Anandamide acts as a vasodilator of dural blood vessels in vivo by activating TRPV1 receptors

Abstract

Migraine pathophysiology is believed to involve the release of neuropeptides via the activation of trigeminal afferents that innervate the cranial vasculature. Anandamide, the endogenous ligand to the cannabinoid receptor, is able to inhibit neurogenic dural vasodilatation, calcitonin gene-related peptide (CGRP)-induced and nitric oxide-induced dural vessel dilation in the intravital microscopy model. In an in vitro setting anandamide is also able to activate the vanilloid type 1 (TRPV1) receptor and cause vasodilation, via the release of CGRP. In this study we used intravital microscopy to study whether anandamide behaves as a TRPV1 receptor agonist in the trigeminovascular system. We examined if anandamide-induced dural vasodilation involves CGRP release that can be reversed by the CGRP receptor antagonist, CGRP(8-37), and whether like capsaicin the anandamide effect could be reversed by the TRPV1 receptor antagonist, capsazepine. Anandamide 1 (19+/-9%, n=12), 3 (29+/-5%, n=37), 5 (74+/-7%, n=13) and 10 mg kg(-1) (89+/-18%, n=6) was able to cause a dose-dependent increase in dural vessel diameter. Capsazepine (3 mg kg(-1), t(5)=6.2, P<0.05) and CGRP(8-37) (300 micrograms kg(-1), t(6)=11.1, P<0.05) attenuated the anandamide-induced dural vessel dilation when compared to control (Student's paired t-test). AM251 (3 mg kg(-1)), a cannabinoid type 1 (CB(1)) receptor antagonist, was unable to reverse this anandamide-induced dilation. The study demonstrates that anandamide acts as a TRPV1 receptor agonist in the trigeminovascular system, activating TRPV1 receptors that promote CGRP release and cause vasodilation independent of any action at the CB(1) receptor. Anandamide has been shown previously to inhibit trigeminovascular neurons and prevent vasodilation, through an action at CB(1) receptors.

Figure 1

Effects of increasing dose of anandamide (1, 3, 5 and 10 mg kg⁻¹) on dural blood vessel diameter.

Figure 2

Effect of anandamide response on dural blood vessel diameter with capsazepine or AM251 intervention. Following control responses to anandamide (5 mg kg⁻¹) rats were injected with either capsazepine (3 mg kg⁻¹) or in a separate series of experiments, AM251 (3 mg kg⁻¹) and anandamide injection repeated after 5 min. ^*P<0.05 significance when compared to the control dilation of anandamide using Student's t-test.

Figure 3

Effect of anandamide response on dural blood vessel diameter with CGRP_8–37 intervention. Following control responses to anandamide (5 mg kg⁻¹), rats were injected with either CGRP_8–37 (300 μg kg⁻¹) and anandamide injection repeated after 5 and 20 min. ^*P<0.05 significance when compared to the control dilation of anandamide using Student's t-test. ^#P<0.05 significance when compared to CGRP_8–37 induced inhibition of anandamide using Student's t-test.

Figure 4

Example traces showing the effects of anandamide on arterial blood pressure and dural blood vessel diameter, as well as the effects of AM251 and capsazepine on the effects of anandamide. Anandamide given was alone, and then anandamide was preceded by either AM251 or capsazepine.