Introduction

Endocannabinoids (ECs) refer to every chemical substance (Table 1) produced endogenously that resembles cannabinoids and does bind with the cannabinoid receptors (CRs) of the body and central nervous system (CNS). The first discovered endocannabinoid was anandamide (1992) [1]. ECs produce the same effects in our body (Table 1) as do the cannabinoids of Cannabis sativa (Marijuana) like delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD). Marijuana is well-known worldwide for its substantial psychoactive properties. It has a tremendous ability to alleviate pain, upsurge appetite, and relieves nausea and apprehension [1]. As per the report of the National Conference of State Legislatures (2021), Marijuana is medically legal in 36 states of the USA [2]. It is also approved for recreational purposes in Washington DC [2]. There is enough research in support of the role ECs impart in energy regulation throughout the body. The corresponding data steadily supports the assumption that ECs` signaling is deeply connected with an increase in energy intake and its storage [3, 4]. Physical activity mobilizes ECs that add to the replenishment of energy reserves as well as anti-depressant effects. The other conditions where the concentrations of circulating ECs vary very significantly include irregular sleep, pain, and inflammation. ECS signaling is stress-responsive and imparts a vital role in maintaining homeostasis after stress. Some studies have shown that the concentration of circulating ECs change drastically in stress-related disorders. Thus evaluation of circulating ECs can contribute safe data about ECS signaling which may help us to test/define the hypothesis. However, a range of associated physiological variables makes it very complex, the reason why we cannot imply circulating ECs as a biomarker for a specific disorder. Challenges in the pharmacology of cannabinoids arise from their pharmacokinetics, off-target effects, and psychoactive effects. The two main types of CRs are CB1 and CB2 (G protein-coupled receptors, GPCR) which form protein complexes with other GPCRs and non-GPCRs and play a distinctive role in cell-signaling as shown in Figure 1. Some non–CB1/CB2 receptors have been hypothesized, founded on indirect data, however, these are not characterized so far. Ryberg et al., however, characterized GPR55 . CB1 and CB2 trigger cyclic adenosine monophosphate (AMP). CB1 is concentrated highly in the CNS (brain and spinal cord), and the peripheral nerve cells. It is very abundant in both gamma-aminobutyric acid (GABA) and glutaminergic-releasing neurons. The former is excitatory and the latter is inhibitory. Thus, the activation of CB1 triggers reactivation of neurotransmitter discharge, which may be stimulating or inhibiting conditional to its location in the brain [5]. Interestingly, CB1 gene assortment has been established but its functional impact is not well understood. Some of its translatory effects are linked with anxiety and melancholy [6, 7]. CB2 is positioned in peripheral neurons highly concentrated in immune-moderating cells, together with microglia (brain). CB2 is expected to play a modulating role against autoimmunity [6, 7]. Based on some animal studies, it is believed that CB2 is associated with the regulation of depression.

Materials and Methods

Databases like Science Direct, PubMed, Google Scholar, and Scopus were explored for the terms; endocannabinoids, cannabinoid receptors CB₁ and CB₂ and their clinical and pharmacological role in modulating stress, obesity, anxiety, pain, hypo/hypertension, tumorogenesis, hepatic steatosis, gamete formation and osteoclastogenesis; and Pharmacologic potential of endocannabinoids between the years 2010 and October 2021 to prepare the current review.

Physiological and pathological role of CB₁ and CB₂

Stress

The stress response can save one`s life when prompted, however, prolonged stress is taxing and has dangerous health consequences. The contribution of ECS is well-recognized to control the sensitivity and intensity of response to stress [6, 7]. Under critical circumstances of stress, vibrant fluctuations in ECS autocrine and paracrine signals lead to both the initiation and dissolution of the response and malfunctioning of ECS may either fasten or delay the normal stress response. Given this ability to regulate stress response, it is very much expected that the ECS can affect an organism's capacity to demonstrate resilience or to be at risk of chronic stress.

The ECS is a chief stress modulator; it controls the discharge of acetylcholine, and adrenaline and also regulates some hormones like norepinephrine. ECS mediates its role in the transformation of emotional stress, cognition, and function of the hypothalamic-pituitary-adrenal axis (Figure 1). The ECS may also intervene in the effect of glucocorticoids that may induce liver steatosis. Hence, the ECS may be considered a vital junction and therapeutic objective to alleviate the negative impact of chronic stress [13].

Obesity

CB₁ is critical for maintaining energy balance in the body. With fasting/starvation, 2-Arachidonoylglycerol (2-AG) concentration upsurges significantly in the forebrain including the hypothalamus which in turn activates CB_1. This gives rise to an urge to consume more food and thus affects the energy balance of the body [14, 15]. CB₁ also increases food appetite by increasing the odor sensitivity of the olfactory pathway [15]. Some people may be overweight because of the extreme CB₁ activation [15]. FAAH (Fatty acid amide hydrolase) is an enzyme that destroys AEA (anandamide). The mutation in this enzyme may increase the concentration of AEA multiple times and stimulate the hypothalamus to increase appetite. A hyperactive ECS was observed in wild-type mice that had developed food-induced obesity [16, 17]. It also increases levels of circulating ECs and receptor sensitivity. However, in the case of pre-satiated mice, the introduction of AEA injection inside the hypothalamus prompted considerable hyperphagia. CB₁inactivation decreases levels of insulin and leptin which significantly metabolizes energy reserves in the body [16, 17].

Nervous system

ECs are fatty acid derivatives that are involved in cell-cell and paracrine intercellular signaling. The action of ECs and associated receptors along the line in the neuromodulation of synapses and its flexibility throughout the CNS has been demonstrated  9 ]. ECs seem to work predominantly through retrograde signaling that involves the release of postsynaptic ECs and stimulation of presynaptic receptors (Figure 2) [18].

The ECS shows its significance in maintaining homeostasis of the CNS and peripheral nervous system (PNS). Consequently, many neurological maladies may be initiated by the ubiquitous ECS signaling, or the ECS may itself become modified by diseases. For instance, a hyperactive ECS decreases dopamine levels which contributes to the pathophysiology of Parkinson`s disease [19]. Other maladies associated with ECS comprise sclerosis, Alzheimer's disease, seizure disorders, Huntington's disease, and schizophrenia [5, 20]. The studies on pain analysis that evaluate the incidence of hypersensitivity towards the stimulus of pain after blocking CB₁ through the application of several drugs support the hypothesis of pain suppression by ECs [21]. Undeniably, the ECS is spread throughout the CNS and PNS in a manner that proposes its role as a modulator in pain dispensation. CB₁ and CB₂ are linked to the dorsal root in the spinal cord and specific areas of the brain that are sensitive to pain perception [10]. Dorsal root ganglion cells also modulate CB₁ and CB₁ to peripheral cutaneous neurons [10]. Intravenous administration of CBs may aid in modulating pain through the action of the spinal cord [10]. Pain is recognized as a vital target for ECs. The agonists of CB₁ can relieve pain by modulating its action through interneurons of the spinal cord Moreover, CB₂ agonists also have proved their efficiency in plummeting swelling and reversing the hypersensitivity toward pain, skin ailments, and inflammation [22].

Circulatory system

The ECS is extensively strewn all over the circulatory system. ECs show marginal activity in the normal circulatory system but are implicated in many circulatory malfunctions. The ECs released into the bloodstream activates CB₁ and modulate the corresponding hypotension when prompted by the stimulus of shock [23]. The ECS is cardio-protective and can induce both vasodilation and vasorelaxation. In the cardiovascular system, ECs induce vessel dilation by stimulating multiple target sites, inhibiting Ca²⁺ channels, and activating K⁺channels [23].

CB₁ activation demonstrates its role in blood pressure regulation by assisting in blood vessel dilation and heart contractions. CB₁ activation could thus be considered a therapeutic strategy for treating low blood pressure and thrombosis [23].

Cancer

ECS has been evaluated at the forefront of the fight against cancer over the past decade [24]. The sheer amount of data published about its dual-action both in tumorogenesis and reticence of tumor proliferation and metastatic growth in in-vitro conditions has made it a striking new objective for the handling of various types of cancer. Although searching for cancer biomarkers usually reaps a single target which allows for the manipulation of biochemical or hereditary defects, tagging the large ECS as a pharmaceutical-led business brings many benefits alongside problems.

Cannabis and ECs show many similar characteristics both in vitro and in vivo like both are anti-depressive, and anti-metastatic in many types of cancer. The ECs are also anti-proliferative and anti-apoptotic in a few cancers, however, they are apoptotic in the majority of cancers and promote tumor inhibition. Currently, cannabinoid therapy is restricted to nausea and pain, however, further research is required to explore its full pharmaceutical prospectives [23, 25, 26].

Liver

The ECS is implicated in liver fibrosis. Nevertheless, these studies are based on animal models or laboratory trials. Moreover, the roles of CB₁ and CB₂ in chronic hepatitis B are not evaluated fully yet. However, Dai et al., 2017 [27] have shown that CB₁ and CB₂ are translated in all patients with chronic hepatitis B and the level of fibrosis was translation dependent. Correspondingly, the higher the expression of CB₁ and CB₂, the more was the level of fibrosis in chronic hepatitis B patients. CB₁ helps in controlling hepatic lipogenesis in the liver. The activation of CB₁ aids in the synthesis of fatty acids, which leads to hepatic steatosis and ultimately obesity. Moreover, CB₁ endorses hepatic fibrosis and adds further complications to liver cirrhosis. However, the CB₂ activation reduces inflammation and limits lipid peroxidation, thus CB₂ activity is defensive against liver cirrhosis. Thus, further understanding of the hepatic-ECS relationship may provide breakthroughs in healing many liver ailments [28].

Reproduction

Both male and female reproductive systems are regulated significantly along many lines through the junction of the hypothalamus-pituitary-gonads. The ECS is deeply involved in reproduction via cell signaling. A list of possible mediators involved in this event is still booming. During the last two decades, the role of lipid coordinators sharing the same functions as delta-9-tetrahydrocannabinol (Δ9-THC) of Marijuana has been established. The role of these lipid modulators— radically altered through the action of biosynthetic machinery and hydrolysis—controls the different events of reproduction which includes the discharge of gonadotropins, biosynthesis of steroid hormones, synthesis and discharge of male/female gametes, and effective pregnancy [29].

The CB₁ is present both in males (Leydig cells) and females (ovary, oviducts, and uterus). Moreover, ECS may also be required for normal gametogenesis in males (spermatogenesis) and females (oogenesis). It has been learned that the CB₁ also found in the placenta is essential for embryo implantation [25]. Marijuana use is linked to implantation failures, miscarriages, fetal growth restraint, and premature delivery of babies. More studies are required to unravel the complex role of the ECS in the reproductive system.

Skeletal system

In recent times, immense interest is growing in understanding the potential of cannabinoid receptors in the skeletal system. In addition to immunomodulatory pathways, CB₁ is involved in maintaining proper bone formation and maintaining skeletal growth [30]. CB₂ is abundant in osteocytes, osteoclasts, and osteoblasts and is often age and sex-dependent. CB₂ agonists enhance endocortical osteoblast reproduction and activation while inhibiting osteoclastogenesis Figure 1. Schematic representation of stimulus (on the left side) versus induced endocannabinoids mediated response (on the right side). CB₁ and CB₂ are G protein-coupled receptors (GPCR) that may form protein complexes with other GPCRs or sometimes non-GPCRs and play an active role in the endocannabinoid-mediated cell-signaling response. This concept is supported by many studies discussed in this review.

Pharmacologic therapy

Cannabinoid receptor agonists

Epidolex

US Food and Drug Administration (FDA) 2018 approved the use of Epidolex (an oral CBD pharmaceutical syrup) against Lennox-Gastaut`s syndrome and Dravet`s syndrome (Table 2). These syndromes are two rare and serious types of infant-onset epilepsy. Epidolex (cannabidiol) has the potential to cure seizures implicated by these syndromes [31]. After witnessing its encouraging results, FDA stretched the usage of this oral solution in the treatment of seizures related to tuberous sclerosis complex (TSC). TSC is another erratic genetic malady that leads to a benign tumor in the brain and other body parts (heart, lungs, kidneys, eyes, and skin [32].

Sativex

FDA has approved the use of Sativex (an oral spray that contains both THC and CBD) for treating neurological pain endured during multiple sclerosis (Figure 2). Its bioavailability is not determined yet. The drug has been given license in Canada and 25 other countries including many parts of Europe [10, 33-35]. Because of the ongoing COVID-19 pandemic, the phase-3 trial of this drug for confirming its biosafety and efficiency has been delayed for time being [36]. However, its placebo-control trial with about 446 adults suffering from multiple sclerosis/or spasticity is anticipated to be accomplished by late 2022 [36].

Dronabinol

Dronabinol is a synthetic analog of THC that binds with the receptors CB₁ and CB_2. It has low bioavailability (less than10%) [37]. FDA has approved its medical installation as an antiemetic for chemotherapy patients. It is also used to stimulate the appetite of acquired immunodeficiency syndrome (AIDS) patients. However, significant negative effects are associated with its good deeds, the drug is reported to be toxic for CNS particularly [38].

Nabilone (cesamet)

As seen in Table 2, Nabilone is a synthetic derivative of THC. It binds with the CB₁ and CB₂ receptors and is FDA approved for its antiemetic properties to take care of those chemotherapy patients where other alternatives have proven unsuccessful. It is not recommended for patients suffering from upper motor neuron syndrome [22, 39].

CB₁ receptor antagonists

CB₁ receptor activates the dopaminergic pathway. The drugs like nicotine, THC, alcohol, opiates and other usually abused analgesics share a common mechanism i.e. the dopaminergic gush in the accumbens nucleus. CB₁ antagonism has shown appreciative results with animal models (human beings and mice). CB₁ antagonists in the case of humans have proved to be beneficial in terms of smoking cessation, reduced alcohol consumption, and decline in cocaine-quest behavior.

Rimonabant (also known as Acomplia or Zimulti)

It is a discerning CB₁ receptor antagonist (SR141716) (Table 2), that binds with centrally acting CB₁. It was marketed in Europe with the trade name Acomplia for treating obesity. However, in the USA, the drug was disapproved and had to be withdrawn due to its serious negative psychiatric effects, particularly depression [40-42].

Nonetheless, ECS is a common controller of cellular activity for both health and sickness. The potential pharmacologic objectives for ECS therapeutic intervention (both agonists and antagonists) are pain, cough, cachexia, antinausea, glaucoma, neurological disorders (for example Parkinson's disease, amyotrophic lateral sclerosis, alcohol-prompted neurological inflammation/degeneration, stroke, Huntington`s disease, multiple sclerosis, trauma, and seizures), autoimmune diseases (inflammatory bowel sickness, autoimmune uveitis, and systemic sclerosis), infection (HIV-1 brain infection), cardiovascular disease (atherosclerosis), psychiatric diseases (anorexia nervosa, anxiety-related sicknesses, substance abuse disorder, impulsivity, attention-scarcity/hyperactivity malady, bipolar disorder, addictive and personality disorders), gastrointestinal diseases (gut motility ailments, liver cirrhosis, bowel inflammation syndrome), cancer (cancer of breast, colon, lymph nodes, skin, and pancreas), osteoporosis and diabetic nephropathy [43].