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 [1α,2β-(R)- 5α]-(-)-5-(1,1-dimethyl)-2-[5-hydroxy-2-(3-hydroxypropyl) cyclohexyl-phenol], and the psychoactive constituent of marijuana, Δ⁹-tetrahydrocannabinol (Δ⁹-THC), on the function of homomeric α₇-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 μM) 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 Δ⁹-THC did not alter α₇-nACh receptor function. The inhibition of α₇-mediated currents by 2-AG was found to be non-competitive and voltage-independent. Additional experiments using endocannabinoid metabolites suggested that arachidonic acid, but not ethanolamine or glycerol, could also inhibit the α₇-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 α₇-nACh-5-hydroxytryptamine (5-HT)₃ receptors comprised of the amino-terminal domain of the α₇-nACh receptor and the transmembrane and carboxyl-terminal domains of 5-HT ₃ receptors indicated that the site of interaction of the endocannabinoids with the α₇-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 α₇-nACh receptor function, whereas the synthetic cannabinoid ligands, and Δ⁹-THC, are without effect, or are relatively ineffective at inhibiting these receptors.