Researchers have been studying how the compounds in the Cannabis plant act on individual cells, both in the brain and elsewhere in the body. This knowledge is crucial to determining exactly how cannabis and its constituents affect users. Studies indicate that cannabinoids produce most of their effects by binding to proteins, called receptors, on the surfaces of certain types of cells. Many different types of receptor proteins stud the exterior membranes of cells throughout the human body. Each receptor recognises only a few specific molecules, known collectively as ligands. When the appropriate ligand binds to its receptor, it typically sets off a chain of biochemical reactions inside the cell. Many compounds, including hormones and neurotransmitters, exert their effects by acting as ligands at different receptors.
The cellular receptors that bind cannabinoids and their chemical relatives are known as cannabinoid receptors, and all vertebrate animals have similar types of cannabinoid receptors on their cells. Some invertebrates, such as molluscs and leeches, also have cannabinoid receptors, an indication that the receptors fulfill similar functions in a broad range of animal species. Moreover, it suggests that cannabinoid receptors have existed at least since vertebrates first evolved, more than 500 million years ago. To date, scientists have identified two main types of cannabinoid receptors, known as CB1 (cloned in 1990) and CB2 (cloned in 1993). CB1 receptors are extraordinarily abundant in the brain, with ten times as many cannabinoid receptors as opioid receptors (responsible for the effects of heroin and other opiates as well as the body’s own endorphins). CB2 receptors, on the other hand, are relatively scarce in the brain but plentiful in the immune system, white blood cells, tonsils, spleen, bone marrow and pancreas. CB2 receptors may also be expressed by certain central and peripheral neurons.
The molecular structure of CB1 is shown as a yellow ribbon with the bound stabilising antagonist AM6538 as orange sticks. The active ingredient in cannabis, THC, is shown as yellow sticks.
In an October 2016 study, ‘Crystal Structure of the Human Cannabinoid Receptor CB1‘, the Director of the Center for Drug Discovery, Northeastern University in Boston (US) and co-author said, “We found that the CB1 receptor consists of multiple sub-pockets and channels. This complex structure will allow chemists to design diverse compounds that specifically target portions of the receptor to produce desired effects”.
A human cannabinoid receptor (blue) with the cannabinoid inhibitor taranabant (magenta) bound at the receptor’s binding pocket sitting on a molecular surface (grey)
This 3-D illustration of a human cannabinoid receptor was published online in November 2016 in a study, ‘Highest-resolution model to date of brain receptor behind marijuana*’s high‘ out of the University of Texas (US) Southwestern Medical Center. Lead author, Dr Daniel Rosenbaum, Assistant Professor of Biophysics and Biochemistry said, “What is most exciting from a therapeutic standpoint is that the same receptor pocket that binds tetrahydrocannabinol (THC) also binds cannabinoid inhibitors that have been studied as possible treatments for conditions such as obesity. The structure is an important step toward explaining how cannabinoids initiate signals in the brain that affect the release of neurotransmitters that relay messages between the brain’s neurons. This 3-D structure provides high-resolution details of the binding pocket in the CB1 receptor, where plant cannabinoids like THC, cannabinoids made in the body (endocannabinoids) and synthetic cannabinoid inhibitors all work to modulate receptor function and physiology“. He said the CB1 receptor is the target for cannabinoid inhibitor compounds now under study as possible treatments for epilepsy, pain control, obesity and other conditions.
Ligands activating the CB1 and CB2, which are G protein-coupled receptors (GPCR‘s, the largest family of membrane receptors translating extracellular into intracellular signals) include the phytocannabinoid Δ9-tetrahydrocannabinol (THC) and numerous endogenous (produced in the body) compounds known as endocannabinoids. Some of these ligands activate or block one type of cannabinoid receptor more potently than the other type. Cannabinoid receptor ligands undergo orthosteric (binds at the active site) or allosteric (binds elsewhere) with non-CB1, non-CB2 established GPCR’s, deorphanised receptors such as GPR55, ligand-gated ion channels, transient receptor potential (TRP) channels, other ion channels or peroxisome proliferator-activated receptors (PPAR’s, receptor proteins that function as transcription factors regulating the expression of genes). From these data, it is clear some ligands that interact similarly with CB1 and/or CB2 receptors are likely to display significantly different pharmacological profiles.
Cells bearing cannabinoid receptors respond to ligand binding in a variety of ways. When THC binds CB1 receptors in some nerve cells, for example, it triggers a cascade of reactions that ultimately slow down nerve impulses. This process could dull pain signals travelling along the same nerves, providing pain relief. Likewise, when THC binds CB2 receptors on white blood cells, it may impede their natural response to infection, a bad thing if it lowers a person’s resistance to disease but a good thing if it reduces painful inflammation. Although CB1 and CB2 share some structural and functional similarities, the two receptor types are different enough that it may be possible to design ligands that, unlike THC, would act on only one of them. Medicines based on these ligands would be expected to have fewer side effects due to their greater precision. Nervous system responses to THC and other cannabinoid receptor agonists include therapeutically beneficial effects of analgesia, attenuation of the nausea and vomiting in cancer chemotherapy (CINV), appetite stimulation in wasting syndromes and decreased intestinal motility. Side effects accompanying these therapeutic responses include temporary alterations in cognition and memory, euphoria and sedation.
The CB1 and CB2 cannabinoid receptors are activated by three major chemical classes of ligands;
- endocannabinoids (eicosanoids), N-arachidonoylethanolamine (Anandamide, AEA) and 2-Arachidonoylglycerol (2-AG),
- aminoalkylindoles (cannabimimetic).
CB1 is expressed in all brain structures and in decreasing amounts in the olfactory bulb, cerebellum, hippocampus, basal ganglia, cortex, amygdala, hypothalamus, thalamus and brain stem. Overall, CB1 is known to be the most abundant GPCR in the mammalian brain and for this reason it used to be referred to as the “brain cannabinoid receptor”. In addition to the brain, CB1 is also expressed in the peripheral nervous system and in almost all mammal tissues and organs including the gastrointestinal tract, heart, liver, adipose tissue, lungs, adrenal glands, smooth and skeletal muscle, male and female reproductive systems, bone and skin. The crucial role of this receptor in the maintenance of homoeostasis during several mammalian functions has been demonstrated by the use of both pharmacological and genetic tools. Many studies have reported that the loss of CB1 receptor function may be associated with disorders affecting both central and peripheral organs.
Like CB1 receptors, CB2 receptors modulate an array of signalling pathways. Activation of CB2 receptors by natural or synthetic ligands favours a range of receptor conformations that can variably affect different signalling pathways. It appears CB2 receptors are abundantly expressed in cells belonging to the immune system. In these cells, CB2 receptor activation reduces release of pro-inﬂammatory factors. It appears CB2 receptors have the ability to control the activation and migration of immune cells and represent key regulators of inﬂammatory and nociceptive (relating to or denoting pain arising from stimulation of nerve cells) responses.
Signalling between nerve cells
Signal transmission between two neurons (nerve cells) begins as the sending neuron releases chemical messengers called neurotransmitters. Neurotransmitter molecules move across the gap to the receiving neuron, where they are bound by receptors on its surface. Binding may activate the receptor, triggering a chain of events that can alter thought and behaviour. The magnified view shows a variety of ligands binding to different types of receptors present on neurons. Anandamide, which is produced by the body, and THC, the main neuro-active ingredient in cannabis, can function as neurotransmitters. Both compounds bind and activate cannabinoid receptors on nerve cells, much as other neurotransmitters bind and activate their own specific receptors.
In recent years researchers have discovered natural ligands that bind only to CB1 or CB2 and have synthesised a few selective ligands. These compounds represent an encouraging start toward developing novel medicines based on cannabinoids. When researchers identify a receptor in the human body that binds a particular compound, such as THC, they try to find molecules that naturally interact with the receptor in order to learn more about how the receptor functions and what purposes it serves. Scientists have identified several chemicals produced in the body that act on the cannabinoid receptors, CB1 and CB2. The best studied among these compounds, anandamide (from ananda, the Sanskrit word for “bliss”), appears to act throughout the body, especially on the central nervous system (CNS).
Anandamide is present in high concentrations, along with abundant CB1 receptors in areas of the brain that control learning, memory, movement, coordination and responses to stress. Significant amounts of anandamide are also found in the spleen, which has numerous CB2 receptors, and the heart. Compared with THC, anandamide binds cannabinoid receptors weakly. As a result, the reactions that anandamide provokes are probably milder than those triggered by THC. Moreover, enzymes in the body quickly break down anandamide, so its effects are also relatively short lived. Another factor that limits anandamide’s activity is a phenomenon known as re-uptake, the rapid re-absorption of certain types of neurotransmitters after their release from nerve cells, which protects neighbouring nerve cells from over-stimulation. In some cases, this protection system can be adjusted to provide a therapeutic benefit.
Locations and functions of brain regions with abundant cannabinoid receptors. Several regions of the brain, which govern a wide range of body functions, contain high concentrations of cannabinoid receptors. Abundant cannabinoid receptors are also present in the following areas not shown in this view of the brain: the basal ganglia, which controls movement; the nucleus of the solitary tract, which governs visceral sensation, nausea and vomiting; the nucleus acumbens, the brain’s reward centre; and the central grey area, which registers pain relief.
In addition to anandamide, researchers have identified several chemicals produced by the human body that bind to cannabinoid receptors and they are continually finding more. These compounds are thought to perform a broad range of functions in the brain. Over the next few years scientists are likely to learn much more about these naturally occurring endogenous cannabinoids. Researchers have also noted that cannabinoids can affect the body without binding to receptors. Both THC and cannabidiol (CBD) have been shown to reduce toxic forms of oxygen that build up in tissues under stress, as do the antioxidant vitamins A and C. Also, because cannabinoids dissolve easily in the fatty membranes enclosing every cell, they may alter membrane function and the activity of enzymes and proteins embedded in cell membranes. These properties, too, may prove medically useful.
Several reviews have comprehensively considered the range of CB2 receptor ligands that have been synthesised and characterised, ‘Therapeutic utility of cannabinoid receptor type 2 (CB2) selective agonists’, in 2013 and, ‘Latest progress in the identification of novel synthetic ligands for the cannabinoid CB2 receptor’, in 2014), for example. An interesting development in the identification of naturally occurring ligands for CB2 is the existence of a number of abundant phytochemicals that engage CB2 receptors. Perhaps the best example of this is the terpene, β-caryophyllene. A 2008 study, ‘Beta-caryophyllene is a dietary cannabinoid’, and a 2014 study, ‘Functionalization of β-caryophyllene generates novel poly-pharmacology in the endocannabinoid system’, offer a starting point for novel compounds that influence endocannabinoid signalling. A key concept to keep in mind when evaluating experiments conducted with CB2 ligands is that many of the commonly used CB2 ligands are only relatively selective with regard to CB1. This is because most of the commonly encountered CB2 ligands were evolved from molecules that have appreciable affinity for CB1 receptors. Therefore, the concentrations of CB2-preferring agonists that are commonly encountered in the literature can result in significant occupancy of CB1 receptors, with subsequent signalling. Similarly, CB2-preferring antagonists can substantially antagonise CB1-mediated responses.
Activation of either CB1 or CB2 results in a sustained increase of ceramide, which triggers apoptosis (programmed cell death in which a sequence of events leads to the elimination of cells without releasing harmful substances), thus activation of CB1 receptors can result in programmed cell death (cancer is one of the scenarios where too little apoptosis occurs, resulting in malignant cells that will not die). Anandamide (AEA) can also activate the intracellular binding site of TRPV1 receptors, leading to apoptosis whereas CB1 activation can prevent the effects induced by TRPV1 activation, resulting in protection against apoptosis.
Expanded from Marijuana As Medicine?: The Science Beyond the Controversy, with Cannabinoid receptors: Introduction, Granny Storm Crow’s List – 2016, Endogenous cannabinoids revisited: A biochemistry perspective, Effects of Cannabinoids on Neurotransmission, Classification of cannabinoid receptors, The different ways through which specificity works in orthosteric and allosteric drugs, Deorphanisation of G protein-coupled receptors … new insights in nervous system pathophysiology ..., Endocannabinoids and Endocannabinoid Related Mediators … Role In Neurological Disorders, Introduction to Cannabinoids, and CB2 Cannabinoid Receptors as a Therapeutic Target—What Does the Future Hold?
*Cannabis sativa L., is the correct botanical term, marijuana is a North American colloquialism