Monoacylglycerol lipase inhibition by organophosphorus compounds leads to elevation of brain 2-arachidonoylglycerol and the associated hypomotility in mice.
Three components of the cannabinoid system are sensitive to selected organophosphorus (OP) compounds: monoacylglycerol (MAG) lipase that hydrolyzes the major endogenous agonist 2-arachidonoylglycerol (2-AG); fatty acid amide hydrolase (FAAH) that cleaves the agonist anandamide present in smaller amounts; the CB1 receptor itself. This investigation considers which component of the cannabinoid system is the most likely contributor to OP-induced hypomotility in mice. Structure-activity studies by our laboratory and others rule against major involvement of a direct toxicant-CB1 receptor interaction for selected OPs. Attention was therefore focused on the OP sensitivities of MAG lipase and FAAH, assaying 19 structurally diverse OP chemicals (pesticides, their metabolites and designer compounds) for in vitro inhibition of both enzymes. Remarkably high potency and low selectivity is observed with three O-alkyl (C1, C2, C3) alkylphosphonofluoridates (C8, C12) (IC50 0.60-3.0 nM), five S-alkyl (C5, C7, C9) and alkyl (C10, C12) benzodioxaphosphorin oxides (IC50 0.15-5.7 nM) and one OP insecticide metabolite (chlorpyrifos oxon, IC50 34-40 nM). In ip-treated mice, the OPs at 1-30 mg/kg more potently inhibit brain FAAH than MAG lipase, but FAAH inhibition is not correlated with hypomotility. However, the alkylphosphonofluoridate-treated mice show dose-dependent increases in severity of hypomotility, inhibition of MAG lipase activity and elevation of 2-AG. Moderate to severe hypomotility is accompanied by 64 to 86% MAG lipase inhibition and about 6-fold elevation of brain 2-AG level. It therefore appears that OP-induced MAG lipase inhibition leads to elevated 2-AG and the associated hypomotility.
Quistad
Gary B
GB
Environmental Chemistry and Toxicology Laboratory, Department of Environmental Science, Policy and Management, University of California, 115 Wellman Hall, Berkeley, CA 94720-3112, USA.
Klintenberg
Rebecka
R
Caboni
Pierluigi
P
Liang
Shannon N
SN
Casida
John E
JE
D000581:Amidohydrolases / Q000187:drug effects; D000818:Animals; D001095:Arachidonic Acids / Q000378:metabolism*; D001921:Brain / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D063388:Endocannabinoids; D004791:Enzyme Inhibitors / Q000633:toxicity*; D005989:Glycerides / Q000378:metabolism*; D018476:Hypokinesia / Q000139:chemically induced / Q000201:enzymology*; D020128:Inhibitory Concentration 50; D008297:Male; D051379:Mice; D008994:Monoacylglycerol Lipases / Q000037:antagonists & inhibitors*; D010755:Organophosphates / Q000633:toxicity*; D010575:Pesticides / Q000633:toxicity; D000099252:Fatty Acid Amide Hydrolases
D000581:Amidohydrolases; D001095:Arachidonic Acids; D063388:Endocannabinoids; D004791:Enzyme Inhibitors; D005989:Glycerides; D008994:Monoacylglycerol Lipases; D010755:Organophosphates; D010575:Pesticides; D000099252:Fatty Acid Amide Hydrolases; C094503:glyceryl 2-arachidonate