Efflux of osmolyte amino acids during isovolumic regulation in hippocampal slices

Hippocampal and Cortical Pyramidal Neurons Swell in Parallel with Astrocytes during Acute Hypoosmolar Stress

Normal nervous system function is critically dependent on the balance of water and ions in the extracellular space (ECS). Pathological reduction in brain interstitial osmolarity results in osmotically-driven flux of water into cells, causing cellular edema which reduces the ECS and increases neuronal excitability and risk of seizures. Astrocytes are widely considered to be particularly susceptible to cellular edema due to selective expression of the water channel aquaporin-4 (AQP4). The apparent resistance of pyramidal neurons to osmotic swelling has been attributed to lack of functional water channels. In this study we report rapid volume changes in CA1 pyramidal cells in hypoosmolar ACSF (hACSF) that are equivalent to volume changes in astrocytes across a variety of conditions. Astrocyte and neuronal swelling was significant within 1 min of exposure to 17 or 40% hACSF, was rapidly reversible upon return to normosmolar ACSF, and repeatable upon re-exposure to hACSF. Neuronal swelling was not an artifact of patch clamp, occurred deep in tissue, was similar at physiological vs. room temperature, and occurred in both juvenile and adult hippocampal slices. Neuronal swelling was neither inhibited by TTX, nor by antagonists of NMDA or AMPA receptors, suggesting that it was not occurring as a result of excitotoxicity. Surprisingly, genetic deletion of AQP4 did not inhibit, but rather augmented, astrocyte swelling in severe hypoosmolar conditions. Taken together, our results indicate that neurons are not osmoresistant as previously reported, and that osmotic swelling is driven by an AQP4-independent mechanism.

Turning down the volume: Astrocyte volume change in the generation and termination of epileptic seizures

Approximately 1% of the global population suffers from epilepsy, a class of disorders characterized by recurrent and unpredictable seizures. Of these cases roughly one-third are refractory to current antiepileptic drugs, which typically target neuronal excitability directly. The events leading to seizure generation and epileptogenesis remain largely unknown, hindering development of new treatments. Some recent experimental models of epilepsy have provided compelling evidence that glial cells, especially astrocytes, could be central to seizure development. One of the proposed mechanisms for astrocyte involvement in seizures is astrocyte swelling, which may promote pathological neuronal firing and synchrony through reduction of the extracellular space and elevated glutamate concentrations. In this review, we discuss the common conditions under which astrocytes swell, the resultant effects on neural excitability, and how seizure development may ultimately be influenced by these effects.

Imaging Taurine in the Central Nervous System Using Chemically Specific X-ray Fluorescence Imaging at the Sulfur K-Edge

A method to image taurine distributions within the central nervous system and other organs has long been sought. Since taurine is small and mobile, it cannot be chemically "tagged" and imaged using conventional immuno-histochemistry methods. Combining numerous indirect measurements, taurine is known to play critical roles in brain function during health and disease and is proposed to act as a neuro-osmolyte, neuro-modulator, and possibly a neuro-transmitter. Elucidation of taurine's neurochemical roles and importance would be substantially enhanced by a direct method to visualize alterations, due to physiological and pathological events in the brain, in the local concentration of taurine at or near cellular spatial resolution in vivo or in situ in tissue sections. We thus have developed chemically specific X-ray fluorescence imaging (XFI) at the sulfur K-edge to image the sulfonate group in taurine in situ in ex vivo tissue sections. To our knowledge, this represents the first undistorted imaging of taurine distribution in brain at 20 μm resolution. We report quantitative technique validation by imaging taurine in the cerebellum and hippocampus regions of the rat brain. Further, we apply the technique to image taurine loss from the vulnerable CA1 (cornus ammonis 1) sector of the rat hippocampus following global brain ischemia. The location-specific loss of taurine from CA1 but not CA3 neurons following ischemia reveals osmotic stress may be a key factor in delayed neurodegeneration after a cerebral ischemic insult and highlights the significant potential of chemically specific XFI to study the role of taurine in brain disease.

Multiple functions of neuronal plasma membrane neurotransmitter transporters

Removal from receptors of neurotransmitters just released into synapses is one of the major steps in neurotransmission. Transporters situated on the plasma membrane of nerve endings and glial cells perform the process of neurotransmitter (re)uptake. Because the density of transporters in the membranes can fluctuate, transporters can determine the transmitter concentrations at receptors, thus modulating indirectly the excitability of neighboring neurons. Evidence is accumulating that neurotransmitter transporters can exhibit multiple functions. Being bidirectional, neurotransmitter transporters can mediate transmitter release by working in reverse, most often under pathological conditions that cause ionic gradient dysregulations. Some transporters reverse to release transmitters, like dopamine or serotonin, when activated by 'indirectly acting' substrates, like the amphetamines. Some transporters exhibit as one major function the ability to capture transmitters into nerve terminals that perform insufficient synthesis. Transporter activation can generate conductances that regulate directly neuronal excitability. Synaptic and non-synaptic transporters play different roles. Cytosolic Na(+) elevations accompanying transport can interact with plasmalemmal or/and mitochondrial Na(+)/Ca(2+) exchangers thus generating calcium signals. Finally, neurotransmitter transporters can behave as receptors mediating releasing stimuli able to cause transmitter efflux through multiple mechanisms. Neurotransmitter transporters are therefore likely to play hitherto unknown roles in multiple therapeutic treatments.

Intracellular levels of glutamate in swollen astrocytes are preserved via neurotransmitter reuptake and de novo synthesis: implications for hyponatremia

Hyponatremia and several other CNS pathologies are associated with substantial astrocytic swelling. To counteract cell swelling, astrocytes lose intracellular osmolytes, including l-glutamate and taurine, through volume-regulated anion channel. In vitro, when swollen by exposure to hypo-osmotic medium, astrocytes lose endogenous taurine faster, paradoxically, than l-glutamate or l-aspartate. Here, we explored the mechanisms responsible for differences between the rates of osmolyte release in primary rat astrocyte cultures. In radiotracer assays, hypo-osmotic efflux of preloaded [(14) C]taurine was indistinguishable from d-[(3) H]aspartate and only 30-40% faster than l-[(3) H]glutamate. However, when we used HPLC to measure the endogenous intracellular amino acid content, hypo-osmotic loss of taurine was approximately fivefold greater than l-glutamate, and no loss of l-aspartate was detected. The dramatic difference between loss of endogenous taurine and glutamate was eliminated after inhibition of both glutamate reuptake [with 300 μM dl-threo-β-benzyloxyaspartic acid (TBOA)] and glutamate synthesis by aminotransferases [with 1 mM aminooxyacetic acid (AOA)]. Treatment with TBOA+AOA made reductions in the intracellular taurine and l-glutamate levels approximately equal. Taken together, these data suggest that swollen astrocytes actively conserve intracellular glutamate via reuptake and de novo synthesis. Our findings likely also explain why in animal models of acute hyponatremia, extracellular levels of taurine are dramatically elevated with minimal impact on extracellular l-glutamate. We identified mechanisms that allow astrocytes to conserve intracellular l-glutamate (Glu) upon exposure to hypo-osmotic environment. Cell swelling activates volume-regulated anion channel (VRAC) and triggers loss of Glu, taurine (Tau), and other cytosolic amino acids. Glu is conserved via reuptake by Na(+) -dependent transporters and de novo synthesis in the reactions of mitochondrial transamination (TA). These findings explain why, in acute hyponatremia, extracellular levels of Tau can be dramatically elevated with minimal changes in extracellular Glu.

Effects of Putative Epididymal Osmolytes on Sperm Volume Regulation of Fertile and Infertile c-rosTransgenic Mice

Volume regulation by spermatozoa has been demonstrated to be crucial in both mice and men for transport in the female tract. In order to determine the nature of osmolytes used by spermatozoa, they were released from the cauda epididymis of fertile c-ros heterozygous mice into incubation medium of uterine osmolality (representing an osmotic challenge), containing increasing concentrations of compounds that are major epididymal fluid components and known osmolytes in somatic cells. This should nullify the concentration gradients for osmolytes that mediate volume regulation, prevent osmolyte efflux, and lead to swelling. Of the osmolytes tested, K(+) caused the most rapid and extensive volume increases; glutamate, taurine, L-carnitine, and myo-inositol also were effective, but glycerophosphocholine was not. Such effects were not observed in cauda sperm from the infertile knockout mice, demonstrating a defect in normal volume regulation. K(+) concentrations in cauda epididymal fluid were 21 mM higher in the knockout than the heterozygous mice, but no differences were found in caudal fluid glutamate, carnitine, or myo-inositol. The carnitine content of cauda sperm from knockout males was not different from that of fertile males, but lower amounts of glutamate and inositol were found that could explain the poor volume regulation. In heterozygous mice, cauda but not caput sperm responded to the K(+) channel blocker quinine by swelling, demonstrating development of volume regulation during epididymal transit, whereas knockout cauda sperm showed no response, as with the osmolytes. Major epididymal secretions could serve as osmolytes in murine spermatozoa for volume regulation in response to physiological osmotic challenge in the normal fertile mice; the reduced sperm content of inositol and glutamate in the c-ros knockout mice might reflect maturational abnormalities in volume regulation.

Na(+)-dependent transport of taurine is found only on the abluminal membrane of the blood-brain barrier

Luminal and abluminal plasma membranes were isolated from bovine brain microvessels and used to identify and characterize Na(+)-dependent and facilitative taurine transport. The calculated transmembrane potential was -59 mV at time 0; external Na(+) (or choline under putative zero-trans conditions) was 126 mM (T=25 °C). The apparent affinity constants of the taurine transporters were determined over a range of taurine concentrations from 0.24 μM to 11.4 μM. Abluminal membranes had both Na(+)-dependent taurine transport as well as facilitative transport while luminal membranes only had facilitative transport. The apparent K(m) for facilitative and Na(+)-dependent taurine transport were 0.06±0.02 μM and 0.7±0.1 μM, respectively. The Na(+)-dependent transport of taurine was voltage dependent over the range of voltages studied (-25 to -101 mV). The transport was over 5 times greater at -101 mV compared to when V(m) was -25 mV. The sensitivity to external osmolality of Na(+)-dependent transport was studied over a range of osmolalities (229 to 398 mOsm/kg H(2)O) using mannitol as the osmotic agent to adjust the osmolality. For these experiments the concentration of Na(+) was maintained constant at 50mM, and the calculated transmembrane potential was -59 mV. The Na(+)-dependent transport system was sensitive to osmolality with the greatest rate observed at 229 mOsm/kg H(2)O.

Osmosensitive response of glutamate in the substantia nigra

Previous studies have suggested the increase of extracellular glutamate (GLU) in the substantia nigra (SN) as a cause of dopamine-cell degeneration (excitotoxicity) in Parkinson's disease (PD). However, the mechanisms involved in this increase remain unknown. The present work studied osmoregulation as a cause of GLU release in the SN. Microdialysis was used to change extracellular osmolarity, to administer drugs and to quantify the extracellular non-synaptic GLU (EnS-GLU). Two osmolarity modifications were performed, a moderate decrease (5%) resembling physiological modifications and a substantial decrease (>or=20% decrease) similar to that observed under pathological conditions. Hypo-osmolarity induced a dose-response (285-80 mOsm) increase of EnS-GLU which was detected after small osmolarity modifications (15 mOsm) and which was very marked (>1000%) after more intense osmolarity changes. This response disappeared after pre-treating rats with a P2 purinergic-receptor antagonist (pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid; 1 mM) suggesting ATP involvement in the osmosensitive EnS-GLU response. The EnS-GLU increase observed after administration of ATP (0.1-100 microM) and 2-methylthioadenosine triphosphate tetrasodium (P2-receptor agonist; 100 microM) and the lack of effects of adenosine administration (1 mM) suggest that the ATP action on P2 receptors is an amplificatory mechanism in the osmosensitive EnS-GLU response. The marked action of osmolarity on extracellular Glu suggests osmolarity regulation as a possible source for excitotoxicity in the SN.

Two distinct modes of hypoosmotic medium-induced release of excitatory amino acids and taurine in the rat brain in vivo

A variety of physiological and pathological factors induce cellular swelling in the brain. Changes in cell volume activate several types of ion channels, which mediate the release of inorganic and organic osmolytes and allow for compensatory cell volume decrease. Volume-regulated anion channels (VRAC) are thought to be responsible for the release of some of organic osmolytes, including the excitatory neurotransmitters glutamate and aspartate. In the present study, we compared the in vivo properties of the swelling-activated release of glutamate, aspartate, and another major brain osmolyte taurine. Cell swelling was induced by perfusion of hypoosmotic (low [NaCl]) medium via a microdialysis probe placed in the rat cortex. The hypoosmotic medium produced several-fold increases in the extracellular levels of glutamate, aspartate and taurine. However, the release of the excitatory amino acids differed from the release of taurine in several respects including: (i) kinetic properties, (ii) sensitivity to isoosmotic changes in [NaCl], and (iii) sensitivity to hydrogen peroxide, which is known to modulate VRAC. Consistent with the involvement of VRAC, hypoosmotic medium-induced release of the excitatory amino acids was inhibited by the anion channel blocker DNDS, but not by the glutamate transporter inhibitor TBOA or Cd2+, which inhibits exocytosis. In order to elucidate the mechanisms contributing to taurine release, we studied its release properties in cultured astrocytes and cortical synaptosomes. Similarities between the results obtained in vivo and in synaptosomes suggest that the swelling-activated release of taurine in vivo may be of neuronal origin. Taken together, our findings indicate that different transport mechanisms and/or distinct cellular sources mediate hypoosmotic medium-induced release of the excitatory amino acids and taurine in vivo.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Volume-dependent osmolyte efflux from neural tissues: regulation by G-protein-coupled receptors

The CNS is particularly vulnerable to reductions in plasma osmolarity, such as occur during hyponatremia, the most commonly encountered electrolyte disorder in clinical practice. In response to a lowered plasma osmolarity, neural cells initially swell but then are able to restore their original volume through the release of osmolytes, both inorganic and organic, and the exit of osmotically obligated water. Given the importance of the maintenance of cell volume within the CNS, mechanisms underlying the release of osmolytes assume major significance. In this context, we review recent evidence obtained from our laboratory and others that indicates that the activation of specific G-protein-coupled receptors can markedly enhance the volume-dependent release of osmolytes from neural cells. Of particular significance is the observation that receptor activation significantly lowers the osmotic threshold at which osmolyte release occurs, thereby facilitating the ability of the cells to respond to small, more physiologically relevant, reductions in osmolarity. The mechanisms underlying G-protein-coupled receptor-mediated osmolyte release and the possibility that this efflux can result in both physiologically beneficial and potentially harmful pathophysiological consequences are discussed.

Sperm maturation in the epididymis: a new look at an old problem

The osmotic challenges facing maturing spermatozoa and their responses to them are discussed in relation to the concept of sperm maturation, defined as the increased ability of more distally recovered epididymal spermatozoa to fertilize eggs when inseminated into the female tract. One explanation could be that the more distal cells are better able to regulate their volume, and reach the oviducts, as a consequence of uptake of epididymal osmolytes. Increased motility, zona binding and oolemma fusion capacities are also acquired within the epididymis and are necessary for those cells that finally arrive at the site of fertilization.

Substantia nigra osmoregulation: taurine and ATP involvement

An extracellular nonsynaptic taurine pool of glial origin was recently reported in the substantia nigra (SN). There is previous evidence showing taurine as an inhibitory neurotransmitter in the SN, but the physiological role of this nonsynaptic pool of taurine has not been explored. By using microdialysis methods, we studied the action of local osmolarity on the nonsynaptic taurine pool in the SN of the rat. Hypoosmolar pulses (285-80 mosM) administered in the SN by the microdialysis probe increased extrasynaptic taurine in a dose-dependent way, a response that was counteracted by compensating osmolarity with choline. The opposite effect (taurine decrease) was observed when osmolarity was increased. Under basal conditions, the blockade of either the AMPA-kainate glutamate receptors with 6-cyano-7-nitroquinoxaline-2,3-dionine disodium or the purinergic receptors with pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid modified the taurine concentration, suggesting that both receptors modulate the extrasynaptic pool of taurine. In addition, these drugs decreased the taurine response to hypoosmolar pulses, suggesting roles for glutamatergic and purinergic receptors in the taurine response to osmolarity. The participation of purinergic receptors was also supported by the fact that ATP (which, under basal conditions, increased the extrasynaptic taurine in a dose-dependent way) administered in doses saturating purinergic receptors also decreased the taurine response to hypoosmolarity. Taken together, present data suggest osmoregulation as a role of the nonsynaptic taurine pool of the SN, a function that also involves glutamate and ATP and that could influence the nigral cell vulnerability in Parkinson's disease.

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.

Taurine interaction with neurotransmitter receptors in the CNS: an update

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

Endogenous neuro-protectants in ammonia toxicity in the central nervous system: facts and hypotheses

The paper overviews experimental evidence suggestive of the engagement of three endogenous metabolites: taurine, kynurenic acid, and glutathione (GSH) in the protection of central nervous system (CNS) cells against ammonia toxicity. Intrastriatal administration of taurine via microdialysis probe attenuates ammonia-induced accumulation of extracellular cyclic guanosine monophosphate (cGMP) resulting from over-activation of the N-methyl-D: -aspartate/nitric oxide (NMDA/NO) pathway, and this effect involves agonistic effect of taurine on the GABA-A and glycine receptors. Taurine also counteracts generation of free radicals, increased release of dopamine, and its metabolism to dihydroxyphenylacetic acid (DOPAC). Taurine reduces ammonia-induced increase of cell volume (edema) in cerebrocortical slices by a mechanism involving GABA-A receptors. Massive release of radiolabeled or endogenous taurine from CNS tissues by ammonia in vivo and in vitro is thought to promote its neuroprotective action, by making the amino acid available for interaction with cell membranes and/or by driving excess water out of the CNS cells (astrocytes) that underwent ammonia-induced swelling. Ammonia in vivo and in vitro affects in variable ways the synthesis of kynurenic acid (KYNA). Since KYNA is an endogenous NMDA receptor antagonist with a high affinity towards its glycine site, changes in its content may counter over-activation or depression of glutaminergic transmission observed at the different stages of hyperammonemia. GSH is a major antioxidant in the CNS whose synthesis is partly compartmented between neurons and astrocytes: astrocytic GSH is a source of precursors for the synthesis of neuronal GSH. Ammonia in vitro stimulates GSH synthesis in cultured astrocytes, which may compensate for increased GSH consumption (decreased GSH/GSSG ratio) in neurons.

Activation of ?-aminobutyric acid GAT-1 transporters on glutamatergic terminals of mouse spinal cord mediates glutamate release through anion channels and by transporter reversal

The effects of gamma-aminobutyric acid (GABA) on the release of glutamate from mouse spinal cord nerve endings have been studied using superfused synaptosomes. GABA elicited a concentration-dependent release of [3H]D-aspartate ([3H]D-ASP; EC50= 3.76 microM). Neither muscimol nor (-)baclofen mimicked GABA, excluding receptor involvement. The GABA-evoked release was strictly Na+ dependent and was prevented by the GABA transporter inhibitor SKF89976A, suggesting involvement of GAT-1 transporters located on glutamatergic nerve terminals. GABA also potentiated the spontaneous release of endogenous glutamate; an effect sensitive to SKF89976A and low-Na+-containing medium. Confocal microscopy shows that the GABA transporter GAT-1 is coexpressed with the vesicular glutamate transporter vGLUT-1 and with the plasma membrane glutamate transporter EAAT2 in a substantial portion of synaptosomal particles. The GABA effect was external Ca2+ independent and was not decreased when cytosolic Ca2+ ions were chelated by BAPTA. The glutamate transporter blocker DL-TBOA or dihydrokainate inhibited in part (approximately 35%) the GABA (10 microM)-evoked [3H]D-ASP release; this release was strongly reduced by the anion channel blockers niflumic acid and NPPB. GABA, up to 30 microM, was unable to augment significantly the basal release of [3H]glycine from spinal cord synaptosomes, indicating selectivity for glutamatergic transmission. It is concluded that GABA GAT-1 transporters and glutamate transporters coexist on the same spinal cord glutamatergic terminals. Activation of these GABA transporters elicits release of glutamate partially by reversal of glutamate transporters present on glutamatergic terminals and largely through anion channels.

Glycine taken up through GLYT1 and GLYT2 heterotransporters into glutamatergic axon terminals of mouse spinal cord elicits release of glutamate by homotransporter reversal and through anion channels

Glycine concentration-dependently elicited [3H]D-aspartate ([3H]D-ASP) release from superfused mouse spinal cord synaptosomes. Glycine effect was insensitive to strychnine or 5,7-dichlorokynurenic acid, but was prevented by the glycine transporter blocker glycyldodecylamide. Glycine also evoked release of endogenous glutamate, which was sensitive to glycyldodecylamide and abolished in low-Na+ medium. Experiments with purified synaptosomes and gliasomes show that the glycine-evoked [3H]D-ASP release largely originates from glutamatergic nerve terminals. The glycine-evoked [3H]D-ASP release was halved by NFPS, a selective blocker of GLYT1 transporters, or by Org 25543, a selective GLYT2 blocker, and almost abolished by a mixture of the two, suggesting that activation of GLYT1 and GLYT2 present on glutamatergic terminals triggers the release of [3H]D-ASP. Accordingly, confocal microscopy experiments show localization of GLYT1 and GLYT2 in purified synaptosomes immuno-stained for the vesicular glutamate transporter vGLUT1. The glycine effect was independent of extra- and intraterminal Ca2+ ions. It was partly inhibited by the glutamate transporter blocker DL-TBOA and largely prevented by the anion channel blockers niflumic acid and NPPB. To conclude, transporters for glycine (GLYT1 or/and GLYT2) and for glutamate coexist on the same spinal cord glutamatergic terminals. Activation of glycine heterotransporters elicits glutamate release partly by homotransporter reversal and largely through anion channels.

Osmotic Tension as a Possible Link between GABAA Receptor Activation and Intracellular Calcium Elevation

Intracellular calcium concentration rises have been reported following activation of GABA(A) receptors in neonatal preparations and attributed to activation of voltage-dependent Ca(2+) channels. However, we show that, in cerebellar interneurons, GABA(A) agonists induce a somatodendritic Ca(2+) rise that persists at least until postnatal day 20 and is not mediated by depolarization-induced Ca(2+) entry. A local Ca(2+) elevation can likewise be elicited by repetitive stimulation of presynaptic GABAergic afferent fibers. We find that, following GABA(A) receptor activation, bicarbonate-induced Cl(-) entry leads to cell depolarization, Cl(-) accumulation, and osmotic tension. We propose that this tension induces the intracellular Ca(2+) rise as part of a regulatory volume decrease reaction. This mechanism introduces an unexpected link between activation of GABA(A) receptors and intracellular Ca(2+) elevation, which could contribute to activity-driven synaptic plasticity.

Extracellular ATP activates chloride and taurine conductances in cultured hippocampal neurons

We investigated regulation by extracellular ATP of channels important for volume regulation of rat hippocampal neurons. Cultures made from fetuses at the eighteenth gestational day were predominantly neuronal after 10-20 days in vitro, as indicated by immunostaining for neuron specific enolase. Neurons recorded with whole-cell patch clamp showed inward currents when membrane voltages were driven to values greater than -50 mV. Chloride conductance increased with 10 microM-100 microM extracellular ATP in a dose-dependent fashion. Similarly, an increase in taurine conductance was observed with 50 microM ATP. These currents were inhibited by the anion channel and purinergic receptor antagonists niflumic acid and suramin, respectively. The chloride conductance response to 10 microM ATP was increased over eight-fold in hypoosmotic medium (250 mOsm); however, chloride conductance in 0 mM ATP was not altered by this osmolality. Thus anion and osmolyte conducting channels activated via purinergic receptors may mediate volume regulation of hippocampal neurons.

The role of inhibitory amino acidergic neurotransmission in hepatic encephalopathy: a critical overview

Gamma-Aminobutyric acid (GABA) is the main inhibitory amino acid in the central nervous system (CNS). Experiments with animal models of HE, and with brain slices or cultured CNS cells treated with ammonia, have documented changes in GABA distribution and transport, and modulation of the responses of both the GABA(A)-benzodiazepine receptor complex and GABA(B) receptors. Although many of the data point to an enhancement of GABAergic transmission probably contributing to HE, the evidence is not unequivocal. The major weaknesses of the GABA theory are (1) in a vast majority of HE models, there were no alterations of GABA content in the brain tissue and/or extracellular space, indicating that exposure of neurons to GABA may not have been altered, (2) changes in the affinity and capacity of GABA receptor binding were either absent or qualitatively different in HE models of comparable severity and duration, and (3) no sound changes in the GABAergic system parameters were noted in clinical cases of HE. Taurine (Tau) is an amino acid that is thought to mimic GABA function because of its agonistic properties towards GABA(A) receptors, and to contribute to neuroprotection and osmoregulation. These effects require Tau redistribution between the different cell compartments and the extracellular space. Acute treatment with ammonia evokes massive release of radiolabeled or endogenous Tau from CNS tissues in vivo and in vitro, and the underlying mechanism of Tau release differs from the release evoked by depolarizing conditions or hypoosmotic treatment. Subacute or chronic HE, and also long-term treatment of cultured CNS cells in vitro with ammonia, increase spontaneous Tau "leakage" from the tissue. This is accompanied by a decreased potassium- or hypoosmolarity-induced release of Tau and often by cell swelling, indicating impaired osmoregulation. In in vivo models of HE, Tau leakage is manifested by its increased accumulation in the extrasynaptic space, which may promote inhibitory neurotransmission and/or cell membrane protection. In chronic HE in humans, decreased Tau content in CNS is thought to be one of the causes of cerebral edema. However, understanding of the impact of the changes in Tau content and transport on the pathogenic mechanisms of HE is hampered by the lack of clear-cut evidence regarding the various roles of Tau in the normal CNS.

Mechanisms counteracting swelling in brain cells during hyponatremia

Water gain in the brain consequent to hyponatremia is counteracted by mechanisms that initially include a compensatory displacement of liquid from the interstitial space to cerebrospinal fluid and systemic circulation and subsequently an active reduction in cell water accomplished by extrusion of intracellular osmolytes to reach osmotic equilibrium. Potassium (K+), chloride (Cl-), amino acids, polyalcohols, and methylamines all contribute to volume regulation, with a major contribution of ions at the early phase and of organic osmolytes at the late phase of the regulatory process. Experimental models in vitro show that osmolyte fluxes occur via leak pathways for organic osmolytes and separate channels for Cl- and K+. Osmotransduction signaling cascades for Cl- and taurine efflux pathways involve tyrosine kinases and phosphoinositide kinases, while Ca2+ and serine-threonine kinases modulate K+ pathways. In-depth knowledge of the cellular and molecular adaptive mechanisms of brain cells during hyponatremia contributes to a better understanding of the associated complications, including the risks of inappropriate correction of the hyponatremic condition.

Osmosensitive release of neurotransmitter amino acids: relevance and mechanisms

Hyposmolarity activates amino acid efflux as part of the corrective volume process in a variety of cells. This review discusses the mechanism of amino acid release in brain cells preparations. Results present evidence of substantial differences between the efflux of taurine and that of GABA and glutamate, which besides a possible role as osmolytes, have a main function as synaptic transmitters. The differences found concern the efflux time course, the sensitivity to C1- channel blockers, the modulation by tyrosine kinases, the influence of PKC and the effect of cytoskeleton disruptive agents. While taurine efflux features fit well with the mechanisms so far described in most cell types, the efflux of GABA and glutamate does not. Alternate mechanisms for the release of these two amino acids are discussed, including a PKC-modulated, actin-dependent exocytosis.

Activation of alpha 6 GABAA receptors on depolarized cerebellar parallel fibers elicits glutamate release through anion channels

Rat cerebellar synaptosomes labeled with [3H]D-aspartate ([3H]D-ASP) were exposed in superfusion to muscimol. The GABA(A) receptor agonist did not affect [3H]D-ASP basal release or the overflow provoked by 15mM K(+); muscimol potentiated the 35mM K(+)-evoked overflow of [3H]D-ASP or endogenous glutamate. Membrane potential measured by Rhodamine 6G fluorescence was -65mV under resting conditions and -32mV in the presence of 35mM K(+). The membrane potential was not significantly affected by muscimol. The muscimol effect on the K(+)(35mM)-evoked [3H]D-ASP overflow was not inhibited by omitting external Ca(2+) or by entrapping BAPTA to chelate cytosolic Ca(2+). Muscimol lost its ability to release glutamate following superfusion with D-aspartate to deplete cytosolic glutamate by heteroexchange suggesting that GABA(A) receptor activation elicits release of cytosolic glutamate. The non-transportable glutamate carrier blockers dihydrokainate or DL-TBOA did not reduce the muscimol potentiation. This was abolished by the anion channel blockers niflumic acid and NPPB. To conclude, when cerebellar parallel fiber terminals are sufficiently depolarized, activation of alpha6 GABA(A) receptors on these terminals mediates glutamate release in addition to that evoked by depolarization. This extra-release does not occur by exocytosis or transporter reversal but involves the opening of anion channels present on parallel fiber terminals.

Isovolumetric regulation mechanisms in cultured cerebellar granule neurons

Cultured cerebellar granule neurons exposed to gradual reductions in osmolarity (-1.8 mOsm/min) maintained constant volume up to -50% external osmolarity (pi(o)), showing the occurrence of isovolumetric regulation (IVR). Amino acids, Cl-, and K+ contributed at different phases of IVR, with early efflux threshold for [3H]taurine, D-[3H]aspartate (as marker for glutamate) of pi(o) -2% and -19%, respectively, and more delayed thresholds of -30% for [3H]glycine and -25% and -29%, respectively, for Cl- (125I) and K+ (86Rb). Taurine seems preferentially involved in IVR, showing the lowest threshold, the highest efflux rate (five-fold over other amino acids) and the largest cell content decrease. Taurine and Cl- efflux were abolished by niflumic acid and 86Rb by 15 mM Ba2+. Niflumic acid essentially prevented IVR in all ranges of pi(o). Cl--free medium impaired IVR when pi(o) decreased to -24% and Ba2+ blocked it only at a late phase of -30% pi(o). These results indicate that in cerebellar granule neurons: (i) IVR is an active process of volume regulation accomplished by efflux of intracellular osmolytes; (ii) the volume regulation operating at small changes of pi(o) is fully accounted for by mechanisms sensitive to niflumic acid, with contributions of both Cl- and amino acids, particularly taurine; (iii) Cl- contribution to IVR is delayed with respect to other niflumic acid-sensitive osmolyte fluxes (osmolarity threshold of -25% pi(o)); and (iv), K+ fluxes do not contribute to IVR until a late phase (< -30% pi(o)).