163,644 research outputs found
Synaptic GABA release prevents GABA transporter type-1 reversal during excessive network activity.
No full textGABA transporters control extracellular GABA, which regulates the key aspects of neuronal and network behaviour. A prevailing view is that modest neuronal depolarization results in GABA transporter type-1 (GAT-1) reversal causing non-vesicular GABA release into the extracellular space during intense network activity. This has important implications for GABA uptake-targeting therapies. Here we combined a realistic kinetic model of GAT-1 with experimental measurements of tonic GABAA receptor currents in ex vivo hippocampal slices to examine GAT-1 operation under varying network conditions. Our simulations predict that synaptic GABA release during network activity robustly prevents GAT-1 reversal. We test this in the 0 Mg(2+) model of epileptiform discharges using slices from healthy and chronically epileptic rats and find that epileptiform activity is associated with increased synaptic GABA release and is not accompanied by GAT-1 reversal. We conclude that sustained efflux of GABA through GAT-1 is unlikely to occur during physiological or pathological network activity
Evidence for a GABAergic system in rodent and human testis: Local GABA production and GABA receptors
No full textThe major neurotransmitter of the central nervous system, gamma-aminobutyric acid (GABA), exerts its actions through GABA(A), GABA(B) and GABA(C) receptors. GABA and GABA receptors are, however, also present in several non-neural tissues, including the endocrine organs pituitary, pancreas and testis. In the case of the rat testis, GABA appears to be linked to the regulation of steroid synthesis by Leydig cells via GABA(A) receptors, but neither testicular sources of GABA, nor the precise nature of testicular GABA receptors are fully known. We examined these points in rat, mouse, hamster and human testicular samples. RT-PCR followed by sequencing showed that the GABA-synthesizing enzymes glutamate decarboxylase (GAD) 65 and/or GAD67, as well as the vesicular GABA transporter vesicular inhibitory amino acid transporter (VIAAT/VGAT) are expressed. Testicular GAD in the rat was shown to be functionally active by using a GAD assay, and Western blot analysis confirmed the presence of GAD65 and GAD67. Interstitial cells, most of which are Leydig cells according to their location and morphological characteristics, showed positive immunoreaction for GAD and VIAAT/VGAT proteins. In addition, several GABA(A) receptor subunits (alpha1-3, beta1-3, gamma1-3), as well as GABAB receptor subunits R1 and R2, were detected by RT-PCR. Western blot analysis confirmed the results for GABA(A) receptor subunits beta2/3 in the rat, and immunohistochemistry identified interstitial Leydig cells to possess immunoreactive GABA(A) receptor subunits beta2/3 and alpha1. The presence of GABA(A) receptor subunit alpha1 mRNA in interstitial cells of the rat testis was further shown after laser microdissection followed by RT-PCR analysis. In summary, these results describe molecular details of the components of an intratesticular GABAergic system expressed in the endocrine compartment of rodent and human testes. While the physiological significance of this peripheral neuroendocrine system conserved throughout species remains to be elucidated, its mere presence in humans suggests the possibility that clinically used drugs might be able to interfere with testicular function. Copyright (C) 2003 S. Karger AG, Basel
An autocrine role for pituitary GABA: Activation of GABA-B receptors and regulation of growth hormone levels
No full textThere is increasing evidence suggesting that the neurotransmitter gamma-aminobutyric acid (GABA) is a local factor involved in the regulation of endocrine organs. Examples of such functions are documented in the pancreas, but recent results suggest that GABA may act in a similar way in the pituitary, in which GABA receptors are expressed and pituitary growth hormone (GH) cells provide a source of GABA. We hypothesised that GABA secreted in somatotropes may act as an autoregulatory signaling molecule. To test this hypothesis we first examined the nature of GABA receptors expressed by GH cells. RT-PCR analysis demonstrated that GABA-B receptor subunits R1 and R2 are present in the whole rat pituitary. Laser microdissection of immunostained GH cells, followed by RT-PCR as well as immunoelectron microscopy, showed that GABA-B receptors are expressed on somatotropes. To investigate GABA-B receptor function in somatotropes, we used rat GH3 adenoma cells, which, like pituitary GH cells, express GABA-B R1 and R2 (as assessed by RT-PCR and immunoelectron microscopy) and produce GABA (checked by high performance liquid chromatography). After inhibition of endogenous GABA synthesis, GH production was stimulated by baclofen, a chromatography). After inhibition of endogenous GABA synthesis, GH production was stimulated by bactofen, a GABA-B receptor agonist. By contrast, blocking GABA-B receptors by an antagonist, phaclofen, decreased GH levels. We conclude that in GH-producing cells, GABA acts as an autocrine factor via GABA-B receptors to control GH levels. Copyright (C) 2002 S. KargerAG, Basel
GABA increases electrical excitability in a subset of human unmyelinated peripheral axons
No full textA proportion of small diameter primary sensory neurones innervating human skin are chemosensitive. They respond in a receptor dependent manner to chemical mediators of inflammation as well as naturally occurring algogens, thermogens and pruritogens. The neurotransmitter GABA is interesting in this respect because in animal models of neuropathic pain GABA pre-synaptically regulates nociceptive input to the spinal cord. However, the effect of GABA on human peripheral unmyelinated axons has not been established
Actions of Agonists, Fipronil and Ivermectin on the Predominant In Vivo Splice and Edit Variant (RDLbd, I/V) of the Drosophila GABA Receptor Expressed in Xenopus laevis Oocytes
No full textIonotropic GABA receptors are the targets for several classes of insecticides. One of the most widely-studied insect GABA receptors is RDL (resistance to dieldrin), originally isolated from Drosophila melanogaster. RDL undergoes alternative splicing and RNA editing, which influence the potency of GABA. Most work has focussed on minority isoforms. Here, we report the first characterisation of the predominant native splice variant and RNA edit, combining functional characterisation with molecular modelling of the agonist-binding region. The relative order of agonist potency is GABA> muscimol> TACA> β-alanine. The I/V edit does not alter the potency of GABA compared to RDLbd. Docking calculations suggest that these agonists bind and activate RDLbdI/V through a similar binding mode. TACA and β-alanine are predicted to bind with lower affinity than GABA, potentially explaining their lower potency, whereas the lower potency of muscimol and isoguvacine cannot be explained structurally from the docking calculations. The A301S (resistance to dieldrin) mutation reduced the potency of antagonists picrotoxin, fipronil and pyrafluprole but the I/V edit had no measurable effect. Ivermectin suppressed responses to GABA of RDLbdI/V, RDLbd and RDLbdI/VA301S. The dieldrin resistant variant also showed reduced sensitivity to Ivermectin. This study of a highly abundant insect GABA receptor isoform will help the design of new insecticides
GABA-Independent GABAA Receptor Openings Maintain Tonic Currents
No full textActivation of GABAA receptors (GABAARs) produces two forms of inhibition: ‘phasic’ inhibition generated by the rapid, transient activation of synaptic GABAARs by presynaptic GABA release, and tonic inhibition generated by the persistent activation of peri- or extrasynaptic GABAARs which can detect extracellular GABA. Such tonic GABAAR-mediated currents are particularly evident in dentate granule cells in which they play a major role in regulating cell excitability. Here we show that in rat dentate granule cells in ex-vivo hippocampal slices, tonic currents are predominantly generated by GABA-independent GABAA receptor openings. This tonic GABAAR conductance is resistant to the competitive GABAAR antagonist SR95531, which at high concentrations acts as a partial agonist, but can be blocked by an open channel blocker picrotoxin. When slices are perfused with 200 nM GABA, a concentration that is comparable to cerebrospinal fluid concentrations but is twice that measured by us in the hippocampus in vivo using zero-net-flux microdialysis, negligible GABA is detected by dentate granule cells. Spontaneously opening GABAARs, therefore, maintain dentate granule cell tonic currents in the face of low extracellular GABA concentrations
Effects of ethylenediamine – a putative GABA-releasing agent – on rat hippocampal slices and neocortical activity in vivo
No full textThe simple diamine diaminoethane (ethylenediamine, EDA) has been shown to activate GABA receptors in the central and peripheral nervous systems, partly by a direct action and partly by releasing endogenous GABA. These effects have been shown to be produced by the complexation of EDA with bicarbonate to form a carbamate. The present work has compared EDA, GABA and [beta]-alanine responses in rat CA1 neurons using extracellular and intracellular recordings, as well as neocortical evoked potentials in vivo. Superfusion of GABA onto hippocampal slices produced depolarisation and a decrease of field epsps, both effects fading rapidly, but showing sensitivity to blockade by bicuculline. EDA produced an initial hyperpolarisation and increase of extracellular field epsp size with no fade and only partial sensitivity to bicuculline, with subsequent depolarisation, while [beta]-alanine produces a much larger underlying hyperpolarisation and increase in fepsps, followed by depolarisation and inhibition of fepsps. The responses to [beta]-alanine, but not GABA or EDA, were blocked by strychnine. In vivo experiments, recording somatosensory evoked potentials, confirmed that EDA produced an initial increase followed by depression, and that this effect was not fully blocked by bicuculline. Overall the results indicate that EDA has actions in addition to the activation of GABA receptors. These actions are not attributable to activation of [beta]-alanine-sensitive glycine receptors, but may involve the activation of sites sensitive to adipic acid, which is structurally equivalent to the dicarbamate of EDA. The results emphasise the complex pharmacology of simple amines in bicarbonate-containing solution
Inhibition of activity of GABA transporter GAT1 by δ-opioid receptor
No full textAnalgesia is a well-documented effect of acupuncture. A critical role in pain sensation plays the nervous system, including the GABAergic system and opioid receptor (OR) activation. Here we investigated regulation of GABA transporter GAT1 by δOR in rats and in Xenopus oocytes. Synaptosomes of brain from rats chronically exposed to opiates exhibited reduced GABA uptake, indicating that GABA transport might be regulated by opioid receptors. For further investigation we have expressed GAT1 of mouse brain together with mouse δOR and μOR in Xenopus oocytes. The function of GAT1 was analyzed in terms of Na(+)-dependent [(3)H]GABA uptake as well as GAT1-mediated currents. Coexpression of δOR led to reduced number of fully functional GAT1 transporters, reduced substrate translocation, and GAT1-mediated current. Activation of δOR further reduced the rate of GABA uptake as well as GAT1-mediated current. Coexpression of μOR, as well as μOR activation, affected neither the number of transporters, nor rate of GABA uptake, nor GAT1-mediated current. Inhibition of GAT1-mediated current by activation of δOR was confirmed in whole-cell patch-clamp experiments on rat brain slices of periaqueductal gray. We conclude that inhibition of GAT1 function will strengthen the inhibitory action of the GABAergic system and hence may contribute to acupuncture-induced analgesia
Regulation of Arabidopsis 14-3-3 gene expression by GABA.
No full textThe function in plants of the non-protein amino acid, gamma-aminobutyric acid (GABA) is poorly understood. In this study, we show that GABA down-regulates the expression of a large sub-set of 14-3-3 gene family members in Arabidopsis thaliana seedlings in a calcium, ethylene and abscisic acid-dependent manner. Gene expression is not affected when seedlings are supplied with glutamate, a precursor of GABA. The repression of 14-3-3 gene expression by GABA is dependent on functional ethylene and abscisic acid signalling pathways, since the response is lost in the etr1-1, abi1-1 and abi2-1 mutants. Calcium measurements show that in contrast to glutamate, GABA does not elicit a cytoplasmic calcium elevation, suggesting that the GABA response is unlikely to be mediated by glutamate receptors, as has been suggested previously. We suggest that in addition to its role as a stress-related metabolite, GABA may regulate gene expression in Arabidopsis, including members of the 14-3-3 gene family
Does GABA act as a signal in plants? : hints from molecular studies.
No full textGABA is a non-protein amino acid that accumulates rapidly in plant tissues in response to biotic and abiotic stress. There have been a number of suggestions as to the role that GABA might play in plants, ranging from a straightforward involvement in N metabolism to a signal mediating plant-animal and plant-microbe interactions. It has also been several proposed that it might function as an intracellular signalling molecule in plants. Here, we discuss recent evidence that plant cells respond at the molecular level to the presence of applied GABA. We argue that these data might serve as the basis for investigating the possible signalling role for GABA in plant development and stress responses in more detail
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