Pharmacological characterization of cannabinoid receptor activity in the rat-isolated ileum myenteric plexus-longitudinal muscle preparation
Background and purpose: Cannabinoid effects on intestinal transit are commonly evaluated in rats. We characterized the cannabinoid receptors mediating the inhibitory effect of 5-(1,1-dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxypropyl)-cyclohexyl]-phenol (CP 55,940), (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212-2), arachidonylethanolamide (AEA) and Δ9-tetrahydrocannabinol (Δ9-THC) on contractions of the rat ileum myenteric plexus-longitudinal muscle (MPLM) preparation. Experimental approach: The interaction of each agonist was examined with the CB1 and CB2 receptor antagonist rimonabant and SR 144,528 respectively, on contractions elicited by electrical field stimulation (EFS) or exogenous ACh. The interaction of AEA with capsazepine, a TRPV1 receptor antagonist, was also investigated. Key results: EFS with single and trains of pulses evoked neurogenic ACh-mediated twitch and rebound contractions respectively. The rank order of potency for inhibition was CP 55,940 = WIN 55,212-2 > AEA > Δ9-THC and AEA > WIN 55,212-2 =Δ9-THC = CP 55,940 respectively. The stereoisomer WIN 55,212-3 was without effect. Rimonabant antagonized the inhibition of the twitches with pKB values of around 8.60, but only antagonized rebound contractions induced by WIN 55,212-2, AEA and Δ9-THC, with pA2 values of around 6.80. Rimonabant increased the twitches but inhibited the rebound contractions. Contractions to exogenous ACh were not altered. These observations extended to the guinea pig ileum MPLM. Conclusions and implications: The rat MPLM contains CB1 receptors and at least two non-CB1-non-CB2-non-TRPV1 receptors attenuating EFS-evoked ACh-mediated contractions in an EFS frequency-dependent pre-synaptic and stereo-specific manner. Augmentation of the twitches by rimonabant may be through antagonism of an endocannabinoid tone or inverse agonism, whereas inhibition of the rebound contractions involved partial agonism.