Cannabis sativa contains a set of compounds known collectively as phytocannabinoids. This study focused on a little-investigated phytocannabinoid, Δ⁹-tetrahydrocannabivarin (THCV), an analogue of Δ⁹-tetrahydrocannabinol with a 3-propyl instead of a 3-pentyl side chain. We explored the ability of THCV to modulate isometric contractions of the mouse vas deferens when administered alone or in combination with a cannabinoid CB₁/CB₂ receptor agonist or with clonidine. The ability of THCV to displace [³H]CP55940 from CB₁ binding sites was also determined.

Vasa defer­entia were obtained from adult MF1 mice (31 to 48 g) and experi­ments with these tissues were performed as described by Thomas et al. (2004). Supra­maxi­mal electrical stimu­li were applied using 0.5 s trains of pulses (train frequency 0.1 Hz, pulse frequency 5 Hz, pulse duration 0.5 ms). Binding experiments were performed with mouse whole brain membranes and [³H]CP55940 (Thomas et al., 2004). THCV was extracted from cannabis and supplied by GW Pharmaceuticals (Porton Down). It was investigated by itself or added 30 min before clonidine, β,γ-methylene-ATP, phenylephrine or the CB₁/CB₂ agonists, WIN55212-2 or anandamide. THCV and anandamide were dissolved in DMSO, WIN55212-2 in a 1:1 mixture of DMSO and a 0.9% aqueous solution of NaCl (saline), and other drugs in saline. The methods used to determine E_max, K_B and K_i values are detailed elsewhere (Thomas et al., 2004). Values are ex­pres­­sed as means and vari­­ability as s.e.mean or 95% confidence limits.

THCV displaced [³H]CP55940 from mouse brain membranes, its mean K_i value with 95% confidence limits shown in brackets being 80 nM (51 & 126 nM; n=6; GraphPad Prism 4). At 10, 100, 316 and 1000 nM, it opposed the ability of WIN55212-2 to inhibit electrically-evoked contractions (n=6 to 9). This it did in a manner that was concentration-related and not accompanied by any significant decrease in the maximum effect (E_max) of WIN55212-2 (P>0.05; ANOVA followed by Dunnett’s test). The dextral shifts produced by THCV in the cumulative log concentration response curve of WIN55212-2 did not deviate significantly from parallelism and yielded a Schild plot with a mean slope (0.99±0.14) not significantly different from unity (P>0.1; one-sample t test; n=4). The mean K_B value of THCV was calculated to be 1.5 nM, its 95% confidence limits being 1.1 and 2.3 nM. At 10 nM, THCV also behaved as a competitive, surmountable antagonist of the endogenous cannabinoid, anandamide (K_B=1.4 nM; 95% confidence limits = 0.36 & 7.5 nM; n=7). However, at 100 nM, THCV did not oppose the ability of clonidine to inhibit electrically-evoked contractions (n=8). By itself, THCV (3 to 1000 nM) did not inhibit electrically-evoked contractions of the vas deferens (n=10). Nor did 1000 nM THCV alter the size of contractions induced by 10 µM β,γ-methylene-ATP or 30 µM phenylephrine ( P >0.05; Student’s paired t test; n=8).

In conclusion, THCV is a potent antagonist of WIN55212-2 and anandamide that exhibits at least some degree of selectivity. As it appears to be more potent in antagonizing WIN55212-2 and anandamide than in displacing [³H]CP55940 from CB₁ receptors , further experiments are required to establish the basis of this antagonism.

Thomas, A. et al. (2004) Eur. J. Pharmacol., 487, 213-221.