Co-operativity in sodium-independent taurine binding to brain membranes in the mouse

Taurine Neuroprotection and Neurogenesis Effect in Chronic Ethanol-Induced Rats

Taurine (2-aminoethanesulfonic acid) is a non-protein β-amino acid essential for cellular homeostasis, with antioxidant, anti-inflammatory, and cytoprotective properties that are crucial for life maintenance. This study aimed to evaluate the effects of taurine administration on hippocampal neurogenesis, neuronal preservation, or reverse damage in rats exposed to forced ethanol consumption in an animal model. Wistar rats were treated with ethanol (EtOH) for a 28-day period (5% in the 1st week, 10% in the 2nd week, and 20% in the 3rd and 4th weeks). Two taurine treatment protocols (300 mg/kg i.p.) were implemented: one during ethanol consumption to analyze neuroprotection, and another after ethanol consumption to assess the reversal of ethanol-induced damage. Overall, the results demonstrated that taurine treatment was effective in protecting against deficits induced by ethanol consumption in the dentate gyrus. The EtOH+TAU group showed a significant increase in cell proliferation (145.8%) and cell survival (54.0%) compared to the EtOH+Sal group. The results also indicated similar effects regarding the reversal of ethanol-induced damage 28 days after the cessation of ethanol consumption. The EtOH+TAU group exhibited a significant increase (41.3%) in the number of DCX-immunoreactive cells compared to the EtOH+Sal group. However, this amino acid did not induce neurogenesis in the tissues of healthy rats, implying that its activity may be contingent upon post-injury stimuli.

Functions of Maternally-Derived Taurine in Fetal and Neonatal Brain Development

Taurine (2-aminoethanesulfonic acid) is a sulfur-containing organic acid, which has various physiological functions, including membrane stabilization, cell-volume regulation, mitochondrial protein translocation, anti-oxidative activity, neuroprotection against neurotoxicity and modulation of intracellular calcium levels. Taurine also activates GABAA receptors and glycine receptors. Mammalian fetuses and infants are dependent on taurine delivered from their mothers via either the placenta or their mother's milk. Taurine is a molecule that links mother-fetus or mother-infant bonding.This review describes the functions of taurine and the mechanisms of action of taurine in fetal and brain development. Taurine is involved in regulating the proliferation of neural progenitors, migration of newly-generated neurons, and the synapse formation of neurons after migration during fetal and neonatal development. In this review, we also discuss the environmental factors that might influence the functional roles of taurine in neural development.

Role of taurine on acid secretion in the rat stomach

Background

Taurine has chemical structure similar to an inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Previous studies on GABA in the stomach suggest GABAergic neuron is involved in acid secretion, but the effects of taurine are poor understood.

Methods

The effects of taurine on acid secretion, signal transduction, and localization of taurinergic neurons were determined in the rat stomach using everted whole stomach, RIA kit and immunohistochemical methods.

Results

We used antibodies against taurine-synthesizing enzyme, cysteine sulfuric acid decarboxylase (CSAD), and taurine. CSAD- and taurine-positive cells were found in the muscle and mucosal layers. Distributions of CSAD- and taurine-positive cells in both mucosal and muscle layers were heterogeneous in the stomach. Taurine at 10^-9~10^-4 M induced acid secretion, and the maximum secretion was at 10^-5 M, 1.6-fold higher than the spontaneous secretion. Taurine-induced acid secretion was completely inhibited by bicuculline and atropine but not by cimetidine, proglumide, or strychnine. Atropine and tetrodotoxin (TTX) completely inhibited the acid secretion induced by low concentrations of taurine and partially inhibited induced by high concentrations. Verapamil, a calcium blocker agent, inhibited acid output elicited by taurine. We assumed all Ca²⁺ channels involved in the response to these secretagogues were equally affected by verapamil. Intracellular cAMP (adenosine 3', 5'-monophosphat) in the stomach significantly increased with taurine treatment in a dose-dependent manner. High correlation (r=0.859, p < 0.001) of taurine concentrations with cAMP was observed.

Conclusions

Our results demonstrated for the first time in taurine-induced acid secretion due to increase intracellular calcium may act through the A type of GABA receptors, which are mainly located on cholinergic neurons though cAMP pathway and partially on nonneuronal cells in the rat stomach.

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

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

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

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

Taurine attenuates D-[3H]aspartate release evoked by depolarization in ischemic corticostriatal slices

Taurine is thought to be protective in ischemia due to its neuroinhibitory effects. The present aim was to assess the ability of taurine to attenuate glutamate release evoked by ischemia and to determine which component of this release is affected. The release of preloaded D-[(3)H]aspartate (a non-metabolized analog of glutamate) from superfused murine corticostriatal slices was used as index of glutamate release. Preincubation of corticostriatal slices with 10 mM taurine reduced the D-[(3)H]aspartate release evoked by either chemical ischemia (0.5 mM NaCN in glucose-free medium) or oxygen-glucose deprivation. The taurine uptake inhibitor guanidinoethanesulfonate (5 mM), the glycine receptor antagonist strychnine (0.1 mM) and the GABA(A) receptor antagonist bicuculline (0.1 mM) did not block the taurine effect. To determine which component of ischemia-induced glutamate release is affected by taurine, three pathways of this release were pharmacologically modeled. Unlabeled D-aspartate (0.5 mM) and hypo-osmotic medium (NaCl reduced by 50 mM) evoked D-[(3)H]aspartate release via homoexchange and hypo-osmotic release pathways, respectively. Taurine did not influence these pathways. However, it suppressed the synaptic release of D-[(3)H]aspartate evoked by the voltage-gated sodium channel opener veratridine (0.1 mM). Taurine thus reduces glutamate release under ischemic conditions by affecting the depolarization-evoked component.

Taurine interaction with neurotransmitter receptors in the CNS: an update

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

Mode of action of taurine as a neuroprotector

Previously, it has been shown that taurine exerts its protective function against glutamate-induced neuronal excitotoxicity through its action in reducing glutamate-induced elevation of intracellular free calcium, [Ca2+]i. Here, we report the mechanism underlying the effect of taurine in reducing [Ca2+]i. We found that taurine inhibited glutamate-induced calcium influx through L-, P/Q-, N-type voltage-gated calcium channels (VGCCs) and NMDA receptor calcium channel. Surprisingly, taurine had no effect on calcium influx through NMDA receptor calcium channel when cultured neurons were treated with NMDA in Mg2+-free medium. Since taurine was found to prevent glutamate-induced membrane depolarization, we propose that taurine protects neurons against glutamate excitotoxicity by preventing glutamate-induced membrane depolarization, probably through its effect in opening of chloride channels and, therefore, preventing the glutamate-induced increase in calcium influx and other downstream events.

Developmental shift in bidirectional functions of taurine-sensitive chloride channels during cortical circuit formation in postnatal mouse brain

Taurine (2-aminoethanesulfonic acid) is the most abundant free amino acid in the developing mammalian cerebral cortex, however, few studies have reported its neurobiological functions during development. In this study, by means of whole-cell patch-clamp recordings, we examined the effects of taurine on chloride channel receptors in neocortical neurons from early to late postnatal stages, which cover a critical period in cortical circuit formation. We show here that taurine activates chloride channels in cortical neurons throughout the postnatal stages examined (from postnatal day 2 to day 36). The physiological effects of taurine changed from excitatory to inhibitory due to variations in the intracellular Cl- concentration during development. An antagonist blocking analysis also demonstrated a developmental shift in the receptor target of taurine, from glycine receptors to GABAA receptors. Taken together, these results may reflect genetically programmed, bidirectional functions of taurine. At the early developmental stage, taurine acting on glycine receptors would serve to promote cortical circuit formation. As cortical circuit has to be regulated in the later stages, taurine would serve as a safeguard against hyperexcitable circuit.

Interactions of taurine and structurally related analogues with the GABAergic system and taurine binding sites of rabbit brain

1. The aim of this study was to find taurinergic compounds that do not interact with brain GABA ergic systems. 2. Washed synaptic membranes (SM) from whole rabbit brain were able to bind [(3)H]muscimol. Saturation experiments of the binding of [(3)H]GABA to GABA(B) receptors showed that SM possess two binding components; twice Triton X-100-treated SM contained 0.048 mmol endogenous taurine/kg protein and bound [(3)H]taurine in a saturable manner (K(d)=249.0+/-6.3 nM and B(max)=3.4+/-1.0 pmol mg(-1) prot). 3. Among the 19 structural analogues of taurine, 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine 1,1-dioxide (TAG), 2-aminoethylarsonic (AEA), 2-hydroxyethanesulfonic (ISE) and (+/-)cis-2-aminocyclohexane sulfonic acids (CAHS) displaced [(3)H]taurine binding (K(i)=0.13, 0.13, 13.5 and 4.0 micro M, respectively). These analogues did not interact with GABA(A) and GABA(B) receptors and did not affect taurine- and GABA-uptake systems and GABA-transaminase activity. 4. 3-Aminopropanesulfonic acid (OMO), beta-alanine, pyridine-3-sulfonic acid, N,N,N-trimethyltaurine (TMT), 2-(guanidino)ethanesulfonic acid (GES), ethanolamine-O-sulphate, N,N-dimethyltaurine (DMT), taurine and (+/-)piperidine-3-sulfonic acid (PSA) inhibited [(3)H]muscimol binding to GABA(A) receptors with different affinities (K(i)=0.013, 7.9, 24.6, 47.5, 52.0, 91.0, 47.5, 118.1 and 166.3 micro M, respectively). Taurine, 2-aminoethylphosphonic acid, DMT, TMT and OMO inhibited the binding of [(3)H]GABA to GABA(B) receptors with K(i)'s in the micro M range (0.8, 3.5, 4.4, 11.3 and 5.0, respectively). GES inhibited taurine uptake (IC(50)=3.72 micro M) and PSA GABA transaminase activity (IC(50)=103.0 micro M). 5. In conclusion, AEA, TAG, ISE and CAHS fulfill the criteria for taurinergic agents.

Effects of taurine and some structurally related analogues on the central mechanism of thermoregulation: a structure-activity relationship study

There is large body of evidences on the role of taurine in the central mechanisms of thermoregulation in mammals, but it is not clear, whether the hypothermic effect of taurine depends on its interaction with GABA receptors or with a specific receptor. In order to answer this question, we have performed a structure-activity relationship study by using both in vitro and in vivo preparations. MicroM amounts of taurine or each of 20 analogues were injected intracerebroventricularly in conscious, restrained rabbits while rectal temperature was recorded. Receptor-binding studies, with synaptic membrane preparations from rabbit brain were used to determine the affinities of these compounds for GABA(A) and GABA(B) receptors. Furthermore, the interaction with presynaptic GABA and taurine uptake systems was studied using crude synaptosomal preparations from rabbit brain. Among the compounds tested, (+/-)-cis-2-aminocyclohexanesulfonic acid, induced hypothermia, but did not interact with GABA(A) and GABA(B) receptors neither did it affect GABA and taurine uptake, thus suggesting that its effect on body temperature is not mediated by the central GABAergic system. Interestingly, the trans-isomer was devoid of effects either in vivo or in vitro. In order to explain (+/-)-cis-2-aminocyclohexanesulfonic acid-induced hypothermia, a stereoscopic model was produced showing its possible interactions with a putative taurine brain receptor.

Taurine as osmoregulator and neuromodulator in the brain

Taurine has been assumed to function as an osmoregulator and neuromodulator in the brain. The pertinent studies are now reviewed in an attempt to formulate a unifying hypothesis as to how taurine could simultaneously act in both roles. Neuromodulatory actions of taurine may also underlie its protective effects against neuronal overexcitation and glutamate agonist-induced neurotoxicity.

Characterization of sodium-independent beta-alanine binding to cerebral cortical membranes from 7-day-old and adult mice

The sodium-independent binding of beta-alanine to cerebral cortical membranes from adult (3- and 12-month-old) and developing (7-day-old) mice was characterized for the first time. The binding was saturable in each age group, consisting of only one component. The affinity for beta-alanine was highest and the number of available binding sites greatest in young animals. The binding was not affected by strychnine, but inhibited by beta-alanine itself, glycine, L-alanine and L-serine, the IC50 values being lower in immature mice. Glycine was shown to be a competitive inhibitor. The binding was also inhibited, albeit only in adults, by N-methyl-D-aspartate receptor antagonists acting at the glycine modulatory site and by some GABAergic substances. It is concluded that even though beta-alanine may possess binding sites of its own, particularly in the immature cerebral cortex, beta-alanine could at least partly bind to strychnine-insensitive glycine sites in the N-methyl-D-aspartate receptor complex.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Taurine receptors in membranes from retinal pigment epithelium cells in culture

[3H]Taurine-specific binding to membranes from retinal pigment epithelium was demonstrated. A single saturable system was found, with KB = 237 nM and Bmax = 2.8 pmol/mg protein. Binding to freshly prepared membranes showed partial Na(+)-dependence while in frozen/thawed membranes, binding remained unchanged in the absence or presence of this ion. A 30-40% increase in binding was observed at physiological temperature (37 degrees C) compared to 4 degrees C in fresh but not in frozen membranes. Accumulation of taurine was followed during differentiation in vitro; results showed that changes in uptake and receptor binding to frozen membranes are not parallel, discarding the possibility of an interaction with uptake sites. Pharmacology of these binding sites suggests that they could be common to other amino acids, since displacement experiments showed that glycine, beta-alanine and strychnine were as potent as taurine itself in displacing [3H]taurine. Our data open the possibility of taurine being involved in the communication between the retina and the retinal pigment epithelium through an interaction with specific receptors.

Modification of chloride flux across brain membranes by inhibitory amino acids in developing and adult mice

The influx of 36Cl- was studied in membrane vesicles prepared from different brain regions from 3-day-old and adult mice. In both age groups the influx was enhanced about threefold by gamma-aminobutyric acid (GABA), which effect was blocked by bicuculline and picrotoxin but not by baclofen, characteristic of a GABAA receptor-mediated event. In samples from the adult brain stem the GABA stimulation was smaller than in samples from the other brain regions. Most of the compounds studied apparently act at the same receptor site with the following order of efficacy: muscimol greater than GABA greater than beta-alanine greater than hypotaurine greater than taurine. A number of anticonvulsant taurine derivatives were not effective and glycine only in the brain stem. The weak modulatory effects of taurine could be of significance in vivo since depolarizing stimuli release massive amounts of taurine in developing brain tissue.

Interactions of taurine with GABAB binding sites in mouse brain

The interactions of taurine with GABAB receptors were studied in membranes from the brain of the mouse by measuring the binding of [3H]baclofen and that of [3H]GABA in the presence of isoguvacine. Taurine displaced ligand binding to GABAB receptors concentration-dependently with an IC50 in the micromolar range. The effects of baclofen on the release of taurine and GABA from slices of cerebral cortex of the mouse were assessed using a superfusion system. Potassium-stimulated release of both [3H]taurine and [3H]GABA was unaffected by baclofen but potentiated by delta-aminovalerate. The enhancement of release of [3H]taurine by delta-aminovalerate was partially antagonized by baclofen, suggesting that baclofen-sensitive receptors could modify the release.

Effects of taurine on cell morphology and expression of low-affinity GABA receptors in cultured cerebellar granule cells

Effects of taurine and THIP were studied on the development of cultured cerebellar granule cells with regard to GABA receptor expression and morphological development. Culturing in the presence of taurine or THIP led to the formation of low affinity GABA receptors as revealed from Scatchard analysis of [3H]GABA binding. This formation of receptors was susceptible to inhibition upon culturing in the simultaneous presence of taurine and bicuculline demonstrating the involvement of the high affinity GABA receptors which are present on the cells regardless of the culture condition. Superfusion experiments on cells cultured under the different conditions demonstrated that the low affinity GABA receptors expressed after culturing in the presence of THIP or taurine mediated an inhibition by GABA of evoked transmitter release from the granule cells. Cells cultured in either plain culture media or in the presence of taurine were indistinguishable with respect to the number of neurite extending cells observed after 4 days in culture. In contrast, culturing in the presence of THIP increased the number of neurite extending cells by 8% relative to the controls.

Potassium-stimulated release of [3H]taurine from cultured GABAergic and glutamatergic neurons

The effect of depolarizing concentrations of potassium (56 mM) on the release of [3H]taurine was examined in two types of cultured neurons from mouse brain: cerebral cortex neurons, which are largely GABAergic, and cerebellar neurons, which after treatment with kainate consist almost entirely of glutamatergic granule cells. The release of [3H]taurine was compared to that of gamma-[3H]aminobutyric acid [( 3H]GABA) in cortical neurons and to that of D-[3H]aspartate in granule cells. Cortical neurons responded to potassium stimulation (1 min or continuously) by an immediate increase in [3H]GABA efflux of more than six times over the basal efflux, followed by a sharp decline despite the persistence of the stimulatory agent. The potassium-induced release of [3H]GABA was largely calcium-dependent. The release of [3H]taurine was considerably less in magnitude, only doubling after the stimulus, with a time course delayed in both onset and decline. The release of [3H]taurine was partially calcium-dependent and was also decreased in low-chloride solutions. In cerebellar granule cells, exposure to potassium resulted in a large (sixfold) and prompt release of D-[3H]aspartate, largely calcium-dependent. A totally different pattern was observed for the release of [3H]taurine. A stimulatory effect occurred only when cells were exposed continuously to potassium. Taurine efflux was very delayed, with a broad stimulus plateau reached after 15-20 min of stimulation. Taurine release was unaffected by omission of calcium, but it was abolished in a low-chloride medium. These results suggest that taurine is released from cells handling other neuroactive amino acids as neurotransmitters.(ABSTRACT TRUNCATED AT 250 WORDS)

Gamma-aminobutyric acid and benzodiazepine receptors in cultured cerebellar granule cells: effects of taurine and its lipophilic derivatives

The effects of taurine and some lipophilic derivatives of taurine on binding to GABA and benzodiazepine receptors were studied in intact cerebellar granule cells. The phenylsuccinylimido derivatives of taurine appeared to increase the binding of muscimol in micromolar concentrations, while taurine decreased it slightly. Only minor changes were seen in the basal binding of flunitrazepam, whereas stimulation of the binding by GABA was strongly reduced by piperidino, benzamido and phenyl-succinylimidotaurine with taurine itself again showing only a weak effect. Diphenylhydantoin, which bears structural resemblance to the phenylsuccinylimido group, had a strong effect on the stimulated binding of flunitrazepam and it also slightly reduced the basal level of binding. Thus, it seems possible that the effects of the phenylsuccinylimido derivatives of taurine on the binding of flunitrazepam were due to this chemical structure and not to the taurine-like core of the molecules. The phthalimido derivative of taurine, taltrimide, which has been tested in clinical trials with epileptic patients, did not show any activity in the binding studies.

Release of endogenous taurine and gamma-aminobutyric acid from brain slices from the adult and developing mouse

The spontaneous and potassium-stimulated release of endogenous taurine and gamma-aminobutyric acid (GABA) from cerebral cortex and cerebellum slices from adult and developing mice was studied in a superfusion system. The spontaneous release of GABA was of the same magnitude in slices from adult and developing mice, but the spontaneous release of taurine was considerably greater in the adults. The potassium-stimulated release of GABA from cerebral cortex slices was about five times greater in adult than in 3-day-old mice, but the potassium-stimulated release of taurine was more than six times greater in 3-day-old than in adult mice. In cerebellar slices from 7-day-old mice, potassium stimulation also evoked a massive release of taurine, whereas the evoked release from slices from adult mice was rather negligible. Also in cerebellar slices the potassium-stimulated release of GABA exhibited the opposite quantitative pattern. The stimulated release of both GABA and taurine was partially calcium dependent. The results suggest that taurine may be an important regulator of excitability in the developing brain.

Taurine and beta-alanine act on both GABA and glycine receptors in Xenopus oocyte injected with mouse brain messenger RNA

The responding pathway (process from agonist binding to channel opening) of taurine and beta-alanine was investigated in Xenopus oocytes injected with mouse brain poly(A)+ RNA. Responses to gamma-aminobutyric acid (GABA), glycine, taurine and beta-alanine were induced in oocytes injected with poly(A)+ RNA extracted from 3 regions, cerebrum, cerebellum and brainstem of the mouse brain. From comparison, responses to these 4 inhibitory amino acids in each regional poly(A)+ RNA-injected oocytes were categorized into at least 3 groups: (1) GABA, (2) glycine, and (3) taurine and beta-alanine. No cross-desensitization was observed between GABA response and glycine response, but taurine and beta-alanine responses cross-desensitized both the GABA and glycine responses. Taurine and beta-alanine responses were partially inhibited by the GABA antagonist, bicuculline, and also by the glycine antagonist, strychnine. The results suggest that the taurine or the beta-alanine response in the brain is caused through both the GABA receptor and the glycine receptor.