Increased GAD-positive neurons in the cortex of taurine-fed mice

Taurine Regulation of Neuroendocrine Function

Taurine (2-aminoethanesulfonic acid) is a sulfur-containing amino acid. It is one of the most abundant free amino acids in many excitable tissues, including the brain, skeletal and cardiac muscles. Physiological actions of taurine are widespread and include regulation of plasma glucose levels, bile acid conjugation, detoxification, membrane stabilization, blood pressure regulation, osmoregulation, neurotransmission, and modulation of mitochondria function and cellular calcium levels. Taurine plays an important role in modulating glutamate and GABA neurotransmission and prevents excitotoxicity in vitro primarily through modulation of intracellular calcium homeostasis. Taurine supplementation prevents age-dependent decline of cognitive functions. Because of the wide spread actions of taurine, its levels are highly regulated through enzymatic biosynthesis or dietary intake. Furthermore, depletion of endogenous or dietary supplementation of exogenous taurine have been shown to induce wide spread actions on multiple organs. Cysteine sulfonic acid decarboxylase (CSAD) was first identified in the liver and is thought to be the rate-limiting enzyme in taurine biosynthesis. CSAD mRNA is expressed in the brain in astrocytes. Homozygous knockout mice lacking CSAD (CSAD-KO) have very reduced taurine content and show severe functional histopathology in the visual system, skeletal system, heart, pancreas and brain. Conversely, dietary supplementation of taurine results in significant health benefits acting through the same organ systems. Fluctuation of taurine bioavailability lead to changes in the expression levels of taurine transporters in neuronal plasma membranes, endothelial cells forming the blood-brain barrier and proximal cells of the kidneys. Suggesting a highly regulated mechanism for maintaining taurine homeostasis and organ systems function. Here we show how alterations in taurine levels directly affect the function of one organ system and through functional interaction and compensatory adaptation; these effects extend to another organ systems with focus on the nervous system.

Taurine activates glycine and GABAA receptor currents in anoxia-tolerant painted turtle pyramidal neurons

Unlike anoxia-intolerant mammals, painted turtles can survive extended periods without oxygen. This is partly accomplished by an anoxia-mediated increase in gamma-aminobutyric acid (GABA) release, which activates GABA receptors and mediates spike arrest in turtle neurons via shunting inhibition. Extracellular taurine levels also increase during anoxia; why this occurs is unknown but it is speculated that glycine and/or GABA_A/B receptors are involved. Given the general importance of inhibitory neurotransmission in the anoxia-tolerant painted turtle brain, we investigated the function of taurine as an inhibitory neuromodulator in turtle pyramidal neurons. Using whole-cell patch-clamp electrophysiological methods to record from neurons within a cortical brain sheet, we found that taurine depolarized membrane potential by ∼8 mV, increased whole-cell conductance ∼2-fold, and induced an inward current that possessed characteristics similar to GABA- and glycine-evoked currents. These effects were mitigated following glycine receptor antagonism with strychnine and GABA_A receptor antagonism with gabazine, bicuculine or picrotoxin, but were unchanged following GABA_B or glutamatergic receptor inhibition. These data indicate that a high concentration of taurine in vitro mediates its effects through both glycine and GABA_A receptors, and suggests that taurine, in addition to GABA, inhibits neuronal activity during anoxia in the turtle cortex.

Effects of Taurine Supplementation on Neuronal Excitability and Glucose Homeostasis

In this study we examined the role of chronic taurine supplementation on plasma glucose homeostasis and brain excitability through activation of the insulin receptor. FVB/NJ male mice were supplemented with taurine in drinking water (0.05% w/v) for 4 weeks and subjected to a glucose tolerance test (7.5 mg/kg BW) after 12 h fasting. We found that taurine-fed mice were slightly hypoglycemic prior to glucose injection and showed significantly reduced plasma glucose at 30 and 60 min post-glucose injection when compared to control mice. Previously, we reported that taurine supplementation induces biochemical changes that target the GABAergic system. Those studies show that taurine-fed mice are hyperexcitable, have reduced GABAA receptors expression and increased GAD and somatostatin expression in the brain. In this study, we found that taurine-fed mice had a significant increase in insulin receptor (IR) immuno-reactivity in the pancreas and all brain regions examined. At the mRNA level, we found that the IR showed differential regional expression. Surprisingly, we found that neurons express the gene for insulin and that taurine had a significant role in regulating insulin gene expression. We propose that increased insulin production and secretion in taurine-fed mice cause an increase activation of the central IR and may be partially responsible for the increased neuronal excitability observed in taurine supplemented mice. Furthermore, the high levels of neuronal insulin expression and its regulation by taurine implicates taurine in the regulation of metabolic homeostasis.

Taurine supplementation to anti-seizure drugs as the promising approach to treat pharmacoresistant epilepsy: A pre-clinical study

Background Pharmacoresistance leads to severe, irreversible disabilities and premature death in ∼30% cases of epilepsy despite adequate and appropriate treatment with available anti-seizure drugs (ASDs) without any underlying cause. In light of the large body of evidence which suggests the anti-seizure action of taurine in experimental animals and its wide safety margins in human, supplementation of this inhibitory amino-sulfonic acid to available ASDs seems promising to treat pharmacoresistant epilepsy.

Methods We examined the anti-seizure effect of lamotrigine (15 mg/kg), levetiracetam (40 mg/kg), carbamazepine (40 mg/kg), phenytoin (35 mg/kg) & taurine (50, 100 & 200 mg/kg) in lamotrigine pre-treated pentylenetetrazole-kindled mice (LPK) which mimic core features of pharmacoresistant epilepsy, either alone ASDs or in combinations whereby three different doses of taurine were supplemented with tested ASDs.

Results Both, the ASDs and the taurine were failed to suppress generalized tonic-clonic seizures in LPK mice. However, taurine supplementation clearly restored the anti-seizure effect of tested ASDs. Further neurochemical studies revealed that higher levels of taurine in the hippocampus and cerebral cortex restored the imbalance between major excitatory neurotransmitters glutamate & its inhibitory counterpart GABA.

Conclusions These findings emphasize that supplementation of taurine with ASDs may be useful to treat pharmacoresistant epilepsy. Thus, further clinical validation is encouraged.

Sex-Specific Effects of Organophosphate Diazinon on the Gut Microbiome and Its Metabolic Functions

BACKGROUND

There is growing recognition of the significance of the gut microbiome to human health, and the association between a perturbed gut microbiome with human diseases has been established. Previous studies also show the role of environmental toxicants in perturbing the gut microbiome and its metabolic functions. The wide agricultural use of diazinon, an organophosphate insecticide, has raised serious environmental health concerns since it is a potent neurotoxicant. With studies demonstrating the presence of a microbiome-gut-brain axis, it is possible that gut microbiome perturbation may also contribute to diazinon toxicity.

OBJECTIVES

We investigated the impact of diazinon exposure on the gut microbiome composition and its metabolic functions in C57BL/6 mice.

METHODS

We used a combination of 16S rRNA gene sequencing, metagenomics sequencing, and mass spectrometry-based metabolomics profiling in a mouse model to examine the functional impact of diazinon on the gut microbiome.

RESULTS

16S rRNA gene sequencing revealed that diazinon exposure significantly perturbed the gut microbiome, and metagenomic sequencing found that diazinon exposure altered the functional metagenome. Moreover, metabolomics profiling revealed an altered metabolic profile arising from exposure. Of particular significance, these changes were more pronounced for male mice than for female mice.

CONCLUSIONS

Diazinon exposure perturbed the gut microbiome community structure, functional metagenome, and associated metabolic profiles in a sex-specific manner. These findings may provide novel insights regarding perturbations of the gut microbiome and its functions as a potential new mechanism contributing to diazinon neurotoxicity and, in particular, its sex-selective effects. Citation: Gao B, Bian X, Mahbub R, Lu K. 2017. Sex-specific effects of organophosphate diazinon on the gut microbiome and its metabolic functions. Environ Health Perspect 125:198-206; http://dx.doi.org/10.1289/EHP202.

Taurine mitigates cognitive impairment induced by chronic co-exposure of male Wistar rats to chlorpyrifos and lead acetate

Organophosphate pesticides and heavy metals are ubiquitous environmental pollutants and neurotoxicants. We investigated the effects of taurine (an antioxidant; TA) on oxidative stress and cognition in male Wistar rats co-treated with chlorpyrifos (an organophosphate pesticide; CPF) and lead acetate (heavy metal; LA). The Wistar rats were divided into 5 groups of 10 rats each. The first two groups were administered with distilled water and soya oil respectively. The remaining three groups were administered with taurine (TA), 50 mg/kg body weight, CPF+LA group [CPF (4.25 mg/kg, 1/20 LD₅₀] and LA (233.25 mg/kg, 1/20 LD₅₀) and TA+CPF+LA group [TA (50 mg/kg), CPF (4.25 mg/kg) and LA (233.25 mg/kg)]. The xenobiotics were administered once daily by oral gavage for 16 weeks. The results showed reductions in the activities of brain antioxidant enzymes and acetylcholinesterase, increased lipoperoxidation and histopathological alterations of the cerebral cortex in the CPF+LA group. However, TA mitigated perturbations in the activities of the antioxidant enzymes and acetylcholinesterase, counteracted oxidative stress and brain lipoperoxidation and attenuated neuronal degeneration induced by joint CPF and LA-induced neurotoxicity. The results suggested that TA is neuroprotective following chronic co-exposure of rats to CPF and LA.

Taurine improves congestive functions in a mouse model of fragile X syndrome

Increased seizure susceptibility is a feature of the mouse model for fragile X that has parallels in the hyperarousal and prevalence of seizures in the fragile X syndrome. Our investigation of the basis for the increased seizure susceptibility of the fragile X mouse indicated a reduction in GABA(A) receptor expression and increased expression of glutamic acid decarboxylase (GAD), the enzyme responsible for GAB(A) synthesis. Taurine-fed mice also show these GABAergic alterations. However, unlike fragile X mice, taurine-fed mice show a significant increase in memory acquisition and retention. This discordance implies that there may be divergent events downstream of the biochemical changes in the GABAergic system in these two mouse models. To investigate the divergence of these two models we fed taurine to fragile X mice. Our preliminary data shows that taurine supplementation to fragile X mice resulted in a significant improvement in acquisition of a passive avoidance task. Since taurine is an agonist for GABA(A) receptor, we suggest that chronic activation of GABA(A) receptors and the ensuing alterations in the GABAergic system may have beneficial effects in ameliorating the learning deficits characteristic of the fragile X syndrome.

Functional implication of taurine in aging

Age-related impairment of central functions is though to result from alterations of neurochemical indices of synaptic function. These neurochemical modifications involve structural proteins, neurotransmitters, neuropeptides and related receptors. Several studies demonstrated that GABA receptors, glutamic acid decarboxylase (GAD65&67), and different subpopulations of GABAergic neurons are markedly decreased in experimental animal brains during aging. Thus, the age-related decline in cognitive functions could be attributable, at least in part, to decrements in the function of the GABAergic inhibitory neurotransmitter system. In this study we show that chronic supplementation of taurine to aged mice significantly ameliorated the age-dependent decline in memory acquisition and retention, and caused alterations in the GABAergic system. These changes include increased levels of the neurotransmitters GABA and glutamate, increased expression of glutamic acid decarboxylase and the neuropeptide somatostatin and increased in the number of somatostatin-positive neurons. These specific alterations of the inhibitory system caused by taurine treatment oppose those naturally-occurring during aging, and suggest a protective role of taurine in this process. 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 might help forestall the age-related decline in cognitive functions through interaction with the GABAergic system.

Selective resistance of taurine-fed mice to isoniazide-potentiated seizures: in vivo functional test for the activity of glutamic acid decarboxylase

Taurine, 2-aminoethanesulfonic acid, is one of the most abundant free amino acids especially in excitable tissues, with wide physiological actions. We have previously reported that in mice, supplementation of the drinking water with taurine induces alterations in the inhibitory GABAergic system. In taurine-fed mice we found that the expression level of glutamic acid decarboxylase (GAD), the enzyme responsible for GABA synthesis, is elevated. Increased expression of GAD was accompanied by increased levels of GABA. Here, we investigated pharmacologically the functional significance of taurine-induced increase in GAD expression by determining the threshold for kainic acid-induced seizures after partial inhibition of GAD activity with isoniazide. We found that taurine-fed mice have elevated GAD expression and showed a higher threshold for seizure onset when compared with age-matched controls. Thus, taurine-fed mice have a functional increase in GAD activity which offers some protection in this seizure model. Furthermore, this pharmacological manipulation can be used to determine the level of GAD activity in other model systems that show alterations in GAD expression.