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Key Event Title
Reduced, Presynaptic release of glutamate
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
Key Event Description
Biological state: Glutamate is an amino acid with neurotransmitter function that is stored in presynaptic vesicles by the action of vesicular glutamate transporters (VGLUTs) and under physiological conditions is found at a concentration of 100 mmol/L per vesicle. Different mechanisms are involved in the release of glutamate (reviewed in Meldrum, 2000). Glutamate is mainly released from the vesicles in a Ca2+-dependent mechanism that involves N- and P/Q-type voltage-dependent Ca2+ channels, closely linked to vesicle docking sites. However, glutamate can also be released by reverse operation during reduction of Na+ and K+ gradient across the membrane like for example during cerebral ischemia. Interestingly, the synaptic release of glutamate is controlled by a wide range of presynaptic receptors that are not only glutamatergic like Group II and Group III of glutamate metabotropic receptors but also cholinergic such as nicotinic and muscarinic, adenosine (A1), kappa opioid, γ-aminobutyric acid (GABA)B, cholecystokinin and neuropeptide Y (Y2) receptors.
The synaptic effects of glutamate are rapidly terminated by action of glutamate transporters (excitatory amino acid transporters [EAATs]) located on the plasma membrane of astrocytes and neurons. Therefore, pre-synaptically released glutamate is mostly transported via EAATs to astrocytes and only a small portion is re-uptaken from the synaptic cleft into the presynaptic terminals(Rundfeldt et al., 1994; Blanke and VanDongen, 2009).
Following its release, glutamate exerts its effects via ionotropic and metabotropic receptors. Although glutamate is available for binding to receptors for a short time, NMDA receptors show high affinity for this specific neurotransmitter that causes their activation compared to other receptors.
Biological compartments: Glutamate is the most abundant amino acid in the diet, consequently is found at higher levels in plasma compared to cerebrospinal fluid. The blood brain barrier prevents the entry of glutamate, meaning that the glutamate present in CNS is derived from de novo synthesis of this neurotransmitter relying on the recycling of the main resources. Glutamine and α-ketoglutarate are the major precursors of glutamate. Glutamine is converted via phosphate-activated glutaminase to glutamate and ammonia, whereas α-ketoglutarate is transaminated into glutamate (Platt, 2007). In glial cells, the glutamate is metabolised via glutamine synthase into glutamine or metabolised into α-ketoglutarate. These products are actively transported out of the glial cells and back into the pre-synaptic terminals for subsequent re-synthesis and storage of glutamate.
Five transporters of glutamate have been identified in the CNS. Two are expressed predominantly in glia and three in neurons (reviewed in Meldrum, 2000). The presence of glutamate has also been demonstrated in other tissues and organs as glutamate receptors have been found to be expressed in pancreatic β-cells, osteoblasts and osteoclasts of bones (Nedergaard et al., 2002).
General role in biology: In mature nervous system, glutamate is known to play important role in synaptic plasticity. Similarly important is this neurotransmitter during development where it regulates neurogenesis, neurite outgrowth, synaptogenesis and apoptosis (reviewed in Mattson, 1996; Meldrum, 2000; Mattson, 2008).
The proper functioning of the central nervous system relays on the physiological homeostasis between glutamate and GABA, creating the opposite excitatory/inhibitory forces in the brain. Together, these two neurotransmitters constitute more than 90% of all neurotransmission. If this homeostasis is disturbed it could lead to anxiety disorders (Wieronska et al., 2015).
How It Is Measured or Detected
Methods that have been previously reviewed and approved by a recognized authority should be included in the Overview section above. All other methods, including those well established in the published literature, should be described here. Consider the following criteria when describing each method: 1. Is the assay fit for purpose? 2. Is the assay directly or indirectly (i.e. a surrogate) related to a key event relevant to the final adverse effect in question? 3. Is the assay repeatable? 4. Is the assay reproducible?
No OECD methods are available to measure glutamate release.
There are radioactive assays like [3H]glutamate release assay and spectrophotometric commercially available kits to measure glutamate in cell culture medium (release) or intracellular (cell lysate) using LC-MS. Furthermore, neurotransmitters including glutamate can be measured by HPLC with fluorescence detector.
Domain of Applicability
Whereas glutamatergic transmission in vertebrates is excitatory, mediated by glutamate-gated cation channels, glutamate serves as both an excitatory and an inhibitory transmitter in invertebrates (Cleland, 1996).
Blanke ML, VanDongen AMJ. (2009) Activation Mechanisms of the NMDA Receptor. In: Van Dongen AM, editor. Biology of the NMDA Receptor. Boca Raton (FL): CRC Press; Chapter 13. Available from: http://www.ncbi.nlm.nih.gov/books/NBK5274/
Cleland TA. (1996) Inhibitory glutamate receptor channels. Mol Neurobiol. 13: 97-136.
Mattson MP. (1996) Calcium and free radicals: mediators of neurotrophic factor and excitatory transmitter regulated developmental plasticity and cell death. Perspect Dev Neurobiol. 3: 79-91.
Mattson MP. (2008) Glutamate and neurotrophic factors in neuronal plasticity and disease. Ann N Y Acad Sci. 1144: 97-112.
Meldrum BS. (2000) Glutamate as a neurotransmitter in the brain: review of physiology and pathology. J Nutr. 130: 1007S-1015S.
Nedergaard M, Takano T, Hansen AJ. (2002) Beyond the role of glutamate as a neurotransmitter. Nat Rev Neurosci. 3: 748-755.
Platt SR. (2007) The role of glutamate in central nervous system health and disease-a review. Vet J. 173: 278-286.
Rundfeldt C, Wlaź P, Löscher W. (1994) Anticonvulsant activity of antagonists and partial agonists for the NMDAR-associated glycine site in the kindling model of epilepsy. Brain Res. 653: 125-130.
Wieronska JM, Kłeczek N, Woźniak M, Gruca P, Łasoń-Tyburkiewicz M, Papp M, Brański P, Burnat G, Pilc A. (2015) mGlu₅-GABAB interplay in animal models of positive, negative and cognitive symptoms of schizophrenia.Neurochem Int. 88: 97-109.