Relative preservation of neurons
Neuroprotection refers to the relative preservation of neuronal structure and/or function. In the case of an ongoing insult (a neurodegenerative insult) the relative preservation of neuronal integrity implies a reduction in the rate of neuronal loss over time, which can be expressed as a differential equation.
Mechanisms in neurodegeneration, and associated treatments.
It is a widely explored treatment option for many central nervous system disorders including neurodegenerative diseases, stroke, traumatic brain injury, spinal cord injury, and acute management of neurotoxin consumption (i.e. methamphetamine overdoses). Neuroprotection aims to prevent or slow disease progression and secondary injuries by halting or at least slowing the loss of neurons.
Despite differences in symptoms or injuries associated with CNS disorders, many of the mechanisms behind neurodegeneration are the same. Common mechanisms of neuronal injury include decreased delivery of oxygen and glucose to the brain, energy failure, increased levels in oxidative stress, mitochondrial dysfunction, excitotoxicity, inflammatory changes, iron accumulation, and protein aggregation. Of these mechanisms, neuroprotective treatments often target oxidative stress and excitotoxicity—both of which are highly associated with CNS disorders. Not only can oxidative stress and excitotoxicity trigger neuron cell death but when combined they have synergistic effects that cause even more degradation than on their own. Thus limiting excitotoxicity and oxidative stress is a very important aspect of neuroprotection. Common neuroprotective treatments are glutamate antagonists and antioxidants, which aim to limit excitotoxicity and oxidative stress respectively.
Excitotoxicity.
Glutamate excitotoxicity is one of the most important mechanisms known to trigger cell death in CNS disorders. Over-excitation of glutamate receptors, specifically NMDA receptors, allows for an increase in calcium ion (Ca2+) influx due to the lack of specificity in the ion channel opened upon glutamate binding. As Ca2+ accumulates in the neuron, the buffering levels of mitochondrial Ca2+ sequestration are exceeded, which has major consequences for the neuron. Because Ca2+ is a secondary messenger and regulates a large number of downstream processes, accumulation of Ca2+ causes improper regulation of these processes, eventually leading to cell death. Ca2+ is also thought to trigger neuroinflammation, a key component in all CNS disorders.
Glutamate antagonists.
Glutamate antagonists are the primary treatment used to prevent or help control excitotoxicity in CNS disorders. The goal of these antagonists is to inhibit the binding of glutamate to NMDA receptors such that accumulation of Ca2+ and therefore excitotoxicity can be avoided. Use of glutamate antagonists presents a huge obstacle in that the treatment must overcome selectivity such that binding is only inhibited when excitotoxicity is present. A number of glutamate antagonists have been explored as options in CNS disorders, but many are found to lack efficacy or have intolerable side effects. Glutamate antagonists are a hot topic of research. Below are some of the treatments that have promising results for the future:
Oxidative stress.
Increased levels of oxidative stress can be caused in part by neuroinflammation, which is a highly recognized part of cerebral ischemia as well as many neurodegenerative diseases including Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis. The increased levels of oxidative stress are widely targeted in neuroprotective treatments because of their role in causing neuron apoptosis. Oxidative stress can directly cause neuron cell death or it can trigger a cascade of events that leads to protein misfolding, proteasomal malfunction, mitochondrial dysfunction, or glial cell activation. If one of these events is triggered, further neurodegradation is caused as each of these events causes neuron cell apoptosis. By decreasing oxidative stress through neuroprotective treatments, further neurodegradation can be inhibited.
Antioxidants.
Antioxidants are the primary treatment used to control oxidative stress levels. Antioxidants work to eliminate reactive oxygen species, which are the prime cause of neurodegradation. The effectiveness of antioxidants in preventing further neurodegradation is not only disease dependent but can also depend on gender, ethnicity, and age. Listed below are common antioxidants shown to be effective in reducing oxidative stress in at least one neurodegenerative disease:
Other approaches to neuroprotection.
Stimulants.
NMDA receptor stimulants can lead to glutamate and calcium excitotoxicity and neuroinflammation. Some other stimulants, in appropriate doses, can however be neuroprotective.
Neuroprotectants (cerebroprotectants) for acute ischemic stroke.
When applied to protecting the brain from the effects of acute ischemic stroke, neuroprotectants are often called cerebroprotectants. Over 150 drugs have been tested in clinical trials, leading to the regulatory approval of tissue plasminogen activator in several countries, and the approval of edaravone in Japan.
Exercise.
Exercise has been put forward as helpful for neuroprotection.
Further neuroprotective treatments.
More neuroprotective treatment options exist that target different mechanisms of neurodegradation. Continued research is being done in an effort to find any method effective in preventing the onset or progression of neurodegenerative diseases or secondary injuries. These include:
References.
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Further reading.
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