The effect of WIN 55212-2 and cannabinoid receptor antagonists on electrically-evoked contractions of the guinea-pig isolated ileum

Leanne Roberts, Mark Gumbleton, William Ford. Welsh School of Pharmacy, Cardiff University, Cardiff, United Kingdom.

Two types of mammalian cannabinoid receptors have been cloned (CB1 and CB2) and there is evidence for an endothelial cannabinoid receptor (CBe)1. Cannabinoids are though to inhibit intestinal motility via activation of CB1 receptors located presynaptically on cholinergic nerve terminals. The aim of this study was to pharmacologically characterise the inhibition of electrical-field stimulated (EFS) contractions induced by the cannabinoid agonist WIN 55,212-2 (WIN) through the use of antagonists selective for CB1 (AM281 and rimonabant), CB2 (AM630) and CBe (0-1918) receptors.2 cm segments of guinea-pig ileum were mounted onto an electrode and stimulated (25V, 0.1hz, pulse width 2ms) in an organ bath filled with Kreb’s solution maintained at 37°C gassed with 5% CO2 in O2. Boluses of WIN were added cumulatively and contraction size was measured after addition of each dose. Where used, 1 μM of an antagonist or vehicle was incubated for 30 minutes before the addition of WIN (n = 4 to 6). WIN showed a concentration dependent reduction in the size of EFS-induced contractions (45±5% at 3 μM). AM281 and rimonabant had no effect on low concentrations of WIN but significantly increased the inhibitory effect of high WIN concentrations. WIN 3μM reduced contractions by 69±9, 74± 6 and 41 ± 8% in the presence of AM281, rimonabant and vehicle respectively. Conversely, AM630 increased the effect of low WIN concentrations but had no effect on high concentrations. WIN 3x10-11M reduced contractions by 18 ± 2% in the presence of AM630 but had no significant effect in the presence of the vehicle. 01918 did not modify the effect of any of the WIN concentrations examined. In conclusion, WIN inhibited electrically-evoked contractions of the guinea-pig isolated ileum but AM281, rimonabant, AM630 and 01918 did not antagonise this inhibition. This suggests that the inhibitory effect of WIN is not mediated through activation of the CB1, CB2 or CBe receptor.

1. Offertáler et al. Selective ligands and cellular effectors of a G protein-coupled endothelial cannabinoid receptor. Mol.Pharmacol 2003; 63: 699-705

The authors would like to thank Norgine for funding this study.