Endocannabinoids, most notably 2-arachidonoyl glycerol (2-AG) and
N-arachidonoyl ethanolamine (AEA, anandamide), are endogenous lipid ligands for the cannabinoid receptors CB
1 and CB
2.
11–14 These lipids have several notable characteristics, one being that they are produced enzymatically and on demand,
15 and much effort has gone into identification of the responsible enzymes both on the production and metabolism side. Another important feature of these endocannabinoids is that they have multiple congeners that differ by the length and saturation of their carbon backbone. The 2-AG–related lipid species 2-oleoyl glycerol (2-OG), 2-palmitoyl glycerol (2-PG), and 2-linolenoyl glycerol (2-LG) are all present in the CNS at comparable quantities.
16,17 Some of them may have their own roles as ligands: 2-OG has been proposed as an endogenous ligand for GPR119.
18 Because a given enzyme may metabolize an entire class of lipids (e.g., monoacylglycerol lipase (MAGL) metabolizes not only 2-AG but also 2-PG, 2-OG, and 2-LG,
19 the picture of signaling is muddied. In the case of 2-AG and AEA, the congeners have been proposed to play the role of “entourage” compounds, competing for breakdown and thereby enhancing the signaling of the chief compound,
20 although this is still a subject of debate.
21 Because of this complexity, it is important to study not only the disposition of the lipids of interest but also of the related lipid species. Enzymatic processes are best conceived of as dynamic flows; blockade or elimination of key enzymes can block these flows the way a dam might block a river. The accumulation of molecules—the reservoir that builds behind the dam, to extend the metaphor—may offer insights into the nature of those flows. We therefore examined the changes that result from the blockade of two enzymes: fatty acid amide hydrolase (FAAH) and
N-acyl phosphatidylethanolamine-specific phospholipase D (NAPE-PLD). These enzymes have been implicated in the metabolism of anandamide, which has been detected in various ocular tissues.
22,23 The role of FAAH in breaking down anandamide was established relatively early,
24 and anandamide metabolism, presumed to occur via FAAH activity, has been studied in the eye.
25,26 Importantly in our case, FAAH and anandamide are implicated in the production of
N-arachidonoyl glycine (NAGly), a candidate GPR18 agonist.
27,28 NAPE-PLD was proposed early on as the enzymatic source of anandamide,
29,30 but the first knockout study was discouraging, setting off a search for other candidate pathways.
31 Recently this question has been revisited and further support has been lent to a central role of NAPE-PLD in the production of anandamide in particular but also other acylethanolamines.
32