Towards A Molecular Understanding of The Cannabinoid Related Orphan Receptor GPR18: A Focus on Its Constitutive Activity.


“The orphan G-protein coupled receptor (GPCR), GPR18, has been recently proposed as a potential member of the cannabinoid family as it recognizes several endogenous, phytogenic, and synthetic cannabinoids. Potential therapeutic applications for GPR18 include intraocular pressure, metabolic disorders, and cancer. GPR18 has been reported to have high constitutive activity, i.e., activation/signaling occurs in the absence of an agonist. This activity can be reduced significantly by the A3.39N mutation. At the intracellular (IC) ends of (transmembrane helices) TMH3 and TMH6 in GPCRs, typically, a pair of oppositely charged amino acids form a salt bridge called the “ionic lock”. Breaking of this salt bridge creates an IC opening for coupling with G protein. The GPR18 “ionic lock” residues (R3.50/S6.33) can form only a hydrogen bond. In this paper, we test the hypothesis that the high constitutive activity of GPR18 is due to the weakness of its “ionic lock” and that the A3.39N mutation strengthens this lock. To this end, we report molecular dynamics simulations of wild-type (WT) GPR18 and the A3.39N mutant in fully hydrated (POPC) phophatidylcholine lipid bilayers. Results suggest that in the A3.39N mutant, TMH6 rotates and brings R3.50 and S6.33 closer together, thus strengthening the GPR18 “ionic lock”.”

Some Prospective Alternatives for Treating Pain: The Endocannabinoid System and Its Putative Receptors GPR18 and GPR55.

Image result for frontiers in pharmacology“Marijuana extracts (cannabinoids) have been used for several millennia for pain treatment.

Regarding the site of action, cannabinoids are highly promiscuous molecules, but only two cannabinoid receptors (CB1 and CB2) have been deeply studied and classified.

Thus, therapeutic actions, side effects and pharmacological targets for cannabinoids have been explained based on the pharmacology of cannabinoid CB1/CB2 receptors. However, the accumulation of confusing and sometimes contradictory results suggests the existence of other cannabinoid receptors.

Different orphan proteins (e.g., GPR18, GPR55, GPR119, etc.) have been proposed as putative cannabinoid receptors.

According to their expression, GPR18 and GPR55 could be involved in sensory transmission and pain integration.

This work summarized novel data supporting that, besides cannabinoid CB1 and CB2receptors, GPR18 and GPR55 may be useful for pain treatment.

Conclusion: There is evidence to support an antinociceptive role for GPR18 and GPR55.”

Abnormal cannabidiol confers cardioprotection in diabetic rats independent of glycemic control.

Cover image

“Chronic GPR18 activation by its agonist abnormal cannabidiol (trans-4-[3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]-5-pentyl-1,3-benzenediol; abn-cbd) improves myocardial redox status and function in healthy rats.

Here, we investigated the ability of abn-cbd to alleviate diabetes-evoked cardiovascular pathology and the contribution of GPR18 to this effect.

Collectively, the current findings present evidence for abn-cbd alleviation of diabetes-evoked cardiovascular anomalies likely via GPR18 dependent restoration of cardiac adiponectin-Akt-eNOS signaling and the diminution of myocardial oxidative stress.”

Evidence for a GPR18 Role in Diurnal Regulation of Intraocular Pressure.


Image result for Invest Ophthalmol Vis Sci

“The diurnal cycling of intraocular pressure (IOP) was first described in humans more than a century ago. This cycling is preserved in other species. The physiologic underpinning of this diurnal variation in IOP remains a mystery, even though elevated pressure is indicated in most forms of glaucoma, a common cause of blindness. Once identified, the system that underlies diurnal variation would represent a natural target for therapeutic intervention.

We now report that NAPE-PLD and FAAH mice do not exhibit a diurnal cycling of IOP. These enzymes produce and break down acylethanolamines, including the endogenous cannabinoid anandamide. The diurnal lipid profile in mice shows that levels of most N-acyl ethanolamines and, intriguingly, N-arachidonoyl glycine (NAGly), decline at night: NAGly is a metabolite of arachidonoyl ethanolamine and a potent agonist at GPR18 that lowers intraocular pressure. The GPR18 blocker O1918 raises IOP during the day when pressure is low, but not at night. Quantitative PCR analysis shows that FAAH mRNA levels rise with pressure, suggesting that FAAH mediates the changes in pressure.



Our results support FAAH-dependent NAGly action at GPR18 as the physiologic basis of the diurnal variation of intraocular pressure in mice.”

The endogenous lipid N-arachidonoyl glycine is hypotensive and nitric oxide-cGMP-dependent vasorelaxant.

Image result for Eur J Pharmacol

“N-arachidonoyl glycine (NAGLY), is the endogenous lipid that activates the G protein-couple receptor 18 (GPR18) with vasodilatory activity in resistance arteries. This study investigates its hemodynamic effects and mechanisms of vasorelaxation.

NAGLY is an endothelium-dependent vasodilator and hypotensive lipid. The vasorelaxation is predominantly via activation of nitric oxide-cGMP pathway and NCX and probably mediated by the “endothelial anandamide” receptor, while the hypotensive effect of NAGLY appears not to involve the anandamide receptor. NAGLY also potentiates carbachol-induced vasorelaxation, the mechanism of which might involve stimulation of NO release.”

The Effect of Chronic Activation of the Novel Endocannabinoid Receptor GPR18 on Myocardial Function and Blood Pressure in Conscious Rats.

Image result for journal of cardiovascular pharmacology

“While acute activation of the novel endocannabinoid receptor GPR18 causes hypotension, there are no reports on GPR18 expression in the heart or its chronic modulation of cardiovascular function. In this study, after demonstrating GPR18 expression in the heart, we show that chronic (2 weeks) GPR18 activation with its agonist abnormal cannabidiol (abn-cbd; 100 µg/kg/day; i.p) produced hypotension, suppressed the cardiac sympathetic dominance, and improved left ventricular (LV) function (increased the contractility index dp/dtmax, and reduced LV end diastolic pressure, LVEDP) in conscious rats. Ex vivo studies revealed increased: (i) cardiac and plasma adiponectin (ADN) levels; (ii) vascular (aortic) endothelial nitric oxide synthase (eNOS) expression, (iii) vascular and serum nitric oxide (NO) levels; (iv) myocardial and plasma cyclic guanosine monophosphate (cGMP) levels; (v) phosphorylation of myocardial protein kinase B (Akt) and extracellular signal regulated kinase 1/2 (ERK1/2) along with reduced myocardial reactive oxygen species (ROS) in abn-cbd treated rats. These biochemical responses contributed to the hemodynamic responses and were GPR18-mediated because concurrent treatment with the competitive GPR18 antagonist (O-1918) abrogated the abn-cbd evoked hemodynamic and biochemical responses. The current findings present new evidence for a salutary cardiovascular role for GPR18, mediated, at least partly, via elevation in the levels of ADN.”

G protein-coupled receptor 18: A potential role for endocannabinoid signalling in metabolic dysfunction.

“Endocannabinoids are products of dietary fatty acids that are modulated by an alteration in food intake levels.

Overweight and obese individuals have substantially higher circulating levels of the arachidonic acid-derived endocannabinoids, anandamide and 2-arachidonoyl glycerol, and show an altered pattern of cannabinoid receptor expression.

These cannabinoid receptors are part of a large family of G protein-coupled receptors (GPCRs).

GPCRs are major therapeutic targets for various diseases within the cardiovascular, neurological, gastrointestinal and endocrine systems, as well as metabolic disorders such as obesity and type 2 diabetes mellitus.

Obesity is considered a state of chronic low grade inflammation elicited by an immunological response.

Interestingly, the newly deorphanised G protein-coupled receptor GPR18, which is considered to be a putative cannabinoid receptor, is proposed to have an immunological function.

In this review, the current scientific knowledge on GPR18 is explored including its localisation, signalling pathways and pharmacology.

Importantly, the involvement of nutritional factors and potential dietary regulation of GPR18 and its (patho)physiological roles are described.

Further research on this receptor and its regulation will enable a better understanding of the complex mechanisms of GPR18 and its potential as a novel therapeutic target for treating metabolic disorders.”

Δ(9)-THC and N-arachidonoyl glycine regulate BV-2 microglial morphology and cytokine release plasticity: implications for signaling at GPR18.

“Microglial cells are extremely plastic and undergo a variety of CNS-prompted shape changes relative to their location and current role. Signaling molecules from neurons also regulate microglial cytokine production. Neurons are known to employ the endogenous cannabinoid system to communicate with other cells of the CNS.

N-arachidonoyl glycine (NAGly) and Δ(9)-tetrahydrocannabinol (Δ(9)-THC) signaling via GPR18 has been introduced as an important new target in microglial-neuronal communication…

These data add to an emerging profile that emphasizes NAGly as a component of an endogenous system present in the CNS that tightly integrates microglial proliferation, recruitment, and adhesion with neuron-glia interactivity and tissue remodeling.”

The Novel Endocannabinoid Receptor GPR18 is Expressed in the Rostral Ventrolateral Medulla and Exerts Tonic Restraining Influence on Blood Pressure.

“Systemic administration of the GPR18 agonist abnormal cannabidiol (Abn CBD) lowers blood pressure (BP).

These findings are the first to demonstrate GPR18 expression in the RVLM, and to suggest sympathoinhibitory role for this receptor. The findings yield new insight into the role of a novel cannabinoid receptor (GPR18) in central BP control.”

Δ(9) -Tetrahydrocannabinol and N-arachidonyl glycine are full agonists at GPR18 receptors and induce migration in human endometrial HEC-1B cells.



Endometriosis is a disorder in which the endometrium forms growths outside the uterus and is associated with chronic pain. Recent evidence suggests that endometrial motility plays a role in the aetiology of endometriosis. The endocannabinoid system regulates cellular migration. Given the growing involvement of the endocannabinoids in reproduction, we investigated the role of the endocannabinoid system in migration of endometrial cells.


Migration of the human endometrial HEC-1B cells was assayed. Standard PCR techniques were used to determine the presence of the GPCR, GPR18, in HEC-1B cells, and p44/42 MAPK was assayed in stably transfected HEK293-GPR18 cells to determine receptor specificity for known cannabinoid agonists and antagonists. N-arachidonoyl ethanolamine (AEA) metabolism was measured, using HPLC/MS/MS for lipid analysis.


AEA, Δ(9) -tetrahydrocannabinol (Δ(9) -THC) and N-arachidonoyl glycine (NAGly) induce migration of HEC-1B cells through cannabinoid CB(1) receptor-independent mechanisms. MAPK activation in HEK293-GPR18 cells revealed novel pharmacology for known CB(1) and CB(2) receptor ligands at GPR18 receptors, including Δ(9) -THC, which activates MAPK at nanomolar concentrations, whereas WIN 55212-2, CP55940, JWH-133 and JWH-015, and arachidonyl-1-hydroxy-2-propylamide (R1-methanandamide) had no effect. Moreover, HEC-1B migration and MAPK activation by NAGly and Δ(9) -THC were antagonized by Pertussis toxin, AM251 and cannabidiol.


An understanding of the function and regulation of GPR18 and its molecular interactions with endogenous ligands, and how phytocannabinoids play a role with GPR18 signalling is vital if we are to comprehensively assess the function of the cannabinoid signalling system in human health and disease. LINKED ARTICLES: This article is commented on by Alexander, pp. 2411-2413 of this issue and is part of a themed section on Cannabinoids in Biology and Medicine. To view Alexander visit To view the other articles in this section visit To view Part I of Cannabinoids in Biology and Medicine visit”