Major Phytocannabinoids and Their Related Compounds: Should We Only Search for Drugs That Act on Cannabinoid Receptors?

2.2.1. Cannabigerol (CBG)-Type Compounds

Cannabigerol-type compounds represent one of the most structurally diverse classes of phytocannabinoids, being the second most abundant in the Cannabis plant, making up 16.3% of the phytocannabinoid content, with the most important compounds being represented by: cannabigerol (CBG), cannabigerolic acid (CBGA), cannabigerovarin (CBGV), cannabigerovarinic acid (CBGVA), ortho-methyl cannabigerol, and cannabigerolic acid methyl ether [8].

Cannabigerol (CBG), a minor cannabinoid present in small amounts (<1%) in the Cannabis plant, serves as the direct precursor to cannabidiol (CBD) and tetrahydrocannabinol (THC), and it was the first natural cannabinoid to be synthesized [64]. The compound was purified from Cannabis in the same year as THC (1964) by Gaoni and Mechoulam, and, soon enough, it was found that CBG does not have the same psychotropic effects as THC [105].

Cannabigerolic acid (CBGA) is one of many minor cannabinoids produced by the Cannabis plant; however, CBGA is at the top of the cascade reaction that produces the three major cannabinoids: THC, CBD, and CBC. In small proportions, CBGA may convert to CBG, but most of the CBGA produced in the Cannabis plant converts into either THC or CBD [106].

Cannabigerivarin (CBGV) and cannabigerivarinic acid (CBGVA) were isolated by Shoyama et al. between 1975 and 1977: CBGV from the benzene extract of Cannabis, and CBGVA from an extract of dried leaves of Thai Cannabis [107,108].

Cascio et al. found that CBG binds the CB1 receptor from mouse brain membranes with Ki = 381 nM, and the CB2 receptor from CHO cells expressing the human receptor with Ki = 2.6 μM [86]. In 2014, Rosenthaler et al. obtained Ki values of 897 nM for CB1 and 153 nM for CB2 in competition assays [69]. Additionally, CBG did not produce psychotropic effects in the in vivo tests, but it did affect endocannabinoid function indirectly by inhibiting anandamide uptake, contributing to increasing the levels of anandamide [76].

In a 2020 study, it was demonstrated that CBGA has a very low affinity for both CB1R and CB2R using [3H]-CP-55940 as a ligand, and thus the compound lacks pharmacological effects due to CB receptor modulation, while CBGV was the compound with greater affinity. From this class of phytocannabinoids, CBGV remains the only one with questionable behavior regarding cannabinoid receptors because, in the same study, the authors concluded that CBGV has a complex behavior, acting as a potent agonist, through Gi and MAPK pathway activation, studied in vitro, but with the observation that, in vivo, CBGV acts as an inverse agonist of cannabinoid receptors [67].

Because these compounds showed only a marginal affinity for CB receptors, they were not taken into account in studies until recently, but it seems that they have a lot of other pharmacological effects with possible applications in the treatment of neurodegenerative diseases, inflammatory disease, cancer, anxiety, and depression [72], as well as infectious diseases, metabolic disorders, and psoriasis [64,109,110].

It was found that this class of compounds along with derivative compounds presents many different actions such as neuromodulatory, neuroprotective, anti-inflammatory, analgesic, anxiolytic, antidepressant, and antibacterial activity, or that they can be used to treat cancer, psoriasis, metabolic diseases such as diabetes and their complications, anorexia, or cancer-induced cachexia by mechanisms that do not involve CB receptors [72,74,86,111,112].

Transient receptor potential ankyrin 1 (TRPA1) is a good target for the discovery of novel medicines to treat pain because it has a primary role in nociceptive transduction and neurogenic inflammation, contributes to noxious cold sensation, and plays a role in neuropathic and inflammatory pain [113]. TRPA1 is activated by CBG with an EC50 value of 3.4 ± 1.0 µM (0.6 ± 0.1), but it was found that through cyclization, the activity on this receptor is increased, and the cyclized analog of CBG could be considered for future studies as an analgesic and anti-inflammatory agent [75].

Transient Receptor Potential Melastatin-8 (TRPM8) is a non-selective cation channel activated by cold temperature and by cooling agents, and a neuronal sensor that plays a role in cold and mechanical allodynia associated with neuropathic pain secondary to trauma. It was proved that this channel is involved in pain perception, and that TRPM8 activation or deactivation can modulate analgesia [114]. De Petrocellis demonstrated in his paper that CBG is the second most efficient TRPM8 antagonist, and he found that a “CBG-free” extract from the Cannabis plant (with the exact quantity of CBG that was extracted from the plant) was inactive per se, but when added to pure CBG, the activity of the CBG-enriched extract was significantly increased, and it was more efficient in antagonizing TRPM8 than pure CBG. Thus, he suggested that there might be a synergistic effect between pure CBG and some of the components of its corresponding Cannabis extract [72].

Transient receptor potential vanilloid-3 (TRPV3) and transient receptor potential vanilloid-type 4 (TRPV4) are indirectly involved in gastrointestinal inflammation from inflammatory bowel diseases because they function as sensors of harmless and non-harmless chemical or physical stimuli [115]. It was demonstrated on in vitro models of overexpressed TRPV3 channels (against carvacrol) and overexpressed TRPV4 channels (against 4α-phorbol-12,13-didecanoate(4α-PDD)) that CBG produced a significant TRPV3 and TRPV4 desensitization as a result of their activation, which was associated with an anti-inflammatory effect [76]. In another in vivo model of murine colitis produced by intracolonic administration of dinitrobenzene sulphonic acid (DNBS), CBG had an anti-inflammatory effect associated with the downregulation of cytokines and inducible nitric oxide synthase expression (iNOS) levels [77].

In a study from 2012, the same author measured the activity of CBGA and CBGV on in vitro models of overexpressed TRPV3 channels (against carvacrol) and overexpressed TRPV4 channels (against 4α-phorbol-12,13-didecanoate(4α-PDD)). The results suggested that these two compounds desensitize TRPV3 and TRPV4 channels at lower doses than those at which they stimulate these channels. These findings have led to the conclusion that these compounds are good candidates for studies on in vivo models of conditions involving overexpression of TRPV3 and TRPV4 channels, which involve pain and inflammation in inflammatory bowel disease (IBD) [76].

TRPV1 is activated by heat, protons, and proinflammatory cytokines and is associated with pain and inflammation. The results of several studies suggest the potential of TRPV1 agonists, antagonists, and positive allosteric modulators in the treatment of pain [104,105,106]. It seems that CBGA can stimulate human TRPV1 sufficiently in order to be a potent candidate for future studies on TRPV1-modulating effects (EC50 = 1.0–2.0 mM) to treat pain and inflammation [72].

In a 2011 study, CBG and CBGA showed inhibition of more than 30% of cyclooxygenase-1 (COX-1) and cyclooxygenase-1 (COX-2) in a concentration-dependent manner, thus having enhanced anti-inflammatory effects [62]. CBG has also been found to have anti-inflammatory action by interfering with the prostaglandin E2 (PGE2) synthesis pathways at various levels: by inhibiting the enzyme cytosolic phospholipase A2 (PLA2), which catalyzes the production of arachidonic acid from membrane phospholipids, or by inhibiting monoacylglycerol lipase (MAGL), which catalyzes the production of arachidonic acid from 2-arachidoylglycerol [25].

Animal studies (mice) have shown that CBG has antinociceptive and anti-inflammatory effects on several acute pain models such as those induced by using intraperitoneally administered formalin and carrageenan. Both models are mediated by α2-adrenoceptor because they are blocked by yohimbine, an α2-adrenoceptor antagonist [25].

In a computational model of α2A, α2B, and α2C isoforms of murine and human adrenoceptors, CBG affinity for the receptor appeared higher than that of the α2-adrenergic receptor agonist clonidine. This affinity of CBG seems to be associated with sedation and analgesia and needs future research [116].

CBG exhibits the best growth inhibition against human oral epithelioid carcinoma cell lines and fibroblasts and has antiproliferative action in mouse skin melanoma cells [117,118].

It was also shown that TRPM8 expression strongly increases in prostatic cancer, and in neuroendocrine tumor cells from pancreatic cancer and colon cancer. CBG seems to be a potent TRPM8 antagonist (IC50 = 0.16 ± 0.02), which suggests it may have a therapeutic effect in these types of cancers [72].

In another study published in 2014, it was shown that CBG inhibits the growth of colorectal cancer cells primarily through a pro-apoptotic mechanism and blocks colon carcinogenesis in vivo. This effect of CBG is associated with reactive oxygen species overproduction, and the authors suggested that CBG can be a promising curative and preventive pharmacological agent for colorectal cancer [111].

Transient receptor potential cation channel subfamily V member 1 (TRPV1), also named “capsaicin receptor”, was recently reported to be aberrantly expressed in many tumor types such as breast cancer, skin tumors, or colon cancer. It was found in several studies that its activation by capsaicin was associated with antiproliferative effects. That is why the TRPV1 channel offers new treatment possibilities for cancer [119]. In a study of the effects of cannabinoids on ionotropic TRP channels conducted by De Petrocellis in 2011, it was demonstrated that CBGA stimulated human TRPV1 enough to be a potent candidate for future studies on agents that modulate TRPV1 [72].

CBGV is the most important antagonist of TRPV2 (EC50 = 1.7 μM), enough to be considered as a potential pharmacological agent to treat cancer because the overexpression of TRPV2 has been linked to several cancer types and cell lines such as urothelial cancers, prostate cancer, breast cancer, esophageal squamous cell carcinoma, and benign hepatoma, and in hepato-carcinomas and hematological malignancies, such as myeloma or acute myeloid leukemia [72]. TRPV2 channels control multiple processes involved in cancer progression by modulating survival, cell proliferation, angiogenesis, migration, and invasion in different cancer types. Clinical data suggest there might be a direct relationship between altered TRPV2 expression and a negative prognosis [81].

A recently published article concluded that CBGA is a good candidate for colorectal cancer treatment, with increased toxic activity on colorectal cancer cells and reduced activity on normal colon cell lines. In the same study, it was found that CBGA acts synergistically with THCA against neoplastic cells, but with a CBGA fraction that is greater than the THCA fraction. The cytotoxic activity of CBGA and THCA+CBGA most often led to cell cycle arrest and apoptotic cell death for cancer cells. CBGA, THCA, and CBGA+THCA are also active on adenomatous polyps, suggesting another possible therapeutic value [120].

TRPV1 is involved in the modulation of anxiety and may have implications in the treatment of depression [121]. TRPV1 may affect the locomotor activity and plays a role in thermosensation as it can be activated by noxious heat (>42 °C) [122,123]. Due to all these findings and considering De Petrocellis’s studies on the modulatory action of CBGA on TRPV1, we can assume that CBGA could be a potent candidate for future studies on TRPV1-modulating effects to treat the neurological disorders in which these channels are involved, such as anxiety or depression [72,76].

In an experiment performed on mouse brain membranes, it was proved that CBG can activate α2-adrenoreceptors and block 5-HT1A receptors, antagonizing the 5-HT1A receptor agonist R-(+)-8-hydroxy-2-(di-n-propylamino) tetralin. The authors concluded that CBG has the ability to inhibit noradrenaline uptake in the [35S] GTPγS binding assay, the brain needs further investigation, and more in vivo experiments are required to verify that the in vitro dose is enough to also be effective in vivo without adverse effects [86].

In an in vivo model of multiple sclerosis produced with TMEV (Thaler’s murine encephalomyelitis virus), CBG and CBG-quinone showed anti-inflammatory and neuroprotective effects through the inhibition of IL-1β and IL-6 cytokines, and downregulation of PGE2 synthesis. CBG and CBG-quinone inhibited the microglia inflammatory response, protected neurons from toxic insults in vitro, and restored motor function impairment [124].

Using an in vivo model of Huntington’s disease (HD), created using nitopropionic acid (3-NPA), CBG efficacy was tested using a dose of 10 mg/kg/day administered i.p. and was shown to have neuroprotective effects by downregulating the proinflammatory markers COX-2, iNOS, IL-6, and TNF-α, by preventing neuronal degradation, downregulating disease-associated genes SgKL and CD44, and normalizing specific protein-1 levels. Clinically, specific symptoms such as dystonia and tightening of the hind limbs were visibly improved, and locomotor activity was enhanced [125].

In an in vitro model of neuroinflammation on NSC-34 motor neurons, pretreatment with CBG (7.5 μM) improved viability in treated cells through the inhibition of cell apoptosis, a reduction in IL-1β, TNF-α, IFN-γ, and PPAR-γ proinflammatory protein levels, a reduction in oxidative stress, and upregulation of Nrf-2 levels [74].

CBG blocked cell death, reduced oxidative damage, and prevented neurons from accumulating toxic β-amyloid protein, in an in vitro Alzheimer’s disease model [126].

In an in vitro study on the effect of CBGA and CBG on the aldose reductase (ALR2) enzyme, it was found that both drugs showed statistically significant ALR2 inhibitory activity by being able to interact with the major active site of the enzyme. Considering that ALR2 is a key enzyme involved in diabetic complications, the results obtained in this study may have some relevance in medicine to prevent or treat possible diabetic complications [79].

In a 2019 in silico study, a computer simulation was developed in order to assess CBGA’s role in activating peroxisome proliferator-activated receptors (PPARs) that regulate metabolism. Three phytocannabinoids, namely, CBGA, CBDA, and CBG, were thus identified as being PPARα/γ dual agonists. The study established that they act as full or partial agonists on PPARα/γ isoforms. Effects on downstream gene transcription in adipocytes and hepatocytes were also noted, further certifying their roles as functional dual agonists [80]. It is already known that peroxisome proliferator-activated receptor agonists such as rosiglitazone and pioglitazone are used to treat diabetes to improve the pathogenesis of insulin resistance and hyperglycemia; therefore, CBGA can be considered a good candidate for the treatment of diabetes complications [127].

It was found that higher doses (120 to 140 mg/kg) of GBG induce a dose-dependent increase in food intake, increase the number of meals taken, decrease the latency until the first meal, and improve locomotor activity [128]. Another study also found that pure CBG (120 mg/kg) can attenuate weight loss induced by the chemotherapy agent cisplatin [25]. Due to these findings, CBG could be useful for the treatment of feeding disorders and weight loss in cancer anorexia-cachexia syndrome [25,128].

It was found that CBG has antibacterial properties against Gram-positive bacteria, mycobacteria, and fungi [109]. Recently, the antibacterial properties of CBG were tested against various methicillin-resistant Staphylococcus aureus (S. aureus) strains of current clinical relevance, and the results were promising [110].

In a 2020 study on CBG antibacterial activity against methicillin-resistant S. aureus (MRSA), it was found that, in vitro, CBG acts through the disruption of the cytoplasmatic membrane of MRSA. Using an in vivo model of systemic MRSA infection in mice, CBG was shown to express antibacterial action comparable to vancomycin, at a non-toxic dose of 100 mg/kg, suggesting that CBG could be a less toxic alternative for the treatment of methicillin-resistant Gram-negative bacteria [129].

In a study on the effects of CBG in skin conditions, CBG demonstrated inhibitory action on keratinocyte proliferation in a CB1/CB2 receptor-independent manner, being a good candidate for the treatment of psoriasis [130]. Additionally, CBG acted as a transcriptional repressor controlling cell proliferation and differentiation through a mechanism that involved increasing DNA methylation on the keratin-10 gene, making CBG a good candidate for the development of novel therapeutics for skin disease [131].

2.2.2. Cannabidiol (CBD)-Type Compounds

Cannabidiol (CBD) is considered to be the second phytocannabinoid in abundance after Δ9-THC, biosynthesized by the plant Cannabis sativa, with a defensive role against parasites that may affect the plant, being its main non-psychomimetic compound [26,27,132]. Numerous preclinical and clinical studies have reported that, due to the absence of psychoactive effects, CBD could have great potential in the treatment of symptoms characteristic of neuropsychiatric disorders (anxiety, depression, substance use disorders, dependence on various drugs, epilepsy, psychosis, Parkinson’s, Alzheimer’s, multiple sclerosis, Huntington’s disease), migraine, inflammatory diseases, rheumatoid arthritis, etc. [26,27,28,29,133,134].

The CBD acidic precursor cannabidiolic acid (CBDA) is one of the most present phytocannabinoids in European hemp [32]. Of all cannabinoid acids, CBDA seems to be the one with the weakest pharmacological activity, being studied thus far for its effects in pain, inflammation, and nausea but also for its therapeutic potential in treating breast cancer and in relieving the symptoms of Dravet syndrome [92,94,135,136].

Cannabidivarin (CBDV), isolated in 1969 by Vollner et al., is a CBD homolog that has begun to attract the attention of researchers for its pharmacological profile because it has low activity at CB1 receptors, thus lacking the psychotropic effects related to CB1 receptor activity [67,137]. Regarding CB2 receptors, the affinity of CBDV for these receptors is still under scrutiny, the results of the latest studies being contradictory. While Zagzoong et al. reported a high affinity for these receptors measured using Chinese hamster ovary (CHO) cells expressing human CB2 receptors, Navarro reported a low affinity using a heterologous system expressing human versions of CB1 and CB2 receptors. In this situation, the best way to solve this contradiction is by performing more in vivo tests on the effects of CBDV on models such as chronic and acute pain, epilepsy, or anxiety as pathologies in which the activity of CB2 receptors could be involved [65,67].

CBD pharmacokinetics are complex and variable, with low oral bioavailability due to incomplete oral absorption and high hepatic clearance, but can be greatly increased (4-fold) when combined with high-fat or high-calorie meals. Due to its low bioavailability, CBD shows a high pharmacokinetic variability with consequences on clinical response. Due to its highly lipophilic nature, it has a large volume of distribution (23–43 L/kg), is highly bound to plasma proteins (>94%), and the time required to reach the maximum plasma concentration when administered as a single dose is 3–5 h. Its plasma half-life depends on the dose and route of administration, with great variability between humans and rodents, ranging from 18 to 32 h, making dosing difficult at this time. It is excreted via feces [67,87].

The numerous effects of CBD have increased the interest in its pharmacological properties. It has a low affinity for cannabinoid receptors but can act as a negative allosteric modulator of CB receptors. Although it has a low affinity for these receptors, it is nevertheless interesting that CBD may favor increases in endogenous endocannabinoid levels by indirect mechanisms. However, its actions are much more varied: for example, it intervenes in modulating the activity of some neurotransmitter transporters such as dopamine, norepinephrine, adenosine, and glutamate [84,138].

It can be an agonist of transient receptors (TRPV1), of 5-HT1A receptor, and of PPARγ receptor, or it can modulate the activity of numerous ion channels and various enzymes as well as modulating G55 protein-coupled receptor (GPR55) [84].

CBDV has been studied in vitro and has been shown to stimulate TRPA1, TRPV1, and TRPV2 channels in a dose-dependent manner [72,88], to act as an antagonist for TRPM8 [72] channels and GPR55 [134], and to act as an inverse agonist of GPR6 [90]. CBDV may also indirectly affect CBR signaling, by inhibiting the cellular uptake of AEA, or by inhibiting diacylglycerol-lipase-α (DAGLα) [72]. In vivo, CBDV also acts as a partial agonist for dopamine D2-like receptors [139].

CBD, CBDA, and CBDV have been found to have numerous beneficial effects on the relief of various symptoms, especially in central nervous system disorders. CBD was tested in various neurological disorders such as depression and mood disorders, schizophrenia, Dravet syndrome (DS), Lennox–Gastaut syndrome (LGS), and Alzheimer’s, and in other types of pathologies such as neuropathic pain and inflammation [29,87,91,135,138,140,141]. CBDV has also become a good candidate as a therapeutic agent for neurological diseases such as epilepsy and autism spectrum disorders, Rett syndrome, ischemic strokes, or inflammatory pathologies such as inflammatory bowel disease (IBD) [91,142,143,144].

The first preclinical study showing that CBD could be effective in relieving depressive symptoms was published in 2010; in a murine (mouse) model, CBD reduced the immobility time in mice undergoing a forced swimming test, the effect being similar to that produced by antidepressants such as imipramine. The authrors concluded that the effects of CBD are most likely initiated by the activation of 5-HT (1A) receptors, receptors involved in the neurobiology of depression [85].

Based on the idea that CBD does not bind directly to CB1 receptors, the antidepressant effect may be due to indirect modulation of the endocannabinoid system in the prefrontal cortex with subsequent activation of CB1 receptors, thus promoting 5-HT(1A) activation in cortical and limbic brain regions. Both in vivo and in vitro studies have shown that chronic CBD treatment promotes hippocampal neurogenesis and synaptogenesis by increasing anandamide signaling in the hippocampus, while endocannabinoid system signaling promotes neurogenesis via CB1 and CB2 receptors. These observations may represent evidence to support CBD as a potential drug to treat mood disorders [26,84].

A recent randomized, double-blind, placebo-controlled clinical trial evaluated the acute effects of tetrahydrocannabinol, cannabidiol, and their combination on the Emotional Recognition Facial Affect Test which showed that inhalation of a 16 mg single dose improved subjects’ performance on this test by 60% in emotion intensity [145]. Other clinical studies have shown that a single dose of CBD (300/600 mg/kg) reduced anxiety in healthy volunteers during public speaking, effects that may depend on changes in brain regions involved in emotional processing [26].

In recent years, research in the field has been increasingly focused on understanding the pathophysiological mechanisms involved in drug addiction, a serious public health problem, with CBD recently being considered as a potential therapeutic approach. In this regard, a preclinical study published in 2021 investigated the possible beneficial effects of CBD on relapse symptoms after amphetamine re-exposure, drug relapse being the most difficult clinical factor to control during addiction treatment. The study was conducted on 43-day-old rats, an age that was selected as it corresponds to the adolescent period, a highly vulnerable period for the development of drug abuse conditions. CBD treatment was able to prevent amphetamine relapse behavior in rats that had previously exhibited amphetamine-conditioned place preference and modulated immunoreactivity of dopaminergic targets in the prefrontal cortex and ventral striatum, areas with major involvement in drug dependence. Amphetamine impairs dopaminergic neurotransmission by altering dopamine transport, but in contrast, this study showed that CBD was able to maintain dopamine transport levels. However, this study could not state whether CBD treatment reversed the molecular changes underlying amphetamine conditioning [27].

Other preclinical and clinical studies suggest that CBD could also be useful in treating schizophrenia, since this drug seems to exert antipsychotic effects in various animal models. Thus, CBD may restore deficiencies in prepulse inhibition (PPI) of the startle reflex. PPI is described by the response decrement that occurs when an acoustic stimulus is preceded by one below the subthreshold. In schizophrenia, this behavioral modification is proposed to reflect the impaired sensorimotor gating. Furthermore, the disruption of PPI may also be induced by compounds that facilitate the inhibition of glutamatergic or dopaminergic neurotransmissions such as N-methyl-d-aspartate (NMDA) receptor antagonist, amphetamine (AMPH), or dizocilpine (MK-801) [132]. Other studies showed that CBD reduced apomorphine-induced stereotypy and amphetamine- and ketamine-induced hyperlocomotion, and enhanced extracellular dopamine levels in the nucleus accumbens, with doses required for its antipsychotic action being higher compared to those used to induce anxiolytic effects [26].

Epidiolex, a prescription medicine that contains CBD, was the first drug approved in June 2018 by the Food and Drug Administration (FDA) [84,140,146], and in 2019, it was also approved by the European Medicines Agency (EMA), in the treatment of seizures associated with Dravet syndrome (DS) and Lennox–Gastaut syndrome (LGS), as well as seizures associated with tuberous sclerosis [26,87,147].

The mechanisms involved in the anticonvulsant action of CBD are still unclear but may involve antagonism of G55 protein-coupled receptor (GPR55), inhibition of adenosine reuptake, and desensitization of vanilloid type 1 receptor (TRPV1) [87].

GPR55 is currently considered an orphan receptor whose activation triggers a series of events followed by intracellular Ca²⁺ release with modulation of neurotransmitter release and neuronal excitability [141]. It acts as an antagonist of GPR55, reducing the frequency, severity, and duration of spontaneous seizures, which has been demonstrated and validated in a genetic mouse model of Dravet syndrome [89].

Rett syndrome (RTT) is a rare genetic neurological disorder characterized by severe impairments affecting the ability to speak, walk, eat, and even breathe easily; it is most often diagnosed in children [143]. GPR55 was found to be increased in the Rett syndrome mouse hippocampus, suggesting that GPR55 antagonists could be potential pharmacological agents for this pathology [144]. Given that CBDV proved to have antagonistic proprieties on GPR55 [148], several studies on RTT mouse models were performed in order to study the effects of CBDV on disease symptoms. After 14 days of daily administration, CBDV proved to attenuate brain alterations, restore the compromised general status, increase sociability, and partially restore motor coordination in treated mice [144], and after 4–9 weeks of administration, CBDV delayed the appearance of neurological defects [143].

In a preclinical study using rats with autism-like behavior, created through prenatal valproic acid exposure, CBDV proved to ameliorate behavioral abnormalities, restore hippocampal endocannabinoid signaling, and decrease neuroinflammation, indicating that CBDV could be a potential therapeutic agent for autism spectrum disorders [149].

As for vanilloid type 1 receptor desensitization, CBD can modulate the intracellular Ca²⁺ concentration. The effect of TRPV1 activation and inhibition on the seizure threshold is complex, and although it is mainly a TRPV1 agonist, CBD causes rapid desensitization of the channel with a role in the antiseizure activity.

In the brain, adenosine acts as an endogenous anticonvulsant by stimulating A1 and other adenosine receptors. CBD is a potent inhibitor of the equilibrative nucleoside transporter that mediates adenosine reuptake followed by an increased extracellular concentration [147].

Another CNS pathology is Alzheimer’s disease, a condition mainly characterized by the formation of senile β-amyloid plaques and neurofibrillary tangles caused by hyperphosphorylation of tau proteins. In in vitro studies, CBD inhibited tau hyperphosphorylation and reduced Aβ production, and in in vivo studies, it reversed cognitive impairments on a double AD transgenic mouse model (APP/PS1) [26].

The acidic form of CBD was studied for its anticonvulsant effect in a mouse model of Dravet syndrome, and it was found that CBDA exhibited significant anticonvulsant properties through a mechanism that could involve the 5-HT1A, GPR55, or TRPV1 receptors [136].

It is important to note that CBD does not only act on CNS structures. Available data suggest that CBD has a potent anti-inflammatory effect, which could make it a promising candidate for various diseases, being a modulator of the immune system, causing a decrease in proinflammatory cytokines (interleukin 1-β, interleukin 6, interferon-β) in lipopolysaccharide-activated microglial cells as well as interleukin 10 and 12 in murine macrophages [150].

Recent studies have shown that CBD has promising potential in chronic, neuropathic, and inflammatory pain [82]. Cannabidiol modulates chronic neuropathic pain and depression-specific behavior by activating 5-HT1A and CB1 receptors in the prefrontal cortex, a fact which has been demonstrated in animal models. A preclinical study in Wistar rats aimed to create a model of neuropathic pain induced by sciatic nerve injury. There is a close link between chronic neuropathic pain and depression, with the prelimbic division of the medial prefrontal cortex being directly involved in both conditions. The authors found that local cannabidiol administration attenuated mechanical allodynia as well as depression-like behavior. This study showed that acute systemic administration of cannabidiol increases extracellular serotonin levels through activation of 5HT1A and CB1 in the ventromedial prefrontal cortex, involved in the regulation of emotional impairment, as cannabidiol may be proposed as a potential drug with therapeutic indications for the treatment of depressive disorders associated with chronic neuropathic pain [83].

A 2021 study concluded that CBDV and CBG (discussed previously) have neuroprotective and anti-inflammatory properties on an in vitro model of ischemic stroke obtained by exposing cells to ischemic conditions through oxygen–glucose deprivation [151].

The anti-inflammatory activity of CBDV was studied previously on an IBD mouse model of DNBS- and dextran sulfate sodium (DSS)-induced colitis. It was found that the administration of CBDV (orally or intraperitoneally) reduced the specific signs of colon inflammation–neutrophil infiltration and increased colon weight and intestinal permeability [91]. In the same study, human colonic tissues from children with active ulcerative colitis were treated in vitro with CBGV, and it was shown that this treatment produced a significant reduction in the proinflammatory cytokine levels (IL-1β) [91].

CBDA was also reported to produce anti-inflammatory and anti-hyperalgesia effects on carrageenan-induced hyperalgesia and edema in rodent models of inflammatory pain when administrated i.p. 60 min before carrageenan [135].

CBDA was also studied for its anticancer properties, and the authors reported that this compound is a potent MDA-MB-231 breast cancer cell migration inhibitor, through a mechanism that is supposed to involve the activation of RhoA (an inhibitor of cancer cell mobility), and, at the same time, the inhibition of cAMP-dependent protein kinase A [94].

In two studies, from 2013 and 2020, researchers found that CBDA can potently suppress nausea and vomiting in rats through the activation of the serotonin 1A receptor (5-HT1A) using animal models of acute lithium chloride-induced nausea [92,93].

However, CBD pharmacology is complex, affecting many different targets. It appears to bind to the same transmembrane protein site as cholesterol, interacting directly with a wide range of targets. Both cholesterol and CBD alter membrane elasticity and can specifically or non-specifically inhibit a wide range of transmembrane targets [136]. Cannabinoids such as CBD are transported to the endoplasmic reticulum by fatty acid-binding protein, the endoplasmic reticulum being the site of cholesterol homeostasis. CBD appears to alter cholesterol homeostasis by modulating the PPARγ receptor which lowers cholesterol levels by reducing HMG-reductase and increasing CYP7A1, but this mechanism is inconsistent with recent evidence that CBD can increase cholesterol levels [140].

As side effects, CBD can cause drowsiness/sedation, diarrhea, decreased appetite, rash, fatigue, sleep disturbances, increased liver transaminases, anemia, viral infections, and behavioral changes. Due to hepatic metabolism, it may cause drug–drug interactions in combination with drugs metabolized by the cytochrome P450 superfamily, such as warfarin-type anticoagulants, direct oral anticoagulants (dabigatran), antiaggregants (clopidogrel), and various antiepileptic drugs (clobazam, topiramate, zonisamide) [29,82,87,146]. In a 2019 study, Andreson et al. showed that CBD–clobazam interaction could be used to improve the anticonvulsant effect of CBD, but only when an anticonvulsant dose of CBD is used, meaning a sub-anticonvulsant dose of CBD did not potentiate the effects of clobazam, despite the presence of pharmacokinetic interaction (Anderson et al., 2019). These are not the only drug interactions of CBD, and its metabolism can be inhibited by ketoconazole (CYP3A4 inhibitor) or induced by rifampicin. Other reports suggest that CBD may increase plasma concentrations of tacrolimus and everolimus [29,82,87,146,147].