Taurine Targets the GluN2b-Containing NMDA Receptor Subtype

The impact of chronic fentanyl administration on the cerebral cortex in mice: Molecular and histological effects

PURPOSE

Fentanyl, a fully synthetic opioid, is widely used for severe pain management and has a huge abuse potential for its psychostimulant effects. Unlike other opioids, the neurotoxic effects of chronic fentanyl administration are still unclear. In particular, little is known about its effect on the cerebral cortex. The current study aims to test the chronic toxicity of fentanyl in the mice model.

METHODS

Adult male Balb/c mice were chronically treated with low (0.05 mg/kg, i.p) and high (0.1 mg/kg, i.p) doses of fentanyl for 5 consecutive weeks, and various neurotoxic parameters, including apoptosis, oxidative stress, and neuroinflammatory response were assessed in the cortex. Potential histological as well as neurochemical changes were also evaluated.

RESULTS

The results of this study show that chronic fentanyl administration induced intense levels of apoptosis, oxidative stress, and neuroinflammation in the cerebral cortex. These findings were found to be correlated with histopathological characteristics of neural degeneration and white matter injury. Moreover, fentanyl administration was found to reduce the expression of both NMDA receptor subunits and dopamine receptors and elevate the level of epidermal growth factor (EGF).

CONCLUSION

Fentanyl administration induced neurotoxic effects in the mouse cerebral cortex that could be primarily mediated by the evoked oxidative-inflammatory response. The altered expression of NMDA receptors, dopamine receptors, and EGF suggests the pernicious effects of fentanyl addiction that may end in the development of toxic psychosis.

Emergence of taurine as a therapeutic agent for neurological disorders

Taurine is a sulfur-containing, semi-essential amino acid that occurs naturally in the body. It alternates between inflammation and oxidative stress-mediated injury in various disease models. As part of its limiting functions, taurine also modulates endoplasmic reticulum stress, Ca2+ homeostasis, and neuronal activity at the molecular level. Taurine effectively protects against a number of neurological disorders, including stroke, epilepsy, cerebral ischemia, memory dysfunction, and spinal cord injury. Although various therapies are available, effective management of these disorders remains a global challenge. Approximately 30 million people are affected worldwide. The design of taurine formation could lead to potential drugs/supplements for the health maintenance and treatment of central nervous system disorders. The general neuroprotective effects of taurine and the various possible underlying mechanisms are discussed in this review. This article is a good resource for understanding the general effects of taurine on various diseases. Given the strong evidence for the neuropharmacological efficacy of taurine in various experimental paradigms, it is concluded that this molecule should be considered and further investigated as a potential candidate for neurotherapeutics, with emphasis on mechanism and clinical studies to determine efficacy.

Taurine Alleviates Chronic Social Defeat Stress-Induced Depression by Protecting Cortical Neurons from Dendritic Spine Loss

Abnormal amino acid metabolism in neural cells is involved in the occurrence and development of major depressive disorder. Taurine is an important amino acid required for brain development. Here, microdialysis combined with metabonomic analysis revealed that the level of taurine in the extracellular fluid of the cerebral medial prefrontal cortex (mPFC) was significantly reduced in mice with chronic social defeat stress (CSDS)-induced depression. Therefore, taurine supplementation may be usable an intervention for depression. We found that taurine supplementation effectively rescued immobility time during a tail suspension assay and improved social avoidance behaviors in CSDS mice. Moreover, taurine treatment protected CSDS mice from impairments in dendritic complexity, spine density, and the proportions of different types of spines. The expression of N-methyl D-aspartate receptor subunit 2A, an important synaptic receptor, was largely restored in the mPFC of these mice after taurine supplementation. These results demonstrated that taurine exerted an antidepressive effect by protecting cortical neurons from dendritic spine loss and synaptic protein deficits.

Effects of Taurine in Mice and Zebrafish Behavioral Assays With Translational Relevance to Schizophrenia

BACKGROUND

Altered redox state and developmental abnormalities in glutamatergic and GABAergic transmission during development are linked to the behavioral changes associated with schizophrenia. As an amino acid that exerts antioxidant and inhibitory actions in the brain, taurine is a potential candidate to modulate biological targets relevant to this disorder. Here, we investigated in mice and zebrafish assays whether taurine prevents the behavioral changes induced by acute administration of MK-801 (dizocilpine), a glutamate N-methyl-D-aspartate (NMDA) receptor antagonist.

METHODS

C57BL/6 mice were i.p. administered with saline or taurine (50, 100, and 200 mg/kg) followed by MK-801 (0.15 mg/kg). Locomotor activity, social interaction, and prepulse inhibition of the acoustic startle reflex were then assessed in different sets of animals. Zebrafish were exposed to tank water or taurine (42, 150, and 400 mg/L) followed by MK-801 (5 µM); social preference and locomotor activity were evaluated in the same test.

RESULTS

MK-801 induced hyperlocomotion and disrupted sensorimotor gating in mice; in zebrafish, it reduced sociability and increased locomotion. Taurine was mostly devoid of effects and did not counteract NMDA antagonism in mice or zebrafish.

DISCUSSION

Contradicting previous clinical and preclinical data, taurine did not show antipsychotic-like effects in the present study. However, it still warrants consideration as a preventive intervention in animal models relevant to the prodromal phase of schizophrenia; further studies are thus necessary to evaluate whether and how taurine might benefit patients.

New solutions for old challenges in glaucoma treatment: is taurine an option to consider?

Glaucoma is a range of progressive optic neuropathies characterized by progressive retinal ganglion cell loss and visual field defects. It is recognized as a leading cause of irreversible blindness affecting more than 70 million people worldwide. Currently, reduction of intraocular pressure, a widely recognized risk factor for glaucoma development, is the only pharmacological strategy for slowing down retinal ganglion cell loss and disease progression. However, retinal ganglion cell death and visual field loss have been observed in normotensive glaucoma, suggesting that the disease process is partially independent of intraocular pressure. Taurine is one of the agents that have attracted attention of researchers recently. Taurine has been shown to be involved in multiple cellular functions, including a central role as a neurotransmitter, as a trophic factor in the central nervous system development, as an osmolyte, as a neuromodulator, and as a neuroprotectant. It also plays a role in the maintenance of the structural integrity of the membranes and in the regulation of calcium transport and homeostasis. Taurine is known to prevent N-methyl-D-aspartic acid-induced excitotoxic injury to retinal ganglion cells. A recently published study clearly demonstrated that taurine prevents retinal neuronal apoptosis both in vivo and in vitro. Protective effect of taurine may be attributed to direct inhibition of apoptosis, an activation of brain derived neurotrophic factor-related neuroprotective mechanisms and reduction of retinal oxidative and nitrosative stresses. Further studies are needed to fully explore the potential of taurine as a neuroprotective agent, so that it can be applied in clinical practice, particularly for the treatment of glaucoma. The objective of current review was to summarize recent evidence on neuroprotective properties of taurine in glaucoma.

Taurine protects against NMDA-induced retinal damage by reducing retinal oxidative stress

This study aimed to evaluate effect of TAU on NMDA-induced changes in retinal redox status, retinal cell apoptosis and retinal morphology in Sprague-Dawley rats. Taurine was injected intravitreally as pre-, co- or post-treatment with NMDA and 7 days post-treatment retinae were processed for estimation of oxidative stress, retinal morphology using H&E staining and retinal cell apoptosis using TUNEL staining. Treatment with TAU, particularly pre-treatment, significantly increased retinal glutathione, superoxide dismutase and catalase levels compared to NMDA-treated rats; whereas, the levels of malondialdehyde reduced significantly. Reduction in retinal oxidative stress in TAU pre-treated group was associated with significantly greater fractional thickness of ganglion cell layer within inner retina and retinal cell density in inner retina. TUNEL staining showed significantly reduced apoptotic cell count in TAU pre-treated group compared to NMDA group. It could be concluded that TAU protects against NMDA-induced retinal injury in rats by reducing retinal oxidative stress.

Effect of Magnesium Acetyltaurate and Taurine on Endothelin1-Induced Retinal Nitrosative Stress in Rats

PURPOSE

Retinal ganglion cell apoptosis in glaucoma is associated with elevated levels of endothelin-1 (ET1), a potent vasoconstrictor. ET1-induced retinal ischemia leads to altered expression of nitric oxide synthase (NOS) isoforms leading to increased formation of nitric oxide (NO) and retinal nitrosative stress. Since magnesium (Mg) is known to improve endothelial functions and reduce oxidative stress and taurine (TAU) possesses potent antioxidant properties, we investigated the protective effects of magnesium acetyltaurate (MgAT) against ET1-induced nitrosative stress and retinal damage in rats. We also compared the effects of MgAT with that of TAU alone.

METHODS

Sprague Dawley rats were intravitreally injected with ET1. MgAT and TAU were administered as pre-, co-, or posttreatment. Subsequently, the expression of NOS isoforms was detected in retina by immunohistochemistry, retinal nitrotyrosine level was estimated using ELISA, and retinal cell apoptosis was detected by TUNEL staining.

RESULTS

Intravitreal ET1 caused a significant increase in the expressions of nNOS and iNOS while eNOS expression was significantly reduced compared to vehicle treated group. Administration of both MgAT and TAU restored the altered levels of NOS isoform expression, reduced retinal nitrosative stress and retinal cell apoptosis. The effect of MgAT, however, was greater than that of TAU alone.

CONCLUSIONS

MgAT and TAU prevent ET1-induced retinal cell apoptosis by reducing retinal nitrosative stress in Sprague Dawley rats. Addition of TAU to Mg seems to enhance the efficacy of TAU compared to when given alone. Moreover, the pretreatment with MgAT/TAU showed higher efficacy compared to co- or posttreatment.

Taurine as an Essential Neuromodulator during Perinatal Cortical Development

A variety of experimental studies demonstrated that neurotransmitters are an important factor for the development of the central nervous system, affecting neurodevelopmental events like neurogenesis, neuronal migration, programmed cell death, and differentiation. While the role of the classical neurotransmitters glutamate and gamma-aminobutyric acid (GABA) on neuronal development is well established, the aminosulfonic acid taurine has also been considered as possible neuromodulator during early neuronal development. The purpose of the present review article is to summarize the properties of taurine as neuromodulator in detail, focusing on the direct involvement of taurine on various neurodevelopmental events and the regulation of neuronal activity during early developmental epochs. The current knowledge is that taurine lacks a synaptic release mechanism but is released by volume-sensitive organic anion channels and/or a reversal of the taurine transporter. Extracellular taurine affects neurons and neuronal progenitor cells mainly via glycine, GABA(A), and GABA(B) receptors with considerable receptor and subtype-specific affinities. Taurine has been shown to directly influence neurogenesis in vitro as well as neuronal migration in vitro and in vivo. It provides a depolarizing signal for a variety of neuronal population in the immature central nervous system, thereby directly influencing neuronal activity. While in the neocortex, taurine probably enhance neuronal activity, in the immature hippocampus, a tonic taurinergic tone might be necessary to attenuate activity. In summary, taurine must be considered as an essential modulator of neurodevelopmental events, and possible adverse consequences on fetal and/or early postnatal development should be evaluated for pharmacological therapies affecting taurinergic functions.

Taurine and Epidermal Growth Factor Belong to the Signature of First-Episode Psychosis

This study evaluated the levels of two amino acid derivatives taurine and spermine in first-episode psychosis (FEP) patients and their response to antipsychotic treatment. The levels of taurine and spermine were significantly up-regulated in antipsychotic-naïve FEP patients compared to control subjects (CS). Treatment of FEP patients with antipsychotic drugs significantly reduced the positive symptoms of schizophrenia. This positive effect was accompanied by a significant reduction of taurine and spermine to the levels measured in CS. General linear model was used to establish associations of taurine and spermine with the levels of cytokines and growth factors, measured in our previous experiments using the same study sample. There was a strong association between taurine and epidermal growth factor (EGF). Both biomarkers significantly correlated with the disease symptoms as well as with the effectiveness of antipsychotic treatment. Accordingly one can conclude that taurine and EGF belong to the signature of FEP. Most probably they reflect altered oxidative stress and corrupted function of N-methyl-D-aspartate (NMDA) receptors in FEP.

Neuroprotection of taurine against reactive oxygen species is associated with inhibiting NADPH oxidases

It is well established that taurine shows potent protection against glutamate-induced injury to neurons in stroke. The neuroprotection may result from multiple mechanisms. Increasing evidences suggest that NADPH oxidases (Nox), the primary source of superoxide induced by N-methyl-d-aspartate (NMDA) receptor activation, are involved in the process of oxidative stress. We found that 100μM NMDA induced oxidative stress by increasing the reactive oxygen species level, which contributed to the cell death, in vitro. Neuron cultures pretreated with 25mM taurine showed lower percentage of death cells and declined reactive oxygen species level. Moreover, taurine attenuated Nox2/Nox4 protein expression and enzyme activity and declined intracellular calcium intensity during NMDA-induced neuron injury. Additionally, taurine also showed neuroprotection against H2O2-induced injury, accompanying with Nox inhibition. So, we suppose that protection of taurine against reactive oxygen species during NMDA-induced neuron injury is associated with Nox inhibition, probably in a calcium-dependent manner.