Release of GABA and taurine from brain slices

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

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

Enhanced GABA release in cell-damaging conditions in the adult and developing mouse hippocampus

The release of [3H]GABA from hippocampal slices from adult (3-month-old) and developing (7-day-old) mice was studied in cell-damaging conditions in vitro using a superfusion system. Cell damage was induced by modified superfusion media, including hypoxia, hypoglycemia, ischemia, the presence of Free radicals and oxidative stress. The basal release of GABA from the immature and mature hippocampus was generally markedly increased in all cell-damaging conditions. In 7-day-old mice the release was enhanced most in the presence of free radicals. 1.0 mM NaCN and ischemia, whereas in the adults 1.0 mM NaCN provoked the largest release of GABA, followed by ischemia and free radical-containing media. Potassium stimulation (50 mM K+) was still able to potentiate the release in all cell-damaging conditions in both age groups. It was shown by superfusing the slices in Ca- and Na-free media that ischemia-induced GABA release was Ca-independent, occurring by a reversed operation of Na-dependent cell membrane carriers in both adult and developing hippocampus. Glutamate and its receptor agonists, N-methyl-D-aspartate (NMDA), kainate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), potentiated GABA release only in the immature hippocampus by a receptor-mediated mechanism. The enhancement by kainate and AMPA receptors also operated under ischemic conditions. The massive amount of GABA released simultaneously with excitatory amino acids in the mature and immature hippocampus may be an important protective mechanism against excitotoxicity, counteracting harmful effects that lead to neuronal death. The GABA release induced by activation of presynaptic glutamate receptors may contribute particularly to the maintenance of homeostasis in the hippocampus upon impending hyperexcitation.

Volume regulation and the efflux of amino acids from cells in incubated slices of rat cerebral cortex. I. Characteristics of transport mechanisms

The characteristics of amino acid efflux from pre-loaded cells in incubated slices of rat cerebral cortex have been investigated under basal conditions (isosmotic media, 315 mosmol/kg) and following mild hyposmotic shock (265 mOsmol/kg). Rates of efflux have been correlated with the extent of cell swelling in hyposmotic media. Hyposmolality accelerated the slow phase of cellular efflux of L-aspartate (+ 29%), gamma-aminoisobutyric acid (GABA) (+ 38%), L-glutamate (+ 28%) and glycine (+ 26%). The anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-sulfonic acid (DIDS, 25 or 100 microM) as well as trifluoperazine (TFP, 25 microM), an inhibitor of calmodulin activation, both retarded efflux in hyposmotic media, with associated cell swelling (increase in slice non-inulin space). The effects of DIDS and TFP were not additive. N-Ethylmaleimide (NEM, 100 microM) significantly retarded the efflux of neutral amino acids, with cell swelling: these effects were less pronounced in cells loaded with acidic amino acids. It is concluded that the hyposmotically-activated efflux of carboxylic amino acids, and associated cell swelling limitation, requires calmodulin activation and the presence of free sulfydryl groups.

Relation of taurine transport and brain edema in rats with simple hyperammonemia or liver failure

Taurine (Tau), an amino acid that abounds in brain, has been implicated in inhibitory neuromodulation and osmoregulation, the latter function being manifested by Tau release along with osmotically obligated water in response to brain tissue edema. A previous study (Hilgier and Olson: J. Neurochem. 62:197-204, 1994) had shown that simple hyperammonemia (HA) induced in rats by daily administration of ammonium acetate resulted in a decrease of both tissue specific gravity indicative of edema and Tau content, in basal ganglia (BG) but not in cerebral cortex (CC). By contrast, rats with hepatic encephalopathy (HE) following administration of a hepatotoxin, thioacetamide, were characterized by CC edema and an increased Tau content in both BG and CC. In the present study, we tested the following parameters that may potentially have affected Tau distribution in the two models: a) spontaneous, and stimulated (hypoosmolarity-induced) release of loaded [3H] Tau in vitro from CC and BG slices; b) blood Tau content; and c) uptake of [14C] Tau in vivo from blood to brain corrected for [3H] water passage-the so-called brain uptake index (BUI). The two edema-affected structures: BG in the HA model and CC in the HE model, showed increased spontaneous Tau release. Edema-associated spontaneous release of Tau may favor inhibitory neurotransmission contributing to the pathomechanism of HA or HE. Stimulated release, reflecting the ability of the tissue to reduce water content, was decreased in the BG from HA rats, in agreement with the postulated role of Tau in osmoregulation. Stimulated release was unchanged in CC of HE rats. Neither spontaneous nor stimulated release of Tau were affected in CC of HA rats or in BG of HE rats. HE, but not HA, was associated with elevated blood content and increased BUI for TAU, which in combination, contributed to the increase of Tau content in CC. The latter phenomenon adds to the list of metabolic changes distinguishing simple HA from toxic liver damage, reemphasizing the crucial role of factors other than ammonia in the pathomechanism of HE.

Characteristics of nitric oxide-evoked [3H]taurine release from cerebral cortical neurons

Pharmacological characteristics of [3H]taurine release evoked by nitric oxide (NO) were investigated using mouse cerebral cortical neurons in primary culture. NO generators such as S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP) dose-dependently increased [3H]taurine release from neurons. Such stimulatory effects of NO generators were completely abolished by hemoglobin, a NO radical scavenger, indicating that these [3H]taurine releases might be due to NO liberated from SNAP and SNP. Sodium withdrawal from incubation buffer significantly inhibited the SNAP- and SNP-induced [3H]taurine releases, whereas the removal of calcium showed no alterations in the [3H]taurine release evoked by NO generators. Beta-Alanine and guanidinoethane sulfonate, inhibitors of carrier-mediated taurine transport system, inhibited the SNAP- and SNP-evoked releases of [3H]taurine in a dose-dependent manner. These results indicate that the NO-evoked [3H]taurine release from cerebral cortical neurons is mediated by the reverse process of sodium-dependent carrier-mediated taurine transport system.

Kinetic analysis of taurine influx into cerebral cortical slices from adult and developing mice in different incubation conditions

The influx of taurine into cerebral cortical slices was studied with 3-day-old and 3-month-old mice in different ionic environments in incubation medium. In standard Krebs-Ringer medium the influx comprised two saturable uptake components, high- and low-affinity, and non-saturable penetration. In isoosmotic medium potassium stimulation abolished the high-affinity uptake in both age groups. In hyperosmotic medium the high-affinity uptake disappeared totally in 3-day-old mice and partially in 3-month-old mice. The high-affinity uptake was also obliterated in hypoosmotic medium and in the absence of chloride ions in both age groups. The low-affinity uptake was abolished by potassium stimulation in 3-month-olds and strongly inhibited in 3-day-olds. Hypoosmotic and chloride-free media also inhibited the low-affinity uptake at both ages. Non-saturable influx was greatly diminished in chloride-free media. The taurine uptake systems are thus strongly inhibited in incubation conditions which simultaneously evoke apparent release of taurine from cerebral cortical slices. This inhibition contributes to the magnitude of the estimated release, which in vitro represents overflow of released taurine molecules which escape recapture by the membrane carriers. In vivo the same mechanism may underlie the delayed and spreading neuromodulatory actions of taurine.

Nitric oxide-evoked [3H]taurine release is mediated by reversal of the Na(+)-dependent carrier-mediated taurine transport system

The pharmacological characteristics of [3H]taurine release evoked by nitric oxide (NO) were investigated using mouse cerebral cortical neurons in primary culture. N-Methyl-D-aspartate (NMDA) and S-nitroso-N-acetylpenicillamine (SNAP) dose-dependently increased [3H]taurine release from neurons. The NMDA-evoked release of [3H]taurine was reduced to the basal level by N omega-nitro-L-arginine, a NO synthase inhibitor, and MK-801, a noncompetitive antagonist for NMDA receptors. The NMDA- and SNAP-evoked releases of [3H]taurine were completely abolished by hemoglobin, indicating that these [3H]taurine releases were evoked by NO produced by NMDA receptor activation and liberated from SNAP. Withdrawal of Na+ from incubation buffer significantly inhibited the NMDA- and SNAP-induced [3H]taurine releases, whereas removal of Ca2+ produced no alteration in the SNAP-evoked [3H]taurine release. In addition, beta-alanine and guanidinoethane sulfonate, antitransporters of the carrier-mediated taurine transport system, reduced the NMDA- and SNAP-evoked releases of [3H]taurine in a dose-dependent manner. These results indicate that the NO-evoked [3H]taurine release from cerebral cortical neurons is mediated by a reversal of the Na(+)-dependent carrier-mediated taurine transport system.

Endogenous excitatory amino acid release from brain slices and astrocyte cultures evoked by trimethyltin and other neurotoxic agents

Trimethyltin (TMT) is a toxic alkyltin compound that is known to produce neuronal necrosis in the CNS. The present study examined the effects of TMT on the release of excitatory amino acids (EAA) from cortical slices prepared from adult and aged (24 months old) rats. The calcium dependence of TMT-induced EAA efflux was evaluated and compared to other neurotoxic agents. The actions of TMT were also evaluated in an astrocyte culture model to assess glial contributions to TMT-induced EAA efflux. TMT (10-1000 microM) evoked a dose-related increase in GLU and ASP efflux during a 30 min incubation period and this efflux was sustained or slightly higher during a 15 min recovery period. TMT-stimulated GLU efflux was not altered in aged rats. TMT-induced GLU efflux was significantly reduced by removing extracellular calcium and including 10 microM EGTA in the incubation media. Calcium channel blockers (nifedipine, verapamil, flunarizine, amiloride, neomycin) and MK-801 did not significantly attenuate TMT-induced GLU efflux. Diltiazem (25 microM) produced modest but inconsistent reductions in TMT-induced GLU efflux from brain slices, and significantly inhibited the leakage of lactate dehydrogenase (LDH) from TMT-treated astrocyte cultures. TMT did not increase GLU efflux from glial cultures during a 30 min incubation period, but did significantly elevate GLU efflux during the 15 min recovery period. TMT evoked the release of EAA by both calcium dependent and independent mechanisms in brain slices. TMT at high concentrations also produced a delayed increase in glial GLU efflux. These studies suggest that excitotoxic mechanisms may contribute to TMT-induced neurotoxicity.

Taurine efflux and cell volume regulation in cerebral cortical slices during chronic hypernatraemia

Efflux of cellular taurine from pre-loaded cerebral cortical slices incubated in hypo- and hyperosmotic media has been studied in normal and chronically hypernatraemic rats. Significant differences in transport mechanisms between the two groups has been noted. Hyperosmotic media retard efflux in cells from normal animals, with associated cell shrinkage, but accelerate efflux in cells from hypernatraemic rats, in which cell volumes are well maintained at pre-hypernatraemic levels. In hypernatraemic rats an anionic component of taurine efflux, present in normal animals, is lacking. Conversely, a distinct, calmodulin-dependent component which in normal rats is stimulated only in hypo-osmotic media, is present in both hypo- and hyperosmotically incubated slices from hypernatraemic rats, and inhibition of calmodulin-activation leads to cell swelling. This altered pattern of efflux and cell volume-regulation persists for at least 5 h following recovery from hypernatraemia, but remits by 30 h, indicating slow down-regulation of the hypernatraemically activated calmodulin-dependent efflux pathway and re-expression of anionic taurine transport.

Characterisation of extracellular amino acids in striatum of freely moving rats by in vivo microdialysis

To investigate the characteristics of extracellular amino acids released from the striatum, we performed in vivo microdialysis in non-anaesthetised, freely moving rats. Amino acids were determined after precolumn derivatisation with o-phthalaldehyde by high-performance liquid chromatography and fluorescence detection. The omission of Ca2+ in the perfusion medium partially decreased the basal concentration of aspartate and glutamate. This shows that a small fraction of basal concentration of aspartate and glutamate is of neuronal origin. The effect of high K+ and veratrine stimulation was evaluated in the presence or absence of Ca2+ or tetrodotoxin (1 microM). High K+ and veratrine caused a remarkable increase in the aspartate and glutamate efflux. The omission of Ca2+ only partially decreased K(+)-stimulated aspartate and glutamate efflux. Tetrodotoxin completely antagonised veratrine-stimulated aspartate and glutamate efflux. Although glycine and taurine releases were stimulated by high K+ and veratrine, their release was not always antagonised with Ca2+ omission or tetrodotoxin inclusion. Thus, the neuronal origin of stimulated release of glycine and taurine is unclear. Although tetrodotoxin sensitivity and Ca2(+)-dependency are regarded as a basic criterion for classical neurotransmitters in microdialysis experiments, they should not be adapted to the physiological characteristics of the release of amino acids.

Ammonia stimulates the release of taurine from cultured astrocytes

The effect of ammonia on the release of the neuroactive amino acids taurine (TAU), gamma-aminobutyric acid (GABA) and D-aspartate (D-ASP), an analog of L-glutamate (L-GLU), from cultured rat cortical astrocytes was studied. NH4Cl (1 and 5 mM) induced the release of TAU. TAU release was reduced when Na+ was removed, and was almost completely abolished when Cl- was omitted. In contrast, TAU basal release was enhanced upon removal of Na+ or Cl-. Ammonia inhibited the release of GABA and D-ASP. Ammonia-induced release of astroglial TAU may modify the neuronal excitability accompanying hyperammonemic conditions.

Glutamate agonists and [3H]GABA release from rat hippocampal slices: involvement of metabotropic glutamate receptors in the quisqualate-evoked release

The effects of glutamate agonists and their selective antagonists on the Ca(2+)-dependent and independent releases of [3H]GABA from rat coronal hippocampal slices were studied in a superfusion system. The Ca(2+)-dependent release evoked by glutamate, kainate and N-methyl-D-aspartate (NMDA) gradually declined with time despite the continuous presence of the agonists. Quisqualate (QA) caused a sustained release which exhibited no tendency to decline within the 20-min period of stimulation. This release was enhanced in Ca(2+)-free medium. The release evoked by QA in Ca(2+)-containing medium was significantly inhibited by (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohept-5,10-imine hydrogen maleate (MK-801) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), showing that QA activates NMDA receptors directly or indirectly through (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. The inhibition of MK-801 was slightly diminished and that of CNQX totally abolished in Ca(2+)-free medium. Verapamil inhibited the QA-activated release in both Ca(2+)-containing and Ca(2+)-free media. The effect of QA but not that of AMPA was blocked in Ca(2+)-free medium by L(+)-2-amino-3-phosphonopropionate (L-AP3), a selective antagonist of the metabotropic glutamate receptor. It is suggested that the sustained release of GABA is also mediated partly by activation of metabotropic receptors and mobilization of Ca2+ form intracellular stores.

Regulation of D-aspartate release by glutamate and GABA receptors in cerebral cortical slices from developing and ageing mice

The basal release of D-[3H]aspartate, an unmetabolized analogue of glutamate, from cerebral cortical slices remained at the same level from three-day-old to 24-month-old mice, but the response to K+ stimulation (50 mM) was smaller in young than in adult or aged mice. Kainate, N-methyl-D-aspartate and quisqualate (0.1 mM) stimulated the basal release of D-aspartate in the cerebral cortex of seven-day-old mice, the effects of kainate and N-methyl-D-aspartate being reduced by their antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and dizocilpine maleate, respectively, indicating that in the immature cerebral cortex the kainate and N-methyl-D-aspartate types of the glutamate receptor are involved in the basal release. The K(+)-stimulated release was not affected by glutamate agonists in developing mice, though they markedly attenuated the evoked release in adults. The inhibitory amino acids GABA, taurine and glycine depressed the K(+)-stimulated release only in the adult cerebral cortex. The action of GABA was abolished by bicuculline, demonstrating the involvement of presynaptic GABAA receptors. The glycine effect was strychnine-insensitive, characteristic of the glycine modulatory site in the N-methyl-D-aspartate receptor. This kind of regulation by both kainate and N-methyl-D-aspartate receptors could be of physiological significance, particularly in the immature cerebral cortex.

Net taurine transport and its inhibition by a taurine antagonist

P2-fractions were isolated from rat brain, and used to study net taurine transport. The fractions were incubated in increasing concentrations of [3H]taurine and the intraterminal concentration measured by liquid scintillation and amino acid analysis. The membrane potential of the isolated fractions was estimated using 86Rb+ as a marker for intracellular K+. Taurine was synthesized in the P2-fraction when incubated in taurine free medium. At external taurine concentrations below 370 microM a significant amount of the endogenous taurine was released to the incubation medium. Net taurine uptake into the P2-fraction was achieved at external taurine concentrations exceeding 370 microM. The taurine antagonist 6-aminomethyl-3-methyl-4H, 1, 2, 4-benzothiadiazine-1, 1-dioxide (TAG) competitively inhibited taurine and [3H]taurine transport into the P2-fraction. As the external concentration of taurine was increased, the accumulation of 86Rb+ into the P2-fraction was facilitated. This indicated an increasing hyperpolarization of the neuronal membrane as taurine transport shifted from release towards uptake. TAG reduced the hyperpolarization that paralleled taurine accumulation, in a dose dependent manner. Our results indicate that relatively low transmembranal gradients of taurine may be maintained by an electrogenic taurine transporter having a large transport capacity. Such a transporter may well serve the needs of osmotic regulation, i.e. to transport large amounts of taurine in any direction across the neuronal membrane.

The effect of a vitamin E-deficient diet on amino acid levels in the substantia nigra, striatum and hippocampus of rats

The effects of a vitamin E deficiency diet for 15 days on amino acid concentrations have been studied in the substantia nigra, striatum and hippocampus of the rat. The substantia nigra showed an increase in glutamate and GABA and a decrease of tryptophan concentration compared with controls. In the striatum, aspartate and glycine decreased, no changes were found in the amino acid concentrations in the hippocampus. The substantia nigra and striatum showed opposite results-an increase and decrease of amino acids respectively. The increase of glutamate found in substantia nigra is particularly interesting as it may suggest possible links to degenerative processes. These results suggest that vitamin E could play a crucial role in substantia nigra degeneration and that the substantia nigra could be more sensitive to an oxidative stress than other brain structures.

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.

Na+ influx through Ca2+ channels can promote striatal GABA efflux in Ca(2+)-deficient conditions in response to electrical field depolarization

Electrical field depolarization releases gamma-aminobutyric acid (GABA) in rat striatal slices in the absence of external Ca2+. omega-Conotoxin GVIA (omega-CgTx; 1-50 nM), a neuronal Ca2+ channel blocker, inhibits electrically evoked efflux of newly taken up [3H]GABA in a concentration-dependent manner in either normal or Ca(2+)-free medium. This suggests that ion influx occurs through Ca2+ channels in the absence of external Ca2+ and contributes to the efflux of GABA. Reducing external Na+ concentration to 27.25 mM (low [Na+]o medium) by equimolarly substituting choline chloride for sodium chloride has differential effects on electrically evoked GABA efflux depending on the external Ca2+ concentrations. In normal Ca2+ medium, electrically evoked GABA efflux increases whereas, in Ca(2+)-free medium, it is greatly inhibited when [Na+]o is reduced to 27.25 mM. In low [Na+]o medium, GABA efflux is largely tetrodotoxin (TTX)-sensitive, however, spike firing evoked by antidromic stimulation of striatal cells is inhibited. In Na(+)-free medium, resting GABA efflux increases 17-fold whereas evoked GABA efflux diminishes. In Ca(2+)-free medium, 70 min of incubation with 1-2-bis-(1-aminophenoxy)ethane-N,N,N',N' tetraacetoxy methyl ester (BATPA-AM, 1 microM), an intracellular calcium chelator, increases both resting GABA efflux and electrically evoked GABA overflow by approximately 100%. These results suggest that: (1) in Ca(2+)-free conditions, Na+ permeability of cells increases via Ca2+ channels and this profoundly affects GABA efflux. (2) Electrical field depolarization is likely to release GABA by directly depolarizing axon terminals. (3) Ca(2+)-independent GABA efflux is not promoted by an increase in intracellular free Ca2+ concentration via Na+/Ca2+ exchange processes from internal pools.

Taurine release in the hypothalamus is altered by blood pressure changes and neuroactive drugs

The release of endogenous taurine was determined in the posterior hypothalamus of the conscious, freely moving rat by using the push-pull superfusion technique. At the start of superfusion, the outflow of endogenous taurine declined rapidly with a half-time of 7.9 min and reached a steady state after approximately 1 h. Thereafter, the release rate was constant and amounted to 2.6 +/- 0.3 pmol/min. During depolarization either with K+ (50 or 90 mM) or veratridine (1 or 10 microM), taurine outflow was increased in a concentration-dependent way. Hypothalamic superfusion with tetrodotoxin (1 microM) elicited a sustained decrease in the taurine release to 60% of the control values. Intravenous infusion of noradrenaline led to a rise in blood pressure (45 mm Hg) and enhanced the release of taurine in the hypothalamus. A fall of blood pressure (30 mm Hg) caused by an intravenous infusion of nitroprusside diminished taurine outflow. The results suggest that a considerable amount of the taurine detected is released from hypothalamic neurons. Changes in the release rate of taurine by experimentally induced alterations of blood pressure indicate that, in the posterior hypothalamus, the amino acid might play an important role as a neurotransmitter or neuromodulator possessing a hypotensive function.

Spontaneous and evoked release of [3H]taurine from a P2 subcellular fraction of the rat retina

The effects of spontaneous and evoked [3H]taurine release from a P2 fraction prepared from rat retinas were studied. The P2 fraction was preloaded with [3H]taurine under conditions of high-affinity uptake and then examined for [3H]taurine efflux utilizing superfusion techniques. Exposure of the P2 fraction to high K+ (56 mM) evoked a Ca(2+)-independent release of [3H]taurine. Li+ (56 mM) and veratridine (100 microM) had significantly less effect (8-15% and 15-30%, respectively) on releasing [3H]taurine compared to the K(+)-evoked release. 4-Aminopyridine (1 mM) had no effect on the release of [3H]taurine. The spontaneous release of [3H]taurine was also Ca(2+)-independent. When Na+ was omitted from the incubation medium K(+)-evoked [3H]taurine release was inhibited by approximately 40% at the first 5 minute depolarization period but was not affected at a second subsequent 5 minute depolarization period. The spontaneous release of [3H]taurine was inhibited by 60% in the absence of Na+. Substitution of Br- for Cl- had no effect on the release of either spontaneous or K(+)-evoked [3H]taurine release. However, substitution of the Cl- with acetate, isethionate, or gluconate decreased K(+)-evoked [3H]taurine release. Addition of taurine to the superfusion medium (homoexchange) resulted in no significant increase in [3H]taurine efflux. The taurine-transport inhibitor guanidinoethanesulfonic acid increased the spontaneous release of [3H]taurine by approximately 40%. These results suggest that the taurine release of [3H]taurine is not simply a reversal of the carrier-mediated uptake system. It also appears that taurine is not released from vesicles within the synaptosomes but does not rule out the possibility that taurine is a neurotransmitter. The data involving chloride substitution with permeant and impermeant anions support the concept that the major portion of [3H]taurine release is due to an osmoregulatory action of taurine while depolarization accounts for only a small portion of [3H]taurine release.