Evidence for both inverse agonism at the cannabinoid CB1 receptor and the lack of an endogenous cannabinoid tone in the rat and guinea-pig isolated ileum myenteric plexus-longitudinal muscle preparation

Background and purpose:

Cannabinoid receptor agonists reduce intestinal propulsion in rodents through the CB₁ receptor. In addition to its antagonistic activity at this receptor, rimonabant (N-(piperidino)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxyamide) alone augments intestinal transit. Using rat and guinea-pig ileum MPLM (myenteric plexus-longitudinal muscle) preparations, we investigated whether the latter effect was through inverse agonism or antagonism of endocannabinoid agonist(s).

Experimental approach:

Inverse agonism was investigated by comparing the maximal enhancement of electrically evoked contractions of the MPLM by two CB₁ receptor antagonists, AM 251 (N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide) and O-2050 [(6aR,10aR)-3-(1-methanesulphonylamino-4-hexyn-6-yl)-6a,7,10,10a-tetrahydro-6,6,9-trimethyl-6-H-dibenzo[b,d]pyran], with that produced by rimonabant. To reveal ongoing endocannabinoid activity, effects of inhibiting endocannabinoid hydrolysis by fatty acid amide hydrolase (FAAH) using AA-5HT (arachidonyl-5-hydroxytryptamine), PMSF (phenylmethylsulphonyl fluoride) or URB-597 (3′-carbamoyl-biphenyl-3-yl-cyclohexylcarbamate), or putative uptake using VDM-11 [(5Z,8Z,11Z,14Z)-N-(4-hydroxy-2-methylphenyl)-5,8,11,14-eicosatetraenamide] was evaluated.

Key results:

The presence of CB₁ receptors was revealed by antagonism of exogenous anandamide, arachidonylethanolamide (AEA) and WIN 55,212-2 [(R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)-pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate] by rimonabant. The rank order of potentiation of contractions was AM 251 > rimonabant > O-2050. Neither the FAAH inhibitors nor VDM-11 affected electrically evoked contractions. Each FAAH inhibitor increased the potency of AEA but not WIN 55,212-2. VDM-11 did not alter the inhibitory effect of AEA.

Conclusions and implications:

The different levels of maximal potentiation of contractions by the CB₁ receptor antagonists suggest inverse agonism. The potentiation of the action of AEA by the FAAH inhibitors showed that FAAH was present. The lack of effect of FAAH inhibitors and VDM-11 alone on electrically evoked contractions, and on the potency of exogenous AEA suggests that pharmacologically active endocannabinoids were not released and the endocannabinoid transporter was absent. Thus, the CB₁ receptor antagonists behave as inverse agonists.This article is part of a themed issue on Cannabinoids. To view the editorial for this themed issue visit http://dx.doi.org/10.1111/j.1476-5381.2010.00831.x