Improvement of the Closed Cranial Window Model in Rats by Intracarotid Infusion of Signalling Molecules Implicated in Migraine

Abstract

Intravital microscopy on a closed cranial window allows one to measure change in the diameter of cranial blood vessels after intravenous (i.v.) administration of pharmacodynamic substances. Putative targets being pursued in migraine are large vasodilating peptide molecules such as calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase polypeptide (PACAP)-38. High i.v. doses are required to study their craniovascular pharmacology. Unfortunately, this leads to a drop in blood pressure (BP) that subsequently causes blood vessels to dilate by autoregulation. Hence it is difficult to decipher what effect is caused by direct receptor agonist interaction or contributed by autoregulation. In the present study we infused substances with an ingenious indwelling catheter in the common carotid artery in rats. Intracarotidly seven-, 12- and 17-fold lower doses of CGRP, PACAP-38 and capsaicin were required, respectively, compared with i.v. infusion to induce the same dilation in dural artery. Dilating intracarotid (i.c.) doses caused no or a minimal fall in BP, whereas equi-responsive i.v. doses caused a marked BP reduction. The CGRP blocking potential of olcegepant was amplified by > 20 times on i.c. infusion. Pial artery responses to CGRP did not change with i.c. infusion, demonstrating that dilations after i.v. CGRP are mediated by autoregulation rather than through specific receptors. We applied CGRP topically, which induced concentration-dependent dural vasodilation, but no effect on pial artery or on BP. In conclusion, this new approach offers an improvement of the existing model by allowing more accurate assessment of effects of pharmaca on the cranial vasculature without inducing significant systemic effects.

Effects of Voluntary Locomotion and Calcitonin Gene-Related Peptide on the Dynamics of Single Dural Vessels in Awake Mice

“…Thus, intravenous injection of CGRP dilated dural vessels but not pial vessels. This result is consistent with other studies using closed cranial window in anesthetized rats that CGRP topical perfusion only induce dural vasodilation but not pial vessels (Williamson et al, 1997a;Gupta et al, 2010). Dural vessel dilations were blocked by coinjection of sumatriptan, a 5-HT receptor agonist (Peroutka, 1990; Fig.…”

“…Smaller cortical arterioles are more reactive (dilate more) in response to hypercapnia (Lee et al, 2001) and sensory stimulation (Drew et al, 2011;Gao et al, 2015;Huo et al, 2015a). Cerebral arteries that perfuse different territories can also have heterogeneous responses to the same vasodilator or vasoconstrictor (Hamel et al, 1988), and similar location dependence of responses may apply to dural arterioles. The size-and position-dependent reactivity of arterioles could be the cause of differences between our measurements in mice and those made in humans (Amin et al, 2013b).…”

“…If CGRP can leave dural vessels and enter into the CSF in which it can cause relaxation of dural vessels, it should also have similar effects on cerebral arteries if they were responsive to CGRP, because CGRP innervation of vessels is on the outermost adventitial layer of the vessel (Nakakita, 1990). Although we chose the dose (10 Ϫ3 mg/kg) and method of delivery (intravenous injection) of CGRP to mimic human studies (Lassen et al, 2002), other animals studies have used intracerebroventricular or intraparenchymal injections of CGRP (Williamson et al, 1997a;Gupta et al, 2010;Kaiser et al, 2012). Although some of these animal studies report dilation of pial vessels and behavioral effects, their results are hard to compare with the human intravenous infusion experiments because CGRP receptors are present on central neurons (Oliver et al, 1998;Han et al, 2005).…”

“…Although we chose the dose (10 Ϫ3 mg/kg) and method of delivery (intravenous injection) of CGRP to mimic human studies (Lassen et al, 2002), other animals studies have used intracerebroventricular or intraparenchymal injections of CGRP (Williamson et al, 1997a;Gupta et al, 2010;Kaiser et al, 2012). Although some of these animal studies report dilation of pial vessels and behavioral effects, their results are hard to compare with the human intravenous infusion experiments because CGRP receptors are present on central neurons (Oliver et al, 1998;Han et al, 2005). Any behavioral change or vessel dilation elicited by injection of CGRP into the brain is likely a result of the change in neural activity, rather than the perception of pain or direct vasodilatory action.…”

“…Although it has been studied heavily, the locus or loci of vascular action of CGRP in the awake animal is still not clear (Williamson et al, 1997a;Schoonman et al, 2008;Asghar et al, 2010Asghar et al, , 2011Gupta et al, 2010). Voluntary locomotion drove the constriction of dural vessels, distinct from the dynamics of pial vessels, suggesting that the two sets of vessels do not respond uniformly.…”