Taurine and GABA binding in mouse brain: effects of freezing, washing and Triton X-100 treatment on membranes

Potential protective effect of taurine against dibromoacetonitrile-induced neurotoxicity in rats

Dibromoacetonitrile (DBAN) is a disinfection by-product of water chlorination. Epidemiological studies indicate that it might present a potential hazard to human health. The present study aimed to investigate the possible neurotoxicity of DBAN in rats and possible protection by taurine. Based on initial dose-response experiment, DBAN (60 mg/kg) was administrated orally for 7 days. DBAN administration significantly impaired behavior of rats. Further, DBAN produced significant decrease of monoamines, γ-aminobutyric acid (GABA), glutamate contents, acetylcholinestrase (AChE) and aspartate aminotransferase (AST) activities, in rat brain. On the other hand, a significant increase in malondialdehyde (MDA), nitric oxide (NO) contents and lactic dehydrogenase (LDH) activity was observed. Co-administration of taurine (200mg/kg, i.p.) with DBAN mitigated most tested parameters. In conclusion, the present study indicates that DBAN has the propensity to cause significant oxidative damage in rat brain. However, taurine has a promising role in attenuating the obtained hazardous effects of DBAN.

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.

Triton X-100 inhibits agonist-induced currents and suppresses benzodiazepine modulation of GABA(A) receptors in Xenopus oocytes

Changes in lipid bilayer elastic properties have been proposed to underlie the modulation of voltage-gated Na(+) and L-type Ca(2+) channels and GABA(A) receptors by amphiphiles. The amphiphile Triton X-100 increases the elasticity of lipid bilayers at micromolar concentrations, assessed from its effects on gramicidin channel A appearance rate and lifetime in artificial lipid bilayers. In the present study, the pharmacological action of Triton-X 100 on GABA(A) receptors expressed in Xenopus laevis oocytes was examined. Triton-X 100 inhibited GABA(A) alpha(1)beta(3)gamma(2S) receptor currents in a noncompetitive, time- and voltage-dependent manner and increased the apparent rate and extent of desensitization at 10 muM, which is 30 fold below the critical micelle concentration. In addition, Triton X-100 induced picrotoxin-sensitive GABA(A) receptor currents and suppressed allosteric modulation by flunitrazepam at alpha(1)beta(3)gamma(2S) receptors. All effects were independent of the presence of a gamma(2S) subunit in the GABA(A) receptor complex. The present study suggests that Triton X-100 may stabilize open and desensitized states of the GABA(A) receptor through changes in lipid bilayer elasticity.

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.

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.

Metabotropic glutamate receptors modulate GABA release from mouse hippocampal slices

The effects of metabotropic glutamate receptor agonists on the basal and potassium (50 mM K+)-stimulated release of [3H]GABA from mouse hippocampal slices were investigated using a superfusion system. The group I agonist (1+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate enhanced the basal GABA release and reduced the K+-evoked release by a mechanism antagonized by (RS)-1-aminoindan-1,5-dicarboxylate in both cases. The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine failed to have any effect on the basal release, but inhibited the stimulated release. This inhibition was not affected by the antagonist (2S)-2-ethylglutamate. The group III agonists L(+)-amino-4-phosphonobutyrate and O-phospho-L-serine inhibited the basal GABA release, which effects were blocked by the antagonist (RS)-2-cyclopropyl-4-phosphonophenylglycine. Moreover, the suppression of the K+-evoked release by L(+)2-amino-4-phosphonobutyrate was apparently receptor-mediated, being blocked by (RS)-2-cyclopropyl-4-phosphonophenylglycine. The results show that activation of metabotropic glutamate receptors of group I is able to potentiate the basal release of GABA, whereas activation of groups I and III receptors reduce K+-stimulated release in mouse hippocampal slices.

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.

Interference of S-alkyl derivatives of glutathione with brain ionotropic glutamate receptors

The effects of glutathione, glutathione sulfonate and S-alkyl derivatives of glutathione on the binding of glutamate and selective ligands of ionotropic N-methyl-D-aspartate (NMDA) and non-NMDA receptors were studied with mouse synaptic membranes. The effects of glutathione and its analogues on 45Ca2+ influx were also estimated in cultured rat cerebellar granule cells. Reduced and oxidized glutathione, glutathione sulfonate, S-methyl-, -ethyl-, -propyl-, -butyl- and -pentylglutathione inhibited the Na+-independent binding of L-[3H]glutamate. They strongly inhibited also the binding of (S)-2-amino-3-hydroxy-5-[3H]methyl-4-isoxazolepropionate [3H]AMPA (IC50 values: 0.8-15.9 microM). S-Alkylation of glutathione rendered the derivatives unable to inhibit [3H]kainate binding. The NMDA-sensitive binding of L-[3H]glutamate and the binding of 3-[(R)-2-carboxypiperazin-4-yl][1,2-(3)H]propyl-1-phosphonate ([3H]CPP, a competitive antagonist at NMDA sites) were inhibited by the peptides at micromolar concentrations. The strychnine-insensitive binding of the NMDA coagonist [3H]glycine was attenuated only by oxidized glutathione and glutathione sulfonate. All peptides slightly enhanced the use-dependent binding of [3H]dizocilpine (MK-801) to the NMDA-gated ionophores. This effect was additive with the effect of glycine but not with that of saturating concentrations of glutamate or glutamate plus glycine. The glutamate- and NMDA-evoked influx of 45Ca2+ into cerebellar granule cells was inhibited by the S-alkyl derivatives of glutathione. We conclude that besides glutathione the endogenous S-methylglutathione and glutathione sulfonate and the synthetic S-alkyl derivatives of glutathione act as ligands of the AMPA and NMDA receptors. In the NMDA receptor-ionophore these glutathione analogues bind preferably to the glutamate recognition site via their gamma-glutamyl moieties.

Glutathione is an endogenous ligand of rat brain N-methyl-D-aspartate (NMDA) and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors

A study was made of the effects of reduced (GSH) and oxidized (GSSG) glutathione on the Na(+)-independent and N-methyl-D-aspartate (NMDA) displaceable bindings of glutamate, on the binding of kainate, 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), and ligand of the brain NMDA receptor-ionophore complex: glycine, dizocilpine (MK-801) and (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP). GSH and GSSG strongly inhibited the binding of glutamate, CPP and AMPA, kainate and glycine binding being less affected. Both peptides enhanced the binding of dizocilpine in a time- and concentration-dependent manner. This activatory effect was not additive to that of saturating concentrations of glutamate or glutamate plus glycine. The activation of dizocilpine binding by GSH and GSSG was prevented by the competitive NMDA and glycine antagonists DL-2-amino-5-phosphonovalerate and 7-chlorokynurenate. GSH and GSSG may be endogenous ligands of AMPA and NMDA receptors, binding preferably to the glutamate recognition site via their gamma-glutamyl moieties. In addition to this, at millimolar concentrations they may regulate the redox state of the NMDA receptor-ionophore complex.

Age-related changes in the uptake and release of glutamate and aspartate in the mouse brain

The studies were carried out on 3- and 7-day, 3-, 6-, 12-, 18- and 24-month-old mice. The levels of glutamate and aspartate increased in most brain areas in developing mice and then decreased gradually during ageing, the changes depending, however, on the brain region. The maximal velocity (V) of high-affinity uptake of [3H]glutamate was markedly reduced in cerebral cortical synaptosomes at the age of 24 months, indicating an age-related loss in the number of transport sites. The transport constant (Km) was also diminished in aged mice, indicating a compensatory increase in the affinity of the remaining transport sites. The basal and K(+)-stimulated (50 mM) release of endogenous glutamate and aspartate varied depending on the brain region. The responses to K+ stimulation generally increased during maturation, whereas the other age-related changes were more variable. The basal unstimulated release of glutamate remained fairly constant in the cerebral cortex during ageing, but K+ depolarization liberated more glutamate in 24-month-olds than in 3-month-olds. The decreased uptake capacity together with an increased release of glutamate may contribute to the degenerative changes associated with normal brain ageing and also to the pathogenesis of age-related neurodegenerative disorders.

Glutamate and N-methyl-D-aspartate binding sites in the guinea pig hippocampus: ontogeny and effect of acute in vitro ethanol exposure

The objectives of this study were to characterize the ontogeny of the L-glutamate (glutamate) and N-methyl-D-aspartate (NMDA) binding sites in the developing guinea pig hippocampus, and to determine the effect of acute in vitro ethanol exposure on these binding sites. Specific [3H]glutamate binding and NMDA-sensitive [3H]glutamate binding were determined using a guinea pig hippocampal synaptic membrane preparation (HSMP). To characterize the ontogeny of the density (Bmax) and affinity (Kd) of the glutamate and NMDA binding sites, saturation analysis was conducted on HSMP of guinea pigs at gestational day (GD) 50 (immature fetus; term, GD 68), GD 62 (mature, near-term fetus), postnatal day (PD) 13 (neonate), and PD > 60 (adult). To examine the effect of ethanol on the glutamate and NMDA binding sites, HSMP of guinea pigs at GD 50, GD 62, PD 13, and PD > 60 was incubated with ethanol (0-100 mM), followed by determination of specific [3H]glutamate binding and NMDA-sensitive [3H]glutamate binding. To determine the effect of 50 mM ethanol on the Bmax and Kd of the glutamate and NMDA binding sites, HSMP of guinea pigs at GD 62 and PD > 60 was incubated with 0 or 50 mM ethanol followed by saturation analysis. The Bmax values of the hippocampal glutamate and NMDA binding sites were greater at GD 62 and PD 13 compared with GD 50 and PD > 60, but there was no change in the Kd of the binding sites throughout development.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of phencyclidine on [3H]GABA and [3H]flunitrazepam binding in the brain of naive and handling-habituated rats

The effects of handling and handling combined with phencyclidine (PCP) treatment on GABAergic neurotransmission were studied in Sprague-Dawley rats. The animal material consisted of handling-habituated (HH, for 11 d), acutely handled (naive, N), handling-habituated and PCP-treated (10 mg kg-1 i.p., HH + PCP) and acutely handled (naive) PCP-treated (N + PCP) and unhandled 'control' rats. The binding of [3H]GABA and [3H]flunitrazepam (FLU) was studied with membranes and the release of [3H]GABA with slices prepared from the striatum and frontal cortex. In the striatum the maximal binding capacity (Bmax) and the binding constant (KD) of [3H]GABA were the same in N and HH rats, but in the frontal cortex KD was lower in N rats. KD constants of [3H]FLU were significantly lower in both brain areas in N rats than in HH rats. After PCP treatment both Bmax and KD for [3H]FLU increased in these two brain areas in handling-habituated rats, whereas Bmax of [3H]GABA diminished. Neither handling nor PCP had any effect on [3H]GABA release from striatal and frontal cortical slices. Handling prior to killing thus affects differently the GABAergic parameters studied and modulates the PCP-induced effects.

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.

Taurine in the developing cat: uptake and release in different brain areas

Taurine is an important modulator of neuronal activity in the immature brain. In kittens, taurine deficiency causes serious dysfunction in the cerebellar and cerebral visual cortex. The processes of taurine transport in vitro were now studied for the first time in different brain areas in developing and adult cats. The uptake of taurine consisted initially of two saturable components, high- and low-affinity, in synaptosomal preparations from the developing cerebral cortex and cerebellum, but the high-affinity uptake component completely disappeared during maturation. The release of both endogenous and preloaded labeled taurine from brain slices measured in a superfusion system was severalfold stimulated with a slow onset by depolarizing K+ (50 mM) concentrations. K+ stimulation released markedly more taurine from the cerebral cortex, cerebellum and brain stem in kittens than in adult cats. The responses were largest in the cerebellum. Both uptake and release of taurine are thus highly efficient in the brain of kittens and may be of significance in view of the vulnerability of cats to taurine deficiency.

Taurine release and swelling of cerebral cortex slices from adult and developing mice in media of different ionic compositions

The release of preloaded radiolabeled taurine was studied in superfused cerebral cortex slices obtained from adult and 3-day-old mice in media of varying ionic composition. Our aim was to establish whether the release of taurine from slices evokable by high concentrations of K+ could be attributed solely to cell volume changes or whether it results directly from depolarization of cell membranes. In both age groups hypoosmotic media enhanced the release of taurine. The enhancement was greater in 3-day-old than in adult mice. The K(+)-evoked release of taurine was likewise greater in slices from 3-day-old mice than in slices from adult mice. The K+ stimulation was totally preserved in adult mice and partially preserved in 3-day-old mice when the slices were superfused with Cl(-)-free media, with media in which the K+ x Cl- ionic product was kept constant and with hyperosmotic high-K+ media. The results were practically the same when the permeant anion acetate and the impermeant anion gluconate were used to replace the Cl- deficit. The unstimulated release of preloaded taurine was greatly enhanced in Cl(-)-free media in both age groups. There obtained no statistically significant correlation between the intracellular swelling of slices and the magnitude of taurine release under the present different experimental conditions in either age group. The results show that the K(+)-evoked release of taurine from superfused cerebral cortex slices cannot be solely attributed to depolarization-induced cell swelling. At least a part of the release results directly from membrane depolarization which besides exocytosis apparently also enhances the carrier-mediated release of taurine and inhibits the reuptake of taurine liberated from intracellular compartments.

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)

Differential taurine uptake in central and peripheral regions of goldfish retina

The transport system of taurine was investigated in fragments of goldfish retina prepared from the total tissue and from concentric regions: center and periphery. A high-affinity, saturable, sodium-dependent system was demonstrated in the three types of fragments. The Km for one-site analysis was similar in the two regions and the total retina. The analysis for two sites revealed a significant higher Km for the high-affinity site in fragments from the central region. The maximal uptake rate was higher in the central zone than in the total retina or the periphery. The Hill slopes obtained from saturation experiments of fragments of total retina, center, and periphery were similar to one other and near to 1. The slope of the time course uptake was intermediate for total retina and higher in the center than in the periphery. Hypotaurine and beta-alanine were found to inhibit taurine uptake, but GABA was a weak inhibitor. The values of Ki for hypotaurine by one- and two-site analysis were lower in the central region. The disruption of photoreceptors by shaking did not modify significantly the uptake of the amino acid. Remotion of endogenous taurine by dialysis of central and peripheral fragments increased the uptake in the periphery, but not in the center. The differences observed among the three samples revealed less affinity and high capacity for taurine uptake in the center, plus a higher sensitivity of inhibition. In addition, the peripheral zone had a greater affinity for taurine, and the maximal velocity of the entrance seems to be inhibited by the higher concentration of the amino acid in this zone. These observations may reflect differences between proliferating and non-proliferating regions of the retina (i.e., periphery and center).

Excitatory amino acids evoke taurine release from cerebral cortex slices from adult and developing mice

Glutamate, aspartate and the agonists of the excitatory amino acid receptors, N-methyl-D-aspartate, kainate and quisqualate, evoked more release of both endogenous and preloaded exogenous taurine from cerebral cortical slices from three-day-old than from specimens from adult mice. The N-methyl-D-aspartate- and quisqualate-evoked release was blocked by D-2-amino-5-phosphonovalerate and glutamatediethylester, respectively, in three-day-old mice but not in the adults. The kainate-evoked release was not affected by gamma-D-glutamyltaurine and gamma-D-glutamylglycine in either age group. Exposure of the slices to excitatory amino acids and their agonists caused intracellular swelling of the slices, which was directly proportional to the increase in taurine release in adult mice. In three-day-old mice the correlation between the swelling and taurine release was less pronounced. The excitatory amino acid receptors seem to modify more effectively the release of taurine in the developing than the adult brain. In the adults the evoked release of taurine may be related to cell volume regulation in the context of the excitation-coupled ionic and water movements across plasma membranes.

[3H]thienylcyclohexylpiperidine binding activity in brain synaptic membranes treated with Triton X-100

Binding activity of [3H]thienylcyclohexylpiperidine was examined using rat brain synaptic membranes treated with Triton X-100. This compound is proposed to be a noncompetitive antagonist for the N-methyl-D-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors. The activity decreased in proportion to increasing concentrations of the detergent up to 0.08%. In vitro addition of L-glutamate (Glu) partially restored the decreased activity caused by this Triton treatment, whereas further addition of glycine (Gly) entirely reversed the loss of activity to the level found in membranes extensively washed but not treated with a detergent. These stimulatory effects were found to be due to the acceleration of the association of ligand. The rank order of potentiation of the activity coincided well with that of the affinity for the NMDA-sensitive subclass among numerous Glu analogs. The potentiation by Gly as well as Glu was invariably prevented by competitive NMDA antagonists, such as DL-2-amino-5-phosphonovalerate and (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate, but not by strychnine. No significant difference was observed between pharmacological profiles of the activities in synaptic membranes treated and not treated with Triton X-100, except haloperidol. The potency of this sigma-ligand to inhibit the activity was greatly reduced by the Triton treatment in the presence of both Glu and Gly. These results suggest that the regulatory properties of Triton-treated synaptic membranes remain unchanged in terms of the interaction within the NMDA receptor complex.

Profiles of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine binding in brain synaptic membranes treated with Triton X-100

Binding of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine (TCP) was examined using rat brain synaptic membranes treated with a low concentration of Triton X-100. This compound is assumed to be a non-competitive antagonist for the N-methyl-D-aspartate(NMDA)-sensitive subclass of central excitatory amino acid receptors. Binding was quite low but detectable in Triton-treated membranes irrespective of the incubation temperature, and the temperature-dependent portion of the binding was greatly reduced in these Triton-treated membranes. However, binding was drastically potentiated by the inclusion of L-glutamate and its analogous amino acids in a concentration-dependent manner at a concentration range of 10 nM to 0.1 mM. Agonists for the NMDA-sensitive subclass also potentiated binding, with agonists for the other subclasses being ineffective. Glycine at a concentration above 10 nM was not only effective as a stimulant of potentiated binding by glutamate, but was also active in enhancing binding in the absence of added glutamate. Glycine increased both the association and dissociation rates without significantly affecting the dissociation constant. Pharmacological profiles of binding in Triton-treated membranes were not significantly different from those in untreated membranes, except for that of haloperidol. Haloperidol is proposed to be highly selective for brain sigma-receptors on the basis of a potent inhibition of sigma-receptor binding. The inhibitory potency of this sigma-ligand was markedly attenuated in the presence of both glutamate and glycine in Triton-treated membranes, as compared with that in untreated membranes. These results suggest that [3H]TCP binding in Triton-treated membranes is a useful biochemical tool to evaluate predominantly the activated state of ion channels associated with the NMDA-sensitive receptors in terms of freedom from the confounding effects of endogenous amino acids.

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.

Interaction of strychnine-insensitive glycine binding with MK-801 binding in brain synaptic membranes

Strychnine-insensitive [3H]glycine binding was detected in brain synaptic membranes treated with Triton X-100 using a filtration assay method. The binding was a time-dependent, inversely temperature-dependent, and reversible process with a relatively high affinity for the neuroactive amino acid. Scatchard analysis revealed that Triton treatment doubled both the affinity and density of the binding sites, which consisted of a single component. The binding was not only displaced by structurally-related amino acid such as D-serine and D-alanine, but also inhibited by some peptides containing glycine, including glycine methylester and N-methylglycine. These ligands invariably potentiated the binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]- cyclohepten-5,10-imine ([3H]MK-801), a noncompetitive antagonist for the N-methyl-D-aspartate-sensitive subclass of the central excitatory amino acid receptors, in a concentration-dependent manner. Among various endogenous tryptophan metabolites, kynurenic acid significantly inhibited the strychnine-insensitive [3H]glycine binding. The Triton treatment did not affect the pharmacological profile of [3H]MK-801 binding sites. These results suggest that brain synaptic membranes treated with Triton X-100 are useful in evaluating the strychnine-insensitive and kynurenate-sensitive binding sites of glycine, which are functionally linked to N-methyl-D-aspartate- sensitive receptor channels.

Temperature-independent binding of [3H](+-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid in brain synaptic membranes treated by Triton X-100

Specific binding of [3H](+-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), a highly selective antagonist for N-methyl-D-aspartic acid (NMDA) receptors, was examined in brain synaptic membranes treated with Triton X-100 by using a filtration assay method. Elevation of incubation temperature from 2 to 30 degrees C markedly diminished the binding. The binding reached a plateau within 5 min after the initiation of incubation at 2 degrees C, while the time required to attain an equilibrium was 1 min at 30 degrees C. The binding at 2 degrees C was rapidly dissociated by the addition of an excess of unlabeled CPP, NMDA and L-glutamic acid (L-Glu). The binding was also saturable with increasing concentrations of the ligand and displaced by various amino acids structurally related to L-Glu in a stereospecific manner. Competitive but not noncompetitive antagonists for the NMDA receptors invariably inhibited the binding. However, the binding was not prominently affected by agonists for the other subclasses of the brain excitatory amino acid receptors. Both reduced and oxidized forms of glutathione significantly displaced the binding. Scatchard analysis revealed that Triton treatment increased the affinity and density of binding sites which consisted of a single component. Among some endogenous tryptophan metabolites, kynurenic, anthranilic and quinolinic acids inhibited the binding. These results suggest that a filtration assay method is also useful to detect the binding of NMDA receptors in the brain.

Displacement of excitatory amino acid receptor ligands by acidic oligopeptides

A number of L-glutamyl and L-aspartyl dipeptides, glutathione, gamma-D-glutamylglycine and gamma-D-glutamyltaurine, were tested for their efficacy to displace ligands specific for different subtypes of excitatory amino acid receptors from rat brain synaptic membranes. In general, the L enanthiomorphs of gamma-glutamyl peptides were more potent displacers than gamma-D-glutamylglycine and -taurine but the latter were more specific for the quisqualate type of receptors. gamma-L-glutamyl-L-glutamate was the most effective dipeptide in displacing the binding of glutamate, 2-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA) and 2-amino-5-phosphonoheptanoate (APH), whereas gamma-L-glutamyl-L-aspartate was the most effective in the binding of kainate. Both oxidized and reduced glutathione were inhibitory, being most potent in the binding of AMPA. gamma-L-Glutamylaminomethylsulphonate was most effective in the binding of APH. The most potent gamma-L-glutamyl peptides (glutathione, gamma-L-glutamyl-L-glutamate, -L-aspartate, and -glycine) may act as endogenous modulators of excitatory aminoacidergic neurotransmission.

Strychnine-insensitive binding of [3H]glycine to synaptic membranes in rat brain, treated with Triton X-100

Binding of radiolabelled glycine, a putative inhibitory neurotransmitter in mammalian lower central structures, was examined by using the synaptic membranes of the brain of rat, treated with Triton X-100. This treatment with Triton markedly potentiated the binding of [3H]glycine detected at 2 degrees C and 30 degrees C. However, this binding was not affected by three different convulsants, strychnine, picrotoxin and bicuculline. The binding was saturable at 2 degrees C, with increasing concentrations of [3H]glycine up to 1 microM. Scatchard analysis revealed that the binding sites consisted of a single component with a Kd of 202 nM and a Bmax of 1.74 pmol/mg protein. The binding was inhibited, not only by various amino acids structurally related to glycine, including D- and L-serine and D-, L- and beta-alanine, but was also eliminated by some peptides containing glycine, such as gamma-D- and gamma-L-glutamylglycine, glycine methylester and N-methyl-glycine. In addition, the strychnine-insensitive binding of [3H]glycine was significantly abolished by numerous quinoxaline antagonists for excitatory amino acid receptors in the brain. These results suggest that synaptic membranes of brain, treated with Triton X-100, are useful to detect the strychnine-insensitive binding of [3H]glycine and superior to untreated membranes in terms of the freedom from the confounding effects of some endogenous amino acids.

Labeling of NMDA receptor channels by [3H]MK-801 in brain synaptic membranes treated with Triton X-100

Binding activity of [3H](+)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d] cyclohepten-5, 10-imine maleate (MK-801) was examined by using rat brain synaptic membranes treated with Triton X-100. This compound is known as a non-competitive antagonist for one subclass of the central excitatory amino acid receptors, N-methyl-D-aspartic acid (NMDA)-sensitive receptors. Triton treatment completely abolished the temperature-dependent portion of the binding activity, with a concomitant reduction of membranous protein content. Addition of L-glutamic acid (Glu), however, markedly potentiated the activity at concentrations higher than 10 nM in a temperature-dependent manner. Similarly significant potentiation was induced by structurally related amino acids as well as agonists for the NMDA-sensitive receptors, but not by agonists for the other subclasses. The rank order of this stimulatory potency was well consistent with that of the displacing activity of these Glu analogues on NMDA-sensitive [3H]Glu binding. Competitive NMDA antagonists, such as (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid and D-2-amino-5-phosphonovaleric acid, were all effective in preventing the potentiation of [3H]MK-801 binding activity by L-Glu. The latter Glu-dependent activity was additionally enhanced by glycine and its analogues at concentrations above 10 nM in a temperature- and Glu-dependent fashion. Additional enhancement by glycine was also antagonized by competitive NMDA antagonists, but not by a classical glycine antagonist strychnine. These results suggest that Triton X-100-treated membranes are suitable for the study of the interaction of the non-competitive antagonists with NMDA receptor channels, and may be superior to non-detergent-treated membranes in terms of freedom from the confounding effects of endogenous amino acids.

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.

N-methyl-D-aspartate-sensitive [3H]glutamate binding sites in brain synaptic membranes treated with Triton X-100

Specific binding activity of radiolabeled L-glutamic acid, a putative central excitatory neutrotransmitter, was drastically increased with increasing concentrations of Triton X-100 used for pretreatment of rat brain synaptic membranes. The binding in these Triton-treated membranes was a protein dependent, inversely temperature-dependent, stereospecific, structure-selective and saturable process with a high affinity for the amino acid. The binding activity was invariably inhibited by agonists and antagonists for the N-methyl-D-aspartic acid (NMDA)-sensitive subclass, but not by agonists for the other subclasses of excitatory amino acid neurotransmitter receptors in the brain. Scatchard analysis revealed that the binding sites consisted of a single component with a Kd of 24.4 +/- 2.5 nM and a Bmax of 0.94 +/- 0.09 pmol/mg protein. Some endogenous tryptophan metabolites such as kynurenic acid and quinolinic acid also inhibited the binding. These results suggest that synaptic membranes may indeed contain the NMDA-sensitive receptors which are disclosed by Triton X-100 treatment.

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.

Release of taurine and GABA from cerebellar slices from developing and adult mice

The properties of the release of exogenous radioactive taurine and GABA from cerebellar slices from developing and adult mice were investigated using a superfusion system. Potassium stimulation (50 mM K+) caused, approximately, a 1.4-fold enhancement in the release of preloaded taurine from slices from adult mice, while the response to potassium in 7-day-old mice was about 6-fold. The potassium stimulation caused, approximately, a 3-fold increase in the release of preloaded GABA from cerebellar slices from 7-day-old mice, whereas the enhancement was about 10-fold in the adult. The actual molar amount of taurine released from the immature cerebellum was strikingly large, about 16 times larger than the amount of GABA released upon the same stimulus. Spontaneous taurine efflux was potentiated by taurine and GABA, the responses being more pronounced in the 7-day-old cerebellum, suggesting that the immature cerebellum is more prone to stimulation by homo- and heteroexchange than the mature cerebellum. Potassium-stimulated taurine release was inhibited by GABAergic substances in the adult but not in the developing cerebellum. Potassium-stimulated GABA release from the adult cerebellum was greatly increased by GABA and also moderately by muscimol and bicuculline, the effect of the latter being antagonized by taurine and hypotaurine. Taurine was thus able to modulate GABA release through bicuculline-sensitive receptors, but this modulation was not evident in cerebellar slices from 7-day-old mice. An exposure of the slices to sodium-free media greatly enhanced taurine and GABA release in both age groups. The stimulated release of GABA generally exhibited a similar calcium dependency in the adult and 7-day-old cerebella but in 7-day-old mice the stimulated release of taurine was not strictly calcium-dependent.

Release of taurine from cultured cerebellar granule cells and astrocytes: co-release with glutamate

The properties of the release of preloaded [3H]taurine and endogenous taurine were studied with cultured cerebellar granule cells (7-8 days in vitro) and astrocytes (14-15 days in vitro) from the rat. The spontaneous release of taurine from both cell types was slow. The release from both neurons and astrocytes was significantly enhanced by 0.1 mM veratridine, the stimulatory effect being more pronounced in granule cells than in astrocytes. No homo or heteroexchange with extracellularly added taurine or its structural analogues could be detected, suggesting that the efflux is probably not mediated via the membrane transport sites. Kainate stimulated the release more from granule cells than from astrocytes, the effect apparently being mediated by kainate-sensitive receptors. Depolarization of cell membranes by 50 mM K+ induced co-release of endogenous taurine and glutamate from both cell types. Preloaded [3H]taurine was readily released from astrocytes by potassium stimulation. Stimulated release occurred from granule cells if they had been cultured for 4 days with the label but not from the cells preloaded for only 15 min.

A re-examination of the Na+-independent binding of [3H]beta-alanine to rat brain stem-spinal cord

The Na+-independent binding of [3H]beta-alanine to rat brain stem plus spinal cord was reinvestigated, in order to study in more detail the characteristics of previously described beta-alanine binding processes. Binding was absent when amino acid-free postnuclear supernatants or crude synaptic membranes were used. Experiments performed with several other Na+-free preparations showed a sole binding component, irrespective of the preparation used. Biochemical characterization of this Na+-independent binding, using frozen/thawed/washed synaptosomal-mitochondrial fractions, showed that binding reached a plateau between 7 min and 13 min, increasing thereafter. Binding was linear with fraction protein over a range of 200-415 micrograms/ml incubation medium. Binding was completely inhibited by glycine, alanine, alpha-aminobutyric acid, beta-aminoisobutyric acid, hypotaurine and strychnine, and to a lesser extent by 2,2-dimethyl-beta-alanine, brucine and gelsemine. It was insensitive to taurine, gamma-aminobutyric acid (GABA), 2-guanidinoethanesulfonic acid (GES), carnosine, and bicuculline methiodide. Binding was reversible, saturable (KD 20 microM), and heat sensitive.

Taurine and GABA release from mouse cerebral cortex slices: potassium stimulation releases more taurine than GABA from developing brain

The release of exogenous taurine and gamma-aminobutyric acid (GABA) was studied with slices from the developing mouse cerebral cortex. The spontaneous efflux of GABA increased with the cerebral GABA content during postnatal development, while the spontaneous efflux of taurine was approximately the same in both neonate and adult mice, in spite of a several-fold higher cerebral taurine content in the former. GABA, taurine and their structural analogues caused marked homo- and hetero-trans-stimulation of the release in both adult and developing mice, probably via membrane transport sites. The release was greatly enhanced by both 0.01 mM veratridine and exposure to sodium-free medium, the effects being more pronounced with GABA in the adults and with taurine in the neonates. The excitatory amino acids homocysteate, aspartate and kainate enhanced taurine release particularly from the developing cerebral cortex but were not effective on GABA release in the adults. The potassium stimulation of taurine release had a strikingly slow time course in both adult and developing mice. The responses in GABA release were also fairly slow in the neonates. Potassium stimulation evoked a large release of GABA in adult but not in developing mice. The evoked taurine release was in developing mice several-fold greater than the evoked GABA release, decreasing in magnitude with age. The potassium-stimulated release was only partially calcium dependent, more so with GABA in the adults and with taurine in the neonates, but a high magnesium ion concentration inhibited the release of both amino acids more strongly in the latter age group. Verapamil (0.1 mM) almost abolished the potassium stimulation of GABA release in both adult and neonate mice and was more effective on taurine release in neonate mice. The results suggest that taurine, not GABA, is the major inhibitor of excitability in developing mouse brain.

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

Sodium-independent taurine binding to mouse brain membranes treated twice with Triton X-100 exhibited properties of positive co-operativity, suggesting that two or more taurine molecules interact at the binding site. The proposed taurine antagonist 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide and the new anticonvulsant taurine derivative taltrimide as well as glycine and GABA with their antagonists displaced taurine binding, strychnine being the most effective.