Development of in vitro smooth muscle preparations as suitable models for elucidating the mechanism of action of cannabinoids
The suitability of the electrically stimulated guinea-pig MP-LM preparation, mouse isolated vas deferens and urinary bladder for the study of cannabinoid receptor ligands was investigated. Cannabinoid receptor agonists produced concentration-related inhibition of the contractile responses in all three tissue preparations, demonstrating high potency, chemical- and stereo-selectivity. The rank order of the inhibitory potency of the cannabinoid agonists in all three tissue preparations correlated with their binding affinity for specific cannabinoid CB1 binding sites in rat brain tissue. These results suggested a receptor-mediated mechanism of action for cannabinoid receptor agonists via prejunctional functional cannabinoid CB1 receptors in these three models, in the absence of an antagonist. The endogenous cannabinoid receptor ligand anandamide, also produced concentration-related inhibitory effects in all three tissue preparations. However, anandamide was found to be metabolically less stable in the guinea-pig MP-LM preparation. SR141716A, a potent, CB1 selective cannabinoid receptor antagonist was found to attenuate the inhibitory effects of cannabinoid receptor agonists investigated in all three tissue preparations. This provided further evidence for a receptor-mediated mechanism of action for cannabinoid receptor ligands in these three tissue preparations. However, further studies with SR141716A suggests that, it may be acting as an inverse agonist rather than a pure antagonist in these three preparations. Finally, this study was further extended to characterise some novel cannabinoid receptor ligands in the guinea-pig ML-LM preparation and mouse isolated vas deferens.