A stroke occurs when the supply of blood to a region of the brain is interrupt. It is the second death cause in the world and its victims could suffer from permanent disability or even death. The pathophysiology of this neurodegenerative disease is complex, Stroke involves excitotoxicity mechanisms. The ultimate result of ischemic pathway generated by acute stroke is neuronal death along with an irreversible loss of neuronal function. The therapeutic strategy in stroke is to focus on: reinstallation of cerebral blood flow and to reduce the destruction effect on neurons (Deb et al., 2009).
The intensity of brain injury reasoned by a stroke varies considerably and depends on the blood vessel involved, duration, location and the severity of the ischemic region. A stroke can remain undiagnosed if simply a small blood vessel is blocked, however if a large blood vessel is blocked it can direct to long-term disability or death. There are two sorts of strokes: Ischemic and haemorrhage. The former is the most common sort, usually caused by a blood clot or a piece of fatty material blocking an artery limiting its blood supply. Haemorrhagic stroke occurs due to the weakening of the blood vessel's walls in the brain causing them to burst resulting in bleeding and brain injury.
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Stroke is an emergency medical situation and immediate treatment is crucial. To function appropriately, a continuous flow of blood is required to deliver oxygen and nutrients to brain cells. Interestingly the brain, unlike any other organs, is unable to synthesise or stock energy substance and depends completely on blood-borne glucose for its energy supply. When the blood flow to a region of the brain is all of a sudden blocked, ischemia cascade is triggered and substantial fall in energy sources production, such as adenine triphosphate (ATP). This has a considerably negative and irreversible effect on neurons survival and sets off a series of connected events ending in brain cells injury or death. This is due to an increase of both extracellular glutamate concentration and a sensitisation of neurones through a mechanism of excitotoxicity (Doble, 1999).
Excitatory amino acid neurotransmitter:
Glutamate is the predominant excitatory neurotransmitter in the human central nervous system. With levels that are 1000-10000 folds higher than other neurotransmitters, for example serotonin, dopamine and norepinephrine (Bear et al., 2001).
Its concentration in the CNS either comes from glucose via Krebs cycle or from glutamine from astrocytes taken up by neurons. This latter supply of glutamate is described as glutamine-glutamate cycle (Figure.1). Glutamate is released from the neurons into the synaptic cleft and then it is up taken by the astrocytes via glutamate transportes. After producing glutamine from glutamate through glutamine synthase in the astrocytes it is released again into the synaptic cleft, where it is innocuous and will be up taken by nerve terminals where it is retransformed into glutamate by glutaminase enzyme effect (Rang et al., 2007).
During rapid excitatory synaptic transmission process, glutamate is released from glutaminergic nerve terminals, upon depolarisation, into the synaptic cleft where it binds with postsynaptic receptors. These receptors are ion channel membrane when activated they allow cation influx into postsynaptic neurons after depolarisation. Therefore, fires of action potential are generated when depolarisation reaches a certain threshold (Doble, 1999).
It is well known that glutamate is essential for normal brain development and function, such as in the process of learning and memory and in synaptic plasticity. However, overload of this neurotransmitter will lead to excessive activation of its receptors. This action ultimately results in neuronal death and will affect other cells expressing glutamate receptors (Sitte and Freissmuth, 2006).
Normally most of the glutamate is concentrated in the intracellular medium, whereas in the extracellular medium its concentration is about million-folds less; this steep concentration difference is crucial for efficient stimulation. During rapid synaptic transmission this neurotransmitter is released into the synaptic cleft and stimulates glutamate receptors.
To maintain the concentration of glutamate in the synaptic cleft suitably low, glutamate uptake mechanism from the extracellular medium is necessary. There is no evidence indicating presence of metabolic enzymes in the extracellular medium that transform glutamate to an inactive form. Subsequently, there is only one method that rapidly removes glutamate from the extracellular medium that is by cellular uptake via glutamate transporters. For example: glutamate aspartate transporter (GLAST), glutamate transportr subtype-1 (GLT-1) and excitatory amino acid carrier-1 (EAAC1). This process is completed through Sodium-dependent glutamate transporters that are present mainly on astrocytes membrane and to a minor level on neurons. Once glutamate is taken up, it undergoes either metabolic process or enters glutamate-glutamine cycle and is reused as a transmitter. Glutamate transporters play a major role in preventing excitotoxicity from happening under pathological conditions. This critical neurotransmitter clearance mechanism is inhibited in neurological diseases (Sitte and Freissmuth, 2006; Danbolt, 2001).
Excitatory Amino Acid Receptors:
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Glutamate receptors are found in almost all neurons and on many glial cells in the CNS. They are classified into two main groups, according to the signal transduction mechanism:
* Iontropic glutamate receptors are directly coupled to ion channels on membrane.
* Metabotropic glutamate receptors are G-proteins-coupled receptors, which in a direct or indirect way controls ion channels and enzymes (this group is not generally thought to contribute in excitotoxicity process for this reason it is not further discussed) (Rang et al., 2007).
Ionotropic receptors are further classified in to three main classes: N-methyl-D-aspartate receptors (NMDARs), α-amino-3-hydroxy-5-methyl-4-isoxazole proprionate receptors (AMPARs) and Kainate receptors. Each class is again classified into several subtype receptors. They vary in subunit composition, their ligand recognition properties and the biophysical properties of the ion conductance that their activation caused (Jonas and Monyer, 1999).
N-methyl-D-aspartate receptors (NMDARs):
They are ligand-gated ion channels, tetramers consisting of NR1and NR2 subunits; evidence shows that NR2 subunit presents the binding site for glutamate, while NR1 subunit compromises the binding site for glycine. They are both pre- and postsynaptic receptors expressed on astrosytes and neurons membranes. These receptors are characterized with high conductance allowing Na and Ca ions entry and K ion efflux from neurons.
NMDARs are activated by NMDA itself, Quinolinic acid and Ibotenic acid and blocked by Kitamine, phencyclidine and MK-801; these receptors are desensitised slowly.
At resting membrane potentials, NMDARs are blocked by Mg ions, and this prevents them from being activated by glutamate, NR2 subunit is believed to be responsible for this voltage-dependent blockade. For this reason, NMDARs are not thought to be involved in primary synaptic transmission at glutamatergic synapses.
However, this blockade by Mg ions is voltage-dependent, and therefore, NMDARs can be stimulated by glutamate, in the presence of glycine as a co-transmitter, consequence to the neurone depolarisation by another excitatory mechanism.
Thus, NMDARs contribute as a slow secondary excitatory synaptic potential at glutamatergic synapses, once the neurone has been depolarised through the excitation of AMPA/kainate receptors.
The voltage-dependent blockade of NMDARs characterises this receptor with a unique- dependent activation mechanism (Jonas and Monyer, 1999; Bear et al., 2006). Such a mechanism has been hypothesised to motivate cellular mechanisms of learning. NMDARs have been concerned in many plasticity events within the CNS, such as long-term potentiation, long-term depression, and stimulus-dependent plasticity (Doble, 1999).
α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPARs) and Kainate receptors:
They are ligand-gated cation channels allowing the entry of Na ions and the exit of K ions. They coexist with NMDARs, present pre- and postsynaptically on astrosytes and neurons membranes.
AMPARs are tetramers assembled from four subunits GluR1-4 can expressed as homomeric and heteromeric receptors, it should usually consists of two different subunits per functional receptor channel. AMPARs that contain GluR2 subunit are Ca ion impermeable but those that lack GluR2 subunit are Ca ion permeable. The former receptors are much more predominant than the latter receptors (Jonas and Monyer, 2006).
Kinate receptors can be divided into two groups: the first group compromising GluR5-7 subunits, these have low affinity kainite-binding sites. Whereas, the second group consists of KA1 and KA2 which have high affinity binding sites for Kainate (Jonas and Monyer, 2006).
It is difficult to separate AMPARs from Kainate receptors since they are mutually activated by the same endogenous agonists.
AMPA/kainate receptors can be activated by glutamate, AMPA, and kainic acid. AMPA/kainate channels have a (5-10)-fold lower elementary conductance than NMDARs.
Responses of AMPARs desensitise rapidly to the endogenous neurotransmitter glutamate, as well as to AMPA and quisqualic acid. While, kainic acid desensitises slower and less completed on these receptors (Doble, 1999).
They mediate fast excitatory synaptic transmission. Upon AMPA/Kinate receptors activation by glutamate they exchange Na ion with K ions that results in neuron depolarisation. This neuronal depolarisation is essential for the Mg ion release from NMDARs channels (Bear et al., 2006).
The following table presents main a comparison between ionotropic glutamate receptors:
1. Permits Na, k ions (and Ca ions in receptors lacking GluR2 subunit).
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3. Requires glutamate only for activation.
4. Subunits: GluR1-4.
5. Fast synaptic transmission.
1. Permits Na and Ca ions.
2. Voltage and ligand-gated.
3. Requires glycine as well as glutamate for activation.
4. Subunits: NR1, NR2.
5. Slow synaptic transmission.
It describes a process of neuronal death caused by an increase in the amount or duration of glutamenergic receptors activation. The resultant accumulation of Ca causes destruction of glial cells, in which they are the major supply of glutamate transporters for the removal of glutamate. This starts by disturbing Ca homeostasis that destruct the Mitochondria and the production of free radicals.
The role of excitotoxicity in the aetiology and progression of neurodegenerative diseases has been strongly proposed but not always productive research efforts have been seen. Examples of these diseases are stroke, Alzheimer's disease, amyotrophic lateral sclerosis (ALS), trauma, epilepsy, Huntington disease (Rang et al., 2007).
This process is generated by glutamate, the critical neurotransmitter. Its high concentration in the extracellular medium is due to:
* Increase of its release into the extracellular medium.
* Decline of glutamate uptake or transporters responsible for its clearance from the synaptic cleft.
* Blunt discharge of glutamate from injured neurons as observed in stroke.
The latter cause massively promotes extracellular glutamate concentration from 0.6µmol/L through normal conditions up to 2-5µmol/L during neuronal injury (Mark et al., 2001).
The massive amount of glutamate in the extracellular medium causes constant depolarisation of neurons. This triggeres a pathway that eventually results in cell death. This pathway depends on three main points: Na influx, Ca influx and exocytosis of glutamate.
Depolarisation is initially caused by stimulating AMPARs and subsequently by stimulating voltage-dependent sodium channels. This results in influx of Na ions and further depolarisation. If the cell becomes fully depolarised, the NMADRs releases the blockade voltage dependent Mg ion and become obtainable for stimulation by glutamate and glycine as a co-factor. Its stimulation allows Ca influx which represents the main Ca entry into the cell.
Osmotic balance of the cell will be disturbed upon continuous depolarisation due to the Na level rise. This is followed by passive influx of K ions and then the entry of water following the osmotic pressure. The final consequence is cell lysis, however, this excitotoxicity is reversible when depolarisation effect is removed (Jonas and Monyer, 2006).
Cell dpolarisation results in the following consequences, which ultimately leads to excitotoxicity:
* Unblocks NMDA channels, allowing Ca ions enter the cell.
* Activates voltage-dependent calcium channel, permitting further Ca entry.
* Stimulates voltage-sensitive sodium channels, leading influx of Na and cause further depolarisation.
* Inhibits glutamate uptake, resulting in an increase of glutamate extracellular level (Rang et al., 2007).
Excitotoxicity and Intracellular Calcium:
Increased concentration of Ca in the intracellular medium is considered as the secondary factor that enhances excitotoxicity and inevitably causing cell death, yet this factor is not reversible even after inhibiting the depolarizing action (Doble, 1999).
Calcium is regard as one of the most important signal ions in the CNS; it is a powerful effecter and excess accumulation of Ca in the intracellular cause cell death, so it is vital to maintain a low cytoplasm concentration. Normally intracellular concentration is about less than 100nM as compared to about 1mM in the extracellular, it is the only ion that could be increased up to ten-fold easily (Rang et al., 2007).
However, when the cell is excessively depolarised, Ca ion concentration increases. This occurs mainly through voltage-dependent calcium channels (VDCC), which are highly selective to Ca ion and permit Ca ion influx whenever the membrane is depolarised.
Main types of VDCC are named according to their specific characteristics: T (transient current), N (present on neurons), L (long duration current, large conductance channels) and P (present on purkinje cells of the cerebellum). The most important type in neuronal injury is the L type as it excessively promotes long duration Ca influx with the activation of these channels (Mark et al., 2001).
Also increase in Ca ion concentration occurs during elevated glutamate levels in the extracellular medium, which activates NMDARs that are the primary pathway for Ca ion influx after depolarization to eject the Mg ion that blocks these receptors. Nevertheless, this Ca ion influx could only last for a short time as the VDCC as well as the NMDARs are desensitize rapidly (Rang et al., 2007).
The consequences of Ca elevation are:
* Increase glutamate release, Ca influx stimulate the glutamate vesicles to cross synapse by exocsytosis.
* Overactivation of glutamate receptors effect a number of enzymes which cause membrane damage and results in neuronal death, for example: nitric oxide synthase, protein kinase Ð†I, phospholipases, proteases, endonucleases, phosphatases and ornithine decarboxylase.
* Disturbs Ca homeostatic mechanisms (Mark et al., 2001).
Slow Excitotoxicity and Mtabolic Impariment:
There is another form of excitotoxicty, called slow excitotoxicty which is generated by the consequence of Ca influx into the mitochondria in the absence of high glutamate level stimulation. However, this process source extracellular glutamate level elevation that culminates in neurons death (Doblt, 1999). In other words, excitotoxicity of glutamate-induced level or Ca-induced level occur consecutively and the outcome of one process will inevitably trigger the other.
Mitochondria play a major role in energy production within the cell, in the form of ATP. Additionally they compensate the rise of intracellular Ca concentration whenever it exceeds the critical level (Rang et al., 2007).
Ca ions will accumulate from the cytosol into the mitochondria when [Ca]i reaches 0.5 µM, resulting in mitochondria depolarisation (Greenwood and Connolly, 2007). Eventually damage will occur to the mitochondria due to the electron transport chain or by physical disruption. Mitochondria impairment will interrupt or reverse the ATP production leading to energy production failure. In the neuron, ATP is essential in controlling ATP-dependent processes, particularly Na/K-ATPase ion channel, the major route of Na extrusion, which is the main source of maintaining the resting potential membrane (Greenwood and Connolly, 2007).
Accordingly, an increase of Na ion in cytosol, from both pathways, by Ca and Na influx via VDCC and by inhibition of Na ion efflux via Na/K-ATPase channels, will cause neuron depolarisation. This depolarisation will further enhance VDCC and permit excess Ca and Na ions entry. This course of action will prompt the adverse consequence of Ca elevation in neuronal death as well as further neuronal depolarisation through Na increase. Moreover, depolarisation will cause the release of the voltage-dependent Mg blockade in NMDARs. In the presence of extracellular glutamate, NMDARs are activated allowing Ca influx, which is the main route of Ca source (Bear et al., 2001).
Overall, these series of progressions will all sensitise the neurons to the toxic effect of glutamate which excite NMDARs resulting in massive increase of Ca causing rupture of mitochondria membrane and releasing [Ca]i, leading to apoptosis or necrosis of the neurons.
Free Radicals and Excitotoxicity:
It is considered as the third cause that contribute in excitotoxicity process as a consequence in the alterations in [Ca]i. Free radicals are produced according to the activation of calcium-dependent enzymes, for instance: nitric oxide, phospholipase A and from impaired mitochondria, which produces high amount of free radicals upon massive [Ca]i (Doblt, 1999).
During excitotoxicity the increase of influx Ca activate nitric oxide synthesis (NOS) release from post synaptic neurons; this is through Ca binding to Calmodulin, which is a co-factor for NOS. Upon neuronal depolarisation and the stimulation of glutamate, NMDARs are activated permitting further Ca influx. Increase Ca levels produce neuronal NOS which form NO radical that activates NMDARs. During hypoxia and ischemia, injured neurons are accumulate with the formed reactive oxygen species (ROS) causing oxidative stress and molecular damage This will have a huge impact affecting ATP production and causing oxidative damage of DNA, eventually causing cell death (Nelson et al., 2002).
Schulz et al. (1995) studies implicated that NOS inhibitors can protect against glutamate and NMADRs neurotoxicity but not AMPARs nor Kainate receptors. This proves that NMDARs play a major role in excitotoxicity process caused by free radical formation.
Another enzyme that participates in producing free radicals upon [Ca]i alevation is phospholipases A. Its activation will produce platelet-activating factor and arachidonic acid. The former will directly increase glutamate release while the latter will inhibit glutamate uptake. Leading in further stimulation of iontropic glutamate receptors and more arachidonic acid production. This increase production cause oxygen free radicals formation, which activates phospholipases A. and the process continues in a circle resulting in neuronal self digestion through free radical formation, protein breakdown and lipid peroxidation (Mark et al., 2001).
It is widely accepted that glutamate is the major excitatory neurotransmitter in the CNS, and excessive activation of its inotropic receptors load massive amounts of calcium into neurons leading to excitotoxicity, a process in which inevitably causes irreversible cell death. The concept of excitotoxicity is an area of intensive investigation in neuronal injury as it is the final pathway of most neurodegenerative diseases, in which stroke is one of them. The best anti-excitotoic therapy suggested is NMDARs antagonists, however, until a full understanding of the pathogenesis and the ability to demonstrate the efficacy of the hypothetical anti-excitotoxic agents, its treatment will remain elusive. This is a very interesting field and further neuroscientific research is required for screening new therapies for this particular disease.