Neuroprotection by the endogenous cannabinoid anandamide and arvanil against in vivo excitotoxicity in the rat: Role of vanilloid receptors and lipoxygenases

Type 1 vanilloid receptors (VR1) have been identified recently in the brain, in which they serve as yet primarily undetermined purposes. The endocannabinoid anandamide ( AEA) and some of its oxidative metabolites are ligands for VR1, and AEA has been shown to afford protection against ouabain-induced in vivo excitotoxicity, in a manner that is only in part dependent on the type 1 cannabinoid (CB1) receptor. In the present study, we assessed whether VR1 is involved in neuroprotection by AEA and by arvanil, a hydrolysis-stable AEA analog that is a ligand for both VR1 and CB1. Furthermore, we assessed the putative involvement of lipoxygenase metabolites of AEA in conveying neuroprotection. Using HPLC and gas chromatography/mass spectroscopy, we demonstrated that rat brain and blood cells converted AEA into 12-hydroxy-N-arachidoylethanolamine (12-HAEA) and 15-hydroxy-N-arachidonoylethanolamine (15-HAEA) and that this conversion was blocked by addition of the lipoxygenase inhibitor nordihydroguaiaretic acid. Using magnetic resonance imaging we show the following: ( 1) pretreatment with the reduced 12-lipoxygenase metabolite of AEA, 12-HAEA, attenuated cytotoxic edema formation in a CB1 receptor-independent manner in the acute phase after intracranial injection of the Na+/ K+-ATPase inhibitor ouabain; ( 2) the reduced 15-lipoxygenase metabolite, 15-HAEA, enhanced the neuroprotective effect of AEA in the acute phase; ( 3) modulation of VR1, as tested using arvanil, the VR1 agonist capsaicin, and the antagonist capsazepine, leads to neuroprotective effects in this model, and arvanil is a potent neuroprotectant, acting at both CB1 and VR1; and ( 4) the in vivo neuroprotective effects of AEA are mediated by CB1 but not by lipoxygenase metabolites or VR1.