Objective:

Anandamide is an endogenous lipid mediator that has agonist activities on cannabinoid and vanilloid (TRPV1) receptors. This study evaluates the importance of neural contribution to its dilator action on blood vessels of the rat knee joint.

Methods:

Experiments were performed on Sprague-Dawley rats (250–300 g) anaesthetized with urethane (1.8 g/kg). Drugs were administered topically onto the exposed rat knee joints, and changes in blood flow were monitored by a laser Doppler perfusion imager.

Results:

Anandamide produced dose-dependent (10 nmol, 100 nmol, 1 mmol) increases in knee joint blood flow. However, the vehicle (35% Tocrisolve) for the high dose of anandamide (1 mmol) also produced significant vasodilator action on its own. Therefore, effects of antagonists were tested on the vasodilator response to 100 nmol anandamide. At this dose, anandamide produced a maximum 44.53.7% increase on basal blood flow; this was achieved at 2 mins and blood flow returned to basal level at 15 mins. In the presence of a tachykinin NK ₁ receptor antagonist (RP67580, 2nmol) and a calcitonin gene-related peptide (CGRP) receptor antagonist (CGRP_8–37, 2nmol), the maximum increase was reduced to 13.02.8% (i.e., 71%; P<0.001) and 15.24.9% (i.e., 66% inhibition; P<0.001), respectively. In the presence of a mixture of receptor antagonists consisted of TRPV1 (capsazepine, 100 nmol), cannabinoid CB₁ (AM201, 10 nmol), CB₂ (AM630, 10 nmol), and a putative endothelial anandamide/abnormal-cannabidiol receptor antagonist (O-1918, 10 nmol), the maximum increase was reduced to 30.33.9% (i.e., 32% inhibition; P<0.05). However, denervation of the rat knees did not significantly affect the maximum increase; 41.22.9% (i.e., 5.7% inhibition; P>0.05).

Conclusions:

These findings suggest a major part of the vasodilator action of anandamide is mediated by substance P and CGRP, and these peptides are probably derived from non- neuronal sources.