Cannabinoids are several structural classes of compounds found in the cannabis plant primarily and most animal organisms or as synthetic compounds. The most notable cannabinoid is the phytocannabinoid tetrahydrocannabinol (THC) (delta-9-THC), the primary psychoactive compound in cannabis. Cannabidiol (CBD) is also a major constituent of temperate cannabis plants and a minor constituent in tropical varieties. At least 113 distinct phytocannabinoids have been isolated from cannabis, although only four have been demonstrated to have a biogenetic origin. It was reported in 2020 that phytocannabinoids can be found in other plants such as rhododendron, licorice and liverwort, and earlier in Echinacea.
Cannabinoid receptors, located throughout the body, are part of the endocannabinoid system of vertebrates– a class of cell membrane receptors in the G protein-coupled receptor superfamily. As is typical of G protein-coupled receptors, the cannabinoid receptors contain seven transmembrane spanning domains. Cannabinoid receptors are activated by three major groups of ligands:
Tetrahydrocannabivarin is a homologue of tetrahydrocannabinol (THC) having a propyl (3-carbon) side chain instead of pentyl (5-carbon), making it non-psychoactive in lower doses. It has been shown to exhibit neuroprotective activity, appetite suppression, glycemic control and reduced side effects compared to THC, making it a potential treatment for management of obesity and diabetes. THCV was studied by Roger Adams as early as 1942.
The endocannabinoid system (ECS) is a biological system composed of endocannabinoids, which are endogenous lipid-based retrograde neurotransmitters that bind to cannabinoid receptors, and cannabinoid receptor proteins that are expressed throughout the vertebrate central nervous system and peripheral nervous system. The endocannabinoid system remains under preliminary research, but may be involved in regulating physiological and cognitive processes, including fertility, pregnancy, pre- and postnatal development, various activity of immune system, appetite, pain-sensation, mood, and memory, and in mediating the pharmacological effects of cannabis. The ECS plays an important role in multiple aspects of neural functions, including the control of movement and motor coordination, learning and memory, emotion and motivation, addictive-like behavior and pain modulation, among others.
2-Arachidonoylglycerol (2-AG) is an endocannabinoid, an endogenous agonist of the CB1 receptor and the primary endogenous ligand for the CB2 receptor. It is an ester formed from the omega-6 fatty acid arachidonic acid and glycerol. It is present at relatively high levels in the central nervous system, with cannabinoid neuromodulatory effects. It has been found in maternal bovine and human milk. The chemical was first described in 1994–1995, although it had been discovered some time before that. The activities of phospholipase C (PLC) and diacylglycerol lipase (DAGL) mediate its formation. 2-AG is synthesized from arachidonic acid-containing diacylglycerol (DAG).
The transient receptor potential cation channel subfamily V member 1 (TRPV1), also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the TRPV1 gene. It was the first isolated member of the transient receptor potential vanilloid receptor proteins that in turn are a sub-family of the transient receptor potential protein group. This protein is a member of the TRPV group of transient receptor potential family of ion channels. Fatty acid metabolites with affinity for this receptor are produced by cyanobacteria, which diverged from eukaryotes at least 2000 million years ago (MYA). The function of TRPV1 is detection and regulation of body temperature. In addition, TRPV1 provides a sensation of scalding heat and pain (nociception). In primary afferent sensory neurons, it cooperates with TRPA1 to mediate the detection of noxious environmental stimuli.
Transient receptor potential cation channel subfamily M (melastatin) member 8 (TRPM8), also known as the cold and menthol receptor 1 (CMR1), is a protein that in humans is encoded by the TRPM8 gene. The TRPM8 channel is the primary molecular transducer of cold somatosensation in humans. In addition, mints can desensitize a region through the activation of TRPM8 receptors.
N-Arachidonyl glycine receptor, also known as G protein-coupled receptor 18 (GPR18), is a protein that in humans is encoded by the GPR18 gene. Along with the other previously "orphan" receptors GPR55 and GPR119, GPR18 has been found to be a receptor for endogenous lipid neurotransmitters, several of which also bind to cannabinoid receptors. It has been found to be involved in the regulation of intraocular pressure.
G protein-coupled receptor 55 also known as GPR55 is a G protein-coupled receptor that in humans is encoded by the GPR55 gene.
Cannabinoid receptor 1 (CB1), is a G protein-coupled cannabinoid receptor that in humans is encoded by the CNR1 gene. The human CB1 receptor is expressed in the peripheral nervous system and central nervous system. It is activated by endogenous cannabinoids called endocannabinoids, a group of retrograde neurotransmitters that include lipids, such as anandamide and 2-arachidonoylglycerol (2-AG); plant phytocannabinoids, such as docosatetraenoylethanolamide found in wild daga, the compound THC which is an active constituent of the psychoactive drug cannabis; and synthetic analogs of THC. CB1 is antagonized by the phytocannabinoid tetrahydrocannabivarin (THCV).
G protein-coupled receptor 119 also known as GPR119 is a G protein-coupled receptor that in humans is encoded by the GPR119 gene.
N-Arachidonoyl dopamine (NADA) is an endocannabinoid that acts as an agonist of the CB1 receptor and the transient receptor potential V1 (TRPV1) ion channel. NADA was first described as a putative endocannabinoid (agonist for the CB1 receptor) in 2000 and was subsequently identified as an endovanilloid (agonist for TRPV1) in 2002. NADA is an endogenous arachidonic acid based lipid found in the brain of rats, with especially high concentrations in the hippocampus, cerebellum, and striatum. It activates the TRPV1 channel with an EC50 of approximately of 50 nM which makes it the putative endogenous TRPV1 agonist.
Abnormal cannabidiol (Abn-CBD) is a synthetic regioisomer of cannabidiol, which unlike most other cannabinoids produces vasodilator effects, lowers blood pressure, and induces cell migration, cell proliferation and mitogen-activated protein kinase activation in microglia, but without producing any psychoactive effects.
N-Arachidonylglycine (NAGly) is a carboxylic metabolite of the endocannabinoid anandamide (AEA). Since it was first synthesized in 1996, NAGly has been a primary focus of the relatively contemporary field of lipidomics due to its wide range of signaling targets in the brain, the immune system and throughout various other bodily systems. In combination with 2‐arachidonoyl glycerol (2‐AG), NAGly has enabled the identification of a family of lipids often referred to as endocannabinoids. Recently, NAGly has been found to bind to G-protein coupled receptor 18 (GPR18), the putative abnormal cannabidiol receptor. NaGly is an endogenous inhibitor of fatty acid amide hydrolase (FAAH) and thereby increases the ethanolamide endocannabinoids AEA, oleoylethanolamide (OEA) and palmitoylethanolamide (PEA) levels. NaGly is found throughout the body and research on its explicit functions is ongoing.
O-1602 is a synthetic compound most closely related to abnormal cannabidiol, and more distantly related in structure to cannabinoid drugs such as THC. O-1602 does not bind to the classical cannabinoid receptors CB1 or CB2 with any significant affinity, but instead is an agonist at several other receptors which appear to be related to the cannabinoid receptors, particularly GPR18 and GPR55. These previously orphan receptors have been found to be targets for a number of endogenous and synthetic cannabinoid compounds, and are thought to be responsible for most of the non-CB1, non-CB2 mediated effects that have become evident in the course of cannabinoid research. O-1602 produces some effects shared with classical cannabinoid compounds such as analgesic and antiinflammatory effects and appetite stimulation, but it does not produce sedation or psychoactive effects, and has several actions in the gut and brain that are not shared with typical cannabinoid agonists.
Lysophosphatidylinositol, or L-α-lysophosphatidylinositol, is an endogenous lysophospholipid and endocannabinoid neurotransmitter. LPI, along with its 2-arachidonoyl- derivative, 2-arachidonoyl lysophosphatidylinositol (2-ALPI), have been proposed as the endogenous ligands of GPR55.
CID16020046 is a compound which acts as an inverse agonist at the former orphan receptor GPR55, and may be the first selective inverse agonist characterised for this receptor. It was found to block a number of GPR55 mediated responses such as wound healing and activation of immune system T-cells and B-cells, as well as showing inverse agonist activity in the absence of GPR55 agonist stimulation. However while it was found to have good selectivity over the related CB1 and CB2 cannabinoid receptors as well as a number of other targets, CID16020046 has not yet been tested against another related receptor GPR18, so its selectivity for GPR55 over this target has not been established. It has antiinflammatory actions, has been used to study the interaction between GPR55 mediated and CB1 mediated activity, and research using this compound has revealed a role for GPR55 in learning and memory.
PSB-CB5 (CID-85469571) is a compound which acts as an antagonist at the former orphan receptor GPR18, and is the first selective antagonist characterised for this receptor, with an IC50 of 279nM, and good selectivity over related receptors (over 36x selectivity vs CB1 and GPR55, and 14x vs CB2.) As all previously known antagonists for GPR18 also antagonise GPR55, it has been difficult to separate the effects of these two receptor targets, so the discovery of a selective GPR18 antagonist is expected to be useful in research into the actions of this receptor.
Cannabidiol-dimethylheptyl (CBD-DMH or DMH-CBD) is a synthetic homologue of cannabidiol where the pentyl chain has been replaced by a dimethylheptyl chain. Several isomers of this compound are known. The most commonly used isomer in research is (−)-CBD-DMH, which has the same stereochemistry as natural cannabidiol, and a 1,1-dimethylheptyl side chain. This compound is not psychoactive and acts primarily as an anandamide reuptake inhibitor, but is more potent than cannabidiol as an anticonvulsant and has around the same potency as an antiinflammatory. Unexpectedly the “unnatural” enantiomer (+)-CBD-DMH, which has reversed stereochemistry from cannabidiol, was found to be a directly acting cannabinoid receptor agonist with a Ki of 17.4nM at CB1 and 211nM at CB2, and produces typical cannabinoid effects in animal studies, as does its 7-OH derivative.
Cannabicitran (CBTC) is a phytocannabinoid first isolated in 1974 as a trace component of Cannabis sativa, Structurally related compounds can be found in some other plants. It is not psychoactive, but was found to reduce intraocular pressure in tests on rabbits, which may reflect agonist activity at the NAGly receptor that is known to be a target of many structurally related cannabinoids.
