT1 and MCT1/4, and neurons expressing MCT2 at postoperative day 3 compared with those in the vehicle-treated rats.

CONCLUSIONS

Inhibition of ANLS alleviates p-IONX-related neuronal, behavioural and immunohistochemical changes, which suggests that ANLS plays an important role in trigeminal neuropathic pain.

GABA Type A receptors expressed in triple negative breast cancer cells mediate chloride ion flux

Triple negative breast cancer (TNBC) is known for its heterogeneous nature and aggressive onset, limited unresponsiveness to hormone therapies and immunotherapy as well as high likelihood of metastasis and recurrence. Since no targeted standard treatment options are available for TNBC, novel and effective therapeutic targets are urgently needed. Ion channels have emerged as possible novel therapeutic candidates for cancer therapy. We previously showed that GABAA β3 subunit are expressed at higher levels in TNBC cell lines than non-tumorigenic MCF10A cells. GABAA β3 subunit knockdown causes cell cycle arrest in TNBC cell lines via decreased cyclin D1 and increased p21 expression. However, it is not known if the upregulated GABAAR express at the cell-surface in TNBC and mediate Cl- flux. Cl- ions are known to play a role in cell-cycle progression in other cancers such as gastric cancer. Here, using surface biotinylation and (N-(Ethoxycarbonylmethyl)-6-Methoxyquinolinium Bromide) MQAE-dye based fluorescence quenching, we show that upregulated GABAAR are on the cell-surface in TNBC cell lines and mediate significantly higher chloride (Cl-) flux as compared to non-tumorigenic MCF10A cells. Moreover, this GABAAR mediated Cl- flux can be modulated by pharmacological agents and is decreased in TNBC cells with GABAA β3 subunit knockdown. Further, treatment of TNBC cells with bicuculline, a GABAAR antagonist reduced cell viability in TNBC cells Overall, these results point to an unexplored role of GABAAR mediated Cl- flux in TNBC.

Perinatal exposure to bisphenol A impairs cognitive function via the gamma-aminobutyric acid signaling pathway in male rat offspring

Bisphenol A (BPA) is a common synthetic endocrine disruptor that can be utilized in the fabrication of materials such as polycarbonates and epoxy resins. Numerous studies have linked BPA to learning and memory problems, although the precise mechanism remains unknown. Gamma-aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in the vertebrate central nervous system, and it is intimately related to learning and memory. This study aims to evaluate whether altered cognitive behavior involves the GABA signaling pathway in male offspring of rats exposed to BPA during the prenatal and early postnatal periods. Pregnant rats were orally given BPA (0, 0.04, 0.4, and 4 mg/kg body weight (BW)/day) from the first day of pregnancy to the 21st day of breastfeeding. Three-week-old male rat offspring were selected for an open-field experiment and a new object recognition experiment to evaluate the effect of BPA exposure on cognitive behavior. Furthermore, the role of GABA signaling markers in the cognition affected by BPA was investigated at the molecular level using western blotting and real-time polymerase chain reaction (RT-PCR). The research demonstrated that BPA exposure impacted the behavior and memory of male rat offspring and elevated the expression of glutamic acid decarboxylase 67 (GAD67), GABA type A receptors subunit (GABAARα1), and GABA vesicle transporter (VGAT) in the hippocampus while decreasing the expression levels of GABA transaminase (GABA-T) and GABA transporter 1 (GAT-1). These findings indicate that the alteration in the expression of GABA signaling molecules may be one of the molecular mechanisms by which perinatal exposure to BPA leads to decreased learning and memory in male rat offspring.

Advancements in the use of xenopus oocytes for modelling neurological disease for novel drug discovery

INTRODUCTION

Introduced about 50 years ago, the model of Xenopus oocytes for the expression of recombinant proteins has gained a broad spectrum of applications. The authors herein review the benefits brought from using this model system, with a focus on modeling neurological disease mechanisms and application to drug discovery.

AREAS COVERED

Using multiple examples spanning from ligand gated ion channels to transporters, this review presents, in the light of the latest publications, the benefits offered from using Xenopus oocytes. Studies range from the characterization of gene mutations to the discovery of novel treatments for disorders of the central nervous system (CNS).

EXPERT OPINION

Development of new drugs targeting CNS disorders has been marked by failures in the translation from preclinical to clinical studies. As progress in genetics and molecular biology highlights large functional differences arising from a single to a few amino acid exchanges, the need for drug screening and functional testing against human proteins is increasing. The use of Xenopus oocytes to enable precise modeling and characterization of clinically relevant genetic variants constitutes a powerful model system that can be used to inform various aspects of CNS drug discovery and development.

Discovery of GABA Aminotransferase Inhibitors via Molecular Docking, Molecular Dynamic Simulation, and Biological Evaluation

γ-Aminobutyric acid aminotransferase (GABA-AT) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that degrades γ-aminobutyric (GABA) in the brain. GABA is an important inhibitory neurotransmitter that plays important neurological roles in the brain. Therefore, GABA-AT is an important drug target that regulates GABA levels. Novel and potent drug development to inhibit GABA-AT is still a very challenging task. In this study, we aimed to devise novel and potent inhibitors against GABA-AT using computer-aided drug design (CADD) tools. Since the crystal structure of human GABA-AT was not yet available, we utilized a homologous structure derived from our previously published paper. To identify highly potent compounds relative to vigabatrin, an FDA-approved drug against human GABA-AT, we developed a pharmacophore analysis protocol for 530,000 Korea Chemical Bank (KCB) compounds and selected the top 50 compounds for further screening. Preliminary biological analysis was carried out for these 50 compounds and 16 compounds were further assessed. Subsequently, molecular docking, molecular dynamics (MD) simulations, and binding free energy calculations were carried out. In the results, four predicted compounds, A07, B07, D08, and H08, were found to be highly potent and were further evaluated by a biological activity assay to confirm the results of the GABA-AT activity inhibition assay.

The importance of ligand gated ion channels in sleep and sleep disorders

On average, humans spend about 26 years of their life sleeping. Increased sleep duration and quality has been linked to reduced disease risk; however, the cellular and molecular underpinnings of sleep remain open questions. It has been known for some time that pharmacological modulation of neurotransmission in the brain can promote either sleep or wakefulness thereby providing some clues about the molecular mechanisms at play. However, the field of sleep research has developed an increasingly detailed understanding of the requisite neuronal circuitry and key neurotransmitter receptor subtypes, suggesting that it may be possible to identify next generation pharmacological interventions to treat sleep disorders within this same space. The aim of this work is to examine the latest physiological and pharmacological findings highlighting the contribution of ligand gated ion channels including the inhibitory GABA_A and glycine receptors and excitatory nicotinic acetylcholine receptors and glutamate receptors in the sleep-wake cycle regulation. Overall, a better understanding of ligand gated ion channels in sleep will help determine if these highly druggable targets could facilitate a better night's sleep.

Altered Development of Prefrontal GABAergic Functions and Anxiety-like Behavior in Adolescent Offspring Induced by Prenatal Stress

Maternal stress can afflict fetal brain development, putting the offspring at risk of cognitive deficits, including anxiety. The prefrontal cortex (PFC), a protracted maturing region, is notably affected by prenatal stress (PS). However, it remains unclear how PS interferes with the maturation of the GABAergic system, considering its functional adjustment in the PFC during adolescence. The present study thus investigated the long-lasting consequences of PS on the prefrontal GABAergic functions of adolescent offspring. Pregnant Sprague–Dawley rats were divided into controls and the PS group, which underwent restraint stress during the last week of gestation. Male pups from postnatal days (PND) 40–42 were submitted to the elevated plus maze (EPM) test. Proteins essentially involved in GABAergic signaling were then examined in PFC tissues, including the K+-Cl− cotransporter (KCC2), Na+-K+-Cl− cotransporter (NKCC1), α1 and α5 subunits of GABA type A receptors (GABAA receptors), and parvalbumin (PV), along with cAMP response element-binding protein phosphorylation (pCREB), which reacts in the plasticity regulation of PV-positive interneurons. The results revealed that the higher anxiety-like behavior of PS adolescent rats concurred with the significant decreases of the KCC2 and α1 subunits, with PV- and pCREB-lowered levels. The findings suggested that PS disrupts the continuance of PFC maturity by reducing the essential elements of GABAergic functions. These changes likely underlie the anxiety emerging in adolescence, possibly progressing to mental disorders.

The Critical Modulatory Role of Spiny Stellate Cells in Seizure Onset Based on Dynamic Analysis of a Neural Mass Model

Growing evidence suggests that excitatory neurons in the brain play a significant role in seizure generation. Nonetheless, spiny stellate cells are cortical excitatory non-pyramidal neurons in the brain, whose basic role in seizure occurrence is not well understood. In the present research, we study the critical role of spiny stellate cells or the excitatory interneurons (EI), for the first time, in epileptic seizure generation using an extended neural mass model inspired by a thalamocortical model originally introduced by another research group. Applying bifurcation analysis on this modified model, we investigated the rich dynamics corresponding to the epileptic seizure onset and transition between interictal and ictal states caused by EI connectivity to other cell types. Our results indicate that the transition between interictal and ictal states (preictal signal) corresponds to a supercritical Hopf bifurcation, and thus, the extended model suggests that before seizure onset, the amplitude and frequency of neural activities gradually increase. Moreover, we showed that (1) the altered function of GABAergic and glutamatergic receptors of EI can cause seizure, and (2) the pathway between the thalamic relay nucleus and EI facilitates the transition from interictal to ictal activity by decreasing the preictal period. Thereafter, we considered both sensory and cortical periodic inputs to study model responses to various harmonic stimulations. Bifurcation analysis of the model, in this case, suggests that the initial state of the model might be the main cause for the transition between interictal and ictal states as the stimulus frequency changes. The extended thalamocortical model shows also that the amplitude jump phenomenon and non-linear resonance behavior result from the preictal state of the modified model. These results can be considered as a step forward to a deeper understanding of the mechanisms underlying the transition from normal activities to epileptic activities.

Impaired learning and memory in mice induced by nano neodymium oxide and possible mechanisms

A growing number of individuals are now exposed to neodymium (Nd) owing to its extensive applications. However, the biological effects of Nd on humans, especially on learning and memory, remain elusive. To investigate whether Nd exposure affects learning and memory, in this study female ICR mice were exposed to nano Nd₂ O₃ via intranasal instillation at doses of 50, 100, and 150 mg/kg body weight, daily for 45 days. According to Morris water maze data, learning and memory parameters were significantly reduced in the 150 mg/kg nano-Nd₂ O₃ group than the sham control. Furthermore, inductively coupled plasma-mass spectroscopy analysis revealed that Nd levels were significantly higher in the hippo campus of the 100 and 150 mg/kg exposed group than the sham control; however, no significant differences were observed in the hippocampal histopathology between these groups. Furthermore, reactive oxygen species were elevated in hippocampal tissues of experimental groups than the sham control, 447.3 in high dose group and 360.0 in control group; however, malondialdehyde levels were significantly increased and superoxide dismutase activities were decreased only in mice exposed to 100 and 150 mg/kg Nd₂ O₃ . High-performance liquid chromatography data demonstrated that levels of glutamic acid, glycine, and gamma-aminobutyric acid were higher in the hippocampus of mice exposed to 150 mg/kg Nd₂ O₃ than the sham control. Our findings indicated that the neuronal injury was induced by disruption of the oxidation-antioxidation homeostasis and altered amino acid neurotransmitter levels in the hippocampus, which could result in the poor cognitive performance demonstrated by exposed mice.

Pharmacological modulation of GABAA receptors

Ligand-gated ion channels are integral membrane proteins that activate through a change in conformation upon transmitter binding and were identified as key players of brain function. GABA_A receptors are major inhibitory ligand-gated ion channels of this protein family. They are the target of many therapeutic compounds widely used in the clinic and continue to attract the attention of academic and pharmaceutical laboratories. Advances in the knowledge of the structure of GABA_A receptors at the molecular level with unprecedented resolution enabled the determination of the binding sites of many allosteric modulators revealing the nature of their interactions with the receptors. Herein, we review the latest findings on allosteric modulation of GABA_A receptors and their relevance to drug discovery.

[123I]Iomazenil SPECT Detects a Reversible Lesion of the Left Medial Temporal Lobe in a Case of Global Autobiographical Amnesia

Global autobiographical amnesia is a rare disorder that is characterized by a sudden loss of autobiographical memories covering many years of an individual's life. Generally, routine neuroimaging studies such as CT and MRI yield negative findings in individuals with global autobiographical amnesia. However, in recent case reports, functional analyses such as SPECT and fMRI have revealed changes in activity in various areas of the brain when compared with controls. Studies using iomazenil (IMZ) SPECT with individuals with global autobiographical amnesia have not been reported. We report the case of a 62-year-old Japanese woman with global autobiographical amnesia who had disappeared for ∼4 weeks. [123I]-IMZ SPECT showed reduced IMZ uptake in her left medial temporal lobe and no significant reduction on N-isopropyl-[123I] p-iodoamphetamine (IMP) SPECT in the identical region. Because IMZ binds to the central benzodiazepine receptor, this dissociation between IMZ and IMP SPECT was thought to reflect the breakdown of inhibitory neurotransmission in the left medial temporal lobe. Moreover, when the woman recovered most of her memory 32 months after fugue onset, the IMZ SPECT-positive lesion had decreased in size. Because the woman had long suffered verbal abuse from her former husband's sister and brother, which can also cause global autobiographical amnesia, it is difficult to conclude whether the IMZ SPECT-positive lesion in the left medial temporal lobe was the cause or the result of her global autobiographical amnesia. Although only one case, these observations suggest that IMZ SPECT may be useful in uncovering the mechanisms underlying global autobiographical amnesia.

Allosteric Modulation of GABAA Receptors in Rat Basolateral Amygdala Blocks Stress-Enhanced Reacquisition of Nicotine Self-Administration

Stress is a major determinant of relapse to smoked tobacco. In a rat model, repeated stress during abstinence from nicotine self-administration (SA) results in enhanced reacquisition of nicotine SA, which is dependent on the basolateral amygdala (BLA). We postulate that repeated stress during abstinence causes hyperexcitability of the BLA principal output neurons (PNs) due to disinhibition of the PNs from reduced inhibitory regulation by local GABAergic interneurons. To determine if enhanced GABAergic regulation of the BLA PNs can lessen the effects of stress on nicotine intake, positive allosteric modulators (PAMs) of GABA_A receptors were infused into the BLA immediately prior to reacquisition of nicotine SA. Three selective PAMs [NS 16085 (binds the benzodiazepine site on α2/α3 GABA_A); DCUK-OEt (binds a novel, benzodiazepine site on α1 or α5, β2 or β3, γ2 or δ GABA_A); DS2 (binds exclusively to δ GABA_A] with varied GABA_A subunit specificities abolished the stress-induced amplification of nicotine taking during reacquisition. These studies indicate that highly selective PAMS targeting α3 or δ subunit-containing GABA_A in the BLA may be effective in ameliorating the stress-induced relapse to smoked tobacco during abstinence from cigarettes.

Synapse and Receptor Alterations in Two Different S100B-Induced Glaucoma-Like Models

Glaucoma is identified by an irreversible retinal ganglion cell (RGC) loss and optic nerve damage. Over the past few years, the immune system gained importance in its genesis. In a glaucoma-like animal model with intraocular S100B injection, RGC death occurs at 14 days. In an experimental autoimmune glaucoma model with systemic S100B immunization, a loss of RGCs is accompanied by a decreased synaptic signal at 28 days. Here, we aimed to study synaptic alterations in these two models. In one group, rats received a systemic S100B immunization (n = 7/group), while in the other group, S100B was injected intraocularly (n = 6–7/group). Both groups were compared to appropriate controls and investigated after 14 days. While inhibitory post-synapses remained unchanged in both models, excitatory post-synapses degenerated in animals with intraocular S100B injection (p = 0.03). Excitatory pre-synapses tendentially increased in animals with systemic S100B immunization (p = 0.08) and significantly decreased in intraocular ones (p = 0.04). Significantly more N-methyl-d-aspartate (NMDA) receptors (both p ≤ 0.04) as well as gamma-aminobutyric acid (GABA) receptors (both p < 0.03) were observed in S100B animals in both models. We assume that an upregulation of these receptors causes the interacting synapse types to degenerate. Heightened levels of excitatory pre-synapses could be explained by remodeling followed by degeneration.

Carisoprodol pharmacokinetics and distribution in the nucleus accumbens correlates with behavioral effects in rats independent from its metabolism to meprobamate

Carisoprodol (Soma®) is a centrally-acting skeletal-muscle relaxant frequently prescribed for treatment of acute musculoskeletal conditions. Carisoprodol's mechanism of action is unclear and is often ascribed to that of its active metabolite, meprobamate. The purpose of this study was to ascertain whether carisoprodol directly produces behavioral effects, or whether metabolism to meprobamate via cytochrome P450 (CYP450) enzymatic reaction is necessary. Rats were trained to discriminate carisoprodol (100 mg/kg) to assess time course and whether a CYP450 inhibitor (cimetidine) administered for 4 days would alter the discriminative effects of carisoprodol. Additionally, pharmacokinetics of carisoprodol and meprobamate with and without co-administration of cimetidine were assessed via in vivo microdialysis combined with liquid-chromatography-tandem mass spectrometry from blood and nucleus accumbens (NAc). The time course of the discriminative-stimulus effects of carisoprodol closely matched the time course of the levels of carisoprodol in blood and NAc, but did not match the time course of meprobamate. Administration of cimetidine increased levels of carisoprodol and decreased levels of meprobamate consistent with its interfering with metabolism of carisoprodol to meprobamate. However, cimetidine failed to alter the discriminative-stimulus effects of carisoprodol. Carisoprodol penetrated into brain tissue and directly produced behavioral effects without being metabolized to meprobamate. These findings indicate that understanding the mechanism of action of carisoprodol independently of meprobamate will be necessary to determine the validity of its clinical uses.

Efficient production of gamma-aminobutyric acid by engineered Saccharomyces cerevisiae with glutamate decarboxylases from Streptomyces

Gamma-aminobutyric acid (GABA) is an industrially valuable natural product. This study was aimed to establish an efficient food-grade production process of GABA by engineering Saccharomyces cerevisiae that is generally recognized as safe (GRAS). GABA can be produced by catalytic decarboxylation of l-glutamate (l-Glu) by glutamate decarboxylase (GAD, EC4.1.1.15). Two GADs, SsGAD from Streptomyces sp. MJ654-NF4 and ScGAD from Streptomyces chromofuscus ATCC 49982, were heterologously expressed in S. cerevisiae BJ5464. The engineered yeast strains were used as whole-cell biocatalysts for GABA production. S. cerevisiae BJ5464/SsGAD exhibited significantly higher efficient catalytic activity than that of S. cerevisiae BJ5464/ScGAD. The optimal bioconversion system consisted of a cell density of OD₆₀₀ 30, 0.1 M l-Glu, and 0.28 mM pyridoxal phosphate in 0.2 M Na₂ HPO₄ -citric acid buffer with pH 5.4, and the reactions were performed at 50 °C for 12 H. S. cerevisiae BJ5464/SsGAD cells can be reused, and the accumulated GABA titer reached 62.6 g/L after 10 batches with an overall molar conversion rate of 60.8 mol%. This work thus provides an effective production process of GABA using engineered yeast for food and pharmaceutical applications.

Receptor variants and the development of centrally acting medications

The progressive changes in research paradigms observed in the largest pharmaceutical companies and the burgeoning of biotechnology startups over the last 10 years have generated a need for outsourcing research facilities. In parallel, progress made in the fields of genomics, protein expression in recombinant systems, and electrophysiological recording methods have offered new possibilities for the development of contract research organizations (CROs). Successful partnering between pharmaceutical companies and CROs largely depends upon the competences and scientific quality on offer for the discovery of novel active molecules and targets. Thus, it is critical to review the knowledge in the field of neuroscience research, how genetic approaches are augmenting our knowledge, and how they can be applied in the translation from the identification of potential molecules up to the first clinical trials. Taking these together, it is apparent that CROs have an important role to play in the neuroscience of drug discovery.


Reactive Oxygen Species in the Regulation of the GABA Mediated Inhibitory Neurotransmission

Reactive oxygen species (ROS) are best known for being involved in cellular metabolism and oxidative stress, but also play important roles in cell communication. ROS signaling has become increasingly recognized as a mechanism implicated in the regulation of synaptic neurotransmission, under both physiological and pathological conditions. Hydrogen peroxide (H₂O₂) and superoxide anion are the main biologically relevant endogenous ROS in the nervous system. They are predominantly produced in the mitochondria of neurons and glial cells and their levels are tightly regulated by the antioxidant cell machinery, which allows for dynamic signaling through these agents. Physicochemical and biological properties of H₂O₂ enable it to effectively play an important role in signaling. This review brings up some or the most significant evidence supporting ROS as signaling agents in the nervous system and summarizes data showing that ROS modulate γ-aminobutyric acid (GABA)-mediated neurotransmission by pre- and postsynaptic mechanisms. ROS induce changes on both, the activity of phasic and tonic GABA_A receptors and GABA release from presynaptic terminals. Based on these facts, ROS signaling is discussed as a possible selective mechanism linking cellular metabolism to inhibitory neurotransmission through the direct or indirect modulation of the GABA_A receptor function. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

GABA is a modulator, rather than a classical transmitter, in the medial nucleus of the trapezoid body-lateral superior olive sound localization circuit

KEY POINTS

The lateral superior olive (LSO), a brainstem hub involved in sound localization, integrates excitatory and inhibitory inputs from the ipsilateral and the contralateral ear, respectively. In gerbils and rats, inhibition to the LSO reportedly shifts from GABAergic to glycinergic within the first three postnatal weeks. Surprisingly, we found no evidence for synaptic GABA signalling during this time window in mouse LSO principal neurons. However, we found that presynaptic GABAB Rs modulate Ca2+ influx into medial nucleus of the trapezoid body axon terminals, resulting in reduced synaptic strength. Moreover, GABA elicited strong responses in LSO neurons that were mediated by extrasynaptic GABAA Rs. RNA sequencing revealed highly abundant δ subunits, which are characteristic of extrasynaptic receptors. Whereas GABA increased the excitability of neonatal LSO neurons, it reduced the excitability around hearing onset. Collectively, GABA appears to control the excitability of mouse LSO neurons via extrasynaptic and presynaptic signalling. Thus, GABA acts as a modulator, rather than as a classical transmitter.

ABSTRACT

GABA and glycine mediate fast inhibitory neurotransmission and are coreleased at several synapse types. Here we assessed the contribution of GABA and glycine in synaptic transmission between the medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO), two nuclei involved in sound localization. Whole-cell patch-clamp experiments in acute mouse brainstem slices at postnatal days (P) 4 and 11 during pharmacological blockade of GABAA receptors (GABAA Rs) and/or glycine receptors demonstrated no GABAergic synaptic component on LSO principal neurons. A GABAergic component was absent in evoked inhibitory postsynaptic currents and miniature events. Coimmunofluorescence experiments revealed no codistribution of the presynaptic GABAergic marker GAD65/67 with gephyrin, a postsynaptic marker for GABAA Rs, corroborating the conclusion that GABA does not act synaptically in the mouse LSO. Imaging experiments revealed reduced Ca2+ influx into MNTB axon terminals following activation of presynaptic GABAB Rs. GABAB R activation reduced the synaptic strength at P4 and P11. GABA appears to act on extrasynaptic GABAA Rs as demonstrated by application of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol, a δ-subunit-specific GABAA R agonist. RNA sequencing showed high mRNA levels for the δ-subunit in the LSO. Moreover, GABA transporters GAT-1 and GAT-3 appear to control extracellular GABA. Finally, we show an age-dependent effect of GABA on the excitability of LSO neurons. Whereas tonic GABA increased the excitability at P4, leading to spike facilitation, it decreased the excitability at P11 via shunting inhibition through extrasynaptic GABAA Rs. Taken together, we demonstrate a modulatory role of GABA in the murine LSO, rather than a function as a classical synaptic transmitter.

Pediatric Epilepsy Mechanisms: Expanding the Paradigm of Excitation/Inhibition Imbalance

Mechanisms underlying seizures and epilepsy have traditionally been considered to involve abnormalities of ion channels or synaptic function. Those considerations gave rise to the excitation/inhibition (E/I) imbalance theory, whereby increased excitation, decreased inhibition, or both favor a hyperexcitable state and an increased propensity for seizure generation and epileptogenesis. Several recent findings warrant reconsideration and expansion of the E/I hypothesis: novel genetic mutations have been identified that do not overtly affect E/I balance; neurotransmitters may exert paradoxical effects, especially during development; anti-seizure medications do not necessarily work by decreasing excitation or increasing inhibition; and metabolic factors participate in the regulation of neuronal and network excitability. These novel conceptual and experimental advances mandate expansion of the E/I paradigm, with the expectation that new and exciting therapies will emerge from this broadened understanding of how seizures and epilepsy arise and progress.

Acute and long-lasting effects of oxytocin in cortico-limbic circuits: consequences for fear recall and extinction

The extinction of conditioned fear responses entrains the formation of safe new memories to decrease those behavioral responses. The knowledge in neuronal mechanisms of extinction is fundamental in the treatment of anxiety and fear disorders. Interestingly, the use of pharmacological compounds that reduce anxiety and fear has been shown as a potent co-adjuvant in extinction therapy. However, the efficiency and mechanisms by which pharmacological compounds promote extinction of fear memories remains still largely unknown and would benefit from a validation based on functional neuronal circuits, and the neurotransmitters that modulate them. From this perspective, oxytocin receptor signaling, which has been shown in cortical and limbic areas to modulate numerous functions (Eliava et al. Neuron 89(6):1291-1304, 2016), among them fear and anxiety circuits, and to enhance the salience of social stimuli (Stoop Neuron 76(1):142-59, 2012), may offer an interesting perspective. Experiments in animals and humans suggest that oxytocin could be a promising pharmacological agent at adjusting memory consolidation to boost fear extinction. Additionally, it is possible that long-term changes in endogenous oxytocin signaling can also play a role in reducing expression of fear at different brain targets. In this review, we summarize the effects reported for oxytocin in cortico-limbic circuits and on fear behavior that are of relevance for the modulation and potential extinction of fear memories.

International Union of Basic and Clinical Pharmacology. CVI: GABAA Receptor Subtype- and Function-selective Ligands: Key Issues in Translation to Humans

GABAA receptors are the major inhibitory transmitter receptors in the brain. They are ligand-gated chloride channels and the site of action of benzodiazepines, barbiturates, neuroactive steroids, anesthetics, and convulsants. GABAA receptors are composed of five subunits that can belong to different subunit classes. The existence of 19 homologous subunits and their distinct regional, cellular, and subcellular distribution gives rise to a large number of GABAA receptor subtypes with distinct pharmacology, which modulate different functions of the brain. A variety of compounds have been identified that were claimed to modulate selectively individual GABAA receptor subtypes. However, many of these compounds have only incompletely been investigated or, in addition to a preferential modulation of a receptor subtype, also modulate other subtypes at similar concentrations. Although their differential efficacy at distinct receptor subtypes reduced side effects in behavioral experiments in rodents, the exact receptor subtypes mediating their behavioral effects cannot be unequivocally delineated. In addition, the discrepant in vivo effects of some of these compounds in rodents and man raised doubts on the applicability of the concept of receptor subtype selectivity as a guide for the development of clinically useful drugs. Here, we provide an up-to-date review on the currently available GABAA receptor subtype-selective ligands. We present data on their actual activity at GABAA receptor subtypes, discuss the translational aspect of subtype-selective drugs, and make proposals for the future development of ligands with better anxioselectivity in humans. Finally, we discuss possible ways to strengthen the conclusions of behavioral studies with the currently available drugs.

Methods for the Discovery of Novel Compounds Modulating a Gamma-Aminobutyric Acid Receptor Type A Neurotransmission

This manuscript presents a step-by-step protocol for screening compounds at gamma-aminobutyric acid type A (GABA_A) receptors and its use towards the identification of novel molecules active in preclinical assays from an in vitro recombinant receptor to their pharmacological effects at native receptors in rodent brain slices. For compounds binding at the benzodiazepine site of the receptor, the first step is to set up a primary screen that consists of developing radioligand binding assays on cell membranes expressing the major GABA_A subtypes. Then, taking advantage of the heterologous expression of rodent and human GABA_A receptors in Xenopus oocytes or HEK 293 cells, it is possible to explore, in electrophysiological assays, the physiological properties of the different receptor subtypes and the pharmacological properties of the identified compounds. The Xenopus oocyte system will be presented here, starting with the isolation of the oocytes and their microinjection with different mRNAs, up to the pharmacological characterization using two-electrode voltage clamps. Finally, recordings conducted in rodent brain slices will be described that are used as a secondary physiological test to assess the activity of molecules at their native receptors in a well-defined neuronal circuit. Extracellular recordings using population responses of multiple neurons are demonstrated together with the drug application.

Neuroprotective effects of honokiol: from chemistry to medicine

The incidence of neurological disorders is growing in developed countries together with increased lifespan. Nowadays, there are still no effective treatments for neurodegenerative pathologies, which make necessary to search for new therapeutic agents. Natural products, most of them used in traditional medicine, are considered promising alternatives for the treatment of neurodegenerative diseases. Honokiol is a natural bioactive phenylpropanoid compound, belonging to the class of neolignan, found in notable amounts in the bark of Magnolia tree, and has been reported to exert diverse pharmacological properties including neuroprotective activities. Honokiol can permeate the blood brain barrier and the blood-cerebrospinal fluid to increase its bioavailability in neurological tissues. Diverse studies have provided evidence on the neuroprotective effect of honokiol in the central nervous system, due to its potent antioxidant activity, and amelioration of the excitotoxicity mainly related to the blockade of glutamate receptors and reduction in neuroinflammation. In addition, recent studies suggest that honokiol can attenuate neurotoxicity exerted by abnormally aggregated Aβ in Alzheimer's disease. The present work summarizes what is currently known concerning the neuroprotective effects of honokiol and its potential molecular mechanisms of action, which make it considered as a promising agent in the treatment and management of neurodegenerative diseases. © 2017 BioFactors, 43(6):760-769, 2017.

An Overview of the Mechanisms of Abnormal GABAergic Interneuronal Cortical Migration Associated with Prenatal Ethanol Exposure

GABAergic Interneuronal migration constitutes an essential process during corticogenesis. Derived from progenitor cells located in the proliferative zones of the ventral telencephalon, newly generated GABAergic Interneuron migrate to their cortical destinations. Cortical dysfunction associated with defects in neuronal migration results in severe developmental consequences. There is growing evidence linking prenatal ethanol exposure to abnormal GABAergic interneuronal migration and subsequent cortical dysfunction. Investigating the pathophysiological mechanisms behind disrupted GABAergic interneuronal migration encountered with prenatal alcohol exposure is crucial for understanding and managing fetal alcohol spectrum disorders. This review explores the molecular pathways regulating GABAergic interneuronal cortical migration that might be altered by prenatal ethanol exposure thus opening new avenues for further research in this topic.

Optogenetic inhibition of behavior with anion channelrhodopsins

Optogenetics uses light exposure to manipulate physiology in genetically modified organisms. Abundant tools for optogenetic excitation are available, but the limitations of current optogenetic inhibitors present an obstacle to demonstrating the necessity of neuronal circuits. Here we show that anion channelrhodopsins can be used to specifically and rapidly inhibit neural systems involved in Drosophila locomotion, wing expansion, memory retrieval and gustation, thus demonstrating their broad utility in the circuit analysis of behavior.