Neuroprotective role of taurine during aging.

Aging of the brain is characterized by several neurochemical modifications involving structural proteins, neurotransmitters, neuropeptides and related receptors. Alterations of neurochemical indices of synaptic function are indicators of age-related impairment of central functions, such as locomotion, memory and sensory performances. Several studies demonstrate that ionotropic GABA receptors, glutamate decarboxylase (GAD), and somatostatinergic subpopulations of GABAergic neurons are markedly decreased in experimental animal brains during aging. Additionally, levels of several neuropeptides co-expressed with GAD decrease during aging. Thus, the age-related decline in cognitive functions could be attributable, at least in part, to decrements in GABA inhibitory neurotransmission. In this study, we showed that chronic supplementation of taurine to aged mice significantly ameliorated the age-dependent decline in spatial memory acquisition and retention. We also demonstrated that concomitant with the amelioration in cognitive function, taurine caused significant alterations in the GABAergic and somatostatinergic system. These changes included (1) increased levels of the neurotransmitters GABA and glutamate, (2) increased expression of both isoforms of GAD (65 and 67) and the neuropeptide somatostatin, (3) decreased hippocampal expression of the ß3 subunits of the GABAA receptor, (4) increased expression in the number of somatostatin-positive neurons, (5) increased amplitude and duration of population spikes recorded from CA1 in response to Schaefer collateral stimulation and (6) enhanced paired pulse facilitation in the hippocampus. These specific alterations of the inhibitory system caused by taurine treatment oppose those naturally occurring in the aging brain, suggesting a protective role of taurine in this process. An increased understanding of age-related neurochemical changes in the GABAergic system will be important in elucidating the underpinnings of the functional changes of aging. Taurine supplementation might help forestall the age-related decline in cognitive functions through interaction with the GABAergic system.

The effects of chronic taurine supplementation on motor learning.

Taurine is one of the most abundant nonprotein amino acids shown to be essential for the development, survival, and growth of vertebrate neurons. We previously demonstrated that chronic taurine supplementation during neonatal development results in changes in the GABAergic system (El Idrissi, Neurosci Lett 436:19-22, 2008). The brains of mice chronically treated with taurine have decreased levels of GABA(A)ß subunits and increased expression of GAD and GABA, which contributes to hyperexcitability. This down regulation of GABA(A)receptor subunit expression and function may be due to a sustained interaction of taurine with GABA(A)receptors. This desensitization decreases the efficacy of the inhibitory synapses at the postsynaptic membrane. If changes occur in the GABAergic system as a possible compensatory mechanism due to taurine administration, then it is important to study all aspects by which taurine induces hyperexcitability and affects motor behavior. We therefore hypothesized that modification of the GABAergic system in response to taurine supplementation influences motor learning capacity in mice. To test this hypothesis, the rotarod task was employed after chronic taurine supplementation in drinking water (0.05% for 4 weeks). Control animals receiving no taurine supplementation were also tested in order to determine the difference in motor learning ability between groups. Each animal was trained on the rotarod apparatus for 7 days at an intermediate speed of 24 rpm in order to establish baseline performance. On the testing day, each animal was subjected to eight different predefined speeds (5, 8, 15, 20, 24, 31, 33, and 44 rpm). From our observations, the animals that underwent chronic taurine supplementation appeared to have a diminished motor learning capacity in comparison to control animals. The taurine-fed mice displayed minor improvements after repeated training when compared to controls. During the testing session the taurine-fed mice also exhibited a shorter latency to fall, as the task requirements became more demanding.

Taurine's effects on the neuroendocrine functions of pancreatic ß cells.

Taurine plays significant physiological roles, including those involved in neurotransmission. Taurine is a potent γ-aminobutyric acid (GABA) agonist and alters cellular events via GABA(A) receptors. Alternately, taurine is transported into cells via the high affinity taurine transporter (TauT), where it may also play a regulatory role. We have previously demonstrated that treatment of Hit-T15 cells with 1 mM taurine for 24 h significantly decreases insulin and GABA levels. We have also demonstrated that chronic in vivo administration of taurine results in an up-regulation of glutamic acid decarboxylase (GAD), the key enzyme in GABA synthesis. Here, we wished to test if administration of 1 mM taurine for 24 h may increase release of another ß cell neurotransmitter somatostatin (SST) and also directly impact up-regulation of GAD synthesis. Treatment with taurine did not significantly alter levels of SST (p > 0.05) or GAD67 (p > 0.05). This suggests that taurine does not directly affect SST release, nor does it directly affect GAD synthesis. Taken together with our observation that taurine does promote GABA release via large dense-core vesicles, the data suggest that taurine may alter membrane potential, which in turn would affect calcium flux. We show here that 1 mM taurine does not alter intracellular Ca(2+) concentrations from 20 to 80 s post treatment (p > 0.05), but does increase Ca(2+) flux between 80 and 200 s post-treatment (p < 0.005). This suggests that taurine may induce a biphasic response in ß cells. The initial response of taurine via GABA(A) receptors hyperpolarizes ß cell and sequesters Ca(2+). Subsequently, taurine may affect Ca(2+) flux in long term via interaction with K(ATP) channels.

Assimilation of elements and digestion in grass shrimp pre-exposed to dietary mercury.

Grass shrimp Palaemonetes pugio were fed mercury (Hg)-contaminated oligochaetes for 15 days and analyzed for Hg, cadmium (Cd), and carbon assimilation efficiencies (AE) as well as toxicological end points related to digestion. Disproportionate increases in stable Hg concentrations in shrimp did not appear to be related to partitioning to trophically available Hg in worms. Hg AE by pre-exposed shrimp reached a plateau (approximately 53 %), whereas Cd AE varied (approximately 40-60 %) in a manner that was not dose-dependent. Carbon AE did not differ among treatments (approximately 69 %). Gut residence time was not impacted significantly by Hg pre-exposure (grand median approximately 465 min), however, there was a trend between curves showing percentages of individuals with markers in feces over time versus treatment. Feces-elimination rate did not vary with dietary pre-exposure. Extracellular protease activity varied approximately 1.9-fold but did not exhibit dose-dependency. pH increased over the range of Hg pre-exposures within the anterior (pH approximately 5.33-6.51) and posterior (pH approximately 5.29-6.25) regions of the cardiac proventriculus and Hg assimilation exhibited a negative relationship to hydrogen ion concentrations. The results of this study indicate that previous Hg ingestion can elicit post-assimilatory impacts on grass shrimp digestive physiology, which may, in turn, influence Hg assimilation during subsequent digestive cycles.

Carbon assimilation and digestive toxicity in naïve grass shrimp (Palaemonetes pugio) exposed to dietary cadmium.

Naïve grass shrimp Palaemonetes pugio were pulse-fed cadmium-contaminated meals containing carbon-14, fluorescent or near-infrared markers and analyzed for carbon assimilation efficiency, gut residence time, feces elimination rate, extracellular digestive protease activity or gut pH. Carbon assimilation efficiency (~83%), minimum gut residence time (~435 min) and proventriculus pH (~5.29 to ~6.01) were not impacted significantly by cadmium ingestion. A dose-dependent decrease in feces elimination rate (from ~14.4 to ~6.4 mm h(-1)) was observed for shrimp for 2 h following minimum gut residence time. Protease activities increased ~2.4-fold over the range of dietary cadmium exposures, however, this variation was not dose-dependent. Differential impacts of cadmium exposure on carbon and cadmium assimilation reported previously are consistent with work involving shrimp subjected to chronic field exposure. The influence of ingested cadmium on feces elimination rate may be related to pre-assimilatory impacts on packaging, intestinal transport or release of feces. Protease activities may have been influenced by pre-assimilatory interactions between available cadmium ions in gut fluid and enzyme-secreting cells of the hepatopancreatic epithelium or direct impacts on active enzymes.

Digestive toxicity in grass shrimp collected along an impact gradient.

Ingested pollutants may elicit digestive toxicity following incorporation into consumer tissues. This post-assimilatory toxicity may include tissue damage influencing synthesis of digestive enzymes, gut transit time and absorption of nutrients as well as pollutants by the gut epithelium. This study investigated impacts of chronic field exposure on gut residence time (GRT), feces elimination rate (FER), extracellular digestive protease activities and gut pH in grass shrimp Palaemonetes pugio. Adult shrimp were collected from differentially impacted sites within the New York/New Jersey Harbor Estuary and fed prepared meals containing fluorescent or near-infrared markers and analyzed for digestive toxicity. Relationships between digestive parameters and assimilation efficiencies (AE) for Cd, Hg and organic carbon reported previously were also analyzed. Minimum GRT did not vary significantly for field-collected shrimp, but was positively correlated with Cd, but not Hg or carbon, AE. FER was not impacted by field exposure. Digestive protease activities exhibited a marked decrease in grass shrimp from impacted field sites relative to reference shrimp. Relationships between the assimilation of elements and digestive physiology in field-collected shrimp suggest that digestive plasticity (increasing GRT) may be important in compensating for post-assimilatory digestive toxicity (reduced protease activities) in order to maintain nutrient assimilation. Stress-induced variability in digestive function among grass shrimp populations may, in turn, enhance the assimilation of non-essential elements, such as Cd.

Taurine regulates insulin release from pancreatic beta cell lines.

BACKGROUND: Pancreatic beta-cells release insulin via an electrogenic response triggered by an increase in plasma glucose concentrations. The critical plasma glucose concentration has been determined to be approximately 3 mM, at which time both insulin and GABA are released from pancreatic beta-cells. Taurine, a beta-sulfonic acid, may be transported into cells to balance osmotic pressure. The taurine transporter (TauT) has been described in pancreatic tissue, but the function of taurine in insulin release has not been established. Uptake of taurine by pancreatic beta-cells may alter membrane potential and have an effect on ion currents. If taurine uptake does alter beta-cell current, it might have an effect on exocytosis of cytoplasmic vesicle. We wished to test the effect of taurine on regulating release of insulin from the pancreatic beta-cell. METHODS: Pancreatic beta-cell lines Hit-TI5 (Syrian hamster) and Rin-m (rat insulinoma) were used in these studies. Cells were grown to an 80% confluence on uncoated cover glass in RPMI media containing 10% fetal horse serum. The cells were then adapted to a serum-free, glucose free environment for 24 hours. At that time, the cells were treated with either 1 mM glucose, 1 mM taurine, 1 mM glucose + 1 mM taurine, 3 mM glucose, or 3 mM glucose + 1 mM taurine. The cells were examined by confocal microscopy for cytoplasmic levels of insulin. RESULTS: In both cell lines, 1 mM glucose had no effect on insulin levels and served as a control. Cells starved of glucose had a significant reduction (p<0.001) in the level of insulin, but this level was significantly higher than all other treatments. As expected, the 3 mM glucose treatment resulted in a statistically lower (p<0.001) insulin level than control cells. Interestingly, 1 mM taurine also resulted in a statistically lower level of insulin (p<0.001) compared to controls when either no glucose or 1 mM glucose was present. Cells treated with 1 mM taurine plus 3 mM glucose showed a level of insulin similar to that of 3 mM glucose alone. CONCLUSIONS: Taurine administration can alter the electrogenic response in beta-cell lines, leading to a change in calcium homeostasis and a subsequent decrease in intracellular insulin levels. The consequence of these actions could represent a method of increasing plasma insulin levels leading to a decrease in plasma glucose levels.

Pharmacological characterization of GABAA receptors in taurine-fed mice.

BACKGROUND: Taurine is one of the most abundant free amino acids especially in excitable tissues, with wide physiological actions. Chronic supplementation of taurine in drinking water to mice increases brain excitability mainly through alterations in the inhibitory GABAergic system. These changes include elevated expression level of glutamic acid decarboxylase (GAD) and increased levels of GABA. Additionally we reported that GABAA receptors were down regulated with chronic administration of taurine. Here, we investigated pharmacologically the functional significance of decreased / or change in subunit composition of the GABAA receptors by determining the threshold for picrotoxin-induced seizures. Picrotoxin, an antagonist of GABAA receptors that blocks the channels while in the open state, binds within the pore of the channel between the beta2 and beta3 subunits. These are the same subunits to which GABA and presumably taurine binds. METHODS: Two-month-old male FVB/NJ mice were subcutaneously injected with picrotoxin (5 mg kg-1) and observed for a) latency until seizures began, b) duration of seizures, and c) frequency of seizures. For taurine treatment, mice were either fed taurine in drinking water (0.05%) or injected (43 mg/kg) 15 min prior to picrotoxin injection. RESULTS: We found that taurine-fed mice are resistant to picrotoxin-induced seizures when compared to age-matched controls, as measured by increased latency to seizure, decreased occurrence of seizures and reduced mortality rate. In the picrotoxin-treated animals, latency and duration were significantly shorter than in taurine-treated animas. Injection of taurine 15 min before picrotoxin significantly delayed seizure onset, as did chronic administration of taurine in the diet. Further, taurine treatment significantly increased survival rates compared to the picrotoxin-treated mice. CONCLUSIONS: We suggest that the elevated threshold for picrotoxin-induced seizures in taurine-fed mice is due to the reduced binding sites available for picrotoxin binding due to the reduced expression of the beta subunits of the GABAA receptor. The delayed effects of picrotoxin after acute taurine injection may indicate that the two molecules are competing for the same binding site on the GABAA receptor. Thus, taurine-fed mice have a functional alteration in the GABAergic system. These include: increased GAD expression, increased GABA levels, and changes in subunit composition of the GABAA receptors. Such a finding is relevant in conditions where agonists of GABAA receptors, such as anesthetics, are administered.

Dibutylphthalate and Tween 80 alter ultrastructure in Candida albicans: implications for peroxisome proliferation.

Phthalates are ubiquitous environmental pollutants associated with endocrine disruption and peroxisome proliferation in experimental animals. In yeasts exposed to environmental chemicals, including phthalates, alterations in cell growth, cellular morphology, and H2O2 detoxification occur. Nutrient availability also influences diverse cellular processes. Differences in responses to environmental stress between Candida albicans and the model yeast, Saccharomyces cerevesiae, have been reported. In this study, we chose C. albicans as an alternate model for testing estrogen-like chemicals because of its high affinity estrogen-binding protein and, in contrast to S. cerevesiae, estrogens are not growth inhibitory for C. albicans. Cultures were grown in either yeast nitrogen dextrose (YND; phosphate limiting) or YNDP (YND plus 100 mmol/L inorganic phosphate). For chemical testing, 0.5% dibutylphthalate (DBP), 0.05% Tween 80, or a combination of the two (DBPT) were incorporated in growth media to investigate the effects of these estrogenic agents on cell proliferation, morphology, and catalase demonstration. We observed significant differences in cell growth related to DBP and changes in cell wall thickness related to both Tween 80 and phosphate. We describe ultrastructural changes including detachment of the outer yeast cell wall layer and presence of putative peroxisomes. Our findings support the proposal that C. albicans may be particularly suitable for use in studies involving cellular responses associated with exposure to estrogenic chemicals contained in complex mixtures.