Chronic administration of the cannabinoid CP55,940 inhibits the vasorelaxant effects of cannabinoids in the isolated rat aorta S.E.O'Sullivan, D.A. Kendall & M.D. Randall, School of Biomedical Sciences, University of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH, UK.	Print Abstract Search PubMed for: O'Sullivan SE Kendall DA Randall MD

The involvement of currently recognised cannabinoid (CB) receptors in vasorelaxation to cannabinoids is unclear. Studies reporting inhibition of vasorelaxation to anandamide by the CB₁ receptor antagonist SR141716A have found that other CB₁ antagonists are ineffective (White et al., 2001; Harris et al., 2002). We have now examined vasorelaxation to a number of cannabinoids after induction of tolerance by chronic cannabinoid treatment (Rubino et al., 1994).

Male Wistar rats (250-350g) were stunned and killed by cervical dislocation. The thoracic aorta was removed and all adherent tissue stripped from the vessels. The aortae were mounted in oxygenated Krebs-Henseleit solution at 37^oC and connected, using hooks and thread to a force transducer for isometric recording (Tep-areenan et al., 2003). Vessels were set at a baseline tension of 10 mN and allowed to equilibrate for an hour. The thromboxane-mimetic U46619 (10-300 nM) was added to increase tension by at least 5 mN. Some aortae came from animals chronically treated with the synthetic cannabinoid receptor agonist CP55,940 (0.4 mg/kg/day for 11 days, Rubino et al., 1994). Tolerance was defined as a lack of hypothermic response to CP55,940 injection. Vehicle controls animals were injected with ethanol:cremaphor:saline (1:1:18). The effects of anandamide, N-arachidonoyl-dopamine (NADA) and
⁹-tetrahydrocannabinol (THC) were assessed ex vivo.

In control aortae, all cannabinoids caused vasorelaxation of similar magnitude (anandamide pEC₅₀=5.66 ± 0.37 (mean ±SEM, R_max=20.6 ± 3.0 %, n=8; NADA pEC₅₀=5.38 ± 0.38, R_max=20.8 ± 3.8 %, n=8; THC pEC₅₀= 5.49 ± 0.28, R_max=23.4 ± 0.3%, n=8). Vessels from animals chronically treated with CP55,940 displayed a small reduction in maximal vasorelaxation response to anandamide (100 µM; 14.7 ± 4.6 % relaxation, n=6, P<0.05, Student's t-test), but a non-significant reduction in the vasorelaxant response to NADA (R_max=17.79 ± 13.32 %, n=6, relaxation). Vasorelaxation to THC was virtually abolished in animals treated with CP55,940 (R_max=7.72 ± 3.47 %, n=6, P<0.0001). The vasorelaxant responses of aortae from vehicle control animals were not different from those in control animals (n=6).

These data show, for the first time, that cannabinoid-tolerant animals have inhibited vasorelaxant responses to THC and, to a lesser extent, anandamide. Vasorelaxation to THC and endocannabinoids appears to be mediated by a cannabinoid receptor in the aorta that is desensitised by CP55,940.

This study was funded by the British Heart Foundation (PG2001/150).

Harris D, McCulloch AI, Kendall DA et al. (2002) J Physiol 539, 893-902.
Rubino T, Massi P, Patrini G et al. (1994) Neuroreport 5, 2493-6.
Tep-areenan P, Kendall DA & Randall MD (2003) Eur. J. Pharmacol. 465, 125-132.
White R, Ho WS, Bottrill FE et al. (2001) Br J Pharmacol 134, 921-9.