Edwin W Willems

Effects of the CGRP receptor antagonist BIBN4096BS on capsaicin-induced carotid haemodynamic changes in anaesthetised pigs

Calcitonin gene‐related peptide (CGRP), a potent vasodilator released from capsaicin‐sensitive trigeminal sensory nerves, seems to be involved in the pathogenesis of migraine. Hence, CGRP receptor antagonists may serve as a novel treatment for migraine. This study was therefore designed to investigate the effects of BIBN4096BS (100, 300 and 1000 μg kg−1, i.v.), a potent and selective CGRP receptor antagonist, on capsaicin‐induced carotid haemodynamic changes in anaesthetised pigs. Both vagosympathetic trunks were cut and phenylephrine was infused into the carotid artery (i.c.) to support carotid vascular tone. Infusions of capsaicin (0.3, 1, 3 and 10 μg kg−1 min−1, i.c.) did not alter the heart rate, but dose‐dependently increased the mean arterial blood pressure. This moderate hypertensive effect was not modified by BIBN4096BS. Capsaicin infusion (10 μg kg−1 min−1, i.c.) increased total carotid, arteriovenous anastomotic and tissue blood flows and conductances as well as carotid pulsations, but decreased the difference between arterial and jugular venous oxygen saturations. These responses to capsaicin were dose‐dependently blocked by BIBN4096BS. Capsaicin infusion (10 μg kg−1 min−1, i.c.) more than doubled the jugular venous plasma concentration of CGRP. This effect was not blocked, but rather increased, by BIBN4096BS. The above results show that BIBN4096BS behaves as a potent antagonist of capsaicin‐induced carotid haemodynamic changes that are mediated via the release of CGRP. Therefore, this compound may prove effective in the treatment of migraine.

Investigation of the role of 5-HT1B and 5-HT1D receptors in the sumatriptan-induced constriction of porcine carotid arteriovenous anastomoses.

It has previously been shown that the antimigraine drug sumatriptan constricts porcine carotid arteriovenous anastomoses via 5‐HT1‐like receptors, identical to 5‐HT1B/1D receptors. The recent availability of silent antagonists selective for the 5‐HT1B (SB224289) and 5‐HT1D (BRL15572) receptor led us to further analyse the nature of receptors involved. In pentobarbitone‐anaesthetized, bilaterally vagosympathectomized pigs, sumatriptan (30, 100 and 300 μg kg−1, i.v.) dose‐dependently decreased carotid arteriovenous anastomotic conductance by up to 70±5%. The dose‐related decreases in carotid arteriovenous anastomotic conductance by sumatriptan (30, 100 and 300 μg kg−1, i.v.) remained unchanged in animals treated (i.v.) with 1 mg kg−1 of BRL15572 (maximum decrease: 72±3%), but were significantly attenuated by 1 mg kg−1 (maximum decrease: 30±11%) and abolished by 3 mg kg−1 (maximum decrease: 3±7%) of SB224289. The highest dose of SB224289 did not attenuate the hypertension, tachycardia or increases in carotid blood flow induced by bolus injections of noradrenaline (0.1–3 μg kg−1, i.v.). The results indicate that sumatriptan constricts porcine carotid arteriovenous anastomoses primarily via 5‐HT1B, but not via 5‐HT1D receptors.

Effects of BIBN4096BS on cardiac output distribution and on CGRP-induced carotid haemodynamic responses in the pig

Calcitonin gene related peptide (CGRP) seems to be involved in the pathogenesis of migraine, since plasma CGRP levels increase during the headache phase. In the present study, we investigated the effects of a novel CGRP receptor antagonist, BIBN4096BS (1-piperidinecarboxamide, N-[2-[[5-amino-1-[[4-(4-pyridinyl)-1-piperazinyl]carbonyl] pentyl] amino]-1-[(3,5-dibromo-4-hydroxyphenyl) methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2H)-quinazolinyl)-, [R-(R*,S*)]-), on the regional cardiac output distribution and on the carotid haemodynamic changes induced by alpha-CGRP in anaesthetised pigs. Treatment with BIBN4096BS (100, 300 and 1000 microg kg(-1), i.v.) did not affect the heart rate, mean arterial blood pressure or systemic vascular conductance, but a small decrease in cardiac output was noticed; the latter was, however, not significantly different from that in vehicle-treated animals. The highest dose of BIBN4096BS moderately decreased vascular conductance in the lungs, kidneys, spleen and adrenals. Vascular conductance in other tissues including the brain, heart, gastrointestinal system, skin and skeletal muscles remained unchanged. Intracarotid artery infusions of alpha-CGRP (10, 30 and 100 pmol kg(-1) min(-1) during 3 min) increased the total carotid blood flow and conductance, but decreased the arterial blood pressure. These responses were dose-dependently blocked by BIBN4096BS. The above results show that BIBN4096BS is a CGRP receptor antagonist in the porcine carotid and systemic circulations, but the endogenous CGRP does not seem to play an important physiological role in regulating basal vascular tone. These findings suggest that BIBN4096BS may have therapeutic usefulness in migraine.

The role of several α1‐ and α2‐adrenoceptor subtypes mediating vasoconstriction in the canine external carotid circulation

It has recently been shown that both α1‐ and α2‐adrenoceptors mediate vasoconstriction in the canine external carotid circulation. The present study set out to identify the specific subtypes (α1A, α1B and α1D as well as α2A, α2B and α2C) mediating the above response. Consecutive 1 min intracarotid infusions of phenylephrine (α1‐adrenoceptor agonist) and BHT933 (α2‐adrenoceptor agonist) produced dose‐dependent decreases in external carotid blood flow, without affecting mean arterial blood pressure or heart rate. The responses to phenylephrine were selectively antagonized by the antagonists, 5‐methylurapidil (α1A) or BMY7378 (α1D), but not by L‐765,314 (α1B), BRL44408 (α2A), imiloxan (α2B) or MK912 (α2C). In contrast, only BRL44408 or MK912 affected the responses to BHT933. The above results support our contention that mainly the α1A, α1D, α2A and α2C‐adrenoceptor subtypes mediate vasoconstriction in the canine external carotid circulation.

Pharmacological evidence that α1- and α2-adrenoceptors mediate vasoconstriction of carotid arteriovenous anastomoses in anaesthetized pigs

Vasoconstriction of carotid arteriovenous anastomoses may be involved in the therapeutic action of acutely acting antimigraine agents, including the triptans and ergot alkaloids. While 5‐HT1B/1D receptors mediate the effect of triptans, ergotamine and dihydroergotamine also interact with α‐adrenoceptors. In the present study, we investigated the potential role of α1‐ and α2‐adrenoceptors in mediating vasoconstriction of carotid arteriovenous anastomoses in anaesthetized pigs. Ten minute intracarotid infusions of phenylephrine (1, 3 and 10 μg kg−1 min−1) or BHT 933 (3, 10 and 30 μg kg−1 min−1) produced dose‐dependent decreases in total carotid and arteriovenous anastomotic conductances; no changes were observed in the capillary fraction. The carotid vascular effects of phenylephrine and BHT 933 were selectively abolished by prazosin (100 μg kg−1, i.v.) and rauwolscine (300 μg kg−1, i.v.), respectively. The responses to phenylephrine and BHT 933 were not affected by the selective 5‐HT1B/1D receptor antagonist GR127935 (500 μg kg−1, i.v.). These results show that both α1‐ and α2‐adrenoceptors can mediate vasoconstriction of carotid arteriovenous anastomoses in anaesthetized pigs. Since vasoconstrictor activity in this in vivo model is predictive of anti‐migraine activity, an agonist activity at particularly the α2‐adrenoceptor subtypes, in view of their less ubiquitous nature, could provide migraine abortive potential. Thus, the present results may aid further understanding of the mode of action of some current anti‐migraine agents and may eventually be helpful in the development of future treatment in migraine.

Possible role of α-adrenoceptor subtypes in acute migraine therapy

Even though the underlying mechanisms for the pathophysiology of migraine attacks are not completely understood, little doubt exists that the headache phase is explained by dilatation of cranial, extracerebral blood vessels. In this context, experimental models predictive for anti-migraine activity have shown that both triptans and ergot alkaloids, which abort migraine headache, produce vasoconstriction within the carotid circulation of different species. In contrast to the well-established role of serotonin (5-hydroxytryptamine; 5-HT) 5-HT1B receptors in the common carotid vascular bed, the role of α-adrenoceptors and their subtypes has been examined only relatively recently. Using experimental animal models and α1- and α2-adrenoceptor agonists (phenylephrine and BHT933, respectively) and antagonists (prazosin and rauwolscine, respectively), it was shown that activation of either receptor produces a cranioselective vasoconstriction. Subsequently, investigations employing relatively selective antagonists at α1- (α1A, α1B, α1D) and α2- (α2A, α2B, α2C) adrenoceptor subtypes revealed that specific receptors mediate the carotid haemodynamic responses in these animals. From these observations, together with the potential limited role of α1B-and α2C-adrenoceptors in the regulation of systemic haemodynamic responses, it is suggested that selective agonists at these receptors may provide a promising novel avenue for the development of acute anti-migraine drugs.

Pharmacological identification of the major subtypes of adrenoceptors involved in the canine external carotid vasoconstrictor effects of adrenaline and noradrenaline

This study investigated the potential effects of adrenaline and noradrenaline on the external carotid blood flow of vagosympathectomised dogs and the receptor mechanisms involved. One minute (1 min) intracarotid infusions of adrenaline and noradrenaline produced dose-dependent decreases in external carotid blood flow without changes in blood pressure or heart rate. These responses, which remained unaffected after saline, were: (i) mimicked by the adrenoceptor agonists, phenylephrine (alpha1) and BHT933 (6-Ethyl-5,6,7,8-tetrahydro-4H-oxazolo [4,5-d] azepin-2-amine dihydrochloride; alpha2); (ii) abolished after phentolamine (2000 microg/kg) unmasking a vasodilator component (subsequently blocked by propranolol; 1000 microg/kg); and (iii) partly blocked by rauwolscine (30 and 100 microg/kg), and subsequently abolished by prazosin (100 microg/kg). Accordingly, rauwolscine (100 and 300 microg/kg) markedly blocked the responses to BHT933 without affecting those to phenylephrine; likewise, prazosin (100 microg/kg) markedly blocked the responses to phenylephrine without affecting those to BHT933. These results show that both alpha1- and alpha2-adrenoceptors mediate vasoconstriction within the canine external carotid circulation. Moreover, after blockade of alpha1/alpha2-adrenoceptors, both adrenaline and noradrenaline exhibit a beta-adrenoceptor-mediated vasodilator component.

Porcine carotid vascular effects of eletriptan (UK-116,044): a new 5-HT1B/1D receptor agonist with anti-migraine activity

It has been suggested that opening of cephalic arteriovenous anastomoses may be involved in the headache phase of migraine. Indeed, a number of acutely acting anti-migraine drugs, including the ergot alkaloids and sumatriptan, constrict porcine carotid arteriovenous anastomoses. In this study, using pentobarbital anaesthetised pigs, we investigated the effects of eletriptan, a close structural analogue of sumatriptan, on the distribution of common carotid artery blood flow into arteriovenous anastomotic and nutrient (capillary) fractions. Eletriptan (10, 30, 100, 300 and 1000 µg kg–1, i.v.) decreased the total carotid blood flow, exclusively by decreasing cephalic arteriovenous anastomotic blood flow; nutrient blood flow, particularly to the ear, skin and fat, was significantly increased. The doses of eletriptan needed to reduce arteriovenous anastomotic blood flow and conductance by 50% (ED50) were, respectively, 117±21 µg kg–1 (251±45 nmol kg–1) and 184±42 µg kg–1 (396±91 nmol kg–1); the highest dose caused reductions of 84±3% and 77±4%, respectively. The eletriptan-induced changes in carotid haemodynamics were clearly attenuated by pretreating the pigs with the selective 5-HT1B/1D receptor antagonist GR127935 (0.5 mg kg–1). On the basis of these results, we conclude that (1) the eletriptan-induced constriction of cephalic arteriovenous anastomoses as well as the arteriolar dilatation in head tissues is predominantly mediated by 5-HT1B/1D receptors, and (2) eletriptan should be effective in aborting migraine headache. Clinical studies have already demonstrated its therapeutic action in migraine patients.

Effects of sumatriptan on capsaicin-induced carotid haemodynamic changes and CGRP release in anaesthetized pigs

It is suggested that during a migraine attack capsaicin-sensitive trigeminal sensory nerves release calcitonin gene related peptide (CGRP), resulting in cranial vasodilatation and central nociception. Hence, inhibition of trigeminal CGRP release may prevent the above vasodilatation and, accordingly, abort migraine headache. Therefore, this study investigated the effects of sumatriptan (100 and 300 μg/kg, i.v.) on capsaicin-induced carotid haemodynamic changes and on CGRP release. Intracarotid (i.c.) infusions of capsaicin (10 μg/kg/min, i.c.) increased total carotid, arteriovenous anastomotic and capillary conductances as well as carotid pulsations, but decreased the difference between arterial and jugular venous oxygen saturations. Except for some attenuation of arteriovenous anastomotic changes, the capsaicin-induced responses were not affected by sumatriptan. Moreover, i.c. infusions of capsaicin (0.3, 1, 3 and 10 μg/kg/min, i.c.) dose-dependently increased the jugular venous plasma concentrations of CGRP, which also remained unaffected by sumatriptan. The above results support the contention that the therapeutic action of sumatriptan is mainly due to cranial vasoconstriction rather than trigeminal (CGRP release) inhibition.

Effects of donitriptan on carotid haemodynamics and cardiac output distribution in anaesthetized pigs.

We investigated the effects of donitriptan, which possesses a uniquely high affinity and efficacy at 5-HT1B/1D receptors, on carotid and systemic haemodynamics in anaesthetized pigs. Donitriptan (0.16-100 μg kg-1, i.v.) dose-dependently decreased total carotid blood flow and vascular conductance (maximum response: -25 ± 3%). This effect was entirely due to a selective reduction in the cephalic arteriovenous anastomotic fraction (maximum response: -63 ± 3%; ED50%: 92 ± 31 nmol/kg); the nutrient vascular conductance increased. Donitriptan did not decrease vascular conductances in or blood flow to a number of organs, including the heart and kidneys; in fact, vascular conductances in the skin, brain and skeletal muscles increased. Cardiac output was slightly decreased by donitriptan, but this effect was confined to peripheral arteriovenous anastomoses. The haemodynamic effects of donitriptan were substantially reduced by the 5-HT1B/1D receptor antagonist GR127935. These results show that donitriptan selectively constricts arteriovenous anastomoses via 5-HT1B receptor activation. The drug should be able to abort migraine headaches and it is unlikely to compromize blood flow to vital organs.