Glutamic Acid: Synthesis, Pathways, Receptors, Uses And Conditions

Glutamic acid is an excitatory amino acid, while the others are inhibitory amino acids.  Every neuron in the CNS is innervated by GABA and glutamate neurons. The concentration of synaptic GABA and glutamate is in the range of millimolar, while the concentration of synaptic biogenic amines such as dopamine, serotonin, or peptide neurotransmitters is in the range of micromolar or less.  

Glutamic acid mediates fast excitatory neurotransmission in the CNS. It transmits 80% of brain synapses, especially the dendritic spines.  Its concentration in the brain is 10 millimolar. It is the highest of all the amino acid neurotransmitters and represents a 20% neurotransmitter pool of glutamate in the body. 

Synthesis Of Glutamic acid

Glutamic acid is excluded from the brain by the blood-brain barrier since it is cytotoxic. It is produced in the brain from glucose. The synthesis is with the help of 2 pathways:

First Pathway: The tricarboxylic acid cycle converts glucose into ?-ketoglutarate, which is subsequently converted to glutamic acid by transaminase reaction.  Glutamate and α-ketoglutarate are present in the brain in equilibrium.  20% of glutamate is dedicated to neurotransmission. Vesicular glutamate transporter (VGLUT) puts these glutamate in storage vessels.

The 2nd metabolic pathway for replenishing synaptic glutamate is related to astrocytes. Astrocytic endfeet envelopes glutaminergic synapse. Astrocytes are one of the glial cells in the CNS. It expresses glutamate transporters EAAT (excitatory amino acid transporter) 1 and 2. There are other glutamate transporters, such as EAAT 3,4,5. EATT 1 and 2 remove glutamate from the synapse, resulting in the termination of its action. 

The glutamate present in the glutaminergic neurons of the vesicular glutamate transporter( VGLUT) is released in the synapse. After its action on the receptors, it is taken by the astrocytes with the help of EAAT transporters.  The ATP-dependent enzyme 'glutamine synthetase' in astrocytes converts glutamate into glutamine. 

Glutamine is released into the glutamate neurons with the help of a specific glial neutral amino acid transporter( SNAT).  Further, glutamine is again converted to glutamate by a phosphate-activated enzyme called glutaminase found in the mitochondria.  This cycle is known as the glutamine cycle. This accounts for 40 % of glutamate turnover.  It is an important pathway for the replenishment of glutamate in neurons. 

Major Glutaminergic Pathways in the Brain

All primary sensory afferent systems appear to use glutamate as a neurotransmitter, including retinal ganglion cells, cochlear cells, trigeminal nerves, and spinal afferents. The cortico-brainstem glutaminergic pathway starts from cortical pyramidal neurons and goes to various regions in the brainstem, especially those associated with other neurotransmitters.

For example, from cortex to raphe, associated with serotonin, the ventral tegmental area and substantia nigra are associated with dopamine, and locus coeruleus is associated with norepinephrine. Glutamate stimulates the release of these neurotransmitters, while GABA inhibits it. Other glutaminergic pathways include thalamocortical projections.

The temporal lobe circuit is important in developing new memories and is a series of 4 glutaminergic synapses. It starts from the perforant pathway, innervates hippocampus granules, and then goes to CA3 pyramidal cells and CA1 pyramidal cells.  Other glutaminergic pathways are climbing fibers innervating the cerebellar cortex, corticospinal tract, and cortico-cortical glutamate pathways. 

Receptors Of Glutamic acid

Two categories of receptors mediate the postsynaptic effects of glutamine. These are glutamate-gated cation channels used for fast neurotransmission and mGLuR(metabotropic glutamate receptors), which are G protein-coupled receptors primarily used to modulate glutaminergic neurotransmission. 

Ionotropic Glutamate Receptor

Ionotropic glutamate receptors are tetrameric ligand-gated ion channels. There are ligand-gated ion channels of other neurotransmitters, which are ?4?2 in acetylcholine, GABA, and 5HT3. 

These are pentameric ligand-gated ion channels. One single subunit of the ion channel has 3 full transmembrane regions, and 4^th is the re-entrant loop. 4 of these subunits combine to form a tetrameric ligand-gated ion channel, and the functional ion channel is in the middle.

The three families of ionotropic glutamate receptors are the AMPA receptor, the kainate receptor family, and the NMDA receptor. These are named after the agonists that act on these receptors. 

Mnemonic: I KAN REMEMBER

A. AMPA receptor 

AMPA, that is ⍺- amino-3-hydroxy-5 methyl-4-isoxazole-propionic acid receptor, mediates depolarizing postsynaptic currents. It consists of 4 subunit- glutamate receptors 1, 3, 4, and 2.  The subunits 1,3 and 4 have glutamine Q residue, resulting in high Ca²⁺ conduction.  The subunit 2 has arginine R, which severely restricts Ca²⁺ passage and conducts only Na⁺. 

B. Kainate receptor

The kainate receptor family consists of 5 subunits Glut 5, 6, 7 and Kainate 1, 2. The Glut 5-7 subunits form glutamate-gated cation channels, and KA 1-2 subunits are associated with glut 5-7 to form high-affinity kainate receptors. The presynaptic localization on glutamate terminals causes reduced glutamate transmission. A common allelic variant of glutamate receptor 7 (GRIK 3) is associated with an increased risk of major depressive disorder.

AMPA and kainate receptors mediate fast and excitatory neurotransmission, allowing Na⁺ to enter, causing depolarization. A prolonged agonist binding causes desensitization and closes the channel, leading to transient unresponsiveness to the agonist. 

C. NMDA receptor

NMDA is an N-methyl-D-aspartic acid receptor and has several unique features. The various subunits of the NMDA receptor are encoded by 7 genes. Mg²⁺ blocks the channel at resting membrane potential, where Mg²⁺ acts as a negative allosteric modulator. 

The receptors are silent till activated by AMPA receptors. AMPA receptors depolarize neuronal membranes sufficiently to relieve Mg²⁺ blockade. NMDA receptor requires simultaneous binding of 2 ligands at 2 separate recognition sites to open the channel. 

The NR1 subunit forms the channel, and the NR2 subunit (NR2A-D) is a non-channel binding site. The NR1 subunit has a glycine modulatory site with two endogenous ligands, Glycine and  D-serine, whereas the NR2 subunit has a glutamate binding site.  The ligands should bind to the two binding sites, glycerine and D-serine, for the functioning of the receptor. 

In the diagram, Mg²⁺ binds to the NR1 and blocks the entering of Ca²⁺. There are other binding sites, such as ketamine and spermine, which have binding sites on NR2.   The NMDA receptor is also called a coincidence detector as 3 events must occur simultaneously, such as the channel to open, allowing Ca²⁺ to enter, to trigger postsynaptic actions by glutamate. 

3 Important events to occur: first is glutamate binds to NR2. Glycine or D-serine binds to NR1, which is released from the neighboring astrocytes. Glycine is prominent in the brainstem, whereas D-serine is present in the forebrain. These two bind to the respective sites, but the receptor does not function as Mg²⁺ blocks. The synaptic membrane should be sufficiently depolarized to release Mg²⁺. 

These 3 events should occur simultaneously for the functioning of the NMDA receptor. When NMDA Ca²⁺ channels are open, the intracellular Ca²⁺ activates several kinases and shows the effect through gene expression in the neurons. Important signals are activated by long-term potentiation and synaptic plasticity. 

NMDA receptors are tightly regulated as they are important in learning and excitotoxicity. The glycine reuptake from the synapse and reverse release is by GLY T2, which is type 2 glycine transporter, while D-serine reuptake is from the synapse and revere release is by D-SERT-T(D-serine transport). 

Glycine is synthesized from L-serine in the astrocyte with the help of an enzyme SHMT(serine hydroxymethyl transferase). It is then released into the synapse by GLY T2 by reverse transport. Reuptake is again by the GLY T2 transporter. D-serine is produced from Glycine, which is converted to L-serine by the enzyme SHMT and then converted into D-serine with the help of the enzyme D-serine racemase, or it can be directly produced from L-serine.

 It is then released in the synapse by the reverse D-SERT transporter, and there is reuptake by the same transporter in the astrocytes. The D-serine is inactivated by the enzyme DAAO (D-amino acid oxidase) and is converted to inactive hydroxy pyruvate that is released.  

Metabotropic Glutamate Receptor

The G proteins mediate the effects of the metabotropic glutamate receptors. There are 8 metabotropic glutamate receptors in the superfamily of G protein-coupled receptors.  The structure is of seven transmembrane regions.  The agonist acts on the receptor, which is coupled with G protein, and the enzyme leads to the production of secondary messengers. There are 3 classes of the metabotropic glutamate receptors. These are group I, group II, and group III. 

Group I contains mGLuR 1 and 5, group II includes mGLuR 2 and 3, and group III has mGLuR 4,6,7,8. Group I activates phospholipase C through Gq, while groups II and III inhibit adenylyl cyclase via Gi protein. 

The receptors can be remembered with a mnemonic 'I KAN ReMember' where I stands for ionotropic glutamate receptors, M stands for Metabotropic glutamate receptor, K is kainate receptor, A is AMPA receptor, and N is NMDA receptor. 

Also Read: 5 Effective Study Strategies for Psychiatry Residency Exams

Postsynaptic Density

Multiprotein complexes for intercellular signaling pathways contain scaffolding proteins(PSD-95), cell adhesion molecules, and various proteins.  Postsynaptic density contains several regions that bind to other proteins. There are 3 PDZ domains. P stands for PSD 95, D for Disc large, and Z for Zona occludens 1.  

NMDA binds to PSD-95 by NR2. The enzyme nitric oxide synthase(NOS) and calmodulin-activated protein kinase II(CaMKII) are closely related to PSD 95, so the calcium released can affect these enzymes.  AMPA receptors are bound by some intermediary proteins such as glutamate receptor interacting protein (GRIP), protein interacting with c-kinase 1(PICK 1), and synapse-associated protein of 97KDa (SAP-97) are subsequently attached to PSD-95. Postsynaptic mGluR is bound to some scaffolding proteins such as homer, shank, and PSD-95 consecutively, except for mGluR 7. PSD-95 is also bound to ?-actinin and F-actin to stabilize the structure. 

Neuroglin binds to PDZ and extends into the synaptic cleft, which is attached to β-neurexin, which further binds to the presynaptic component.  This arrangement stabilizes synapses by connecting pre and postsynaptic components. Mutations in neurexin, neuroglin, and shank are implicated in autism.

 Any disorganization in the structure will affect the stability of synapses, which can subsequently lead to various disorders.  

Role of Astrocytes

The specialized endfeet of astrocytes surrounds glutaminergic synapses. They express two Na⁺ - dependent glutamate transporters that primarily remove glutamate from the synapse, terminating its action. EAAT 1 and EAAT 2 are excitatory amino acid transporters.

 Astrocytes express AMPA receptors to monitor synaptic glutamate release.  The glycine transporters on the astrocytes maintain a sub saturating Glycine concentration in the synapse. It is expressed on the astrocyte plasma membranes. 3 Na⁺ is transported out, and 1 glycine molecule is transported in the astrocyte. 

The glutamate in the synapse stimulates AMPA receptors on astrocytes, which causes depolarization of astrocytes, and Glycine is released in the synapse.  Hence, glycine release in the synapse is coordinated with glutaminergic neurotransmission. 

Astrocytes express AMPA receptors to monitor synaptic glutamate releaseD-Serine levels determined by: D'Amino Acid Oxidase (DAAO). The stimulation of AMPA astrocyte causes grip dissociation and binds to serine; with the help of D-serine racemase, it synthesizes D-Serine. 

D-serine levels are determined by D-amino acid oxidase (DAAO). D-serine is the primary modulator of NMDA in the forebrain as there is a low DAAO enzyme. Glycine is a prominent modulator in the brainstem and cerebellum, as the glycine transporter is highest here. 

Plasticity in Glutaminergic Neurotransmission

Plasticity is the ability to change. Glutaminergic neurotransmission is associated with learning and memory.  Learning and memory involve use-dependent changes in synaptic efficacy. A brief period of intense stimulation of glutaminergic Schaffer collateral synapse on hippocampal CA1 pyramidal cells leads to a persistent increase in the efficacy of synaptic neurotransmission known as long-term potentiation(LTP).

A stable low-frequency stimulation of glutaminergic axon decreases the efficacy of neurotransmission, known as long-term depression (LTD). The LTP in the hippocampus requires stimulation of NMDA. NMDA is blocked by NMDA antagonists such as ketamine and PCP. The blockade of LTP in the hippocampus impairs the acquisition of new memories. 

A persistent change in synaptic efficacy in LTP and LTD results from inserting or removing AMPA receptors from postsynaptic densities of affected synapses.  The insertion of AMPA causes long-term potentiation (LTP), and removal causes long-term depression( LTD).

The extinction of conditioned fear is an active process mediated by the stimulation of NMDA receptors in the amygdala. In rats, NMDA receptor antagonists prevent the extinction of conditioned fear. D-cycloserine, an antibiotic used in TB, is a robust enhancer of fear if given with cognitive behavior therapy.  

It is a glycine modulatory site partial agonist that facilitates the extinction of conditioned fear. The glutamate-mediated synaptic plasticity is not only functional but also structural. Fragile X mental retardation protein (FMRP) is deficient in patients with fragile X syndrome.

These are synthesized in the spine during the stimulation of the NMDA receptor and play a role in transporting specific mRNA for translation. Treatment with mGLuR 5 antagonists in mice reverses fragile X-like phenotypes. 

Excitotoxicity

Excitotoxicity can also be seen with glutamate neurotransmission. The direct injection of ionotropic glutamate receptor agonists, e.g., kainic acid and N-methyl-D-aspartic acid, causes neuronal degeneration, affecting neurons with their cell bodies near the injection site.

Persistent and overwhelming activation of AMPA/ kainate or NMDA receptors increases the influx of Na⁺, Ca²⁺, and the secondary influx of H₂O, causing cellular edema and, eventually, necrotic cell death.  The sites distant from the injection site are also affected. A persistent increase in Ca²⁺ disrupts mitochondria to release cytochrome C and activates caspases, leading to apoptosis, which is known as programmed cell death. 

Neuropsychiatric Implications

Schizophrenia

There is hypofunction of the NMDA receptor in patients with schizophrenia. It is observed that PCP and ketamine (NMDA receptor blockers) produce symptoms of schizophrenia. The putative gene risks for schizophrenia are closely associated with NMDA receptors.

 For example, the DAAO gene encodes a protein that activates D-amino acid oxidase and the allelic variant of serine racemase, which decreases D-serine availability in the cortex and impairs NMDA receptor function. CSF and blood levels of D-serine are significantly decreased in schizophrenia.  The neuregulin 1 (NRG 1) gene directly interacts with the NMDA receptor. 

Dysbindin is expressed in glutaminergic terminal mGLuR 3, which down-regulates glutamine release. There are treatment trials related to drugs affecting glutamate, such as agonists at the glycine modulatory site on the NMDA receptor in patients concurrently receiving antipsychotics.

These include partial agonist D-cycloserine, an antibiotic in tuberculosis that decreases negative symptoms and improves cognition.. Also, high doses of Glycine 30-60g/day have been shown to decrease negative symptoms and positive symptoms and improve cognition. Moreover, endogenous glycine transport inhibitors such as sarcosine are seen to be effective. 

Depression

Through various MRS studies, it is found that glutamate is decreased in the prefrontal cortex in depressive disorder. NMDA antagonists such as ketamine have antidepressant effects and improve depressive symptoms. 

Alcohol Dependence

Ethanol intoxication increases GABA receptor function and attenuates NMDA receptor function. Persistent use of ethanol leads to dependence and causes downregulation of the GABA receptor and upregulation of the NMDA receptor.  A person consuming alcohol regularly has the CNS depressant effects of alcohol. 

Therefore, sudden withdrawal leads to a hyper-excited state, causing symptoms like hallucinations, tremors, seizures, or delirium tremens. There is supersensitivity of NMDA receptors in thiamine deficiency, which may contribute to excitotoxic neuron degeneration of Wernicke-Korsakoff syndrome.

 Acamprosate is a derivative of homotaurine, which appears to inhibit NMDA.It is used in the treatment of alcohol dependence. Microcephaly may be due to the inhibition of NMDA receptor function, causing widespread neuronal apoptosis in the immature cortex, leading to fetal alcohol syndrome.NMDA stimulation is essential for immature neuron survival and differentiation. 

Also Read: Thought Disorder: Causes, Types, Symptoms, Risk Factors, Diagnosis And Treatment

Hope you found this blog helpful for your Psychiatry residency Basic Sciences preparation. For more informative and interesting posts like these, keep reading PrepLadder’s blogs.