Glutamatergic system and PET
L-glutamate is a non-essential amino acid, key component in cellular metabolism, and a major constituent of proteins. Glutaminase converts L-glutamine to glutamate. Glutamate dehydrogenase (GDH) breaks glutamate into ammonia and α-ketoglutarate, which enters the TCA.
L-glutamate is the most abundant neurotransmitter in the nervous system, used in >90% of synapses, and linked to other neurotransmitter systems. Glutamate can be transported across the blood brain barrier by a high affinity transport system, but its concentration in the extracellular space is kept at a low level and tightly controlled in the synapse. Glutamate is an excitatory neurotransmitter, while γ-aminobutyric acid (GABA), which is synthesized from glutamate by the enzyme L-glutamic acid decarboxylase, is the main inhibitory neurotransmitter. Excess concentration of glutamate can induce hyperexcitability, and even excitotoxicity and cell death, in postsynaptic neurons.
Dipeptide N-acetyl-L-aspartyl-L-glutamate (NAAG) is one of the neuropeptides and an abundant neurotransmitter that activates metabotropic glutamate receptor mGluR3. NAAG is a substrate of glutamate carboxypeptidase (PSMA); PSMA expression may be involved with glutamate excitotoxicity (Bařinka et al., 2012).
There are two very different glutamate receptor groups, ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs, respectively). Ionotropic glutamate receptors are fast-acting ion channels, which activate when glutamate binds to the receptor, and the flow of ions triggers membrane depolarization in the post-synaptic cell, inducing signal transmission. The actions of G-protein-coupled metabotropic glutamate receptors on the ion channels are indirect and slow, involving a secondary messenger system, gene expression, and protein synthesis; mGluRs may function as enhancers of the excitability of the neuron, or on presynaptic side, as inhibitor of neurotransmitter release.
Glutamate receptors are located on the dendrites of postsynaptic neurons, but also on astrocytes, oligodendrocytes, and on endothelial cells. iGluRs are also found in heart in cardiac nerve terminals, ganglia, conducting fibres, and possibly in myocardiocytes; in the pancreas, modulating the secretion of insulin and glucagon; in the kidneys; in lungs; in lymphocytes and platelets; in the skin; in bone osteoclasts and osteoblasts; and in the gastrointestinal tract.
Ionotropic glutamate receptor channels consist of heterotetrameric or homotetrameric subunits. The subunits are divided into three families, named after the ligand that specifically binds to the subunit type: AMPA, kainic acid (kainate), and NMDA. AMPA subunits include GluA1, GluA2, GluA3, GluA4 (previously GluR1-4); kainate subunits include GluK1 (GluR5), GluK2 (GluR6), GluK3 (GluR7), GluK4 (KA-1), and GluK5 (KA-1); and NMDA subunits include GluN1 (NR1), GluN2A (NR2A), GluN2B (NR2B), GluN2C (NR2C), GluN2D (NR2D), GluN3A (NR3A), and GluN3B (NR3B).
AMPA receptors (AMPARs) consist of four AMPA subunits. [11C]K-2 is a promising PET radioligand for assessing AMPARs in the brain (Miyazaki et al., 2020; Hatano et al., 2021). TARP γ-8 constitutes an auxiliary subunit of AMPARs, and [11C]TARP-2105 is a promising radioligand for its assessment in the brain (Yu et al., 2022; Yamasaki et al., 2023).
NMDA receptors (NMDARs) contain only NMDA subunits, and have different subunit distributions in different brain regions. Ketamine is an NMDA receptor agonist. Activation of neuronal NMDARs may require binding of glutamate to GluN2 subunit and co-agonist, such as glycine or D-serine, to GluN1 subunit. Actions of glycine are restricted to extrasynaptic sites, while D-serine is the key NMDAR modulator in synapses. GluN2B antagonists may have less side effects than drugs used to block ion channels. [18F]PF-BN1 and (R)-[18F]OF-MeNB1 are promising radioligands for imaging GluN2B subunits (Ahmed et al., 2019; Zheng et al., 2022; Smart et al., 2022), as well as [18F]GE-179 (McGinnity et al., 2014; Vibholm et al., 2021a and 2021b; Galovic et al., 2021) and (R)-[11C]Me-NB1 (Rischka et al., 2022).
Delta family (δ receptors) can be considered as the fourth iGluR family; these receptors are mainly found in cerebellar Purkinje cells.
There are eight subtypes of metabotropic glutamate receptors, mGluR1-8. mGluR1, mGluR4 and mGluR6-8 increase the [Ca2+] in cytoplasm. mGluR5 activates K+ channels, enabling the release of K+. mGluR2 and mGluR3 inhibit adenylyl cyclase, and decrease cAMP concentration. Group 1 receptors (mGluR1 and mGluR5) are located only in postsynaptic neurons. Group 2 receptors (mGluR2 and mGluR3) are located also on presynaptic neurons, possibly suppressing glutamate transmission. Group 3 receptors (mGluR4 and mGluR6-8) are located on presynaptic neurons, inhibiting neurotransmitter release. Presynaptic mGluRs may play a role in anxiety disorders, group 1 receptors in learning and memory problems. Changes in mGluR5 are seen in depressive disorders and long-term antidepressant use.
The mGluR1 is expressed heterogeneously in the human brain, most predominantly in the cerebellum. PET radioligands for this receptor include [18F]FIMX (Zanotti-Fregonara et al., 2016), [18F]FITM (Yamasaki et al., 2012), [11C]ITMM (Sakata et al., 2017) and [18F]MK-1312 (Hostetler et al., 2011).
PET radiopharmaceuticals for mGluR2 are being developed. Problem has been that radioligands with excellent binding profile in vitro show poor brain uptake or low mGluR2 specificity in vivo. For instance, [11C]JNJ-42491293 does not seem to bind mGluR2 in vivo in humans, macaques, or marmosets (Kang et al., 2023). [18F]mG2P026 and [11C]mG2P001 have shown promising results in animal studies (Yuan et al., 2022 and 2023).
Several PET radiopharmaceuticals for mGluR5 have been developed, including [18F]FPEB (Wong et al., 2013; Sullivan et al., 2013; Park et al., 2015; Leurquin-Sterk et al., 2016), [18F]PSS232 (Müller Herde et al., 2015; Warnock et al., 2018), [11C]ABP688 (Elmenhorst et al., 2010; DeLorenzo et al., 2011; DuBois et al., 2016; Esterlis et al., 2018), and [18F]FMTEB (Krzyczmonik et al., 2023).
The mGluR5R binding of [18F]FPEB and [11C]ABP688 in the brain has high within-day variation (DeLorenzo et al., 2017). This suggests that physiological processes modify glutamate levels. A combined MRS and [11C]ABP688 PET study has shown that glutamate levels in human brains are dynamically altered depending on food intake and changed plasma glucose levels (Kubota et al., 2021). Age-related reduction in [18F]FPEB brain binding appears to be caused by tissue loss (Mecca et al., 2021).
Since the level of mGluR5 in white matter and cerebellum is low, reference region methods can be applied in the analysis. Venous blood sampling cannot be used instead of arterial sampling, at least not with [18F]FPEB, since parent fractions and total blood activity is lower in venous blood than in arterial blood (Sullivan et al., 2013). Low affinity (Z)-isomer of [11C]ABP688 reduces estimates of binding potential (Smart et al., 2019).
Glutamate transporters are located on both pre- and postsynaptic neurons, and on astrocytes (astroglia) in the central nervous system (CNS), but are also found in other tissues, including heart and liver. Glutamate that is released into the synapse is sequestered into neurons and glia by high-affinity glutamate transporters, and metabolized by astrocytes to glutamine.
Excitatory amino acid transporters (EAATs) belong to the SLC1 family of transporters. There are five subtypes of EAATs, EAAT1-5. EAATs remove glutamate from extracellular spaces into neurons and other cells. In addition to L-glutamate, they also transport L- and D-aspartate. In CNS, subtypes EAAT1 (GLAST) and EAAT2 (GLT-1) are found mainly on glial cells. EAAT2 is responsible of most of the glutamate reuptake. Additionally, glial cells convert glutamate into glutamine, which is taken up by presynaptic neurons, converted back to glutamate, and stored in vesicles. EAAT3 and EAAT4 are expressed only on neurons, and EAAT5 only in the retina. EAAT3 is present also in Schwann cells of peripheral nervous system and in kidneys and intestine.
In the presence of decreased extracellular Na+ these transporters can function in reverse, releasing glutamate to the extracellular space.
Vesicular glutamate transporters (VGluTs) belong to the SLC17 family of transporters. These intracellular transporters concentrate cytoplasmic L-glutamate into synaptic vesicles. The three vesicular glutamate transporters VGluT1-3 are dependent on the proton gradient between the cytosol and vesicles, and the vesicular glutamate concentration is ∼0.1 M.
Cystine-glutamate antiporter (xCT, xC-, SLC7A11) is located in plasma membranes, and it supports non-vesicular release of glutamate. Glutamate released by system xC- activates extrasynaptic but not synaptic NMDA iGluRs. During brain ischaemia, an excessive release of glutamate, mainly via xCT, triggers neuronal death through the over-activation of NMDA iGluRs. EPO can block glutamate release and prevent cell death. PET imaging with [18F]FSPG has shown increased xC- function in ischaemic rats (Soria et al., 2014; Domercq et al., 2016) and in rat model of MS (Martín et al., 2016). xCT is vital to antioxidant defence in the brain, and its expression and activity is rapidly upregulated under oxidative stress.
- xC- antiporter
- GABAergic system
- Cholinergic system
- Endocannabinoid system
- Dopaminergic system
- [13N]NH4+ in brain
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Updated at: 2023-08-02
Created at: 2017-10-03
Written by: Vesa Oikonen