Amino acid transmitters in the mammalian central nervous system

Astrocytic modulation of neuronal signalling

Neuronal signalling is a key element in neuronal communication and is essential for the proper functioning of the CNS. Astrocytes, the most prominent glia in the brain play a key role in modulating neuronal signalling at the molecular, synaptic, cellular, and network levels. Over the past few decades, our knowledge about astrocytes and their functioning has evolved from considering them as merely a brain glue that provides structural support to neurons, to key communication elements. Astrocytes can regulate the activity of neurons by controlling the concentrations of ions and neurotransmitters in the extracellular milieu, as well as releasing chemicals and gliotransmitters that modulate neuronal activity. The aim of this review is to summarise the main processes through which astrocytes are modulating brain function. We will systematically distinguish between direct and indirect pathways in which astrocytes affect neuronal signalling at all levels. Lastly, we will summarize pathological conditions that arise once these signalling pathways are impaired focusing on neurodegeneration.

Age-related differences of γ-aminobutyric acid (GABA)ergic transmission in human colonic smooth muscle

BACKGROUND

Enteric neurons undergo to functional changes during aging. We investigated the possible age-associated differences in enteric γ-aminobutyric acid (GABA)ergic transmission evaluating function and distribution of GABAergic receptors in human colon.

METHODS

Mechanical responses to GABA and GABA receptor agonists on slow phasic contractions were examined in vitro as changes in isometric tension in colonic muscle strips from young (<65 years old) and aged patients (>65 years old). GABAergic receptor expression was assessed by quantitative RT-PCR.

KEY RESULTS

In both preparations GABA induced an excitatory effect, consisting in an increase in the basal tone, antagonized by the GABAA receptor antagonist, bicuculline, and potentiated by phaclofen, GABAB receptor antagonist.Tetrodotoxin (TTX) and atropine-sensitive contractile responses to GABA and GABAA receptor agonist, muscimol, were more pronounced in old compared to young subjects. Baclofen, GABAB receptor agonist, induced a TTX-sensitive reduction of the amplitude of the spontaneous. Nω-nitro-l-arginine methyl ester (L-NAME), nitric oxide (NO) synthase inhibitor abolished the inhibitory responses in old preparations, but a residual responses persisted in young preparations, which in turn was abolished by suramin, purinergic receptor antagonist. α3-GABAA receptor subunit expression tends to change in an age-dependent manner.

CONCLUSIONS AND INFERENCES

Our results reveal age-related differences in GABAergic transmission in human colon. At all the age tested GABA regulates muscular contractility modulating the activity of the intrinsic neurons. Activation of GABAA receptor, through acetylcholine release, induces contraction, which increases in amplitude with age. GABAB receptor activation leads to neural release of NO and purines, being a loss of purinergic-component in aged group.

Physiological synaptic activity and recognition memory require astroglial glutamine

Presynaptic glutamate replenishment is fundamental to brain function. In high activity regimes, such as epileptic episodes, this process is thought to rely on the glutamate-glutamine cycle between neurons and astrocytes. However the presence of an astroglial glutamine supply, as well as its functional relevance in vivo in the healthy brain remain controversial, partly due to a lack of tools that can directly examine glutamine transfer. Here, we generated a fluorescent probe that tracks glutamine in live cells, which provides direct visual evidence of an activity-dependent glutamine supply from astroglial networks to presynaptic structures under physiological conditions. This mobilization is mediated by connexin43, an astroglial protein with both gap-junction and hemichannel functions, and is essential for synaptic transmission and object recognition memory. Our findings uncover an indispensable recruitment of astroglial glutamine in physiological synaptic activity and memory via an unconventional pathway, thus providing an astrocyte basis for cognitive processes.

The authors present a fluorescent probe that tracks glutamine in live cells. They demonstrate the capabilities of the probe by providing direct visual evidence of an activity-dependent glutamine supply from astroglial networks to presynaptic structures under physiological conditions.

Tomatoes: An Extensive Review of the Associated Health Impacts of Tomatoes and Factors That Can Affect Their Cultivation

Simple Summary

The research outlined in this review paper discusses potential health benefits associated with a diet enriched with tomatoes and tomato products. This includes details of previous studies investigating the anticancer properties of tomatoes, protection against cardiovascular and neurodegenerative diseases and diabetes, maintenance of a healthy gut microbiome, and improved skin health, fertility, immune response, and exercise recovery. The specific parts of a tomato fruit that contribute these health benefits are also outlined. The potential disadvantages to a tomato-rich diet are detailed, especially the consumption of supplements that contain compounds found in tomatoes, such as lycopene. This review also discusses how the cultivation of tomato plants can affect the nutritional value of the fruit harvested. Different environmental growing conditions such as light intensity, growing media, and temperature are explained in terms of the impact they have on the quality of fruit, its nutrient content, and hence the potential health benefits acquired from eating the fruit.

Abstract

This review outlines the health benefits associated with the regular consumption of tomatoes and tomato products. The first section provides a detailed account of the horticultural techniques that can impact the quality of the fruit and its nutritional properties, including water availability, light intensity, temperature, and growing media. The next section provides information on the components of tomato that are likely to contribute to its health effects. The review then details some of the health benefits associated with tomato consumption, including anticancer properties, cardiovascular and neurodegenerative diseases and skin health. This review also discusses the impact tomatoes can have on the gut microbiome and associated health benefits, including reducing the risk of inflammatory bowel diseases. Other health benefits of eating tomatoes are also discussed in relation to effects on diabetes, the immune response, exercise recovery, and fertility. Finally, this review also addresses the negative effects that can occur as a result of overconsumption of tomato products and lycopene supplements.

The Conformationally Sensitive Spatial Distance Between the TM3-4 Loop and Transmembrane Segment 7 in the Glutamate Transporter Revealed by Paired-Cysteine Mutagenesis

Excitatory amino acid transporters can maintain extracellular glutamate concentrations lower than neurotoxic levels by transferring neurotransmitters from the synaptic cleft into surrounding glial cells and neurons. Previous work regarding the structural studies of Glt_Ph, Glt_TK, excitatory amino acid transporter 1 (EAAT1), EAAT3 and alanine serine cysteine transporter 2 described the transport mechanism of the glutamate transporter in depth. However, much remains unknown about the role of the loop between transmembrane segment 3 and 4 during transport. To probe the function of this loop in the transport cycle, we engineered a pair of cysteine residues between the TM3-TM4 loop and TM7 in cysteine-less EAAT2. Here, we show that the oxidative cross-linking reagent CuPh inhibits transport activity of the paired mutant L149C/M414C, whereas DTT inhibits the effect of CuPh on transport activity of L149C/M414C. Additionally, we show that the effect of cross-linking in the mutant is due to the formation of the disulfide bond within the molecules of EAAT2. Further, L-glutamate or KCl protect, and D,L-threo-β-benzyloxy-aspartate (TBOA) increases, CuPh-induced inhibition in the L149C/M414 mutant, suggesting that the L149C and M414C cysteines are closer or farther away in the outward- or inward-facing conformations, respectively. Together, our findings provide evidence that the distance between TM3-TM4 loop and TM7 alter when substrates are transported.

A meta-analysis of ultra-high field glutamate, glutamine, GABA and glutathione 1HMRS in psychosis: Implications for studies of psychosis risk

Ultra-high field proton magnetic resonance spectroscopy (1HMRS) offers a unique opportunity to measure the concentration of neurometabolites implicated in psychosis (PSY). The extant 7 T 1HMRS literature measuring glutamate-associated neurometabolites in the brain in PSY in vivo is small, but a comprehensive, quantitative summary of these data can offer insight and guidance to this emerging field. This meta-analysis examines proton spectroscopy (1HMRS) measures of glutamate (Glu), glutamine (Gln), glutamate+glutamine (Glx), gamma aminobutyric acid (GABA), and glutathione (GSH) across 255 individuals with PSY (121 first episode) and 293 healthy comparison participants (HC). While all five neurometabolites were lower in PSY as compared to HC, only Glu (Cohen's d = -0.18) and GSH (Cohen's d = -0.21) concentrations were significantly lower in PSY, whereas concentrations of Gln, Glx, and GABA did not significantly differ between groups. Notably, 1HMRS methodological choices and sample demographic characteristics did not impact study-specific effect sizes for PSY-related Glu or GSH differences. This review thus provides further evidence of neurometabolite dysfunction in first episode and chronic PSY, and thereby suggests that Glu and GSH abnormalities may additionally play a role in more incipient stages of the disorder: in clinical high risk stages. Additional 7 T neurochemical imaging studies in larger, longitudinal, and unmedicated samples and in youth at risk for developing psychosis are needed. Such studies will be critical for elucidating the neurodevelopmental and clinical time course of PSY-related neurometabolite alterations, and for assessing the potential for implicated metabolites to serve as druggable targets for decreasing PSY risk.

The journey from white rice to ultra-high hydrostatic pressurized brown rice: an excellent endeavor for ideal nutrition from staple food

Although brown rice (BR) contains significantly higher levels of nutrients than the traditionally used polished white rice (WR), its consumption among the population is still not noteworthy. WR and BR are essentially same grain. The only difference between the two is the application of an exhaustive milling procedure during the processing of WR that removes all other layers of the grain except the portion of its white endosperm. BR, on the other hand, is prepared by removing only the outer hull of the rice seed. Thus, in addition to its inner endosperm, the bran and germ are also left on the BR. Hence, BR retains all its nutrients, including proteins, lipids, carbohydrates, fibers, vitamins, minerals, tocopherols, tocotrienols, γ-oryzanol, and γ-aminobutyric acid (GABA) packed into the bran and germ of the seed. Since BR tastes nutty and takes longer to cook than WR, it is not appreciated by the consumers. However, these problems have been circumvented using non-thermal ultra-high hydrostatic pressure (UHHP)-processing for the treatment of BR. A superior modification in the physicochemical and functional qualities of UHHPBR, along with its ability to curb human diseases may make it a more palatable and nutritious choice of rice over WR or the untreated-BR. Here, we have reviewed the mechanism by which UHHP treatment leads to the modification of nutrients such as proteins, lipids, carbohydrates, and fibers. We have focused on the effects of rice on cell and animal models of different conditions such as hyperlipidemia, diabetes, and hypertension and the possible mechanisms. Finally, we have emphasized the effects of UHHPBR in human cases with rare conditions such as osteoporosis and brain cognition - two age-related degenerative diseases of the elderly population.

A quantitative meta-analysis of brain glutamate metabolites in aging

Glutamate (Glu) is a key molecule in cellular metabolism, the most abundant excitatory neurotransmitter in the brain, and the principal neurotransmitter of cortical efferents. Glutamate dysfunction, on the other hand, is common in neurodegenerative disorders, and likely contributes to age-related declines in behavioral and cognitive functioning. Nonetheless, the extant literature measuring age-related changes in brain glutamate in vivo has yet to be comprehensively and quantitatively summarized. This meta-analysis examines proton spectroscopy (¹HMRS) measures of Glu-related brain metabolites across 589 healthy young and older adults. Glu (Cohen's d = -0.82) and Glu+glutamine (Cohen's d = -0.51) concentrations were significantly lower in older compared with younger adults, whereas the concentration of glutamine (d = 0.43) was significantly higher in older individuals. Notably, ¹HMRS methodological choices impacted effect sizes for age-related Glu differences. Glu metabolite change appears to be a robust marker of aging-related neurological change; however, additional studies are needed to elucidate age-related trajectories of glutamatergic alterations and their relationship to cognitive phenotypes.

The Pharmacology of Visual Hallucinations in Synucleinopathies

Visual hallucinations (VH) are commonly found in the course of synucleinopathies like Parkinson's disease and dementia with Lewy bodies. The incidence of VH in these conditions is so high that the absence of VH in the course of the disease should raise questions about the diagnosis. VH may take the form of early and simple phenomena or appear with late and complex presentations that include hallucinatory production and delusions. VH are an unmet treatment need. The review analyzes the past and recent hypotheses that are related to the underlying mechanisms of VH and then discusses their pharmacological modulation. Recent models for VH have been centered on the role played by the decoupling of the default mode network (DMN) when is released from the control of the fronto-parietal and salience networks. According to the proposed model, the process results in the perception of priors that are stored in the unconscious memory and the uncontrolled emergence of intrinsic narrative produced by the DMN. This DMN activity is triggered by the altered functioning of the thalamus and involves the dysregulated activity of the brain neurotransmitters. Historically, dopamine has been indicated as a major driver for the production of VH in synucleinopathies. In that context, nigrostriatal dysfunctions have been associated with the VH onset. The efficacy of antipsychotic compounds in VH treatment has further supported the notion of major involvement of dopamine in the production of the hallucinatory phenomena. However, more recent studies and growing evidence are also pointing toward an important role played by serotonergic and cholinergic dysfunctions. In that respect, in vivo and post-mortem studies have now proved that serotonergic impairment is often an early event in synucleinopathies. The prominent cholinergic impairment in DLB is also well established. Finally, glutamatergic and gamma aminobutyric acid (GABA)ergic modulations and changes in the overall balance between excitatory and inhibitory signaling are also contributing factors. The review provides an extensive overview of the pharmacology of VH and offers an up to date analysis of treatment options.

Tetramethylenedisulfotetramine neurotoxicity: What have we learned in the past 70 years?

Tetramethylenedisulfotetramine (tetramine, TETS, TMDT) is a seizure-producing neurotoxic chemical formed by the condensation of sulfamide and formaldehyde. Serendipitously discovered through an occupational exposure in 1949, it was promoted as a rodenticide but later banned worldwide due to its danger to human health. However, exceptional activity of the agent against rodent pests resulted in its clandestine manufacture with large numbers of inadvertent, intentional, and mass poisonings, which continue to this day. Facile synthesis, extreme potency, persistence, lack of odor, color, and taste identify it as an effective food adulterant and potential chemical agent of terror. No known antidote or targeted treatment is currently available. In this review we examine the origins of tetramethylenedisulfotetramine, from its identification as a neurotoxicant 70 years ago, through early research, to the most recent findings including the risk it poses in the post-911 world. Included is the information known regarding its in vitro pharmacology as a GABA_A receptor channel antagonist, the toxic syndrome it produces in vivo, and its effect upon vulnerable populations. We also summarize the available information about potential therapeutic countermeasures and treatment strategies as well as the contribution of clinical development of TMDT poisoning to our understanding of epileptogenesis. Finally we identify gaps in our knowledge and suggest potentially fruitful directions for continued research on this dangerous, yet intriguing compound.

NLRX1 Enhances Glutamate Uptake and Inhibits Glutamate Release by Astrocytes

Uptake of glutamate from the extracellular space and glutamate release to neurons are two major processes conducted by astrocytes in the central nervous system (CNS) that protect against glutamate excitotoxicity and strengthen neuronal firing, respectively. During inflammatory conditions in the CNS, astrocytes may lose one or both of these functions, resulting in accumulation of the extracellular glutamate, which eventually leads to excitotoxic neuronal death, which in turn worsens the CNS inflammation. NLRX1 is an innate immune NOD-like receptor that inhibits the major inflammatory pathways. It is localized in the mitochondria and was shown to inhibit cell death, enhance ATP production, and dampen oxidative stress. In the current work, using primary murine astrocyte cultures from WT and Nlrx1^-/- mice, we demonstrate that NLRX1 potentiates astrocytic glutamate uptake by enhancing mitochondrial functions and the functional activity of glutamate transporters. Also, we report that NLRX1 inhibits glutamate release from astrocytes by repressing Ca²⁺-mediated glutamate exocytosis. Our study, for the first time, identified NLRX1 as a potential regulator of glutamate homeostasis in the CNS.

Glutamate Transport and Preterm Brain Injury

Preterm birth complications are the leading cause of child death worldwide and a top global health priority. Among the survivors, the risk of life-long disabilities is high, including cerebral palsy and impairment of movement, cognition, and behavior. Understanding the molecular mechanisms of preterm brain injuries is at the core of future healthcare improvements. Glutamate excitotoxicity is a key mechanism in preterm brain injury, whereby the accumulation of extracellular glutamate damages the delicate immature oligodendrocytes and neurons, leading to the typical patterns of injury seen in the periventricular white matter. Glutamate excitotoxicity is thought to be induced by an interaction between environmental triggers of injury in the perinatal period, particularly cerebral hypoxia-ischemia and infection/inflammation, and developmental and genetic vulnerabilities. To avoid extracellular build-up of glutamate, the brain relies on rapid uptake by sodium-dependent glutamate transporters. Astrocytic excitatory amino acid transporter 2 (EAAT2) is responsible for up to 95% of glutamate clearance, and several lines of evidence suggest that it is essential for brain functioning. While in the adult EAAT2 is predominantly expressed by astrocytes, EAAT2 is transiently upregulated in the immature oligodendrocytes and selected neuronal populations during mid-late gestation, at the peak time for preterm brain injury. This developmental upregulation may interact with perinatal hypoxia-ischemia and infection/inflammation and contribute to the selective vulnerability of the immature oligodendrocytes and neurons in the preterm brain. Disruption of EAAT2 may involve not only altered expression but also impaired function with reversal of transport direction. Importantly, elevated EAAT2 levels have been found in the reactive astrocytes and macrophages of human infant post-mortem brains with severe white matter injury (cystic periventricular leukomalacia), potentially suggesting an adaptive mechanism against excitotoxicity. Interestingly, EAAT2 is suppressed in animal models of acute hypoxic-ischemic brain injury at term, pointing to an important and complex role in newborn brain injuries. Enhancement of EAAT2 expression and transport function is gathering attention as a potential therapeutic approach for a variety of adult disorders and awaits exploration in the context of the preterm brain injuries.

Astrocytes Maintain Glutamate Homeostasis in the CNS by Controlling the Balance between Glutamate Uptake and Release

Glutamate is one of the most prevalent neurotransmitters released by excitatory neurons in the central nervous system (CNS); however, residual glutamate in the extracellular space is, potentially, neurotoxic. It is now well-established that one of the fundamental functions of astrocytes is to uptake most of the synaptically-released glutamate, which optimizes neuronal functions and prevents glutamate excitotoxicity. In the CNS, glutamate clearance is mediated by glutamate uptake transporters expressed, principally, by astrocytes. Interestingly, recent studies demonstrate that extracellular glutamate stimulates Ca2+ release from the astrocytes' intracellular stores, which triggers glutamate release from astrocytes to the adjacent neurons, mostly by an exocytotic mechanism. This released glutamate is believed to coordinate neuronal firing and mediate their excitatory or inhibitory activity. Therefore, astrocytes contribute to glutamate homeostasis in the CNS, by maintaining the balance between their opposing functions of glutamate uptake and release. This dual function of astrocytes represents a potential therapeutic target for CNS diseases associated with glutamate excitotoxicity. In this regard, we summarize the molecular mechanisms of glutamate uptake and release, their regulation, and the significance of both processes in the CNS. Also, we review the main features of glutamate metabolism and glutamate excitotoxicity and its implication in CNS diseases.

Analgesic potential of PF-06372865, an α2/α3/α5 subtype-selective GABAA partial agonist, in humans

BACKGROUND

This study investigated the analgesic effects of two doses (15 and 65 mg) of PF-06372865, a novel α2/α3/α5 gamma-aminobutyric acid A (GABA_A) subunit selective partial positive allosteric modulator (PAM), compared with placebo and pregabalin (300 mg) as a positive control.

METHODS

We performed a randomised placebo-controlled crossover study (NCT02238717) in 20 healthy subjects, using a battery of pain tasks (electrical, pressure, heat, cold and inflammatory pain, including a paradigm of conditioned pain modulation). Pharmacodynamic measurements were performed at baseline and up to 10 h after dose.

RESULTS

A dose of 15 mg PF-06372865 increased pain tolerance thresholds (PTTs) for pressure pain at a ratio of 1.11 (90% confidence interval [CI]: 1.02, 1.22) compared with placebo. A dose of 65 mg PF-06372865 led to an increase in PTT for the cold pressor at a ratio of 1.17 (90% CI: 1.03, 1.32), and pressure pain task: 1.11 (90% CI: 1.01, 1.21). Pregabalin showed an increase in PTT for pressure pain at a ratio of 1.15 (95% CI: 1.06, 1.26) and cold pressor task: 1.31 (90% CI: 1.16, 1.48).

CONCLUSION

We conclude that PF-06372865 has analgesic potential at doses that do not induce significant sedation or other intolerable adverse events limiting its clinical use. In addition, the present study established the potential role for this battery of pain tasks as a tool in the development of analgesics with a novel mechanism of action, for the treatment of various pain states including neuropathic pain and to establish proof-of-concept.

CLINICAL TRIALS REGISTRATION

NCT0223871.

Miniature pig magnetic resonance spectroscopy model of normal adolescent brain development

BACKGROUND

We are developing the miniature pig (Sus scrofa domestica), an in-vivo translational, gyrencephalic model for brain development, as an alternative to laboratory rodents/non-human primates. We analyzed longitudinal changes in adolescent pigs using proton magnetic resonance spectroscopy (¹H-MRS) and examined the relationship with white matter (WM) integrity derived from diffusion weighted imaging (DWI).

NEW METHOD

Twelve female Sinclair^™ pigs underwent three imaging/spectroscopy sessions every 23.95 ± 3.73 days beginning at three months of age using a clinical 3 T scanner. ¹H-MRS data were collected using 1.2 × 1.0 × 3.0 cm voxels placed in left and right hemisphere WM using a Point Resolved Spectroscopy sequence (TR = 2000 ms, TE = 30 ms). Concentrations of N-acetylaspartate, myo-inositol (MI), glutamate + glutamine, choline, creatine, and macromolecules (MM) 09 and 14 were averaged from both hemispheres. DWI data were collected using 15 shells of b-values (b = 0-3500 s/mm2) with 32 directions/shell and fit using the WM Tract Integrity model to calculate fractional anisotropy (FA), kurtosis anisotropy (KA) and permeability-diffusivity index.

RESULTS

MI and MM09 significantly declined with age. Increased FA and KA significantly correlated with decline in MI and MM09. Correlations lost significance once corrected for age.

COMPARISON WITH EXISTING METHODS

MRI scanners/protocols can be used to collect 1H-MRS and DWI data in pigs. Pigs have a larger, more complex, gyrencephalic brain than laboratory rodents but are less complex than non-human primates, thus satisfying the "replacement" principle of animal research.

CONCLUSIONS

Longitudinal effects in MRS measurements were similar to those reported in adolescent humans. MRS changes correlated with diffusion measurements indicating ongoing WM myelination/maturation.

GABAA receptor: Positive and negative allosteric modulators

gamma-Aminobutyric acid (GABA)-mediated inhibitory neurotransmission and the gene products involved were discovered during the mid-twentieth century. Historically, myriad existing nervous system drugs act as positive and negative allosteric modulators of these proteins, making GABA a major component of modern neuropharmacology, and suggesting that many potential drugs will be found that share these targets. Although some of these drugs act on proteins involved in synthesis, degradation, and membrane transport of GABA, the GABA receptors Type A (GABAAR) and Type B (GABABR) are the targets of the great majority of GABAergic drugs. This discovery is due in no small part to Professor Norman Bowery. Whereas the topic of GABABR is appropriately emphasized in this special issue, Norman Bowery also made many insights into GABAAR pharmacology, the topic of this article. GABAAR are members of the ligand-gated ion channel receptor superfamily, a chloride channel family of a dozen or more heteropentameric subtypes containing 19 possible different subunits. These subtypes show different brain regional and subcellular localization, age-dependent expression, and potential for plastic changes with experience including drug exposure. Not only are GABAAR the targets of agonist depressants and antagonist convulsants, but most GABAAR drugs act at other (allosteric) binding sites on the GABAAR proteins. Some anxiolytic and sedative drugs, like benzodiazepine and related drugs, act on GABAAR subtype-dependent extracellular domain sites. General anesthetics including alcohols and neurosteroids act at GABAAR subunit-interface trans-membrane sites. Ethanol at high anesthetic doses acts on GABAAR subtype-dependent trans-membrane domain sites. Ethanol at low intoxicating doses acts at GABAAR subtype-dependent extracellular domain sites. Thus GABAAR subtypes possess pharmacologically specific receptor binding sites for a large group of different chemical classes of clinically important neuropharmacological agents. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Delineation of the Role of Astroglial GABA Transporters in Seizure Control

Studies of GABA transport in neurons and astrocytes have provided evidence that termination of GABA as neurotransmitter is brought about primarily by active transport into the presynaptic, GABAergic nerve endings. There is, however, a considerable transport capacity in the astrocytes surrounding the synaptic terminals, a transport which may limit the availability of transmitter GABA leading to a higher probability of seizure activity governed by the balance of excitatory and inhibitory neurotransmission. Based on this it was hypothesized that selective inhibition of astrocytic GABA transport might prevent such seizure activity. A series of GABA analogs of restricted conformation were synthesized and in a number of collaborative investigations between Prof. Steve White at the University of Utah and medicinal chemists and pharmacologists at the School of Pharmacy and the University of Copenhagen, Denmark, GABA analogs with exactly this pharmacological property were identified. The most important analogs identified were N-methyl-exo-THPO (N-methyl-3-hydroxy-4-amino-4,5,6,7-tetrahydro-1,2-benzisoxazole) and its lipophilic analog EF-1502 ((RS)-4-[N-[1,1-bis(3-methyl-2-thienyl)but-1-en-4-yl]-N-methylamino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol) both of which turned out to be potent anticonvulsants in animal models of epilepsy.

Individual Differences in Resting Corticospinal Excitability Are Correlated with Reaction Time and GABA Content in Motor Cortex

Individuals differ in the intrinsic excitability of their corticospinal pathways and, perhaps more generally, their entire nervous system. At present, we have little understanding of the mechanisms underlying these differences and how variation in intrinsic excitability relates to behavior. Here, we examined the relationship between individual differences in intrinsic corticospinal excitability, local cortical GABA levels, and reaction time (RT) in a group of 20 healthy human adults. We measured corticospinal excitability at rest with transcranial magnetic stimulation, local concentrations of basal GABA with magnetic resonance spectroscopy, and RT with a behavioral task. All measurements were repeated in two separate sessions, and tests of reliability confirmed the presence of stable individual differences. There was a negative correlation between corticospinal excitability and RT, such that larger motor-evoked potentials (MEPs) measured at rest were associated with faster RTs. Interestingly, larger MEPs were associated with higher levels of GABA in M1, but not in three other cortical regions. Together, these results suggest that individuals with more excitable corticospinal pathways are faster to initiate planned responses and have higher levels of GABA within M1, possibly to compensate for a more excitable motor system.SIGNIFICANCE STATEMENT This study brings together physiological, behavioral, and neurochemical evidence to examine variability in the excitability of the human motor system. Previous work has focused on state-based factors (e.g., preparedness, uncertainty), with little attention given to the influence of inherent stable characteristics. Here, we examined how the excitability of the motor system relates to reaction time and the regional content of the inhibitory neurotransmitter GABA. Importantly, motor pathway excitability and GABA concentrations were measured at rest, outside a task context, providing assays of intrinsic properties of the individuals. Individuals with more excitable motor pathways had faster reaction times and, paradoxically, higher concentrations of GABA. We propose that greater GABA capacity in the motor cortex counteracts an intrinsically more excitable motor system.

Kinetics of Inhibitory Feedback from Horizontal Cells to Photoreceptors: Implications for an Ephaptic Mechanism

Inhibitory feedback from horizontal cells (HCs) to cones generates center-surround receptive fields and color opponency in the retina. Mechanisms of HC feedback remain unsettled, but one hypothesis proposes that an ephaptic mechanism may alter the extracellular electrical field surrounding photoreceptor synaptic terminals, thereby altering Ca(2+) channel activity and photoreceptor output. An ephaptic voltage change produced by current flowing through open channels in the HC membrane should occur with no delay. To test for this mechanism, we measured kinetics of inhibitory feedback currents in Ambystoma tigrinum cones and rods evoked by hyperpolarizing steps applied to synaptically coupled HCs. Hyperpolarizing HCs stimulated inward feedback currents in cones that averaged 8-9 pA and exhibited a biexponential time course with time constants averaging 14-17 ms and 120-220 ms. Measurement of feedback-current kinetics was limited by three factors: (1) HC voltage-clamp speed, (2) cone voltage-clamp speed, and (3) kinetics of Ca(2+) channel activation or deactivation in the photoreceptor terminal. These factors totaled ∼4-5 ms in cones meaning that the true fast time constants for HC-to-cone feedback currents were 9-13 ms, slower than expected for ephaptic voltage changes. We also compared speed of feedback to feedforward glutamate release measured at the same cone/HC synapses and found a latency for feedback of 11-14 ms. Inhibitory feedback from HCs to rods was also significantly slower than either measurement kinetics or feedforward release. The finding that inhibitory feedback from HCs to photoreceptors involves a significant delay indicates that it is not due to previously proposed ephaptic mechanisms.

SIGNIFICANCE STATEMENT

Lateral inhibitory feedback from horizontal cells (HCs) to photoreceptors creates center-surround receptive fields and color-opponent interactions. Although underlying mechanisms remain unsettled, a longstanding hypothesis proposes that feedback is due to ephaptic voltage changes that regulate photoreceptor synaptic output by altering Ca(2+) channel activity. Ephaptic processes should occur with no delay. We measured kinetics of inhibitory feedback currents evoked in photoreceptors with voltage steps applied to synaptically coupled HCs and found that feedback is too slow to be explained by ephaptic voltage changes generated by current flowing through continuously open channels in HC membranes. By eliminating the proposed ephaptic mechanism for HC feedback regulation of photoreceptor Ca(2+) channels, our data support earlier proposals that synaptic cleft pH changes are more likely responsible.

Tetramethylenedisulfotetramine: pest control gone awry

Incidences of pesticide poisonings are a significant cause of morbidity and mortality worldwide. The seizure-inducing rodenticide tetramethylenedisulfotetramine is one of the most toxic of these agents. Although banned, it has been responsible for thousands of accidental, intentional, and mass poisonings in mainland China and elsewhere. An optimal regimen for treatment of poisoning has not been established. Its facile synthesis from easily obtained starting materials, extreme potency, and lack of odor, color, or taste make it a potential chemical threat agent. This review describes the toxicologic properties of this agent, more recent advances in our understanding of its properties, and recommendations for future research.

The peroxisome proliferator activated receptor gamma agonist pioglitazone increases functional expression of the glutamate transporter excitatory amino acid transporter 2 (EAAT2) in human glioblastoma cells

Glioma cells release glutamate through expression of system xc-, which exchanges intracellular glutamate for extracellular cysteine. Lack of the excitatory amino acid transporter 2 (EAAT2) expression maintains high extracellular glutamate levels in the glioma microenvironment, causing excitotoxicity to surrounding parenchyma. Not only does this contribute to the survival and proliferation of glioma cells, but is involved in the pathophysiology of tumour-associated epilepsy (TAE). We investigated the role of the peroxisome proliferator activated receptor gamma (PPARγ) agonist pioglitazone in modulating EAAT2 expression in glioma cells. We found that EAAT2 expression was increased in a dose dependent manner in both U87MG and U251MG glioma cells. Extracellular glutamate levels were reduced with the addition of pioglitazone, where statistical significance was reached in both U87MG and U251MG cells at a concentration of ≥ 30 μM pioglitazone (p < 0.05). The PPARγ antagonist GW9662 inhibited the effect of pioglitazone on extracellular glutamate levels, indicating PPARγ dependence. In addition, pioglitazone significantly reduced cell viability of U87MG and U251MG cells at ≥ 30 μM and 100 μM (p < 0.05) respectively. GW9662 also significantly reduced viability of U87MG and U251MG cells with 10 μM and 30 μM (p < 0.05) respectively. The effect on viability was partially dependent on PPARγ activation in U87MG cells but not U251MG cells, whereby PPARγ blockade with GW9662 had a synergistic effect. We conclude that PPARγ agonists may be therapeutically beneficial in the treatment of gliomas and furthermore suggest a novel role for these agents in the treatment of tumour associated seizures through the reduction in extracellular glutamate.

Pituitary Adenylate cyclase-activating polypeptide orchestrates neuronal regulation of the astrocytic glutamate-releasing mechanism system xc (.)

Glutamate signaling is achieved by an elaborate network involving neurons and astrocytes. Hence, it is critical to better understand how neurons and astrocytes interact to coordinate the cellular regulation of glutamate signaling. In these studies, we used rat cortical cell cultures to examine whether neurons or releasable neuronal factors were capable of regulating system xc (-) (Sxc), a glutamate-releasing mechanism that is expressed primarily by astrocytes and has been shown to regulate synaptic transmission. We found that astrocytes cultured with neurons or exposed to neuronal-conditioned media displayed significantly higher levels of Sxc activity. Next, we demonstrated that the pituitary adenylate cyclase-activating polypeptide (PACAP) may be a neuronal factor capable of regulating astrocytes. In support, we found that PACAP expression was restricted to neurons, and that PACAP receptors were expressed in astrocytes. Interestingly, blockade of PACAP receptors in cultures comprised of astrocytes and neurons significantly decreased Sxc activity to the level observed in purified astrocytes, whereas application of PACAP to purified astrocytes increased Sxc activity to the level observed in cultures comprised of neurons and astrocytes. Collectively, these data reveal that neurons coordinate the actions of glutamate-related mechanisms expressed by astrocytes, such as Sxc, a process that likely involves PACAP. A critical gap in modeling excitatory signaling is how distinct components of the glutamate system expressed by neurons and astrocytes are coordinated. In these studies, we found that system xc (-) (Sxc), a glutamate release mechanism expressed by astrocytes, is regulated by releasable neuronal factors including PACAP. This represents a novel form of neuron-astrocyte communication, and highlights the possibility that pathological changes involving astrocytic Sxc may stem from altered neuronal activity.

Field trial of GABA-fortified rice plants and oral administration of milled rice in spontaneously hypertensive rats

Hypertension is one of the most critical risk factors accompanying cardiovascular diseases. γ-Aminobutyric acid (GABA) is a non-protein amino acid that functions as a major neurotransmitter in mammals and also as a blood-pressure lowering agent. We previously produced GABA-fortified rice lines of a popular Japonica rice cultivar 'Koshihikari' by genetic manipulation of GABA shunt-related genes. In the study reported here, we grew these same novel rice lines in a field trial and administered the milled rice orally to rats. The yield parameters of the transgenic rice plants were almost unchanged compared to those of untransformed cv. 'Koshihikari' plants, while the rice grains of the transgenic plants contained a high GABA content (3.5 g GABA/kg brown rice; 0.75-0.85 GABA g/kg milled rice) in a greenhouse trial. Oral administration of a diet containing 2.5% GABA-fortified rice, with a daily intake for 8 weeks, had an approximately 20 mmHg anti-hypertensive effect in spontaneous hypertensive rats but not in normotensive Wistar-Kyoto rats. These results suggest that GABA-fortified rice may be applicable as a staple food to control or prevent hypertension.

Analysis of γ-aminobutyric acid (GABA) type A receptor subtypes using isosteric and allosteric ligands

The GABAA receptors (GABAARs) play an important role in inhibitory transmission in the brain. The GABAARs could be identified using a medicinal chemistry approach to characterize with a series of chemical structural analogues, some identified in nature, some synthesized, to control the structural conformational rigidity/flexibility so as to define the 'receptor-specific' GABA agonist ligand structure. In addition to the isosteric site ligands, these ligand-gated chloride ion channel proteins exhibited modulation by several chemotypes of allosteric ligands, that help define structure and function. The channel blocker picrotoxin identified a noncompetitive channel blocker site in GABAARs. This ligand site is located in the transmembrane channel pore, whereas the GABA agonist site is in the extracellular domain at subunit interfaces, a site useful for low energy coupled conformational changes of the functional channel domain. Also in the trans-membrane domain are allosteric modulatory ligand sites, mostly positive, for diverse chemotypes with general anesthetic efficacy, namely, the volatile and intravenous agents: barbiturates, etomidate, propofol, long-chain alcohols, and neurosteroids. The last are apparent endogenous positive allosteric modulators of GABAARs. These binding sites depend on the GABAAR heteropentameric subunit composition, i.e., subtypes. Two classes of pharmacologically very important allosteric modulatory ligand binding site reside in the extracellular domain at modified agonist sites at other subunit interfaces: the benzodiazepine site, and the low-dose ethanol site. The benzodiazepine site is specific for certain subunit combination subtypes, mainly synaptically localized. In contrast, the low-dose (high affinity) ethanol site(s) is found at a modified benzodiazepine site on different, extrasynaptic, subtypes.

The neurobiology of alcohol consumption and alcoholism: an integrative history

Studies of the neurobiological predisposition to consume alcohol (ethanol) and to transition to uncontrolled drinking behavior (alcoholism), as well as studies of the effects of alcohol on brain function, started a logarithmic growth phase after the repeal of the 18th Amendment to the United States Constitution. Although the early studies were primitive by current technological standards, they clearly demonstrated the effects of alcohol on brain structure and function, and by the end of the 20th century left little doubt that alcoholism is a "disease" of the brain. This review traces the history of developments in the understanding of ethanol's effects on the most prominent inhibitory and excitatory systems of brain (GABA and glutamate neurotransmission). This neurobiological information is integrated with knowledge of ethanol's actions on other neurotransmitter systems to produce an anatomical and functional map of ethanol's properties. Our intent is limited in scope, but is meant to provide context and integration of the actions of ethanol on the major neurobiologic systems which produce reinforcement for alcohol consumption and changes in brain chemistry that lead to addiction. The developmental history of neurobehavioral theories of the transition from alcohol drinking to alcohol addiction is presented and juxtaposed to the neurobiological findings. Depending on one's point of view, we may, at this point in history, know more, or less, than we think we know about the neurobiology of alcoholism.

Metabolic maturation of the human brain from birth through adolescence: insights from in vivo magnetic resonance spectroscopy

Between birth and late adolescence, the human brain undergoes exponential maturational changes. Using in vivo magnetic resonance spectroscopy, we determined the developmental profile for 6 metabolites in 5 distinct brain regions based on spectra from 309 children from 0 to 18 years of age. The concentrations of N-acetyl-aspartate (an indicator for adult-type neurons and axons), creatine (energy metabolite), and glutamate (excitatory neurotransmitter) increased rapidly between birth and 3 months, a period of rapid axonal growth and synapse formation. Myo-inositol, implicated in cell signaling and a precursor of membrane phospholipid, as well as an osmolyte and astrocyte marker, declined rapidly during this period. Choline, a membrane metabolite and indicator for de novo myelin and cell membrane synthesis, peaked from birth until approximately 3 months, and then declined gradually, reaching a plateau at early childhood. Similarly, taurine, involved in neuronal excitability, synaptic potentiation, and osmoregulation, was high until approximately 3 months and thereafter declined. These data indicate that the first 3 months of postnatal life are a critical period of rapid metabolic changes in the development of the human brain. This study of the developmental profiles of the major brain metabolites provides essential baseline information for future analyses of the pediatric health and disease.

Amino acids that centrally influence blood pressure and regional blood flow in conscious rats

Functional roles of amino acids have increasingly become the focus of research. This paper summarizes amino acids that influence cardiovascular system via the brain of conscious rats. This paper firstly describes why amino acids are selected and outlines how the brain regulates blood pressure and regional blood flow. This section includes a concise history of amino acid neurotransmitters in cardiovascular research and summarizes brain areas where chemical stimulations produce blood pressure changes mainly in anesthetized animals. This is followed by comments about findings regarding several newly examined amino acids with intracisternal stimulation in conscious rats that produce changes in blood pressure. The same pressor or depressor response to central amino acid stimulations can be produced by distinct mechanisms at central and peripheral levels, which will be briefly explained. Thereafter, cardiovascular actions of some of amino acids at the mechanism level will be discussed based upon findings of pharmacological and regional blood flow measurements. Several examined amino acids in addition to the established neurotransmitter amino acids appear to differentially activate brain structures to produce changes in blood pressure and regional blood flows. They may have physiological roles in the healthy brain, but pathological roles in the brain with cerebral vascular diseases such as stroke where the blood-brain barrier is broken.

Expression of GABAergic receptors in mouse taste receptor cells

Background

Multiple excitatory neurotransmitters have been identified in the mammalian taste transduction, with few studies focused on inhibitory neurotransmitters. Since the synthetic enzyme glutamate decarboxylase (GAD) for gamma-aminobutyric acid (GABA) is expressed in a subset of mouse taste cells, we hypothesized that other components of the GABA signaling pathway are likely expressed in this system. GABA signaling is initiated by the activation of either ionotropic receptors (GABA_A and GABA_C) or metabotropic receptors (GABA_B) while it is terminated by the re-uptake of GABA through transporters (GATs).

Methodology/Principal Findings

Using reverse transcriptase-PCR (RT-PCR) analysis, we investigated the expression of different GABA signaling molecules in the mouse taste system. Taste receptor cells (TRCs) in the circumvallate papillae express multiple subunits of the GABA_A and GABA_B receptors as well as multiple GATs. Immunocytochemical analyses examined the distribution of the GABA machinery in the circumvallate papillae. Both GABA_A-and GABA_B- immunoreactivity were detected in the peripheral taste receptor cells. We also used transgenic mice that express green fluorescent protein (GFP) in either the Type II taste cells, which can respond to bitter, sweet or umami taste stimuli, or in the Type III GAD67 expressing taste cells. Thus, we were able to identify that GABAergic receptors are expressed in some Type II and Type III taste cells. Mouse GAT4 labeling was concentrated in the cells surrounding the taste buds with a few positively labeled TRCs at the margins of the taste buds.

Conclusions/Significance

The presence of GABAergic receptors localized on Type II and Type III taste cells suggests that GABA is likely modulating evoked taste responses in the mouse taste bud.

Kinetic analysis of evoked IPSCs discloses mechanism of antagonism of synaptic GABAA receptors by picrotoxin

BACKGROUND AND PURPOSE

Although picrotoxin is a well-established antagonist of GABA(A) receptors, detailed studies of its action on inhibitory synaptic transmission have not previously been made.

EXPERIMENTAL APPROACH

Electrophysiological techniques were used to study the action of picrotoxin on inhibitory postsynaptic currents (IPSCs) evoked in hippocampal neurones, in culture and slice preparations prepared from Wistar rat embryos and juveniles, respectively.

KEY RESULTS

Picrotoxin gradually reduced the amplitude of GABA(A) receptor-mediated eIPSCs in a concentration-dependent manner. This was accompanied by a marked acceleration of the eIPSC decay kinetics, which, in contrast to the effect on amplitude, developed immediately and was completely reversed on washing. The decaying phase of the IPSC could be resolved into two components; 30 microM picrotoxin reduced tau(fast) by 34% and increased its relative amplitude, while tau(slow) was reduced by 38%, and its relative amplitude decreased. The area under the decaying phase of the normalized eIPSC showed an immediate reduction by 36% in 30 microM picrotoxin. With increasing concentrations of picrotoxin, this normalized area converged towards 55% of the control, indicating that the rate of relaxation and block has a finite maximum. This implies that picrotoxin does not act by a pore-occluding mechanism (open-channel blocking), and suggests allosteric stabilization of desensitized receptor states as a more likely alternative. This was corroborated by modelling, based on two established microscopic GABA(A) receptor transition schemes.

CONCLUSIONS AND IMPLICATIONS

Although the identity of the stabilized state has not been determined unequivocally, picrotoxin effectively traps synaptic GABA(A) receptors in a desensitized state.

The Effects of gamma-Aminobutyric Acid, Vinegar, and Dried Bonito on Blood Pressure in Normotensive and Mildly or Moderately Hypertensive Volunteers

The purpose of this study was to examine the effects of γ-aminobutyric acid (GABA) in fermented drinking water prepared from sodium glutamate, vinegar, and dried bonito (FDWG) compared with placebo [vinegar and dried bonito without GABA (FDW)] and its safety in normotensive and mildly or moderately hypertensive volunteers. A double-blind, placebo-controlled, randomized study was conducted involving volunteers with normal (group-N) and mildly or moderately high (group-H) blood pressure (BP). After a pretreatment period of 2 weeks (weeks –2), the subjects received FDWG or FDW for 12 weeks followed by 4 weeks of no intake (weeks 16). In group-H, both FDWG and FDW significantly decreased systolic (SBP, −7.6 ± 4.0 and −5.5 ± 1.5 mmHg, p<0.05, respectively) and diastolic (DBP, −10.6 ± 4.0 and −7.6 ± 1.7 mmHg, p<0.01, respectively) BP compared to the baseline (0-week) value at 12 weeks, respectively. There were no abnormal changes in hematological or blood chemistry variables, urinalysis, heart rate, or body weight in the study groups. These findings indicated that vinegar and dried bonito with or without GABA might have an effect on BP in mildly or moderately hypertensive patients.

Seed-specific expression of truncated OsGAD2 produces GABA-enriched rice grains that influence a decrease in blood pressure in spontaneously hypertensive rats

Gamma-aminobutyric acid (GABA) is a four-carbon amino acid that is commonly present in living organisms and functions as a major inhibitory neurotransmitter in mammals. It is understood to have a potentially anti-hypertensive effect in mammals. GABA is synthesized from glutamate by glutamate decarboxylase (GAD). In plants, GAD is regulated via its calmodulin-binding domain (CaMBD) by Ca²⁺/CaM. We have previously reported that a C-terminal truncated version of one of the five rice GAD isoforms, GAD2ΔC, revealed higher enzymatic activity in vitro and that its over-expression resulted in exceptionally high GABA accumulation (Akama and Takaiwa, J Exp Bot 58:2699–2607, 2007). In this study, GAD2ΔC, under the control of the rice glutelin promoter (GluB-1), was introduced into rice cells via Agrobacterium-mediated transformation to produce transgenic rice lines. Analysis of the free amino acid content of rice grains revealed up to about a 30-fold higher level of GABA than in non-transformed rice grains. There were also very high levels of various free protein amino acids in the seeds. GABA-enriched rice grains were milled to a fine powder for oral administration to spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto rats (WKYs). Six weeks of administration showed that transgenic rice brings about a 20 mmHg decrease in blood pressure in two different kinds of SHRs, while there was no significant hypotensive effect in WKYs. These results suggest an alternative way to control and/or cure hypertension in humans with GABA-enriched rice as part of a common daily diet.

Alterations of striatal glutamate transmission in rotenone-treated mice: MRI/MRS in vivo studies

Animal models treated with agricultural chemicals, such as rotenone, reproduce several degenerative features of human central nervous system (CNS) diseases. Glutamate is the most abundant excitatory amino acid transmitter in the mammalian central nervous system and its transmission is implicated in a variety of brain functions including mental behavior and memory. Dysfunction of glutamate neurotransmission in the CNS has been associated with a number of human neurodegenerative diseases, either as a primary or as a secondary factor in the excitotoxic events leading to neuronal death. Since many human CNS disorders do not arise spontaneously in animals, characteristic functional changes have to be mimicked by toxic agents. Candidate environmental toxins bearing any direct or indirect effects on the pathogenesis of human disease are particularly useful. The present longitudinal Magnetic Resonance Imaging (MRI) studies show, for the first time, significant variations in the properties of brain ventricles in a rotenone-treated (2 mg/kg) mouse model over a period of 4 weeks following 3 days of rotenone treatment. Histopathological analysis reveals death of stria terminalis neurons following this short period of rotenone treatment. Furthermore, in vivo voxel localized (1)H MR spectroscopy also shows for the first time significant bio-energetic and metabolic changes as well as temporal alterations in the levels of glutamate in the degenerating striatal region. These studies provide novel insights on the effects of environmental toxins on glutamate and other amino acid neurotransmitters in human neurodegenerative diseases.

The glutamate story

Glutamatergic synaptic transmission in the mammalian central nervous system was slowly established over a period of some 20 years, dating from the 1950s. Realisation that glutamate and like amino acids (collectively known as excitatory amino acids (EAA)) mediated their excitatory actions via multiple receptors preceded establishment of these receptors as synaptic transmitter receptors. EAA receptors were initially classified as N-methyl-D-aspartate (NMDA) and non-NMDA receptors, the latter subdivided into quisqualate (later AMPA) and kainate receptors after agonists that appeared to activate these receptors preferentially, and by their sensitivity to a range of differentially acting antagonists developed progressively during the 1970s. NMDA receptors were definitively shown to be synaptic receptors on spinal neurones by the sensitivity of certain excitatory pathways in the spinal cord to a range of specific NMDA receptor antagonists. Importantly, specific NMDA receptor antagonists appeared to be less effective at synapses in higher centres. In contrast, antagonists that also blocked non-NMDA as well as NMDA receptors were almost universally effective at blocking synaptic excitation within the brain and spinal cord, establishing both the existence and ubiquity of non-NMDA synaptic receptor systems throughout the CNS. In the early 1980s, NMDA receptors were shown to be involved in several central synaptic pathways, acting in concert with non-NMDA receptors under conditions where a protracted excitatory postsynaptic potential was effected in response to intense stimulation of presynaptic fibres. Such activation of NMDA receptors together with non-NMDA receptors led to the phenomenon of long-term potentiation (LTP), associated with lasting changes in synaptic efficacy (synaptic plasticity) and considered to be an important process in memory and learning. During the 1980s, it was shown that certain glutamate receptors in the brain mediated biochemical changes that were not susceptible to NMDA or non-NMDA receptor antagonists. This dichotomy was resolved in the early 1990s by the techniques of molecular biology, which identified two families of glutamate-binding receptor proteins (ionotropic (iGlu) and metabotropic (mGlu) receptors). Development of antagonists binding to specific protein subunits is currently enabling precise identification of discrete iGlu or mGlu receptor subtypes that participate in a range of central synaptic processes, including synaptic plasticity.

Electrogenic glutamate transporters in the CNS: molecular mechanism, pre-steady-state kinetics, and their impact on synaptic signaling

Glutamate is the major excitatory neurotransmitter in the mammalian CNS. The spatiotemporal profile of the glutamate concentration in the synapse is critical for excitatory synaptic signalling. The control of this spatiotemporal concentration profile requires the presence of large numbers of synaptically localized glutamate transporters that remove pre-synaptically released glutamate by uptake into neurons and adjacent glia cells. These glutamate transporters are electrogenic and utilize energy stored in the transmembrane potential and the Na+/K+-ion concentration gradients to accumulate glutamate in the cell. This review focuses on the kinetic and electrogenic properties of glutamate transporters, as well as on the molecular mechanism of transport. Recent results are discussed that demonstrate the multistep nature of the transporter reaction cycle. Results from pre-steady-state kinetic experiments suggest that at least four of the individual transporter reaction steps are electrogenic, including reactions associated with the glutamate-dependent transporter halfcycle. Furthermore, the kinetic similarities and differences between some of the glutamate transporter subtypes and splice variants are discussed. A molecular mechanism of glutamate transport is presented that accounts for most of the available kinetic data. Finally, we discuss how synaptic glutamate transporters impact on glutamate receptor activity and how transporters may shape excitatory synaptic transmission.

Pharmacological and functional characterization of astrocytic GABA transport: a short review

GABA neurotransmission is terminated by high affinity transport mediated by a number of carriers on neurons and astrocytes. So far four different carriers have been cloned and their cellular distribution has been partly worked out. It is generally believed that GAT-1 (mouse homologue GAT1) is the quantitatively most important of the transporters and it is primarily present on GABAergic neurons but also to some extent on astrocytes. The pharmacological properties of neuronal and astrocytic GABA uptake have been studied extensively and recently the GABA analogue N-methyl-Exo-THPO has been reported to act as a selective and potent (IC50 28 microM) astroglial GABA transport inhibitor with a 15-fold selectivity. It has moreover been reported to act as an anticonvulsant in animal models of epilepsy. This may underline the functional importance of astrocytic GABA uptake in relation to seizure activity.

GABA(B) receptors inhibit mechanosensitivity of primary afferent endings

The modulatory effects of baclofen on the sensitivity of peripheral afferent endings to mechanical stimulation were investigated using an in vitro ferret gastroesophageal vagal afferent preparation. Changes in sensitivity of three types of gastroesophageal vagal afferent endings previously categorized as mucosal, tension, and tension-mucosal (TM) receptors according to their mechanoreceptive field characteristics were investigated. Baclofen (30-200 microM) dose dependently reduced responses of mucosal afferents to mucosal stroking with calibrated von Frey hairs (10-1000 mg). This was reversed by the GABA(B) receptor antagonist SCH50911 (1 microM). TM afferent responses to mucosal stroking (10-1000 mg) were unaffected by baclofen (30-200 microM). However, baclofen (30-200 microM) significantly inhibited the response of 11 of 18 TM afferents to circumferential tension. This was reversed by SCH50911 (1 microM). Baclofen (100 and 200 microM) significantly inhibited the response of all tension receptor afferents to circumferential tension in the lower range (1-3 gm) but not in the higher range (4-7 gm). This inhibition was reversed by SCH50911 (1 microM; n = 3). This study provides the first direct evidence for the inhibitory modulation of peripheral mechanosensory endings by the G-protein-coupled GABA(B) receptor. Inhibition was dose-dependent, pharmacologically reversible, and selective to certain aspects of mechanosensitivity. These findings have important relevance to strategies for selective reduction of sensory input to the CNS at a peripheral site.

A reluctant gating mode of glycine receptor channels determines the time course of inhibitory miniature synaptic events in zebrafish hindbrain neurons

Miniature IPSCs (mIPSCs) recorded in the Mauthner (M)-cell of zebrafish larvae have a broad amplitude distribution that is attributable only partly to the functional heterogeneity of postsynaptic glycine receptors (GlyRs). The role of the kinetic properties of GlyRs in amplitude fluctuation was investigated using fast-flow application techniques on outside-out patches. Short applications of a saturating glycine concentration evoked outside-out currents with a biphasic deactivation phase as observed for mIPSCs, and they were consistent with a rapid clearance of glycine from the synaptic cleft. Patch currents declined slowly during continuous applications of 3 mM glycine, but the biphasic deactivation phase of mIPSCs cannot reflect a desensitization process because paired-pulse desensitization was not observed. The maximum open probability (Po) of GlyRs was close to 0.9 with 3 mM glycine. Analyses of the onset of outside-out currents evoked by 0.1 mM glycine are consistent with the presence of two equivalent binding sites with a Kd of O.3-O.4 mM. Activation and deactivation properties of GlyRs were better described with a kinetic model, including two binding states, a doubly liganded open state, and a reluctant gating mode leading to another open state. The 20-80% rise time of mIPSCs was independent of their amplitude and is identical to that of outside-out currents evoked by the applications of a saturating concentration of glycine (>1 mM). These results support the hypothesis that GlyR kinetics determines the time course of synaptic events at M-cell inhibitory synapses and that large mIPSC amplitude fluctuations are mainly of postsynaptic origin.

Glutamate transporter EAAC-1-deficient mice develop dicarboxylic aminoaciduria and behavioral abnormalities but no neurodegeneration

Four L-glutamate neurotransmitter transporters, the three Na(+)-dependent GLAST-1, GLT-1 and EAAC-1, and the Cl(-)-dependent EAAT-4, form a new family of structurally related integral plasma membrane proteins with different distribution in the central nervous system. They may have pivotal functions in the regulation of synaptic L-glutamate concentration during neurotransmission and are believed to prevent glutamate neurotoxicity. To investigate the specific physiological and pathophysiological role of the neuronal EAAC-1, which is also expressed in kidney and small intestine, we have generated two independent mouse lines lacking EAAC-1. eaac-1(-/-) mice develop dicarboxylic aminoaciduria. No neurodegeneration has been observed during a period of >12 months, but homozygous mutants display a significantly reduced spontaneous locomotor activity.

The rapid L- and D-aspartate uptake in cultured astrocytes

Astrocytes in primary culture possess a rapid L-aspartate saturable transport system (K(m) = 93 microM; V(max) = 81 nmol/min/mg protein), which shows certain stereospecificity since V(max) was 36% lower for D-aspartate uptake. These are values obtained at short incubation time (15 seconds), to obtain approximate initial rate conditions. Metabolic energy inhibitors, rotenone and iodoacetate very potently inhibited the L- and D-aspartate uptake processes, indicating that the transport process is an active one. However, the accumulation of L-aspartate was "enhanced" by inhibitors of L-aspartate metabolism, such as the aspartate aminotransferase inhibitor, aminooxyacetate and L-methionine sulfoximine, an inhibitor of glutamine synthetase, whereas D-aspartate (a non-metabolizable analog of L-aspartate) uptake was not affected. The accumulated levels of L-aspartate in the presence of aminooxyacetate were similar to the levels of D-aspartate. These effects of L-aspartate metabolic inhibitors, suggest that due to metabolism of the the L-aspartate, short incubation time (eg., 15 seconds) is necessary to measure the initial rate of L-aspartate uptake, in order to obtain the "true" kinetic parameters.

Characteristics of GABAB receptor binding sites on rat whole brain synaptic membranes. 1983

Saturable binding of (±)-[³H]-baclofen and [³H]-γ-aminobutyric acid ([³H]-GABA) to rat brain crude synaptic membranes has been examined by means of a centrifugation assay.

The binding of [³H]-baclofen could be detected in fresh or previously frozen tissue and was dependent on the presence of physiological concentrations of Ca²⁺ or Mg²⁺ although a lower affinity Na⁺-dependent component could also be observed. Both components probably reflect binding to receptor recognition sites.

The saturable portion of bound [³H]-baclofen formed 20.3 ± 6.9% of total bound ligand. This could be displaced by GABA (IC₅₀ = 0.04 μm), (–)-baclofen (0.04 μm) and to a much lesser extent by (+)-baclofen (33 μm). Isoguvacine, piperidine-4-sulphonic acid and bicuculline methobromide were inactive (up to 100 μm) and muscimol was only weakly active (IC₅₀ = 12.3 μm).

Saturable binding of [³H]-GABA increased on adding CaCl₂ or MgSO₄ (up to 2.5 Mm and 5.0 Mm respectively) to the Tris-HCl incubation solution. This binding (GABA_B site binding) was additional to the bicuculline-sensitive binding of GABA (GABA_A site binding) and could be completely displaced by (–)-baclofen (IC₅₀ = 0.13 μm).

Increasing the Ca²⁺ concentration (0 to 2.5 Mm) increased the binding capacity of the membranes without changing their affinity for the ligand.

The binding of [³H]-GABA to GABA_B sites could be demonstrated in fresh as well as previously frozen membranes with a doubling of the affinity being produced by freezing. Further incubation with the non-ionic detergent Triton-X-100 (0.05% v/v) reduced the binding capacity by 50%.

The pharmacological profile of displacers of [³H]-GABA from GABA_B sites correlated well with that for [³H]-baclofen displacement. A correlation with data previously obtained in isolated preparations of rat atria and mouse vas deferens was also apparent.

It is concluded that [³H]-baclofen or [³H]-GABA are both ligands for the same bicuculline-insensitive, divalent cation-dependent binding sites in the rat brain.

Glycine induces two distinct membrane currents in neonatal rat sympathetic preganglionic neurones in vitro

1. The effects of glycine (Gly) on neonatal (12- to 16-day-old) rat sympathetic preganglionic neurones (SPNs) in transverse (500 microns) thoracolumbar spinal cord slices were studied by whole-cell patch-clamp techniques. 2. Gly elicited three types of membrane currents when applied to SPNs by pressure ejection (100 mM; 20-180 ms pulse duration): (1) an outward current (20/94 cells); (2) an inward current (30/94); and (3) a biphasic response (44/94) consisting of an outward followed by an inward current. 3. The Gly-induced outward current (IGly,(out)) had a mean reversal potential of -67 mV, was reversed in a low (5.7 mM) chloride Krebs solution, and was reversibly eliminated by strychnine (0.1-1 microM). 4. The Gly-induced inward current (IGly,(in)) had a mean reversal potential of -41 mV, was reduced in a Na(+)-free and increased in a high (15 mM) K+ solution; strychnine at the high concentration of 1 microM reduced the response by an average of 53%. 5. The electrophysiological and pharmacological characteristics of the biphasic response suggest that it was a combined response of outward and inward currents. 6. The results show that Gly elicits two distinct membrane currents, a Cl(-)-dependent outward current and a cationic inward current, which are mediated by strychnine-sensitive and strychnine-resistant Gly receptors. Functionally, activation of both types of Gly receptors reduces neuronal excitability and attenuates synaptic transmission.

Characteristics and regulation of proline transport in cultured glioblastoma cells

L-Proline transport in C6 glioblastoma cells takes place mainly via a saturable Na(+)-dependent mechanism. The uptake process can be discriminated into two components, system A and system ASC. A minor proportion of L-proline transport is carried out by the ASC system, which appears to be constitutively expressed by the cell, but most is by system A which shows adaptive responses to amino acid deprivation and sensitivity to N-methyl-alpha-aminoisobutyric acid. The transport system is inhibited by proline derivatives, such as methyl and benzyl esters, and also hydroxyproline, and is stereospecific. Incubation of glioblastoma cells with phorbol 12-myristate 13-acetate led to concentration- and time-dependent decreases in L-proline transport. This effect could be mimicked by exogenous phospholipase C. Proline transport is significantly stimulated in the presence of Ca(2+)-mobilization agents and strongly inhibited in the absence of Ca2+. The present data suggest a complex regulation of L-proline transport by different kinases in glioblastoma cells.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Maintenance of cell volume in the central nervous system

Maintenance of the ionic and osmotic composition and volume of intra- and extracellular fluids in the brain is crucial for normal functioning of the central nervous system (CNS). Osmoregulation in the CNS is mediated by solute and water transport across the blood-brain barrier, choroid plexus and plasma membrane of glial cells and neurons. Despite its clinical and physiological significance, however, little is known about the underlying cellular and molecular mechanisms by which CNS osmotic and ionic balance is maintained. In this review, I will discuss our current understanding of cell volume regulation in the CNS and how it relates to various disease processes, such as hyponatremia, renal failure and hypernatremia. A detailed understanding of brain osmoregulatory processes represents a fundamental physiological problem and is required for the treatment of numerous disease states, particularly those encountered in the practice of nephrology.

The minimal inhibitory synaptic currents evoked in neonatal rat motoneurones

1. Tight-seal whole-cell recordings were made from lumbar motoneurones visually identified in thin slices of neonatal rat spinal cord. The inhibitory postsynaptic currents (IPSCs) were evoked by extracellular stimulation of a neighbouring internuncial neurone in the presence of glutamate receptor antagonists. 2. Glycinergic IPSCs were recorded in the presence of bicuculline. The IPSCs appeared in an all-or-none manner as the graded stimulus intensity exceeded a certain threshold. Their latencies showed a unimodal distribution with a mean of 0.81 ms at 37 degrees C. Thus, the observed IPSCs are suggested to be monosynaptically evoked unitary IPSCs. The mean conductance of unitary IPSCs was 2.9 +/- 1.2 nS (+/- S.D.). 3. When the external Ca2+ concentration ([Ca2+]o) was reduced, the number of failures in response to stimulation increased, thereby reducing the mean amplitude of IPSCs. The mean amplitude of IPSCs was linearly related to the [Ca2+]o (0.35-1.4 mM) with a mean slope of 3.1 +/- 0.67 on double logarithmic co-ordinates. 4. The amplitude of individual IPSCs decreased with decrease in [Ca2+]o. However, below 0.7 mM [Ca2+]o, the mean amplitude of IPSCs (excluding failures) reached a steady minimum level. The mean conductance of these IPSCs measured in 0.5 mM [Ca2+]o was 657 +/- 281 pS. 5. The minimal IPSCs had a coefficient of variation of 0.50 +/- 0.13. No clear correlation was observed between the rise time and the amplitude of minimal IPSCs evoked in individual motoneurones, indicating that the amplitude variability is not due to the different synaptic locations. 6. Spontaneous miniature IPSCs were recorded from motoneurones in the presence of tetrodotoxin. The miniature IPSCs had a mean conductance of 739 +/- 278 pS, being comparable to the minimal evoked IPSCs. 7. Under various internal and external Cl- concentration, the reversal potential of the IPSCs (EIPSC) approximately coincided with the Cl- equilibrium potential. A 730-fold change in the potassium concentration gradient across the membrane did not affect the EIPSC. The permeability ratio of K+ to Cl- (Pk/PCl) was less than 0.05. 8. It is concluded that the IPSCs are carried almost exclusively by Cl- and that the minimal evoked IPSCs represent the quantal response of the transmitter.

On the inhibitory actions of baclofen and gamma-aminobutyric acid in rat ventral midbrain culture

1. Whole-cell voltage-clamp recordings were used to study the effects of (-)-baclofen and of gamma-aminobutyric acid (GABA) on neurones cultured from the ventral midbrain of embryonic rats. 2. Baclofen induced an outward current (IBac) at a holding potential of -60 mV. The maximal current was 80 pA, and half-maximal current was evoked by 5 microM-baclofen. The proportion of cells affected by baclofen was greater in 25-day-old cultures than in 14-day-old cultures. 3. IBac was blocked by barium (1 mM), and it reversed polarity at a potential that changed according to the Nernst equation when the extracellular potassium concentration was changed. The reversal potential was not different when recording electrodes contained caesium instead of potassium. 4. GABA (10-20 microM), in the presence of picrotoxin (50 microM) and bicuculline (50 microM), also evoked a small potassium current at -60 mV. There was no correlation between the amplitude of the potassium current caused by GABA and that caused by baclofen measured in the same neurones. 5. Spontaneous synaptic currents (up to hundreds of picoamps) were observed that were blocked by picrotoxin (20 microM; IPSCs) or by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM; EPSCs); the amplitude and frequency were strongly reduced by baclofen and by GABA. 6. Spontaneous synaptic currents of lower amplitudes (up to 60 pA) remained in the presence of tetrodotoxin. IPSCs (blocked by picrotoxin, reversal at -50 mV) and EPSCs (blocked by CNQX, reversal at 0 mV) were reduced in frequency by baclofen. GABA, in the presence of bicuculline and picrotoxin, had a similar effect on the EPSCs. This action of baclofen persisted in barium (1 mM), and was observed as readily in cells cultured for 14 days as those cultured for 25 days. 7. Some spontaneous synaptic currents remained in the presence of tetrodotoxin and cadmium (100 microM). Their frequency was reduced by baclofen. The effectiveness of baclofen was greater on cells that had been longer in culture. 8. It is concluded that activation of GABAB receptors has two main effects on neurones cultured from rat ventral midbrain. These are potassium conductance increase, and inhibition of the spontaneous release of GABA and excitatory amino acids; both effects can be observed in tetrodotoxin and cadmium.

GABAB receptor modulation of the release of substance P from capsaicin-sensitive neurones in the rat trachea in vitro

1. The role of gamma-aminobutyric acid (GABA) as an inhibitory transmitter in the central nervous system is well documented. Recently, GABAA and GABAB receptors have been identified in the peripheral nervous system, notably on primary afferent neurones (PAN). We have utilised a multi-superfusion system to investigate the effect of selective GABA receptor agonists and antagonists on the release of substance P (SP) from the rat trachea in vitro. 2. GABA (1-100 microM) did not affect spontaneous release of SP-like immunoreactivity (LI) but caused dose-related inhibition of calcium-dependent potassium (60 mM)-stimulated SP-LI release. The greatest inhibition of 77.7 +/- 18.8% was observed at 100 microM. 3. The inhibitory effect of GABA was mimicked by the GABAB receptor agonist, (+/-)-baclofen (1-100 microM), but not the GABAA receptor agonist, 3-amino-1-propane-sulphonic acid (3-APS, 1-100 microM). Baclofen (100 microM) had no effect on SP-LI release stimulated by capsaicin (1 microM). 4. The inhibitory effect of baclofen (30 microM) was significantly reduced by prior and concomitant exposure to the GABAB receptor antagonist, phacolofen (100 microM) but not the GABAA receptor antagonist, bicuculline (10 microM). Neither antagonist, alone, affected spontaneous or potassium-stimulated SP-LI release. 5. We conclude that activation of pre-synaptic GABAB receptors on the peripheral termini of PANs in the rat trachea inhibits SP-LI release and suggest that GABAB receptor agonists may be of value in the therapeutic treatment of asthma.

Light and electron microscopic localization of GABAA-receptors on cultured cerebellar granule cells and astrocytes using immunohistochemical techniques

GABAA-receptors were localized in explant cultures of rat cerebellum and in dissociated primary cultures of rat cerebellar granule cells and rat cerebellar astrocytes using the monoclonal antibody bd-17 directed against the beta-subunit of the GABAA/benzodiazepine/chloride channel complex. At the light microscope level specific staining of GABAA-receptors was localized in various types of neurones in explant cultures of rat cerebellum using the indirect peroxidase-antiperoxidase (PAP) technique, whereas no specific staining was found in astrocytes. At the electron microscope level labeling of GABAA-receptors was observed in the plasma membrane of both the cell bodies and processes in dissociated primary cultures of cerebellar granule cells using an indirect preembedding immunogold staining technique which in contrast to the classical PAP technique allows quantitative estimations to be performed. Quantification of the labeling intensity revealed a higher concentration of GABAA-receptors per microns plasma membrane in the cell bodies than in the processes. In discrete areas an extremely high density of the GABAA-receptors was observed. No specific labeling of GABAA-receptors was observed in dissociated primary cultures of cerebellar astrocytes.

Microionophoretic study with milacemide, a glycine precursor, on mammalian central nervous system cells

1. The effect of milacemide, a glycine percursor known to increase gamma-aminobutyric acid (GABA) and glycine content in the brain, and to have anticonvulsant properties, was tested by ionophoresis on 247 neurones situated in the cerebral cortex and in deeper structures of cats and rats anaesthetized with urethane. 2. Virtually all the neurones, either firing spontaneously or exogenously driven by the excitatory amino acids, glutamate, N-methyl-D-aspartate (NMDA), kainate and quisqualate or by acetylcholine, were reversibly depressed in a dose-dependent fashion. The same depressant effect was observed in animals pretreated with the monoamine oxidase B inhibitor (IMAO-B) deprenyl which is known to reduce milacemide metabolism into glycinamide and glycine. Intravenous administration of milacemide (10 to 100 mg kg-1) also depressed the firing induced by glutamate, NMDA and acetylcholine. 3. When compared to GABA, milacemide was a weaker depressant. However, its effect could still be observed in the presence of the reversible GABAA antagonist, SR 95531, and thus milacemide is unlikely to act through GABA receptors. In addition, on cells unaffected by glycine, milacemide also had a depressant effect, and on cells inhibited by glycine, it was still capable of depressing cell firing during reversible blockade by strychnine of the glycine inhibitory action; thus milacemide is unlikely to act through glycine receptors. Simultaneous release of milacemide and GABA or of milacemide and glycine, did not show potentiation of the inhibitory amino acid action. However, the depressant effect of milacemide was additive with that of GABA and glycine. 4. No consistent depression of glutamate-induced firing was obtained by ionophoresis of glycinamide, the first metabolite of milacemide. 5. It is concluded that milacemide by itself is a depressant agent and that its depressant effect does not necessarily require its metabolism into glycine, or its stimulator effect on the production of GABA.

Immunocytochemical localization of mitochondrial and cytosolic aspartate aminotransferase isozymes in the vestibular end-organs of rats

The localization of mitochondrial (m-) and cytosolic (c-) aspartate aminotransferase (AAT) was examined in the vestibular ganglion neurons and sensory cells in the vestibular end-organs of rats by an indirect immunohistochemical method using antibodies specific for m- and c-AAT. Neurons in the vestibular ganglion were stained by both m- and c-AAT antibodies, but the vestibular sensory cells exhibited only m-AAT-like immunoreactivity and were not labeled by c-AAT. These findings suggested that aspartate is a neurotransmitter in the hair cells of the vestibular end-organs.