21/3/2019  Extracts from Article by ProfofPot 
 
ProfOfPot wrote a comprehensive article on all the different types of cannabinoid receptors within our body.  Whilst the most well known are CB1 and CB2, endocannabinoids can bind to at least 8 more receptors. We have summarised key information below.  Quite fascinating ...
 

Cannabinoid CB1 Receptor

The CB1 receptor is hands down the most famous of the endocannabinoid system. This receptor sits within the cell membrane and upon activation, start a signaling cascade within the cell that leads to specific effects. The two most common endocannabinoids (produced within our body) to activate CB1 are anandamide and 2-AG.
The highest levels of CB1 expression are in the central nervous system (CNS). In fact, there are more CB1 receptors in the brain than any other type of GPCR. However, despite descriptions as the “brain receptor” it is also found throughout the body in many different tissues: cardiovascular, reproductive, immune, gastrointestinal, and peripheral nerves to name a few important ones. Given the wide distribution of the CB1 receptor, it is not surprising that it seems to be involved in, well, just about everything. 
 
Regulates learning and memory
Neuronal development & synaptic plasticity
Regulates reward and addiction
Reduces pain
Reduces neuroinflammation and degeneration
Regulates metabolism & food intake
Regulates bone mass
Cardiovascular effects


Cannabinoid CB2 Receptor

The CB2 receptor is located primarily in the periphery instead of the CNS (central nervous system). It is mainly expressed in immune cells, giving it an important role in inflammation. However, we now know that CB2 is expressed in a variety of cells, including those in the CNS, liver, and bone. CB1 is no longer considered to be the only cannabinoid receptor that affects memory and cognition. Using mice with the genetically-deleted receptor, many functions of CB2 have been elucidated. Interestingly, mice which lacked CB2 receptors had  more severe conditions in a variety of disease models including:
 
Allergic and autoimmune inflammatory diseases
Osteoporosis (loss of bone mass)
Neurodegenerative diseases
Ischemic injury from stroke or heart attack
Chronic pain
Hepatic (liver) injury and disease
Alcohol and nicotine addiction
Weight gain
Stress responses

Based on this animal data, there is no guarantee that activation of CB2 receptors will help these conditions in humans. For many of these conditions, there is additional supportive nonclinical and clinical evidence, but that is out of the scope of this article.
 

GPR18

This receptor was discovered in 1997, but for several years it was an “orphan receptor”, meaning that they did not know what its ligand was. In 2006, a surprising discovery was made – this receptor could be activated by endocannabinoids!
GPR18 can be activated by anandamide, but it’s main endocannabinoid ligand appears to be N-arachidonyl glycine (NAGly), which is a metabolite of anandamide.
The GPR18 receptor is expressed highly in the spinal cord, small intestine, immune cells, spleen, bone marrow, thymus, lungs, testis and cerebellum.
GPR18 activation can lower blood pressure. It also has significant functions in immune cells. It acts as a powerful chemoattractant – meaning it induces migration of immune cells.
 

GPR55

This receptor has a similar story to GPR18. It was an orphan receptor for many years until its ligands were discovered. GPR55 is activated by the endocannabinoids 2-AG and anandamide, but its main ligand appears to be another putative endocannabinoid called lysophosphatidylinositol (LPI).
This receptor is expressed at high levels in the central nervous system, as well as adrenal glands, gastrointestinal tract, lung, liver, uterus, bladder and kidneys. It’s wide tissue distribution gives it roles in a variety of body systems.
GPR55 activation causes hypotension (lowers blood pressure), is anti-inflammatory, and is in some cases anti-nociceptive (pain blocking). GPR55 regulates energy intake and expenditure, which could impact diseases such as obesity and diabetes. It is also expressed in bone cells with a possible role in osteoporosis. GPR55 is neuroprotective and decreased neurodegeneration in models of multiple sclerosis
 

GPR119

 GPR119 expression is restricted to a limited number of tissues. It is primarily found in the pancreas and gastrointestinal tract – hinting that its role is the regulation of energy and metabolism.
GPR119 is activated primarily by the endocannabinoid OEA, with minimal activation by other endocannabinoids such as anandamide and 2-AG.
Activation reduces food intake, improves handling of blood sugar, and decreases body weight. These effects appear to be mediated through regulation of hormones such as insulin and GLP-1.
 

Vanilloid Receptors

Transient receptor potential vanilloid 1 (TRPV1) is an ion channel expressed both on sensory neurons and in the brain. In sensory nerves, TRPV1 acts a sensor for things that could potentially cause tissue damage. It is activated in response to heat and proinflammatory substances, sending a pain signal to the brain. The most famous activator of TRPV1 is capsaicin, the ingredient found in chili peppers that causes a burning pain. Dysregulation of TRPV1 is also involved in chronic pain.
Interestingly, anandamide is an activator of the TRPV1 channel. Sensory neurons often co-express both the CB1 receptor and the TRPV1 receptor, making the role of anandamide in generating pain signals unclear.
TRPV1 plays a very different role in the brain, where its activation by anandamide seems to reduce pain.
 

Serotonin Receptors

There are many different serotonin (5-HT) receptor subtypes that mediate the different effects of serotonin. 
The 5-HT3 receptor is most well-known for mediated nausea and vomiting, particularly after chemotherapy. Several anti-nausea drugs work by inhibiting this ion channel. It also has a role in neuropathic pain.
Anandamide can directly bind to the 5-HT3 receptor and inhibit its activation. However, it doesn’t work by blocking the main serotonin binding site on the receptor. Instead, it binds to a different site and acts as a negative allosteric modulator. In other words, it changes the conformation of the receptor to minimize activation by 5-HT.
This inhibition of 5-HT3 is at least partly responsible for the analgesic effects of cannabinoids that are not mediated through the traditional CB1 or CB2 receptors.
 

Glycine Receptors

Glycine receptors (GlyRs) are ligand-gated ion channels which inhibit nerve activation. GlyRs are expressed in spinal interneurons, where they regulate pain transmission to the brain.
Anandamide is capable of directly binding to GlyRs and increasing channel activation. Anandamide does not bind the main agonist site nor can it activate GlyRs by itself. Like with 5-HT3 receptors, anandamide acts as an allosteric modulator. It binds a different site on the GlyR and enhances activation by glycine
This is another mechanism, independent of the CB1 and CB2 receptors, that endocannabinoids may reduce pain by acting at the spinal level.
 

Peroxisome Proliferator-Activated Receptors

Peroxisome proliferator-activated receptors (PPARs) are fundamentally different than the receptors described above. Rather than sit within the cell membrane, PPARs reside within the cell and can directly bind to DNA sequences and change transcription of targeted genes. There are three isoforms of PPAR: α, β, and γ.
Anandamide and 2-AG are potentially able to activate PPARα, but activation is much stronger by the endocannabinoids OEA and PEA. Anandamide and 2-AG may also be able to activate PPARγ.
PPARs regulate cellular functions in almost every tissue. Some of the effects of endocannabinoids which may be at least partially attributed to either PPARα or PPARγ activation include neuroprotection against ischemia and neurodegeneration, reduced nicotine addiction, analgesia, anti-tumor effects, vasorelaxation, weight reduction, and reduced inflammation.
 

Other Possible Endocannabinoid Targets

Other potential targets for endocannabinoids have been identified. However, it is not clear if these play a significant role in the effects of endocannabinoids. These include voltage-gated ion channels, NMDA receptors, acetylcholine receptors, and glycine transporters.
 
To read the full article, click HERE.