Release of GABA and taurine from brain slices

A review on epilepsy, current treatments, and potential of medicinal plants as an alternative treatment

Epilepsy is considered common neurological diseases that threaten the lives of millions of people all around the world. Since ancient times, different forms of medications have been used to treat this condition. Adverse events associated with treatments and the residence time of available drugs caused to search for safer and more efficient therapies and drugs remain one of the major areas of research interest for scientists. As one of the therapeutics with fewer side effects, plants and their essential oils can be considered replacements for existing treatments. Medicinal plants have proven to be an effective natural source of antiepileptic drugs; most of them have their mechanism of action by affecting GABA receptors in different paths. Cannabis indica and Cymbopogon winterianus are well-known plant species with antiepileptic activities. The current review presenting a list of plants with antiepileptic effects aims to pave the way for finding alternative drugs with fewer side effects for scientists.

Taurine and Astrocytes: A Homeostatic and Neuroprotective Relationship

Taurine is considered the most abundant free amino acid in the brain. Even though there are endogenous mechanisms for taurine production in neural cells, an exogenous supply of taurine is required to meet physiological needs. Taurine is required for optimal postnatal brain development; however, its brain concentration decreases with age. Synthesis of taurine in the central nervous system (CNS) occurs predominantly in astrocytes. A metabolic coupling between astrocytes and neurons has been reported, in which astrocytes provide neurons with hypotaurine as a substrate for taurine production. Taurine has antioxidative, osmoregulatory, and anti-inflammatory functions, among other cytoprotective properties. Astrocytes release taurine as a gliotransmitter, promoting both extracellular and intracellular effects in neurons. The extracellular effects include binding to neuronal GABAA and glycine receptors, with subsequent cellular hyperpolarization, and attenuation of N-methyl-D-aspartic acid (NMDA)-mediated glutamate excitotoxicity. Taurine intracellular effects are directed toward calcium homeostatic pathway, reducing calcium overload and thus preventing excitotoxicity, mitochondrial stress, and apoptosis. However, several physiological aspects of taurine remain unclear, such as the existence or not of a specific taurine receptor. Therefore, further research is needed not only in astrocytes and neurons, but also in other glial cells in order to fully comprehend taurine metabolism and function in the brain. Nonetheless, astrocyte’s role in taurine-induced neuroprotective functions should be considered as a promising therapeutic target of several neuroinflammatory, neurodegenerative and psychiatric diseases in the near future. This review provides an overview of the significant relationship between taurine and astrocytes, as well as its homeostatic and neuroprotective role in the nervous system.

Long-lasting alterations of hippocampal GABAergic neurotransmission in adult rats following perinatal Δ9-THC exposure

The long-lasting effects of gestational cannabinoids exposure on the adult brain of the offspring are still controversial. It has already been shown that pre- or perinatal cannabinoids exposure induces learning and memory disruption in rat adult offspring, associated with permanent alterations of cortical glutamatergic neurotransmission and cognitive deficits. In the present study, the risk of long-term consequences induced by perinatal exposure to cannabinoids on rat hippocampal GABAergic system of the offspring, has been explored. To this purpose, pregnant rats were treated daily with Delta⁹-tetrahydrocannabinol (Δ⁹-THC; 5mg/kg) or its vehicle. Perinatal exposure to Δ⁹-THC induced a significant reduction (p<0.05) in basal and K⁺-evoked [³H]-GABA outflow of 90-day-old rat hippocampal slices. These effects were associated with a reduction of hippocampal [³H]-GABA uptake compared to vehicle exposed group. Perinatal exposure to Δ⁹-THC induced a significant reduction of CB1 receptor binding (B_max) in the hippocampus of 90-day-old rats. However, a pharmacological challenge with either Δ⁹-THC (0.1μM) or WIN55,212-2 (2μM), similarly reduced K⁺-evoked [³H]-GABA outflow in both experimental groups. These reductions were significantly blocked by adding the selective CB1 receptor antagonist SR141716A. These findings suggest that maternal exposure to cannabinoids induces long-term alterations of hippocampal GABAergic system. Interestingly, previous behavioral studies demonstrated that, under the same experimental conditions as in the present study, perinatal cannabinoids exposure induced cognitive impairments in adult rats, thus resembling some effects observed in humans. Although it is difficult and sometimes misleading to extrapolate findings obtained from animal models to humans, the possibility that an alteration of hippocampus aminoacidergic transmission might underlie, at least in part, some of the cognitive deficits affecting the offspring of marijuana users, is supported.

Properties of Taurine Release in Glucose-Free Media in Hippocampal Slices from Developing and Adult Mice

The release of preloaded [(3)H]taurine from hippocampal slices from developing 7-day-old and young adult 3-month-old mice was studied in a superfusion system in the absence of glucose. These hypoglycemic conditions enhanced the release at both ages, the effect being markedly greater in developing mice. A depolarizing K(+) concentration accentuated the release, which indicates that it was partially mediated by exocytosis. The anion channel blockers were inhibitory, witnessing the contribution of ion channels. NO-generating agents fomented the release as a sign of the participation of excitatory amino acid receptors. The other second messenger systems were apparently less efficient. The much greater taurine release could be a reason for the well-known greater tolerance of developing nervous tissue to lack of glucose.

Mechanisms Underlying Taurine Protection Against Glutamate-Induced Neurotoxicity

Taurine appears to exert potent protections against glutamate (Glu)-induced injury to neurons, but the underlying molecular mechanisms are not fully understood. The possibly protected targets consist of the plasma membrane and the mitochondrial as well as endoplasmic reticulum (ER) membranes. Protection may be provided through a variety of effects, including the prevention of membrane depolarization, neuronal excitotoxicity and mitochondrial energy failure, increases in intracellular free calcium ([Ca2+]i), activation of calpain, and reduction of Bcl-2 levels. These activities are likely to be linked spatially and temporally in the neuroprotective functions of taurine. In addition, events that occur downstream of Glu stimulation, including altered enzymatic activities, apoptotic pathways, and necrosis triggered by the increased [Ca2+]i, can be inhibited by taurine. This review discusses the possible molecular mechanisms of taurine against Glu-induced neuronal injury, providing a better understanding of the protective processes, which might be helpful in the development of novel interventional strategies.

Protection of taurine and granulocyte colony-stimulating factor against excitotoxicity induced by glutamate in primary cortical neurons

Abstracts

EFFECTS OF ISOFLURANE ON GLUTAMATE AND TAURINE RELEASES, BRAIN SWELLING AND INJURY DURING TRANSIENT ISCHEMIA AND REPERFUSION

The volatile anesthetic agent isoflurane was thought to provide neuroprotection against ischemic damage; however, this effect remains controversial. Using the middle cerebral artery occlusion model and intracerebral microdialysis, the authors monitored the variations of glutamate and taurine concentrations in the extra-cellular space in male rats anesthetized with pentobarbital or isoflurane. Brain injury and edema were evaluated 24 h after ischemia. Isoflurane prevented the ischemia-induced efflux of glutamate and reduced the release of taurine. No difference in the size of the brain lesions was observed with both anesthetics, and isoflurane induced the formation of a bigger brain edema and reduced taurine release. These results suggest that inhibiting glutamate release during ischemia may not be sufficient to improve brain outcome after transient ischemia.

Characteristics of GABA Release Induced by Free Radicals in Mouse Hippocampal Slices

The release of the inhibitory neurotransmitter GABA is generally enhanced under potentially cell-damaging conditions. The properties and regulation of preloaded [3H]GABA release from mouse hippocampal slices were now studied in free radical-containing medium in a superfusion system. Free radical production was induced by 0.01% of H2O2 in the medium. H2O2 markedly potentiated GABA release, which was further enhanced about 1.5-fold by K+ stimulation (50 mM). In Ca2+-free media this stimulation was not altered, indicating that the release was mostly Ca2+-independent. Moreover, omission of Na+ increased the release, suggesting that it is mediated by Na+-dependent transporters operating outwards, a conception confirmed by the enhancement with GABA homoexchange. Inhibition of the release with the ion channel inhibitors diisothiocyanostilbene-2,2'-disulphonate and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate indicates that Cl(-) channels also participate in the process. This release was not modified by the adenosine receptor (A1 and A2a) agonists and ionotropic glutamate receptor agonists kainate, N-methy-D: -aspartate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, whereas the agonists of metabotropic glutamate receptors of group I [(S)-3,5-dihydroxyphenylglycine] and of group II [(2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate] enhanced it by receptor-mediated mechanisms, the effects being abolished by their respective antagonists. The group III agonist L+-2-amino-4-phosphonobutyrate reduced the evoked GABA release, but this was not affected by the antagonist. Furthermore, the release was reduced by activation of protein kinase C by 4 beta-phorbol 12-myristate 13-acetate and by inhibition of tyrosine kinase by genistein and of phoshoplipase by quinacrine. On the other hand, increasing cGMP levels with the phosphodiesterase inhibitor zaprinast, selective for PDE5, 6 and 9, and NO production with the NO-generating compounds hydroxylamine, sodium nitroprusside and S-nitroso-N-penicillamine enhanced the release. The regulation of GABA release induced by free radical production proved thus to be rather complex. Under potentially cell-damaging conditions, the potentiation of GABA release may be a mechanism to counteract hyperactivity and reduce the effects of excitatory amino acid release. On the other hand, reduction of GABA release could be harmful and contribute to excitotoxic damage and neuronal degeneration.

Nitric oxide is involved in taurine release in the mouse brain stem under normal and ischemic conditions

Nitric oxide (NO) has been shown to regulate neurotransmitter release in the brain; both inhibitory and excitatory effects have been seen. Taurine is essential for the development and survival of neural cells and protects them under cell-damaging conditions. In the brain stem, it regulates many vital functions such as cardiovascular control and arterial blood pressure. Now we studied the effects of the NO-generating compounds hydroxylamine (HA), S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP) on the release of preloaded [(3)H]taurine under normal and ischemic conditions in slices prepared from the mouse brain stem from developing (7-day-old) to young adult (3-month-old) mice. In general, the effects of NO on the release were somewhat complex and difficult to explain, as expected from the multifunctional role of NO in the central nervous system. The basal initial release under normal conditions was enhanced by the NO donors 5 mM HA and 1.0 mM SNAP at both ages, but SNP was inhibitory in developing mice. The release was markedly enhanced by K(+) stimulation. The effects of HA, SNAP and SNP on the basal release were not antagonized by the NO synthase inhibitor N(G)-nitro-L-arginine (L-NNA, 1.0 mM), demonstrating that mechanisms other than NO synthesis are involved. Taurine release in developing mice in the presence of SNP was reduced by the inhibitor of soluble guanylate cyclase, 1H-(1,2,3)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), indicating the possible involvement of cGMP. In normoxia, N-methyl-D-aspartate (NMDA, 1.0 mM) enhanced the SNAP- and HA-evoked taurine release in developing mice and the HA-evoked release in adults. In ischemia, both K(+) stimulation and NMDA potentiated the NO-induced release, particularly in the immature mice, probably without the involvement of the NO synthase or cGMP. The substantial release of taurine in the developing brain stem evoked by NO donors together with NMDA might represent signs of important mechanisms against excitotoxicity which protect the brain stem under cell-damaging conditions.

Influences of different developmental periods of taurine supplements on synaptic plasticity in hippocampal CA1 area of rats following prenatal and perinatal lead exposure

Background

Previous study has demonstrated that dietary taurine supplement protected rats from impairments of synaptic plasticity induced by postnatal lead exposure. However, little is known about the role of taurine in the presence of prenatal and perinatal lead exposure. We investigated the possible effect of taurine supplement on prenatal and perinatal lead-induced synaptic plasticity deficit and determined developmental periods critical for the effect of taurine.

Results

In the present study, taurine was administrated to prenatal and perinatal lead-exposed rats in different developmental periods: from prenatal to weaning (Lead+PW-Tau), from weaning to life (Lead+WL-Tau), and from prenatal to life (Lead+PL-Tau). We examined the input-output (I/O) function, paired-pulse facilitation (PPF) and the long-term potentiation (LTP) of field excitatory postsynaptic potential (fEPSP) in the hippocampal CA1 area of rats on postnatal days 18–25 (P18–25) or days 60–75 (P60–75). We found that (1) on P18–25, taurine had no evident effect on I/O functions and PPF ratios of lead-exposed rats but caused a 12.0% increase in the LTP amplitudes of these animals; (2) on P60–75, taurine significantly elevated lead depressed I/O functions and PPF ratios in Lead+PW-Tau and Lead+PL-Tau rats, but failed in Lead+WL-Tau rats. The amplitudes of LTP of lead-exposed rats were all significantly increased by additional taurine supplement in any developmental period compared with untreated rats. Thus, taurine appeared to have the most effect during the prenatal and lactation periods and its effects on younger rats would not be manifest until the adult life; and (3) the level of lead deposition in hippocampus was evidently reduced by additional treatment of taurine in lead-exposed rats, compared with untreated rats.

Conclusion

Taurine supplement can protect the adult rats from synaptic plasticity deficits following prenatal and perinatal lead exposure, and the protective effects are critical for the prenatal and lactation periods of lead-exposed rats.

Modulation of GABA release by second messenger substances and NO in mouse brain stem slices under normal and ischemic conditions

GABA is the inhibitory neurotransmitter in most brain stem nuclei. The properties of release of preloaded [(3)H]GABA were now investigated with slices from the mouse brain stem under normal and ischemic (oxygen and glucose deprivation) conditions, using a superfusion system. The ischemic GABA release increased about fourfold in comparison with normal conditions. The tyrosine kinase inhibitor genistein had no effect on GABA release, while the phospholipase inhibitor quinacrine reduced both the basal and K(+)-evoked release in normoxia and ischemia. The activator of protein kinase C (PKC) 4beta-phorbol 12-myristate 13-acetate had no effects on the releases, whereas the PKC inhibitor chelerythrine reduced the basal release in ischemia. When the cyclic guanosine monophosphate (cGMP) levels were increased by superfusion with zaprinast and other phosphodiesterase inhibitors, GABA release was reduced under normal conditions. The NO donors S-nitroso-N-acetylpenicillamine (SNAP) and hydroxylamine (HA) enhanced the basal and K(+)-stimulated release by acting directly on presynaptic terminals. Under ischemic conditions GABA release was enhanced when cGMP levels were increased by zaprinast. This effect was confirmed by inhibition of the release by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The NO-producing agents SNAP, HA, and sodium nitroprusside potentiated GABA release in ischemia. These effects were reduced by the NO synthase inhibitor N(G)-nitro-L: -arginine, but not by ODQ. The results show that particularly NO and cGMP regulate both normal and ischemic GABA release in the brain stem. Their effects are however complex.

Taurine release in mouse brain stem slices under cell-damaging conditions

Taurine has been thought to be essential for the development and survival of neural cells and to protect them under cell-damaging conditions. In the brain stem taurine regulates many vital functions, including cardiovascular control and arterial blood pressure. We have recently characterized the release of taurine in the adult and developing brain stem under normal conditions. Now we studied the properties of preloaded [3H]taurine release under various cell-damaging conditions (hypoxia, hypoglycemia, ischemia, the presence of metabolic poisons and free radicals) in slices prepared from the mouse brain stem from developing (7-day-old) and young adult (3-month-old) mice, using a superfusion system. Taurine release was greatly enhanced under these cell-damaging conditions, the only exception being the presence of free radicals in both age groups. The ischemia-induced release was characterized to consist of both Ca2+-dependent and -independent components. Moreover, the release was mediated by Na+-, Cl--dependent transporters operating outwards, particularly in the immature brain stem. Cl- channel antagonists reduced the release at both ages, indicating that a part of the release occurs through ion channels, and protein kinase C appeared to be involved. The release was also modulated by cyclic GMP second messenger systems, since inhibitors of soluble guanylyl cyclase and phosphodiesterases suppressed ischemic taurine release. The inhibition of phospholipases also reduced taurine release at both ages. This ischemia-induced taurine release could constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.

Characteristics of taurine release in slices from adult and developing mouse brain stem

Taurine has been thought to function as a regulator of neuronal activity, neuromodulator and osmoregulator. Moreover, it is essential for the development and survival of neural cells and protects them under cell-damaging conditions. Taurine is also involved in many vital functions regulated by the brain stem, including cardiovascular control and arterial blood pressure. The release of taurine has been studied both in vivo and in vitro in higher brain areas, whereas the mechanisms of release have not been systematically characterized in the brain stem. The properties of release of preloaded [(3)H]taurine were now characterized in slices prepared from the mouse brain stem from developing (7-day-old) and young adult (3-month-old) mice, using a superfusion system. In general, taurine release was found to be similar to that in other brain areas, consisting of both Ca(2+)-dependent and Ca(2+)-independent components. Moreover, the release was mediated by Na(+)-, Cl(-)-dependent transporters operating outwards, as both Na(+)-free and Cl(-) -free conditions greatly enhanced it. Cl(-) channel antagonists and a Cl(-) transport inhibitor reduced the release at both ages, indicating that a part of the release occurs through ion channels. Protein kinases appeared not to be involved in taurine release in the brain stem, since substances affecting the activity of protein kinase C or tyrosine kinase had no significant effects. The release was modulated by cAMP second messenger systems and phospholipases at both ages. Furthermore, the metabotropic glutamate receptor agonists likewise suppressed the K(+)-stimulated release at both ages. In the immature brain stem, the ionotropic glutamate receptor agonists N-methyl-D-aspartate (NMDA) and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) potentiated taurine release in a receptor-mediated manner. This could constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.

Characteristics of GABA release in mouse brain stem slices under normal and ischemic conditions

GABA is known to be the inhibitory neurotransmitter in the majority of brain stem nuclei. The release of GABA has been extensively studied both in vivo and in vitro in higher brain areas, whereas the mechanisms of release in the brain stem have not been systemically characterized. The properties of preloaded [3H]GABA were now investigated in mouse brain stem slices, using a superfusion system. The basal release was enhanced by K+ stimulation (50 mM K+) and under various cell-damaging conditions (ischemia, hypoglycemia, the presence of free radicals and metabolic poisons). No K+-stimulated release was discernible in the absence of Ca2+, indicating that the release was at least partly Ca2+-dependent. Moreover, the release was increased when Na+ or Cl- was omitted from the superfusion medium. GABA and beta-alanine stimulated the release, confirming the involvement of the reversed function of GABA transporters. Incubation of the slices with the anion channel inhibitors diisothiocyanostilbene and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate and with the Cl- uptake inhibitor 9-anthracenecarboxylic acid also reduced GABA release, demonstrating that a part of it comprises leakage through anion channels. All these mechanisms were involved in the ischemia-induced GABA release, which was over 4-fold greater than the release in normoxia. Contrary to the other brain areas, GABA release in the brain stem was not affected by ionotropic glutamate receptors but may be modulated by metabotropic receptors. This ischemia-induced GABA release might constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.

Activation of GABAA receptors by taurine and muscimol blocks the neurotoxicity of β-amyloid in rat hippocampal and cortical neurons

The beta-amyloid peptide (Abeta) is centrally related to the pathogenesis of Alzheimer's disease (AD) and is potently neurotoxic to central nervous system neurons. The neurotoxicity of Abeta has been partially related to the over activation of glutamatergic transmission and excitotoxicity. Taurine is a naturally occurring beta-amino acid present in the mammalian brain. Due to its safety and tolerability, taurine has been clinically used in humans in the treatment of a number of non-neurological disorders. Here, we show that micromolar doses of taurine block the neurotoxicity of Abeta to rat hippocampal and cortical neurons in culture. Moreover, taurine also rescues central neurons from the excitotoxicity induced by high concentrations of extracellular glutamate. Neuroprotection by taurine is abrogated by picrotoxin, a GABA(A) receptor antagonist. GABA and muscimol, an agonist of the GABA(A) receptor, also block neuronal death induced by Abeta in rat hippocampal and cortical neurons. These results suggest that activation of GABA(A) receptors protects neurons against Abeta toxicity in AD-affected regions of the mammalian brain and that taurine should be investigated as a novel therapeutic tool in the treatment of AD and of other neurological disorders in which excitotoxicity plays a relevant role.

The role of protein kinase C and cyclic AMP in the ammonia-induced shift of the taurine uptake/efflux balance towards efflux in C6 cells

A previous study showed that treatment of C6 glioma cells with 10 mM ammonium chloride monia") for 24 h decreases taurine uptake and evokes sodium-dependent taurine efflux, indicating reversal of the taurine transporter (TauT)-mediated transport as an underlying mechanism. Consistent with the involvement of TauT we now show that the ammonia-induced changes in Tau uptake and efflux are inhibited by the protein kinase C (PKC) activator phorbol 12,13-dibutyrate (PDBu). Ammonia treatment of C6 cells resulted in increased intracellular accumulation of cAMP. Incubation of the cells with dibutyryl cAMP (dbcAMP) mimicked the effects of ammonia on both taurine uptake and efflux. The effects of dbcAMP on taurine uptake and efflux were additive to the effects of ammonia. Collectively, the results suggest that the effects of ammonia on taurine uptake and efflux may be partly mediated by cAMP. Consistent with this mechanism, the adenyl cyclase inhibitor, miconazole reduced the stimulation of efflux by ammonia.

Interstitial concentrations of amino acids in the rat striatum during global forebrain ischemia and potassium-evoked spreading depression

The early detection and appropriate treatment of brain ischemia is of paramount importance. The interstitial concentrations of neurotransmitter amino acids are often used as an index of neuronal injury. However, monitoring of non-neurotransmitter amino acids may be equally important. We have studied the behavior of 10 amino acids during K(+)-induced spreading depression (application of 70 mM KCl during 40 min) and global forebrain ischemia (two-vessel occlusion with hypotension during 20 min). The concentrations of glutamate, aspartate, taurine, GABA, glycine, and alanine, measured in the rat striatum by microdialysis, increased during both ischemia and spreading depression, whereas glutamine concentrations decreased in both cases. Only ischemia, but not spreading depression, led to enhanced release of serine, threonine, and asparagine. We thus conclude that an elevation in the interstitial concentrations of non-neurotransmitter amino acids is specific to deep ischemic injury to nervous tissue. We propose the monitoring of serine, asparagine, and threonine, together with excitatory amino acids, as an index of the degree of ischemic brain injury.

Role of the GABA transporter in epilepsy

The GABA transporter plays a well-established role in reuptake of GABA after synaptic release. The anticonvulsant effect of tiagabine appears to result largely from blocking this reuptake. However, there is another side to the GABA transporter, contributing to GABA release by reversing in response to depolarization. We have recently shown that this form of GABA release is induced by even small increases in extracellular [K+], and has a powerful inhibitory effect on surrounding neurons. This transporter-mediated GABA release is enhanced by the anticonvulsants gabapentin and vigabatrin. The latter drug also potently increases ambient [GABA], inducing tonic inhibition of neurons. Here we review the evidence in support of a physiological role for GABA transporter reversal, and the evidence that it is increased by high-frequency firing. We postulate that the GABA transporter is a major determinant of the level of tonic inhibition, and an important source of GABA release during seizures. These recent findings indicate that the GABA transporter plays a much more dynamic role in control of brain excitability than has previously been recognized. Further defining this role may lead to a better understanding of the mechanisms of epilepsy and new avenues for treatment.

The role of inhibitory amino acidergic neurotransmission in hepatic encephalopathy: a critical overview

Gamma-Aminobutyric acid (GABA) is the main inhibitory amino acid in the central nervous system (CNS). Experiments with animal models of HE, and with brain slices or cultured CNS cells treated with ammonia, have documented changes in GABA distribution and transport, and modulation of the responses of both the GABA(A)-benzodiazepine receptor complex and GABA(B) receptors. Although many of the data point to an enhancement of GABAergic transmission probably contributing to HE, the evidence is not unequivocal. The major weaknesses of the GABA theory are (1) in a vast majority of HE models, there were no alterations of GABA content in the brain tissue and/or extracellular space, indicating that exposure of neurons to GABA may not have been altered, (2) changes in the affinity and capacity of GABA receptor binding were either absent or qualitatively different in HE models of comparable severity and duration, and (3) no sound changes in the GABAergic system parameters were noted in clinical cases of HE. Taurine (Tau) is an amino acid that is thought to mimic GABA function because of its agonistic properties towards GABA(A) receptors, and to contribute to neuroprotection and osmoregulation. These effects require Tau redistribution between the different cell compartments and the extracellular space. Acute treatment with ammonia evokes massive release of radiolabeled or endogenous Tau from CNS tissues in vivo and in vitro, and the underlying mechanism of Tau release differs from the release evoked by depolarizing conditions or hypoosmotic treatment. Subacute or chronic HE, and also long-term treatment of cultured CNS cells in vitro with ammonia, increase spontaneous Tau "leakage" from the tissue. This is accompanied by a decreased potassium- or hypoosmolarity-induced release of Tau and often by cell swelling, indicating impaired osmoregulation. In in vivo models of HE, Tau leakage is manifested by its increased accumulation in the extrasynaptic space, which may promote inhibitory neurotransmission and/or cell membrane protection. In chronic HE in humans, decreased Tau content in CNS is thought to be one of the causes of cerebral edema. However, understanding of the impact of the changes in Tau content and transport on the pathogenic mechanisms of HE is hampered by the lack of clear-cut evidence regarding the various roles of Tau in the normal CNS.

Effects of ammonia and hepatic failure on the net efflux of endogenous glutamate, aspartate and taurine from rat cerebrocortical slices: modulation by elevated K+ concentrations

Cerebrocortical minislices derived from control rats ("control slices") and from rats with thioacetamide (TAA)-induced hepatic failure showing moderate hyperammonemia and symptoms of hepatic encephalopathy (HE) ("HE slices"), were incubated with physiological saline in the absence or presence of 5 mM ammonium acetate ("ammonia"), at potassium ion (K+) concentrations ranging from 5 to 15 mM. The efflux of endogenous aspartate (Asp), glutamate (Glu) and taurine (Tau) to the incubation medium was assayed by HPLC. At 5 mM K+, perfusion of control slices with ammonia did not affect Glu and slightly depressed Asp efflux. Raising K+ concentrations in the incubation medium to 7.5 led to inhibition of Glu and Asp efflux by ammonia and the inhibitory effect was further potentiated at 10 mM K+. The inhibition was also significant at 15 mM K+. This suggests that, depression of excitatory neurotransmission associated with acute hyperammonemia is more pronounced under conditions of intense neuronal activity than in the resting state. HE moderately increased the efflux of Glu and Asp, and the stimulatory effect of HE on Glu and Asp efflux showed virtually no variation upon changing K+ concentration up to 15 mM. Ammonia strongly, and HE moderately, increased Tau efflux at 5 mM K+. However, both the ammonia- and HE-dependent Tau efflux decreased with increasing K+ concentration in the medium and was no longer significant at 10 mM concentration, indicating that intense neuronal activity obliterates the neuroprotective functions of this amino acid triggered by hyperammonemia.

Effects of acute toxic doses of psychostimulants on extracellular levels of excitatory amino acids and taurine in rats: comparison of d-amphetamine and sydnocarb

We used microdialysis to study how acute toxic doses of d-amphetamine and sydnocarb [3-(beta-phenylisopropyl)-N-phenylcarbamoylsydnonimine], an original Russian psychostimulant, affect extracellular levels of glutamate, aspartate, and taurine in the neostriatum of halothane-anesthetized male Sprague-Dawley rats. The administration of d-amphetamine (5.0 mg/kg x 4 i.p.) caused gradual fivefold increases in the extracellular glutamate and taurine levels and moderate increases in the extracellular aspartate level. Sydnocarb administration (23.8 mg/kg x 4 i.p., a dose equimolar to 5.0 mg/kg d-amphetamine) elicited a marked increase in the extracellular aspartate level and a small increase in the extracellular level of glutamate. The extracellular taurine level increased only after the last (fourth) injection. We conclude that a massive increase in extracellular taurine reflects hyperactivation of glutamatergic neurotransmission elicited by acute toxic dose of d-amphetamine. Sydnocarb seems to be less neurotoxic than d-amphetamine, because it elicits lesser changes in the extracellular levels of glutamate and taurine.

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.

Effects of sydnocarb and D-amphetamine on the extracellular levels of amino acids in the rat caudate-putamen

The neurotoxic effects of psychostimulants at high dosages limit their clinical applicability but the mechanism of neurotoxicity is still unsettled. We now studied by microdialysis how acute and subchronic (four times at 2-h intervals) administrations of D-amphetamine and sydnocarb [3-(beta-phenylisopropyl)-N-phenylcarbamoylsydnonimine], an original novel Russian psychostimulant, affected the extracellular levels of amino acids in the caudate-putamen of halothane-anesthetized male Sprague-Dawley rats. Acute D-amphetamine administration (5.0 mg/kg, i.p.) produced a moderate accumulation of extracellular glutamate and aspartate. Sydnocarb (23.8 mg/kg, i.p., a dose equimolar to 5.0 mg/kg D-amphetamine) also increased extracellular glutamate and alanine. Subchronic D-amphetamine administration (5.0 mg/kg x 4, i.p.) caused gradual fivefold increases in the glutamate and taurine levels and moderate increases in the aspartate and alanine levels. Subchronic sydnocarb administration (23.8 mg/kg x 4, i.p.) elicited a marked increase in the aspartate level and a small increase in the level of glutamate. The alanine level increased temporarily after each administration of sydnocarb, while the taurine level increased only after the last injection. We conclude that the mode of action of sydnocarb differs from that of D-amphetamine. Sydnocarb also seems to be less neurotoxic than D-amphetamine, since it elicits lesser changes in the extracellular level of glutamate.

Lubeluzole attenuates K(+)-evoked extracellular accumulation of taurine in the striatum of healthy rats and rats with hepatic failure

Lubeluzole is a newly designed neuroprotectant which has proved effective in the treatment of experimental stroke in rats, mainly by inhibition of the glutamate-activated NO pathway, but also by counteracting osmotic stress by a mechanism associated with the release of the osmotically active amino acid taurine (Tau). Here we show that lubeluzole administered i.p. decreases by 25% the high (50 mM) K+-evoked accumulation of Tau in striatal microdialysates of healthy rats and by 34% in rats with thioacetamide-induced hepatic failure, where the increased extracellular accumulation of Tau signifies ongoing hepatic encephalopathy. Lubeluzole does not affect the nonstimulated accumulation of Tau in either group of rats. The results indicate that lubeluzole may be effective in ameliorating ionic or osmotic stress in a range of pathological conditions involving the rise of extracellular K+, and also in decreasing the vulnerability to stress in rats with hepatic failure.

GABA transaminase inhibition induces spontaneous and enhances depolarization-evoked GABA efflux via reversal of the GABA transporter

The GABA transporter can reverse with depolarization, causing nonvesicular GABA release. However, this is thought to occur only under pathological conditions. Patch-clamp recordings were made from rat hippocampal neurons in primary cell cultures. Inhibition of GABA transaminase with the anticonvulsant gamma-vinyl GABA (vigabatrin; 0.05-100 microm) resulted in a large leak current that was blocked by bicuculline (50 microm). This leak current occurred in the absence of extracellular calcium and was blocked by the GABA transporter antagonist SKF-89976a (5 microm). These results indicate that vigabatrin induces spontaneous GABA efflux from neighboring cells via reversal of GABA transporters, subsequently leading to the stimulation of GABA(A) receptors on the recorded neuron. The leak current increased slowly over 4 d of treatment with 100 microm vigabatrin, at which time it reached an equivalent conductance of 9.0 +/- 4.9 nS. Blockade of glutamic acid decarboxylase with semicarbazide (2 mm) decreased the leak current that was induced by vigabatrin by 47%. In untreated cells, carrier-mediated GABA efflux did not occur spontaneously but was induced by an increase in [K(+)](o) from 3 to as little as 6 mm. Vigabatrin enhanced this depolarization-evoked nonvesicular GABA release and also enhanced the heteroexchange release of GABA induced by nipecotate. Thus, the GABA transporter normally operates near its equilibrium and can be easily induced to reverse by an increase in cytosolic [GABA] or mild depolarization. We propose that this transporter-mediated nonvesicular GABA release plays an important role in neuronal inhibition under both physiological and pathophysiological conditions and is the target of some anticonvulsants.

Endogenous glutamate-taurine interaction in striatum and nucleus accumbens of the freely moving rat: studies during the normal process of aging

Using microdialysis, the effects of endogenous glutamate on extracellular concentrations of taurine, in striatum and nucleus accumbens of the awake rat during the normal process of aging were investigated. The glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) was perfused through the microdialysis probe to increase the endogenous concentration of glutamate. Young (2-4 months), middle aged (12-14 months), aged (27-32 months) and very aged (37 months) male Wistar rats were used. PDC (1, 2 and 4 mM) produced a dose-related increase of extracellular concentrations of glutamate and taurine in striatum and nucleus accumbens in all groups of age. Increases of glutamate, but not of taurine, decreased during aging in striatum. In nucleus accumbens there were no age-related changes in the increases of glutamate and taurine induced by PDC. In all age groups, increases of extracellular taurine were significantly correlated with increases of extracellular glutamate. However, when these taurine-glutamate correlations for all groups of age were compared, no statistical differences were found. These results show first that a decrease in the increases of glutamate produced by the glutamate reuptake inhibitor in striatum, but not in nucleus accumbens, is produced by age; second that glutamate-taurine interaction in striatum and nucleus accumbens does not change during the normal process of aging.

Extracellular concentrations of taurine, glutamate, and aspartate in the cerebral cortex of rats at the asymptomatic stage of thioacetamide-induced hepatic failure: modulation by ketamine anesthesia

Subclinical hepatic encephalopathy (SHE) was produced in rats by two intraperitoneal injections of TAA at 24 h intervals and the animals were examined 21 days later. Concentrations of the neuroactive amino acids taurine (Tau), glutamate (Glu) and aspartate (Asp), were measured in the cerebral cortical microdialysates of thioacetamide (TAA)-treated and untreated control rats. During microdialysis some animals were awake while others were anesthetized with ketamine plus xylazine. There was no difference in the water content of cerebral cortical slices isolated from control and SHE rats, indicating a recovery from cerebral cortical edema that accompanies the acute, clinical phase of hepatic encephalopathy in this model. When microdialysis was carried out in awake rats, dialysate concentrations of all the three amino acids were 30% to 50% higher in SHE rats than in control rats. Ketamine anesthesia caused a 2.2% increase of water content of cerebral cortical slices and increased Asp, Glu, and Tau concentration in microdialysates of control rats. In SHE rats, ketamine anesthesia produced a similar degree of cerebral edema, however, it did not alter Asp and Glu concentrations in the microdialysates. These data may reflect on one hand a neuropathological process of excitotoxic neuronal damage related to increased Glu and Asp, on the other hand neuroprotection from neuronal swelling indicated by Tau redistribution in the cerebral cortex. The reduction of the effects of SHE on Glu and Asp content in ketamine-anesthesized rats is likely to be due to interference of ketamine with the NMDA receptor-mediated component of the SHE-evoked excitatory neurotransmitter efflux and/or reuptake of the two amino acids. By contrast, the SHE-related increase of Tau content was not affected by ketamine anesthesia, indicating that the mechanism(s) underlying SHE-evoked accumulation of Tau must be different from the mechanism causing release of excitatory amino acids. The results with ketamine advocate caution when using this anesthetic in studies employing the cerebral microdialysis technique for measurement of extracellular amino acids.

Enhanced taurine release in cultured cerebellar granule cells in cell-damaging conditions

The release of taurine from cultured cerebellar granule neurons was studied in different cell-damaging conditions, including hypoxia, hypoglycemia, ischemia, oxidative stress and in the presence of free radicals. The effects of both ionotropic and metabotropic glutamate receptor agonists on the release were likewise investigated. The release of [3H]taurine from the glutamatergic granule cells was increased by K+ (50 mM) and veratridine (0.1 mM), the effect of veratridine being the greater. Hypoxia and ischemia produced an initial increase in release compared to normoxia but resulted in a diminished response to K+. Hypoglycemia, oxidative stress and free radicals enhanced taurine release, and subsequent K+ treatment exhibited a correspondingly greater stimulation. A common feature of taurine release in all the above conditions was a slow response to the stimulus evoked by K+ and particularly to that evoked by veratridine. All ionotropic glutamate receptor agonists potentiated taurine release, but only the action of kainate seemed to be receptor-mediated. Metabotropic receptor agonists of group I slightly stimulated the release. The prolonged taurine release seen in both normoxia and cell-damaging conditions may be of importance in maintaining homeostasis in the cerebellum and reducing excitability for a longer period than other neuroprotective mechanisms.

Characteristics of ischemia-induced taurine release in the developing mouse hippocampus

Taurine release in the developing hippocampus is markedly potentiated in ischemia. The mechanisms of the ischemia-induced release were studied in hippocampal slices from seven-day-old mice using a superfusion system. The basal release of [3H]taurine was significantly increased in media under normal conditions, but the ischemia-evoked release decreased in Na+ -free media, indicating the participation of Na+ -dependent transport processes. The involvement of taurine transporters in the release was confirmed with the structural analogs, hypotaurine and beta-alanine. These amino acids potentiated the release by trans-stimulation, but not in Na+ -free media. In the absence of Ca2+, the basal taurine release was markedly increased in normoxia but diminished in ischemia, indicating that a part of basal taurine release in ischemia is Ca2+ dependent. On the other hand, the K+ stimulation of taurine release was preserved in Ca2+ -free medium. The phospholipase and protein kinase inhibitors had no effect on ischemia-induced taurine release, nor did the chloride channel blockers 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate (2 mM) and diisothiocyanostilbene-2,2'-disulfonate (0.1 mM) affect the release in ischemia. The increase in extracellular levels of taurine in the immature hippocampus in ischemia may serve as an important protective mechanism against excitotoxicity, to which the developing brain is particularly vulnerable, and contribute to the resistance of the immature brain to hypoxia.

Taurine release is enhanced in cell-damaging conditions in cultured cerebral cortical astrocytes

The release of preloaded [3H]taurine from cultured cerebral cortical astrocytes was studied under various cell-damaging conditions, including hypoxia, ischemia, aglycemia and oxidative stress, and in the presence of free radicals. Astrocytic taurine release was enhanced by K+ (50 mM), veratridine (0.1 mM) and the ionotropic glutamate receptor agonist kainate (1.0 mM). Metabotropic glutamate receptor agonists had only weak effects on taurine release. Similarly to the swelling-induced taurine release the efflux in normoxia seems to be mediated mainly by DIDS-(diisothiocyanostilbene-2,2'-disulphonate) and SITS-(4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate) sensitive CI- channels, since these blockers were able to reduce both basal and K+ -stimulated release. The basal release of taurine was moderately enhanced in hypoxia and ischemia, whereas the potentiation in the presence of free radicals was marked. The small basal release from astrocytes signifies that taurine release from brain tissue in ischemia may originate from neurons rather than glial cells. On the other hand, the release evoked by K+ in hypoxia and ischemia was greater than in normoxia, with a very slow time-course. The enhanced release of the inhibitory amino acid taurine from astrocytes in ischemia may be beneficial to surrounding neurons, outlasting the initial stimulus and counteracting overexcitation.

Dual action of the benzodiazepine receptor inverse agonist RU34347 on responses to exogenously applied GABA in the rat cerebellar slice

The benzodiazepine receptor inverse agonist has been shown to produce agonist-like effects at low concentrations. RU34347 has both inverse agonist (attenuation of GABA-responses) and agonist-like (reduction of spontaneous Purkinje cell firing rate) in the cerebellar slice preparation. The benzodiazepine antagonist flumazenil prevented the inverse agonist actions, but only partially reduced the agonist-like effects. Further, brief application of RU34347 to slices mimicked the response to GABA, and pharmacological investigation determined that this action was mediated through increased GABA through action at a site proximal to the parallel fiber-basket cell synapse, at an as yet undetermined receptor.

Effects of ammonia in vitro on endogenous taurine efflux and cell volume in rat cerebrocortical minislices: influence of inhibitors of volume-sensitive amino acid transport

Rat cerebrocortical minislices were incubated with physiological saline in the absence or presence of 5 mM ammonium acetate ("ammonia") and/or inhibitors of osmosensitive amino acid transport: 50 microM niflumic acid and 100 microM N-ethyl-maleimide for 60 min, with medium changes after 20 min and 40 min. The efflux of endogenous taurine, glutamate and glutamine was assayed by high-performance liquid chromatography, and steady-state cell volumes were monitored in the slices with the [14C]inulin method. In the absence of ammonia, niflumic acid abolished taurine efflux but did not affect glutamate or glutamine efflux at all time-points, and increased cell volume at 20 min and 60 min. N-Ethyl-maleimide increased taurine, glutamine and glutamate efflux at 20 min and 40 min, inhibited taurine and glutamine efflux at 60 min, and increased cell volume at 20 min. Ammonia strongly stimulated taurine (by 380% at 20 min), and only moderately glutamate (30% at 20 min) or glutamine efflux (76% at 20 min). Ammonia increased cell volume above the control level at all time-points. Niflumic acid inhibited, but did not abolish ammonia-dependent taurine and glutamine efflux, and did not change glutamate efflux. The effects of ammonia + niflumic acid on cell volume did not differ from the effects of each compound separately. N-Ethyl-maleimide inhibited ammonia-dependent efflux of all three amino acids except for stimulation of glutamate efflux at 20 min. N-Ethyl-maleimide + ammonia decreased the cell volumes more than did each compound separately. It is concluded that although ammonia-induced taurine efflux is accompanied by an increase in cell volume, the underlying mechanism is not simply a cell volume regulatory response normally observed in hypoosmotic stress. Increased efflux of taurine, which is an inhibitory amino acid and a cell membrane protectant, may serve to counteract the deleterious effects of increased excitatory transmission accompanying acute hyperammonemic insult.

Taurine release modified by nitric oxide-generating compounds in the developing and adult mouse hippocampus

The effects of the nitric oxide-generating compounds hydroxylamine, sodium nitroprusside and S-nitroso-N-acetylpenicillamine, and the nitric oxide synthase inhibitors nitroarginine and 7-nitroindazole on taurine release from hippocampal slices from adult (three-month-old) and developing (seven-day-old) mice were characterized using a superfusion system. The basal release of [3H]taurine was enhanced when the nitric oxide donors were added at the beginning of superfusion, more markedly in the adult than in the immature hippocampus. The effect of hydroxylamine was clearly concentration-dependent. Hydroxylamine also markedly enhanced the release of endogenous taurine. The K+-stimulated (50 mM) release of taurine was generally inhibited by the nitric oxide-generating compounds in both age groups. Nitric oxide is thus able to act directly at presynaptic terminals, modulating taurine release as a retrograde messenger. The N-methyl-D-aspartate-evoked taurine release was reduced by the nitric oxide donors, particularly by sodium nitroprusside, in the adult hippocampus, while hydroxylamine and S-nitroso-N-acetylpenicillamine markedly potentiated the release in developing mice. In the immature hippocampus the hydroxylamine-enhanced taurine release seems to involve a Ca2+-independent, Na+-dependent and carrier-mediated process while in adult mice only a part of the hydroxylamine-enhanced release is mediated by the same mechanism. The results show that nitric oxide-generating compounds modify the basal, K+- and N-methyl-D-aspartate-evoked releases of taurine in both adult and immature hippocampus. The enhanced N-methyl-D-aspartate receptor-evoked release may be an important mechanism protecting the immature brain against excitotoxicity.

Changes in the extracellular profiles of neuroactive amino acids in the rat striatum at the asymptomatic stage of hepatic failure

Rats were treated with a hepatotoxin thioacetamide (TAA) and examined 21 days later, when they showed moderate fatty metamorphosis of the liver and morphological changes in brain indicative of excitotoxic neuronal damage, but no evident biochemical or neurophysiological symptoms of hepatic encephalopathy (HE). High-performance liquid chromatography (HPLC) analysis of extracellular amino acids in striatal microdialysates of TAA-treated rats revealed a significant increase in the excitatory amino acids glutamate (Glu) and aspartate (Asp) and their amino acid metabolites glutamine (Gln) and alanine (Ala). Microdialysis in the presence of 50 mM K+ triggered in TAA-treated rats an accumulation of Asp and Glu, and diminished the accumulation of Gln. These effects were virtually absent in control rats. None of the treatments affected the accumulation of the nontransmitter amino acid leucine (Leu). The above changes mirror those previously described in symptomatic HE and are likely to contribute to excitotoxic damage. The basal microdialysate content of taurine (Tau), an amino acid with antioxidant and volume regulatory properties, was 60% lower in TAA-treated rats than in control rats despite its increased blood-to-brain transport. The decrease in extracellular Tau may thus reflect Tau redistribution to adjacent central nervous system (CNS) cells manifesting a cell-protective response. Stimulation with 50 mM K+ increased extracellular Tau in control rats by 182% and in TAA-treated rats by 322%. Stimulation with 100 microM N-methyl-D-aspartate (NMDA) increased extracellular Tau in control rats by 27 % and in TAA-treated rats by as much as 250%. The increase of K+- or NMDA-dependent Tau release may reflect improved cell volume regulation and neuroprotection and contribute to attenuation of neurologic symptoms in rats with liver failure.

Beta-alanine release from the adult and developing hippocampus is enhanced by ionotropic glutamate receptor agonists and cell-damaging conditions

The release of the inhibitory amino acid beta-alanine was investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice, using a superfusion system. The release was enhanced by beta-alanine itself and the structural analogs taurine and y-aminobutyrate. It was dependent on Na+, but independent of Ca2+ in both mature and immature hippocampus, being thus mostly mediated by uptake carriers operating in an outward direction. The release was potentiated in the developing mice, but not affected in the adults, by the ionotropic glutamate receptor agonists N-methyl-D-aspartate, kainate, 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and tetrazolylglycine in a receptor-mediated manner. Cell-damaging conditions, including hypoxia, hypoglycemia, ischemia, oxidative stress and the presence of free radicals, greatly enhanced beta-alanine release at both ages, but more markedly in the adults. The great amounts of beta-alanine, together with the inhibitory amino acids taurine and gamma-aminobutyrate, released simultaneously with the excitatory amino acids in the hippocampus may constitute an important protective mechanism against excitotoxicity, which leads to neuronal death.

Activation of adenosine A2 receptors enhances high K(+)-evoked taurine release from rat hippocampus: a microdialysis study

The present study was designed to examine which type of adenosine receptors was involved in enhancement of high K(+)-evoked taurine release from in vivo rat hippocampus using microdialysis. Perfusion with 0.5 or 5.0 mM adenosine enhanced high K(+)-evoked taurine release. Perfusion with 2 microM R(-)-N6-2-phenylisopropyladenosine (PIA), a selective adenosine A1 receptor agonist, did not modulate taurine release. Perfusion with 1 microM 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a selective adenosine A1 receptor antagonist, increased taurine release. On the other hand, perfusion with 20 microM 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethyl-carboxamide-adenos ine (CGS21680), a selective adenosine A2A receptor agonist, enhanced taurine release, while perfusion with 1 mM 3,7-dimethyl-propagylxanthine (DMPX), an adenosine A2 receptor antagonist, did not affect taurine release. These results demonstrate that adenosine enhances high K(+)-evoked taurine release via activation of adenosine A2A receptors from both neurons and glial cells of in vivo rat hippocampus.

Release of endogenous glutamate, aspartate, GABA, and taurine from hippocampal slices from adult and developing mice under cell-damaging conditions

The releases of endogenous glutamate, aspartate, GABA and taurine from hippocampal slices from 7-day-, 3-, 12-, and 18-month-old mice were investigated under cell-damaging conditions using a superfusion system. The slices were superfused under hypoxic conditions in the presence and absence of glucose and exposed to hydrogen peroxide. In the adult hippocampus under normal conditions the basal release of taurine was highest, with a response only about 2-fold to potassium stimulation (50 mM). The low basal releases of glutamate, aspartate, and GABA were markedly potentiated by K+ ions. In general, the release of the four amino acids was enhanced under all above cell-damaging conditions. In hypoxia and ischemia (i.e., hypoxia in the absence of glucose) the release of glutamate, aspartate and GABA increased relatively more than that of taurine, and membrane depolarization by K+ markedly potentiated the release processes. Taurine release was doubled in hypoxia and tripled in ischemia but K+ stimulation was abolished. In both the mature and immature hippocampus the release of glutamate and aspartate was greatly enhanced in the presence of H2O2, that of aspartate particularly in developing mice. In the immature hippocampus the increase in taurine release was 10-fold in hypoxia and 30-fold in ischemia, and potassium stimulation was partly preserved. The release processes of the four amino acids in ischemia were all partially Ca2+-dependent. High concentrations of excitatory amino acids released under cell-damaging conditions are neurotoxic and contribute to neuronal death during ischemia. The substantial amounts of the inhibitory amino acids GABA and taurine released simultaneously may constitute an important protective mechanism against excitatory amino acids in excess, counteracting their harmful effects. In the immature hippocampus in particular, the massive release of taurine under cell-damaging conditions may have a significant function in protecting neural cells and aiding in preserving their viability.

Effects of 4-aminopyridine on extracellular concentrations of glutamate in striatum of the freely moving rat

4-aminopyridine (4-AP) is a voltage-sensitive K(+)-channel blocker extensively used in in vitro experiments as a depolarizing agent for the release of glutamate (GLU). This research investigated whether 4-AP could be used in in vivo experiments using microdialysis. For that, the effects of 4-AP on the extracellular concentrations of glutamate (GLU), glutamine (GLN), taurine (TAU) and citrulline (CIT) in striatum of the freely moving rat were investigated. The effects of 4-AP were compared with those produced by perfusion with a high K+ (100 mM) medium. Intrastriatal perfusion with 4-AP (1, 5 and 10 mM) produced no effects on extracellular [GLU], [TAU] and [CIT], but decreased extracellular [GLN]. Perfusion with a high K+ (100 mM) medium increased extracellular [GLU] and [TAU], decreased extracellular [GLN], and had no effects on [CIT]. To test whether the lack of effects of 4-AP on extracellular [GLU] was due to GLU uptake mechanisms, 4-AP was perfused after a previous inhibition of GLU uptake with L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC). Under the effects of PDC (1 mM), 4-AP (1 mM) had no effects on extracellular [GLU], [TAU] and [CIT], but decreased extracellular [GLN]. These results show that 4-AP decreased extracellular [GLN] but failed to produce a significant release of GLU in striatum of the freely moving rat. Thus, 4-AP can not be used as a depolarizing agent for stimulating the release of GLU in in vivo studies using microdialysis.