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Excitotoxicity
Excitotoxicity is the pathological process by which nerve cells are damaged or killed by excessive stimulation by neurotransmitters such as glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate (glutamate receptors) such as the NMDA receptor and AMPA receptor are overactivated by glutamatergic storm. Excitotoxins like NMDA and kainic acid which bind to these receptors, as well as pathologically high levels of glutamate, can cause excitotoxicity by allowing high levels of calcium ions (Ca2+) to enter the cell,.〔〔Manev H, Favaron M, Guidotti A, and Costa E. Delayed increase of Ca2+ influx elicited by glutamate: role in neuronal death. ''Molecular Pharmacoloy.'' 1989 Jul;36(1):106-112. PMID 2568579. Retrieved on January 31, 2007.〕 Ca2+ influx into cells activates a number of enzymes, including phospholipases, endonucleases, and proteases such as calpain. These enzymes go on to damage cell structures such as components of the cytoskeleton, membrane, and DNA.
Excitotoxicity may be involved in spinal cord injury, stroke, traumatic brain injury, hearing loss (through noise overexposure or ototoxicity) and in neurodegenerative diseases of the central nervous system (CNS) such as multiple sclerosis, Alzheimer's disease, amyotrophic lateral sclerosis (ALS), Parkinson's disease, alcoholism or alcohol withdrawal and especially over-rapid benzodiazepine withdrawal, and also Huntington's disease.〔Kim AH, Kerchner GA, and Choi DW. Blocking Excitotoxicity or Glutamatergic Storm. Chapter 1 in ''CNS Neuroprotection''. Marcoux FW and Choi DW, editors. Springer, New York. 2002. Pages 3-36〕 Other common conditions that cause excessive glutamate concentrations around neurons are hypoglycemia. Blood sugars are the primary glutamate removal method from inter-synaptic spaces at the NMDA and AMPA receptor site. Persons in excitotoxic shock must never fall into hypoglycemia. Patients should be given 5% glucose (dextrose) IV drip during excitotoxic shock to avoid a dangerous build up of glutamate around NMDA and AMPA neurons. When 5% glucose (dextrose) IV drip is not available high levels of fructose are given orally. Treatment is administered during the acute stages of excitotoxic shock along with glutamate antagonists. Dehydration should be avoided as this also contributes to the concentrations of glutamate in the inter-synaptic cleft and "status epilepticus can also be triggered by a build up of glutamate around inter-synaptic neurons".
==History==
The harmful effects of glutamate on the central nervous system (CNS) were first observed in 1954 by T. Hayashi, a Japanese scientist who noted that direct application of glutamate to the CNS caused seizure activity, though this report went unnoticed for several years. The toxicity of glutamate was then observed by D. R. Lucas and J. P. Newhouse in 1957, when the subcutaneous injection of monosodium glutamate into newborn mice destroyed the neurons in the inner layers of the retina. Later, in 1969, John Olney discovered that the phenomenon was not restricted to the retina, but occurred throughout the brain, and coined the term excitotoxicity. He also assessed that cell death was restricted to postsynaptic neurons, that glutamate agonists were as neurotoxic as their efficiency to activate glutamate receptors, and that glutamate antagonists could stop the neurotoxicity. Subsequent research by Mark Mattson provided evidence for the involvement of excitotoxicity in Alzheimer's disease, and other age-related neurodegenerative conditions that involve oxidative stress and cellular energy deficits.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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