Mechanism Of Taurine Protecting Retinal Ganglion Cells Against Glutamate-induced Excitotoxicity In Rat

As the major neurotransmitter in mammalian central nervous system (CNS), Glutamate (Glu) contributes to excitatory neurotransmission in many central synapses. However, extracellular glutamate in excess can lead to neuronal injury or death, this so-called glutamate-induced excitotoxicity has been implicated as an important mechanism underlying a variety of brain injuries such as ischemia, trauma,and neurodegenerative diseases of CNS. The key reason for this excitotoxicity is because glutamate can increase the concentration of intracellular Ca2+ ([Ca2+];) in neurons, and thus finally induce neuronal death by activating proteases, phospholipases, and endonucleases. Several mechanisms explain how glutamate increases intracellular Ca2+ level. The activation of Ca2+-permeable N-methyl-D-aspartate (NMDA) receptors, opening of voltage-dependent Ca2+ channels following membrane depolarization induced by activation of 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors, and/or activation of metabotropic glutamate receptors(mGluRs) linked to phosphoinositide hydrolysis, which releases Ca2+ from intracellular stores are leading factors.Glutamate is the principal excitatory neurotransmitter in retina as well as in CNS, and retinal ganglion cells (RGCs) are primarily glutamatergic. Glutamate-induced excitotoxicity has been implicated as an important cause of death of RGCs in retinal ischemia, glaucoma, and degenerative retinopathy. Over-stimulating NMDA receptors contributes to the death of glutamate-induced inner retinal neurons include RGCs. Some researches indicate that low concentration glutamate can activate Ca2+-permeable AMPA-KA receptors in cultured RGCs, leading to increases in [Ca2+], and decreased survival, suggesting that the activation of AMPA-KA receptors may also play a role in neurotoxicity in retinal neurons. Accumulating evidence suggests that blocking glutamate excitotoxicity can protect RGCs and reduce RGCs death, which is the theoretical foundation of the current two methods that are provided to protect RGCs from glutamate-induced excitotoxicity: (1) Glutamate -receptor antagonists. However, since NMDA-receptor activities are essential for normal neuronal function, potential neuroprotective agents that block virtually all NMDA-receptoractivity will have unacceptable clinical side effects. (2) Agents to reduce Ca2+ overload, such as Ca2+-channel blocker. It is just the beginning of these kinds of research.Taurine, another one of the most abundant free amino acid in CNS other than glutamate, is a 0 -amino acid. As a matter of fact, the concentration of taurine in the brain even exceeds that of glutamate in many mammals. Taurine is known to act as an osmoregulator and an inhibitory neuromodulator in CNS. It relieves epileptic seizures and serves as a neuroprotector, which may explain its protective effect against glutamate-induced excitotoxicity. There is a consensus that taurine is a powerful agent in regulating and reducing the intracellular calcium levels in neurons, it functions in retina as osmoregulatory agent, anti-oxidant, and Ca2* modulator. In short, taurine is an essential nutrient agent in retinal development, but the effects of taurine on glutamate-induced excitotoxicity in RGCs are still poorly understood.Based on the researches from both at home and abroad, the purpose of this study was to determine whether taurine has neuroprotective effects in experimental in vitro and in vivo models. During in vivo study, we used a rat model of retinal glutamate excitotoxicity induced by intravitreal injection of 40nmol glutamate. The retinal damage was evaluated by counting the number of cells in the ganglion cell layer (GCL), examining the a- and b-waves in the electroretinogram (ERG), Hematoxylin-eosin (HE) staining, and observing ultrastructure of retina. For the purpose of advanced research in the protective effects of taurine, situ TUNEL test, immunohistochemistry, western blotting, and RT-PCR methods were used to study the RGCs apoptosis, the expression of glutamate, Thy-1, Neuro...