Anandamide is a partial agonist at native vanilloid receptors in acutely isolated mouse trigeminal sensory neurons

The endogenous fatty acid anandamide (AEA) is a partial agonist at cannabinoid CB1 receptors and has been reported to be a full agonist at the recombinant vanilloid receptor, VR1. Whole cell voltage clamp techniques were used to examine the efficacy of AEA and related analogues methanandamide and N ‐(4‐hydroxyphenyl)‐arachidonylamide (AM404) at native VR1 receptors in acutely isolated mouse trigeminal neurons. Superfusion of the VR1 agonist capsaicin onto small trigeminal neurons voltage clamped at +40 mV produced outward currents in most cells, with a pEC 50 of 6.3±0.1 (maximum currents at 10–30 μ M ). AEA produced outward currents with a pEC 50 of 5.6±0.1. Maximal AEA currents (30–100 μ M ) were 38±2% of the capsaicin maximum. AEA currents were blocked by the VR1 antagonist capsazepine (30 μ M ), but unaffected by the CB1 antagonist SR141716A (1 μ M ). Methanandamide and AM404 were less potent than AEA at activating VR1. Methanandamide (100 μ M ) produced currents 37±6% of the capsaicin maximum, the highest concentration of AM404 tested (100 μ M ) produced currents that were 55±9% of the capsaicin maximum. Capsazepine abolished the currents produced by AM404 (100 μ M ) and strongly attenuated (>70%) those produced by methanandamide (100 μ M ). Co‐superfusion of AEA (30 μ M , methanandamide (100 μ M ) or AM404 (100 μ M ) with capsaicin (3 μ M ) resulted in a significant reduction of the capsaicin current. These data indicate that AEA, methanandamide and AM404 activate native VR1 receptors, but that all three compounds are partial agonists when compared with capsaicin.British Journal of Pharmacology (2002) 137 , 421–428. doi: 10.1038/sj.bjp.0704904