A few other receptors have recently been identified and characterized as cannabinoids interacting and binding receptors. PPARs are a family of nuclear receptors that include PPARα, PPARγ, and PPARβ/δ, acting as transcription factors primarily involved in the transcriptional regulation of genes linked with inflammation and metabolism
[25]. The increased transcriptional activity of PPARα after binding the endocannabinoids 2-AG and oleoyl ethanolamide (OEA) has been shown to regulate genes associated with metabolism and anti-inflammatory effects (
Figure 1)
[26]. Similarly, different phytocannabinoids can bind, interact, and regulate PPAR activity when natural and synthetic phytocannabinoids, including THC, CBD, cannabimovone (CBM), and quinone derivatives, are all characterized as PPARγ agonists (
Figure 1)
[26][27].
TRP channels, known as modulators of ion entry, mediate various neuronal signals controlling the sensation of pressure, temperature, and smell and are also regulated by cannabinoids
[28]. In recent years, it has come to light that the TRP channel also retains a functional extra-neuronal expression in immune and epithelial cells with important implications for mucosal immunology. For example, TRP channels maintain intracellular calcium homeostasis, regulating various immunological functions, such as producing and releasing inflammatory mediators, phagocytosis, or even immune-cell migration
[29]. Correspondingly with the role of ECS in immunity, AEA was identified as an endogenous agonist of TRPV1
[30], while later, N-arachidonoyl dopamine (NADA) and AEA were also characterized as TRPM8 antagonists (
Figure 1)
[31]. Furthermore, phytocannabinoids can bind and regulate TRPV channels when CBD is a potent agonist at TRPV1 and TRPM8 (
Figure 1)
[32], while delta9-THC mainly interacts with TRPV2 but also moderately modulates with TRPV3, TRPV4, TRPA1, and TRPM8 activities (
Figure 1)
[28][32]. Thus, understanding EC’s mechanism in influencing these channels is essential for various immunological and therapeutic processes, such as inflammation and its resolution and host immunity
[28].
The G protein-coupled receptors (GPCRs) are prevalent and popular pharmaceutical targets for drug design, representing one of the largest and most versatile cell surface receptors family
[33]. The activation of GPCRs is achieved by a diverse range of ligands, including chemokines, lipids, vitamins, hormones, protein, peptides, and neurotransmitters
[34]. Therefore, GPCRs are involved in many fundamental cellular and physiological processes, such as the ability to sense light, taste and smell, metabolism, endocrine, and exocrine secretion
[35].
In recent years, GPCRs have been found to play an important role also in the immune system
[35]. GPCRs, such as GPR18, GPR55, and GPR119, regulate signals associated with cell proliferation and migration
[36]. Moreover, GPR18 is expressed on human leukocytes and is believed to be responsible for regulating immune system cells during inflammation
[37]. The endocannabinoid N-arachidonoyl-l-serine (ARA-S) mediates angiogenesis and wound healing through GPR-55 (
Figure 1)
[38]. CBD in a mouse model of dravet syndrome shows antagonism of GPR-55 effectively reduced seizures
[39].
3. Phytocannabinoids and Their Use in Medicine
Phytocannabinoids are bioactive natural products found in some flowering plants, liverworts, and fungi and can be used to treat diseases or undesirable physiological symptoms
[40].
Cannabis sativa is an abundant and most studied source of phytocannabinoids
[40]. Currently, approximately 130 distinct phytocannabinoids have been isolated from cannabis plants and classified into several groups: cannabigerols (CBGs), cannabichromenes (CBCs), cannabidiols (CBDs), (−)-delta9-trans-tetrahydrocannabinols (delta9-THCs), (−)-delta8-trans-tetrahydrocannabinols (delta8-THCs), cannabicyclols (CBLs), cannabielsoins (CBEs), cannabinols (CBNs), cannabinodiols (CBNDs), cannabitriols (CBTs), and a few other miscellaneous phytocannabinoids
[40]. The most abundant
Cannabis sativa constituents are trans-delta 9-THC, CBD, CBC, and CBG, together with their respective acid forms (delta9-tetrahydrocannabinolic acid A (delta9-THCA), CBDA, CBCA, and CBGA)
[40]. However, it is essential to note that phytocannabinoids and ECs mediate their effects as antagonists or agonists by interacting with the same specific cell-surface CBRs, essentially acting as ECs.
To date, the most utilized, popular, and intensively studied phytocannabinoids are delta9-THC and CBD, although lately, other phytocannabinoids derivatives, such as Δ9-tetrahydrocannabivarin (delta9-THCV), cannabinol (CBN), cannabidivarin (CBDV), cannabigerol (CBG), and cannabichromene (CBC), have become of significant interest
[41]. Discovered in 1971, THC, the primary psychoactive compound of cannabis, was initially not widely used, but later led to the discovery of CBR-1 and a better definition of the ECS
[42]. Today, THC is an approved medical therapy drug
[43]. Nabiximols, a cannabis-based preparation containing a standardized ratio of THC and CBD, is an approved treatment for people with MS and has been used in the United Kingdom since 2010, then in Spain, Germany, Denmark, Sweden, Italy, Austria, and Poland since 2011–2012. France legalized cannabinoids in medicine in 2013
[44], and the brand name Sativex has been approved in Canada for cancer-related pain and was recently legalized in Ukraine. Moreover, THC products are commonly used as an effective remedy in a range of neurodegenerative and neurological disorders, such as Huntington, Parkinson’s, Alzheimer’s, and Tourette syndromes
[45]. In addition, currently in the U.S., there are two chemically synthesized, cannabis-based drugs approved by the U.S. Food and Drug Administration (FDA) for use primarily in oncological patients. Dronabinol (Marinol) is a gelatin capsule containing an isomer of THC, used to relieve nausea and vomiting caused by chemotherapy or enhance the appetite to achieve weight gain in AIDS patients. Although it is a synthetic-based cannabinoid, Nabilone (Cesamet), acting much as THC, is another FDA-approved alternative drug to treat nausea and vomiting caused by chemotherapy
[46].
CBD, the second most abundant phytocannabinoid, accounts for up to 40% of the plant’s extract and shows almost no affinity for CBR-1 and CBR-2, but it can antagonize the effect of THC and AEA to CBR-1
[47]. CBD does not have the same psychoactivity as THC and, thus far, CBD oil is mainly used to treat pain by affecting non-cannabinoid receptors, such as TRP channels and PPARs
[47]. Although the U.S. removed hemp and hemp extracts (including CBD) from the list of controlled substances in 2018, and the FDA approved the first CBD-containing medical product, Epidiolex
®, for the treatment of two rare epilepsy disorders, proving the tremendous medical potential of cannabis-based products
[48], the marketing and sales of CBD formulations remain a grey area in many states. Contrary to the U.S., European Union member states allow CBD products legally sold as an antioxidant, anti-sebum, skin protection, and skincare (cosmetics) product, but they are restricted from being marketed as medical products improving health
[48].