Taurine release evoked by NMDA receptor activation is largely dependent on calcium mobilization from intracellular stores

Changes in the neurotransmitter profile in the central nervous system of marine medaka (Oryzias melastigma) after exposure to brevetoxin PbTx-1 - A multivariate approach to establish exposure biomarkers

A strategy to construct multivariate biomarkers for exposure to algal neurotoxins via correlating changes to the profiles of a series of neurotransmitters and their metabolites in the central nervous system (CNS) of exposed test organism is reported. 3-Month-old marine medaka (Oryzais melastigma) were exposed to waterborne brevetoxin PbTx-1 at two sub-lethal dose levels (0.5 and 2.5 μg-PbTx-1 L^-1) for a duration of 12 h before quantification of 43 selected neurotransmitters and metabolites in their CNS were measured via dansyl chloride derivatization and LC-MS/MS determination. The profiling data were analyzed by multivariate statistical analyses, including principle component analysis (PCA), projection on latent structure-discriminate analysis (PLS-DA) and orthogonal projection on latent structure-discriminate analysis (OPLS-DA). Neurotransmitters and metabolites related to activation of voltage-gated sodium channels (VGSCs), N-methyl-D-aspartic acid receptors (NMDARs) and cholinergic neurotransmission were found to contribute significantly to class separation in the corresponding OPLS-DA models. Those models obtained from different exposure dosages were correlated by the Shared and Unique Structures Plot (SUS-plot) to identify appropriate variables for the construction of exposure biomarkers in the form of multivariate predictive scores. The established biomarkers for male and female medaka fish were able to predict acute sub-lethal exposure to PbTx-1 with good sensitivity and specificity (male fish: sensitivity 94.7%, specificity 80.0%; female fish: sensitivity 91.4%, specificity 83.3%). Neurotransmitter profiles in the CNS of medaka fish that should have recovered from exposure to PbTx-1 have also been determined to reveal long-term impacts to the CNS of the affected organism even after the exposure has been interrupted.

Taurine protects against NMDA-induced retinal damage by reducing retinal oxidative stress

This study aimed to evaluate effect of TAU on NMDA-induced changes in retinal redox status, retinal cell apoptosis and retinal morphology in Sprague-Dawley rats. Taurine was injected intravitreally as pre-, co- or post-treatment with NMDA and 7 days post-treatment retinae were processed for estimation of oxidative stress, retinal morphology using H&E staining and retinal cell apoptosis using TUNEL staining. Treatment with TAU, particularly pre-treatment, significantly increased retinal glutathione, superoxide dismutase and catalase levels compared to NMDA-treated rats; whereas, the levels of malondialdehyde reduced significantly. Reduction in retinal oxidative stress in TAU pre-treated group was associated with significantly greater fractional thickness of ganglion cell layer within inner retina and retinal cell density in inner retina. TUNEL staining showed significantly reduced apoptotic cell count in TAU pre-treated group compared to NMDA group. It could be concluded that TAU protects against NMDA-induced retinal injury in rats by reducing retinal oxidative stress.

Taurine interaction with neurotransmitter receptors in the CNS: an update

Taurine appears to have multiple functions in the brain participating both in volume regulation and neurotransmission. In the latter context it may exert its actions by serving as an agonist at receptors of the GABAergic and glycinergic neurotransmitter systems. Its interaction with GABAA and GABAB receptors as well as with glycine receptors is reviewed and the physiological relevance of such interactions is evaluated. The question as to whether local extracellular concentrations of taurine are likely to reach the threshold level for the pertinent receptor populations cannot presently be answered satisfactorily. Hence more sophisticated analytical methods are warranted in order to obtain a definite answer to this important question.

Mechanisms of enhanced taurine release under Ca2+ depletion

The sulfur-containing amino acid taurine is an inhibitory neuromodulator in the brain of mammals, as well as a key substance in the regulation of cell volumes. The effect of Ca(2+) on extracellular taurine concentrations is of special interest in the context of the regulatory mechanisms of taurine release. The aim of this study was to characterize the basal release of taurine in Ca(2+)-free medium using in vivo microdialysis of the striatum of anesthetized rats. Perfusion of Ca(2+)-free medium via a microdialysis probe evoked a sustained release of taurine (up to 180 % compared to the basal levels). The Ca(2+) chelator EGTA (1mM) potentiated Ca(2+) depletion-evoked taurine release. The substitution of CaCl(2) by choline chloride did not alter the observed effect. Ca(2+)-free solution did not significantly evoke release of taurine from tissue loaded with the competitive inhibitor of taurine transporter guanidinoethanesulfonate (1mM), suggesting that in Ca(2+) depletion taurine is released by the transporter operating in the outward direction. The volume-sensitive chloride channel blocker diisothiocyanostilbene-2,2'-disulfonate (1mM) did not attenuate the taurine release evoked by Ca(2+) depletion. The non-specific blocker of voltage-sensitive Ca(2+) channels NiCl(2) (0.65 mM) enhanced taurine release in the presence of Ca(2+). CdCl(2) (0.25 mM) had no effect under these conditions. However, both CdCl(2) and NiCl(2) attenuated the effect of Ca(2+)-free medium on the release of taurine. The data obtained imply the involvement of both decreased influx of Ca(2+) and increased non-specific influx of Na(+) through voltage-sensitive calcium channels in the regulation of transporter-mediated taurine release in Ca(2+) depletion.

Role of taurine uptake on the induction of long-term synaptic potentiation

Taurine application in the CA1 area of rat hippocampal slices induces a long-lasting potentiation of excitatory synaptic transmission that has some mechanistic similitude with the late phase of long-term potentiation (L-LTP). Previous indirect evidence such as temperature and sodium dependence indicated that taurine uptake is one of the primary steps leading to the taurine-induced synaptic potentiation. We show that taurine-induced potentiation is not related to the intracellular accumulation of taurine and is not impaired by 2-guanidinoethanesulphonic acid, a taurine transport inhibitor that is a substrate of taurine transporter. We have found that taurine uptake in hippocampal synaptosomes was inhibited by SKF 89976A, a GABA uptake blocker that is not transportable by GABA transporters. SKF 89976A prevents the induction of synaptic potentiation by taurine application. This effect is neither mimicked by nipecotic acid, a broad inhibitor of GABA transporters that does not affect taurine uptake, nor by NO-711, a specific and potent inhibitor of GABA transporter GAT-1. In addition, L-LTP induced by trains of high-frequency stimulation is also inhibited by SKF 89976A, and taurine, at a concentration that does not change basal synaptic transmission, overcomes such inhibition. We conclude that taurine induces synaptic potentiation through the activation of a system transporting taurine and that taurine uptake is required for the induction of synaptic plasticity phenomena such as L-LTP.

The role of taurine in cerebral ischemia: studies in transient forebrain ischemia and embolic focal ischemia in rodents

Sudden cessation of blood flow to the brain results in a series of events that either result in rapid loss of brain cells or delayed neuronal injury in certain vulnerable regions of the brain. Research over the last three decades has allowed for a better understanding of how neurons and other brain cells die from the effects of ischemia and hypoxia in the central nervous system. Excitatory and inhibitory neurotransmitters exist in a very precise balance for normal function of the brain. Ischemia very rapidly disrupts this balance resulting in a rapid build-up of excitatory neurotransmitters, especially glutamate in the extracellular space. The increased glutamate together with energy loss opens a number of different types of calcium and sodium channels resulting in the build-up of these ions in neurons, leading to cellular dysfunction and death. While most ischemia research has focused on antagonism of excitatory amino acids, there are some reports on enhancement and amplification of inhibitory responses in focal and global ischemia. The majority of work relates to potentiation of GABA, either endogenous or through GABA potentiating medications. Taurine has neuroinhibitory properties and may also have potential for neuroprotection in cerebral ischemia. This present review focuses on the role of taurine as a neuroprotective agent, possibly acting through several different inhibitory mechanisms. Taurine may inhibit neurotransmitter release and may result in normal intracellular osmolality. In transient global ischemia in gerbils, we studied in vivo microdialysis of amino acids before, during and after ischemia. We were able to show that taurine resulted in attenuation of glutamate during ischemia (however did not reach significance). In similar experiments, neuronal damage was assessed in the hippocampus. Our results show 48% damage in taurine treated animals, 60% in alanine treated animals and 69% in control groups (trend towards protection but again did not reach significance) Focal ischemia was induced by embolizing a thrombus into the distal internal carotid artery and origin of the middle cerebral artery. Again, in studies where we compared taurine to a placebo treated animal, there was no significant decrease in the amount of damage with taurine. There are reports in the literature that taurine may attenuate neuronal injury during ischemia. Our studies in two models of cerebral ischemia in rodents did not reveal neuronal protection. It is possible that higher doses or possibly prolonged use of taurine may show better results. Taurine may also potentially offer additive protective effects when used in combination with thrombolysis or other neuroprotective agents. Further studies are necessary to better understand the potential for taurine as a neuroprotective agent in cerebral ischemia.

Haloperidol impairs auditory filial imprinting and modulates monoaminergic neurotransmission in an imprinting-relevant forebrain area of the domestic chick

In vivo microdialysis and behavioural studies in the domestic chick have shown that glutamatergic as well as monoaminergic neurotransmission in the medio-rostral neostriatum/hyperstriatum ventrale (MNH) is altered after auditory filial imprinting. In the present study, using pharmaco-behavioural and in vivo microdialysis approaches, the role of dopaminergic neurotransmission in this juvenile learning event was further evaluated. The results revealed that: (i) the systemic application of the potent dopamine receptor antagonist haloperidol (7.5 mg/kg) strongly impairs auditory filial imprinting; (ii) systemic haloperidol induces a tetrodotoxin-sensitive increase of extracellular levels of the dopamine metabolite, homovanillic acid, in the MNH, whereas the levels of glutamate, taurine and the serotonin metabolite, 5-hydroxyindole-3-acetic acid, remain unchanged; (iii) haloperidol (0.01, 0.1, 1 mm) infused locally into the MNH increases glutamate, taurine and 5- hydroxyindole-3-acetic acid levels in a dose-dependent manner, whereas homovanillic acid levels remain unchanged; (iv) systemic haloperidol infusion reinforces the N-methyl-d-aspartate receptor-mediated inhibitory modulation of the dopaminergic neurotransmission within the MNH. These results indicate that the modulation of dopaminergic function and its interaction with other neurotransmitter systems in a higher associative forebrain region of the juvenile avian brain displays similar neurochemical characteristics as the adult mammalian prefrontal cortex. Furthermore, we were able to show that the pharmacological manipulation of monoaminergic regulatory mechanisms interferes with learning and memory formation, events which in a similar fashion might occur in young or adult mammals.

Searching for mechanisms of N-methyl-D-aspartate-induced glutathione efflux in organotypic hippocampal cultures

N-Methyl-D-aspartate (NMDA)-receptor stimulation evoked a selective and partly delayed elevated efflux of glutathione, phosphoethanolamine, and taurine from organotypic rat hippocampus slice cultures. The protein kinase inhibitors H9 and staurosporine had no effect on the efflux. The phospholipase A2 inhibitors quinacrine and 4-bromophenacyl bromide, as well as arachidonic acid, a product of phospholipase A2 activity, did not affect the stimulated efflux. Polymyxin B, an antimicrobal agent that inhibits protein kinase C, and quinacrine in high concentration (500 microM), blocked efflux completely. The stimulated efflux after but not during NMDA incubation was attenuated by a calmodulin antagonist (W7) and an anion transport inhibitor (DNDS). Omission of calcium increased the spontaneous efflux with no or small additional effects by NMDA. In conclusion, NMDA receptor stimulation cause an increased selective efflux of glutathione, phosphoethanolamine and taurine in organotypic cultures of rat hippocampus. The efflux may partly be regulated by calmodulin and DNDS sensitive channels.

Taurine-induced synaptic potentiation and the late phase of long-term potentiation are related mechanistically

The application of taurine (2-aminoethanesulfonic acid) induces a long-lasting increase of synaptic efficacy and axon excitability (LLP-TAU) in rat hippocampal CA1 area. After taurine withdrawal, LLP-TAU lasted at least 3 h. This fact prompted us to assess whether the mechanisms involved in the maintenance of this particular potentiation were similar to those implicated in the late phase of long-term potentiation (L-LTP). In the presence of KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase, taurine perfusion (10 mM, 30 min) did not affect the induction of LLP-TAU. However, LLP-TAU maintenance was completely suppressed by KT5720, an inhibitor of the cAMP-dependent protein kinase (PKA). Moreover, the late phase of LLP-TAU was blocked by inhibiting protein synthesis with anisomycin. In addition, taurine perfusion increased the phosphorylation of cAMP response element-binding protein (CREB), although did not affect cAMP levels. These features of LLP-TAU do not appear to be caused by the activation of D1/D5 dopamine receptors, as taurine also induced synaptic potentiation in the presence of SCH23390, an antagonist of this type of receptors. Finally, the late phase of both L-LTP and LLP-TAU occluded mutually. These results suggest that taurine triggers the sequence of some of the molecular events involved in the induction of L-LTP.

Ammonia-induced extracellular accumulation of taurine in the rat striatum in vivo: role of ionotropic glutamate receptors

Accumulation of taurine (Tau), glutamate (Glu) and glutamine (Gln) was measured in vivo in microdialysates of the rat striatum following a direct application to the microdialysis tube of 60 mM ammonium chloride which renders the final ammonia concentration in the extracellular space to approximately 5 mM. The following compounds were coadministered with ammonia to distinguish between the different mechanisms that may underlie the accumulation of amino acids: ion transport inhibitors, diisothiocyanostilbene-2,2'-disulfonate (DIDS) and furosemide, a Glu transport inhibitor L-trans-pyrrolidine-2,4-dicarboxylate (PDC), an NMDA receptor antagonist dizocilpine (MK-801) and an 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (KA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). Ammonia stimulated Tau accumulation in the microdialysates to approximately 250% of the basal value. Furosemide did not significantly affect the stimulation by ammonia and DIDS only moderately depressed the effect. The ammonia-dependent Tau accumulation was increased by approximately 50% in the presence of PDC and reduced by approximately 35% in the presence dizocilpine and DNQX. In the microdialysates ammonia stimulated Glu and Gln accumulation somewhat less than Tau accumulation. Except for stimulation of Gln accumulation by DNQX, the effects were not modified by any of the cotreatments. The results are consistent with the assumption that ammonia stimulates Tau efflux mainly via activation of ionotropic Glu receptors.

Extracellular taurine as a parameter to monitor cerebral insults 'on-line': time courses and mechanisms as studied in vivo

Taurine increases in the zone surrounding a thrombotic infarct which could be prevented by a neuroprotective drug. Therefore, we aimed at studying the possible release mechanisms since the monitoring of taurine might give valuable information on the progress of cerebral insults and the effect of drugs. A microdialysis membrane was implanted into the cortex of anaesthetised rats: As toxic triggers possibly released by the dying cells in the peri-infarct zone, either a glutamatergic agonist (NMDA) or high potassium were applied via the microdialysis probe. Alternatively, a diluted perfusate was applied to induce cell swelling directly. NMDA antagonists or the NO synthase inhibitor L-NAME were applied locally too. NMDA, NO, high potassium or the hypotonic solution stimulated the release of taurine. The effect of high potassium could be prevented by Ketamine, but not by APV. The effect of NMDA could be inhibited by APV or Ketamine or the NO synthase inhibitor L-NAME. The release of taurine induced by the hypotonic solution could not be be reduced by any of the inhibitors. These data suggest that the release of taurine induced by glutamatergic activity is mediated via the NO cascade. The potassium mediated release seems to be related only in part to glutamatergic activity. Thus, other mechanisms seem to be predominate in potassium mediated swelling. Hypoosmotically induced taurine release is not mediated via the NO cascade and also seems to differ from the aforementioned release mechanisms.

IN CONCLUSION

Monitoring of extracellular taurine allows to follow pathological events and to differentiate drug effects.

GABA, taurine and learning: release of amino acids from slices of chick brain following filial imprinting

The intermediate and medial hyperstriatum ventrale (IMHV) is a forebrain region in the domestic chick that is a site of information storage for the learning process of imprinting. We enquired whether imprinting is associated with learning-related increases in calcium-dependent, potassium-stimulated release of neurotransmitter amino acids from the IMHV. Chicks were hatched and reared in darkness until 15-30 h after hatching. They then either remained in darkness or were trained for 2 h by exposure to an imprinting stimulus. One hour later, the chicks were given a preference test and a preference score was calculated from the results of this test, as a measure of imprinting. Chicks were killed 2 h after training. Slices from the left and right IMHV of trained and untrained chicks were superfused with Krebs' solution either with or without calcium and the superfusate assayed for arginine, aspartate, citrulline, GABA, glutamate, glycine and taurine using high-performance liquid chromatography. For calcium-containing superfusates from the left IMHV, preference score was significantly correlated with potassium-stimulated release of (i) GABA (r=0.51, 23 d.f., P=0.008) and (ii) taurine (r=0.77, 23 d.f., P<0.0001). There was no significant difference between the mean values of trained and untrained chicks for either compound. However, examination of the variance of the data indicated that release of both GABA and taurine increased as a result of learning. No significant correlation between preference score and release was found for any of the amino acids from the right IMHV, nor for control tissue from the left IMHV superfused with calcium-free solution. These results demonstrate that the learning process of imprinting is associated with increases in releasable pools of GABA and taurine and/or membrane excitability in the left IMHV.

Cerebral taurine release mechanisms in vivo: pharmacological investigations in rats using microdialysis for proof of principle

Cerebral taurine acts as neurotransmitter, as neuromodulator, or as osmoregulator. To investigate its release mechanisms in vivo, we combined the microdialysis technique with a variety of experimental paradigms. Taurine release was stimulated by either NMDA, NO or a hypotonic solution locally with or without the addition of the NMDA antagonists APV or Ketamine, or the NO synthase inhibitor L-NAME. Alternatively, the neuroprotective drug lubeluzole was applied i.v. NMDA, NO or the hypotonic solution stimulated the release of taurine. NMDA-mediated taurine release was inhibited by either APV, Ketamine or the NO synthase inhibitor L-NAME. Lubeluzole had no effect. Under the hypotonic conditions only lubeluzole was effective. These data confirm in vivo that the NMDA-induced taurine release is mediated via the NO cascade. By contrast, the release after a hypotonic stimulus is not related to the NO cascade. Instead, Na(+)- and/or Ca(2+)-mediated events might have been attenuated by lubeluzole.

Early N-acetylaspartate depletion is a marker of neuronal dysfunction in rats and primates chronically treated with the mitochondrial toxin 3-nitropropionic acid

N-acetylaspartate (NAA) quantification by 1H-magnetic resonance spectroscopy has been commonly used to assess in vivo neuronal loss in neurodegenerative disorders. Here. the authors used ex vivo and in vivo 1H-magnetic resonance spectroscopy in rat and primate models of progressive striatal degeneration induced by the mitochondrial toxin 3-nitropropionate (3NP) to determine whether early NAA depletions could also be associated with neuronal dysfunction. In rats that were treated for 3 days with 3NP and had motor symptoms, the authors found a significant decrease in NAA concentrations, specifically restricted to the striatum. No cell loss or dying cells were found at this stage in these animals. After 5 days of 3NP treatment, a further decrease in striatal NAA concentrations was observed in association with the occurrence of dying neurons in the dorsolateral striatum. In 3NP-treated primates, a similar striatal-selective and early decrease in NAA concentrations was observed after only a few weeks of neurotoxic treatment, without any sign of ongoing cell death. This early decrease in striatal NAA was partially reversed after 4 weeks of 3NP withdrawal. These results demonstrate that early NAA depletions reflect a reversible state of neuronal dysfunction preceding cell degeneration and suggest that in vivo quantification of NAA 1H-magnetic resonance spectroscopy may become a valuable tool for assessing early neuronal dysfunction and the effects of potential neuroprotective therapies in neurodegenerative disorders.

Taurine-induced synaptic potentiation: role of calcium and interaction with LTP

Taurine induces a long-lasting potentiation of excitatory synaptic potentials due to the enhancement of both synaptic efficacy and axon excitability in the CA1 area of rat hippocampal slices. In this study, we characterized the role of Ca2+ in the generation of these long-lasting taurine effects. Taurine perfusion in a free-Ca2+ medium did not induce changes in either field excitatory synaptic potentials (fEPSP) slope or fiber volley (FV) amplitude. Intracellular recordings with a micropipette filled with the Ca2+ chelator BAPTA, prevented the EPSP potentiation induced by taurine in the impaled cell, whereas a long-lasting potentiation of the simultaneously recorded fEPSP was obtained. The depletion of intracellular Ca2+ stores by thapsigargin (1 microM), an inhibitor of endosomal Ca2+-ATPase, transformed the taurine-induced potentiation into a transitory process that declined to basal values after taurine withdrawal. Taurine-induced potentiation was not significantly affected by kynurenate (glutamate receptor antagonist), or nifedipine (high-voltage-activated Ca2+ channel antagonist). But, the presence of nickel (50 microM), an antagonist of low-voltage-activated Ca2+ channel, inhibited the taurine-induced potentiation, indicating that Ca2+ influx through this type of Ca2+ channels could account for the Ca2+ requirement of the taurine-induced potentiation. Occlusion experiments between tetanus-induced long-term potentiation (LTP) and taurine-induced potentiation indicate that both processes share some common mechanisms during the maintenance period.

Glutathione efflux induced by NMDA and kainate: implications in neurotoxicity?

Neurotoxicity in acute as well as chronic neurological diseases may be partly mediated by oxidative stress caused by overactivation of glutamate receptors. A key component of the cellular defense against oxidative stress is reduced glutathione. In our earlier work, we have shown that ischemia in brain induces increased efflux, elevated metabolism, and decreased tissue concentrations of glutathione. In this study, we have evaluated the effect of glutamate receptor activation on the efflux of glutathione from hippocampus in vitro. NMDA and kainate induced a delayed increase in glutathione, taurine, and phosphoethanolamine efflux. Extracellular glutathione was recovered mainly in the reduced form (85-95%); the efflux was dependent on extracellular calcium but unrelated to dantrolene-sensitive intracellular calcium release and independent of glutathione or NO synthesis. The NMDA-induced efflux of glutathione was enhanced by blockage of gamma-glutamyl transpeptidase, indicating an increased transpeptidation of glutathione after NMDA receptor activation. Our results suggest that increased efflux of glutathione could be a factor in initiating nerve cell death via a change in intracellular redox potential and/or a decrease in the intracellular capacity for inactivation of reactive oxygen species.

Taurine release in the rat vas deferens is modulated by Ca2+ and is independent of contractions

Electrical field stimulation induces taurine release in rat vas deferens. In the present study, it was investigated if this release is secondary to contraction. The influence of Ca2+ and of the stimulation conditions was also studied. Contractions evoked by electrical field stimulation (5 Hz/270 pulses, transverse or longitudinal) were recorded and released taurine was quantified by high performance liquid chromatography with fluorimetric detection. Ca2+ removal abolished contractions, but not the overflow of taurine. Overflow elicited by longitudinal electrical field stimulation was higher than that elicited by transverse electrical field stimulation. Increasing the current strength also increased taurine overflow. In Ca2+-free medium, taurine overflow was decreased by caffeine (5 mM) or ryanodine (10 microM) but increased by dantrolene (50 microM). The results indicate that taurine release evoked by electrical field stimulation is (i) independent of contraction, (ii) modulated by Ca2+, (iii) potential dependent, and may be due to a decrease in taurine affinity for the plasma membrane and/or to an increase of Na+-dependent outward transport.

Microdialysis monitoring of extracellular glutamate combined with the simultaneous recording of evoked field potentials in hippocampal organotypic slice cultures

These experiments combined extracellular electrophysiological multirecordings from hippocampal organotypic slice cultures with application of drugs to and sampling of extracellular fluid from a restricted region of the slice using a microdialysis probe. Glutamate (Glu) concentrations were monitored in 0.5 or 2 min microdialysis samples, while evoked field potentials responses (EvFPR) in the CA1 region of the hippocampus (stimulation in the CA3 area) were simultaneously recorded using a multi-electrodes array (Physiocard). Glu was assayed by capillary electrophoresis with laser-induced fluorescence detection combined with a continuous flow derivatization of dialysates. The performance of this combined approach was demonstrated by monitoring extracellular Glu concentrations and EvFPR after K+ induced depolarisation, Glu uptake blockade by trans-pyrrolidine-2,4-dicarboxylic acid (PDC), and electrical stimulation. Such an approach allows a global monitoring of the neuronal functioning with a fine time resolution (up to 30 s) on a simple in vitro brain slice model, to be used as a complement to conventional in vivo microdialysis studies.

Adenosine transport inhibitors enhance high K(+)-evoked taurine release from rat hippocampus

We examined the effects of Ca(2+)-free medium containing 20 mM Mg2+, a non-selective adenosine receptor antagonist, theophylline, and adenosine transport inhibitors, dipyridamole and nitrobenzylthioinosine, on high K(+)-evoked spreading depression, glutamate, and taurine release from the rat hippocampus using brain microdialysis. High K+ alone perfusion evoked spreading depression and increased glutamate release followed by taurine efflux. Perfusion of Ca(2+)-free medium with high K+ never evoked spreading depression and decreased the high K(+)-evoked taurine release. Perfusion of theophylline (1 mM) increased the occurrence of high K(+)-evoked spreading depression and glutamate release, but did not modify taurine release. In contrast, simultaneous perfusion of dipyridamole (100 microM) and nitrobenzylthioinosine (50 microM) reduced the occurrence of spreading depression and the high K(+)-evoked glutamate release, but enhanced significantly the taurine efflux. These findings suggest that endogenous taurine with adenosine may have neuroprotective actions against high K(+)-evoked glutamate release and spreading depression in the rat hippocampus, in addition to its osmoregulatory action.

Characteristics of nitric oxide-evoked [3H]taurine release from cerebral cortical neurons

Pharmacological characteristics of [3H]taurine release evoked by nitric oxide (NO) were investigated using mouse cerebral cortical neurons in primary culture. NO generators such as S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP) dose-dependently increased [3H]taurine release from neurons. Such stimulatory effects of NO generators were completely abolished by hemoglobin, a NO radical scavenger, indicating that these [3H]taurine releases might be due to NO liberated from SNAP and SNP. Sodium withdrawal from incubation buffer significantly inhibited the SNAP- and SNP-induced [3H]taurine releases, whereas the removal of calcium showed no alterations in the [3H]taurine release evoked by NO generators. Beta-Alanine and guanidinoethane sulfonate, inhibitors of carrier-mediated taurine transport system, inhibited the SNAP- and SNP-evoked releases of [3H]taurine in a dose-dependent manner. These results indicate that the NO-evoked [3H]taurine release from cerebral cortical neurons is mediated by the reverse process of sodium-dependent carrier-mediated taurine transport system.

Nitric oxide-evoked [3H]taurine release is mediated by reversal of the Na(+)-dependent carrier-mediated taurine transport system

The pharmacological characteristics of [3H]taurine release evoked by nitric oxide (NO) were investigated using mouse cerebral cortical neurons in primary culture. N-Methyl-D-aspartate (NMDA) and S-nitroso-N-acetylpenicillamine (SNAP) dose-dependently increased [3H]taurine release from neurons. The NMDA-evoked release of [3H]taurine was reduced to the basal level by N omega-nitro-L-arginine, a NO synthase inhibitor, and MK-801, a noncompetitive antagonist for NMDA receptors. The NMDA- and SNAP-evoked releases of [3H]taurine were completely abolished by hemoglobin, indicating that these [3H]taurine releases were evoked by NO produced by NMDA receptor activation and liberated from SNAP. Withdrawal of Na+ from incubation buffer significantly inhibited the NMDA- and SNAP-induced [3H]taurine releases, whereas removal of Ca2+ produced no alteration in the SNAP-evoked [3H]taurine release. In addition, beta-alanine and guanidinoethane sulfonate, antitransporters of the carrier-mediated taurine transport system, reduced the NMDA- and SNAP-evoked releases of [3H]taurine in a dose-dependent manner. These results indicate that the NO-evoked [3H]taurine release from cerebral cortical neurons is mediated by a reversal of the Na(+)-dependent carrier-mediated taurine transport system.

Striatal NMDA receptor subtypes: the pharmacology of N-methyl-D-aspartate-evoked dopamine, gamma-aminobutyric acid, acetylcholine and spermidine release

We have examined the inhibitory potencies of MK 801, memantine, dextromethorphan, Mg2+ and of strychnine-insensitive glycine site antagonists on the N-methyl-D-aspartate (NMDA)-evoked (300 microM) release of [14C]acetylcholine and [3H]spermidine or [14C] gamma-aminobutyric acid [14C]GABA and [3H]dopamine from rat striatal slices. MK 801, dextromethorphan and all glycine antagonists examined (7-chlorokynurenate, L-689,560 ((+/-)-trans-2-carboxy-5,7-dichlorotetrahydroquinoline-4-phenylure a), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 6,7-dichloroquinoxaline-2,3-dione (DNQX), and (+)-HA966 ((3-amino-1-hydroxypyrrolidin-2-one) more potently inhibited NMDA-evoked dopamine and GABA release than acetylcholine and spermidine release by a factor of 3-21. MgCl2, which does not inhibit NMDA-evoked spermidine release, and memantine which only weakly antagonised NMDA-evoked spermidine release, inhibited NMDA-evoked dopamine, acetylcholine and GABA release with similar potencies. No pharmacological differences were observed between NMDA-evoked dopamine and GABA release. These findings extend those suggesting that NMDA-evoked acetylcholine and spermidine release are mediated by different NMDA receptor subtypes in the striatum and suggest a third native subtype with a distinct pharmacology that regulates striatal dopamine and GABA release.

In vivo studies on NMDA-evoked release of amino acids in the rat spinal cord

In the present study, spontaneous and evoked release of selected amino acids in the rat spinal cord was studied using in vivo microdialysis. Perfusion of the microdialysis probe with 100 K+ evoked a 2-4-fold increase in release of the putative neurotransmitters aspartate, glutamate and taurine while glutamine was decreased. K(+)-evoked release of glutamate was almost completely Ca(2+)-dependent while that of aspartate was partially Ca(2+)-dependent. Taurine release was not affected by substituting Ca2+ with Co2+. Perfusion with 5 mM N-methyl-D-aspartate (NMDA) evoked 3-9-fold release of glutamate, glycine and taurine and a small increase in extracellular beta-alanine. No significant changes in glutamine and serine were found. 5 mM of the competitive NMDA antagonist 3-((+/-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) reduced NMDA-evoked release of glutamate and taurine by approx. 50%. 5 mM 3-amino-1-hydroxypyrrolid-2-one (HA-966), an agonist at the glycine site of the NMDA receptor with very low efficacy, completely inhibited NMDA-evoked release of taurine and reduced the levels of released glutamate below baseline, similar to the effect of 1 mM CPP alone. The present results show that in situations of excessive release of excitatory amino acids such as spinal ischemia and trauma. NMDA receptor-evoked release of glutamate may amplify the deleterious process and spread the damage.

Neuronal Ca2+ stores: activation and function

The intracellular concentration of free Ca2+ ([Ca2+]i) displays complex fluctuations in response to a variety of stimuli, and acts as a pluripotent signal for many neuronal functions. It is well established that various 'metabotropic' neurotransmitter receptors can mediate the mobilization of Ca2+ stores via actions of inositol-polyphosphate second messengers, and more recent evidence suggests that 'ionotropic' receptor-mediated Ca2+ signals in neurones might also involve release of Ca2+ from intracellular stores. These two mechanisms of release of Ca2+ enable considerable temporal and spatial complexity of increases in the [Ca2+]i via multiple interactions at the level of intracellular-receptor activation. The complexity of Ca2+ signalling that is elicited via these interconnecting pathways might underlie mechanisms that are central to information transfer and integration within neuronal compartments.