Free amino acids in the synaptosome and synaptic vesicle fractions of different bovine brain areas

Metabolism of Amino Acids in the Brain and Their Roles in Regulating Food Intake

Amino acids (AAs) and their metabolites play an important role in neurological health and function. They are not only the building blocks of protein but are also neurotransmitters. In the brain, glutamate and aspartate are the major excitatory neurotransmitters, whereas γ-aminobutyrate (GABA, a metabolite of glutamate) and glycine are the major inhibitory neurotransmitters. Nitric oxide (NO, a metabolite of arginine), H₂S (a metabolite of cysteine), serotonin (a metabolite of tryptophan) and histamine (a metabolite of histidine), as well as dopamine and norepinephrine (metabolites of tyrosine) are neurotransmitters to modulate synaptic plasticity, neuronal activity, learning, motor control, motivational behavior, emotion, and executive function. Concentrations of glutamine (a precursor of glutamate and aspartate), branched-chain AAs (precursors of glutamate, glutamine and aspartate), L-serine (a precursor of glycine and D-serine), methionine and phenylalanine in plasma are capable of affecting neurotransmission through the syntheses of glutamate, aspartate, and glycine, as well as the competitive transport of tryptophan and tyrosine across from the blood-brain barrier. Adequate consumption of AAs is crucial to maintain their concentrations and the production of neurotransmitters in the central nervous system. Thus, the content and balance of AAs in diets have a profound impact on food intake by animals. Knowledge of AA transport and metabolism in the brain is beneficial for improving the health and well-being of humans and animals.

Glutamate Release

Our aim was to review the processes of glutamate release from both biochemical and neurophysiological points of view. A large body of evidence now indicates that glutamate is specifically accumulated into synaptic vesicles, which provides strong support for the concept that glutamate is released from synaptic vesicles and is the major excitatory neurotransmitter. Evidence suggests the notion that synaptic vesicles, in order to sustain the neurotransmitter pool of glutamate, are endowed with an efficient mechanism for vesicular filling of glutamate. Glutamate-loaded vesicles undergo removal of Synapsin I by CaM kinase II-mediated phosphorylation, transforming to the release-ready pool. Vesicle docking to and fusion with the presynaptic plasma membrane are thought to be mediated by the SNARE complex. The Ca(2+)-dependent step in exocytosis is proposed to be mediated by synaptotagmin.

Comparison of taurine, hypotaurine and β-alanine uptake in brain synaptosomal preparations from developing and adult mouse

The properties of taurine, hypotaurine and β-alanine uptakes were compared in brain synaptosomal preparations from 6-day-old and adult mice. The uptakes of these structurally related amino acids resembled each other, being concentrative, sodium-dependent and inhibited by the same analogues. The absolute sodium requirement of uptake was already evident in developing brain. The affinity of the lowaffinity uptake for taurine was higher in immature than in adult brain. Both affinity and maximal velocity increased in hypotaurine uptake during development, whereas in β-alanine uptake only the maximal velocity did so. The efficient synaptosomal taurine and hypotaurine transport systems in immature brain could contribute to the high taurine level in developing brain.

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

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

Anticonvulsant activity of 2-phthalimidoethanesulphonamides: New derivatives of taurine

A number of 2-phthalimidoethanesulphonamides, new derivatives of the inhibitory neuromodulator taurine, were tested for their anticonvulsant activity in maximal electroshock seizure and pentetrazole seizure threshold tests in mice. Certain lower N-alkylamides showed activity, methylamide, dimethylamide and isopropylamide derivatives and the unsubstituted amide being pharmacologically most promising. Possible interferences with the intracellular uptake, release and membrane binding of taurine and GABA were assessed in an attempt to elucidate their mode of action. Since the uptake and release processes were only minimally affected, but the sodium-independent binding of GABA and/or taurine to synaptic membranes strongly reduced, the compounds studied may preferentially act as taurine or GABA receptor agonists.

Taurine acts as a glycine receptor agonist in slices of rat inferior colliculus

Taurine is an important endogenous amino acid for neural development and for many physiological functions, but little is known about its functional role in the central auditory system. We investigated in young rats (P10-P14) the effects of taurine on the neuronal responses and synaptic transmissions in the central nucleus of the inferior colliculus (ICC) with a brain slice preparation and with whole-cell patch-clamp recordings. Perfusion of taurine at 1mM reliably evoked a current across the membrane and decreased the input resistance in neurons of the ICC. Taurine also depressed the spontaneous and current-evoked firing of ICC neurons. All these effects were reversible after washout and could be blocked by 3 microM strychnine, an antagonist of glycine receptors, but not by 10 microM bicuculline, an antagonist of GABA(A) receptors. When the inhibitory receptors were not pharmacologically blocked, taurine reversibly reduced the postsynaptic currents/potentials evoked by electrically stimulating the commissure of the inferior colliculus or the ipsilateral lateral lemniscus. The results demonstrate that taurine reduces the neuronal excitability and depresses the synaptic transmission in the ICC by activating glycine-gated chloride channels. Our findings suggest that taurine acts as a ligand of glycine receptors in the ICC and can be involved in the information processing of the central auditory system similarly like the neurotransmitter glycine.

Mechanisms of enhanced taurine release under Ca2+ depletion

The sulfur-containing amino acid taurine is an inhibitory neuromodulator in the brain of mammals, as well as a key substance in the regulation of cell volumes. The effect of Ca(2+) on extracellular taurine concentrations is of special interest in the context of the regulatory mechanisms of taurine release. The aim of this study was to characterize the basal release of taurine in Ca(2+)-free medium using in vivo microdialysis of the striatum of anesthetized rats. Perfusion of Ca(2+)-free medium via a microdialysis probe evoked a sustained release of taurine (up to 180 % compared to the basal levels). The Ca(2+) chelator EGTA (1mM) potentiated Ca(2+) depletion-evoked taurine release. The substitution of CaCl(2) by choline chloride did not alter the observed effect. Ca(2+)-free solution did not significantly evoke release of taurine from tissue loaded with the competitive inhibitor of taurine transporter guanidinoethanesulfonate (1mM), suggesting that in Ca(2+) depletion taurine is released by the transporter operating in the outward direction. The volume-sensitive chloride channel blocker diisothiocyanostilbene-2,2'-disulfonate (1mM) did not attenuate the taurine release evoked by Ca(2+) depletion. The non-specific blocker of voltage-sensitive Ca(2+) channels NiCl(2) (0.65 mM) enhanced taurine release in the presence of Ca(2+). CdCl(2) (0.25 mM) had no effect under these conditions. However, both CdCl(2) and NiCl(2) attenuated the effect of Ca(2+)-free medium on the release of taurine. The data obtained imply the involvement of both decreased influx of Ca(2+) and increased non-specific influx of Na(+) through voltage-sensitive calcium channels in the regulation of transporter-mediated taurine release in Ca(2+) depletion.

Depolarization, exocytosis and amino acid release evoked by hyposmolarity from cortical synaptosomes

External osmolarity reduction (20%) led to labelled glutamate, GABA and taurine release from rat brain cortical synaptosomes. A Cl--independent, Na+-dependent, La3+-sensitive and tetrodotoxin (TTX) reduced depolarization of synaptosomes occurred upon hyposmolarity, suggestive of Na+ entry through nonselective cation channels. This depolarization, together with cytosolic Ca2+ ([Ca2+]I) increase, resulted in exocytosis, monitored by FM1-43. The release fraction resulting from these phenomena was estimated, by its decrease, by La3+, EGTA-AM and tetanus toxin (TeTX), as 34-44% for glutamate, 21-29% for GABA and 18-22% for taurine. Protein kinase C (PKC) activation by phorbol-12-myristate-13-acetate (PMA) increased the hyposmolarity-elicited exocytosis and this activation increased glutamate (80%), GABA (51%) and taurine (42%) hyposmotic efflux. Inhibition by chelerythrine reduced glutamate, GABA and taurine efflux by 64%, 50% and 24%, respectively. The Na+-dependence of amino acid release (glutamate 63%, GABA 46% and taurine 29%) may result from both, prevention of the depolarization-exocytosis efflux, and blockade of the carrier reversal operation. Carrier blockade by dl-threo-beta-benzyloxy aspartate (TBOA) and NO-711 resulted in 37% and 28% reduction of glutamate and GABA release, respectively. Contribution of the osmolyte leak pathway to amino acid release, estimated by the influence of Cl- (NPPB) and tyrosine kinase (AG18) blocker, was up to 55% for taurine, but only 10-18% for GABA, with apparently no contribution for glutamate. The predominant osmolyte-type mechanism of taurine release suggest its function in volume control in nerve endings, while glutamate and GABA respond to events concurrent with hyposmolarity by a neurotransmitter-like release mechanism. The hyposmolarity-induced amino acid efflux from nerve endings may have consequences for neuronal excitability during hyponatremia.

Fe2+ decreases the taurine-induced Cl- current in acutely dissociated rat hippocampal neurons

The effects of ferrous ions (Fe(2+)) on taurine-induced Cl(-) current (I(tau)) recorded from single neurons, which was freshly isolated from the rat hippocampal CA1 area, were studied with conventional whole-cell recording under voltage-clamp conditions. Using standard pharmacological approaches, we found that the currents gated by concentrations of taurine (<or=10 mM), which existed in about 90% of the hippocampal neurons tested, were predominantly mediated by strychnine-sensitive glycine receptors. When co-applied with taurine, Fe(2+) effectively depressed I(tau) in a concentration-dependent manner, with an IC(50) of 3.76 mM and Hill coefficient of 1.01, while preincubation with 1 mM Fe(2+) alone did not affect the following membrane currents elicited by taurine. The result suggests that resting taurine-gated channels are insensitive to Fe(2+). Since internal cell dialysis with 3 mM Fe(2+) failed to modify I(tau), it was deduced that the site of action of Fe(2+) is extracellular. Furthermore, the Lineweaver-Burke double reciprocal plot of normalized response to taurine against the concentration of taurine illustrated that the depression of I(tau) was noncompetitive, therefore Fe(2+) may act on the glycine receptor-chloride ionophore complex at a site distinct from where taurine binds. Various concentrations of Fe(2+) ranging from 0.1 to 20 mM depressed I(tau) and this extracellular depression was independent of membrane voltage. These results indicate that Fe(2+) decreases I(tau) in acutely dissociated rat hippocampal neurons and the inhibition of glycine receptors by Fe(2+) might be one possible approach through which Fe(2+) induces seizures.

Mechanisms of adenosine release in the developing and adult mouse hippocampus

Adenosine is a neuromodulator known to inhibit the synaptic release of neurotransmitters, e.g., glutamate, and to hyperpolarize postsynaptic neurons. The release of adenosine is markedly enhanced under ischemic conditions. It may then act as an endogenous neuroprotectant against cerebral ischemia and excitotoxic neuronal damage. The mechanisms by which adenosine is released from nervous tissue are not fully known, particularly in the immature brain. We now characterized the release of [3H]adenosine from hippocampal slices from developing (7-day-old) and adult (3-month-old) mice using a superfusion system. The properties of the release differed only partially in the immature and mature hippocampus. The K(+)-evoked release was Ca2+ and Na+ dependent. Anion channels were also involved. Ionotropic glutamate receptor agonists potentiated the release in a receptor-mediated manner. Activation of metabotropic glutamate receptors enhanced the release in developing mice, with group II receptors alone being effective. The evoked adenosine release apparently provides neuroprotective effects against excitotoxicity under cell-damaging conditions. Taurine had no effect on adenosine release in adult mice, but depressed the release concentration dependently in the immature hippocampus.

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.

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

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

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.

Glutamate transport and storage in synaptic vesicles

Glutamate plays an important metabolic role in virtually every vertebrate cell. In particular, glutamate is the most common excitatory neurotransmitter in the vertebrate central nervous system. As such, the mechanism by which glutamate is diverted from its normal metabolic activities toward its role as a neurotransmitter has, in recent years, been systematically investigated. In glutamatergic nerve endings, synaptic vesicles accumulate and store a proportion of the cellular glutamate pool and, in response to appropriate signals, release glutamate into the synaptic cleft by exocytosis. Glutamate accumulation is accomplished by virtue of a glutamate uptake system present in the synaptic vesicle membrane. The uptake system consists of a transport protein, remarkably specific for glutamate, and a vacuolar-type H+-ATPase, which provides the coupling between ATP hydrolysis and glutamate transport. The precise manner in which the glutamate transporter and H+-ATPase operate is currently the subject of debate. Recent data relevant to this debate are reviewed in this article. Additionally, pharmacological agents thought to specifically interact with the vesicular glutamate transporter are discussed. Finally, a newly discovered, endogenous inhibitor of vesicular uptake, inhibitory protein factor (IPF), is discussed with some speculations as to its potential role as a presynaptic modulator of neurotransmission.

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

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

Effects of uptake carrier blockers SK & F 89976-A and L-trans-PDC on in vivo release of amino acids in rat hippocampus

This report describes the in vivo effects of the uptake carrier blockers 1-(4,4-diphenyl-3-butenyl)-3-piperidine carboxylic acid hydrochloride (SK & F 89976-A) and L-trans-pyrrolidine-2,4-dicarboxylate (L-trans-PDC) on basal and K(+)-evoked extracellular levels of gamma-aminobutyric acid (GABA), glutamate, aspartate and taurine in the hippocampus of anaesthetised rats, using the microdialysis technique. SK & F 89976-A increased extracellular GABA levels under K(+)-depolarised conditions and did not affect extracellular glutamate, aspartate and taurine levels, indicating its selective effect on GABA uptake L-trans-PDC dose dependently increased basal and K(+)-evoked extracellular glutamate levels, and did not affect extracellular GABA levels, but increased basal aspartate and taurine levels. The K(+)-evoked release of GABA and glutamate, measured in the presence of both SK & F 89976-A and L-trans-PDC, was Ca(2+)-dependent for about 50% and 65%, respectively. In contrast, the release of the putative amino acid transmitters aspartate and taurine was not Ca(2+)-dependent. These results indicate that (1) in rat hippocampus uptake carriers actively regulate extracellular GABA and glutamate levels, (2) the GABA and glutamate released by K+ was derived from both Ca(2+)-dependent (presumably vesicular) and Ca(2+)-independent (presumably cytosolic) pools, whereas aspartate and taurine release was exclusively from Ca(2+)-independent pools.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

Involvement of synaptosomal neurotransmitter amino acids in audiogenic seizure-susceptibility and-severity of Rb mice

The involvement of synaptosomal neurotransmitter amino-acids in seizure susceptibility and seizure severity was explored. The amino-acid contents of brain synaptosomes were determined in three sublines of Rb mice differing in their response to an acoustic stimulus: Rb1, clonic-tonic seizure-prone, Rb2, clonic seizure-prone, and Rb3, seizure-resistant. Synaptosomes were prepared from 6 brain areas considered to be involved in seizure activity: olfactory bulbs, amygdala, inferior colliculus, hippocampus, cerebellum, pons-medulla. The steady-state levels of GABA and glycine (Gly), inhibitory amino-acids, of taurine (Tau), an inhibitory neurotransmitter of neuromodulator, of aspartate (Asp) and glutamate (Glu), excitatory amino-acids, as well as of serine (Ser) and glutamine (Gln), two precursors of neurotransmitter amino-acids, were determined by HPLC. Low levels of Tau, GABA, and Ser in hippocampus, Gly in amygdala, Glu in hippocampus, inferior colliculus and pons, Gln and Asp in inferior colliculus appeared to correlate with seizure-susceptibility. GABA and Asp in olfactory bulb, Gln in amygdala, hippocampus and pons, ser in olfactory bulb and pons, appeared to be associated either with seizure-severity or -diversity. A strong involvement of hippocampus (Tau, GABA, Ser, Glu, and Gln) and inferior colliculus (Asp, Glu, Gln) in audiogenic seizure-susceptibility, and of olfactory bulb (GABA, Asp) in seizure-severity and/or -diversity is suggested.

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)

GABA and glycine in synaptic vesicles: storage and transport characteristics

gamma-Aminobutyric acid (GABA) and glycine are major inhibitory neurotransmitters that are released from nerve terminals by exocytosis via synaptic vesicles. Here we report that synaptic vesicles immunoisolated from rat cerebral cortex contain high amounts of GABA in addition to glutamate. Synaptic vesicles from the rat medulla oblongata also contain glycine and exhibit a higher GABA and a lower glutamate concentration than cortical vesicles. No other amino acids were detected. In addition, the uptake activities of synaptic vesicles for GABA and glycine were compared. Both were very similar with respect to substrate affinity and specificity, bioenergetic properties, and regional distribution. We conclude that GABA, glycine, and glutamate are the only major amino acid neurotransmitters stored in synaptic vesicles and that GABA and glycine are transported by similar, if not identical, transporters.

Taurine release from brain slices in thioacetamide-induced hepatic encephalopathy in rats

The possible involvement of taurine in the pathogenesis of hepatic encephalopathy was studied in rats injected with thioacetamide. Spontaneous release of exogenous labeled taurine was not affected in any brain area studied, but the potassium-stimulated release was enhanced in the striatum in thioacetamide-treated rats. High concentrations of ammonium ions also evoked greater release of taurine from striatal slices in rats with thioacetamide-induced hepatic encephalopathy.

Alterations in synaptosomal neurotransmitter amino acids in "petit-mal" rats at a daytime and a nighttime

The synaptosomal fractions of 6 brain areas-olfactory tubercles (OT), frontal cortex (FC), striatum (Sr), amygdala (A), thalamus (Th), hypothalamus (Hy) - have been analyzed for their neurotransmitter amino acids (AA) content in Wistar rats exhibiting "petit-mal" epilepsy (PM-E) and in controls (C). The analysis was carried out at 11 p.m. (nighttime corresponding to the acrophase for the hourly number of spike-wave complexes) and at 11 a.m. (daytime). A day versus night rhythmicity is recorded for synaptosomal inhibitory AA in control and in PM-E rats. However, day versus night variations are more frequent and more prominent in C rats than in PM-E rats. Two day versus night variations exist only in PM-E rats: increases of GABA level in Sr and of Asp in Hy. Differences between PME-and C in synaptosomal AA content are more likely to be present during the nighttime. During this period lower AA values for PM-E rats are found for one or several inhibitory AA in OT, Th, and FC. It seems that the differences between PM-E and C concerning the inhibitory AA correlate with the number of spike-wave discharges. Only in one brain area is there a similar difference for PM-E and C during daytime and nighttime: a decreased GABA content for PM-E rats in OT. The decrease is larger in nighttime than in daytime. This difference may serve as a marker for this epileptic disorder. Moreover, it is in OT that the greatest number of PM-E versus C differences in synaptosomal neurotransmitter AA are observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural description of glutamate-, aspartate-, taurine-, and glycine-like immunoreactive terminals from five rat brain regions

The ultrastructural localization of putative excitatory (glutamate, aspartate) and inhibitory (taurine, glycine) amino acid neurotransmitters is described in several selected rat brain regions. In general, axon terminal profiles immunoreactive for excitatory amino acids formed asymmetric synapses with non-immunoreactive small diameter dendritic profiles or dendritic spines. In the cerebellum, both mossy fiber terminals and parallel fiber terminals were immunoreactive for glutamate and aspartate. In the hippocampus, mossy fiber terminals within the stratum lucidum of the CA3 region were immunoreactive for glutamate. Localization of glutamate and aspartate to cerebellar parallel and mossy fibers, as well as the identification of glutamate in hippocampal mossy fibers, is consistent with the excitatory nature of these fibers as described in previous physiological studies. Glutamate-like immunoreactive terminals were also identified in subnucleus caudalis of the spinal trigeminal nucleus and in the dorsal horn of the spinal cord. Immunoreactive axon terminals for two putative inhibitory neurotransmitters, glycine and taurine, displayed a greater number of morphological variations in synaptic structure. In the cerebellum, taurine-like immunoreactivity was present in both basket cell axon terminals which formed symmetric synapses with Purkinje cell neurons, and in a few mossy fiber terminals which formed asymmetric synapses with dendritic spines. In the area dentata of the hippocampus, taurine-like immunoreactive profiles formed asymmetric synapses with dendritic elements. Glycine-like immunoreactive terminals formed symmetric synapses with cell perikarya in both the ventral horn of the spinal cord and in the cochlear nuclei, and on axon terminals in the spinal trigeminal and cochlear nuclei. In contrast, some glycine-like immunoreactive terminals formed asymmetric synapses with distal dendritic profiles in the spinal cord and spinal trigeminal nucleus. The localization of taurine to cerebellar basket cell axons and glycine to axon terminals that synapse on ventral horn motor neuron perikarya is consistent with the hypothesis that these amino acids are functioning as inhibitory neurotransmitters at these synapses. Taurine localization to cerebellar mossy fibers and to fibers in the molecular layer of the dentate gyrus may be more consistent with a proposed neuromodulator role of taurine.

Accumulated glutamate levels in the synaptic vesicle are not maintained in the absence of active transport

We have investigated factors which may affect accumulated glutamate levels in synaptic vesicles and glutamate efflux. Agents which dissipate the electrochemical proton gradient resulted in a rapid reduction of steady-state vesicular glutamate levels, which was prevented by N-ethylmaleimide. Glutamate efflux was found to occur even in the presence of an electrochemical proton gradient, but was effectively inhibited by N-ethylmaleimide. These results suggest that accumulated glutamate levels in synaptic vesicles are not maintained unless glutamate is taken up continuously by an active transport mechanism, and they could provide an explanation for the lack of convincing evidence for the enrichment of endogenous glutamate in isolated synaptic vesicles.

Activation of arginine metabolism to glutamate in rat brain synaptosomes in thioacetamide-induced hepatic encephalopathy: an adaptative response?

Crude (P2) synaptosomes derived from rats with acute hepatic encephalopathy (HE) induced with thioacetamide showed a slightly increased uptake of radiolabeled arginine (ARG) and a 2.5-fold enhanced conversion of newly taken-up ARG to both glutamate (GLU) and gamma-aminobutyric acid (GABA) as compared with control synaptosomes. Pulse treatment of the preloaded synaptosomes with a high potassium medium decreased their radioactive GLU and GABA content without affecting the content of the precursor ARG. This result, which was identical with control or HE preparations, appears to indicate that ARG contributes at least, in part, to the synthesis of neurotransmitter GLU or GABA. As measured in purified synaptosomal preparations, HE increased by about 50% the activities of arginase and ornithine-delta-aminotransferase--the two enzymes of the ARG to GLU shunt. It is postulated that increased conversion of ARG to GLU may compensate for excessive utilization of the latter amino acid as an ammonia trap during HE and, as such, may be considered as an adaptative response of the synaptic compartment to this pathological condition.

Involvement of protein kinase C in the Ca2+-dependent vesicular release of GABA from central and enteric neurons of the guinea pig

The involvement of protein kinase C (PKC) in the release of endogenous gamma-aminobutyric acid (GABA) was studied using slices of deep cerebellar nucleus and strips of small intestine from the guinea pig. 12-O-tetradecanoylphorbol 13-acetate (TPA), but not 4 alpha-phorbol-12,13-didecanoate (4 alpha-PDD), potentiated the high K+-evoked release of GABA from both preparations in the presence of tetrodotoxin. Ouabain evoked the release of GABA from both preparations, and this release was not altered by TPA. Therefore, the activation of protein kinase C potentiates the Ca2+-dependent vesicular release of GABA from nerve terminals of the central and enteric GABAergic neurons of the guinea pig.

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.

Immunocytochemical localization of taurine in the mammalian retina

This paper describes the first demonstration of taurine-like immunoreactivity in the mammalian retina using an antiserum raised in rabbits. In rat, cat and guinea pig retina a peroxidase-antiperoxidase immunocytochemical technique showed high levels of taurine immunostaining in photoreceptor inner segments and synaptic terminals, in subpopulations of amacrine and bipolar somata and their synaptic processes in the inner plexiform layer, including numerous large terminals near and on ganglion cell somata. Using the Protein A-gold technique for ultrastructural studies in the rat, the presence of synaptic ribbons confirmed that some of these taurine-containing terminals were from bipolar cells. Lower levels of immunostaining were seen in the pigment epithelium and distal parts of glial cells.

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.

Quantitative assessment of taurine-like immunoreactivity in different cell types and processes in rat cerebellum: an electronmicroscopic study based on a postembedding immunogold labelling procedure

Ultrathin sections of plastic-embedded rat cerebella were incubated with an antiserum against conjugated taurine and subsequently treated with a secondary antibody coupled to colloidal gold. The density of gold particles in various cellular profiles was calculated with the assistance of a computer. In the cerebellar cortex the highest density was found in the somata, dendrites, and dendritic spines of the Purkinje cells, supporting parallel light-microscopical observations in postembedding stained semithin sections from the same tissue blocks. The remaining profiles could be divided into three groups according to their immunolabelling intensity, in descending order: 1) somata and processes of granule and Golgi cells; 2) somata and processes of stellate, basket, and glial cells, and 3) mossy fiber terminals. In a representative experiment, the structures in the first and second groups showed gold particle densities in the range of 19-25%, and 4-11%, respectively, of that in the Purkinje cell somata (values corrected for background) whereas the particle density in the mossy fiber terminals was not significantly above background level. In the cerebellar nuclei, taurine-like immunoreactivity was concentrated in terminals that typically established symmetric or intermediate type contacts with weakly labelled dendrites and cell bodies. These terminals, which shared the ultrastructural features of Purkinje cell terminals, showed an average gold particle density that was about 60% higher than that of the Purkinje somata. For specificity control, ultrathin sections containing a series of different amino acid conjugates were incubated in the same drops of sera as the tissue sections. The highly selective labelling of the taurine conjugate indicated that the distribution of gold particles in the tissue was not confounded by crossreactivity with GABA, glutamate or other common amino acids but adequately reflected the distribution of fixed taurine. For additional control of specificity, the taurine antiserum was applied to the soluble fraction of a rat brain extract separated by thin layer chromatography. In this system the taurine antiserum stained a single spot that comigrated with free taurine. The present results suggest that all cell types and processes in the rat cerebellum (with the exception of the mossy fiber terminals) contain taurine. However, the concentration of taurine appears to vary considerably among the different cell types and may also differ between different parts of the same neuron.

Uptake of gamma-aminobutyric acid by a synaptic vesicle fraction isolated from rat brain

Gamma-Aminobutyric acid (GABA) was taken up by a MgATP-dependent mechanism into synaptic vesicles isolated by hypoosmotic shock and density gradient centrifugation. The properties of the vesicular uptake differed clearly from those of synaptosomal and glial uptake, both with respect to Na+, Mg2+, and ATP dependence and with respect to response to general GABA uptake inhibitors such as nipecotic acid, diaminobutyric acid, and beta-alanine. The uptake showed a Km of 5.6 mM and a net uptake rate of 1,500 pmol/min/mg of protein. It is suggested that the vesicular uptake of GABA is driven by an electrochemical proton gradient generated by a Mg2+-ATPase.

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.

Biochemical and clinical studies on epileptic patients during two phase I trials with the novel anticonvulsant taltrimide

Taltrimide (2-phthalimidoethanesulphon-N-isopropylamide), a lipophilic derivative of taurine and a potent anticonvulsant in animal studies, was administered in daily doses of 1 and 2 g for 2 weeks with an interval of 2.5 months in 2 phase I clinical trials to 9 drug-resistant epileptic patients. Seizures and EEG were recorded, and routine laboratory studies conducted. Concentrations of antiepileptic drugs in plasma, of amino acids in urine and plasma, and contents of amino acids, homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA) and cyclic nucleotides in the cerebrospinal fluid were determined. Although no clinical or neurophysiological effects were observed, an increase in the cerebrospinal fluid contents of HVA and cyclic nucleotides and changes in the concentrations of antiepileptic drugs and amino acids were found. The concentrations of HVA correlated with those of 5-HIAA and also with those of the main active metabolite of taltrimide. Biochemical changes due to taltrimide treatment resembled only partly those found after taurine treatment.