When compared to other central nervous system (CNS) health issues, chronic neurodegenerative diseases can be far more complicated. Foremostly, because the compromised mitochondrial function has been demonstrated in many neurodegenerative diseases, the resulting problems in energy sources are not as severe as the energy collapse in ischemic stroke. Therefore, if excitotoxicity contributes to neurodegeneration, a different time of chronic excitotoxicity needs to be assumed. In the following article, we will outline what is known about the pathways that may cause excitotoxicity in neurodegenerative diseases. We will specifically discuss that in amyotrophic lateral sclerosis (ALS), Alzheimer’s disease (AD) and Huntington’s disease (HD) as fundamental examples with sufficiently validated animal models in research studies.
Alzheimer’s disease (AD) is one of the main causes of dementia among older adults in the United States. Neuropathologically, AD is characterized as neurodegeneration with extracellular senile plaques made up of ? amyloid (A?) and intraneuronal neurofibrillary tangles of aggregated tau, which initially appear in the hippocampus than then spread as the health issue progresses. Prominent microglial cell activation can also be associated with AD. Hereditary types of AD occur due to mutations in the A? precursor protein, A?PP, or in the presenilins, which are part of the multi-protein complex involved in A? generation. The pathophysiology of AD is complicated and a variety of pathways are included in the synaptic and the cellular degeneration in AD, such as abnormalities in signaling pathways through glycogen synthase kinase-3 beta or mitogen-activated protein kinases, cell cycle re-entry, oxidative stress, or decreased transport of trophic factors and adrenal dysregulation. However, evidence suggests that L-glutamate dysregulation plays a critical role in Alzheimer’s disease.
Research studies demonstrated that primary neurons from transgenic mice overexpressing mutant presenilin are far more sensitive to excitotoxic stimulation in vitro. In vitro, aggregated A? increases both NMDA and kainate receptor-mediated L-glutamate toxicity, perhaps by interrupting neuronal calcium homeostasis. Others have demonstrated that A? can increase neuronal excitability by changing the capacity of glycogen synthase kinase 3? inhibition to decrease NMDA receptor-mediated pathways. Soluble A? oligomers were demonstrated to cause L-glutamate release from astrocytes resulting in dendritic spine loss through over-activation of extrasynaptic NMDA receptors. Moreover, extracellular L-glutamate concentrations were demonstrated to increase in a triple transgenic mouse model of AD, in which a 3-month treatment with the NMDA receptor inhibitor ultimately affected synapse loss. However, further research studies are still required.
Numerous mouse research studies have demonstrated the consequences of AD-like pathology on EAAT expression and/or function. In acute hippocampal slice preparations, A? was shown to interrupt the clearance of synaptically released L-glutamate by diminishing membrane insertion of EAAT2, a result perhaps mediated by oxidative stress. In aged A?PP23 mice, research studies revealed the downregulation of EAAT2 expression in the frontal cortex and hippocampus, which in the frontal cortex was associated with an increase in xCT expression. These changes were associated with a strong tendency toward improved extracellular L-glutamate amounts as measured by microdialysis. In triple transgenic AD mice expressing the amyloid precursor protein mutations K670N and M671L, the presenilin 1 mutation M146V and the tau P301L mutation, a strong and age-dependent decrease of EAAT2 expression was demonstrated. Restoration of EAAT2 activity in the AD mice following treatment with all the ?-lactam antibiotic Cef was associated with a decrease in cognitive impairment and reduced tau pathology. In human AD brains, decreased expression of EAAT2 protein and a decrease in EAAT action was determined. However, this outcome measure could not be replicated by other researchers. On the transcriptome level, research studies discovered exon-skipping splice variations of EAAT2 which reduce glutamate transport activity to be upregulated in human AD brains. From the CSF, several groups demonstrated an increase in glutamate concentrations in AD patients where other groups demonstrated absolutely no change or even diminished levels of L-glutamate associated with Alzheimer’s disease.
In vitro, A? causes L-glutamate discharge from primary microglia through the upregulation of program x?c. Others discovered that it also triggered L-glutamate release from astrocytes through the activation of the ?7 nicotinic acetylcholine receptor. Additionally, xCT, the specific subunit of system x?c is upregulated at the region of senile plaques, possibly in microglial cells, in Thy1-APP751 mice (TgAPP) expressing human APP bearing the Swedish (S: KM595/596NL) and London (L: V6421) mutations after A? injection in the hippocampus. Semiquantitative immunoblot evaluations revealed an upregulation of xCT protein expression in the frontal cortex in elderly A?PP23 mice compared to wild-type controls.
Postmortem research studies show that KYN metabolism affects AD elevated concentrations of KYNA while also discovered in the basal ganglia of both AD sufferers. Utilizing immunohistochemistry, research studies demonstrated immunoreactivity for both IDO and QUIN upregulated in AD brains, particularly in the vicinity of plaques. A? causes IDO expression in human primary macrophages and microglia. Systemic inhibition of KMO ultimately increases brain KYNA levels and ameliorated the phenotype of a mouse model of AD, indicating an upregulation of KYNA may be an endogenous protective reaction, including the IDO inhibitor, coptisine, decreased microglial, astrocytic activation and cognitive impairment in AD mice.
Taken together, along with many other harmful changes, there is evidence for chronic excitotoxicity in AD which can be driven by numerous variables, including the central sensitization of both NMDA receptors, a decrease in L-glutamate and L-aspartate reuptake capacity and an increase in glutamate release through system x?c, as shown in Figure 4. Although the KYN pathway seems to be upregulated in AD, no specific conclusions can be drawn regarding glutamatergic neurotransmission from the upregulation of the two QUIN which was neurotoxic and neuroprotective KYNA.
In many research studies, evidence and outcome measures have demonstrated that glutamate dysregulation and excitotoxicity in many neurological diseases, including AD, HD, and ALS, ultimately lead to neurodegeneration and a variery of symptoms associated with the health issues. The purpose of the following article is to discuss and demonstrate the role that glutamate dysregulation and excitotoxicity plays on neurodegenerative diseases. The mechanisms for excitotoxicity are different for every health issue. – Dr. Alex Jimenez D.C., C.C.S.T. Insight – Dr. Alex Jimenez D.C., C.C.S.T. Insight
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In the article above, we outlined what is known about the pathways which may cause excitotoxicity in neurodegenerative diseases. We also discussed that in amyotrophic lateral sclerosis (ALS), Alzheimer’s disease (AD) and Huntington’s disease (HD) as fundamental examples with sufficiently validated animal models in research studies. The scope of our information is limited to chiropractic, musculoskeletal and nervous health issues as well as functional medicine articles, topics, and discussions. We use functional health protocols to treat injuries or chronic disorders of the musculoskeletal system. To further discuss the subject matter above, please feel free to ask Dr. Alex Jimenez or contact us at 915-850-0900 .
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- Lewerenz, Jan, and Pamela Maher. Chronic Glutamate Toxicity in Neurodegenerative Diseases-What Is the Evidence? Frontiers in Neuroscience, Frontiers Media S.A., 16 Dec. 2015, www.ncbi.nlm.nih.gov/pmc/articles/PMC4679930/.
Additional Topic Discussion: Chronic Pain
Sudden pain is a natural response of the nervous system which helps to demonstrate possible injury. By way of instance, pain signals travel from an injured region through the nerves and spinal cord to the brain. Pain is generally less severe as the injury heals, however, chronic pain is different than the average type of pain. With chronic pain, the human body will continue sending pain signals to the brain, regardless if the injury has healed. Chronic pain can last for several weeks to even several years. Chronic pain can tremendously affect a patient’s mobility and it can reduce flexibility, strength, and endurance.
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