Another system that has been recently shown to be targeted by OPs is the cannabinoid system. The endocannabinoid anandamide binds to the cannabinoid receptor (CB1) in brain, which is also the target for Δ9-tetrahydocannabinol, the principal psychoactive ingredient of marijuana [116]. Both anandamide and the endogenous sleep-induced agent oleamide are hydrolyzed by fatty acid amide hydrolase (FAAH). Various OPs are potent inhibitors of FAAH [117], and may increase CB1-mediated responses, such as hypomotility, analysis, catalepsy and hypothermia [118]. Of several OPs tested (both oxons and parent compounds) only chlorpyrifos oxon was a potent inhibitor of CB1 binding in vitro [119]. However, in vivo administration of symptomatic doses only caused a 25% inhibition [119]. All OPs tested were more potent toward FAAH then CB1, but the relevance of these targets in OP toxicity is unclear [111].