Endocannabinoid tone versus constitutive activity of cannabinoid receptors

Abstract

This review evaluates the cellular mechanisms of constitutive activity of the cannabinoid (CB) receptors, its reversal by inverse agonists, and discusses the pitfalls and problems in the interpretation of the research data. The notion is presented that endogenously produced anandamide (AEA) and 2-arachidonoylglycerol (2-AG) serve as autocrine or paracrine stimulators of the CB receptors, giving the appearance of constitutive activity. It is proposed that one cannot interpret inverse agonist studies without inference to the receptors' environment vis-à-vis the endocannabinoid agonists which themselves are highly lipophilic compounds with a preference for membranes. The endocannabinoid tone is governed by a combination of synthetic pathways and inactivation involving transport and degradation. The synthesis and degradation of 2-AG is well characterized, and 2-AG has been strongly implicated in retrograde signalling in neurons. Data implicating endocannabinoids in paracrine regulation have been described. Endocannabinoid ligands can traverse the cell's interior and potentially be stored on fatty acid-binding proteins (FABPs). Molecular modelling predicts that the endocannabinoids derived from membrane phospholipids can laterally diffuse to enter the CB receptor from the lipid bilayer. Considering that endocannabinoid signalling to CB receptors is a much more likely scenario than is receptor activation in the absence of agonist ligands, researchers are advised to refrain from assuming constitutive activity except for experimental models known to be devoid of endocannabinoid ligands. LINKED ARTICLESThis article is part of a themed issue on Cannabinoids in Biology and Medicine. To view the other articles in this issue visit http://dx.doi.org/10. 1111/bph.2011.163.issue-7 Abbreviations 2-AG, 2-arachidonoylglycerol; ABHD4,6 or 12, a/b hydrolase domain 4 (6 or 12); AEA, anandamide or N-arachidonylethanolamide; BRET, bioluminescence resonance energy transfer; CHO, Chinese hamster ovary cells; DAGL, diacylglycerol lipase; DSI or DSE, depolarization-induced suppression of inhibition or excitation; EPSP or IPSP, excitatory or inhibitory post-synaptic potential; ER, endoplasmic reticulum; FAAH, fatty acid amide hydrolase; FABP, fatty acid-binding protein; GPCR, G protein-coupled receptor; GP-NAE, glycerophospho-N-acylethanolamine; HEK293, human embryonic kidney cells clone 293; HFS, high-frequency stimulation; HSP, heat shock protein; IL3, intracellular loop 3; LPS, lipopolysaccharide; LTP, long-term potentiation; MAGL, monoacylglycerol lipase; MAPK, mitogen-activated protein kinase; NAE, N-acylethanolamine; NAPE, N-acyl phosphatidylethanolamine; NArPE, N-arachidonyl phosphatidylethanolamine NAT, N-acyl transferase; NMDA, N-methyl-D-aspartate; OEA, N-oleoylethanolamine; PEA, Npalmitoylethanolamine; PLC, phospholipase C; PLD, phospholipase D; POPC, palmitoyl, oleoyl-phosphatidylcholine; PTP, protein tyrosine phosphatase; TMH, transmembrane helix BJP British Journal of Pharmacology DOI:10.1111DOI:10. /j.1476DOI:10. -5381.2...

Fatty Acid-binding Proteins Transport N-Acylethanolamines to Nuclear Receptors and Are Targets of Endocannabinoid Transport Inhibitors

“…FABPs also mediate endocannabinoid hydrolysis by FAAH (6), indicating that they may carry out context-dependent functions within the endocannabinoid system. For example, the paracrine nature of NAE signaling (31) likely mandates a complementary cellular pattern of PPAR␣ and FAAH expression, with FABPs undertaking signaling and metabolic trafficking functions in the respective cells.…”

Type-1 Cannabinoid Receptor Signaling in Neuronal Development

“…constitutive) activation of CB1Rs. Indeed, the possibility of this direct entry of eCBs from the lipid bilayer to the CB1R-binding site was recently confirmed by biochemical tools as well as molecular simulation (reviewed in Howlett et al [57]). As seen above, CB1Rs display structural characteristics [63] that may contribute, in addition to the ubiquitously present endocannabinoid molecules, to the well-described constitutive stimulation of downstream-signaling pathways in absence of exogenous cannabinoid ligands [57,64].…”

“…These results are in line with the view that an important intracellular pool, reported also for several constitutively active GPCRs [49,50,51,52,53,54,55,56], is a dynamic phenomenon and is related to constitutive activation of CB1Rs. However, as detailed below, in addition to putative structural determinants, eCBs, which are ubiquitously expressed in neurons [57], may contribute to constitutive activation of CB1Rs.…”

Fatty Acid-binding Proteins (FABPs) Are Intracellular Carriers for Δ9-Tetrahydrocannabinol (THC) and Cannabidiol (CBD)

“…Endocannabinoid signaling is believed to be controlled through a balance of on demand synthesis and prompt catabo- lism (52)(53)(54)(55). AEA is inactivated through hydrolysis by intracellular FAAH (56,57).…”