Levels, metabolism, and pharmacological activity of anandamide in CB1 cannabinoid receptor knockout mice:: Evidence for non-CB1, non-CB2 receptor-mediated actions of anandamide in mouse brain

Anandamide [arachidonylethanolamide (AEA)] appears to be an endogenous agonist of brain cannabinoid receptors (CB1), yet some of the neurobehavioral effects of this compound in mice are unaffected by a selective CB1 antagonist. We studied the levels, pharmacological actions, and degradation of AEA in transgenic mice lacking the CB1 gene. We quantified AEA and the other endocannabinoid, 2-arachidonoyl glycerol, in six brain regions and the spinal cord by isotope-dilution liquid chromatography-mass spectrometry. The distribution of endocannabinoids and their inactivating enzyme, fatty acid amide hydrolase, were found to overlap with CB1 distribution only in part. In CB1 knockout homozygotes (CB1-/-), the hippocampus and, to a lesser extent, the striatum exhibited lower AEA levers as compared with wild-type (CB1+/+) controls. These data suggest a ligand/receptor relationship between AEA and CB1 in these two brain regions, where tonic activation of the receptor may tightly regulate the biosynthesis of its endogenous ligand. 2-Arachidonoyl glycerol levels and fatty acid amide hydrolase activity were unchanged in CB1-/- with respect to CB1 +/+ mice in all regions. AEA and Delta (9)-tetrahydrocannabinol (THC) were tested In CB1-/- mice for their capability of inducing analgesia and catalepsy and decreasing spontaneous activity. The effects of AEA, unlike THC, were not decreased in CB1-/- mice. AEA, but not THC, stimulated GTP gammaS binding in brain membranes from CB1-/- mice, and this stimulation was insensitive to CB1 and CB2 antagonists. We suggest that non-CB1, non-CB2 G protein-coupled receptors might mediate in mice some of the neurobehavioral actions of AEA.