Differential effects of endogenous and synthetic cannabinoids on α7-nicotinic acetylcholine receptor-mediated responses in Xenopus oocytes

The effects of endogenous and synthetic cannabinoid receptor agonists, including 2-arachidonoylglycerol (2-AG), R-methanandamide, WIN55,212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)1-( 1-naphthalenylcarbonyl)-6H-pyrrolo[3,2,1ij] quinolin-6-one], and CP 55,940 [1alpha, 2beta-(R)-5alpha]-(-)-5-(1,1-dimethyl)-2-[5-hydroxy- 2-(3-hydroxypropyl) cyclohexyl- phenol], and the psychoactive constituent of marijuana, Delta(9)-tetrahydrocannabinol (Delta(9)-THC), on the function of homomeric alpha(7)-nicotinic acetylcholine (nACh) receptors expressed in Xenopus oocytes was investigated using the two-electrode voltage-clamp technique. The endogenous cannabinoid receptor ligands 2-AG and the metabolically stable analog of anandamide ( arachidonylethanolamide), R-methanandamide, reversibly inhibited currents evoked with ACh ( 100 muM) in a concentration-dependent manner (IC50 values of 168 and 183 nM, respectively). In contrast, the synthetic cannabinoid receptor agonists CP 55,940, WIN55,212-2, and the phytochemical Delta(9)-THC did not alter alpha(7)-nACh receptor function. The inhibition of alpha(7)-mediated currents by 2-AG was found to be noncompetitive and voltage-independent. Additional experiments using endocannabinoid metabolites suggested that arachidonic acid, but not ethanolamine or glycerol, could also inhibit the alpha(7)-nACh receptor function. Whereas the effects of arachidonic acid were also noncompetitive and voltage-independent, its potency was much lower than 2-AG and anandamide. Results of studies with chimeric alpha(7)-nACh-5-hydroxytryptamine (5-HT)(3) receptors comprised of the amino-terminal domain of the alpha(7)-nACh receptor and the transmembrane and carboxyl-terminal domains of 5-HT3 receptors indicated that the site of interaction of the endocannabinoids with the alpha(7)-nAChR was not located on the N-terminal region of the receptor. These data indicate that cannabinoid receptor ligands that are produced in situ potently inhibit alpha(7)-nACh receptor function, whereas the synthetic cannabinoid ligands, and Delta(9)-THC, are without effect, or are relatively ineffective at inhibiting these receptors.