The use ofL. for medical purposes dates back to an Egyptian medical papyrus (circa 1550 BC) [ 1 ]. Among the multitude of cannabinoids present in this plant, Δtetrahydrocannabinol (Δ9-THC), cannabidiol (CBD) and cannabinol (CBN) are the three main cannabinoids that are the most present and the best described components due also to their significant presence [ 2 3 ]. Cannabis is used in three different forms with different THC concentrations: marijuana, hashish, and hash oil [ 4 ].
Cannabinoids recognize and bind to specific receptors, the main ones being recognized in the CBand CBreceptors. They are G-protein-coupled. Their polypeptide chain crosses the cell membrane seven times. The amine end remains on the extracellular side, while the carboxyl end remains on the intracellular side. They are characterized by three extracellular loops and three intracellular loops ( Figure 1 ).
The CBreceptor consists of a longer polypeptide chain than CB(472 amino acids in CB, and 360 amino acids in CB). The amino-terminal (extracellular) domain of CBis shorter. The complete amino acid sequence of the two receptors is homologous in 44% of them, while in the transmembrane domains the sequence is equal in 68% of them [ 5 6 ] ( Figure 2 ).
Another receptor belonging to the GPCR family that binds to ECs is the G-protein coupled receptor 55 (GPR55), also known as CB3. It is supposed to modulate memory, motor activity, and cognitive function because of its high expression in the brain, particularly in the cerebellum [ 7 8 ]. At the peripheral level, GPR55s, being present in osteoblasts and osteoclasts, would modulate bone metabolism [ 9 ].
Widely present in humans is GPR119, which has been shown to represent another cannabinoid receptor that is encoded by the GPR119 gene [ 8 ]. It is present predominantly found in pancreatic (beta cells) and gastrointestinal cells. Recent studies attributed to GPR119’s therapeutic effects on diabetes and obesity highlight its direct action on insulin release in the pancreatic cells and indirectly at the level of intestinal enteroendocrine cells on the production of glucagon-like peptide 1 (GLP-1) [ 10 11 ].
Δ9-THC is the main psychoactive principle of cannabis and is known as the canonical agonist of both cannabinoid receptors, namely the CBand CBreceptors, but with a relatively higher intrinsic affinity for CBthan for CB. THC is a hydrophobic and lipophilic compound [ 12 13 ]. Thus, many studies have been performed focusing on the pharmacology, therapeutic potential, and toxicity of Δ9-THC as a classical cannabinoid molecule in the last 70 years. These studies promoted the discovery and characterization of the endocannabinoid system (ECS) [ 14 ]. The ECS is made up of G-protein coupled (GPCR) cannabinoid receptors (CBand CB) and their endogenous ligands, anandamide (AEA) and 2-arachidonoylglycerol (2AG), which fall into the category of endocannabinoids (ECs) [ 14 ]. In addition to the CBand CBreceptors, the ECS also includes the peroxisome proliferator-activated receptor alpha (PPARα), GPR119, GPR55, and the transient receptor potential vanilloid 1 (TRPV1) receptors [ 15 ]. ECs are metabolized by multiple specific and non-specific enzymes. Those in the former category include fatty acid amide hydrolase (FAAH, for metabolism of AEA) and monoacylglycerol lipase (MAGL, for metabolism of 2-AG) [ 16 ]. Interestingly, since ECs share many structural similarities with prostaglandins, several interactions have between shown between the metabolic pathways for endocannabinoids and inflammatory lipids, including lipoxygenases and cyclooxygenases [ 16 ].
The CBand CBreceptors, encoded by theandgenes, respectively, are the main receptors of ECs. They have different functionalities despite sharing more than 44% of amino acid sequences [ 17 ]. CBreceptors are present in the central nervous system (the cerebellum, cerebral cortex, hippocampus, etc.), and act on cognitive functions, including memory, locomotion, and pain. At the peripheral level, CBreceptors are present in multiple locations, cardiac cells, lung cells, immune cells, reproductive tissues, gastrointestinal system tissues, in the ganglia of the sympathetic nervous system, in the urinary bladder, and in adrenal gland cells, where their functions have been recognized but not well defined [ 18 19 ]. Peripherally, CBreceptors are localized in monocytes/macrophages and poly-morphonuclear neutrophils, lymphocytes and natural killer cells in the testis, skeleton, liver and spleen. In the central nervous system (CNS), CBreceptors are localized in the hippocampus and substantia nigra and the neuronal, glial, and endothelial cells of the cortex. At the CNS level, the functions of the CBreceptor are not yet clear, but it is assumed that it may affect the neuro-immunological system [ 19 ] ( Figure 3 ).
CBD has been isolated and described earlier than Δ9-THC [ 20 ]. However, it has remained a less studied molecule of cannabis, forgetting its participation in the psychotropic effects of this plant, since CBD use is not associated with the typical behavioral effects of cannabinoids [ 21 ]. CBD is a unisomer of THC, and is known to have a better effect on anxiety, cognition, and pain, with little psychoactivity. Compared with THC, CBD has a better affinity for CBand CBreceptors, with the predominance of the latter, and could also interfere with the activity of THC [ 22 24 ].
Cannabinoids are mainly synthetized as acidic forms (A), thus Δ9-THC(A) and CBD(A) are the end-products of the enzymatic biosynthesis of cannabinoids. When exposed to heat (pyrolysis during smoking or baking), radiation, or spontaneously during storage, the compounds undergo decarboxylation and ‘spontaneous rearrangement’ reactions [ 25 ].accumulates THC and CBD in glandular trichomes in the aerial parts of the plant, but not on the root surface. Upon trichome thermal or mechanical ruptures, its contents form a sticky coating on the plant surface due to the viscous, non-crystallizing properties of cannabinoids, which will protect the plant from desiccation and/or potential herbivores [ 26 ]. The amount of cannabinoids formed in the trichomes correlates positively with increased temperatures and imposed heat stress, as well as with low soil moisture and poor mineral nutrient content [ 27 ]. Cannabinoid production may also provide an evolutionary advantage by functioning as sunscreens that absorb biologically destructive UV-B radiation (280–315 nm), as significantly increased cannabinoid production was measured in cannabis flowers after UV-B-induced stress [ 28 ]. Furthermore, cannabinoids in general [ 29 ], and CBD in particular [ 30 ], have a significant antimicrobial action, which confers high climate resistance and soil adaptability to cannabis. Thus, phytocannabinoids convey various biologically beneficial properties for the plant.
The use of CBD has always represented a complicated legal issue worldwide, and this has often restricted the scientific studies and professional awareness about its therapeutic applications. Apart from this, numerous individual findings suggest some therapeutic effects of CBD, which have been reported to include antipsychotic, anticonvulsant, neuroprotective, anxiolytic, and sleep-promoting effects [ 31 ]. Furthermore, pre-clinical and clinical studies attributed a desirable safety profile to CBD [ 31 ], associated with its anti-inflammatory effects [ 32 ]. However, elucidating the pharmacodynamics of CBD has always proven to be difficult for scientists, beginning with initial reports in which CBD was shown to weakly bind to cannabinoid receptor orthosteric sites when compared to canonical agonists [ 33 ], indicating that CBD’s effects might be independent of the cannabinoid receptors. This conclusion was proved to be partially true by other studies, where researchers found a direct interaction of CBD with several receptors, enzymes and ion channels. Recently, however, some reports found both a direct and an indirect modulation of ECS activity from CBD [ 33 ]. Taken together, these findings point out CBD as a novel promising phytocannabinoid-based medicine. Indeed, the therapeutic uses of CBD are mostly linked to its anti-inflammatory, antioxidant, and analgesic properties [ 34 ]. Thus, CBD is endowed with many potential applications, such as in bone tissue processes [ 35 36 ], neuroprotection, epilepsy, anxiety, and cancer [ 37 ]. CBD also has some other effects that have not been fully studied, including relaxation, improved sleep, and stress relief, given edible, tincture, and vape formulations of the drug ( Figure 4 ).
In recent years, thanks to various public and private institutions, much research and development of CBD have been performed, especially with regard to its therapeutic uses. Indeed, approximately USD 30 bilions is expected to be reached by the CBD market in 2025 [ 38 ]. Amongst the various potential uses of cannabinoids [ 39 ], and CBD in particular [ 40 ], dentistry and oral medicine have recently attracted greater attention [ 15 ]. In particular, studies have been exploring the possible medical applications of CBD use in the oral cavity [ 41 ], together with the functional and anatomical characterization of the ECS in this part of the body [ 15 43 ], in addition to its modulation by pathological status [ 15 44 ]. The aim of this narrative review is to provide an historical overview on cannabis use and the ECS, as well as to explore the mechanism of action of CBD and to summarize the recent scientific and technological discoveries of current CBD use and its possible future applications in the field of oral health ( Figure 5 ).
1.1. Cannabis and the Endocannabinoid System in Human History
Consumed in the form of Marijuana, hashish, or bhang, cannabis sativa extracts are the most widely used recreational drugs, with more than 200 million cannabis users worldwide (World Drug Report 2020, United Nations). Its recreational and therapeutic use are due to its psychoactive effects, amongst others, such as changes in sensory perception, relaxation, and euphoria [ 45 ]. However, the Cannabis Sativa plant is one of the first plants that was used by man for fiber, food, medicine, religious, or recreational contexts. The first reference to the use of cannabis as medicine comes from a Chinese pharmacological treatise attributed to Emperor Shen Nung (3000 BCE), which makes cannabis one of the recreational drugs with the longest recorded history of human use [ 45 ].
The medicinal use of cannabis was present in most ancient civilizations, and was used by the Assyrians, Egyptians, Greeks, and Romans. The Aryan and Indo-European populations who lived in ancient Iran and India (source Treccani.it, Britannica.com), used cannabis in their societies, and given their migrations in prehistoric times, they might have passed on their knowledge to other groups [ 46 ]. Cannabis had several applications: as a bandage for swelling and bruising; in fumes for arthritis; either as a drink or in the food for depression, for kidney stones, for impotence, and for annulling witchcraft. In ancient India, it was prepared in the form of a mild drink, called bhang, and it was described as an anti-anxiety drug thanks to its power “to free people from distress” (circa 1500 BCE) [ 46 ].
Ancient civilizations were aware of the dual nature of cannabis and its psychoactive proprieties, and some texts defined it as “the drug which takes away the mind”. Similarly, accounts of its nefarious effects were reported: “hashish eases the muscles of the limbs, but it produces senseless talks”, and “if taken in excess it produces hallucinations and a staggering gait”. If taken for long periods of time it causes people to communicate with spirits [ 47 ]. Nonetheless, in some traditions, such as Indian medicine, the use of cannabis has persisted for centuries. Around 1840, William O’Shaughnessy, an Irish doctor working in India with the British Army, observed the proprieties of cannabis-derived drugs in the treatment of cramps, headaches, convulsions, neuralgia, sciatica, and tetanus [ 46 ]. The medical use of cannabis was then re-introduced in Europe, and experimental work suggested that the Indian claims about cannabis-based treatment were indeed likely spread [ 23 ].
49, The beginning of the 20th century saw the use of medicinal cannabis curtailed due to its chemical and physical properties which made the creation of standardized and reliable preparations impossible. In the same period, the development of synthetic fibers such as nylon led to a sharp decline in cannabis cultivation for textile purposes [ 48 ]. Although cannabis used by textile industries represents a variety without psychoactive properties, known as hemp, its application was significantly associated with marijuana. Indeed, in 1937 a US federal law, the Marijuana Tax Act, restricted the usage and cultivation of all cannabis, without distinction between hemp and marijuana. After that, given its popularization for recreational use around the world, cannabis was classified as a substance of abuse, and any application of the plant was prohibited [ 48 50 ].
The prohibition of cannabis also had a negative impact on scientific research. However, investigations into the chemistry and pharmacology of the plant did not completely stop, and the analysis of resin extracts allowed for the identification of several compounds. Among these, tetrahydrocannabinol (THC) was suspected to be the main psychoactive constituent of cannabis, but its structure was not fully characterized [ 51 ]. Only 20 years after this, the development of Nuclear Magnetic Resonance (NMR) spectroscopy allowed for the designation of Δ9-tetrahydrocannabino l [ 52 ]. This event opened new frontiers in the understanding of cannabis proprieties and its related neuronal substrates. Indeed, cannabis is the source of at least 66 compounds now known as cannabinoids [ 53 ]. CBN;, which is probably formed from THC during the conservation of harvested cannabis, was the first plant cannabinoids (phytocannabinoids) to be discovered at the end of the 19th century, and comes from a red oil extract of cannabis. CBN structures were determined in the early 1930s by R.S. Cahn, with its chemical synthesis first achieved in 1940 in the laboratories of R. Adams in the USA and Lord Todd in the UK. A second phytocannabinoid, (−) CBD, was first obtained from cannabis in the same year by Adams and colleagues, probably associated to cannabidiolic acid, while THCs were first extracted from cannabis in 1942 by Wollner, Matchett, Levine and Loewe, most likely as a mixture of (−)-Δ8- and (−)-Δ9-THC. Both THC and CBD are present in cannabis, mainly as decarboxylated acids upon the heating and combustion of cannabis. The structures and stereochemistry of CBD and Δ9-THC, naturally occurring as an (−)-enantiomer, were discovered by Raphael Mechoulam and colleagues, for CBD in 1963 and for Δ9-THC in 1964, respectively. It was also in Mechoulam’s laboratory, in 1965, that (±)-Δ9-THC and (±)-CBD were first synthesized, a development that was soon followed by the synthesis of the (+)- and (−)-enantiomers, both of these cannabinoids, and of Δ8-THC [ 54 ].
In the last decade, cannabis legislation has changed in several countries. A large number of countries around the world approved the legalization of medicinal cannabis and hemp (finally discerned from marijuana). Additionally, Uruguay, Canada, Georgia, Mexico, South Africa, and 18 US states legalized recreational cannabis consumption, and in many other countries, mostly in Asia, the use of cannabis has been decriminalized [ 55 ]. Globally, the public acceptance of legalizing cannabis and its medical application has increased, and therefore a better understanding of its plethora of effects on the human brain and body is of central interest.
After the THC discovery, extensive studies on the pharmacology and biochemistry of cannabis were carried out, with particular interest regarding the mode of action of THC and other cannabinoids of the plant. Two mechanisms were postulated: the first hypothesis was based on the lipophilic nature of cannabinoids, suggesting that they might act via the chemical interaction with biological membranes, modifying their proprieties; the second one suggested that cannabinoids might act through still undiscovered receptors, thereby modulating cellular signaling. This second hypothesis was based on the observation that THC acts by reducing the activity of adenylyl cyclase (AC), but only in particular cell types, indicating the specific and not ubiquitous action of THC, as expected for cannabinoid-induced membrane fluidity changes [ 56 ].
The development of synthetic cannabinoids helped to address this issue. Indeed, compounds such as CP-55,940, which is 10–100 times more potent in vivo than THC, allowed the autoradiography of cannabinoid-specific binding sites in brain sections from several mammalian species, including humans. The study revealed a specific and conserved labeling profile, suggesting the presence of a specific receptor [ 56 57 ].