Modulation of opioids via protection of anandamide degradation by fatty acid amide hydrolase

Lack of involvement of the opioid system with the endocannabinoid, arachidonylethanolamide (anandamide) was possibly due to hydrolysis by fatty acid amide hydrolase (FAAH). Cyclohexylcarbamic acid 3′-carbamoyl-biphenyl-3-yl ester (URB597) is an inhibitor of FAAH, increases brain anandamide levels and enhances anandamide-induced antinociception in male ICR mice (25–30 g). The combination of URB597 (10 mg/kg, i.p.) and anandamide (40 mg/kg, i.p.) produced maximal antinociception in the mouse tail-flick test [68.7 ± 16.8 percent maximum possible effect (%MPE)], versus either substance alone (27.3 ± 7.9%MPE and 4.6 ± 2.3%MPE, respectively) and is significantly blocked (p < 0.05) by the cannabinoid CB1 receptor antagonist, SR141716A (rimonabant), the kappa opioid receptor-selective antagonist, nor-Binaltorphimine (10 μg i.t.; 12.7 ± 4.0%MPE) and the mu opioid receptor antagonist, naloxone (1 mg/kg, s.c.; 6.0 ± 3.8%MPE), but not by the delta opioid receptor-selective antagonist, naltrindole (2 mg/kg, s.c.; 29.7 ± 8.2%MPE) or the cannabinoid CB2 receptor antagonist, SR144528. In addition, nor-BNI (10 μg i.t) administration to FAAH−/− knockout mice produced a nociceptive response. The URB597/anandamide combination was not active in the CB1−/− knockout mice, but retained activity in the MOR−/− knockout mice. The sub-active combination of (URB597 10 mg/kg, i.p/anandamide 10 mg/kg, i.p.; 15.5 ± 4.3%MPE) shifted the dose response curve of morphine to the left (morphine alone ED50 = 4.6 mg/kg [3.7–5.6] versus morphine/URB597/anandamide (ED50 = 2.5 mg/kg [1.9–3.4]). These data are the first demonstration that anandamide, if protected from degradation, acts via the CB1 receptor to interact with kappa opioid receptor systems in opioid analgesia.