PET imaging of endocannabinoid system
The endocannabinoid system consists of at least two G-protein coupled receptors (GPCRs), CB1R and CB2R, of endogenous agonists for these receptors (endocannabinoids, including anandamide and 2-arachidonoyl glycerol), and the enzymes for the synthesis and degradation of the endocannabinoids. Endocannabinoids are produced from phospholipids, and secreted through extracellular membrane vesicles produced mainly by microglial cells in the brain, and by vascular endothelium, circulating macrophages and platelets. Anandamide (AEA) is degraded by fatty acid amide hydrolase 1 (FAAH). 2-arachidonoyl glycerol (2-AG) is produced by phospholipase C and diacylglycerol lipase (DAGL), and degraded mostly by monoacylglycerol lipase (MAGL). Endocannabinoids can also be oxidized by cyclooxygenase 2 (COX2). Intracellular Fatty acid binding proteins transport endocannabinoids to nuclear receptors and for enzymatic degradation.
CB1 receptors are abundant in the central and peripheral nervous system, also in the cerebellum, in glutamatergic and GABAergic pre- and postsynaptic terminals. Activation of presynaptic CB1Rs inhibits the neurotransmitter release, including glutamate. Low expression of CB1Rs is also seen in adipocytes and hepatocytes, gastrointestinal tract, and in the heart and skeletal muscle. Activation of CB1Rs promotes conservation of energy by increasing appetite and fat storage, and decreasing thermogenesis. PET imaging with [11C]OMAR has shown increased CB1R expression in the heart of obese humans and mice (Valenta et al., 2018). [18F]FMPEP-2 imaging has shown upregulation of CB1Rs in acute activation of brown adipose tissue (Lahesmaa et al., 2018).
CB2 receptors are predominantly expressed on cells of the immune system, and found in the brain only in small quantities, but overexpressed in activated microglia and astrocytes. Some CB2R expression is also seen in the cardiac muscle, adipose tissue, spleen, and in the pancreas. CB2Rs in tonsils may be visible in brain imaging. During inflammation the CB2Rs are upregulated in immune system cells, and activation of CB2Rs by agonists dampens the inflammation. Platelets contain both CB1 and CB2 receptors.
Anandamide and N-arachidonoyl-dopamine (NADA), another endocannabinoid, are also ligands for TRPV1 (capsaicin receptor, vanilloid receptor 1), which has a diverse tissue distribution; it is found in the brain, in glial cells, liver, inflammatory cells, smooth muscles, bladder urothelium, and in keratinocytes of the epidermis. AEA activates TRPV1, leading to an increase in intracellular [Ca2+]. Additionally, TRPV1 may contribute to or regulate the uptake of AEA into endothelial cells.
Endocannabinoids and THC are lipophilic, and therefore bound to lipoproteins in the circulation. High lipophilicity also causes extensive binding to glassware and plastics (Garrett and Hunt, 1974). Transport of anandamide and fatty acids into and from erythrocytes is very fast, but the high plasma protein binding explains that only 10-20% of THC in blood resides in or on the surface of erythrocytes (Agurell et al., 1986). Plasma-to-blood ratios are about 1.5-1.7 for THC, 11-OH-THC, and THC-COOH when measured from fresh blood samples (Giroud et al., 2001; Schwope et al., 2011; Karschner et al., 2012); ratios from frozen blood samples may be markedly higher. Thus RBC-to-plasma ratio should be about 0.08-0.26.
Several PET tracers for CB1 receptors have been introduced, including [18F]FMPEP-2, [18F]MK-9470, [11C]MePPEP, [11C]SD5024, and [11C]OMAR (also called [11C]JHU75528). There may be marked sex difference in the uptake of the CB1R radioligands, which should be taken into account in design of studies (Laurikainen et al., 2019).
The CB2 receptors show less homology between species than CB1R. The existence of different isoforms of CB2R and their tissue and species specific expression patterns, and their intracellular activation (Brailoiu et al., 2014) may hinder the development of CB2R ligands for use in humans. Some PET tracers for CB2R have been developed, such as [11C]NE40, [11C]A-836339, and [11C]KP23, but currently those are only in preclinical use.
Several PET tracers have been introduced for quantification of the activity of FAAH enzyme, including [11C]CURB (Rusjan et al., 2013; Boileau et al., 2015), [11C-carbonyl]PF-04457845, [18F]DOPP, [18F]FCHC, [11C]MFTC, and [11C]MK-3168.
In humans, a single-nucleotide polymorphism in the FAAH gene, C385A (rs324420), encodes a threonine instead of conserved proline at amino-acid position 129 (P129T). This leads to reduced expression and functionality of the endocannabinoid inactivating enzyme FAAH and therefore higher endocannabinoid levels. Proline129 homozygotes have stronger placebo effects than C385A carriers. Approximately 38% of population of European descent have one or more copies of the variant form of the enzyme. This polymorphism may affect the results obtained with PET tracers for FAAH, such as [11C]CURB, but it also provides a naturally occurring probe to examine the role of endocannabinoids in humans (Boileau et al., 2015).
Lysosomal enzyme NAAA (N-acylethanolamine-hydrolyzing acid amidase) catalyses the same reaction as FAAH.
MAGL is an important regulator of the endocannabinoid system in the central nervous system, where 2-AG is the main endocannabinoid, and MAGL is therefore a target for active drug development. MAGL inhibitors have been radiolabelled, and suitable tracers for in vivo PET imaging are being developed (Rempel et al., 2017). SAR127303 is a suicide inhibitor that binds covalently to MAGL, and [11C]SAR127303 is a promising PET tracer (Wang CN et al., 2016; Wang L et al., 2016).
GPR55 is a G-protein coupled receptor which is sometimes described as the third cannabinoid receptor, although it lacks the cannabinoid binding pocket found in CB1 and CB2 receptors, and there is no consensus on whether endocannabinoids can actually activate GPR55 (Liu et al., 2015). Instead, L-α-lysophosphatidylinositol (LPI) may be the endogenous (non-cannabinoid) ligand of GPR55. GPR55 is expressed in the central nervous system and wide range of peripheral tissues, including pancreas, spleen, adrenals, bone, gastrointestinal tract, and adipose tissue (Liu et al., 2015). Some ligands aimed to bind to cannabinoid receptors may also bind to GPR55.
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Created at: 2015-09-20
Updated at: 2018-10-31
Written by: Vesa Oikonen