Volume regulation in astrocytes: a role for taurine as an osmoeffector

Astroglial and Microglial Purinergic P2X7 Receptor as a Major Contributor to Neuroinflammation during the Course of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune inflammatory disease of the central nervous system that leads to the progressive disability of patients. A characteristic feature of the disease is the presence of focal demyelinating lesions accompanied by an inflammatory reaction. Interactions between autoreactive immune cells and glia cells are considered as a central mechanism underlying the pathology of MS. A glia-mediated inflammatory reaction followed by overproduction of free radicals and generation of glutamate-induced excitotoxicity promotes oligodendrocyte injury, contributing to demyelination and subsequent neurodegeneration. Activation of purinergic signaling, in particular P2X7 receptor-mediated signaling, in astrocytes and microglia is an important causative factor in these pathological processes. This review discusses the role of astroglial and microglial cells, and in particular glial P2X7 receptors, in inducing MS-related neuroinflammatory events, highlighting the importance of P2X7R-mediated molecular pathways in MS pathology and identifying these receptors as a potential therapeutic target.

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.

Hyponatremia and the Brain

Hyponatremia is defined by low serum sodium concentration and is the most common electrolyte disorder encountered in clinical practice. Serum sodium is the main determinant of plasma osmolality, which, in turn, affects cell volume. In the presence of low extracellular osmolality, cells will swell if the adaptation mechanisms involved in the cell volume maintenance are inadequate. The most dramatic effects of hyponatremia on the brain are seen when serum sodium concentration decreases in a short period, allowing little or no adaptation. The brain is constrained inside a nonextensible envelope; thus, brain swelling carries a significant morbidity because of the compression of brain parenchyma over the rigid skull. Serum sodium concentration is an important determinant of several biological pathways in the nervous system, and recent studies have suggested that hyponatremia carries a significant risk of neurological impairment even in the absence of brain edema. The brain can also be affected by the treatment of hyponatremia, which, if not undertaken cautiously, could lead to osmotic demyelination syndrome, a rare demyelinating brain disorder that occurs after rapid correction of severe hyponatremia. This review summarizes the pathophysiology of brain complications of hyponatremia and its treatment.

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.

Hepatic encephalopathy: An approach to its multiple pathophysiological features

Hepatic encephalopathy (HE) is a neuropsychiatric complex syndrome, ranging from subtle behavioral abnormalities to deep coma and death. Hepatic encephalopathy emerges as the major complication of acute or chronic liver failure. Multiplicity of factors are involved in its pathophysiology, such as central and neuromuscular neurotransmission disorder, alterations in sleep patterns and cognition, changes in energy metabolism leading to cell injury, an oxidative/nitrosative state and a neuroinflammatory condition. Moreover, in acute HE, a condition of imminent threat of death is present due to a deleterious astrocyte swelling. In chronic HE, changes in calcium signaling, mitochondrial membrane potential and long term potential expression, N-methyl-D-aspartate-cGMP and peripheral benzodiazepine receptors alterations, and changes in the mRNA and protein expression and redistribution in the cerebral blood flow can be observed. The main molecule indicated as responsible for all these changes in HE is ammonia. There is no doubt that ammonia, a neurotoxic molecule, triggers or at least facilitates most of these changes. Ammonia plasma levels are increased two- to three-fold in patients with mild to moderate cirrhotic HE and up to ten-fold in patients with acute liver failure. Hepatic and inter-organ trafficking of ammonia and its metabolite, glutamine (GLN), lead to hyperammonemic conditions. Removal of hepatic ammonia is a differentiated work that includes the hepatocyte, through the urea cycle, converting ammonia into GLN via glutamine synthetase. Under pathological conditions, such as liver damage or liver blood by-pass, the ammonia plasma level starts to rise and the risk of HE developing is high. Knowledge of the pathophysiology of HE is rapidly expanding and identification of focally localized triggers has led the development of new possibilities for HE to be considered. This editorial will focus on issues where, to the best of our knowledge, more research is needed in order to clarify, at least partially, controversial topics.

Astroglial excitability and gliotransmission: an appraisal of Ca2+ as a signalling route

Astroglial cells, due to their passive electrical properties, were long considered subservient to neurons and to merely provide the framework and metabolic support of the brain. Although astrocytes do play such structural and housekeeping roles in the brain, these glial cells also contribute to the brain's computational power and behavioural output. These more active functions are endowed by the Ca²⁺-based excitability displayed by astrocytes. An increase in cytosolic Ca²⁺ levels in astrocytes can lead to the release of signalling molecules, a process termed gliotransmission, via the process of regulated exocytosis. Dynamic components of astrocytic exocytosis include the vesicular-plasma membrane secretory machinery, as well as the vesicular traffic, which is governed not only by general cytoskeletal elements but also by astrocyte-specific IFs (intermediate filaments). Gliotransmitters released into the ECS (extracellular space) can exert their actions on neighbouring neurons, to modulate synaptic transmission and plasticity, and to affect behaviour by modulating the sleep homoeostat. Besides these novel physiological roles, astrocytic Ca²⁺ dynamics, Ca²⁺-dependent gliotransmission and astrocyte–neuron signalling have been also implicated in brain disorders, such as epilepsy. The aim of this review is to highlight the newer findings concerning Ca²⁺ signalling in astrocytes and exocytotic gliotransmission. For this we report on Ca²⁺ sources and sinks that are necessary and sufficient for regulating the exocytotic release of gliotransmitters and discuss secretory machinery, secretory vesicles and vesicle mobility regulation. Finally, we consider the exocytotic gliotransmission in the modulation of synaptic transmission and plasticity, as well as the astrocytic contribution to sleep behaviour and epilepsy.

Regulated exocytosis in astrocytic signal integration

Astrocytes can be considered as signal integrators in central nervous system activity. These glial cells can respond to signals from the heterocellular milieu of the brain and subsequently release various molecules to signal to themselves and/or other neighboring neural cells. An important functional module that enables signal integration in astrocytes is exocytosis, a Ca(2+)-dependent process consisting of vesicular fusion to the plasma membrane. Astrocytes utilize regulated exocytosis to release various signaling molecules stored in the vesicular lumen. Here we review the properties of exocytotic release of three classes of gliotransmitters: (i) amino acids, (ii) nucleotides and (iii) peptides. Vesicles may carry not only lumenal cargo, but also membrane-associated molecules. Therefore, we also discuss exocytosis as a delivery mechanism for transporters and receptors to the plasma membrane, where these proteins are involved in astrocytic intercellular signaling.

Gliotransmission: Exocytotic release from astrocytes

Gliotransmitters are chemicals released from glial cells fulfilling a following set of criteria: (i) they are synthesized by and/or stored in glia; (ii) their regulated release is triggered by physiological and/or pathological stimuli; (iii) they activate rapid (milliseconds to seconds) responses in neighboring cells; and (iv) they play a role in (patho)physiological processes. Astrocytes can release a variety of gliotransmitters into the extracellular space using several different mechanisms. In this review, we focus on exocytotic mechanism(s) underlying the release of three classes of gliotransmitters: (i) amino acids, such as, glutamate and d-serine; (ii) nucleotides, like adenosine 5'-triphosphate; and (iii) peptides, such as, atrial natriuretic peptide and brain-derived neurotrophic factor. It is becoming clear that astrocytes are endowed with elements that qualify them as cells communicating with neurons and other cells within the central nervous system by employing regulated exocytosis.

Diffusion in brain extracellular space

Diffusion in the extracellular space (ECS) of the brain is constrained by the volume fraction and the tortuosity and a modified diffusion equation represents the transport behavior of many molecules in the brain. Deviations from the equation reveal loss of molecules across the blood-brain barrier, through cellular uptake, binding, or other mechanisms. Early diffusion measurements used radiolabeled sucrose and other tracers. Presently, the real-time iontophoresis (RTI) method is employed for small ions and the integrative optical imaging (IOI) method for fluorescent macromolecules, including dextrans or proteins. Theoretical models and simulations of the ECS have explored the influence of ECS geometry, effects of dead-space microdomains, extracellular matrix, and interaction of macromolecules with ECS channels. Extensive experimental studies with the RTI method employing the cation tetramethylammonium (TMA) in normal brain tissue show that the volume fraction of the ECS typically is approximately 20% and the tortuosity is approximately 1.6 (i.e., free diffusion coefficient of TMA is reduced by 2.6), although there are regional variations. These parameters change during development and aging. Diffusion properties have been characterized in several interventions, including brain stimulation, osmotic challenge, and knockout of extracellular matrix components. Measurements have also been made during ischemia, in models of Alzheimer's and Parkinson's diseases, and in human gliomas. Overall, these studies improve our conception of ECS structure and the roles of glia and extracellular matrix in modulating the ECS microenvironment. Knowledge of ECS diffusion properties is valuable in contexts ranging from understanding extrasynaptic volume transmission to the development of paradigms for drug delivery to the brain.

Regulation of taurine transport in rat hippocampal neurons by hypo-osmotic swelling

Taurine, an important mediator of cellular volume regulation in the central nervous system, is accumulated into neurons and glia by means of a highly specific sodium-dependent membrane transporter. During hyperosmotic cell shrinkage, net cellular taurine content increases as taurine transporter activity is enhanced via elevated gene expression of the transporter protein. In hypo-osmotic conditions, taurine is rapidly lost from cells by means of taurine-conducting membrane channels. We reasoned that changes in taurine transporter activity also might accompany cell swelling to minimize re-accumulation of taurine from the extracellular space. Thus, we determined the kinetic and pharmacological characteristics of neuronal taurine transport and the response to osmotic swelling. Accumulation of radioactive taurine is strongly temperature dependent and occurs via saturable and non-saturable pathways. At concentrations of taurine expected in extracellular fluid in vivo, 98% of taurine accumulation would occur via the saturable pathway. This pathway obeys Michaelis-Menten kinetics with a Km of 30.0 +/- 8.8 microm (mean +/- SE) and Jmax of 2.1 +/- 0.2 nmol/mg protein min. The saturable pathway is dependent on extracellular sodium with an effective binding constant of 80.0 +/- 3.1 mm and a Hill coefficient of 2.1 +/- 0.1. This pathway is inhibited by structural analogues of taurine and by the anion channel inhibitors, 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS) and 5-nitro-2-(3 phenylpropylamino) benzoic acid (NPPB). NPPB, but not DIDS, also reduces the ATP content of the cell cultures. Osmotic swelling at constant extracellular sodium concentration reduces the Jmax of the saturable transport pathway by approximately 48%, increases Kdiff for the non-saturable pathway by 77%, but has no effect on cellular ATP content. These changes in taurine transport occurring in swollen neurons in vivo would contribute to net reduction of taurine content and resulting volume regulation.

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.

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.

Inhibition of release of taurine and excitatory amino acids in ischemia and neuroprotection

Volume regulated anion channels (VRAC) have been extensively studied in purified single cell systems like cell cultures where they can be activated by cell swelling. This provides a convenient way of analyzing mechanisms and will likely lead to the holy grails of the field, namely the nature or natures of the volume sensor and the nature or natures of VRACs. Important reasons for such an understanding are that these channels are ubiquitous and have important physiological functions which under pathological conditions convert to deleterious effects. Here we summarize data showing the involvement of VRACs in ischemia-induced release of excitatory amino acids (EAAs) in a rat model of global ischemia. Using microdialysis studies we found that reversal of the astrocytic glutamate transporter and VRACs contribute about equally to the large initial release of EAAs and together account for around 80% of the total release. We used the very potent VRAC blocker, tamoxifen, to see if such inhibition of EAA release via VRACs led to significant neuroprotection. Treatment in the focal rat MCA occlusion model led to around 80% reduction in infarct size with an effective post initiation of ischemia therapeutic window of three hours. However, the common problem of other effects for even the most potent inhibitors pertains here, as tamoxifen has other, potentially neuroprotective, effects. Thus it inhibits nitrotyrosine formation, likely due to its inhibition of nNOS and reduction of peroxynitrite formation. Although tamoxifen cannot therefore be used as a test of the "VRAC-excitotxicity" hypothesis it may prove successful for translation of basic stroke research to the clinic because of its multiple targets.

Brain amino acids during hyponatremia in vivo: clinical observations and experimental studies

Hyponatremia is a highly morbid condition, present in a wide range of human pathologies, that exposes patients to encephalopathic complication and the risk of permanent brain damage and death. Treating hyponatremia has proved to be difficult and still awaits safe management, avoiding the morbid sequelae of demyelinizing and necrotic lesions associated with the use of hypertonic solutions. During acute and chronic hyponatremia in vivo, the brain extrudes the excessive water by decreasing its content of electrolytes and organic osmolytes. At the cellular level, a similar response occurs upon cell swelling. Among the organic osmolytes involved in both responses, free amino acids play a prominent role because of the large intracellular pools often found in nerve cells. An overview of the changes in brain amino acid content during hyponatremia in vivo is presented and the contribution of these changes to the adaptive cell responses involved in volume regulation discussed. Additionally, new data are provided concerning changes in amino acid levels in different regions of the central nervous system after chronic hyponatremia. Results favor the role of taurine, glutamine, glutamate, and aspartate as the main amino acid osmolytes involved in the brain adaptive response to hyponatremia in vivo. Deeper knowledge of the adaptive overall and cellular brain mechanisms activated during hyponatremia would lead to the design of safer therapies for the hyponatremic patient.

Osmoregulation of taurine transporter function and expression in retinal pigment epithelial, ganglion, and müller cells

PURPOSE

To determine whether taurine transporter (TauT) activity and expression are regulated by hyperosmolarity in RPE, ganglion, and Müller cells.

METHODS

Uptake of taurine was measured in ARPE-19 cells cultured in DMEM-F12 medium without or with the addition of 50 mM NaCl or 100 mM mannitol. The kinetics of the transport were analyzed. RT-PCR and Northern and Western blot analyses were used to assess TauT mRNA and protein levels. The influence of hyperosmolarity on the uptake of taurine, myo-inositol, and gamma-aminobutyric acid GABA was studied in RPE, RGC-5, and rMC1 cells.

RESULTS

TauT activity was abundant in RPE and was stimulated (3.5-fold) when the cells were exposed to hyperosmolar conditions (DMEM-F12 culture medium plus 50 mM NaCl or 100 mM mannitol). Peak stimulation of taurine uptake occurred after 17 hours of exposure to hyperosmolar medium. Kinetic analysis revealed that the hyperosmolarity-induced stimulation was associated with an increase in V(max) of TauT with no change in K(m). TauT mRNA and protein levels increased in RPE cells exposed to hyperosmolar conditions. Hyperosmolarity also stimulated the uptake of myo-inositol ( approximately 15-fold); GABA uptake was influenced less markedly. Immunofluorescence and functional studies showed that TauT is present in cultured RGC-5 and rMC1 cells. TauT activity was robust in these cells in normal osmolar conditions and increased by approximately twofold in hyperosmolar conditions.

CONCLUSIONS

These studies provide the first evidence that hyperosmolarity regulates TauT activity and expression in RPE and that TauT is present in ganglion and Müller cells and is regulated by hypertonicity. The data are relevant to diseases such as diabetes, macular degeneration, and neurodegeneration, in which retinal cell volumes may fluctuate dramatically.

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.

Recovery from lactacidosis-induced glial cell swelling with the aid of exogenous anion channels

Hypotonic challenge induces transient swelling in glial cells, which is typically followed by a regulatory volume decrease (RVD). In contrast, lactic acidosis (lactacidosis) induces persistent cell swelling in astrocytes without an accompanying RVD. In the present study, we studied the mechanisms by which lactacidosis interferes with normal volume regulation in rat astrocytic glioma C6 cells. Following exposure of C6 cells to a hypotonic challenge, a current was detected that exhibited properties consistent with those of volume-sensitive outwardly rectifying (VSOR) anion channels. When exposed to in vitro conditions designed to simulate lactacidosis, C6 cells failed to respond to hypotonic stress with an RVD, and VSOR anion currents were not activated. When added to C6 cells, an anion channel-forming protein purified from Helicobacter pylori, VacA, was found to form anion-selective channels in the plasma membrane, and the activity of the VacA channel was not affected by lactacidosis (pH 6.2). Cells preincubated with VacA and then exposed to lactacidotic conditions underwent transient swelling followed by RVD. In contrast, application of a cation ionophore, gramicidin, failed to inhibit lactacidosis-induced persistent cell swelling. From these results, we conclude that inhibition of a volume-sensitive anion channel contributes to persistent swelling induced by lactacidosis in glial cells. Introduction of anion channel activity into glial cells might provide a novel approach for treating cerebral edema, which is associated with lactacidosis in cerebral ischemia or head injury.

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.

Taurine increases rat survival and reduces striatal damage caused by 3-nitropropionic acid

Taurine acts as an antioxidant protecting neurons from free radical-mediated cellular damage. 3-nitropropionic acid (3-NP) inhibits energy metabolism, initiating oxidative stress. With the objective to examine whether taurine can protect glia and neurons from damage produced by 3-NP, male Wistar and Sprague-Dawley rats were treated with either (1) saline, (2) taurine (3) 3-NP and saline, or (4) 3-NP and taurine for 4 days. Survival was determined and brains were processed immunohistochemically. Large striatal lesions and increased GFAP, SOD, and taurine immunoreactivity were detected in the 3-NP group when compared with control groups. In contrast, animals receiving 3-NP and taurine exhibited less GFAP, SOD, and taurine immunoreactivity, along with increased survival rates. Results indicate that taurine treatment after 3-NP administration protects the striatum from damage.

Spinal taurine levels are increased 7 and 30 days following methylprednisolone treatment of spinal cord injury in rats

The amino acid taurine serves many functions in the nervous system serving as inhibitory neurotransmitter/neuromodulator, neurotrophin, antioxidant, and osmolyte. Taurine levels are increased following brain injury and glucocorticoid administration. Thus, the purpose of this study was to examine spinal taurine concentrations following spinal cord injury (SCI) and methylprednisolone (MP) treatment of SCI. A total of 44 adult male Sprague-Dawley rats were divided into control and lesion groups. Control rats received a T6 vertebral laminectomy while lesioned rats received a laminectomy followed by complete spinal transection. Half of the animals in each group received MP intravenously following sham-operation or SCI. Rats survived for 7 or 30 days and concentrations of taurine in spinal gray and white matter, in spinal segments both near and distant from the injury epicenter, were resolved by HPLC analysis. Taurine levels were increased 7 and 30 days following transection in spinal segments immediately adjacent to the lesion and were further elevated by MP treatment. No increases were seen in far rostral/caudal segments, and MP treatment alone had no effect on spinal taurine levels. These findings demonstrate that spinal injury results in increased taurine concentrations in spinal segments undergoing the greatest degree of cellular reactivity and tissue reorganization and that MP therapy potentiates these increases. These findings are significant in that they further characterize the effects of acute MP therapy in spinal tissue. Since taurine is thought to be involved in neuroprotection and/or regeneration following injury, the potentiation of taurine levels by MP treatment may relate to its therapeutic properties.

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.

Activation of volume-regulated Cl(-) channels by ACh and ATP in Xenopus follicles

Osmolarity-dependent ionic currents from follicle-enclosed Xenopus oocytes (follicles) were studied using electrophysiological techniques. Whole follicle currents were monitored using a two-electrode voltage clamp and single-channel activity was measured using the patch-clamp technique. In follicles held at -60 mV two chloride currents were activated in external hyposmotic solutions. One was the habitual volume-regulated current elicited by external hyposmolarity (ICl,swell), and the second was a slow and smooth current (Sin) generated by ACh or ATP application. In follicles, the permeability ratios for different anions with respect to Cl- were similar for both ICl,swell and Sin, with a sequence of: SCN- > I- > Br- >= NO3- >= Cl- > gluconate >= cyclamate > acetate > SO42-. Extracellular ATP blocked the outward component of Sin. Also, extracellular pH modulated the inactivation kinetics of Sin elicited by ACh; e.g. inactivation at +80 mV was approximately 100 % slower at pH 8.0 compared with that at pH 6.0. Lanthanides inhibited ICl, swell and Sin. La3+ completely inhibited ICl,swell with a half-maximal inhibitory concentration (IC50) of 17 +/- 1.9 microM, while Sin was blocked up to 55 % with an apparent IC50 of 36 +/- 2.6 microM. Patch-clamp recordings in follicular cells showed that hyposmotic challenge opened inward single-channel currents. The single channel conductance (4.7 +/- 0.4 pS) had a linear current-voltage relationship with a reversal membrane potential close to -20 mV. This single-channel activity was increased by application of ACh or ATP. The ICl,swell generation was not affected by pirenzepine or metoctramine, and did not affect the purinergic activation of the chloride current named Fin. Thus, ICl,swell was not generated via neurotransmitters released during cellular swelling. All together, equal discrimination for different anions, similar modulatory effects by extracellular pH, the blocking effects by ATP and La3+, and the same single-channel activity, strongly suggest that ICl,swell and Sin currents depend on the opening of the same type or a closely related class of volume-regulated chloride channels.

Isovolumetric regulation of rat glial cells during development and correction of hypo-osmolality

Rat C6 glioma cells undergo regulatory volume decrease (RVD) following sudden exposure to hypo-osmolality, but little or no regulatory volume increase (RVI) is observed when cells cultured in hypo-osmotic media are suddenly returned to isoosmolality. Because C6 glioma cells would rarely be exposed to sudden large changes in osmolality in vivo, we examined the ability of these cells to maintain their volume, termed 'isovolumetric regulation', when exposed to gradual changes in osmolality. When osmolality was gradually reduced by reduction of NaCl concentration from 300 to 250 mOsmol/kg at a rate of 0.4 mOsmol/kg/min or less cells were able to maintain their volume, while at higher rates, the cells swelled. Cells which were cultured in hypo-osmotic (200 mOsmol/kg) media for 3 days exhibited isovolumetric regulation at rates of osmolality increase of 0.5 mOsmol/kg/min or less over the range of 200-250 mOsmol/kg. We conclude that rat C6 glioma cells can sensitively regulate their volume over the osmolality range of pathophysiologic interest at rates of osmolality change which are faster than those generally seen in clinical conditions.

Effects of the anion channel blocker DIDS on ouabain- and high K(+)-induced release of amino acids from the rat cerebral cortex

Amino acid release from the rat cerebral cortex was analyzed using an in vivo cortical cup perfusion model. Topical applications of ouabain or high extracellular K(+) were used to mimic two dimensions of ischemic conditions which promote cell swelling and amino acid release. Ouabain (30 microM) induced significant releases of taurine, gamma-aminobutyric acid (GABA), aspartate, glutamate and phosphoethanolamine. The anion channel blocker, 4, 4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS; 1 mM), inhibited ouabain-induced release of all these amino acids except for glutamate. Exposure to high extracellular K(+) (75 mM) induced a delayed rise in the levels of taurine in the superfusates and an immediate increase in GABA levels. There were no significant releases of other amino acids. The release of taurine and GABA was sensitive to the blocking of anion channels with DIDS. Both ouabain- and high K(+)-induced taurine release is likely to be mediated by DIDS sensitive anion channels. The extracellular accumulation of the other amino acids, where insensitive to DIDS, may be mediated by mechanisms other than swelling-induced anion channels.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Gene expression of taurine transporter and taurine biosynthetic enzymes in brain of rats with acute or chronic hyperosmotic plasma. A comparative study with gene expression of myo-inositol transporter, betaine transporter and sorbitol biosynthetic enzyme

Cells exposed to hyperosmotic conditions maintain their volume by accumulating organic osmolytes. Taurine is considered as an osmolyte in brain cells. Accumulation of other osmolytes (sorbitol, myo-inositol and betaine), was shown in renal cells to result from an upregulation of the expression of the genes regulating osmolyte cell content. We have investigated the gene expression of the taurine transporter (TauT) and of the taurine biosynthetic enzymes, cysteine dioxygenase (CDO) and cysteine sulfinate decarboxylase (CSD) by measuring their mRNA levels in brain of salt-loaded rats. mRNA levels of genes previously identified as osmosensitive, namely aldose reductase (AR), myo-inositol transporter (SMIT) and betaine transporter (BGT1) were also determined. In whole brain, TauT-, SMIT- and BGT1-mRNA levels were significantly increased following acute salt-loading but SMIT-mRNA levels only remained elevated following chronic salt-loading while CDO-, CSD- and AR-mRNA levels remained unchanged in both conditions. Following acute salt-loading, mRNA levels of TauT, CDO, CSD, SMIT, BGT1 and AR were increased in cerebral cortex while SMIT- and BGT1-mRNA levels only were increased in striatum and habenula.TauT, CDO and CSD genes may be upregulated in brain of salt-loaded rats but the upregulation of the TauT gene appears more widespread. TauT, CDO and CSD are thus putative osmosensitive genes. However the actual pattern (amplitude, time course and regional occurrence) of the upregulation of each of the putative (TauT, CDO and CSD) and established (AR, SMIT and BGT1) osmosensitive genes differs markedly. This indicates that there exist other factors in brain cells which can selectively prevent the upregulation of these genes by hyperosmolarity.

The role of taurine in neuronal protection following transient global forebrain ischemia

Osmoregulation and post ischemic glutamate surge suppression (PIGSS) are important mechanisms in the neuroprotective properties of taurine. We studied the role of taurine in PIGSS following transient global forebrain ischemia (TGFI). A group of gerbils received a high dose of continuous intracerebral taurine during the peri-ischemic period. Beta-alanine was given similarly to a negative control group. The control group consisted of animals undergoing only TGFI. On the fourth day following commencement of drug administration, TGFI was induced. Concurrently, half the animals from each group receiving an agent had intracerebral microdialysis. All animals underwent histological assessment at day 7. The microdialysis and histological data was analyzed. Our results showed that taurine treatment did not cause PIGSS. The histological difference between the three groups was statistically insignificant. We conclude that intracerebral taurine in the dosage administered during peri-ischemic period, does not result in PIGSS or histologically evident neuroprotection.

Effects of taurine on ozone-induced memory deficits and lipid peroxidation levels in brains of young, mature, and old rats

To determine the antioxidant effects of taurine on changes in memory and lipid peroxidation levels in brain caused by exposure to ozone, we carried out two experiments. In the first experiment, 150 rats were separated into three experimental blocks (young, mature, and old) with five groups each and received one of the following treatments: control, taurine, ozone, taurine before ozone, and taurine after ozone. Ozone exposure was 0.7-0.8 ppm for 4 h and taurine was administered ip at 43 mg/kg, after or before ozone exposure. Subsequently, rats were tested in passive avoidance conditioning. In the second experiment, samples from frontal cortex, hippocampus, striatum, and cerebellum were obtained from 60 rats (young and old), using the same treatments with 1 ppm ozone. Results show both an impairment in short-term and long-term memory with ozone and an improvement with taurine after ozone exposure, depending on age. In contrast to young rats, old rats showed peroxidation in all control groups and an improvement in memory with taurine. When taurine was applied before ozone, we found high peroxidation levels in the frontal cortex of old rats and the hippocampus of young rats; in the striatum, peroxidation caused by ozone was blocked when taurine was applied either before or after ozone exposure.

Hyposmotically induced amino acid release from the rat cerebral cortex: role of phospholipases and protein kinases

In an evaluation of the contribution of swelling-induced amino acid release, through the regulatory volume decrease (RVD) process, to cerebral ischemic injury, studies of the role of phospholipases and protein kinases in the response to hyposmotic stress were undertaken using an in vivo rat cortical cup model. Hyposmotic stress induced significant releases of aspartate, glutamate, glycine, phosphoethanolamine, taurine and GABA from the rat cerebral cortex. Taurine release was most affected, exhibiting a greater than 9-fold increase during the hyposmotic stimulus. The phospholipase A2 (PLA2) inhibitors 4-bromophenacyl bromide (1 microM) and 7,7-dimethyleicosadienoic acid (5 microM) had no significant effects on hyposmotically induced amino acid release. AACOCF3 (50 microM), an inhibitor of cytosolic PLA2 decreased taurine release to 84% of DMSO controls. The release of the other amino acids was not affected. The phospholipase C inhibitor U73122 (5 microM) had no significant effects on amino acid release. The protein kinase C (PKC) inhibitor chelerythrine (5 microM) significantly reduced hyposmotically induced taurine release to 72% of saline controls but had no significant effects on the other amino acids. Stimulation of PKC with phorbol 12-myristate, 13-acetate (10 microM) did not significantly change taurine, glutamate, glycine or phosphethanolamine release. The releases of aspartate and GABA were enhanced 2 to 3 fold. Phorbol 12,13-didecanoate (10 microM), another potent stimulator of PKC, significantly increased taurine release to 122% of DMSO controls. The releases of aspartate, glutamate and glycine were enhanced 2.5 to 3.5 fold. Similarly, stimulation of protein kinase A with forskolin (100 microM) significantly increased taurine, aspartate, and glycine release 1.5- to 2-fold compared to DMSO controls. In summary, phospholipases may play a minor role in volume regulation. These studies also support the hypothesis that protein kinases play a modulatory role in the RVD response. The results show that although RVD may play a role, additional mechanisms, including phospholipase activation, must be involved in the ischemia-evoked release of excitotoxic amino acids.

Activation and inactivation of taurine efflux in hyposmotic and isosmotic swelling in cortical astrocytes: role of ionic strength and cell volume decrease

A decrease in intracellular ionic strength appears involved in the activation of swelling-elicited 3H-taurine efflux in cortical cultured astrocytes. Hyposmotic (50%) or isosmotic urea-induced swelling leading to a decrease of intracellular ionic strength, activated 3H-taurine efflux from a rate constant of about 0.008 min(-1) to 0.33 min(-1) (hyposmotic) and 0.59 min(-1) (urea). This efflux rate was markedly lower (maximal 0.03 min(-1)) in isosmotic swelling caused by K+ accumulation, where there is no decrease in ionic strength, or in cold (10 degrees C) hyposmotic medium (maximal 0.18 min(-1)), where swelling is reduced and consequently intracellular ionic strength is less affected. Also, astrocytes pretreated with hyperosmotic medium, which recover cell volume by ion accumulation, did not release 3H-taurine when they swelled by switching to isosmotic medium, but when volume was recovered by accumulation of urea, taurine release was restored. These results point to a key role of ionic strength in the activation of osmosensitive 3H-taurine efflux. In contrast, its inactivation was independent of the change in ionic strength but appears related to the reduction in cell volume after swelling, since despite the extent or direction of the change in ionic strength, the 3H-taurine efflux did not inactivate in isosmotic KCl-elicited swelling when cell volume did not recover nor in hyposmotic swelling when RVD was impaired by replacing NaCl in the medium by permeant osmolytes.

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.

Glial cells in neurotoxicity development

Neuroglial cells of the central nervous system include the astrocytes, oligodendrocytes, and microglia. Their counterparts in the peripheral nervous system are the Schwann cells. The term neuroglia comes from an erroneous concept originally coined by Virchow (1850), in which he envisioned the neurons to be embedded in a layer of connective tissue. The term, or its shortened form--glia, has persisted as the preferred generic term for these cells. A reciprocal relationship exists between neurons and glia, and this association is vital for mutual differentiation, development, and functioning of these cell types. Therefore, perturbations in glial cell function, as well as glial metabolism of chemicals to active intermediates, can lead to neuronal dysfunction. The purpose of this review is to explore neuroglial sites of neurotoxicant actions, discuss potential mechanisms of glial-induced or glial-mediated central nervous system and peripheral nervous system damage, and review the role of glial cells in neurotoxicity development.

Effects of anion channel blockers on hyposmotically induced amino acid release from the in vivo rat cerebral cortex

A cortical cup model with continuous perfusion of artificial cerebrospinal fluid (containing 134 mM NaCl) was used to investigate the effects of anion channel blockers on the hyposmotically-induced release of amino acids from the in vivo rat cerebral cortex. The hyposmotic stimulus (25 mM NaCl) evoked a release of taurine, glutamate, aspartate, glycine, phosphoethanolamine and GABA. Topically applied anion channel blockers 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (1 mM); 4-acetamido-4'-isothiocyanatostilbene-2,2-disulfonic acid (2 mM); 5-nitro-2-(3-phenylpropylamino) benzoic acid (350 microM); niflumic acid (500 microM); tamoxifen (20 microM) and arachidonic acid (0.5 microM) all significantly reduced the hyposmotically-induced release of taurine. The releases of glutamate, aspartate, glycine, phosphoethanolamine and GABA were variably susceptible to inhibition by these compounds. These results demonstrate that osmoregulatory processes in cortical cells, in vivo, involve amino acids, with taurine playing a dominant role. The efflux of taurine and, to a lesser extent, the other amino acids may be mediated by anion channels.

Analysis of hyposmolarity-induced taurine efflux pathways in the bullfrog sympathetic ganglia

Hyposmolarity-induced taurine release was dependent on the decrease in medium osmolarity (5-50%) in the satellite glial cells of the bullfrog sympathetic ganglia. Release of GABA induced by hyposmolarity was much less than that of taurine. Omission of external Cl- replaced with gluconate totally suppressed taurine release, but only slightly suppressed GABA release. Bumetanide and furosemide, blockers of the Na+/K+/2Cl- cotransport system, inhibited taurine release by about 40%. Removal of external Na+ by replacement with choline, or omission of K+, suppressed taurine release by 40%. Antagonists of the Cl-/HCO3 exchange system, SITS, DIDS and niflumic acid, significantly reduced taurine release. The carbonic anhydrase inhibitor, acetazolamide, reduced the taurine release by 34%. Omission of external HCO3 by replacement with HEPES caused a 40% increase in the hyposmolarity-induced taurine release. Hyposmolarity-induced GABA release was not affected by bumetanide or SITS. Chloride channel blockers, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and N-phenylanthranilic acid (DPC), practically abolished taurine release. Blockers of K+ channels, clofilium and quinidine, had no effect on the taurine release. The hyposmolarity-induced taurine release was considerably enhanced by a simultaneous increase in external K+. GABA was not mediated by the same transport pathway as that of taurine. These results indicate that Cl- channels may be responsible for the hyposmolarity-induced taurine release, and that Na+/K+/2Cl- cotransporter and Cl-/HCO3 exchanger may contribute to maintain the intracellular Cl- levels higher than those predicted for a passive thermodynamic distribution in the hyposmolarity-induced taurine release.

Osmolyte contents of cultured astrocytes grown in hypoosmotic medium

Primary rat cerebral astrocyte cultures were grown for 2 weeks in isoosmotic medium (305 mosmol) and then placed in similar medium with a reduced NaCl concentration. During the first hour of growth in this moderately hypoosmotic medium (240 mosmol), the cells lose 88% of their taurine contents, 62% of their alanine contents, and 54% of their aspartate contents while regaining normal volume. Loss of these amino acids accounts for 43% of observed volume regulation. Contents of these amino acids remain decreased during 24 h of growth in hypoosmotic medium. In contrast, potassium, glutamate, glutamine, and asparagine contents are not changed, relative to cells in isoosmotic medium, at time points between 1 h and 24 h of hypoosmotic exposure. The data suggest astrocytes contribute to net loss of amino acids, but not potassium, from brains exposed to hypoosmotic conditions in situ.

Glial swelling and astrogliosis produce diffusion barriers in the rat spinal cord

Cell swelling and astrogliosis (manifested as an increase in GFAP) were evoked in isolated rat spinal cords of 4-21-day-old rats by incubation in either 50 mM K+ or hypotonic solution (235 mosmol kg(-1)). Application of K+ and hypotonic solution resulted at first in a decrease of extracellular space (ECS) volume fraction alpha (ECS volume/total tissue volume) and an increase in tortuosity lambda (lambda2 = free/apparent diffusion coefficient) in spinal gray (GM) and white matter (WM). These changes resulted from cell swelling, since the total water content (TW) in spinal cord was unchanged and the changes were blocked in Cl- -free solution and slowed down by furosemide and bumetanide. Diffusion in WM was anisotropic, i.e., more facilitated along fibers (x-axis) than across them (y- or z-axis). The increase of lambda(y,z) was greater than that of lambda(x), reaching unusually high values above 2.4. In GM only, during continuous 45 min application, alpha and lambda started to return towards control values, apparently due to cell shrinkage of previously swollen cells since TW remained unchanged. This return was blocked by fluoroacetate, suggesting that most of the changes were due to the swelling of glia. A 45 min application of 50 mM K+ and, to a lesser degree, of hypotonic solution evoked astrogliosis, which persisted after washing out these solutions with physiological saline. During astrogliosis lambda increased again to values as high as 2.0, while alpha either returned to or increased above control values. This persistent increase in lambda after washout was also found in WM, and, in addition, the typical diffusion anisotropy was diminished. Our data show that glial swelling and astrogliosis are associated with a persistent increase in ECS diffusion barriers. This could lead to the impairment of the diffusion of neuroactive substances, extrasynaptic transmission, "crosstalk" between synapses and neuron-glia communication.

Alterations in glial cell metabolism during recovery from chronic osmotic stress

NMR spectroscopy of F98 glioma cell extracts showed that chronic hypertonic conditions largely increased the intracellular content of small, osmotically active molecules. Moreover, hypertonic stress decreased the incorporation of 13C-labeled amino acids into the cellular proteins albeit their cytosolic concentrations were increased, which reflects an inhibition of protein synthesis under these conditions. Reincubation with isotonic medium restored almost completely the control values for the cytosolic metabolites but not for amino acid incorporation into the protein. An increased amount of 13C label was found in the phospholipids, which indicates stimulation of membrane synthesis processes due to the recovery-induced cell swelling. On the other hand, chronic hypotonic conditions largely decreased the steady state concentration and synthesis of small, cytosolic molecules, whereas the effect on the incorporation of 13C-labeled amino acids into the cellular proteins was variable. Reincubation with isotonic medium partially restored the depressed cytosolic metabolite content and also the incorporation of labeled amino acids into cellular protein, but induced an inhibition of phospholipid synthesis. The results verify that 'readaptation' of glial cell metabolism during recovery from chronic osmotic stress is impaired or at least seriously retarded.

Methylmercury-induced inhibition of regulatory volume decrease in astrocytes: characterization of osmoregulator efflux and its reversal by amiloride

Swelling of neonatal rat primary astrocyte cultures by hypotonic media leads to regulatory volume decrease (RVD) and the resumption of resting cell volume. RVD is associated with activation of conductive K+ and Cl- channels, allowing for the escape of KCl, as well as the release of osmoregulators, such as taurine and myoinositol. As we have previously shown [D. Vitarella, H.K. Kimelberg, M. Aschner, Inhibition of RVD in swollen rat primary astrocyte cultures by methylmercury (MeHg) is due to increase amiloride-sensitive Na+ uptake, Brain Res. 732 (1996) 169-178.], MeHg, when added to hypotonic buffer inhibits RVD, primarily due to increased cellular permeability to Na+ via the Na+/H+ antiporter. The present study was, therefore, undertaken to assess the ability of cation-anion cotransport blockers to reverse the inhibitory effect of MeHg on RVD in swollen astrocytes, and to further characterize MeHg-induced changes in astrocytic osmoregulatory release processes. The studies demonstrate the following: (1) MeHg-induced inhibition of RVD is partially inhibited by the Na+/H+ antiporter blocker, amiloride, but not SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid), DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid), furosemide or bumetanide; (2) exposure of swollen astrocytes to MeHg is associated with specific effects on osmoregulatory release, leading to significant inhibition of taurine release and a significant increase in potassium and myoinositol release compared with release in hypotonic conditions.

The release of amino acids from rat neostriatum and substantia nigra in vivo: a dual microdialysis probe analysis

It has previously been demonstrated, in dual probe microdialysis studies, that stimulation of the neostriatum with kainic acid causes the release of GABA both locally within the neostriatum and distally in the substantia nigra, observations that are consistent with the known anatomy of the basal ganglia. The object of the present study was to further examine the characteristics of GABA release and to determine whether taurine, which has been proposed to be present in striatonigral neurons, has similar characteristics of release, and to examine the release of excitatory amino acids under the same conditions. To this end, dual probe microdialysis studies were carried out on freely-moving rats. The application of kainic acid to neostriatum enhanced the release of GABA, taurine, aspartate and glutamate locally in the neostriatum and distally in the substantia nigra. The distal release of each amino acid in the substantia nigra was sensitive to the administration of 6,7-dinitroquinoxaline-2,3-dione and tetrodotoxin to the neostriatum. Similarly the local release of GABA, aspartate and glutamate but not taurine was sensitive to the intrastriatal application of 6,7-dinitroquinoxaline-2,3-dione or tetrodotoxin. It is concluded that the release of taurine from the substantia nigra has similar characteristics to that of GABA and may be released from the terminals of striatonigral neurons following the stimulation of their cell bodies in the neostriatum. The release of taurine in the neostriatum however, is likely to be mediated mainly by different mechanisms and not related to neuronal activity. The release of excitatory amino acids is likely to involve indirect effects in the neostriatum and polysynaptic pathways in the substantia nigra.

Methylmercury-induced astrocytic swelling is associated with activation of the Na+/H+ antiporter, and is fully reversed by amiloride

Astrocytes are a known 'sink' for brain methylmercury (MeHg) deposition. Yet, the significance of the preferential accumulation of MeHg within these cells is imprecisely defined. To determine whether MeHg in isotonic buffer has the potential to interfere with homeostatic functions, we measured its effect on astrocytic volume using an electrical impedance method [E.R. O'Connor, H.K. Kimelberg, C.R. Keese, I. Giaever, Electrical impedance method for measuring volume changes in astrocytes, Am. J. Physiol. 264 (1993) C471-C478.]. In addition, we have characterized the alterations in astrocytic ion permeability associated with exposure to this organometal. The results show that MeHg rapidly induces astrocytic swelling, and that this effect is secondary to increased astrocytic Na+ uptake. Furthermore, the effect of MeHg on astrocytic swelling is completely inhibited by amiloride, but not by SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid), furosemide, or bumetanide. Accordingly, increased cellular permeability to Na+ via the Na+/H+ antiporter is invoked as the primary mechanism of MeHg-induced astrocytic swelling.