Long-lasting enhancement of corticostriatal neurotransmission by taurine

The Dark Side of Energy Drinks: A Comprehensive Review of Their Impact on the Human Body

In recent years, the consumption of energy drinks by young adults and athletes has risen significantly, but concerns have been raised about the potential health risks associated with excessive consumption. These concerns include cardiovascular problems, nervous system disorders, and the potential for addiction. This review aims to examine the reported effects of acute or chronic abuse of energy drinks on human health. The analysis shows a significant prevalence of adverse effects, particularly on the cardiovascular and neurovegetative systems. In particular, the analysis identified nine cases of cardiac arrest, three of which were fatal. The aetiology of these adverse effects is attributed to the inherent neurostimulant properties of these beverages, of which caffeine is the predominant component. A comparison of documented effects in humans with experimental studies in animal models showed an overlap in results. This review highlights the need for greater rigour in the assessment of sudden cardiac death, particularly in young people, as legal substances such as energy drinks may be involved. We propose stricter limits on the consumption of these beverages than for caffeine, based on the evidence found and the data in the literature. This review also calls for the establishment of regulations governing the consumption of these products in view of their potential impact on human health.

Maternal taurine as a modulator of Cl- homeostasis as well as of glycine/GABAA receptors for neocortical development

During brain and spinal cord development, GABA and glycine, the inhibitory neurotransmitters, cause depolarization instead of hyperpolarization in adults. Since glycine and GABAA receptors (GABAARs) are chloride (Cl-) ion channel receptor, the conversion of GABA/glycine actions during development is influenced by changes in the transmembrane Cl- gradient, which is regulated by Cl- transporters, NKCC1 (absorption) and KCC2 (expulsion). In immature neurons, inhibitory neurotransmitters are released in a non-vesicular/non-synaptic manner, transitioning to vesicular/synaptic release as the neuron matures. In other word, in immature neurons, neurotransmitters generally act tonically. Thus, the glycine/GABA system is a developmentally multimodal system that is required for neurogenesis, differentiation, migration, and synaptogenesis. The endogenous agonists for these receptors are not fully understood, we address taurine. In this review, we will discuss about the properties and function of taurine during development of neocortex. Taurine cannot be synthesized by fetuses or neonates, and is transferred from maternal blood through the placenta or maternal milk ingestion. In developing neocortex, taurine level is higher than GABA level, and taurine tonically activates GABAARs to control radial migration as a stop signal. In the marginal zone (MZ) of the developing neocortex, endogenous taurine modulates the spread of excitatory synaptic transmission, activating glycine receptors (GlyRs) as an endogenous agonist. Thus, taurine affects information processing and crucial developmental processes such as axonal growth, cell migration, and lamination in the developing cerebral cortex. Additionally, we also refer to the possible mechanism of taurine-regulating Cl- homeostasis. External taurine is uptake by taurine transporter (TauT) and regulates NKCC1 and KCC2 mediated by intracellular signaling pathway, with-no-lysine kinase 1 (WNK1) and its subsequent kinases STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) and oxidative stress response kinase-1 (OSR1). Through the regulation of NKCC1 and KCC2, mediated by the WNK-SPAK/OSR1 signaling pathway, taurine plays a role in maintaining Cl- homeostasis during normal brain development.

Effects of Energy Drink Consumption on Physical Performance and Potential Danger of Inordinate Usage

The rise in energy drink (ED) intake in the general population and athletes has been achieved with smart and effective marketing strategies. There is a robust base of evidence showing that adolescents are the main consumers of EDs. The prevalence of ED usage in this group ranges from 52% to 68%, whilst in adults is estimated at 32%. The compositions of EDs vary widely. Caffeine content can range from 75 to 240 mg, whereas the average taurine quantity is 342.28 mg/100 mL. Unfortunately, exact amounts of the other ED elements are often not disclosed by manufacturers. Caffeine and taurine in doses 3–6 mg/kg and 1–6 g, respectively, appear to be the main ergogenic elements. However, additive or synergic properties between them seem to be implausible. Because of non-unified protocol design, presented studies show inconsistency between ED ingestion and improved physical performance. Potential side effects caused by abusive consumption or missed contraindications are the aspects that are the most often overlooked by consumers and not fully elucidated by ED producers. In this review, the authors aimed to present the latest scientific information on ED components and their possible impact on improving physical performance as well as to bring emphasis to the danger of inordinate consumption.

Expedition into Taurine Biology: Structural Insights and Therapeutic Perspective of Taurine in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are characterized by the accumulation of misfolded proteins. The hallmarks of protein aggregation in NDs proceed with impairment in the mitochondrial function, besides causing an enhancement in endoplasmic reticulum (ER) stress, neuroinflammation and synaptic loss. As accumulation of misfolded proteins hampers normal neuronal functions, it triggers ER stress, which leads to the activation of downstream effectors formulating events along the signaling cascade—referred to as unfolded protein response (UPRER) —thereby controlling cellular gene expression. The absence of disease-modifying therapeutic targets in different NDs, and the exponential increase in the number of cases, makes it critical to explore new approaches to treating these devastating diseases. In one such approach, osmolytes (low molecular weight substances), such as taurine have been found to promote protein folding under stress conditions, thereby averting aggregation of the misfolded proteins. Maintaining the structural integrity of the protein, taurine-mediated resumption of protein folding prompts a shift in folding homeostasis more towards functionality than towards aggregation and degradation. Together, taurine enacts protection in NDs by causing misfolded proteins to refold, so as to regain their stability and functionality. The present study provides recent and useful insights into understanding the progression of NDs, besides summarizing the genetics of NDs in correlation with mitochondrial dysfunction, ER stress, neuroinflammation and synaptic loss. It also highlights the structural and functional aspects of taurine in imparting protection against the aggregation/misfolding of proteins, thereby shifting the focus more towards the development of effective therapeutic modules that could avert the development of NDs.

Supplemental taurine during adolescence and early adulthood has sex-specific effects on cognition, behavior and neurotransmitter levels in C57BL/6J mice dependent on exposure window

The mammalian brain goes through final maturation during late adolescence and early adulthood with sex differences in timing. The key cellular processes, including changes in neurotransmitter receptor density and synaptic pruning, make this age uniquely vulnerable to neurotoxic insults. Teenagers and young adults are the major consumers of energy drinks, which contain high levels of taurine and caffeine. Taurine is one of the most abundant amino acids in the central nervous system, but the effects of supplemental taurine consumption during adolescence has not been well studied. We conducted an initial short-term exposure study with 0.12% taurine in drinking water and a long-term exposure dose-response study using 0.06 and 0.12% taurine in male and female C57BL/6J mice. We examined a broad range of cognitive functions and behaviors and measured neurotransmitter levels. We found no significant differences in anxiety, open field locomotor activity, or sensorimotor gating. However, we found impairments in novel object recognition and sex differences in Morris water maze. When taurine treatment stopped before behavioral experiments began, male mice had significant impairments in spatial learning and memory. In the dose-response study when taurine treatment continued throughout behavioral experiments, females had significant impairments. We also found sex differences in neurotransmitter levels with females having higher levels of glutamate, DOPAC and 5-HIAA. We conclude that both females and males are at risk from excess taurine consumption during final brain maturation.

Taurine as an Essential Neuromodulator during Perinatal Cortical Development

A variety of experimental studies demonstrated that neurotransmitters are an important factor for the development of the central nervous system, affecting neurodevelopmental events like neurogenesis, neuronal migration, programmed cell death, and differentiation. While the role of the classical neurotransmitters glutamate and gamma-aminobutyric acid (GABA) on neuronal development is well established, the aminosulfonic acid taurine has also been considered as possible neuromodulator during early neuronal development. The purpose of the present review article is to summarize the properties of taurine as neuromodulator in detail, focusing on the direct involvement of taurine on various neurodevelopmental events and the regulation of neuronal activity during early developmental epochs. The current knowledge is that taurine lacks a synaptic release mechanism but is released by volume-sensitive organic anion channels and/or a reversal of the taurine transporter. Extracellular taurine affects neurons and neuronal progenitor cells mainly via glycine, GABA(A), and GABA(B) receptors with considerable receptor and subtype-specific affinities. Taurine has been shown to directly influence neurogenesis in vitro as well as neuronal migration in vitro and in vivo. It provides a depolarizing signal for a variety of neuronal population in the immature central nervous system, thereby directly influencing neuronal activity. While in the neocortex, taurine probably enhance neuronal activity, in the immature hippocampus, a tonic taurinergic tone might be necessary to attenuate activity. In summary, taurine must be considered as an essential modulator of neurodevelopmental events, and possible adverse consequences on fetal and/or early postnatal development should be evaluated for pharmacological therapies affecting taurinergic functions.

L-DOPA Reverses the Increased Free Amino Acids Tissue Levels Induced by Dopamine Depletion and Rises GABA and Tyrosine in the Striatum

Perturbations in the cerebral levels of various amino acids are associated with neurological disorders, and previous studies have suggested that such alterations have a role in the motor and non-motor symptoms of Parkinson's disease. However, the direct effects of chronic L-DOPA treatment, that produces dyskinesia, on neural tissue amino acid concentrations have not been explored in detail. To evaluate whether striatal amino acid concentrations are altered in peak dose dyskinesia, 6-hydroxydopamine (6-OHDA)-lesioned hemiparkinsonian mice were treated chronically with L-DOPA and tissue amino acid concentrations were assessed by HPLC analysis. These experiments revealed that neither 6-OHDA nor L-DOPA treatment are able to alter glutamate in the striatum. However, glutamine increases after 6-OHDA and returns back to normal levels with L-DOPA treatment, suggesting increased striatal glutamatergic transmission with lack of dopamine. In addition, glycine and taurine levels are increased following dopamine denervation and restored to normal levels by L-DOPA. Interestingly, dyskinetic animals showed increased levels of GABA and tyrosine, while aspartate striatal tissue levels are not altered. Overall, our results indicate that chronic L-DOPA treatment, besides normalizing the altered levels of some amino acids after 6-OHDA, robustly increases striatal GABA and tyrosine levels which may in turn contribute to the development of L-DOPA-induced dyskinesia.

Impact of an Energy Drink on the Structure of Stomach and Pancreas of Albino Rat: Can Omega-3 Provide a Protection?

BACKGROUND AND OBJECTIVES

A controversy developed between the benefits of energy drinks (EDs) versus the possible health threats since its revolution. Lack of information was a call to assess the effect of chronic consumption of Power Horse (PH) as one of the EDs, on the structure of pancreas and fundic mucosa of stomach in rats, and possible protective role of Omega-3.

MATERIALS AND METHODS

Thirty two adult male albino rats were divided equally into 4 groups; control received group which only received a standard diet, Omega-3 group, PH group which given PH and PH plus Omega-3 group received both PH plus Omega-3 for 4 weeks. Biochemical assessment of blood glucose, serum insulin, gastrin, tumor necrosis factor alpha (TNF-α) and inducible nitric oxide synthetase (iNOS) was performed. The antioxidant activity and histopathological examination of both pancreatic tissue and fundic mucosa of stomach were assessed.

RESULTS

Administration of PH significantly increased serum insulin and glucose levels while it significantly reduced serum gastrin level compared to control. PH also caused oxidants/antioxidants imbalance in both pancreas and fundic mucosa. The latter revealed degenerative changes and increased apoptosis which was evident by increased caspase-3 immunoexpression. Pancreas exhibited signs of β-cells overstimulation. Fundic mucosa showed reduced number of parietal cells, gastrin hormone expression compared to control group. Omega-3 administration could alleviate, to some extent, these changes. It significantly decreased TNF-α, iNOS and reduced glutathione (GSH) as well as significantly increasing superoxide dismutase (SOD) and glutathione peroxidase (GPx) activities compared to the group which received PH alone.

CONCLUSION

Power Horse intake significantly injures islet cells, pancreatic acini as well as the glandular cells of the fundic mucosa. Omega-3 decreases these detrimental effects mostly through its antioxidant and anti-inflammatory action.

Assessing Susceptibility to Epilepsy in Three Rat Strains Using Brain Metabolic Profiling Based on HRMAS NMR Spectroscopy and Chemometrics

The possibility that a metabolomic approach can inform about the pathophysiology of a given form of epilepsy was addressed. Using chemometric analyses of HRMAS NMR data, we compared several brain structures in three rat strains with different susceptibilities to absence epilepsy: Genetic Absence Epilepsy Rats from Strasbourg (GAERS), Non Epileptic Control rats (NEC), and Wistar rats. Two ages were investigated: 14 days postnatal (P14) before the onset of seizures and 5 month old adults with fully developed seizures (Adults). The relative concentrations of 19 metabolites were assessed using (1)H HRMAS NMR experiments. Univariate and multivariate analyses including multiblock models were used to identify the most discriminant metabolites. A strain-dependent evolution of glutamate, glutamine, scyllo-inositol, alanine, and glutathione was highlighted during cerebral maturation. In Adults, data from somatosensory and motor cortices allowed discrimination between GAERS and NEC rats with higher levels of scyllo-inositol, taurine, and phosphoethanolamine in NEC. This epileptic metabolic phenotype was in accordance with current pathophysiological hypothesis of absence epilepsy (i.e., seizure-generating and control networks) and putative resistance of NEC rats and was observed before seizure onset. This methodology could be very efficient in a clinical context.

Activity-dependent endogenous taurine release facilitates excitatory neurotransmission in the neocortical marginal zone of neonatal rats

In the developing cerebral cortex, the marginal zone (MZ), consisting of early-generated neurons such as Cajal-Retzius cells, plays an important role in cell migration and lamination. There is accumulating evidence of widespread excitatory neurotransmission mediated by γ-aminobutyric acid (GABA) in the MZ. Cajal-Retzius cells express not only GABA_A receptors but also α2/β subunits of glycine receptors, and exhibit glycine receptor-mediated depolarization due to high [Cl⁻]_i. However, the physiological roles of glycine receptors and their endogenous agonists during neurotransmission in the MZ are yet to be elucidated. To address this question, we performed optical imaging from the MZ using the voltage-sensitive dye JPW1114 on tangential neocortical slices of neonatal rats. A single electrical stimulus evoked an action-potential-dependent optical signal that spread radially over the MZ. The amplitude of the signal was not affected by glutamate receptor blockers, but was suppressed by either GABA_A or glycine receptor antagonists. Combined application of both antagonists nearly abolished the signal. Inhibition of Na⁺, K⁺-2Cl⁻ cotransporter by 20 µM bumetanide reduced the signal, indicating that this transporter contributes to excitation. Analysis of the interstitial fluid obtained by microdialysis from tangential neocortical slices with high-performance liquid chromatography revealed that GABA and taurine, but not glycine or glutamate, were released in the MZ in response to the electrical stimulation. The ambient release of taurine was reduced by the addition of a voltage-sensitive Na⁺ channel blocker. Immunohistochemistry and immunoelectron microscopy indicated that taurine was stored both in Cajal-Retzius and non-Cajal-Retzius cells in the MZ, but was not localized in presynaptic structures. Our results suggest that activity-dependent non-synaptic release of endogenous taurine facilitates excitatory neurotransmission through activation of glycine receptors in the MZ.

The effects of chronic taurine supplementation on motor learning

Taurine is one of the most abundant nonprotein amino acids shown to be essential for the development, survival, and growth of vertebrate neurons. We previously demonstrated that chronic taurine supplementation during neonatal development results in changes in the GABAergic system (El Idrissi, Neurosci Lett 436:19-22, 2008). The brains of mice chronically treated with taurine have decreased levels of GABA(A)β subunits and increased expression of GAD and GABA, which contributes to hyperexcitability. This down regulation of GABA(A)receptor subunit expression and function may be due to a sustained interaction of taurine with GABA(A)receptors. This desensitization decreases the efficacy of the inhibitory synapses at the postsynaptic membrane. If changes occur in the GABAergic system as a possible compensatory mechanism due to taurine administration, then it is important to study all aspects by which taurine induces hyperexcitability and affects motor behavior. We therefore hypothesized that modification of the GABAergic system in response to taurine supplementation influences motor learning capacity in mice. To test this hypothesis, the rotarod task was employed after chronic taurine supplementation in drinking water (0.05% for 4 weeks). Control animals receiving no taurine supplementation were also tested in order to determine the difference in motor learning ability between groups. Each animal was trained on the rotarod apparatus for 7 days at an intermediate speed of 24 rpm in order to establish baseline performance. On the testing day, each animal was subjected to eight different predefined speeds (5, 8, 15, 20, 24, 31, 33, and 44 rpm). From our observations, the animals that underwent chronic taurine supplementation appeared to have a diminished motor learning capacity in comparison to control animals. The taurine-fed mice displayed minor improvements after repeated training when compared to controls. During the testing session the taurine-fed mice also exhibited a shorter latency to fall, as the task requirements became more demanding.

Protective effect of taurine on the decreased biogenic amine neurotransmitter levels in the brain of mice exposed to arsenic

Arsenic (As) exposure has a toxic effect on the central nervous system, especially on learning and memory. Norepinephrine (NE), dopamine (DA), and serotonin (5-HT) play an important role in learning and memory function of the brain. In the present study, the protective effect of taurine on the disturbed biogenic amine neurotransmitter levels in the mouse brain induced by arsenic was examined. Sixty SPF mice were divided into three groups. The As exposure group was administered with 4 ppm As(2)O(3) through drinking water for 60 days. The protective group was treated with both 4 ppm As(2)O(3) and 150 mg/kg taurine. The control group was given drinking water alone. The levels of NE, DA, and 5-HT were determined by HPLC in the cerebrum and cerebellum of mice. Ultrastructure of synapses in brain tissue of mice was observed in these groups by transmission electron microscopy. The mRNA expressions of dopamine beta hydroxylase (DBH), tyrosine hydroxylase (TH), and tryptophan hydroxylase (TPH) as NE, DA, and 5-HT synzymes were also analyzed by real-time RT-PCR. The results showed that the concentrations of NE, DA, and 5-HT; the number of synaptic vesicles; and the expressions of TH, TPH, and DBH genes in the brains of mice exposed to As alone were significantly decreased. However, administration of taurine significantly alleviated the toxic effect on biochemicals detected in the experiment, compared with that in the brain of mice exposed to As alone. These results indicated that taurine was effective in counteracting the decreased biogenic amine neurotransmitter level and the mRNA expressions of their synzymes induced by arsenic.

Region-specific depression of striatal activity in Wistar rat by modest ethanol consumption over a ten-month period

The nucleus accumbens (nAc) is the primary target for the mesolimbic dopamine system and a key brain region for the reinforcing effects displayed by drugs of abuse, including ethanol. During the transition from recreational to compulsive consumption of reinforcing drugs, however, the dorsal striatum seems to be recruited. Understanding how synaptic activity is altered in a sub-region specific manner in the striatum during the course of long-term drug consumption thus could be essential for understanding the long-lasting changes produced by addictive substances, including ethanol. Here we evaluated synaptic activity in the dorsolateral striatum (DLS) and ventral striatum (nucleus accumbens, nAc) of single-housed Wistar rats consuming water, or water and ethanol, for up to 10 months. Even though ethanol intake was moderate, it was sufficient to decrease input/output function in response to stimulation intensity in the DLS, while recorded population spike (PS) amplitudes in the nAc were unaffected. Striatal disinhibition induced by the GABAA receptor antagonist bicuculline had a slower onset in rats that had consumed ethanol for 2 months, and was significantly depressed in slices from rats that had consumed ethanol for 4 months. Bicuculline-induced disinhibition in the nAc, on the other hand, was not significantly altered by long-term ethanol intake. Changes in PS amplitude induced by taurine or the glycine receptor antagonist strychnine were not significantly altered by ethanol in any brain region. Even though input/output function was not significantly affected by age, there was a significant decline in antagonist-induced disinhibition in brain slices from aged rats. The data presented here suggest that even modest consumption of ethanol is sufficient to alter neurotransmission in the striatum, while synaptic activity appears to be relatively well-preserved in the nAc during the course of long-term ethanol consumption.

Sergeeva O. Partial agonism of taurine at gamma-containing native and recombinant GABAA receptors

Taurine is a semi-essential sulfonic acid found at high concentrations in plasma and mammalian tissues which regulates osmolarity, ion channel activity and glucose homeostasis. The structural requirements of GABA_A-receptors (GABA_AR) gated by taurine are not yet known. We determined taurine potency and efficacy relative to GABA at different types of recombinant GABA_AR occurring in central histaminergic neurons of the mouse hypothalamic tuberomamillary nucleus (TMN) which controls arousal. At binary α_1/2β_1/3 receptors taurine was as efficient as GABA, whereas incorporation of the γ_1/2 subunit reduced taurine efficacy to 60–90% of GABA. The mutation γ_2F77I, which abolishes zolpidem potentiation, significantly reduced taurine efficacy at recombinant and native receptors compared to the wild type controls. As taurine was a full- or super- agonist at recombinant α_xβ₁δ-GABA_AR, we generated a chimeric γ₂ subunit carrying the δ subunit motif around F77 (MTVFLH). At α_1/2β₁γ_2(MTVFLH) receptors taurine became a super-agonist, similar to δ-containing ternary receptors, but remained a partial agonist at β₃-containing receptors. In conclusion, using site-directed mutagenesis we found structural determinants of taurine’s partial agonism at γ-containing GABA_A receptors. Our study sheds new light on the β₁ subunit conferring the widest range of taurine-efficacies modifying GABA_AR function under (patho)physiological conditions.

Glycine receptors support excitatory neurotransmitter release in developing mouse visual cortex

Glycine receptors (GlyRs) are found in most areas of the brain, and their dysfunction can cause severe neurological disorders. While traditionally thought of as inhibitory receptors, presynaptic-acting GlyRs (preGlyRs) can also facilitate glutamate release under certain circumstances, although the underlying molecular mechanisms are unknown. In the current study, we sought to better understand the role of GlyRs in the facilitation of excitatory neurotransmitter release in mouse visual cortex. Using whole-cell recordings, we found that preGlyRs facilitate glutamate release in developing, but not adult, visual cortex. The glycinergic enhancement of neurotransmitter release in early development depends on the high intracellular to extracellular Cl(-) gradient maintained by the Na(+)-K(+)-2Cl(-) cotransporter and requires Ca(2+) entry through voltage-gated Ca(2+) channels. The glycine transporter 1, localized to glial cells, regulates extracellular glycine concentration and the activation of these preGlyRs. Our findings demonstrate a developmentally regulated mechanism for controlling excitatory neurotransmitter release in the neocortex.

Ethanol-induced modulation of synaptic output from the dorsolateral striatum in rat is regulated by cholinergic interneurons

The striatum is the largest input nucleus to the basal ganglia and associated with reward-based behavior. We assessed whether acute ethanol (EtOH) exposure could modulate synaptic efficacy in the dorsolateral striatum of juvenile Wistar rats. Since acute EtOH administration can both increase and decrease the probability of release of different neurotransmitters from synaptic terminals, we used field potential recordings to evaluate the net effect of EtOH on striatal output. We showed that 50mM EtOH but not 20, 80 or 100mM, depresses population spike (PS) amplitude in the dorsolateral striatum. This depression of synaptic output is insensitive to the N-methyl-d-aspartic acid (NMDA) receptor inhibitor DL-2-amino-5-phosphonopentanoic acid (AP-5, 50μM), but is blocked in slices treated with glycine receptor antagonists (strychnine, 1μM; PMBA, 50μM), nicotinic acetylcholine receptor antagonists (mecamylamine, 10μM; methyllycaconitine citrate (MLA), 40nM), or GABA(A) receptor inhibitors (picrotoxin, 100μM; bicuculline, 2μM, 20μM). A long-term facilitation of synaptic output, which is more pronounced in slices from adult Wistar rats, is detected following EtOH washout (50, 80, 100mM). This long-term enhancement of PS amplitude is regulated by cholinergic interneurons and completely blocked by mecamylamine, MLA or the non-selective muscarinic antagonist scopolamine (10μM). Administration of 100mM EtOH significantly depresses PS amplitude in scopolamine-treated slices, suggesting that EtOH exerts dual actions on striatal output that are initiated instantly upon drug wash-on. In conclusion, EtOH modulates striatal microcircuitry and neurotransmission in a way that could be of importance for understanding the intoxicating properties as well as the acute reward sensation of EtOH.

Taurine protection of PC12 cells against endoplasmic reticulum stress induced by oxidative stress

Background

Taurine is a free amino acid present in high concentrations in a variety of organs of mammalians. As an antioxidant, taurine has been found to protect cells against oxidative stress, but the underlying mechanism is still unclear.

Methods

In this report, we present evidence to support the conclusion that taurine exerts a protective function against endoplasmic reticulum (ER) stress induced by H₂O₂ in PC 12 cells. Oxidative stress was introduced by exposure of PC 12 cells to 250 uM H₂O₂ for 4 hours.

Results

It was found that the cell viability of PC 12 cells decreased with an increase of H₂O₂ concentration ranging from approximately 76% cell viability at 100 uM H₂O₂ down to 18% at 500 uM H₂O₂. At 250 uM H₂O₂, cell viability was restored to 80% by taurine at 25 mM. Furthermore, H₂O₂ treatment also caused a marked reduction in the expression of Bcl-2 while no significant change of Bax was observed. Treatment with taurine restored the reduced expression of Bcl-2 close to the control level without any obvious effect on Bax. Furthermore, taurine was also found to suppress up-regulation of GRP78, GADD153/CHOP and Bim induced by H₂O₂, suggesting that taurine may also exert a protective function against oxidative stress by reducing the ER stress.

Conclusion

In summary, taurine was shown to protect PC12 cells against oxidative stress induced by H₂O₂. ER stress was induced by oxidative stress and can be suppressed by taurine.

Intermittent high-dose ethanol exposures increase motivation for operant ethanol self-administration: possible neurochemical mechanism

We investigated the neurochemical mechanism of how high-dose ethanol exposure may increase motivation for ethanol consumption. First, we developed an animal model of increased motivation for ethanol using a progressive ratio (PR) schedule. Sprague-Dawley rats were trained to administer 10% ethanol-containing gelatin or plain gelatin (on alternate weeks) in daily 30-min sessions under different fixed ratio (FR) and PR schedules. During FR schedules, rats self-administered about 1 g/kg ethanol, which was decreased to 0.4+/-0.03 g/kg under PR10. Rats then received four pairs of either 3 g/kg ethanol or saline injections during the weeks when the reinforcer was plain gelatin. During subsequent ethanol gel sessions, breakpoints and ethanol consumption rose 40% in the high-dose ethanol group by the fourth set of injections with no change in plain gel responding. Alterations in amino acids in the ventral striatum (VS) during PR10 responding for 10% ethanol gelatin and plain gelatin were measured using microdialysis sampling coupled with capillary electrophoresis and laser-induced fluorescence detection. There was greater release of taurine, glycine and glutamate in the NAC of the high-dose ethanol rats during 10% ethanol-containing gelatin responding, compared to the control rats or during plain gel responding. An increase in the release of glycine in this same brain region has recently been shown to be involved with anticipation of a reward. Thus, it appears that intermittent high-dose ethanol exposure not only increases motivation for ethanol responding but may also change neurotransmitter release that mediates anticipation of reinforcement, which may play a key role in the development of alcoholism.

Protection by taurine of rat brain cortical slices against oxygen glucose deprivation- and reoxygenation-induced damage

Taurine neuroinhibitory features have suggested its potential for neuroprotection. The aim of the present study was to assess whether it prevents or counteracts brain ischemia and reperfusion-induced cell injury. Rat brain cortical slices were subjected to oxygen/glucose deprivation and reperfusion. Tissue damage was assessed by measuring the release of glutamate and lactate dehydrogenase (LDH) during reperfusion and by determining final tissue water gain, taken as an index of cell swelling. When added during the reperfusion period taurine did not significantly affect oxygen/glucose deprivation-induced LDH and glutamate release, while it antagonised tissue water gain in a concentration-dependent manner (IC(50)=46.5 microM). The latter effect was antagonised by 50% when a taurine transport inhibitor, 2-(guanidino)ethanesulphonic acid (GES), or a GABA(A) receptor antagonist, bicuculline, was added together with taurine, while it was completely abolished when both GES and bicuculline or the volume-sensitive outwardly rectifying (VSOR) Cl(-) channel blocker, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), was used. On the contrary, when present throughout the entire experiment, taurine significantly reduced oxygen/glucose deprivation-induced LDH and glutamate release with a maximal effect (45% reduction) between 5 and 20 mM. Taurine antagonised also tissue water gain according to a "U-shaped" concentration-response curve, which was significant within the range of 0.01-1.0 mM concentration. This effect was partially counteracted by GES as well as by bicuculline and fully reverted by NPPB. In conclusion, since brain edema is a major contributing factor to morbidity and mortality in stroke, the present findings give the rational basis for assessing taurine efficacy in reducing brain edema in vivo.

In vivo 1H-magnetic resonance spectroscopy can detect metabolic changes in APP/PS1 mice after donepezil treatment

Background

Donepezil improves cognitive functions in AD patients. Effects on the brain metabolites N-acetyl-L-aspartate, choline and myo-inositol levels have been reported in clinical studies using this drug. The APP/PS1 mouse coexpresses the mutated forms of human β-amyloid precursor protein (APP) and mutated human presenilin 1 (PS1). Consequently, the APP/PS1 mouse model reflects important features of the neurochemical profile in humans. In vivo magnetic resonance spectroscopy (¹H-MRS) was performed in fronto-parietal cortex and hippocampus (ctx/hipp) and in striatum (str). Metabolites were quantified using the LCModel and the final analysis was done using multivariate data analysis. The aim of this study was to investigate if multivariate data analysis could detect changes in the pattern of the metabolic profile after donepezil treatment.

Results

Significant differences were observed in the metabolic pattern of APP/PS1 mice in both str and ctx/hipp before and after donepezil treatment using multivariate data analysis, evidencing a significant treatment effect. A treatment effect was also seen in wild type (wt) mice in str. A significant decrease in the metabolic ratio taurine/creatine (Tau/tCr) was related to donepezil treatment (p < 0.05) in APP/PS1 mice in both brain regions. Furthermore, a significant influence on the choline/creatine (tCho/tCr) level was observed in treated APP/PS1 mice compared to untreated in str (p = 0.011). Finally, there was an increase in glutamate/creatine (Glu/tCr) in str in wt mice treated with donepezil.

Conclusion

Multivariate data analysis can detect changes in the metabolic profile in APP/PS1 mice after donepezil treatment. Effects on several metabolites that are measurable in vivo using MR spectroscopy were observed. Changes in Tau/tCr and tCho/tCr could possibly be related to changed cholinergic activity caused by donepezil treatment.

Glycine receptors mediate excitation of subplate neurons in neonatal rat cerebral cortex

The development of the cerebral cortex depends on genetic factors and early electrical activity patterns that form immature neuronal networks. Subplate neurons (SPn) are involved in the construction of thalamocortical innervation, generation of oscillatory network activity, and in the proper formation of the cortical columnar architecture. Because glycine receptors play an important role during early corticogenesis, we analyzed the functional consequences of glycine receptor activation in visually identified SPn in neocortical slices from postnatal day 0 (P0) to P4 rats using whole cell and perforated patch-clamp recordings. In all SPn the glycinergic agonists glycine, beta-alanine, and taurine induced dose-dependent inward currents with the affinity for glycine being higher than that for beta-alanine and taurine. Glycine-induced responses were blocked by the glycinergic antagonist strychnine, but were unaffected by either the GABAergic antagonist gabazine, the N-methyl-d-aspartate-receptor antagonist d-2-amino-5-phosphonopentanoic acid, or picrotoxin and cyanotriphenylborate, antagonists of alpha-homomeric and alpha1-subunit-containing glycine receptors, respectively. Under perforated-patch conditions, glycine induced membrane depolarizations that were sufficient to trigger action potentials (APs) in most cells. Furthermore, glycine and taurine decreased the injection currents as well as the synaptic stimulation strength required to elicit APs, indicating that glycine receptors have a consistent excitatory effect on SPn. Inhibition of taurine transport and application of hypoosmolar solutions induced strychnine-sensitive inward currents, suggesting that taurine can act as a possible endogenous agonist on SPn. In summary, these results demonstrate that SPn express glycine receptors that mediate robust excitatory membrane responses during early postnatal development.

Phenotype of the Taurine Transporter Knockout Mouse

This chapter reports present knowledge on the properties of mice with disrupted gene coding for the taurine transporter (taut-/- mice). Study of those mice unraveled some of the roles of taurine and its membrane transport for the development and maintenance of normal organ functions and morphology. When compared with wild-type controls, taut-/- mice have decreased taurine levels in skeletal and heart muscle by about 98%, in brain, kidney, plasma, and retina by 80 to 90%, and in liver by about 70%. taut-/- mice exhibit a lower body mass as well as a strongly reduced exercise capacity compared with taut+/- and wild-type mice. Furthermore, taut-/- mice show a variety of pathological features, for example, subtle derangement of renal osmoregulation, changes in neuroreceptor expression, and loss of long-term potentiation in the striatum, and they develop clinically relevant age-dependent disorders, for example, visual, auditory, and olfactory dysfunctions, unspecific hepatitis, and liver fibrosis. Taurine-deficient animal models such as acutely dietary-manipulated foxes and cats, pharmacologically induced taurine-deficient rats, and taurine transporter knockout mouse are powerful tools allowing identification of the mechanisms and complexities of diseases mediated by impaired taurine transport and taurine depletion (Chapman et al., 1993; Heller-Stilb et al., 2002; Huxtable, 1992; Lake, 1993; Moise et al., 1991; Novotny et al., 1991; Pion et al., 1987; Timbrell et al., 1995; Warskulat et al., 2004, 2006b). Taurine, which is the most abundant amino acid in many tissues, is normally found in intracellular concentrations of 10 to 70 mmol/kg in mammalian heart, brain, skeletal muscle, liver, and retina (Chapman et al., 1993; Green et al., 1991; Huxable, 1992; Timbrell et al., 1995). These high taurine levels are maintained by an ubiquitous expression of Na(+)-dependent taurine transporter (TAUT) in the plasma membrane (Burg, 1995; Kwon and Handler, 1995; Lang et al., 1998; Liu et al., 1992; Ramamoorthy et al., 1994; Schloss et al., 1994; Smith et al., 1992; Uchida et al., 1992; Vinnakota et al., 1997; Yancey et al., 1975). Taurine is not incorporated into proteins. It is involved in cell volume regulation, neuromodulation, antioxidant defense, protein stabilization, stress responses, and via formation of taurine-chloramine in immunomodulation (Chapman et al., 1993; Green et al., 1991; Huxtable, 1992; Timbrell et al., 1995). On the basis of its functions, taurine may protect cells against various types of injury (Chapman et al., 1993; Green et al., 1991; Huxtable, 1992; Kurz et al., 1998; Park et al., 1995; Stapleton et al., 1998; Timbrell et al., 1995; Welch and Brown, 1996; Wettstein and Häussinger, 1997). In order to examine the multiple taurine functions, murine models have several intrinsic advantages for in vivo research compared to other animal models, including lower cost, maintenance, and rapid reproduction rate. Further, experimental reagents for cellular and molecular studies are widely available for the mouse. In particular, mice can be easily genetically manipulated by making transgene and knockout mice. This chapter focuses on the phenotype of the TAUT-deficient murine model (taut-/-; Heller-Stilb et al., 2002), which may help researchers elucidate the diverse roles of taurine in development and maintenance of normal organ functions and morphology.

Inhibitory effect of taurine on veratridine-evoked D-[3H]aspartate release from murine corticostriatal slices: involvement of chloride channels and mitochondria

We have previously shown that the inhibitory neuromodulator taurine attenuates the release of preloaded D-[3H]aspartate from murine corticostriatal slices evoked by ischemic conditions or by application of the sodium channel agonist veratridine. The release of D-[3H]aspartate (a non-metabolized analog of glutamate) was used as an index of glutamate release. The aim of the present study was to reveal the molecular mechanisms responsible for this inhibitory effect of taurine. It was shown that 10 mM taurine suppresses D-[3H]aspartate release evoked by 0.1 mM veratridine, but does not affect the high-K+ -(50 mM) or ouabain- (0.1 mM) evoked release. Taurine had no effect in Ca2+ -free medium when the synaptic exocytosis of D-[3H]aspartate was inhibited. Nor did it suppress the release from slices preloaded with the competitive glutamate uptake blocker DL-threo-beta-hydroxyaspartate (THBA), which inhibits D-[3H]aspartate release mediated by the reverse action of glutamate transporters. Omission of Cl- from the incubation medium reduced the effect of taurine, signifying the involvement of a Cl- channel. The glycine receptor antagonist strychnine and the GABA(A) receptor antagonist bicuculline did not block the taurine effect, although picrotoxin, a less specific blocker of agonist-gated chloride channels, completely prevented the effect of taurine on veratridine-induced D-[3H]aspartate release. The respiratory chain blocker rotenone or mitochondrial protonophore carbonyl cyanide 3-chlorophenylhydrazone (CCCP) in combination with the mitochondrial ATPase inhibitor oligomycin, which inhibits the mitochondrial Ca2+ uniporter, also reduced the effect of taurine. The results obtained in the present study show that taurine acts specifically on the release of preloaded D-[3H]aspartate evoked by veratridine, but not on that evoked by other depolarizing agents, and affects the release mediated both by synaptic exocytosis and the reverse action of glutamate transporter. Taurine may attenuate D-[3H]aspartate release by regulation of mitochondrial Ca2+ sequestration and by activation of a chloride channel, but not that governed by GABA(A) or strychnine-sensitive glycine receptors.

1H MRS-detectable metabolic brain changes and reduced impulsive behavior in adult rats exposed to methylphenidate during adolescence

Administration of methylphenidate (MPH, Ritalin) to children affected by attention deficit hyperactivity disorder (ADHD) is an elective therapy, which however raises concerns for public health, due to possible persistent neuro-behavioral alterations. We investigated potential long-term consequences at adulthood of MPH exposure during adolescence, by means of behavioral and brain MRS assessment in drug-free state. Wistar adolescent rats (30- to 44-day-old) were treated with MPH (0 or 2 mg/kg once/day for 14 days) and then left undisturbed until adulthood. Levels of impulsive behavior were assessed in the intolerance-to-delay task: Food-restricted rats were tested in operant chambers with two nose-poking holes, delivering one food pellet immediately, or five pellets after a delay whose length was increased over days. MPH-exposed animals showed a less marked shifting profile from the large/late to the small/soon reward, suggesting reduced basal levels of impulsivity, compared to controls. In vivo MRI-guided 1H MRS examinations at 4.7 T in anaesthetised animals revealed long-term biochemical changes in the dorsal striatum (STR), nucleus accumbens (NAcc), and prefrontal cortex (PFC) of MPH-exposed rats. Notably, total creatine and taurine, metabolites respectively involved in bioenergetics and synaptic efficiency, were up-regulated in the STR and conversely down-regulated in the NAcc of MPH-exposed rats. A strong correlation was evident between non-phosphorylated creatine in the STR and behavioral impulsivity. Moreover, unaltered total creatine and increased phospho-creatine/creatine ratio were detected in the PFC, suggesting improved cortical energetic performance. Because of this enduring rearrangement in the forebrain function, MPH-exposed animals may be more efficient when faced with delay of reinforcement. In summary, MPH exposure during adolescence produced enduring MRS-detectable biochemical modifications in brain reward-related circuits, which may account for increased self-control capacity of adult rats.

Taurine increases mitochondrial buffering of calcium: role in neuroprotection

We have determined the role of mitochondria in the sequestration of calcium after stimulation of cerebellar granule cells with glutamate. In addition we have evaluated the neuroprotective role of taurine in excitotoxic cell death. Mitochondrial inhibitors were used to determine the calcium buffering capacity of mitochondria, as well as how taurine regulates the ability of mitochondria to buffer intracellular calcium during glutamate depolarization and excitotoxicity. We report here that pre-treatment of cerebellar granule cells with taurine (1 mM, 24 h) significantly counteracted glutamate excitotoxicity. The neuroprotective role of taurine was mediated through regulation of cytoplasmic free calcium ([Ca(2+)]( i )), and intra-mitochondrial calcium homeostasis, as determined by fluo-3 and (45)Ca(2+)-uptake. Furthermore, the overall mitochondrial function was increased in the presence of taurine, as assessed by rhodamine accumulation into mitochondria and total cellular ATP levels. We specifically tested the hypothesis that taurine reduces glutamate excitotoxicity through both the enhancement of mitochondrial function and the regulation of intracellular (cytoplasmic and intra-mitochondrial) calcium homeostasis. The role of taurine in modulating mitochondrial calcium homeostasis could be of particular importance under pathological conditions that are characterized by excessive calcium overloads. Taurine may serve as an endogenous neuroprotective molecule against brain insults.

Taurine rescues hippocampal long-term potentiation from ammonia-induced impairment

Hyperammonemia, a major pathophysiological factor in hepatic encephalopathy, impairs long-term potentiation (LTP) of synaptic transmission, a cellular model of learning and memory, in the hippocampus. We have now studied the protective action of taurine on this paradigm by analyzing LTP characteristics in mouse hippocampal slices treated with ammonium chloride (1 mM) in the presence of taurine (1 mM), an ubiquitous osmolyte, antioxidant, and neuromodulator, as well as other substances with such properties. Ammonia-treated slices displayed a significant impairment of LTP maintenance. Taurine and the mitochondrial enhancer l-carnitine, but not the antioxidants (ascorbate, carnosine, and the novel compound GVS-111) or the osmolyte betaine prevented this impairment. The protective effect of taurine was preserved under the blockade of inhibitory GABA(A) and glycine receptors. It is suggested that taurine may rescue the mechanisms of hippocampal synaptic plasticity by improving mitochondrial function under hyperammonemic conditions.

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.

Male transgenic glycine receptor alpha1 (S267Q) mutant mice display a hyperekplexia-like increase in acoustic startle responses

Glycine receptors (GlyR) are ligand-gated ion channels that inhibit neurotransmission in the spinal cord and brainstem, and mutations in GlyR can cause the human disease hyperekplexia, which is characterized by elevated startle responses. Recently, the GlyR alpha1S267Q mutation was shown to disrupt normal GlyR function, and knock-in mice harboring this mutation displayed profoundly increased acoustic startle responses and reduced glycine-stimulated the chloride flux [Findlay, G.S., Phelan, R., Roberts, M.T., Homanics, G.E., Bergeson, S.E., Lopreato, G.F., Mihic, S.J., Blednov, Y.A., Harris, R.A. 2003. Glycine receptor knock-in mice and hyperekplexia: comparisons with the null mutant. J Neurosci 23, 8051-8059.]. In this study, a transgenic mouse model expressing this S267Q mutation was evaluated using similar techniques to determine if these mice are similarly affected. Male transgenic mice displayed increased acoustic startle responses. However, decreases in glycine-stimulated strychnine-sensitive radioactive chloride (36Cl-) uptake were not observed in spinal cord and brainstem synaptoneurosomes from transgenic mice. No changes in habituation or prepulse inhibition of startle responses or spontaneous locomotion in response to taurine were observed as a result of presence of the transgene. Consistent with previous studies using immunoblotting and strychnine binding [Findlay, G.S., Wick, M.J., Mascia, M.P., Wallace, D., Miller, G.W., Harris, R.A., Blednov, Y.A. 2002. Transgenic expression of a mutant glycine receptor decreases alcohol sensitivity of mice. J Pharmacol Exp Ther 300, 526-534.], the glycine-stimulated strychnine-sensitive chloride flux of cortical microsacs in transgenic mice confirmed the ectopic expression of transgenic GlyR. These results support both the idea that transgenic expression of the S267Q mutation produces a less dramatic phenotype as compared to the knock-in mouse model as well as the idea that the in vivo acoustic startle test (as compared to the in vitro chloride flux assay) is particularly sensitive to disruptions in GlyR function.

Mechanisms of long-lasting enhancement of corticostriatal neurotransmission by taurine

The long-lasting enhancement of corticostriatal neurotransmission by taurine, LLE-TAU represents a complex phenomenon requiring concurrent activation of glycine, DA and Ach receptors as well as taurine uptake. The data on the mechanisms of corticostriatal LLE-TAU can be integrated in the following scheme. Taurine interaction with glycine and GABAA receptors causes depolarization of striatal medium spiny cells (Chepkova et al., 2002) which is enhanced by taurine electrogenic uptake by TauT (Sarkar et al., 2003). This depolarization leads to Ca2+ entry via low voltage gated Ca2+ channels. Muscarinic M1 receptors are expressed in medium spiny neurons (Yan et al., 2001) and regulate their excitability mostly via phospholipase C (PLC)/PKC cascade (Lin et al., 2004). Concurrent activation of M1 and PLC-coupled D1 receptors (O'Sullivan et al., 2004) can amplify Ca2+ signal via IP3- stimulated Ca2+ release from intracellular stores and stimulate PKC.

Endogenous neuro-protectants in ammonia toxicity in the central nervous system: facts and hypotheses

The paper overviews experimental evidence suggestive of the engagement of three endogenous metabolites: taurine, kynurenic acid, and glutathione (GSH) in the protection of central nervous system (CNS) cells against ammonia toxicity. Intrastriatal administration of taurine via microdialysis probe attenuates ammonia-induced accumulation of extracellular cyclic guanosine monophosphate (cGMP) resulting from over-activation of the N-methyl-D: -aspartate/nitric oxide (NMDA/NO) pathway, and this effect involves agonistic effect of taurine on the GABA-A and glycine receptors. Taurine also counteracts generation of free radicals, increased release of dopamine, and its metabolism to dihydroxyphenylacetic acid (DOPAC). Taurine reduces ammonia-induced increase of cell volume (edema) in cerebrocortical slices by a mechanism involving GABA-A receptors. Massive release of radiolabeled or endogenous taurine from CNS tissues by ammonia in vivo and in vitro is thought to promote its neuroprotective action, by making the amino acid available for interaction with cell membranes and/or by driving excess water out of the CNS cells (astrocytes) that underwent ammonia-induced swelling. Ammonia in vivo and in vitro affects in variable ways the synthesis of kynurenic acid (KYNA). Since KYNA is an endogenous NMDA receptor antagonist with a high affinity towards its glycine site, changes in its content may counter over-activation or depression of glutaminergic transmission observed at the different stages of hyperammonemia. GSH is a major antioxidant in the CNS whose synthesis is partly compartmented between neurons and astrocytes: astrocytic GSH is a source of precursors for the synthesis of neuronal GSH. Ammonia in vitro stimulates GSH synthesis in cultured astrocytes, which may compensate for increased GSH consumption (decreased GSH/GSSG ratio) in neurons.

Protein tyrosine nitration in hyperammonemia and hepatic encephalopathy

Hepatic encephalopathy is seen as a clinical manifestation of a chronic low grade cerebral edema, which is thought to trigger disturbances of astrocyte function, glioneuronal communication, and finally HE symptoms. In cultured astrocytes, hypoosmotic swelling triggers a rapid oxidative stress response, which involves the action of NADPH oxidase isoenzymes, followed by tyrosine nitration of distinct astrocytic proteins. Oxidative stress and protein tyrosine nitration (PTN) are also observed in response to ammonia, inflammatory cytokines, such as TNF-alpha or interferons, and benzodiazepines with affinity to the peripheral benzodiazepine receptor (PBR). NMDA receptor activation was identified as upstream event in protein tyrosine nitration (PTN). Cerebral PTN is also found in vivo after administration of ammonia, benzodiazepines or lipopolysaccharide and in portocaval shunted rats. PTN predominantly affects astrocytes surrounding cerebral vessels with potential impact on blood-brain-barrier permeability. Among the tyrosine-nitrated proteins, glutamine synthetase, GAPDH, extracellular signal-regulated kinase and the PBR were identified. PTN of glutamine synthetase is associated with inactivation of the enzyme. Thus, factors known to trigger hepatic encephalopathy induce oxidative/nitrosative stress on astrocytes with protein modifications through PTN. The pathobiochemical relevance of astrocytic PTN for the development of HE symptoms remains to be established.

Taurine prevents ammonia-induced accumulation of cyclic GMP in rat striatum by interaction with GABAA and glycine receptors

Previously, we had shown that ammonium chloride (ammonia)-induced accumulation of cyclic GMP in the microdialysates of rat striatum is blocked by taurine. In this study, coinfusion with taurine of a GABAA receptor antagonist bicuculline or a glycine receptor antagonist strychnine (100 microM each), separately, restored ammonia-induced release of cGMP to the extracellular fluid to approximately 29% and 18% of the level measured in the absence of taurine, respectively. Simultaneous coinfusion of both antagonists or of 100 muM picrotoxin, which is an antagonist of both GABAA and Gly receptors, offsets most of the taurine block. Ammonia-induced accumulation of cyclic GMP was attenuated by approximately 12% upon coinfusion of a GABAA receptor agonist muscimol (100 microM). The results suggest that stimulation of both GABAA and glycine receptors is involved in the mechanism by which taurine limits the activation of the NMDA/NO/cGMP pathway by ammonia in the striatum.

Long-Lasting Enhancement of Corticostriatal Transmission by Taurine: Role of Dopamine and Acetylcholine

1. Taurine applied to mouse brain slices evokes a long-lasting enhancement (LLE) of corticostriatal synaptic transmission, LLE(TAU). 2. The occurrence of LLE(TAU) was significantly decreased in the presence of the specific antagonists at either D1 (SCH23390) or D2 (raclopride) dopamine (DA) receptors. 3. LLE(TAU) was prevented by scopolamine, a muscarinic antagonist, and significantly suppressed by the nicotinic antagonist mecamylamine. 4. Thus, dopaminergic and cholinergic mechanisms, in concert with the taurine transporter and glycine receptors, contribute critically to the induction of corticostriatal LLE(TAU).

High Affinity Transport of Taurine by the Drosophila Aspartate Transporter dEAAT2

Excitatory amino acid transporters (EAATs) are structurally related plasma membrane proteins known to mediate the Na(+)/K(+)-dependent uptake of the amino acids l-glutamate and dl-aspartate. In the nervous system, these proteins contribute to the clearance of glutamate from the synaptic cleft and maintain excitatory amino acid concentrations below excitotoxic levels. Two homologues exist in Drosophila melanogaster, dEAAT1 and dEAAT2, which are specifically expressed in the nervous tissue. We previously reported that dEAAT2 shows unique substrate discrimination as it mediates high affinity transport of aspartate but not glutamate. We now show that dEAAT2 can also transport the amino acid taurine with high affinity, a property that is not shared by two other transporters of the same family, Drosophila dEAAT1 and human hEAAT2. Taurine transport by dEAAT2 was efficiently blocked by an EAAT antagonist but not by inhibitors of the structurally unrelated mammalian taurine transporters. Taurine and aspartate are transported with similar K(m) and relative efficacy and behave as mutually competitive inhibitors. dEAAT2 can mediate either net uptake or the heteroexchange of its two substrates, both being dependent on the presence of Na(+) ions in the external medium. Interestingly, heteroexchange only occurs in one preferred substrate orientation, i.e. with taurine transported inwards and aspartate outwards, suggesting a mechanism of transinhibition of aspartate uptake by intracellular taurine. Therefore, dEAAT2 is actually an aspartate/taurine transporter. Further studies of this protein are expected to shed light on the role of taurine as a candidate neuromodulator and cell survival factor in the Drosophila nervous system.

Cellular and subcellular localization of the inhibitory glycine receptor in hippocampal neurons

Inhibitory glycine receptors are most abundant in spinal cord and brainstem, and glycinergic synapses have a well-established role in the regulation of locomotor behavior. Little is known about the function of glycine receptors in cortex and hippocampus, where GABA plays a dominant role in synaptic inhibition. Therefore, we have investigated tissue and cellular expression of glycine receptor alpha-subunits. Western blot and immunohistochemical analyses reveal the presence of glycine receptors in hippocampal tissue. Immunocytochemical experiments in hippocampal cultures show prominent cellular expression of glycine receptors in pyramidal neurons and GAD-positive interneurons similar to the calcium-binding protein VILIP-1 with widespread hippocampal distribution. On the subcellular level we found co-staining of GlyR and the presynaptic marker synapsin I. Furthermore, co-staining with GAD at synaptic terminals indicated partial co-localization of GABA- and glycine receptors.

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.

Taurine activates strychnine-sensitive glycine receptors in neurons freshly isolated from nucleus accumbens of young rats

Although functional glycine receptors (GlyRs) are present in the mature nucleus accumbens (NAcc), an important area of the mesolimbic dopamine system involved in drug addiction, their role has been unclear because the NAcc contains little glycine. However, taurine, an agonist of GlyRs, is abundant throughout the brain, especially during early development. In the present study on freshly dissociated NAcc neurons from young Sprague-Dawley rats (12- to 21-day old), we found that both glycine and taurine can strongly depolarize NAcc neurons and modulate their excitability. In voltage-clamped NAcc neurons, glycine and taurine elicited chloride currents (IGly and ITau) with an EC50 of 0.12 and 1.25 mM, respectively. The reversal potential of IGly or ITau was 0 mV in conventional whole cell mode and -30 mV in gramicidin-perforated mode. At concentrations <1 mM, both glycine and taurine were very effectively antagonized by strychnine and by picrotoxin (with an IC50 of 60 nM and 36.5 microM for IGly, and 40 nM and 42.2 microM for ITau) but were insensitive to 10 microM bicuculline. The currents elicited by taurine (< or =1 mM) showed complete cross-desensitization with IGly, but none with gamma-aminobutyric acid (GABA)-induced currents (IGABA). However, ITau elicited by very concentrated taurine (10 mM) showed partial cross-desensitization with IGABA, and it was substantially antagonized by 10 microM bicuculline. These results indicate that taurine binds mainly to GlyRs in NAcc, but it could be a partial agonist of GABAA receptors. By activating GlyRs, taurine may play an important physiological role in the control of NAcc function, especially during development.

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.

Exogenous glutamate and taurine exert differential actions on light-induced release of two endogenous amino acids in isolated rat retina

A dark-adapted isolated rat retina, preloaded with [(14)C]glutamate ([(14)C]Glu) and [(3)H]taurine ([(3)H]Tau), was superfused with artificial cerebrospinal fluid (ACSF) in the absence and presence of Glu (1 mM) or Tau (1 mM), as well as the Glu uptake inhibitors dihydrokainic acid (DHK, 0.04 mM) and trans-L-pyrrolidine-2,4-dicarboxylate (t-PDC, 0.004 mM). After 10 min of light stimulation, the extracellular level of [(14)C]Glu and [(3)H]Tau was reduced to 82 +/- 2% and 65 +/- 4% of the control, respectively. Basal release was enhanced when Tau and t-PDC were applied together, although none of the compounds had any effect when applied individually. Glu and DHK had no effect. The decrease of [(14)C]Glu efflux evoked by light stimuli was enhanced by t-PDC and Tau, either added separately or together, whereas Glu and DHK were without effect. In contrast, [(3)H]Tau efflux variations induced by light stimuli were reduced markedly by both Tau and Glu. These findings suggest distinctly different roles of Tau and Glu in light-induced responses in mammalian retina, including a possible role for Tau in light adaptation processes.

Taurine-induced long-lasting enhancement of synaptic transmission in mice: role of transporters

Taurine, a major osmolyte in the brain evokes a long-lasting enhancement (LLETAU) of synaptic transmission in hippocampal and cortico-striatal slices. Hippocampal LLETAU was abolished by the GABA uptake blocker nipecotic acid (NPA) but not by the taurine-uptake inhibitor guanidinoethyl sulphonate (GES). Striatal LLETAU was sensitive to GES but not to NPA. Semiquantitative PCR analysis and immunohistochemistry revealed that taurine transporter expression is significantly higher in the striatum than in the hippocampus. Taurine transporter-deficient mice displayed very low taurine levels in both structures and a low ability to develop LLETAU in the striatum, but not in the hippocampus. The different mechanisms of taurine-induced synaptic plasticity may reflect the different vulnerabilities of these brain regions under pathological conditions that are accompanied by osmotic changes such as hepatic encephalopathy.

Taurine reduces ammonia- and N-methyl-D-aspartate-induced accumulation of cyclic GMP and hydroxyl radicals in microdialysates of the rat striatum

Acute ammonia neurotoxicity caused by intraperitoneal administration of ammonium salts is mediated by overactivation of N-methyl-D-aspartate (NMDA) receptors, with ensuing generation of free radicals and extracellular accumulation of cyclic GMP (cGMP) arising from stimulation of nitric oxide (NO) synthesis. In this study, infusion of ammonium chloride or NMDA into the striata of rats via microdialysis probes increased the contents of cyclic GMP and hydroxyl radicals in the microdialysates. Co-infusion of taurine virtually abolished both the ammonia- and NMDA-induced accumulation of cGMP. Taurine also attenuated accumulation of hydroxyl radicals evoked by either treatment. This result is the first evidence of a potential of taurine to attenuate the effects of NMDA receptor overactivation by ammonia in vivo and points to the inhibition of the NMDA receptor-mediated NO synthesis as a possible mechanism of its neuroprotective action. Taurine or its blood-brain barrier penetrating analogues may be applicable in treatment of ammonia-induced neurological deficits.