Pharmacognosy and Effects of Cannabinoids in the Vascular System

Abstract

Understanding the pharmacodynamics of cannabinoids is an essential subject due to the recent increasing global acceptance of cannabis and its derivation for recreational and therapeutic purposes. Elucidating the interaction between cannabinoids and the vascular system is critical to exploring cannabinoids as a prospective therapeutic agent for treating vascular-associated clinical conditions. This review aims to examine the effect of cannabinoids on the vascular system and further discuss the fundamental pharmacological properties and mechanisms of action of cannabinoids in the vascular system. Data from literature revealed a substantial interaction between endocannabinoids, phytocannabinoids, and synthetic cannabinoids within the vasculature of both humans and animal models. However, the mechanisms and the ensuing functional response is blood vessels and species-dependent. The current understanding of classical cannabinoid receptor subtypes and the recently discovered atypical cannabinoid receptors and the development of new synthetic analogs have further enhanced the pharmacological characterization of the vascular cannabinoid receptors. Compelling evidence also suggest that cannabinoids represent a formidable therapeutic candidate for vascular-associated conditions. Nonetheless, explanations of the mechanisms underlining these processes are complex and paradoxical based on the heterogeneity of receptors and signaling pathways. Further insight from studies that uncover the mechanisms underlining the therapeutic effect of cannabinoids in the treatment of vascular-associated conditions is required to determine whether the known benefits of cannabinoids thus currently outweigh the known/unknown risks.

The endocannabinoid system (ECS) consists of novel signaling pathways of lipid mediators (endocannabinoids) that can be produced in essentially all cell types of mammalian species. ECS has been implicated in the synthesis, release, transport, and degradation of many important biological events. This unique system in the human body has been linked with several physiological functions of the nervous system and various peripheral tissues and organs. Its modulation holds therapeutic promise in a wide range of different diseases and pathological conditions.

Cannabinoids encompass all-natural and synthetic compounds that elicit a cannabinoidergic effect via the activation of cannabinoid-specific receptors and other atypical receptors.¹⁻³ Currently, cannabinoids are categorized into three broad groups based on their source, which include: (i) phytocannabinoids, (ii) endocannabinoids, and (iii) synthetic cannabinoids.⁴ Phytocannabinoids are the sole derivatives of Cannabis spp, although other plant sources (Radula and Helichrysum genera) have been reported,⁵ whereas endocannabinoids are naturally synthesized in mammals.^1,6−11 By the end of the 20th century, two endocannabinoids, anandamide (N-arachidonoyl-ethanolamine, AEA) and 2-arachidonoyl-glycerol (2-AG), were fully characterized and remains the most studied endocannabinoid compounds (Figure 1).¹²⁻¹⁴ Three other pharmacologically distinct endocannabinoids, 2-arachidonyl-glyceryl ether (noladin, 2-AGE), O-arachidonoyl-ethanolamine (virhodamine), and N-arachidonoyl-dopamine (NADA), have also been recently described with their characteristics yet to be established.¹⁵⁻¹⁸ In the past decade, many synthetic cannabinoids have also been developed to either mimic or antagonize the effects of the naturally derived cannabinoids.¹⁹

Figure 1.

Endocannabinoid anandamide (AEA) and 2-arachidonoylglycerol (2-AG) are formed from arachidonic acid-containing phospholipids. AEA is formed from a two-step catalysis of phospholipids to form N-arachidonoylphosphatidylethanolamine (NAPE). NAPE is cleaved by phospholipase D (PLD) to form AEA. AEA is metabolized by the enzyme fatty acid amide hydrolase (FAAH) and by N-acylethanolamine-hydrolyzing acid amidase (NAAA) to form arachidonic acid or by COX to form prostaglandin ethanolamide (prostamides). 2-AG is synthesized from diacylglycerol (formed from phosphoinositides by the action of phospholipase C) by the action of diacylglycerol lipase (DAGL). 2-AG is metabolized either via COX to form prostaglandin glycerol esters or by both monoacylglycerol lipase (MAG-L) and α,β-hydrolase domain-containing 6 (ABHD6) to form arachidonic acid. Additionally, arachidonic acid can be synthesized directly from phospholipids by phospholipase A2 (PLA₂) which is further metabolized by lipoxygenase (LOX) to produce leukotrienes (LTs), cyclooxygenase (COX) to form prostaglandin glycerol esters, and cytochrome P450 (CYP) enzymes to form eicosanoids. EETs: epoxyeicosatrienoic acids; HETEs: hydroxyeicosatetraenoic acids.

Many studies have recently implicated the use of cannabis and its derivatives in certain cardiovascular disorders suggesting a positive correlation between cannabinoids and vascular events.²⁰⁻²² However, no unified mechanism(s) have been put forward to explain the development of these vascular-related complications, and a direct link for ECS with cardiovascular morbidity and mortality remains to be determined.^22,23 The vascular effects of the cannabinoids are complex, vary with species, and at different regions of the vascular bed.²⁴⁻²⁶ Moreover, several mechanisms are believed to be responsible for the vascular effects of the cannabinoids which include, but are not limited to, interference with the sympathetic nerve function and direct actions on vascular smooth muscle and endothelial cells.²⁷

Despite the substantial progress being made in narrowing the research in the pharmacology of cannabinoids, further exploration of mechanistic effects in the vascular system remains. Here in this review, we will highlight the distribution, interactions, and stimulatory effect of different cannabinoid ligands on various cannabinoid receptors in the vasculature from existing experimental and human studies, and provide insight into the therapeutic targets for this system in the management of vascular-related complications.

Cannabinoid Receptor Ligands and Signaling

Literature until the mid 1980s indicated no direct evidence for the existence of cannabinoid receptors and seem to suggest that cannabinoids induced their effects in a nonreceptor-dependent manner.²³ The observations of the existence of cannabinoid receptors have been reviewed in detail elsewhere.^23,28 Cannabinoid receptors belong to the G protein-coupled receptor (GPCR) superfamily, and like several other GPCRs, the cannabinoid receptors activate multiple intracellular signal transduction pathways.^29,30

The first cannabinoid (CB1) receptor was identified from several previously cloned “orphan” GPCR of the human cerebral cortex and testis, which was followed by the description of the second cannabinoid (CB2) receptor three years later.³¹⁻³³ However, the very low-affinity binding by some of the naturally occurring cannabinoids, oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), and phytocannabinoids cannabinol (CBN) and cannabidiol (CBD) to CB1 and CB2 receptors seem to suggest the possible existence of a non-CB1/CB2 cannabinoid receptor. This insight led to the description of a potential third cannabinoid receptor, G protein-coupled receptor 55 (GPR55).³⁴ GPR55 is a member of the rhodopsin-like (class A) GCPRs. It is structurally associated with some of the orphan and lysophospholipid-sensitive receptors in the purinoreceptor family as GPR23, GPR92, GPR35, and P2Y5. These receptors are distinct from CB1 and CB2 receptors at the phylogenetic level of a family of lipid receptors.^35,36 Other “orphan” G-protein-coupled receptors, including GPR119, GPR18, and atypical receptors including peroxisome proliferator activated receptor (PPAR), transient receptor protential cation channel (TRPV), and 5-hydroxytryptamine (5-HT) are also believed to mediate cannabinoidergic responses.^34,36−38

CB1 and CB2 Receptor Ligands and Signaling Pathways

The CB1 receptor, although highly expressed in the brain, is not limited to this organ but also present in a variety of other tissues and organs such as skeletal muscle, liver, gastrointestinal tract, pancreas, and adipose tissue,³⁹⁻⁴² although no relationship has been observed between the levels of expression and activity of the receptor in the brain.⁴³ The CB2 receptor is primarily expressed in immune cells, including B cells, monocytes, and T cells as well the hematopoietic systems.^39,44 However, recent studies suggest a low level of expression for CB2 receptors in the brain as well.⁴⁵⁻⁴⁸

The efficiency of the cannabinoid receptor signaling at different sites is dependent on the activating cannabinoid ligand and its affinity to the different CB receptor subtypes.^49,50 With CB1 and CB2 receptors being part of the GPCR family, the cannabinoids have the ability to activate different intrinsic signaling pathways downstream that are generated via the activation of different Gα-subunits, dependent on the type of ligand and activated cannabinoid receptor.^23,51−53

In line with these findings, GPCRs also exhibit “bias signalling”.^51,54−58 Bias signalling posits that different ligands acting on the same receptor could induce different responses through different signalling pathways (Figure 2). The interaction of other cannabinoids eliciting distinct cannabinoidergic effects via bias signaling of cannabinoid receptors makes the precise characterization of the individual signalling contribution challenging. It also limits the potential to harness precise therapeutically relevant GPCR signalling elicited by cannabinoids. Furthermore, the resulting signalling pathways are also dependent on the cell/tissue types, the molecular structure of the ligands, conformational change of the receptor, microenvironment, and the type of stimulus that necessitated the signalling.^51,59

Figure 2.

(A) Binary switch model of GPCRs, where upon stimulation, the GPCR activates heterotrimeric G-proteins causing its dissociation and subsequent formation of effector second messengers and cellular responses with the GRK-β-arrestin pathway serving as a negative feedback loop maintaining homeostasis by desensitization of GPCR interactions with G-proteins. (B) Balanced signaling system: β-arrestin is coupled to numerous signaling mediators aside acting as a negative feedback regulator of GPCR-G-protein signaling, and these two signaling pathways are independent of each other. Biased signaling is when a ligand-receptor-effector complex results in conformations using distinct pathways relative to other pathways. CB receptor signaling can be mediated through β-arrestins and G-proteins suggesting biased signaling. Abbreviations: GRK, G protein-coupled receptor kinase; MAPK, mitogen-activated protein kinase; PI3K, phosphoinositide 3-kinase; AKT, protein kinase B; and EGFR, epidermal growth factor receptor.

Both CB1 and CB2 receptors also appear to activate the arrestin pathway, independent of G-protein signaling.^60,61 Indeed, the activation of either the G-protein and/or the arrestin pathways appears to depend on the type of cannabinoid ligand and the receptor, leading to the difference in specific receptor conformations that affect the downstream signaling pathway.⁶²⁻⁶⁴ The potential of these cannabinoid ligands to selectively engage specific pathways provides the opportunity to develop therapeutically specific treatments to use as the fundamental basis for the subclassification of various group of ligands.

“Orphan” Receptors GPR55, GPR18, and GPR35 Ligands and Signaling Pathways

Beyond the classical CB1 and CB2 cannabinoid receptors, orphan receptors GPR55, GPR18, and GPR35 represent putative cannabinoid receptor sites⁶⁵⁻⁶⁷ for vascular effects of cannabinoids. Up to now, many of these claims remain provisional because of limited success in the lack of direct evidence from specific ligands. However, several cannabinoid ligands have been described to possess an affinity for these so-called cannabinoid “orphan” G-couple protein-coupled receptors. Among phytocannabinoids, CBD, tetrahydrocannabinol (THC), and cannabigerol (CBG) can elicit mild to strong activation of GPR55 receptors.^68,69 However, only a modest number of studies indicate interactions between synthetic cannabinoids and GPR55.⁶⁹ Synthetic radioisomer CBDs, termed abnormal CBDs (abn-CBD), have also recently been determined to elicit effects on the orphan receptors, with vasorelaxant properties without psychoactive effects.⁷⁰

It is also recognized that cannabinergic activation of GPR18 involves variable signal transduction pathways characteristic of a bias agonism.⁷¹ Two active ligands, kynurenic acid and 2-arachidonyl lysophosphatidic acid, are confirmed to activate GPR35 receptors.⁷²⁻⁷⁴ The characterization of the signaling pathway with the activation of GPR35 receptors by kynurenic acid has been linked to the GTPγS-mediated transduction pathway.⁷³ Further studies are warranted to determine the precise signaling of some of these orphan receptors in the vascular system and their potential for therapeutic use.

Other Atypical Cannabinoid Receptors and Ligand Signaling Pathways

Recent studies have suggested the presence of other atypical cannabinoid receptors that elicit cannabinoid effects independent of both CB1 and CB2 receptors in the vascular system. Cannabinoid effects can be mediated by the direct or indirect activation of receptors, including PPARγ, members of the TRPV family and serotonin 5HT_1A receptors.⁷⁵⁻⁷⁹ However, the claim that cannabinoids activate 5-HT receptors remains tentative, with conflicting reports from different studies.⁸⁰⁻⁸² Unlike other cannabinoid receptors, the cannabinoid-activated signal transduction pathway of these atypical cannabinoid receptors remains less defined in the vasculature.

Distribution of Cannabinoid Receptors in the Vascular System

Recently, there has been a significant interest in the expression and pharmacological actions of cannabinoids on the vascular system. The endocannabinoid system is involved in modulating the vasoactivity of both central and peripheral blood vessels. However, the vascular effects of cannabinoids is complex, with the underlying mechanisms for eliciting these effects appearing nascent. Indeed, cannabinoid receptors are variably expressed within different parts of the circulatory system.⁸³⁻¹¹⁵

In addition to the traditional CB1/CB2 receptors in the various vascular beds, there are non-CB1/CB2 receptors in the vascular endothelial cells in various vascular beds.^116−122 However, the pharmacology and molecular signaling mechanisms are yet to be fully established.^123,124 The delayed characterization of the molecular identity of these receptors stem from the inconsistencies of the results obtained from various abn-CBD-receptor vascular studies.^125−128

The vasoactive effects of cannabinoids involve both the well-established CB1/CB2 receptors as well as the yet-to-be-identified non-CB1, non-CB2 G_i/o protein-coupled receptors and appear to be based on the particular vascular bed being investigated.^{123,129−131} Differentiating the pathway(s) utilized by various cannabimimetic ligands in eliciting their effects on the vascular system present an exciting field for further rigorous research to offer targeted treatment options.

Role of Cannabinoids in Vasorelaxation

The primary vascular effect of cannabinoids are believed to be vasorelaxation, although vasoconstrictor effects are reported due to the modulation of specific vasoactive compounds. As well, the varying effect of cannabinoids in different vascular beds is due to multiple mechanisms to elicit their effect,^132−134 as well as on experimental conditions used.^135−138

In vivo experiments suggest the vascular effect of cannabinoids to follow three distinct phases characterized by a vagal-mediated fall in blood pressure (phase I), transient sympathetic pressor response (phase II), and a prolonged hypotensive effect (phase III).^{133,134,139−141}

In general, vasorelaxant effects of cannabinoids involve the stimulation of both classical and nonclassical cannabinoid receptors and the intracellular downstream activation of NO and arachidonic acid metabolites. There is no general concord about the implication of these vascular targets on the effects of cannabinoids, as the precise mechanism(s) of vasorelaxation remains yet unclear and controversial. However, several mechanisms of cannabinoid-mediated vasorelaxation have been proposed. They involve endothelial-mediated vasorelaxation, activation of K channels in the vascular smooth muscle cells, inhibition of the voltage-gated calcium channels of the vascular smooth muscle cell, activation of CB receptors in vascular smooth muscle cells, release of the calcitonin gene-related peptide (CGRP) from sensory neurons coupled to the vanilloid receptor (TRPV1), and inhibition of transmitter release from sympathetic nerve endings at the presynaptic level. We will discuss these in turn.

Endothelium-Mediated Vasorelaxation

Vascular endothelial cells have many endocrine functions, regulating vascular tone under physiological conditions by the production and release of vasoactive substances, including NO, with profound effects on the overall function of the vascular system.^142,143 Crosstalk between NO and the endocannabinoid signaling pathway in normal and pathological conditions play a critical role in affecting vascular health, with emerging evidence suggesting that endocannabinoid mediators regulate NO bioavailability and signaling. The stimulatory or an inhibitory effect of cannabinoids on NO bioavailability depend on the species, vascular bed, and/or activation of specific receptors.^144,145 Deutsch et al.¹⁴⁶ demonstrated the role of the endogenous cannabinoid agonist, AEA, in stimulating a CB1 receptor-mediated release of NO from perfused rat renal arterial segments. Mukhopadhay and his colleagues¹⁴⁷ also reported that AEA and methanandamide evoke vasodilation in juxtamedullary afferent arterioles and aortic rings of rabbits via activation of NO, independent of the cyclooxygenase (COX) pathway (i.e., through the production prostacyclin). Other investigators have further highlighted NO’s role in cannabinoid-induced relaxations in the rat mesenteric vasculature.^130,148,149 Interestingly, NO-induced vasodilation is not universal with all cannabinoids. THC is reported to induce endothelial dysfunction caused by oxidative stress and reduced nitric oxide production in the isolated rat superior mesenteric artery and aortic rings in myography studies.^83,150

There is also evidence to suggest a NO-independent role in cannabinoid-mediated vasorelaxation via the endothelium. In a study conducted by Vanessa and Hiley,¹⁵¹ an abn-CBD-induced NO-independent relaxation of rat mesenteric artery was found, as the relaxation was unaffected by the NO synthase inhibitor, L-N^G-nitroarginine methyl ester (L-NAME). However, in the same study, an endothelium-dependent activation of the K channels was also found through a novel highly selective cannabinoid receptor agonist SR141716A-sensitive pathway.¹⁵¹ A similar observation was made in a study by White and Hiley,¹⁵² who reported a NO-independent vasorelaxant response in mesenteric arterial bed elicited by AEA. AEA has also been shown to dilate coronary arteries independent of NO production.^153,154 The heterogeneous signaling of different cannabinoids in endothelium-dependent relaxation can be attributed to the predominant endothelium factor, such as endothelium-derived hyperpolarizing factor (EDHF), of a given vascular bed and represents a possible target in the therapeutic use of cannabinoids in vascular disorders.

Cannabinoid-Mediated Ion Channels in Vasorelaxation

In addition to the other mechanisms by which cannabinoids mediate vasorelaxation, cannabinoids act on multiple GPCR-independent targets, modulating voltage-gated channels, ligand-gated ion channel receptors, and ion-transporting membrane proteins. This includes the transient potential receptor class (TRP) channels to elicit vasoactive responses.^155−157 These channels are crucial in shaping action potentials and controlling the membrane potential and cell excitability. Therefore, they regulate a wide array of physiological processes and serve as potential therapeutic targets for the treatment of cardiovascular disorders.

Randall et al.¹⁵⁴ suggested that the endogenous cannabinoid, AEA, activates the endothelium-derived hyperpolarizing factor (EDHF) to induce relaxation of smooth muscle cells directly in the isolated perfused rat mesenteric arterial bed which was blocked by the cannabinoid receptor antagonist, SR141716A. However, subsequent studies suggested that vasorelaxant responses of AEA are independent of K channel blockers (which contribute to hyperpolarization); blocking of the K channels with apamin and charybdotoxin did not affect the vasorelaxant responses to anandamide directly in the precontracted rat small mesenteric artery, suggesting a differential sensitivity of AEA to K channel blockers.¹⁵² Further evidence was provided by Zygmunt et al.,¹⁵⁸ who demonstrated that AEA caused smooth muscle hyperpolarization only in the presence of a functional endothelium in rat hepatic arteries.

In rat retinal arterioles, abn-CBD-induced vasorelaxation in response to endothelin-1 (ET-1) mediated vasoconstriction has been shown to be due to a novel endothelial non-CB1/CB2-dependent mechanism (0-1918; a selective, silent antagonist of a putative, sensitive, endothelial anandamide receptor target). It has been shown that it is a consequence of the activation of a small-conductance Ca -activated K channel in rat retinal arterioles.^159,160

Vasodilation produced by the synthetic cannabinoid arachidonylcyclopropylamide (ACPA) have been reported to be, in part, dependent on an intact endothelium and are believed to involve the large-conductance, Ca-activated K channels.^161,162 The role of EDHF-mediated vasodilation thus appear to differ in various vascular beds. However, they are very crucial as a backup mechanism during NO pathway impairment. Markedly different mechanisms utilized by cannabinoids to relax blood vessels, independent of NO or EDHF, also present promising therapeutics targets for endothelial dysfunction and vascular aging.

TRPV1-Mediated Vasorelaxation

TRPV belongs to a diverse superfamily of the transient receptor potential (TRP), with proteins associated with TRP forming cation channels that trigger multiple stimuli. As such, they can act as sensory domains for TRPV response to endogenous ligands, heat, chemicals, mechanical, and osmotic stress.^163−165 TRPV channels are expressed (localized) in the vascular smooth muscle, endothelial cells, and in the perivascular nerves.^166,167

The role of TPRV receptors in the noncannabinoid receptor-associated effects of cannabinoids has been described in several studies within the context of dysregulation in the expression and signaling of TRPs, leading to the development and progression of multiple vascular disorders.^168,169 The vasorelaxant properties of the anandamide-like monounsaturated fatty acid oleoylethanolamide (OEA) are mediated via TRPV1 channels.^148,170

There is a discrepancy as to whether TRPV1-mediated vasorelaxation is endothelium-dependent or endothelium-independent. In a study by Járai and co-workers,¹²³ AEA was reported to induce vasorelaxation in both endothelium-intact and endothelium-denuded rat mesenteric arteries. Only the endothelium-denuded vessels were sensitive to the effects of TRPV1 receptors inhibition. However, Hoi and Hiley reported a capsaicin-sensitive relaxation in the small mesenteric arteries of rats, which were only observed in endothelium-intact blood vessels but not in denuded vessels.¹³⁰ This suggests possible multiple signaling pathways for AEA-induced TRPVI-mediated vasorelaxation.

The activation of TPRV1 by cannabinoids results in the subsequent production of NO and prostacyclin (PGI2) by endothelial cells, opening the intermediate and small conductance K channels (IKca/SKca) and leading to vasodilation of the vasculature.¹⁶⁴ In perivascular sensory nerves, the activation of the TRPV1 channels releases calcitonin gene-related peptide (CGRP), which causes relaxation of vascular smooth muscle cells.¹⁷⁰ Furthermore, AEA has also been reported to induce relaxation in rat mesenteric arteries by stimulating the release of CGRP from capsaicin-sensitive sensory nerves through the activation of TPVR1.^76,171,172 Determining the detailed signaling mechanism involved in cannabinoid TRPV1-mediated vasorelaxation is warranted and could aid in developing therapeutic strategies when the predominant NO and EDHF mechanism is dysfunctional in the vascular system.

Presynaptic Sympathetic Nerve Endings in Cannabinoid-Mediated Vasorelaxation

Activation of presynaptic endocannabinoid receptors has also appeared to mediate the vasoactive response produced by cannabinoids. This presents new insights into how cannabinoids modulate presynaptic cannabinoid receptors and neurotransmitter release and their downstream effect on vascular tone in conditions of sympathetic overstimulation. Their central and peripheral nervous system activity appears to inhibit the excitatory transmitter release from synaptic vesicles.¹⁷³ This effect has been described as occurring via the inhibition of the voltage-dependent Ca channel, the activation of the K channel, and the direct inhibition of the vesicle release of the excitatory transmitters.¹⁷⁴ The CB1 receptor is known to be the primary site that mediates sympathetic nervous system-induced vasorelaxation of cannabinoids as it is highly expressed at the presynaptic nerve endings.¹⁷⁵

Presynaptically located CB1 receptors have been found on the adrenal medulla, suggesting their possible involvement in releasing adrenaline.¹⁷⁶ The stimulation of the presynaptic CB1 receptors in the rat periventricular neurone have been demonstrated to be either dependent and/or independent of the type of catecholamine released from the adrenal medulla.^176−181 Furthermore, the neurogenic vasopressor response of catecholamines appeared to be abolished by administering CB1 receptor agonist WIN 55,212-2 and CP-55,940 in a dose-dependent manner. The effect is thought to be mediated via the activation of CB1 receptors that are located presynaptically on the postganglionic sympathetic nerve fibers innervating resistance vessels.¹⁸² Endocannabinoids have also been suggested to play an important role in the initial phase of lipopolysaccharide-induced septic shock. Activation of the presynaptic inhibitory cannabinoid CB1 receptor, which consequently inhibits the neurogenic vasopressor effect^183,184 is implicated, suggesting a critical neural-cannabinoid role in modulating the vascular system. The sympathoinhibitory effects of cannabinoids play a crucial role in sympathetic and vagal neuroeffector transmission in regulating heart rate and arterial blood pressure in normal and diseased conditions.

Vasoconstriction/Vasopressor Effects of Cannabinoids on the Vascular System

On the basis of reports in the literature, the vascular effects of cannabinoids that are mediated via the activation of CB receptors appear to be primarily associated with vasorelaxation in both animals and humans. Vasoconstriction resulting from the activation of classical cannabinoid receptors is rarely reported, and there is little information on the vascular contractile responses mediated by the activation of CB receptors.¹⁸⁵ Although there is a consensus on the vasoconstrictor effect of cannabinoids, there is no unifying mechanism underlining this effect. For instance, although some cannabinoids elicit their vasopressor effect in the second phase of the so-called “triphase cardiovascular cannabinoid effect” in whole animals, the mechanism underlining this phase is poorly understood as diverse pathways are suggested to be involved. Also, a different mechanism involving activation of atypical cannabinoid receptors by prostanoids is implicated in direct vasoconstrictor response to endocannabinoids.

Wagner et al.¹⁸⁶ first demonstrated the role of CB1 receptors in causing vasoconstriction. AEA was shown to induce vasoconstriction in isolated rat coronary arteries. AEA also produces vasoconstriction via CB1 receptors in spontaneously hypertensive rats but not in normotensive rats in vivo.¹⁸⁷ To corroborate the role of CB1 receptors in producing vasoconstriction, Tamaki et al.¹⁸⁸ further demonstrated that AEA could constrict isolated rat mesenteric arteries via the activation of CB1 receptors at high concentrations followed by long-lasting vasodilatation in a concentration-dependent manner.

Many cannabinoids, including 2-AG, anandamide, and metanandamide have been found to exhibit biphasic and triphasic effects on blood pressure in vivo, of which vasoconstriction/vasopressor response is a critical component.^189−191 As described earlier, the triphasic effect elicited by cannabinoids is characterized by initial bradycardia and hypotensive effect (phase 1), which is followed by a transient pressor effect, vasoconstriction (phase II), and finally, a prolonged decrease in blood pressure (phase III).^133,192 Interestingly, many CB1 receptor agonists, including WIN55212-2 or CP55940, cannot induce the transient vasopressor effect observed as described for phase II.^175,193 It has been suggested that the vasopressor effect of anandamide is mediated via TPRV1 as was reported by Pacher et al. in anesthetized TRPV1 knockout mice.¹⁹⁴ Similarly, methanandamide and capsaicin have been shown to induce a similar pressor effect in anesthetized rats; however, no mechanism was described to be responsible for such effect.¹⁹⁵

Although the mechanism(s) underlining the transient pressor effect is poorly understood, Kwolek et al. provided a possible explanation of the potential mechanism(s) underlining these pressor effects in urethane-anesthetized rats.¹⁹⁶ The first mechanism is thought to be mediated via the central nervous system, as this effect was reversed using a β-adrenoceptor antagonist, propanolol, and an N-methyl-d-aspartate receptor (NMDA) receptor antagonist, MK-801. The second mechanism is suggested to be originating from the periphery, possibly involving blood vessels, and noted to be sensitive to the actions of nifedipine, ruthenium, and pentobarbital, perhaps pointing to the possible involvement of the L-type sensitive Ca and other ion channels.

The indirect target site of action involving the prostanoid receptors TP and EP has also been suggested to be involved in vasoconstriction by means of endocannabinoid metabolites. Lefebvre et al.¹⁸⁹ reported 2-AG induced contractions of the rat aortic rings via the conversion of 2-AG to thromboxane A₂ (TXA₂) (a potent vasoconstrictor), an effect that was blocked entirely by the TXA₂ receptor (TP receptor) antagonist, GR32191.

The definitive pathway utilized in eliciting cannabinoid-induced vasoconstriction is yet to be wholly defined, and this is particularly important as vascular responses to cannabinoids seem to be enhanced in certain pathological conditions such as inflammation and hypertension. Further work is required to establish the extent of vascular actions of cannabinoids and their therapeutic application in physiological and pathophysiological situations

Medicinal Prospect of Cannabinoids in Vascular-Related Pathologies

More recently, cannabinoids have emerged as therapeutic targets for the management of a host of pathologies^197−200 and a subject of investigation in varying experimental protocols. The main avenue of interest in the vascular system has been the preclinical investigations into the therapeutic potential of cannabinoids in the management of vascular-associated complications.

Role of Cannabinoids in Hypertension

The consensus about the potential therapeutic effects of cannabinoids in the vascular system depends on the various cannabinoids used and the stage/model of hypertension. Other factors, such as age and sex, may also be an extra layer to accessing the therapeutic effects of cannabinoids in hypertension. In humans and whole animals, these therapeutic effects can be categorized into short-term and long-term.

Away from the psychoactive effects, cannabinoids have been reported to elicit complex effects on blood pressure involving either a triphasic, which is elicited by endogenous cannabinoids and a biphasic or monophasic (which is reported in phytocannabinoids and synthetic cannabinoids) blood pressure responses in vivo.^201−205

This effect, as described earlier, is characterized by bradycardia, a drop in BP, a brief pressure response resulting from increased cardiac contractility and blood flow in the mesenteric and renal vascular beds and, finally, a prolonged marked decrease in BP in the third phase.²⁰² The biphasic phase involves only phases II and III, while the monophasic phase involves only phase III. The difference in short-term effects is generally reported to depend on the cannabinoids used.

The cardioprotective effect mediated by cannabinoids in hypertension involves the reduction in inotropy and peripheral vascular resistance, which are the critical components of blood pressure control. Presynaptic CB receptors have also been suggested to modulate sympathetic effects on BP.^184,206 This effect results from the activation of both cardiac and vascular CB receptors by endogenous, Phyto, and synthetic cannabinoids.^94,207−214 In a study by Bátkai et al.⁹⁴ using three different rat models of hypertension, the therapeutic effect of cannabinoids was unmasked only in the hypertensive groups involving a decrease in cardiac contractility and reduction in vascular resistance. On the other hand, this suggests that the inhibition of cannabinoid receptors can be a promising therapeutic approach in heart failures by inhibiting the negative inotropy effects of cannabinoids. Furthermore, the difference in cannabinoid effects in the heart and various vascular beds could be as a result of the relative concentration of endogenous ligands and receptors, as well as the type of cannabinoids used.^{94,130,145,185}

Hypertension is a multifactorial disease due to the complex interaction among intrinsic and extrinsic factors, broadly grouped into environmental, genetics, and sex steriods.^215−217 A healthy blood vessel ensures a healthy cardiovascular function and better blood pressure regulation. Arterial remodelling as a cause or consequence of arterial diseases is known to exacerbate hypertension, and the long-term effects of cannabinoids on hypertension involve a yet-to-be-explored role of cannabinoid in modulating/inhibiting the various factors that lead to hypertension beyond this paper.^218,219

In the vascular system, the long-term effects of cannabinoids in regulating blood pressure involve the maintenance of vascular endothelial function. The loss of the endothelial function as a modulator of vascular tone and maintenance of vessel integrity by actively suppressing thrombosis, vascular inflammation, and arterial remodelling alters vascular hemodynamics.^220−222 Cross-talk between NO and cannabinoid signaling pathways (nitrergic signaling) as well as with other vasorelaxant mediators such as prostaglandin, ion channels which are involved in the modulation of vascular tone, suggest a vasoprotective endothelial effect of chronic use of cannabinoids in hypertension.^223−227

The role of cannabinoids in hypertension seems very promising. However, further investigations are needed to assess the therapeutic effects of cannabinoids for short-term and long-term hypertension treatments.

Role of Cannabinoids in Inflammation

For over 2000 years, Chinese healers have claimed the anti-inflammatory properties of cannabinoids, suggesting that it heals rheumatism,²²⁸ and so has Indian folklore medicine, where it has been described as a remedy against inflammation, chronic pain, and asthma.²²⁹ Over the past decade, the potential of cannabinoid pharmacotherapy in inflammation has received much attention. Cannabinoids have been demonstrated to interact with various inflammatory processes, including endothelial inflammatory response,^230−232 chemotaxis,^233−238 adhesion of inflammatory cells to the endothelium,^239−245 as well as its involvement in the release of a variety of proinflammatory mediators.^246−256

Cannabinoids in Atherosclerosis

Atherosclerosis is an inflammatory disease that is characterized by arterial wall lesions containing cholesterol, immune infiltrates, and connective tissue elements.²⁵⁷ Multiple risk factors have been identified, including hypertension, smoking, diabetes, obesity, and genetic predisposition. Atherosclerosis is implicated as a major cause of mortality due to potentiating myocardial ischemia, myocardial infarction, coronary artery diseases, and cerebrovascular diseases.^258−262 Endothelial dysfunction has been implicated as an early indicator for atherosclerosis, as it plays a pivotal role in the degeneration of vascular structure, initiating the pathogenesis and progression of atherosclerosis.^263,264 There is evidence from a number of studies suggesting a role for cannabinoids in the pathogenesis of atherosclerosis, but the exact function of the endocannabinoid system during atherosclerosis is yet to be fully understood. Emerging evidence suggests that CB1 and CB2 receptors play a significant role in the pathogenesis of atherosclerosis.²⁶⁵ It appears that activation of CB2 and inhibition of CB1 receptors reduces the development of atherosclerosis, whereas the activation of the latter receptors may enhance the progression of atherosclerosis.^266,267 A growing body of evidence has established a link between cannabinoids use and endothelial cell functioning. A low dose of THC was shown to reduce the progression of atherosclerosis in both human and mouse atherosclerotic plaque.²⁶⁶ This observation was associated with lowering levels of the T-helper cell 1-derived interferon γ and inhibition of macrophage chemotaxis.²⁶⁶ In a longitudinal epidemiological study, cannabinoids were shown alter not only the function of T-helper cells but also B-lymphocytes.²⁶⁸ However, unlike T-helper cells, the role of B-lymphocytes in the pathogenesis of atherosclerosis in the presence of cannabinoids is less explored.^{244,269−272}

The elucidation of the pharmacodynamics of cannabinoids and their relationship to atherosclerosis could open an alternative therapeutic prospect for the management of the disease.

Role of Cannabinoids in Thrombosis

A fully functional endothelium and vasculature enable the efficient flow of blood to tissues and organs. Thrombotic-induced pathologies are a leading global cause of mortality, accounting for 50% of vascular-related deaths in the western world.^273,274 Cannabinoids play a crucial role in regulating platelet function in thrombosis, and the expression of CB1 receptors in human platelets.²⁷⁵ In general, evidence for the thrombotic effect of cannabinoids appear paradoxical.

Many clinical reports have associated cannabis use with thrombogenic development culminating into acute coronary artery disease.^276−281 However, over the past decade, there have been many discrepancies concerning the influence of endocannabinoids on the formation of thrombus and platelet aggregation. While some studies indicate a cannabinoid-mediated procoagulating effect on human platelet cells,^282,283 additional research has shown that cannabinoids efficiently inhibit platelet aggregation.^{274,284−287}

The prothrombic effect of endogenous cannabinoids reported may be due to different mechanisms of disease development. Some evidence suggests that the development of atherosclerosis may lead to the formation of thrombus.^288,289 However, it appears not to always be the case. In an ex-vivo observation, young individuals produced thrombotic coronary artery occlusion without underlying atherosclerosis^290,291 via a different mechanism not associated with the development of atherosclerosis. Indeed, the in vitro treatment of blood with THC induces direct expression of glycoprotein IIb-IIIa and P-selectin, which are requisites for platelet coagulation²⁸² but not atherosclerosis. These findings are supported by a case report in two patients with coronary artery thrombotic lesions being successfully treated with glycoprotein IIb-IIIa inhibitors.²⁹² The platelet activation by cannabinoids has been suggested not to be mediated via the activation of CB1/2 receptors.²⁹³ Almaghrabi et al.²⁹⁴ showed that vanilloid-like agents (capsaicin, N-arachidonoyldopamine, and N-oleoyldopamine) inhibit platelet aggregation but not via the activation of cannabinoid receptors or TRPV1 channels. Other findings suggest that THC prolongs lipopolysaccharide-activated tissue factor protein expression in activated monocytes, leading to a pro-coagulation effect²⁹⁵ and not via the activation of cannabinoid receptors.

Whatever the mechanism, thrombotic occlusions do play a significant role in atherosclerotic complications, and the pathophysiology of this process is regulated by the key factors responsible for maintaining the integrity of a blood vessel. The role of cannabinoids in modulating these factors, on the other hand, may represent a potentially promising target for the pharmacotherapy of atherosclerotic complications but requires much further investigation.

Conclusion

The pharmacology of cannabinoids in the vascular system is a promising field that should provide further insights into the therapeutic uses of cannabinoids in the vascular system. A literature survey suggests that cannabinoids elicit their vascular response via both classical and atypical receptors. The type of activated receptor(s) is highly dependent on the type of agonist, dose/concentration, type of tissue, and experimental setup (in vivo or in vitro). In addition, the literature at times contains paradoxical findings related to the actions of cannabinoids in relation to the vascular system. Currently, there is no unified evidence for the vasoactive effects of cannabinoids in the vascular system. However, most of the studies demonstrate a vasodilatory role rather than vasoconstrictive effects. This is primarily due to several factors such as the concentration, intrinsic vessel characteristics, baseline vascular tone, experimental conditions, and species. This is also partly due to, but not limited to, the variety of receptors in the vascular system utilized by cannabinoids in eliciting their diverse effects. Further studies to elucidate the various interactions between cannabinoids and the vascular system would provide additional insights into how these interactions facilitate the regulation of normal vascular functions and their modifications in pathophysiological conditions.

In a coda, exploration of the vascular effect of cannabinoids presents an alternative therapeutic option for the management of vascular-associated pathologies and some cardiovascular diseases.

The authors declare no competing financial interest.

Special Issue

Published as part of the ACS Pharmacology & Translational Science virtual special issue “New Drug Modalities in Medicinal Chemistry, Pharmacology, and Translational Science”.