Arachidonoylserotonin and Other Novel Inhibitors of Fatty Acid Amide Hydrolase

Abstract

Fatty acid amide hydrolase (FAAH) catalyzes the hydrolysis of bioactive fatty acid amides and esters such as the endogenous cannabinoid receptor ligands, anandamide (N-arachidonoyl-ethanolamine) and 2-arachidonoylglycerol, and the putative sleep inducing factorcis-9-octadecenoamide (oleamide). Most FAAH blockers developed to date also inhibit cytosolic phospholipase A₂(cPLA₂) and/or bind to the CB1 cannabinoid receptor subtype. Here we report the finding of four novel FAAH inhibitors, two of which, malhamensilipin A and grenadadiene, were screened out of a series of thirty-two different algal natural products, and two others, arachidonoylethylene glycol (AEG) and arachidonoyl-serotonin (AA-5-HT) were selected out of five artificially functionalized polyunsaturated fatty acids. When using FAAH preparations from mouse neuroblastoma N18TG2 cells and [¹⁴C]anandamide as a substrate, the IC₅₀s for these compounds ranged from 12.0 to 26 μM, the most active compound being AA-5-HT. This substance was also active on FAAH from rat basophilic leukaemia (RBL-2H3) cells (IC₅₀=5.6 μM), and inhibited [¹⁴C]anandamide hydrolysis by both N18TG2 and RBL-2H3 intact cells without affecting [¹⁴C]anandamide uptake. While AEG behaved as a competitive inhibitor and was hydrolyzed to arachidonic acid (AA) by FAAH preparations, AA-5-HT was resistant to FAAH-catalyzed hydrolysis and behaved as a tight-binding, albeit non-covalent, mixed inhibitor. AA-5-HT did not interfere with cPLA₂-mediated, ionomycin or antigen-induced release of [³H]AA from RBL-2H3 cells, nor with cPLA₂activity in cell-free experiments. Finally, AA-5-HT did not activate CB1 cannabinoid receptors since it acted as a very weak ligand in in vitro binding assays, and, at 10-15 mg/kg body weight, it was not active in the ‘open field’, ‘hot plate’ and rectal hypothermia tests carried out in mice. Conversely AEG behaved as a cannabimimetic substance in these tests as well as in the ‘ring’ immobility test where AA-5-HT was also active. AA-5-HT is the first FAAH inhibitor reported to date which is inactive both against cPLA₂and at CB1 receptors, whereas AEG represents a new type of cannabinoid receptor agonist.