Elsevier

Neuropharmacology

Volume 76, Part B, January 2014, Pages 228-234
Neuropharmacology

Invited review
More surprises lying ahead. The endocannabinoids keep us guessing

https://doi.org/10.1016/j.neuropharm.2013.07.026Get rights and content

Highlights

  • This article outlines several unanswered questions about the endocannabinoid system.

  • The enzymes involved in anandamide biosynthesis are unknown.

  • This knowledge gap makes it difficult to localize neurons that use anandamide as a transmitter.

  • The enzymes involved in 2-AG biosynthesis and degradation have been identified.

  • 2-AG production is localized to a signaling complex, whose exact composition remain unknown.

Abstract

The objective of this review is to point out some important facts that we don't know about endogenous cannabinoids — lipid-derived signaling molecules that activate CB1 cannabinoid receptors and play key roles in motivation, emotion and energy balance. The first endocannabinoid substance to be discovered, anandamide, was isolated from brain tissue in 1992. Research has shown that this molecule is a bona fide brain neurotransmitter involved in the regulation of stress responses and pain, but the molecular mechanisms that govern its formation and the neural pathways in which it is employed are still unknown. There is a general consensus that enzyme-mediated cleavage, catalyzed by fatty acid amide hydrolase (FAAH), terminates the biological actions of anandamide, but there are many reasons to believe that other as-yet-unidentified proteins are also involved in this process. We have made significant headway in understanding the second arrived in the endocannabinoid family, 2-arachidonoyl-sn-glycerol (2-AG), which was discovered three years after anandamide. Researchers have established some of the key molecular players involved in 2-AG formation and deactivation, localized them to specific synaptic components, and showed that their assembly into a multi-molecular protein complex (termed the ‘2-AG signalosome’) allows 2-AG to act as a retrograde messenger at excitatory synapses of the brain. Basic questions that remain to be answered pertain to the exact molecular composition of the 2-AG signalosome, its regulation by neural activity and its potential role in the actions of drugs of abuse such as Δ9-THC and cocaine.

This article is part of a Special Issue entitled ‘NIDA 40th Anniversary Issue’.

Section snippets

2-AG formation – a new trick for an old dog

Neurons produce 2-AG through a multifunctional lipid pathway that starts with the cleavage of phosphatidylinositol-4,5-bisphosphate (PIP2) and ends (at least temporarily) with the transient accumulation of non-esterified arachidonic acid in cell membranes (Fig. 1). Individual checkpoints along this route can each generate separate sets of signaling molecules (for review, see Piomelli et al., 2007).

The first checkpoint is represented by the β−isoform of phospholipase C (PLC-β), which can be

The trouble with anandamide

In contrast with 2-AG, the reactions leading to the production of anandamide are relatively unprecedented in lipid biochemistry. This state of affairs is not surprising, if we consider that anandamide and other members of its chemical family – the amides of ethanolamine with long-chain fatty acids (known as N-acylethanolamines or fatty acid ethanolamides [FAEs]) – were initially dismissed as being terminal products of post mortem tissue degradation rather than physiologically meaningful

Detours in the path to anandamide

It is quite common for lipid-derived messengers to be produced through multiple biogenetic pathways and anandamide does not make an exception to this rule (Piomelli et al., 2007). Two detours from its canonical biosynthesis have been proposed, both of which utilize N-arachidonoyl-PE as a starting point and substitute NAPE-PLD with different lipid hydrolases (Fig. 2).

Macrophages exposed to the bacterial toxin, lipopolysaccharide (LPS), emit a burst of lipid mediators that include anandamide (Liu

Anandamide degradation

The molecular mechanisms utilized by neural cells to degrade anandamide are reasonably well understood. The compound is a preferred endogenous substrate for the intracellular serine amidase, fatty acid amide hydrolase (FAAH), which catalyzes the cleavage of various long-chain fatty acid amides (Cravatt et al., 1996, Désarnaud et al., 1995, Hillard et al., 1995, Ueda et al., 1995a). Other lipid hydrolases, such as N-acylethanolamine acid amidase (NAAA) (Ueda et al., 2010) and acid ceramidase

2-AG degradation

We mentioned before that the serine lipase MGL catalyzes the cleavage of 2-AG into glycerol and arachidonic acid, which is then recycled into membrane phospholipids or transformed into the eicosanoid family of lipid mediators (Piomelli et al., 2007). There is a general consensus that MGL accounts for the majority of the 2-AG-hydrolyzing activity present in the rodent brain (Blankman et al., 2007, Dinh et al., 2004). This assessment is supported by converging results obtained with MGL inhibitors

Accessing the degradation machinery

The endocannabinoids are released into the extracellular medium (Buczynski et al., 2013, Giuffrida et al., 1999) and exert the majority of their effects by binding to CB1 receptors present on the surface of presynaptic nerve terminals (Mackie, 2008). How do these lipid substances come in contact with the enzymes responsible for their degradation? It turns out that answering this question is, again, relatively straightforward for 2-AG, but much more complex (and controversial) for anandamide.

In

A diffuse release mechanism?

The phospholipids that serve as precursors for anandamide and 2-AG can undergo rapid lateral movements in cell membranes. This high degree of mobility raises the question of whether endocannabinoid production might occur throughout the surface of neuronal cells. This possibility, though inconsistent with classical synaptic transmission, is not unprecedented in brain signaling. Modulatory substances such as adenosine and nitric oxide, which are not stored in synaptic vesicles, are thought to be

Conclusions

Twenty years after the discovery of anandamide, many aspects of the natural history of this substance remain mysterious. The intracellular enzyme responsible for its deactivation, FAAH, has been identified (Cravatt et al., 1996, Désarnaud et al., 1995, Hillard et al., 1995; Ueda et al., 1995a, Ueda et al., 1995b) and potent and selective inhibitors of its activity have been disclosed (for review, see Piomelli, 2005). This has made possible, in turn, to unmask important functions served by

Acknowledgments

Work in the Piomelli lab is funded by NIDA grants R01 DA012413 and DP1 DA031387.

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