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

Attenuation of initial pilocarpine-induced electrographic seizures by methionine sulfoximine pretreatment tightly correlates with the reduction of extracellular taurine in the hippocampus

OBJECTIVE

Initiation and development of early seizures by chemical stimuli is associated with brain cell swelling resulting in edema of seizure-vulnerable brain regions. We previously reported that pretreatment with a nonconvulsive dose of glutamine (Gln) synthetase inhibitor methionine sulfoximine (MSO) mitigates the intensity of initial pilocarpine (Pilo)-induced seizures in juvenile rats. We hypothesized that MSO exerts its protective effect by preventing the seizure-initiating and seizure-propagating increase of cell volume. Taurine (Tau) is an osmosensitive amino acid, whose release reflects increased cell volume. Therefore, we tested whether the poststimulus rise of amplitude of Pilo-induced electrographic seizures and their attenuation by MSO are correlated with the release of Tau from seizure-affected hippocampus.

METHODS

Lithium-pretreated animals were administered MSO (75 mg/kg ip) 2.5 h before the induction of convulsions by Pilo (40 mg/kg ip). Electroencephalographic (EEG) power was analyzed during 60 min post-Pilo, at 5-min intervals. Extracellular accumulation of Tau (eTau) served as a marker of cell swelling. eTau, extracellular Gln (eGln), and extracellular glutamate (eGlu) were assayed in the microdialysates of the ventral hippocampal CA1 region collected at 15-min intervals during the whole 3.5-h observation period.

RESULTS

The first EEG signal became apparent at ~10 min post-Pilo. The EEG amplitude across most frequency bands peaked at ~40 min post-Pilo, and showed strong (r ~ .72-.96) temporal correlation with eTau, but no correlation with eGln or eGlu. MSO pretreatment delayed the first EEG signal in Pilo-treated rats by ~10 min, and depressed the EEG amplitude across most frequency bands, to values that remained strongly correlated with eTau (r > .92) and moderately correlated (r ~ -.59) with eGln, but not with eGlu.

SIGNIFICANCE

Strong correlation between attenuation of Pilo-induced seizures and Tau release indicates that the beneficial effect of MSO is due to the prevention of cell volume increase concurrent with the onset of seizures.

Significance of Taurine in the Brain

Two main functions of taurine in the brain are here discussed: the role of taurine in cell volume regulation and the neuromodulatory actions of taurine liberated by depolarization. Taurine takes part in cell volume regulation with other small-molecular compounds. Extracellular taurine inhibits neuronal firing through GABA and glycine receptors. However, the existence of specific taurine receptors is still not excluded.

Roles of taurine-mediated tonic GABAA receptor activation in the radial migration of neurons in the fetal mouse cerebral cortex

γ-Aminobutyric acid (GABA) depolarizes embryonic cerebrocortical neurons and continuous activation of the GABA_A receptor (GABA_AR) contributes to their tonic depolarization. Although multiple reports have demonstrated a role of GABA_AR activation in neocortical development, including in migration, most of these studies have used pharmacological blockers. Herein, we performed in utero electroporation in GABA synthesis-lacking homozygous GAD67-GFP knock-in mice (GAD67^GFP/GFP) to label neurons born in the ventricular zone. Three days after electroporation, there were no differences in the distribution of labeled cells between the genotypes. The dose–response properties of labeled cells to GABA were equivalent among genotypes. However, continuous blockade of GABA_AR with the GABA_AR antagonist SR95531 accelerated radial migration. This effect of GABA_AR blockade in GAD67^GFP/GFP mice suggested a role for alternative endogenous GABA_AR agonists. Thus, we tested the role of taurine, which is derived from maternal blood but is abundant in the fetal brain. The taurine-evoked currents in labeled cells were mediated by GABA_AR. Taurine uptake was blocked by a taurine transporter inhibitor, 2-(guanidino)ethanesulfonic acid (GES), and taurine release was blocked by a volume-sensitive anion channel blocker, 4-(2-butyl-6,7-dichlor-2-cyclopentylindan-1-on-5-yl) oxobutyric acid, as examined through high-performance liquid chromatography. GES increased the extracellular taurine concentration and induced an inward shift of the holding current, which was reversed by SR95531. In a taurine-deficient mouse model, the GABA_AR-mediated tonic currents were greatly reduced, and radial migration was accelerated. As the tonic currents were equivalent among the genotypes of GAD67-GFP knock-in mice, taurine, rather than GABA, might play a major role as an endogenous agonist of embryonic tonic GABA_AR conductance, regulating the radial migration of neurons in the developing neocortex.

Ischemia induces release of endogenous amino acids from the cerebral cortex and cerebellum of developing and adult mice

Ischemia enhanced release of endogenous neuroactive amino acids from cerebellar and cerebral cortical slices. More glutamate was released in adult than developing mice. Taurine release enhanced by K⁺ stimulation and ischemia was more than one magnitude greater than that of GABA or glutamate in the developing cerebral cortex and cerebellum, while in adults the releases were almost comparable. Aspartate release was prominently enhanced by both ischemia and K⁺ stimulation in the adult cerebral cortex. In the cerebellum K⁺ stimulation and ischemia evoked almost 10-fold greater GABA release in 3-month olds than in 7-day olds. The release of taurine increased severalfold in the cerebellum of 7-day-old mice in high-K⁺ media, whereas the K⁺-evoked effect was rather small in adults. In 3-month-old mice no effects of K⁺ stimulation or ischemia were seen in the release of aspartate, glycine, glutamine, alanine, serine, or threonine. The releases from the cerebral cortex and cerebellum were markedly different and also differed between developing and adult mice. In developing mice only the release of inhibitory taurine may be large enough to counteract the harmful effects of excitatory amino acids in ischemia in both cerebral cortex and cerebellum, in particular since at that age the release of glutamate and aspartate cannot be described as massive.

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.

Histotypic mouse parietal cortex cultures: Excitation/inhibition ratio and ultrastructural analysis

Primary cultures of mouse parietal cortex, prepared between postnatal day 3 (P3) and P9, were studied using transmission electron microscopy and HPLC of excitatory (aspartate and glutamate) and inhibitory neurotransmitters (glycine, GABA and taurine) to determine their morphological and functional development. Relations between excitation and inhibition (E/I) were contrasted with ultrastructural features over the time course of in vitro development. After 6 days in vitro, cultured parietal cortex neurons prepared from mice at P3 had immature morphological characteristics, whereas P5 cultures showed a more developed histological structure but still with scarce synapses. The acquirement of histotypic characteristics was seen in P7 cultures, which contained numerous symmetric and asymmetric synaptic contacts. On P9, the cultures showed signs of tissue damage. In terms of neurotransmitter levels and E/I ratios, P7 cultures had relatively low E/I ratio as compared with the rest of the cultures prepared before or after P7. These results demonstrated that the development of inhibitory synaptic transmission, as indicated in the fall of E/I ratio, marked the maturation of cerebral cortical tissue and that the critical period to obtain histotypic cultures of mouse parietal cerebral cortex coincides between P5 and P7. This work provides useful information regarding the balance between excitation and inhibition as an indicative parameter for in vitro nerve cell survival, differentiation and maturation and reinforces the great value of histotypic cultures in the study of central nervous system development.

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.

Interstitial concentrations of amino acids in the rat striatum during global forebrain ischemia and potassium-evoked spreading depression

The early detection and appropriate treatment of brain ischemia is of paramount importance. The interstitial concentrations of neurotransmitter amino acids are often used as an index of neuronal injury. However, monitoring of non-neurotransmitter amino acids may be equally important. We have studied the behavior of 10 amino acids during K(+)-induced spreading depression (application of 70 mM KCl during 40 min) and global forebrain ischemia (two-vessel occlusion with hypotension during 20 min). The concentrations of glutamate, aspartate, taurine, GABA, glycine, and alanine, measured in the rat striatum by microdialysis, increased during both ischemia and spreading depression, whereas glutamine concentrations decreased in both cases. Only ischemia, but not spreading depression, led to enhanced release of serine, threonine, and asparagine. We thus conclude that an elevation in the interstitial concentrations of non-neurotransmitter amino acids is specific to deep ischemic injury to nervous tissue. We propose the monitoring of serine, asparagine, and threonine, together with excitatory amino acids, as an index of the degree of ischemic brain injury.

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.

Taurine enhances volume regulation in hippocampal slices swollen osmotically

Cell volume regulation has been studied in neuronal and glial cultures but little is known about volume regulation in brain tissue with an intact extracellular space. We investigated volume regulation in hippocampal slices maintained in an interface chamber and exposed to hypo-osmotic medium. Relative changes in intracellular and extracellular volume were measured respectively as changes in light transmittance and extracellular resistance. Slices exposed to hypo-osmotic medium (200-240 mOsm/L) showed a decrease in light transmittance, which occasionally was preceded by a brief transient increase. However, hypo-osmotic exposure was always accompanied by a monotonic increase in extracellular resistance. Peak changes in light transmittance and extracellular resistance occurred at 15-20 min following exposure to hypo-osmotic medium. Optical evidence of volume regulation (RVD) was observed in six of 12 slices and occurred over the next 60-90 min. We hypothesized that the relatively low incidence of RVD was related to depletion of taurine, an osmolyte known to play an important role in volume regulation, during preparation of the slices. Indeed, taurine levels in freshly prepared slices were <50% of those reported in intact hippocampus. Incubation of slices in 1 mM taurine restored taurine to levels observed in situ and increased both the likelihood and magnitude of RVD in hypo-osmotic medium. Inhibition of taurine flux with 100 microM 5-nitro-2-(3 phenylpropylamino) benzoic acid blocked both RVD and the transient undershoot of volume commonly associated with return of swollen slices to iso-osmotic medium. Taurine treatment had no effect on levels of several other amino acids but preserved slice potassium content. The results indicate a critical role for cellular taurine during hypo-osmotic volume regulation in hippocampal slices. Inconsistencies between optical measurements of cellular volume changes and electrical measurements of extracellular space are likely to result from the complex nature of light transmittance in the interface slice preparation.

The possible role of taurine and GABA as endogenous cryogens in the rabbit: changes in CSF levels in heat-stress

We investigated whether heat-stress induced hyperthermia could enhance release of both endogenous taurine and GABA from nerve cells into the extracellular compartment, thus acting like endogenous cryogens. Conscious rabbits were exposed for 1 hr to 40 degrees C (heat stress) while cerebrospinal fluid (CSF) and plasma osmolality and the CSF concentrations of some cations, proteins as well as those of taurine and GABA were determined. Heat stress-induced hyperthermia was accompanied by a significant rise in CSF and plasma osmolality, CSF calcium, taurine and GABA levels. It is suggested that during heat stress taurine and GABA are released in the extracellular space of brain tissues in higher amounts, as compared to control conditions, to counteract the resulting hyperthermia, thus acting as cryogenic agents.

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

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

Effects of ammonia in vitro on endogenous taurine efflux and cell volume in rat cerebrocortical minislices: influence of inhibitors of volume-sensitive amino acid transport

Rat cerebrocortical minislices were incubated with physiological saline in the absence or presence of 5 mM ammonium acetate ("ammonia") and/or inhibitors of osmosensitive amino acid transport: 50 microM niflumic acid and 100 microM N-ethyl-maleimide for 60 min, with medium changes after 20 min and 40 min. The efflux of endogenous taurine, glutamate and glutamine was assayed by high-performance liquid chromatography, and steady-state cell volumes were monitored in the slices with the [14C]inulin method. In the absence of ammonia, niflumic acid abolished taurine efflux but did not affect glutamate or glutamine efflux at all time-points, and increased cell volume at 20 min and 60 min. N-Ethyl-maleimide increased taurine, glutamine and glutamate efflux at 20 min and 40 min, inhibited taurine and glutamine efflux at 60 min, and increased cell volume at 20 min. Ammonia strongly stimulated taurine (by 380% at 20 min), and only moderately glutamate (30% at 20 min) or glutamine efflux (76% at 20 min). Ammonia increased cell volume above the control level at all time-points. Niflumic acid inhibited, but did not abolish ammonia-dependent taurine and glutamine efflux, and did not change glutamate efflux. The effects of ammonia + niflumic acid on cell volume did not differ from the effects of each compound separately. N-Ethyl-maleimide inhibited ammonia-dependent efflux of all three amino acids except for stimulation of glutamate efflux at 20 min. N-Ethyl-maleimide + ammonia decreased the cell volumes more than did each compound separately. It is concluded that although ammonia-induced taurine efflux is accompanied by an increase in cell volume, the underlying mechanism is not simply a cell volume regulatory response normally observed in hypoosmotic stress. Increased efflux of taurine, which is an inhibitory amino acid and a cell membrane protectant, may serve to counteract the deleterious effects of increased excitatory transmission accompanying acute hyperammonemic insult.

Nonsynaptic glycine receptor activation during early neocortical development

Glycine receptors (GlyRs) contribute to fast inhibitory synaptic transmission in the brain stem and spinal cord. GlyR subunits are expressed in the developing neocortex, but a neurotransmitter system involving cortical GlyRs has yet to be demonstrated. Here, we show that GlyRs in immature neocortex are excitatory and activated by a nonsynaptically released endogenous ligand. Of the potential ligands for cortical GlyRs, taurine is by far the most abundant in the developing neocortex. We found that taurine is stored in immature cortical neurons and that manipulations known to elevate extracellular taurine cause GlyR activation. These data indicate that nonsynaptically released taurine activates GlyRs during neocortical development. As fetal taurine deprivation can cause cortical dysgenesis, it is possible that taurine influences neocortical development by activating GlyRs.

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.

Taurine as osmoregulator and neuromodulator in the brain

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

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.

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.

Impaired cell volume regulation in taurine deficient cultured astrocytes

Taurine concentration was reduced by 40 and 65%, respectively in rat cerebellar astrocytes grown in a chemically defined medium or in culture medium containing a blocker of taurine transport (GES). Cell volume in these taurine deficient cells was 10%-16% higher than in controls. When challenged by hyposmotic conditions, astrocytes release taurine and this efflux contributes to the volume regulatory decrease observed in these cells. Taurine deficient astrocytes showed a less efficient volume recovery as compared to controls with normal taurine levels. Exposed to 50% hyposmotic medium, astrocytes with normal taurine concentration recovered 60% of their original volume whereas taurine deficient cells recovered only 30-35%. Similarly, in 30% hyposmotic medium, taurine deficient astrocytes recovered only 40% as compared to 75% in controls. No compensatory increases in the efflux of other osmolytes (free amino acids or potassium) were observed during regulatory volume decrease in taurine deficient astrocytes.

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.