The mouse vas deferens as an in vitro model for elucidating the modes of action of cannabinoids
The suitability of the electrically stimulated mouse isolated vas deferens for the study of cannabinoid receptor ligands was investigated. Cannabinoid receptor agonists produced a concentration-related inhibitory effect on the contractile response of the vas deferens, demonstrating high potency, chemical- and stereo-selectivity. An endogenous cannabinoid receptor ligand, anandamide, was also found to share this inhibitory activity. The potency of the agonists correlated positively with binding affinity at CB 1 receptors and with potency in reducing spontaneous activity in the mouse in vivo. This suggested a receptor-based mode of action for the action of cannabinoid receptor agonist in this model. In order to overcome the non-specific nature of this inhibitory effect of cannabinoid receptor agonists, vasa deferentia were made tolerant to the inhibitory effects of Delta9-tetrahydrocannabinol (THC) by in vivo pretreatment with THC. This tolerance extended to other cannabinoid receptor agonists, synthetic and endogenous, but not to a number of non-cannabinoid twitch inhibitors. This tolerance did not involve a change in the sensitivity of the tissue to noradrenaline or a stable ATP analogue, beta-gamma-methylene-L-ATP (L-AMP-PCP) when exogenously applied. The degree of this tolerance was of the order of a 10 - 30 fold rightward shift of dose response curves, and the duration was greater than 10, but less than 30 days. In conclusion, the electrically stimulated mouse isolated vas deferens appears to be a suitable bioassay for the functional study of cannabinoid receptor ligands acting at CB1 receptors. This model may also be applicable to investigations into the mechanisms of action of cannabinoid tolerance. Pilot studies also demonstrated the possible use of this bioassay for the study of CB2-selective cannabinoid receptor ligands.