02 Stress Induced Plasticity in the Glumatatergic System

Stress, something we experience daily, is believed to have powerful effects on our brain by influencing the neurochemistry within our brain. One of the neurotransmitter systems significantly altered by stress is the glutamate system. Emerging evidence has shown that stress can act on the glutamate system directly or indirectly via corticosteroids, the hormones released by the hypothalamus-pituitary-adrenal (HPA) axis in response to stressors. [1] Profound plastic effects are observed in the limbic and prelimbic areas after both acute and chronic exposure of stress [1]. After both acute and chronic exposure to stress, the overall glutamatergic transmission increases as a result of alterations to multiple mechanisms including the release and the uptake of glutamate. [2],[3],[4],[5] Intracellularly, acute and chronic stress activates different pathways resulting in different cognitive consequences. Acute stress activates serum and glucocorticoid regulated kinase (SGK) mediated pathways and is implicated to have a cognitive enhancement effect. [4] On the other hand, chronic stress promotes cell death by either downregulating genes responsible for cell survival in the CREB mediated pathways6 or upregulating pro-death genes mediated by the FoxO3a. [7],[8] The excitotoxic effect and irreversible alterations as a result of chronic stress are associated with neurodegenerative disease and psychiatric disoders.

Glutamate at physiological conditions
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Retrieved from http://www.decodingdelicious.com/what-is-msg/l-glutamate-structure/

1. Glutamate

Glutamate is one of the major excitatory neurotransmitter in the nervous system and is the most abundant. More than 80% of the neurons in the cerebral cortex release glutamate. [9] Glutamate binds to and activate two types of ionotropic receptors, α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) and N-methyl-D-aspartic acid receptor (NMDAR). The activation of both receptors initiates pathways that are currently believed to be important in learning and memory. [10]

2. Stress induced plasticity in the glutamatergic system

Effect of Acute Stress on Glutamatergic System
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Acute stress enhance glutamate release by a glucocorticoid mediated mechanism.
Additionally, glutamate enhances the activity of AMPAR and NMDAR (not shown in diagram).
Adapted from Figure 2 in [1].

2.1 Acute Stress

Acute stress act to enhance glutamatergic transmission in the cortex mainly by enhancing its synaptic release and enhancing the activity of the receptors that binds it.

2.1.1 Extracellular release

Acute stress (foot shock) has been shown to enhance depolarization evoked glutamate release from synaptosomes in the prefrontal cortex. [2] Glucocorticoid levels were monitored throughout and were found to increase after the stress session. [2] When the glucocorticoid transmission is blocked by glucocorticoid receptor antagonists (RU486), the facilitation of depolarization evoked glutamate release by stress is not observed. [2],[3] Together, it is concluded that acute stress increases extracellular glutamate release by a glucocorticoid mediated mechanism. However, this simple relationship between stress and glucocorticoids are not observed in other brain regions. Acute glucocorticoid injections [12] but not acute foot shock sessions [11] enhances extracellular glutamate release from the hippocampal nerve terminals.

2.1.2 Receptor Activity

Electrophysiological recording studies showed that acute stress mimicked by acute corticosterone (a glucocorticoid) injection increases synaptic NMDAR and AMPAR activity in both the prefrontal cortex4 and the hippocampus. [11] The facilitation of synaptic AMPAR and NMDAR activity is suggested to be mediated by a serum-and-glucocorticoid inducible kinase (SGK) dependent Rab4 (GTPase) mediated mechanism. [4] Acute exposure to behavioral stressors or the administration of corticosterone injection increases SGK activity, allowing it activates Rab4 downstream. [4] When activated, Rab4 is thought to simultaneously promote the synaptic trafficking of AMPARs and NMDARs. [4] Consequently, the activity of the AMPAR and NMDAR increases because more of these receptors are present to respond to incoming stimuli. The involvement of both SGK and Rab4 are crucial to the enhancement of NMDAR and AMPAR activity, as drugs that inhibit either enzyme will block the stress induced enhancement effect. [4]

2.1.2a Cognitive enhancement

Acute stress induced enhancement of glutamate transmission, especially the facilitation of AMPAR and NMDAR activity in the prefrontal cortex has been associated with cognitive enhancement. Young rats exposed to acute behavioral stress has been shown to perform better than ones that were not exposed to stress in a T-maze delayed alternation task, a test of working memory. [4] This enhancement can be blocked by administration of drugs that inhibit SGK activity prior to stress exposure, suggesting that the cognitive enhancement is mediated by a SGK mediated mechanism. [4]

2.1.3 Glutamate Clearance

Acute stress is not believed to have any significant effect on glutamate clearance. Alterations in glutamate clearance are often mediated by changes in the glutamate transporters and glial cells. Most current studies suggest these alterations are associated with chronic exposure of stress but not acute stress exposure. [12],[13]

Effect of Chronic Stress on Glutamatergic System
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Chronic stress enhance glutamate release by a glucocorticoid mediated mechanism.
Chronic stress impair synaptic glutamate clearance mechanisms.
Additionally, glutamate suppress the activity of AMPAR and NMDAR by AMPAR
ubiquitination (not shown in diagram).
Adapted from Figure 10 in [1].

2.2 Chronic Stress

When compared to acute stress, chronic stress tends to cause more long-lasting changes in the glutamatergic system. These changes are often associated with impairments and are implicated to contribute at least in part to the pathology of psychiatric disorders and neurodegenerative disorders.

2.2.1 Extracellular Release

In addition to enhancing depolarization evoked glutamate release, also induced by acute stress, chronic (immobilization) stress has been shown to increase spontaneous glutamate release. [14] A recent study showed that the enhancement effect is attributed to the increased sensitivity of synaptosomes to the depolarizing calcium ions by an unknown N-type and P-type calcium channel dependent mechanism. [14] Another study showed that the enhanced extracellular release induced by chronic stress can be mimicked by chronic glucocorticoid injections and is blocked by glucocorticoid receptor antagonists such as RU486, suggesting that the enhancement is glucocorticoid mediated and dependent. [3] Together, chronic stress seems to enhance extracellular release of glutamate by multiple mechanisms.

2.2.2 Receptor Activity

In contrast to the facilitation of synaptic AMPAR and NMDAR activity induced by acute stress, chronic stress has been implicated to have the opposite effect. Electrophysiological recordings showed a decreased post-synaptic AMPAR and NMDAR response to glutamate release in the prefrontal cortex of chronically (restrain) stressed rats but not the controls. [15] This study further showed that the suppression is caused by a glucocorticoid dependent ubiquitin mediated degradation of synaptic AMPAR. [15]

2.2.2a Cognitive impairment

In sharp contrast to the acute stress mediated cognitive enhancement, chronic stress seems cause cognitive impairment. The group of researchers that previously showed that chronic stress decreases post-synaptic AMPAR and NMDAR responses to glutamate also showed that these chronically stressed rats are impaired at temporal order recognition memory tasks (measure of recognition memory). [15]

2.2.3 Glutamate clearance

Converging evidence suggest that chronic stress alter multiple mechanisms that are directly involved with glutamate clearance from the synapse. As a result, glutamate accumulates in the synapse.

After presynaptic release of glutamate into the synapse, the excitatory neurotransmitter is almost immediately taken up by surrounding glial cells and the synaptosomes that released it [1]. Direct and indirect evidence show that chronic stress exposure decreases glial and synaptosomal uptake, resulting in an accumulation of glutamate in the synapse. [5],[12],[15],[16],[17],[20]

Glial fibrillary acid protein (GFAP) is a filament protein expressed in glial cells and is considered to be a marker for astrocytes, cells that are involved in the uptake of glutamate from the synapse. [15],[17],[20] Multiple studies have shown that chronic (unpredictable) stress reduces GFAP in the prefrontal cortex. [17],[20] More recently, chronic (restraint) stress has been shown to reduce GFAP in the periaqueductal grey area. [21] The decrease in the number of astrocytes, implied by these studies [20],[21],[22], indirectly suggest that the glial uptake of glutamate in the prefrontal cortex and the periaqueductal grey regions are impaired.

More direct evidence comes from studies that utilize radioactively labeled glutamate. After exposing the experimental rats to chronic stress for a period of time, the rats were sacrificed and brain slices were prepared. Tritium, a radioactive isotope of hydrogen, is incorporated into glutamate. The radioactive glutamate is incubated onto the prepared slices. Glutamate uptake is derived from the incorporated radioactivity. Using these similar experimental procedures as above, chronic stress has been shown to decrease glutamate uptake in the frontal cortex [5], the striatum [5], and the hippocampus. [5],[12],[16]

Other studies regarding glutamate clearance specifically looked at the transporters that are responsible for the uptake of glutamate into the synaptosomes or the glial cells. One recent study showed that chronic stress (learned helplessness) decreases the expression of vesicular glutamate transporters (vGluT1) and excitatory amino acid transporters (EAAT2/4) in the hippocampus and the cerebral cortex. [18] The decreased expression of synaptosomal glutamate transporter (vGluT1) and glial glutamate transporters (EAAT2/4) further support the idea that glutamate clearance is impaired with chronic stress resulting in an accumulation of glutamate in the synapse.

However, the effect of chronic stress on the expression of glutamate transporters is complicated by another study with mixed results. Using the maternal separation paradigm, the experimenters showed aged rats that were chronically stress early in life with maternal separation had decreased the expression of vGluT1/2 but increased the expression of EAAT2 in the hippocampus. [19] These rats were also shown to have a higher basal level of corticosterone and consequently implicated to have a higher basal level of glutamate in the synapse [19]. This study affirms the current evidence that synaptosomal glutamate transporters are downregulated by chronic stress but questions the effect of chronic stress on the glial glutamate transporters. Further experimentation is required to resolve the contradictory effect that chronic stress has on glial glutamate transporters.

Extrasynaptic NMDA Mediated Pathways
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The activation of extrasynpatic NMDAR results in the activation of multiple
pathways that will eventually lead to neuronal death. Activation of Jacob and deactivation
of ERK pathways act to counteract the pro-survival effect of synaptic NMDAR activation.
Activation of the FoxO3 pathway induce apoptosis in a more direct manner. Similarly, the
activation of CALC1 also mediate cell death (not shown in diagram).
Adapted from Figure 3 in [20]

2.2.4 Extrasynaptic NMDA Receptor Activation

As a result of chronic stress exposure and alterations to mechanisms responsible for glutamate release and glutamate clearance, synaptic accumulation of glutamate results. The constant presence of glutamate in the synapse over time has many consequences, one of which is the activation of the normally inactive extrasynaptic NMDA receptors. Extrasynaptic NMDA receptors are the glutamate receptors that are localized outside of the synaptic densities. [20] These receptors are further away from the synapse than synaptic NMDA receptors and are less accessible to the glutamate in the synapse. Much of the synaptic glutamate are cleared before they have a chance to diffuse outside of the synapse. Under abnormal conditions where there is a constant overflow of glutamate in the synapse, glutamate is able to diffuse out to bind and activate these extrasynaptic NMDA receptors. [20] Activation of the extrasynaptic NMDA receptors results in the initiation of multiple pathways that will eventually lead to cellular damage and death. Four such pathways will be discussed.

2.2.4a FoxO3a-FasL Pathway

Foxhead transcription factor (FoxO3a) is a transcription factor that is previously shown to upregulate the expression of the Fas ligand (FasL), a protein that is linked with apoptosis. [21] FoxO3a is normally found in the cytoplasm, where it is spatially prevented from upregulating the expression of proteins [22]. FoxO3a has a nuclear localization signal that allows its transport to the nucleus. [22] However, this signal is blocked by the phosphorylation of the serine-253 residue. [22] One recent study showed that the activation of extrasynaptic NMDA receptors in cortical neuronal cell cultures experimentally with the addition of NMDA resulted in the dephosphorylation of the key serine-253 residue, the nuclear localization of FoxO3a, and the increased expression of FasL. [7] Consequently, the activation of extrasynaptic NMDA receptors is implicated to induce neuronal apoptosis by a FoxO3a dependent FasL mediated pathway.

2.2.4b CALC1 Mediated Neuronal Death

CALC1 is previously suggested to be a calcium activated chloride channel. [23] The activation of the CALC1 channel has been previously associated with neuronal cell death and cellular growth arrest in tumor cells. [23] Zhang and colleagues showed that CLAC1 is exclusively activated by calcium ions that enter through the extrasynaptic NMDARs but not synaptic NMDARs. [24] Subsequently, the transfection of CALC1 into hippocampal neuronal culture in vitro resulted in neuronal cell death25. The evidence suggests that extrasynaptic NMDAR initiate an unknown CALC1 mediated pro-death mechanism.

2.2.4c Dephosphorylation of the ERK pathway

Extracellular signal regulated kinase (ERK) pathway is believed to promote cell survival when the ERK enzymes are phosphorylated20. Current evidence suggests that the phosphorylation of the ERK enzymes depends on the competition of the activation of the synaptic NMDA and extrasynaptic NMDA receptors. [25],[26] Synaptic NMDA receptors, when activated, act to promote survival by phosphorylating the ERK enzymes32 while activated extrasynaptic NMDA receptors promote cell death by dephosphorylating the same enzymes. [25],[26],[27],[28] However, the results from multiple studies expose the deficiencies in this simplistic model. One study showed that the activated extrasynaptic NMDA receptors only selectively dephosphorylates the ERK enzymes that were phosphorylated by mechanisms that require the activation of synaptic NMDA receptors but does not dephosphorylates the same ERK enzymes if they are activated by a synaptic NMDAR independent pathway. [25] Another study contradicts the current evidence regarding the role that extrasynaptic NMDARs play in the ERK pathway. Using bath NMDA or glutamate solution to stimulate the activation of the extrasynaptic NMDARs in primary neuronal cell cultures, Léveillé and colleagues showed that extrasynaptic NMDAR activation induces the breakdown of mitochondrial membrane potential and triggers cellular damage while unable to induce any changes to the phosphorylation levels of the ERK enzymes. [28]

Interestingly, the ERK pathway has been linked to another cellular survival pathway. A recent study suggested that the enzyme (Jacob) that previously has been shown to be activated by extrasynaptic NMDARs to dephosphorylate and inactivate the cAMP response element binding protein (CREB) mediated cell survival pathway [6],[29], is actually a substrate of the ERK enzymes. [30] The interactions between the two enzymes and the mechanisms behind the activation of Jacob are still yet to be understood.

2.2.5 Associated Disorders

In general, stress is strongly associated with the development with many pathological conditions such as anxiety disorders, mood disorders and neurodegenerative diseases. Stress contributes to the development of pathological conditions in many ways. Chronic stress induced changes in the glutamatergic system is directly associated with mood disorders and neurodegenerative diseases.

2.2.5a Mood Disorders

Altered glutamatergic transmission is commonly observed in patients suffering mood disorders such as bipolar disorder and depressive disorders. [31] Postmortem studies showed pathological decreases in glial cells across the cerebrum in patients suffering from major depressive disorder. [32],[33] As described previously, chronic stress decreases AMPAR and NMDAR receptor activity in the prefrontal cortex of rats and consequently impairments in recognition memory tasks. [15] Similar mechanisms may be responsible for the memory deficits observed in patients with stress-related psychiatric disorders.

2.2.5b Neurodegenerative Diseases

Altered glutamatergic transmission as a result of chronic stress and the activation of the extrasynaptic NMDA receptors is directly associated with the development of neurodegenerative disease, especially Alzheimer’s disease. [34] Current evidence suggests that there is a complex link between Alzheimer’s disease and glutamate. Multiple neurodegenerative pathways that contribute to Alzheimer’s disease require the activation of extrasynaptic NMDA receptors. [30],[35] Furthermore, amyloid beta peptides, whose excessive deposition is characteristic of Alzheimer’s disease, are shown to increase synaptic glutamate accumulation by downregulating glial glutamate transporter activity and disrupting the glial glutamate metabolism. [36] The synaptic accumulation of glutamate then activates extrasynaptic NMDA receptors and facilitates tau [35] and/or amyloid beta [30] mediated neurodegenerative pathways, resulting in a downward spiral.

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