Taurine as an organic osmolyte in the intact brain: immunocytochemical and biochemical studies

Osmotic gradients and transretinal water flow-a quantitative elemental microanalytical study of frozen hydrated chick eyes

Optical clarity and efficient phototransduction are necessary for optimal vision, however, how the associated processes of osmoregulation and continuous fluid drainage across the whole eye are achieved remains relatively unexplored. Hence, we have employed elemental microanalysis of planed surfaces of light-adapted bulk frozen-hydrated chick eyes to determine the unique intracellular elemental localization, compositions, and hydration states that contribute to maintaining osmotic gradients and water flow from the vitreous, across the retina, retinal pigment epithelium (RPE), to choroid and sclera. As expected, the greatest difference in resultant osmotic concentration gradients, [calculated using the combined concentrations of sodium (Na) and potassium (K)] and tissue hydration [oxygen-defined water concentration], occurs in the outer retina and, in particular, in the RPE where the apical and basal membranes are characterized by numerous bioenergetically active, osmoregulating ion transport mechanisms, aquaporins, and chloride (Cl) channels. Our results also demonstrate that the high intracellular Na+ and K+ concentrations in the apical region of the RPE are partially derived from the melanosomes. The inclusion of the ubiquitous osmolyte taurine to the calculation of the osmotic gradients suggests a more gradual increase in the osmotic transport of water from the vitreous into the ganglion cell layer across the inner retina to the outer segments of the photoreceptor/apical RPE region where the water gradient increases rapidly towards the basal membrane. Thus transretinal water is likely to cross the apical membrane from the retina into the RPE cells down the Na+ and K+ derived osmotic concentration gradient and leave the RPE for the choroid across the basal membrane down the Cl- derived osmotic concentration gradient that is sustained by the well-described bioenergetically active RPE ion transporters and channels.

Sub-chronic taurine administration induces behavioral sensitization but does not influence ethanol-induced dopamine release in the nucleus accumbens

Preclinical studies have shown that the amino acid taurine is of importance for the dopamine elevating properties of ethanol. Taurine intake has escalated over the last decade due to increased consumption of taurine-containing energy drinks and dietary supplements. Whether long-term intake of large amounts of taurine induces adaptations affecting ethanol-induced dopamine elevation is not clear. Thus the aim of the present studies was to explore the impact of repeated administration of large amounts of taurine on ethanol-induced behavior and dopamine neurotransmission. Repeated daily systemic administration of taurine increased taurine-induced locomotor activity and rearing. Acute administration of taurine and ethanol in naïve animals produced an additive effect on extracellular taurine but no alteration of the ethanol-induced dopamine elevation, as measured by in vivo microdialysis. Sub-chronic administration of taurine did not modify the taurine- or dopamine-elevating properties of ethanol. Daily taurine treatment also failed to change the mRNA expression of the taurine transporter and GABA_A- and glycine-receptor subunits, as measured by qPCR in nucleus accumbens tissue. We conclude that systemic administration of taurine may have long lasting central effects, here displayed as behavioral sensitization. However, repeated daily exposure to taurine does not appear to influence the dopamine elevating properties of ethanol.

Cellular and subcellular aquaporin-4 distribution in the mouse neurohypophysis and the effects of osmotic stimulation

Water channel aquaporin-4 (AQP4) is the most abundant water channel in the rodent brain and is mainly expressed in cerebral areas involved in central osmoreception and osmoregulation. The neurohypophysis is the release site of hypothalamic neurohormones vasopressin and oxytocin, which are involved in the regulation of the water balance. The authors investigated the cellular and subcellular distribution of AQP4 in the mouse neurohypophysis before and after chronic osmotic stimulation, using immunofluorescence microscopy and immunoperoxidase electron microscopy. They showed that AQP4 was abundant in the mouse hypophysis, mainly in the neural lobe. AQP4 was discontinuously distributed along pituicytes plasma membranes, in the dense neurosecretory granules and microvesicles of nerve endings and fibers, and along the luminal and abluminal membranes of fenestrated capillary endothelial cells. After chronic osmotic stimulation, AQP4 immunolabeling was enhanced. Taken together, these results suggest that AQP4 could be involved in the pituicyte sensor effect during osmoregulation, the modification and/or maturation mechanism of neurosecretory granules during neurohormone release, and the blood perfusion of the hypophysis.

Temporal patterns of interstitial pyruvate and amino acids after subarachnoid haemorrhage are related to the level of consciousness--a clinical microdialysis study

BACKGROUND

Temporal patterns of brain interstitial amino acids after subarachnoid haemorrhage (SAH) were studied in relation to energy metabolite levels and to the severity of the initial global ischaemia as reflected by the level of consciousness at admission.

METHOD

Intracerebral microdialysis was used to measure brain interstitial amino acids and the energy metabolites glucose, lactate, and pyruvate during five days in 19 patients. Patients who were conscious (n = 11) were compared to those who were unconscious on admission (n = 8).

FINDINGS

Eight non-transmitter amino acids (alanine, asparagine, glutamine, isoleucine, leucine, phenylalanine, serine and tyrosine), as well as glycine and pyruvate showed a pattern of increasing concentrations starting at 60-70 h after the onset of SAH. The conscious patients showed more pronounced elevations of non-transmitter amino acids, glycine, taurine and pyruvate compared to the unconscious patient group. Pyruvate levels were initially critically low for all patients, then normalised in the conscious patients but remained low in the unconscious group.

CONCLUSIONS

There was an increase of the cerebral interstitial levels of non-transmitter amino acids and glycine which correlated temporally to pyruvate levels, more pronounced in patients conscious on admission. Pyruvate levels in these patients normalised, but remained reduced in the unconscious patients. The increase of the non-transmitter amino acids and glycine could reflect an increased amino acid turnover in an attempt at repairing the injured brain, which could have been hampered by the lower pyruvate levels. Interstitial pyruvate may be a useful marker of the energy metabolic situation in the acutely injured brain.

Purinergic activation of anion conductance and osmolyte efflux in cultured rat hippocampal neurons

The majority of mammalian cells demonstrate regulatory volume decrease (RVD) following swelling caused by hyposmotic exposure. A critical signal initiating RVD is activation of nucleotide receptors by ATP. Elevated extracellular ATP in response to cytotoxic cell swelling during pathological conditions also may initiate loss of taurine and other intracellular osmolytes via anion channels. This study characterizes neuronal ATP-activated anion current and explores its role in net loss of amino acid osmolytes. To isolate anion currents, we used CsCl as the major electrolyte in patch electrode and bath solutions and blocked residual cation currents with NiCl(2) and tetraethylammonium. Anion currents were activated by extracellular ATP with a K(m) of 70 microM and increased over fourfold during several minutes of ATP exposure, reaching a maximum after 9.0 min (SD 4.2). The currents were blocked by inhibitors of nucleotide receptors and volume-regulated anion channels (VRAC). Currents showed outward rectification and inactivation at highly depolarizing membrane potentials, characteristics of swelling-activated anion currents. P2X agonists failed to activate the anion current, and an inhibitor of P2X receptors did not block the effect of ATP. Furthermore, current activation was observed with extracellular ADP and 2-(methylthio)adenosine 5'-diphosphate, a P2Y(1) receptor-specific agonist. Much less current activation was observed with extracellular UTP, suggesting the response is mediated predominantly by P2Y(1) receptors. ATP caused a dose-dependent loss of taurine and alanine that could be blocked by inhibitors of VRAC. ATP did not inhibit the taurine uptake transporter. Thus extracellular ATP triggers a loss of intracellular organic osmolytes via activation of anion channels. This mechanism may facilitate neuronal volume homeostasis during cytotoxic edema.

Glutamate counteracts the denaturing effect of urea through its effect on the denatured state

The urea induced equilibrium denaturation behavior of glutaminyl-tRNA synthetase from Escherichia coli (GlnRS) in 0.25 m potassium l-glutamate, a naturally occurring osmolyte in E. coli, has been studied. Both the native to molten globule and molten globule to unfolded state transitions are shifted significantly toward higher urea concentrations in the presence of l-glutamate, suggesting that l-glutamate has the ability to counteract the denaturing effect of urea. d-Glutamate has a similar effect on the equilibrium denaturation of glutaminyl-tRNA synthetase, indicating that the effect of l-glutamate may not be due to substrate-like binding to the native state. The activation energy of unfolding is not significantly affected in the presence of 0.25 m potassium l-glutamate, indicating that the native state is not preferentially stabilized by the osmolyte. Dramatic increase of coefficient of urea concentration dependence (m) values of both the transitions in the presence of glutamate suggests destabilization and increased solvent exposure of the denatured states. Four other osmolytes, sorbitol, trimethylamine oxide, inositol, and triethylene glycol, show either a modest effect or no effect on native to molten globule transition of glutaminyl-tRNA synthetase. However, glycine betaine significantly shifts the transition to higher urea concentrations. The effect of these osmolytes on other proteins is mixed. For example, glycine betaine counteracts urea denaturation of tubulin but promotes denaturation of S228N lambda-repressor and carbonic anhydrase. Osmolyte counteraction of urea denaturation depends on osmolyte-protein pair.

How does glucose generate oxidative stress in peripheral nerve?

Diabetes-associated oxidative stress is clearly manifest in peripheral nerve, dorsal root, and sympathetic ganglia of the peripheral nervous system and endothelial cells and is implicated in nerve blood flow and conduction deficits, impaired neurotrophic support, changes in signal transduction and metabolism, and morphological abnormalities characteristic of peripheral diabetic neuropathy (diabetic peripheral neuropathy). Hyperglycemia has a key role in oxidative stress in diabetic nerve, whereas the contribution of other factors, such as endoneurial hypoxia, transition metal imbalance, and hyperlipidemia, has not been rigorously proven. It has been suggested that oxidative stress, particularly mitochondrial superoxide production, is responsible for sorbitol pathway hyperactivity, nonenzymatic glycation/glycooxidation, and activation of protein kinase C. However, this concept is not supported by in vivo studies demonstrating the lack of any inhibition of the sorbitol pathway activity in peripheral nerve, retina, and lens by antioxidants, including potent superoxide scavengers. Its has been also hypothesized that aldose reductase (AR) detoxifies lipid peroxidation products, and therefore, the enzyme inhibition in diabetes is detrimental rather than benefical. However, the role for AR in lipid peroxdation product metabolism has never been demonstrated in vivo, and the effects of aldose reductase inhibitors and antioxidants on diabetic peripheral neuropathy are unidirectional, i.e., both classes of agents prevent and correct functional, metabolic, neurotrophic, and morphological changes in diabetic nerve. Growing evidence indicates that AR has a key role in oxidative stress in the peripheral nerve and contributes to superoxide production by the vascular endothelium. The potential mechanisms of this phenonmenon are discussed.

Carrier-mediated uptake and release of taurine from Bergmann glia in rat cerebellar slices

Taurine uptake is essential for the maintenance of millimolar intracellular concentrations of taurine, which is released during ischaemia and is thought to be neuroprotective. To determine whether Bergmann glia express functional transporters that can mediate both taurine uptake and efflux, whole-cell patch-clamp recordings were obtained from these cells in rat cerebellar slices. Taurine-induced inward currents can be pharmacologically separated into GABA(A) receptor and taurine transporter currents. In the presence of GABA receptor blockers, residual taurine currents averaged -28 pA at -70 mV and were strictly inwardly rectifying between -70 and +50 mV. These residual currents were also abolished by external Na+ removal and diminished by reduction of external Cl-, consistent with transport currents. Taurine transport currents were reduced by a taurine transporter inhibitor, guanidinoethyl sulphonate (GES). Other classical inhibitors reduced taurine transport currents with an order of potency (hypotaurine > beta-alanine > GES > GABA) similar to that reported for cloned rat taurine transporters. Following intracellular taurine perfusion during the recording, a progressively developing outward current could be observed at -50 mV but not at -70 mV. Intracellular perfusion of taurine also decreased taurine-induced inward currents at both holding potentials. Outward currents induced by intracellular taurine increased in amplitude with depolarization, activated near -50 mV, and were affected by GES. For the first time, these results demonstrate that taurine activates both GABA(A) receptors and Na+/Cl--dependent taurine transporters in Bergmann glia in slices. In addition, our data show that taurine transporters can work in reverse and can probably mediate taurine efflux under ischaemic conditions.

Taurine-, aspartate- and glutamate-like immunoreactivity identifies chemically distinct subdivisions of Kenyon cells in the cockroach mushroom body

The lobes of the mushroom bodies of the cockroach Periplaneta americana consist of longitudinal modules called laminae. These comprise repeating arrangements of Kenyon cell axons, which like their dendrites and perikarya have an affinity to one of three antisera: to taurine, aspartate, or glutamate. Taurine-immunopositive laminae alternate with immunonegative ones. Aspartate-immunopositive Kenyon cell axons are distributed across the lobes. However, smaller leaf-like ensembles of axons that reveal particularly high affinities to anti-aspartate are embedded within taurine-positive laminae and occur in the immunonegative laminae between them. Together, these arrangements reveal a complex architecture of repeating subunits whose different levels of immunoreactivity correspond to broader immunoreactive layers identified by sera against the neuromodulator FMRFamide. Throughout development and in the adult, the most posterior lamina is glutamate immunopositive. Its axons arise from the most recently born Kenyon cells that in the adult retain their juvenile character, sending a dense system of collaterals to the front of the lobes. Glutamate-positive processes intersect aspartate- and taurine-immunopositive laminae and are disposed such that they might play important roles in synaptogenesis or synapse modification. Glutamate immunoreactivity is not seen in older, mature axons, indicating that Kenyon cells show plasticity of neurotransmitter phenotype during development. Aspartate may be a universal transmitter substance throughout the lobes. High levels of taurine immunoreactivity occur in broad laminae containing the high concentrations of synaptic vesicles.

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.

Amine and amino acid transmitters in the eye of the mollusc Bulla gouldiana: an immunocytochemical study

We identified putative transmitters of the photoreceptors and circadian pacemaker neurons and found candidates for efferent control in the eye of the marine mollusc Bulla gouldiana. Established antisera against octopamine, dopamine, serotonin, histamine, glutamate, gamma-aminobutyric acid (GABA), and taurine were used, and central ganglia were processed in parallel to evaluate general staining quality. Photoreceptors and circadian pacemaker cells both expressed immunoreactivity for glutamate and taurine. The eye and its sheath were devoid of GABA-like immunoreactive material, and none of the antisera directed against biogenic amines labelled cells or processes in the nervous tissue of the eye. However, dopamine and octopamine antisera stained large spherical granules (diameter 2-3 microm) contained in granular cells that are located in the connective tissue encapsulating the eye and the optic nerve. The serotonin antiserum revealed a sparse distribution of varicose axon fibers in the optic nerve and eye sheath. No histamine-immunoreactive processes were revealed in the eye. The functional significance of these findings for the molluscan eye and its circadian clock is discussed.

Colocalisation of taurine- with transmitter-immunoreactivities in the nervous system of the migratory locust

The expression of taurine immunoreactivity (TAU-IR) by neurones immunoreactive for octopamine (OA-IR), gamma-aminobutyric acid (GABA-IR), and the C-terminal peptide sequence arginine-phenylalanine (RFamide-IR) was investigated in the migratory locust (Locusta migratoria). TAU-IR is colocalised with OA-IR in the dorsal unpaired median neurones, which are efferent neuroparacrine cells. TAU-IR is not, however, expressed by OA-IR interneurones in the thoracic ganglia and brain. The only other TAU-IR somata found with peripheral axons are the medial neurosecretory cells in abdominal ganglia that project to the neurohaemal organs. These cells exhibit RFamide-IR. The majority of TAU-IR somata in the thoracic abdominal nervous system exhibit GABA-IR. These cells correspond to populations of identified local and intersegmentally projecting inhibitory interneurones. TAU-IR is not, however, exhibited by the well-known GABAergic common inhibitor neurones, which have peripherally projecting axons. This differential distribution of TAU-IR in basically two, functionally different, neuronal subsets (efferent neurosecretory and neuroparacrine cells, inhibitory interneurones) conforms with the concept of taurine acing as a depressive agent to limit excitation during stressful conditions.

Ethanol induces taurine release in the amygdala: an in vivo microdialysis study

The effect of acute IP ethanol injections on the extracellular aspartate, glutamate, taurine and GABA content of the basolateral amygdala microdialysate was investigated in relationship with total brain ethanol. Each acute intraperitoneal injection of ethanol, 0.5, 1.0, 2.0 and 3.0 g/kg body weight, induced an immediate increase in microdialysate taurine; both 0.5 and 1.0 g/kg ethanol evoked an increase during the first 20 minutes following injection which returned to baseline value by 40 minutes, despite the fact that ethanol was detectable in the brain until 60 or 120 minutes, respectively. After either 2.0 or 3.0 g/kg ethanol there was an increase in taurine of gradual intensity which gradually declined to reach baseline values by 100 minutes. In contrast, the ethanol concentration for 2.0 g/kg remained elevated at the end of the 120 minutes; approximately 25 mg ethanol/mg protein. The stimulated release of taurine within the amygdala could participate in the regulation of ethanoli-nduced changes in osmolarity, since taurine is postulated to act as an osmoregulator in the brain. Taurine could also mediate or interact with ethanol-induced central nervous system effects, as it exerts a modulatory action on cell excitability and neurotransmitter processes.