Taurine as osmoregulator and neuromodulator in the brain

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).

Opposite effects of lead exposure on taurine- and HFS-induced LTP in rat hippocampus

The effect of lead exposure on taurine-induced long-term potentiation (LTP(TAU)) was examined and compared with high-frequency stimulation-induced one (LTP(HFS)). Field excitatory postsynaptic potentials (fEPSP) and fiber volley (FV) in area CA1 of hippocampal slice were recorded in control and lead-exposed rats. In contrast to the inhibitory effects of lead exposure on LTP(HFS), the amplitude of LTP(TAU) in the lead-exposed rats (199.3+/-13.7%, n=12) was significantly larger than that in controls (152.3+/-17.0%, n=12). It was also observed that taurine induced greater FV potentiation in lead-exposed rats (162.6+/-9.0%, n=10) than controls (132.1+/-6.9%, n=11). In addition, after a previous HFS, sequent perfusion of taurine could further increase the synaptic efficacy in lead-exposed rats. These results provide the first evidence that chronic lead exposure has opposite effects on the two types of LTP resulting from different lead toxicity sites.

[Physiological function of blood-brain barrier transporters as the CNS supporting and protecting system]

The blood-brain barrier (BBB) segregates the circulating blood from interstitial fluid in the brain and restricts drug permeability into the brain. Our latest studies have revealed that the BBB transporters play important physiological roles in maintaining the brain environment. For an energy-storing system, the creatine transporter localized at the brain capillary endothelial cells (BCECs) mediates the supply of creatine from the blood to the brain. The BBB is involved in the brain-to-blood efflux transport of gamma-aminobutyric acid, and GAT2/BGT-1 mediates this transport process. BCECs also express serotonin and norepinephrine transporters. Organic anion transporter 3 (OAT3) and ASCT2 are localized at the abluminal membrane of the BCECs. OAT3 is involved in the brain-to-blood efflux of a dopamine metabolite, a uremic toxin, and thiopurine nucleobase analogues. ASCT2 plays a role in L-isomer-selective aspartic acid efflux transport at the BBB. Dehydroepiandrosterone sulfate and small neutral amino acids undergo brain-to-blood efflux transport mediated by organic anion transporting polypeptide 2 and ATA2, respectively. The BBB transporters are regulated by various factors: ATA2 by osmolarity, taurine transporter by tumor necrosis factor-alpha, and L-cystine/L-glutamic acid exchange transporter by oxidative stress. Clarifying the physiological roles of BBB transport systems should give important information allowing the development of better central nervous system (CNS) drugs and improving our understanding of the relationship between CNS disorders and BBB function.

The brain-to-blood efflux transport of taurine and changes in the blood–brain barrier transport system by tumor necrosis factor-α

The purpose of this study was to examine whether the efflux transport system for taurine from brain to blood is present at the blood-brain barrier (BBB) by using the brain efflux index (BEI) method and to determine whether the taurine transport system is regulated after central nervous system cell damage by tumor necrosis factor-alpha (TNF-alpha) in vivo. [(3)H]Taurine was microinjected into the parietal cortex area 2 of the rat brain, and was eliminated from the brain with an efflux transport rate of 1.22 x 10(-2) min(-1), and the process is saturable with a K(m) of 39.1 microM. This process was significantly inhibited by taurine transporter (TAUT) inhibitors, such as unlabeled taurine, beta-alanine, betaine, nipecotic acid and gamma-aminobutyric acid (GABA). In addition, the effect of tumor necrosis factor-alpha on [(3)H]taurine transport was investigated. [(3)H]Taurine uptake was increased and its efflux was reduced by pretreatment with tumor necrosis factor-alpha. Also, [(3)H]taurine efflux was reduced by tumor necrosis factor-alpha in a time- and dose-dependent manner. In conclusion, there is the efflux pump for taurine at the blood-brain barrier to reduce taurine concentration in the brain interstitial fluid, and this process was carrier mediated. In addition, the transport system for taurine through the blood-brain barrier was found to be regulated by tumor necrosis factor-alpha in vivo.

Enhanced release of adenosine under cell-damaging conditions in the developing and adult mouse hippocampus

The inhibitory neuromodulator adenosine has been thought to act as an endogenous neuroprotectant against cerebral ischemia and neuronal damage. The release of preloaded [3H]adenosine from hippocampal slices from developing (7-day-old) and adult (3-month-old) mice was characterized using a superfusion system under various cell-damaging conditions, including hypoxia, hypoglycemia, ischemia, oxidative stress, and the presence of free radicals and metabolic poisons. The release of adenosine was greatly potentiated under the above conditions at both ages, with free radicals, metabolic poisons, and ischemia generally having the strongest stimulatory effects. Depolarization by K+ ions (50 mM) could then evoke more release of adenosine only in the immature hippocampus. Omission of Ca2+ from the superfusion media had no effect on the ischemia-induced release in the adults, indicating that it occurs by a Ca2+-independent system. In contrast, the release in the immature hippocampus was partially dependent on extracellular Ca2+. Furthermore, the ischemia-induced adenosine release was reduced in Na+-deficient media and enhanced by ouabain at both ages, pointing to the involvement of Na+-dependent transporters. The release was also reduced by Cl- channel blockers, thus indicating that a part of the evoked release occurs through anion channels. Another inhibitory neuromodulator and cell volume regulator, taurine, was seen to enhance adenosine release in ischemia at both ages. The simultaneous release of taurine and adenosine under cell-damaging conditions could constitute an important protective mechanism against excessive amounts of excitatory amino acids, counteracting their harmful effects and preventing excitation from reaching neurotoxic levels.

Role of taurine accumulation in keratinocyte hydration

Epidermal keratinocytes are exposed to a low water concentration at the stratum corneum-stratum granulosum interface. When epithelial tissues are osmotically perturbed, cellular protection and cell volume regulation is mediated by accumulation of organic osmolytes such as taurine. Previous studies reported the presence of taurine in the epidermis of several animal species. Therefore, we analyzed human skin for the presence of the taurine transporter (TAUT) and studied the accumulation of taurine as one potential mechanism protecting epidermal keratinocytes from dehydration. According to our results, TAUT is expressed as a 69 kDa protein in human epidermis but not in the dermis. For the epidermis a gradient was evident with maximal levels of TAUT in the outermost granular keratinocyte layer and lower levels in the stratum spinosum. No TAUT was found in the basal layer or in the stratum corneum. Keratinocyte accumulation of taurine was induced by experimental induction of skin dryness via application of silica gel to human skin. Cultured human keratinocytes accumulated taurine in a concentration- and osmolarity-dependent manner. TAUT mRNA levels were increased after exposure of human keratinocytes to hyperosmotic culture medium, indicating osmosensitive TAUT mRNA expression as part of the adaptation of keratinocytes to hyperosmotic stress. Keratinocyte uptake of taurine was inhibited by beta-alanine but not by other osmolytes such as betaine, inositol, or sorbitol. Accumulation of taurine protected cultured human keratinocytes from both osmotically induced and ultraviolet-induced apoptosis. Our data indicate that taurine is an important epidermal osmolyte required to maintain keratinocyte hydration in a dry environment.

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.

The selective A2A receptor antagonist SCH 58261 reduces striatal transmitter outflow, turning behavior and ischemic brain damage induced by permanent focal ischemia in the rat

Adenosine A(2A) receptor antagonists have been proved protective in different ischemia models. In this study we verified if the protective effect of the selective A(2A) antagonist, SCH 58261, could be attributed to the reduction of the excitatory amino acid outflow induced by cerebral focal ischemia. A vertical microdialysis probe was inserted into the striatum of male Wistar rats and, after 24 h, permanent right intraluminal middle cerebral artery occlusion (MCAo) was induced. Soon after waking, rats showed a definite contralateral turning behavior, which persisted up to 7 h after MCAo. During 4 h after MCAo, glutamate, aspartate, GABA, adenosine and taurine outflow increased. SCH 58261 (0.01 mg/kg, i.p.), administered 5 min after MCAo, suppressed turning behavior and significantly reduced the outflow of glutamate, aspartate, GABA and adenosine. At 24 h after MCAo, the rats showed severe sensorimotor deficit and damage in both the striatum and cortex. SCH 58261 significantly reduced cortical damage but did not protect against the sensorimotor deficit. The protective effect of SCH 58261 against turning behavior and increased outflow of excitatory amino acids in the first hours after MCAo suggests the potential utility of selective adenosine A(2A) antagonists when administered in the first hours after ischemia. Furthermore, this study, for the first time, proposes that turning behavior after permanent intraluminal MCAo, be used as a precocious index of neurological deficit and neuronal damage.

Colocalization of taurine and glial fibrillary acidic protein immunoreactivity in mouse hippocampus induced by short-term ethanol exposure

Morphological changes of the hippocampus were investigated in mice exhibiting signs of intoxication following short-term exposure to 6% ethanol. These alterations were examined by a double immunofluorescent study using antibodies to taurine and anti-glial fibrillary acidic protein (GFAP) antibody. Antibody-labeled taurine was localized mainly in the astrocytes and endothelial cells of control mice. Ethanol administration resulted in a significant increase in the accumulation of taurine and GFAP immunoreactivity (IR) in the stratum lacunosum-moleculare (sl-m) of the hippocampus. Specifically, the cell bodies of taurine-positive astrocytes were hypertrophied, their processes were elongated in the pericapillary region, and some colocalized with GFAP-IR cells. Furthermore, quantitative analysis revealed that the merged area in ethanol-treated mice was twice that (71.6% vs. 35.8%) of control mice. Since taurine is involved in various neuroprotective functions, the present observations suggest that the expression of taurine IR in reactive astrocytes after ethanol exposure might play an important role in neuroprotective processes.

Regulation of taurine transport at the blood-brain barrier by tumor necrosis factor-alpha, taurine and hypertonicity

Taurine is the abundant sulfur-containing beta-amino acid in brain where it exerts a neuroprotective effect. Although it is known that the blood-brain barrier (BBB) mediates taurine transport, the regulation of taurine transport have not been clarified yet. A conditionally immortalized rat brain capillary endothelial cells (TR-BBB13), an in vitro model of the BBB, exhibited [3H]taurine uptake, which was dependent on both Na+ and Cl-, and inhibited by beta-alanine. Taurine transporter (TAUT) mRNA was detected in TR-BBB13 cells, and TAUT protein was also expressed at 70 kDa. TR-BBB13 cells exposed to 20 ng/mL TNF-alpha and under hypertonic conditions showed a 1.7-fold and 3.2-fold increase in [3H]taurine uptake, respectively. In contrast, lipopolysaccharide and diethyl maleate did not significantly affect taurine uptake. The taurine uptake was reduced by pre-treatment with excess taurine (50 mm). The mRNA level of the TAUT in TNF-alpha and following hypertonic treatment was greater than that in control cells, whereas that under excess taurine conditions was lower than in controls. Therefore, taurine transport activity at the BBB appears to be regulated at the transcriptional level by cell damage, osmolality and taurine in the brain.

Guanidinoethyl sulphonate is a glycine receptor antagonist in striatum

1. Guanidinoethyl sulphonate (GES) is an analogue of taurine and an inhibitor of taurine transport. Interactions of GES with GABA(A) and glycine receptors are studied by whole cell recording and fast drug application in isolated striatal neurons of the mouse. 2. We confirm that GES is a weak agonist at GABA(A) receptors, and is able to antagonize GABA-evoked responses. GES did not gate GlyR. 3. GES antagonized glycine responses in a concentration-dependent and surmountable manner. Glycine dose-response curves were shifted to the right by GES (0.5 mM), yielding EC(50)s and Hill coefficients of 62 micro M and 2.5 in control, 154 micro M and 1.3 in the presence of GES. 4. GlyR-mediated taurine responses were competitively antagonized by GES. Taurine dose-response curves, in contrast to the glycine dose-response curves were shifted by GES to the right in a parallel manner. 5. The GlyR-block by GES was not voltage-dependent. 6. In contrast to our findings in the mouse, in rat striatal neurons which lack expression of the alpha3 GlyR subunit, GES shifted the glycine dose-response curve to the right in a parallel way without affecting the maximal response. Subtype-specificity of the GES action at GlyR must await further investigation in artificial expression systems. 7. We conclude that GES is a competitive antagonist at GlyR. The antagonistic action of GES at inhibitory ionotropic receptors can explain its epileptogenic action. Care must be taken with the interpretation of data on GES evoked taurine release.

Possible ameliorative effect of taurine in the treatment of iron-deficiency anaemia in female university students of Gaza, Palestine

The aim of the study was to evaluate the haematological effects of adding the antioxidant taurine to iron sulfate in the treatment of iron-deficiency anaemia (IDA). A sample of 730 students from Al-Azhar University, Gaza, in Palestine underwent screening with complete blood counts and serum samples. In subjects with microcytosis/hypochromasia, Alpha2 delta2 (HbA2) and serum concentrations of iron, total iron binding capacity (TIBC), ferritin and taurine were determined. Samples from 17 normocytic, normochromic, and non-anaemic subjects were used as baseline controls. At base-line, 81 of the 730 subjects (11.1%) had microcytosis/hypochromasia, 26 (3.6%) were diagnosed as beta-thalassemia carriers, none of which was iron deficient. Four subjects had microcytosis of unknown cause. Fifty-one subjects (all females) had iron-deficiency anaemia and were included in the therapeutic study, which lasted for 20 wk. They were matched for Hb into pairs and were treated with oral iron (325 mg of slow-release iron sulfate). In addition, they were, in a double-blind procedure, randomised to additional oral taurine (1000 mg d(-1) at a cost comparable to that of adding ascorbic acid) or placebo. Mean S-taurine was significantly lower in the IDA subjects than in the controls. After 20 wk of iron supplementation, both the taurine and placebo group significantly improved their Hb concentrations and normalised the markers of iron deficiency. Apart from the expected, albeit in this study mild side-effects of oral iron, no significant side-effects were noted. In the taurine group, there was a statistically significant additive positive change from the baseline values on Hb (2.67 +/- 1.24 g dL(-1)), red blood cell (RBC) count [(0.57 +/- 0.25) x 1012 L(-1)] and serum ferritin (30.33 +/- 17.99 microg L(-1)) as compared to placebo group values, which were 1.80 +/- 1.10 g dL-1, (0.39 +/- 0.36) x 1012 L(-1), and 20.11 +/- 7.34 microg L(-1), respectively. Oral taurine appears to increase the effectiveness of oral iron in the treatment of IDA, and has no significant side-effects. This merits further cost-benefit and clinical analyses.

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.

Ischemia-induced taurine release is modified by nitric oxide-generating compounds in slices from the developing and adult mouse hippocampus

The novel neurotransmitter/neuromodulator nitric oxide (NO), which is linked to the activation of the N-methyl-D-aspartate class of glutamate receptors, has been shown to modify transmitter release in brain tissue. Release of the inhibitory amino acid taurine is also markedly enhanced by N-methyl-D-aspartate and NO-producing agents under normal conditions in the mouse hippocampus. The release of preloaded [3H]taurine from hippocampal slices from adult (3-month-old) and developing (7-day-old) mice was characterized under ischemic conditions in the presence of different NO-generating compounds, hydroxylamine, sodium nitroprusside, and S-nitroso-N-acetylpenicillamine (SNAP), using a superfusion system. The ischemia-induced taurine release at both ages was markedly enhanced by 1.0 mM nitroprusside and 1.0 mM SNAP, whereas 5.0 mM hydroxylamine was effective only in adults. The nitroprusside- and SNAP-induced releases were reduced by the inhibitors of NO synthase (nitroarginine and 7-nitroindazole) and NO-sensitive soluble guanylyl cyclase [1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one], suggesting involvement of the NO/cGMP pathway. The release in ischemia in the absence of Na+ was modified by NO compounds only in adults; the 0.1 mM N-methyl-D-aspartate stimulated taurine release at both ages. The enhanced release of taurine associated with NO production could be beneficial to brain tissue under cell-damaging conditions and corroborates the neuroprotective role of this amino acid, particularly in the immature brain.

Taurine release in the developing and adult mouse hippocampus: involvement of cyclic guanosine monophosphate

The inhibitory neuromodulator taurine is involved in osmoregulation and cell volume adjustments in the central nervous system. In addition, taurine protects neural cells from excitotoxicity and prevents harmful metabolic events evoked by cell-damaging conditions. The release of taurine in nervous cell preparations is greatly enhanced by glutamate receptor agonists and various cell-damaging conditions. NO-generating compounds also increase taurine release in the mouse hippocampus. The further involvement of the NO/cGMP pathway and protein kinases in preloaded [3H]taurine release from hippocampal slices from adult (3-month-old) and developing (7-day-old) mice in normoxia and in ischemia was now studied using a superfusion system. The release was enhanced by 8-Br-cGMP and the phosphodiesterase inhibitor 2-(2-propyloxyphenyl)-8-azapurin-6-one (zaprinast), particularly in the immature hippocampus, indicating that increased cGMP levels induce taurine release. The release was also increased by the inhibitor of soluble guanylyl cyclase, 1H-(1,2,4)oxadiazolo-(4,3a)quinoxalin-1-one (ODQ) and the protein kinase C activator 4beta-phorbol 12-myristate 13-acetate (PMA), but only in the adult hippocampus. The ischemia-induced release was also enhanced by increased cGMP levels in both adult and developing mice, whereas protein kinase inhibitors had no effects in any conditions. The results demonstrate that cGMP is able to modulate hippocampal taurine release in both adult and developing mice, the rise in cGMP levels evoking taurine release in normoxia and in ischemia. This could be part of the neuroprotective properties of taurine, being thus important particularly in cell-damaging conditions and in preventing excitotoxicity.

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.

Activation of human alpha1 and alpha2 homomeric glycine receptors by taurine and GABA

1. Two ligand binding alpha subunits, alpha1 and alpha2, of the human (H) glycine receptor (GlyR) are involved at inhibitory synapses in the adult and neonatal spinal cord, respectively. The ability of homomeric alphaH1 and alphaH2 GlyRs to be activated by glycine, taurine and GABA was studied in Xenopus oocytes or in the human embryonic kidney HEK-293 cell line. 2. In outside-out patches from HEK cells, glycine, taurine and GABA activated both GlyRs with the same main unitary conductance, i.e. 85 +/- 3 pS (n = 6) for alphaH1, and 95 +/- 5 pS (n = 4) for alphaH2. 3. The sensitivity of both alphaH1 and alphaH2 GlyRs to glycine was highly variable. In Xenopus oocytes the EC50 for glycine (EC50gly) was between 25 and 280 microM for alphaH1 (n = 44) and between 46 and 541 microM for alphaH2 (n = 52). For both receptors, the highest EC50gly values were found on cells with low maximal glycine responses. 4. The actions of taurine and GABA were dependent on the EC50gly: (i) their EC50 values were linearly correlated to EC50gly, with EC50tau approximately 10 EC50gly and EC50GABA approximately 500-800 EC50gly; (ii) they could act either as full or weak agonists depending on the EC50gly. 5. The Hill coefficient (n(H)) of glycine remained stable regardless of the EC50gly whereas n(H) for taurine decreased with increasing EC50tau. 6. The degree of desensitization, evaluated by fast application of saturating concentrations of agonist on outside-out patches from Xenopus oocytes, was similar for glycine and taurine on both GlyRs and did not exceed 50 %. 7. Our data concerning the variations of EC50gly and the subsequent behaviour of taurine and GABA could be qualitatively described by the simple del Castillo-Katz scheme, assuming that the agonist gating constant varies whereas the binding constants are stable. However, the stability of the Hill coefficient for glycine was not explained by this model, suggesting that other mechanisms are involved in the modulation of EC50.

Modulation of extracellular neurotransmitter levels in the nucleus accumbens by a taurine uptake inhibitor

Using in vivo microdialysis, we examined the effect of local perfusion of the taurine uptake inhibitor guanidinoethyl sulfonate on extracellular levels of various neurotransmitters in the rat nucleus accumbens. Guanidinoethyl sulfonate (500 microM-50 mM) produced a concentration-dependent increase in extracellular taurine levels. While 500 microM and 5 mM concentrations of guanidinoethyl sulfonate were largely without effect, 50 mM guanidinoethyl sulfonate produced a significant decrease in extracellular levels of aspartate, glutamate and glycine, with no effect on extracellular dopamine levels. These results indicate that guanidinoethyl sulfonate can modulate extracellular amino acid levels in the nucleus accumbens.

Osmotic regulation of neuronal activity: a new role for taurine and glial cells in a hypothalamic neuroendocrine structure

Maintenance of osmotic pressure is a primary regulatory process essential for normal cell function. The osmolarity of extracellular fluids is regulated by modifying the intake and excretion of salts and water. A major component of this regulatory process is the neuroendocrine hypothalamo-neurohypophysial system, which consists of neurons located in the paraventricular and supraoptic nuclei. These neurons synthesize the neurohormones vasopressin and oxytocin and release them in the blood circulation. We here review the mechanisms responsible for the osmoregulation of the activity of these neurons. Notably, the osmosensitivity of the supraoptic nucleus is described including the recent data that suggests an important participation of taurine in the transmission of the osmotic information. Taurine is an amino acid mainly known for its involvement in cell volume regulation, as it is one of the major inorganic osmolytes used by cells to compensate for changes in extracellular osmolarity. In the supraoptic nucleus, taurine is highly concentrated in astrocytes, and released in an osmodependent manner through volume-sensitive anion channels. Via its agonist action on neuronal glycine receptors, taurine is likely to contribute to the inhibition of neuronal activity induced by hypotonic stimuli. This inhibitory influence would complement the intrinsic osmosensitivity of supraoptic neurons, mediated by excitatory mechanoreceptors activated under hypertonic conditions. These observations extend the role of taurine from the regulation of cell volume to that of the whole body fluid balance. They also point to a new role of supraoptic glial cells as active components in a neuroendocrine regulatory loop.

Nitric oxide can differentially modulate striatal neurotransmitter concentrations via soluble guanylate cyclase and peroxynitrite formation

In vivo microdialysis was used to investigate whether nitric oxide (NO) modulates striatal neurotransmitter release in the rat through inducing cyclic GMP formation via soluble guanylate cyclase or formation of peroxynitrite (ONOO(-)). When NO donors, S-nitroso-N-acetyl-DL-penicillamine (SNAP; 1 mM) or (Z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1- ium-1, 2-diolate (NOC-18; 1 mM), were retrodialysed for 15 min, acetylcholine (ACh), serotonin (5-HT), glutamate (Glu), gamma-aminobutyric acid (GABA), and taurine levels were significantly increased, whereas those of dopamine (DA), dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) were decreased. Only effects on ACh, 5-HT, and GABA showed calcium dependency. Inhibition of soluble guanylate cyclase by 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ; 100 and 200 microM) dose-dependently reduced NO donor-evoked increases in ACh, 5-HT, Glu, and GABA levels. Coperfusion of SNAP or NOC-18 with an ONOO(-) scavenger, L-cysteine (10 mM) resulted in enhanced concentrations of Glu and GABA. On the other hand, DA concentrations increased rather than decreased, and no reductions in DOPAC and 5-HIAA occurred. This increase in DA and the potentiation of Glu and GABA were calcium-dependent and prevented by ODQ. Similar to NO, infusions of ONOO(-) (10 or 100 microM) decreased DA, DOPAC, and 5-HIAA. Overall, these results demonstrate that NO increases ACh, 5-HT, Glu, and GABA levels primarily through a cyclic GMP-dependent mechanism. For DA, DOPAC, and 5-HIAA, effects are determined by levels of ONOO(-) stimulated by NO donors. When these are high, they effectively reduce extracellular concentrations through oxidation. When they are low, DA concentrations are increased in a cyclic GMP-dependent manner and may act to facilitate Glu and GABA release further. Thus, changes in brain levels of antioxidants, and the altered ability of NO to stimulate cyclic GMP formation during ageing, or neurodegenerative pathologies, may particularly impact on the functional consequences of NO on striatal dopaminergic and glutamatergic function.

Modulation of the ischemia-induced taurine release by adenosine receptors in the developing and adult mouse hippocampus

The release of the inhibitory amino acid taurine is markedly enhanced under ischemic conditions in both adult and developing hippocampus, together with a pronounced increase in the release of excitatory amino acids and the neuromodulator adenosine. We studied the effects of adenosine receptor agonists and antagonists as well as adenosine transport inhibitors on hippocampal [(3)H]taurine release in normoxia and ischemia, using a superfusion system. Under standard conditions the adenosine A(1) receptor agonists N(6)-cyclohexyladenosine and R(-)N(6)-(2-phenylisopropyl)adenosine potentiated basal taurine release in developing mice and depressed the release in adults in a receptor-mediated manner. Adenosine A(2) receptor compounds had only minor effects on the basal release and the K(+)-stimulated release was not affected by these drugs. The adenosine uptake inhibitor dipyridamole enhanced basal taurine release in the developing hippocampus and reduced it in the adult. In ischemia the adenosine compounds had no marked effects on taurine release in immature animals, whereas A(1) receptor activation was still able to evoke taurine release in adults by a receptor-mediated mechanism. The results show that the basal release of taurine is modulated by A(1) receptors in both mature and immature hippocampus, whereas in ischemia these receptors potentiate taurine release only in adults. The elevated taurine levels together with the depression of excitatory amino acid release by adenosine receptor activation could be beneficial under ischemic conditions, protecting neural cells against excitotoxicity and hyperexcitation.

Involvement of metabotropic glutamate receptors in ischemia-induced taurine release in the developing and adult hippocampus

Metabotropic glutamate receptors have recently been envisaged as involved in both potentiation and prevention of ischemic and excitotoxic neuronal damage. The release of the inhibitory amino acid taurine is markedly enhanced in ischemia in both the immature and mature mouse hippocampus. The modulation of [3H]taurine release by metabotropic receptor agonists and antagonists was studied in hippocampal slices from developing (7-day-old) and adult (3-month-old) mice using a superfusion system. Agonists of group I, II and III metabotropic glutamate receptors generally reduced the ischemia-induced release in adult animals. In the immature hippocampus the group I agonists (S)-3,5-dihydroxyphenylglycine and (1+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate, which mainly enhance neuronal excitation, potentiated initial taurine release in ischemia. Ionotropic glutamate receptor agonists also enhance the ischemia-induced taurine release in developing mice. This glutamate-activated taurine release may thus constitute an important protective mechanism against excitotoxicity in the immature hippocampus.

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

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

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

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

Evidence that taurine modulates osmoregulation by modification of osmolarity sensor protein ENVZ--expression

Although the involvement of taurine in osmoregulation is well-documented and widely accepted, no detailed mechanism for this function has been reported so far. We used subtractive hybridization to study mRNA steady state levels of genes up- or downregulated by taurine. Rats were fed taurine 100 mg/kg body weight per day for a period of three days and hearts (total ventricular tissue) of experimental animals and controls were pooled and used for mRNA extraction. mRNAs from two groups were used for subtractive hybridization. Clones of the subtractive library were sequenced and the obtained sequences were identified by gen bank assignment. Two clones were found to contain sequences which could be assigned to the osmolarity sensor protein envZ, showing homologies of 61 and 65%. EnvZ is an inner membrane protein in bacteria, important for osmosensing and required for porine gene regulation. It undergoes autophosphorylation and subsequently phosphorylates OmpR, which in turn binds to the porine (outer membrane protein) promoters to regulate the expression of OmpF and OmpC, major outer membrane porines. This is the first report of an osmosensing mechanism in the mammalian system, which was described in bacteria only. Furthermore, we are assigning a tentative role for taurine in the osmoregulatory process by modifying the expression of the osmoregulatory sensor protein ENVZ.

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.

Extracellular glutamate and other metabolites in and around RG2 rat glioma: an intracerebral microdialysis study

The current study determined the extracellular content of glutamate, 10 additional amino acids, lactate, glucose and some antioxidants in a rodent model of malignant glioma, its peritumoral space and the adjacent cortex. RG2 tumors were induced in the right frontal cortex of Fischer-344 rats (n = 10) by a standardized procedure to obtain a maximum sagittal tumor width of 3-4 mm diameter. After confirmation of tumor growth and localization by contrast enhanced MRI three microdialysis probes were implanted simultaneously in the cortex: at the tumor implantation site (tumor), 2 mm caudally, brain around tumor (BAT) and 4 mm caudally (cortex) to the site of implantation. Dialysate concentrations of glutamate were increased 3.9-fold in tumor and 2-fold in BAT compared with cortex. Glycine was elevated 11.4-fold in tumor and 2.6-fold in BAT. Lactate was increased 1.7-fold in tumor, 1.2-fold in BAT. Levels of glucose, ascorbic acid and uric acid were not significantly different in tumor, BAT and cortex. The increased dialysate levels of glutamate and glycine in the peritumoral space may contribute to impaired neuronal function and epileptiform activity associated with this tumor type in humans.

Ethanol and amino acids in the central nervous system: assessment of the pharmacological actions of acamprosate

Ethanol induces alterations in the central nervous system by differentially interfering with a number of neurotransmitter systems, although the mechanisms by which such effects are executed are not well understood. The present review therefore, is designed to ascertain the effect of ethanol on both excitatory and inhibitory amino acid neurotransmitters, as well as the sulphonated amino acid taurine, assayed by the microdialysis technique within specific brain regions of rat during different types of alcohol intoxication, acute and chronic, as well as during the withdrawal period. Such an understanding of these pharmacological actions of ethanol on neurotransmitters is essential in order to provide the impetus for the development of appropriate therapeutic intervention to ameliorate the multitude of neurochemical disorders induced by ethanol. In addition the possible mode of action of a new therapeutic drug for the treatment of alcoholism, acamprosate will be discussed. The first part of this review will be limited to studies of the effect of ethanol on both amino acid neurotransmitters and the sulphonated amino acid taurine, a possible neuromodulator. While, the second part will seek to establish the possible mechanism of action of a new therapeutic drug, acamprosate, which is used to combat the effects of ethanol, particularly during the craving period, as well as maintaining abstinence in weaned alcoholics.

Developmental changes in organic osmolytes in prenatal and postnatal rat tissues

At high osmotic pressures, mammalian kidney medulla, heart, lens, and brain utilize organic osmolytes to regulate cell volume. However the types and proportions of these solutes vary among tissues in patterns and for non-osmotic roles not fully elucidated. To clarify these, we analyzed osmolyte-type solute contents in rat tissues at 7 and 2 days prenatal and at 0, 7, 14, 21 (weaning), 35 (juvenile) and 77 (adult) days postnatal. Placentas were dominated by betaine, taurine, and creatine, which decreased between the prenatal times. Fetuses were dominated by glutamate and taurine, which increased between the times. In cerebrum, hindbrain and diencephalon, taurine dominated at early stages, but dropped after postnatal day 7, while myo-inositol, glutamine, creatine and glutamate increased after birth, with the latter two dominating in adults. In olfactory bulb, taurine content declined gradually with age and was equal to glutamate in adults. In all brain regions, glycerophosphorylcholine (GPC) reached a peak in juveniles. In postnatal renal medulla, urea, sodium, GPC, betaine, and taurine increased sharply at day 21. Thereafter, most increased, but taurine decreased. In heart, taurine dominated, and increased with age along with creatine and glutamine, while glutamate decreased after postnatal day 7. In lens, taurine dominated and declined in adults. These patterns are discussed in light of hypotheses on non-osmotic and pathological roles of these solutes.

Characterization of the cDNA coding for rat brain cysteine sulfinate decarboxylase: brain and liver enzymes are identical proteins encoded by two distinct mRNAs

Cysteine sulfinate decarboxylase (CSD) is considered as the rate-limiting enzyme in the biosynthesis of taurine, a possible osmoregulator in brain. Through cloning and sequencing of RT-PCR and RACE-PCR products of rat brain mRNAs, a 2,396-bp cDNA sequence was obtained encoding a protein of 493 amino acids (calculated molecular mass, 55.2 kDa). The corresponding fusion protein showed a substrate specificity similar to that of the endogenous enzyme. The sequence of the encoded protein is identical to that encoded by liver CSD cDNA. Among other characterized amino acid decarboxylases, CSD shows the highest homology (54%) with either isoform of glutamic acid decarboxylase (GAD65 and GAD67). A single mRNA band, approximately 2.5 kb, was detected by northern blot in RNA extracts of brain, liver, and kidney. However, brain and liver CSD cDNA sequences differed in the 5' untranslated region. This indicates two forms of CSD mRNA. Analysis of PCR-amplified products of genomic DNA suggests that the brain form results from the use of a 3' alternative internal splicing site within an exon specifically found in liver CSD mRNA. Through selective RT-PCR the brain form was detected in brain only, whereas the liver form was found in liver and kidney. These results indicate a tissue-specific regulation of CSD genomic expression.

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.

Involvement of metabotropic glutamate receptors in taurine release in the adult and developing mouse hippocampus

The inhibitory amino acid taurine has been held to function as an osmoregulator and modulator of neural activity, being particularly important in the immature brain. Ionotropic glutamate receptor agonists are known markedly to potentiate taurine release. The effects of different metabotropic glutamate receptor (mGluR) agonists and antagonists on the basal and K(+)-stimulated release of [3H]taurine from hippocampal slices from 3-month-old (adult) and 7-day-old mice were now investigated using a superfusion system. Of group I metabotropic glutamate receptor agonists, quisqualate potentiated basal taurine release in both age groups, more markedly in the immature hippocampus. This action was not antagonized by the specific antagonists of group I but by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX), which would suggest an involvement of ionotropic glutamate receptors. (S)-3,5-dihydroxyphenylglycine (DHPG) potentiated the basal release by a receptor-mediated mechanism in the immature hippocampus. The group II agonist (2S, 2'R, 3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) markedly potentiated basal taurine release at both ages. These effects were antagonized by dizocilpine, indicating again the participation of ionotropic receptors. Group III agonists slightly potentiated basal taurine release, as did several antagonists of the three metabotropic receptor groups. Potassium-stimulated (50 mM K+) taurine release was generally significantly reduced by mGluR agents, mainly by group I and II compounds. This may be harmful to neurons in hyperexcitatory states. On the other hand, the potentiation by mGluRs of basal taurine release, particularly in the immature hippocampus, together with the earlier demonstrated pronounced enhancement by activation of ionotropic glutamate receptors, may protect neurons against excitotoxicity.

The role of taurine in the regulation of brain cell volume in chronically hyponatraemic rats

Cells in slices prepared from the superficial cerebral cortex of normonatraemic rats underwent moderate swelling when exposed to low Na+ medium (122 mmol/l) accompanied by a large increase in the rate of efflux of preloaded taurine. In contrast, cells in slices from chronically (4 day) hyponatraemic rats did not increase in volume and the rate of taurine efflux was unchanged. The anion transport inhibitor 4,4'-diisothiocyanato-stilbene-2,2'-sulphonic acid (25 micromol/l) caused marked (-44%) reduction in taurine efflux in cells from normonatraemic rats; this response was strongly attenuated (-16%) by hyponatraemia. When slices from hyponatraemic rats were acutely exposed to medium containing 142 mmol/Na+ cells exhibited marked and paradoxical swelling. This response was completely abolished by the NaCl co-transport inhibitor bumetanide (50 micromol/l) and was not observed in slices that had not been pre-loaded with taurine. Forty eight hours after the start of the remission of hyponatraemia, cells from post-hyponatraemic rats displayed normal responses (i.e., moderate swelling and greatly accelerated taurine efflux) on exposure to 122 mmol/Na+. But at 24 h there was only partial restoration of the efflux response to 122 mmol/Na+, with an enhanced cell swelling response that was not significantly affected by bumetanide. It is concluded that (i) during chronic hyponatraemia, unlike acute hyposmotic stress, cortical cells preserve their volume and that this is not associated with any increase in the rate of taurine loss; there does however, appear to be a decrease in the anionic component of cellular taurine efflux; (ii) acute re-incubation of slices in medium containing 142 mmol/l Na+ is associated with cell swelling that may reflect up-regulation of Na/Cl/taurine co-transport; (iii) following restoration of normonatraemia the pattern of normal cellular response to acute hyposmotic stress is only gradually re-established.

Mechanisms of ischemia-induced taurine release in mouse hippocampal slices

Taurine release in the hippocampus is markedly potentiated in various cell-damaging conditions, including ischemia and excitotoxic damage produced by glutamate. The increase in the levels of taurine may provide an important protective mechanism against excitotoxicity. The mechanisms of the enhanced release were now studied in mouse hippocampal slices using a superfusion system. The basal release of [3H]taurine was significantly increased in Na+-deficient media in normal conditions, whereas the ischemia-evoked release was decreased, indicating the participation of Na+-dependent transport processes. The involvement of taurine transport carriers in the release was confirmed with the structural analogs, hypotaurine and beta-alanine. These amino acids potentiated the release by trans-stimulation in normoxia. In Na+-free conditions, this heteroexchange was not discernible, the carriers not being functional without Na+. In ischemia, the marked potentiation of taurine release by hypotaurine and beta-alanine further indicates that the Na+-requiring transporters also operate in ischemia. The effects of membrane disruption on taurine release due to activation of phospholipases were estimated using phospholipase and protein kinase inhibitors, which had no marked effects on hippocampal taurine release. The chloride channel blockers, 4-acetamido-4'-isothiocyanostilbene-2, 2'-disulphonate (SITS) and diisothiocyanostilbene-2,2'-disulphonate (DIDS), reduced the ischemia-induced release, suggesting that taurine diffusion through an anion channel is partially responsible for the enhanced release in ischemia.

Monoamine neurotransmitters and amino acids in the cerebrum and striatum of immature rats with kaolin-induced hydrocephalus

Hydrocephalus is characterized by enlargement of the cerebral ventricles. The behavioral disturbances are, in some cases, rapidly reversible by surgical treatment suggesting that there may be a functional impairment of neurons. Hydrocephalus was induced in 3-week old rats by kaolin injection into the cisterna magna. Parietal cerebrum and striatum content of monoamine neurotransmitters and amino acids were assayed by high performance liquid chromatography (HPLC), 1, 2, or 4 weeks after induction of hydrocephalus. The ventricles exhibited progressive enlargement which was partially reversed by surgical treatment. Cerebral water content was increased at all stages. Increased levels of cerebral aspartate and glutamate suggest that there is the potential for excitatory neurotoxicity. The increase in cerebral taurine correlated negatively with the increase in water content. Cerebral concentrations of norepinephrine and serotonin, and its metabolite 5-HIAA, were increased at 1 and 2 weeks suggesting an increase in their turnover during the early stages of ventricular dilatation. Dopamine and its metabolite DOPAC were transiently diminished in the striatum at 1 and 2 weeks, respectively, suggesting that axonal projections from the brainstem may be impaired. We conclude that the effect of hydrocephalus on amino acids and monoamines varies regionally. Due to increased water content, there may be dilution effects in whole tissue, therefore, it is important to make determinations on the basis of protein content.

Potassium-stimulated taurine release and nitric oxide synthase activity during quinolinic acid lesion of the rat striatum

The microdialysis technique was used to study the effect of nitric oxide synthase (NOS) activity on taurine release. Taurine release was characterized in rat striatum that was excitotoxically lesioned compared to normal conditions. The basal taurine level of the dialysate decreased during quinolinate (QUIN) lesion in parallel to the cell degeneration process. The K+-stimulated taurine concentration also decreased during QUIN-lesion, but to an extent that was different from that of basal values. K+-stimulated taurine levels were further markedly lowered by coapplication of the NOS inhibitor L-NAME in control and in lesioned animals up to 30 days after QUIN-injection. Postdegenerative tissue did not show any NOS-dependency in K+-induced taurine release. We conclude that a substantial part of K+-induced taurine release depends on NOS-activity both in normal brain tissue and in excitotoxically induced neurodegeneration. The main source of K+-induced taurine release in control rats are neurons but in lesioned animals are activated astroglial cells.

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.

Properties and glial origin of osmotic-dependent release of taurine from the rat supraoptic nucleus

1. Taurine, prominently concentrated in glial cells in the supraoptic nucleus (SON), is probably involved in the inhibition of SON vasopressin neurones by peripheral hypotonic stimulus, via activation of neuronal glycine receptors. We report here the properties and origin of the osmolarity-dependent release of preloaded [3H]taurine from isolated whole SO nuclei. 2. Hyposmotic medium induced a rapid, reversible and dose-dependent increase in taurine release. Release showed a high sensitivity to osmotic change, with a significant enhancement with less than a 5% decrease in osmolarity. Hyperosmotic stimulus decreased basal release. 3. Evoked release was independent of extracellular Ca2+ and Na+, and was blocked by the Cl- channel blockers DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and DPC (N-phenylanthranilic acid), suggesting a diffusion process through volume-sensitive Cl- channels. 4. Evoked release was transient for large osmotic reductions (> or = 15%), probably reflecting regulatory volume decrease (RVD). However, it was sustained for smaller changes, suggesting that taurine release induced by physiological variations in osmolarity is not linked to RVD. 5. Basal and evoked release were strongly inhibited by an incubation of the tissue with the glia-specific toxin fluorocitrate, but were unaffected by a neurotoxic-treatment with NMDA, demonstrating the glial origin of the release of taurine in the SON. 6. The high osmosensitivity of taurine release suggests an important role in the osmoregulation of the SON function. These results strengthen the notion of an implication of taurine and glial cells in the regulation of the whole-body fluid balance through the modulation of vasopressin release.

Alterations in K+ evoked profiles of neurotransmitter and neuromodulator amino acids after focal ischemia-reperfusion

Secondary elevations in extracellular amino acids occur during reperfusion after transient cerebral ischemia. The delayed accumulation of excitatory amino acids may contribute to the progressive development of neuronal injury. In this study, we explored the mechanisms that may be involved in this phenomenon. Microdialysis samples from probes located in rabbit cortex were analysed with a chiral amino acid procedure. Concentrations of neurotransmitters (L-Glu, GABA), N-methyl-D-aspartate receptor modulators (D-Ser, Gly), an inhibitory neuromodulator (Tau), the lipid component phosphoethanolamine, and L-Gln, L-Ser and L-Ala were measured. Depolarization via perfusion with potassium was used to assess the status of release/reuptake systems at 2 and 4 h reperfusion after 2 h transient focal ischemia. Background experiments classified potassium evoked responses as calcium dependent or calcium-independent by inclusion of 30 microM omega-conopeptide MVIIC or by inclusion of 20 mM magnesium and ommision of calcium. During ischemia, large elevations of almost all amino acids occurred. During reperfusion, secondary elevations in transmitter amino acids (L-Glu, GABA) and N-methyl-D-aspartate receptor modulators (D-Ser, Gly) occurred. Tau remained slightly elevated whereas the lipid component phosphoethanolamine remained high and stable during reperfusion. Reperfusion significantly potentiated the potassium response for amino acids with calcium-dependent responses (L-Glu and GABA). In contrast, calcium-independent responses (Tau, phosphoethanolamine, L-Gln) were significantly attenuated. Intermediate behavior was observed with Gly, while no potassium responses were observed for D-Ser, L-Ser or L-Ala. These data demonstrate that perturbations in evoked amino acid profiles after ischemia-reperfusion are selective. Reduction of calcium-independent responses implicate a general decline in efficacy of transporter mechanisms that restore transmembrane gradients of ions and transmitters. Decreased efficacy of transporter systems may reduce transmitter reuptake and account for the amplified release of L-Glu and GABA, thus contributing to progressive neural dysfunction after cerebral ischemia.

Enhanced taurine release in cell-damaging conditions in the developing and ageing mouse hippocampus

Taurine has been shown to be essential for neuronal development and survival in the central nervous system. The release of preloaded [3H]taurine was studied in hippocampal slices from seven-day-, three-month- and 18-22-month-old mice in cell-damaging conditions. The slices were superfused in hypoxic, hypoglycemic and ischemic conditions and exposed to free radicals and oxidative stress. The release of taurine was greatly enhanced in the above conditions in all age groups, except in oxidative stress. The release was large in ischemia, particularly in the hippocampus of aged mice. Potassium stimulation was still able to release taurine in cell-damaging conditions in immature mice, whereas in adult and aged animals the release was so substantial that this additional stimulus failed to work. Taurine release was partially Ca2+-dependent in all cases. The massive release of the inhibitory amino acid taurine in ischemic conditions could act neuroprotectively, counteracting in several ways the effects of simultaneous release of excitatory amino acids. This protection could be of great importance in developing brain tissue, while also having an effect in aged brains.