Olcegepant blocks neurogenic and non-neurogenic CGRPergic vasodepressor responses and facilitates noradrenergic vasopressor responses in pithed rats.

BACKGROUND AND PURPOSE: Olcegepant (BIBN4096BS) is a selective non-peptide CGRP receptor antagonist with acute antimigraine properties. Since systemic vascular tone is modulated by perivascular (primary sensory CGRPergic and sympathetic) nerves, this randomized study investigated in pithed rats the effect of acute i.v. treatment with olcegepant on the neurogenic and non-neurogenic: (i) CGRPergic vasodepressor responses; and (ii) noradrenergic vasopressor responses. The pithed rat is an experimental model predictive of systemic (cardio) vascular side effects. EXPERIMENTAL APPROACH: Seventy-five male Wistar rats (divided into 15 groups, n = 5 each) were pithed, artificially ventilated and prepared for: (i) spinal stimulation (T9 -T12 ; 0.56-5.6 Hz) of the sensory CGRPergic vasodepressor outflow or i.v. bolus injections (0.1-1 µg·kg-1 ) of α-CGRP, substance P or acetylcholine, which induced frequency-dependent or dose-dependent vasodepressor responses; or (ii) spinal stimulation (T7 -T9 ; 0.03-3 Hz) of the sympathetic vasopressor outflow or i.v. bolus injections (0.03-3 µg·kg-1 ) of noradrenaline, which produced frequency-dependent or dose-dependent vasopressor responses. KEY RESULTS: Olcegepant (1000 and 3000 µg·kg-1 , i.v.) dose-dependently blocked the vasodepressor responses to sensory nerve stimulation or i.v. α-CGRP, without affecting those to substance P or acetylcholine. Whereas it potentiated the vasopressor responses to sympathetic nerve stimulation or i.v. noradrenaline. CONCLUSIONS AND IMPLICATIONS: Olcegepant (i.v.) selectively blocked the neurogenic and non-neurogenic CGRPergic vasodepressor responses. This blockade by olcegepant potentiated the neurogenic and non-neurogenic noradrenergic vasopressor responses in pithed rats, an effect that might result in an increased vascular resistance and, consequently, in a prohypertensive action.

Development of an experimental model to study trigeminal nerve-mediated vasodilation on the human forehead.

BACKGROUND: During migraine, trigeminal sensory nerve terminals release calcitonin gene-related peptide (CGRP), inducing nociception and vasodilation. Applied on the skin, capsaicin activates the transient receptor potential vanilloid type 1 (TRPV1) channel and releases CGRP from sensory nerve terminals, thus increasing dermal blood flow (DBF). Using capsaicin application and electrical stimulation of the forehead skin, a trigeminal nerve-innervated dermatome, we aimed to develop a model to measure trigeminal nerve-mediated vasodilation in humans. METHODS: Using laser Doppler imaging, forehead DBF responses to application of capsaicin (0.06 mg/ml and 6.0 mg/ml) and saline, with and without iontophoresis, were studied in healthy subjects. The within-subject coefficient of variation (WCV) of repeated DBF measurements was calculated to assess reproducibility. RESULTS: Maximal DBF responses to 6.0 mg/ml capsaicin with and without iontophoresis did not differ (Emax 459 ± 32 and 424 ± 32 arbitrary units (a.u.), WCV 6 ± 4%). In contrast, DBF responses to 0.06 mg/ml capsaicin were significantly larger with than without iontophoresis (Emax 307 ± 60 versus 187 ± 21 a.u., WCV 21 ± 13%). Saline with iontophoresis significantly increased DBF (Emax: 245 ± 26 a.u, WCV 11 ± 8%), while saline application without iontophoresis did not affect DBF. CONCLUSION: Topical application of capsaicin and electrical stimulation induce reproducible forehead DBF increases and therefore are suitable to study trigeminal nerve-mediated vasodilation in humans.

The role of dopamine D2, but not D3 or D4, receptor subtypes, in quinpirole-induced inhibition of the cardioaccelerator sympathetic outflow in pithed rats.

BACKGROUND AND PURPOSE: Quinpirole (a dopamine D2-like receptor agonist) inhibits the cardioaccelerator sympathetic outflow in pithed rats by sympathoinhibitory D2-like receptors. The present study was designed to identify pharmacologically the specific D2-like receptor subtypes (i.e. D2 , D3 and D4) involved in this sympathoinhibition by quinpirole. EXPERIMENTAL APPROACH: One hundred fourteen male Wistar rats were pithed, artificially ventilated with room air and prepared for either preganglionic spinal (C7-T1) stimulation of the cardioaccelerator sympathetic outflow (n = 102) or i.v. bolus injections of exogenous noradrenaline (n = 12). This approach resulted in frequency-dependent and dose-dependent tachycardic responses, respectively, as previously reported by our group. KEY RESULTS: I.v. continuous infusions of quinpirole (0.1-10 µg kg(-1) min(-1)), but not of saline (0.02 mL min(-1)), dose-dependently inhibited the sympathetically induced tachycardic responses. Moreover, the cardiac sympathoinhibition induced by 3 µg kg(-1) min(-1) quinpirole (which failed to affect the tachycardic responses to i.v. noradrenaline) was: (i) unchanged after i.v. injections of the antagonists SB-277011-A (D3 ; 100-300 µg kg(-1)) or L-745,870 (D4 ; 30-100 µg kg(-1)); and (ii) markedly blocked and abolished by, respectively, 100 and 300 µg kg(-1) of the D2 preferring receptor subtype antagonist L-741,626. These doses of antagonists, which did not affect per se the sympathetically induced tachycardic responses, were high enough to completely block their respective receptors. CONCLUSIONS AND IMPLICATIONS: The cardiac sympathoinhibition induced by 3 µg kg(-1) min(-1) quinpirole involves the dopamine D2 receptor subtype, with no evidence for the involvement of the D3 or D4 subtypes. This provides new evidence for understanding the modulation of the cardioaccelerator sympathetic outflow.

Effects of ionotropic glutamate receptor antagonists on rat dural artery diameter in an intravital microscopy model.

BACKGROUND AND PURPOSE: During migraine, trigeminal nerves may release calcitonin gene-related peptide (CGRP), inducing cranial vasodilatation and central nociception; hence, trigeminal inhibition or blockade of craniovascular CGRP receptors may prevent this vasodilatation and abort migraine headache. Several preclinical studies have shown that glutamate receptor antagonists affect the pathophysiology of migraine. This study investigated whether antagonists of NMDA (ketamine and MK801), AMPA (GYKI52466) and kainate (LY466195) glutamate receptors affected dural vasodilatation induced by alpha-CGRP, capsaicin and periarterial electrical stimulation in rats, using intravital microscopy. EXPERIMENTAL APPROACH: Male Sprague-Dawley rats were anaesthetized and the overlying bone was thinned to visualize the dural artery. Then, vasodilator responses to exogenous (i.v. alpha-CGRP) and endogenous (released by i.v. capsaicin and periarterial electrical stimulation) CGRP were elicited in the absence or presence of the above antagonists. KEY RESULTS: alpha-CGRP, capsaicin and periarterial electrical stimulation increased dural artery diameter. Ketamine and MK801 inhibited the vasodilator responses to capsaicin and electrical stimulation, while only ketamine attenuated those to alpha-CGRP. In contrast, GYKI52466 only attenuated the vasodilatation to exogenous alpha-CGRP, while LY466195 did not affect the vasodilator responses to endogenous or exogenous CGRP. CONCLUSIONS AND IMPLICATIONS: Although GYKI52466 has not been tested clinically, our data suggest that it would not inhibit migraine via vascular mechanisms. Similarly, the antimigraine efficacy of LY466195 seems unrelated to vascular CGRP-mediated pathways and/or receptors. In contrast, the cranial vascular effects of ketamine and MK801 may represent a therapeutic mechanism, although the same mechanism might contribute, peripherally, to cardiovascular side effects.

The role of peripheral 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E and 5-HT1F serotonergic receptors in the reduction of nociception in rats.

This study assessed the possible antinociceptive role of peripheral 5-HT(1) receptor subtypes in the rat formalin test. Rats were injected into the dorsum of the hind paw with 50 microl of diluted formalin (1%). Nociceptive behavior was quantified as the number of flinches of the injected paw. Reduction of flinching was considered as antinociception. Ipsilateral, but not contralateral, peripheral administration of the 5-HT(1) receptor agonists R(+)-UH-301 (5-HT(1A); 0.1-3 microg/paw), CGS-12066A (5-HT(1B); 0.01-0.3 microg/paw), GR46611 (5-HT(1B/1D); 0.3-10 microg/paw), BRL54443 (5-HT(1E/1F); 3-300 microg/paw) or LY344864 (5-HT(1F); 3-300 microg/paw) significantly reduced formalin-induced flinching. The corresponding vehicle was devoid of any effect by itself. The local antinociceptive effect of R(+)-UH-301 (0.3 microg/paw) was significantly reduced by WAY-100635 (30-100 microg/paw; a 5-HT(1A) receptor antagonist). Moreover, the antagonists GR55562 (30-100 microg/paw; 5-HT(1B/D)) or SB224289 (30-100 microg/paw; 5-HT(1B)) dose-dependently reduced the antinociceptive effect of CGS-12066A (0.3 microg/paw) whereas GR55562 (30-100 microg/paw) or BRL15572 (30-100 microg/paw, 5-HT(1D)) reduced the antinociceptive effect of GR46611 (0.3 microg/paw). Interestingly, the effects of BRL54443 and LY344864 (300 microg/paw each) were partially reduced by methiothepin, but not by the highest doses of WAY-100635, SB224289 or BRL15572. The above antagonists did not produce any effect by themselves. These results suggest that peripheral activation of the 5-HT(1A,) 5-HT(1B), 5-HT(1D), 5-HT(1F) and, probably, 5-HT(1E) receptor subtypes leads to antinociception in the rat formalin test. Thus, the use of selective 5-HT(1) receptor agonists could be a therapeutic strategy to reduce inflammatory pain.

Evidence that some imidazoline derivatives inhibit peripherally the vasopressor sympathetic outflow in pithed rats.

Imidazoline derivatives (e.g. clonidine and moxonidine) and alpha(2)-adrenoceptor agonists (e.g. B-HT 933) have been shown to inhibit sympathetically-induced [(3)H]noradrenaline release in several isolated blood vessels. The present study has compared the potential capability of agonists at imidazoline I(1/2) receptors and/or alpha(1/2)-adrenoceptors to inhibit the sympathetically-induced vasopressor responses in pithed rats. For this purpose, male Wistar rats were pithed and prepared for measurement of diastolic blood pressure and heart rate. Then, the vasopressor responses induced by either selective electrical stimulation (2 ms, 60 V; 0.03, 0.1, 0.3, 1 and 3 Hz) of the vascular sympathetic outflow (T(7)-T(9)) or i.v. bolus injections of exogenous noradrenaline (0.03, 0.1, 0.3, 1 and 3 microg/kg) were determined before and during i.v. continuous infusions of the agonists B-HT 933 (alpha(2)), clonidine (alpha(2), I(1)), moxonidine (alpha(2), I(1)), cirazoline (alpha(1), I(2)), agmatine (putative endogenous ligand of imidazoline receptors) and methoxamine (alpha(1)), or equivalent volumes of physiological saline. Electrical sympathetic stimulation elicited frequency-dependent vasopressor responses which were significantly inhibited during the continuous infusions of B-HT 933, clonidine, moxonidine, cirazoline and agmatine, but not of physiological saline. Interestingly, the vasopressor responses to exogenous noradrenaline, which remained unaffected during the infusions of physiological saline, B-HT 933, moxonidine, cirazoline and agmatine, were significantly blocked during the infusions of clonidine or methoxamine. These results suggest that B-HT 933, moxonidine, cirazoline and agmatine induced a prejunctional inhibition of the vasopressor sympathetic outflow in pithed rats, whilst clonidine inhibited the vasopressor sympathetic outflow by both prejunctional and postjunctional mechanisms.

Pharmacological profile of the clonidine-induced inhibition of vasodepressor sensory outflow in pithed rats: correlation with alpha(2A/2C)-adrenoceptors.

BACKGROUND AND PURPOSE: Resistance blood vessels are innervated by sympathetic and primary sensory nerves, which modulate vascular tone through the release of noradrenaline and calcitonin gene-related peptide (CGRP), respectively. Moreover, electrical stimulation of the perivascular sensory outflow in pithed rats results in vasodepressor responses which are mainly mediated by CGRP release. The present study has investigated the role of alpha(2)-adrenoceptors in the inhibition of these vasodepressor responses. EXPERIMENTAL APPROACH: 144 pithed male Wistar rats were pretreated with hexamethonium (2 mg kg(-1) min(-1)) followed by i.v. continuous infusions of either methoxamine (15 and 30 microg kg(-1) min(-1)) or clonidine (3, 10 and 30 microg kg(-1) min(-1)). Under these conditions, electrical stimulation (0.56-5.6 Hz; 50 V and 2 ms) of the spinal cord (T(9)-T(12)) resulted in frequency-dependent decreases in diastolic blood pressure. KEY RESULTS: The infusion of clonidine (10 microg kg(-1) min(-1)), as compared to those of methoxamine (15 or 30 microg kg(-1) min(-1)), inhibited the vasodepressor responses to electrical stimulation without affecting those to i.v. bolus injections of alpha-CGRP (0.1-1 microg kg(-1)). This inhibition by clonidine was: (i) antagonized by 300 microg kg(-1) rauwolscine (alpha(2A/2B/2C)), 300 and 1000 microg kg(-1) BRL44408 (alpha(2A)), or 10 and 30 microg kg(-1) MK912 (alpha(2C)); and (ii) unaffected by 1 ml kg(-1) saline, 100 microg kg(-1) BRL44408, 3000 and 10,000 microg kg(-1) imiloxan (alpha(2B)) or 3 microg kg(-1) MK912. CONCLUSIONS AND IMPLICATIONS: The inhibition produced by 10 microg kg(-1) min(-1) clonidine on the vasodepressor (perivascular) sensory outflow in rats may be mainly mediated by prejunctional alpha(2A)/alpha(2C)-adrenoceptors.

Characterization of the postjunctional alpha 2C-adrenoceptor mediating vasoconstriction to UK14304 in porcine pulmonary veins.

BACKGROUND AND PURPOSE: In terms of postjunctional alpha(2)-adrenoceptors in the pulmonary circulation, no evidence is available with regard to the receptor subtypes mediating vasoconstriction. Therefore, we characterized the alpha(2)-adrenoceptor subtypes mediating contraction in isolated porcine pulmonary veins. EXPERIMENTAL APPROACH: alpha-adrenoceptor-mediated vasoconstriction was studied using a tissue bath protocol. mRNA profile and relative quantification of alpha(2)-adrenoceptor subtypes were determined in porcine pulmonary veins using reverse-transcriptase polymerase chain reaction (RT-PCR) and real-time PCR. KEY RESULTS: In porcine pulmonary veins, noradrenaline, phenylephrine (alpha(1)-adrenoceptor agonist), UK14304 and clonidine (alpha(2)-adrenoceptor agonists) caused concentration-dependent contractions. The rank order of agonist potency was: NA approximately UK14304 approximately clonidine > phenylephrine. UK14304 responses were antagonised by MK912 (noncompetitive antagonist parameter pD'(2): 10.1), rauwolscine (pK(B): 9.5), yohimbine (pK(B): 9.1), WB4101 (pK(B): 8.7), ARC239 (pK(B): 7.5), prazosin (pK(B): 7.1) and BRL44408 (pK(B): 7.0). Antagonist potencies fitted best with radioligand binding data (pK(i)) at the human recombinant alpha(2C)-adrenoceptor (r(2)=0.96, P=0.0001). Correlation with alpha(2B)-adrenoceptors was lower (r(2)=0.74, P>0.01) and no correlation was obtained with alpha(2A)-adrenoceptors. Moreover, RT-PCR studies in porcine pulmonary veins showed mRNA signals for alpha(2A)- and alpha(2C)-adrenoceptors, but not for alpha(2B)-adrenoceptors, whilst real-time PCR studies indicated a prominent expression of alpha(2C)-adrenoceptor mRNA. CONCLUSIONS AND IMPLICATIONS: Postjunctional alpha(2C)-adrenoceptors mediated contraction in porcine pulmonary veins. alpha(1)-Adrenoceptors also seem to be present in this tissue. Since alpha(2)-adrenoceptor responsiveness is increased when pulmonary vascular tone is elevated, alpha(2C)-adrenoceptor antagonists may be beneficial in diseases such as pulmonary hypertension or congestive heart failure.

Donitriptan, but not sumatriptan, inhibits capsaicin-induced canine external carotid vasodilatation via 5-HT1B rather than 5-HT1D receptors.

BACKGROUND AND PURPOSE: It has been 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, trigeminal inhibition may prevent this vasodilatation and abort migraine headache. This study investigated the effects of the agonists sumatriptan (5-HT(1B/1D) water-soluble), donitriptan (5-HT(1B/1D) lipid-soluble), PNU-142633 (5-HT(1D) water-soluble) and PNU-109291 (5-HT(1D) lipid-soluble) on vasodilator responses to capsaicin, alpha-CGRP and acetylcholine in dog external carotid artery. EXPERIMENTAL APPROACH: 59 vagosympathectomized dogs were anaesthetized with sodium pentobarbitone. Blood pressure and heart rate were recorded with a pressure transducer, connected to a cannula inserted into a femoral artery. A precalibrated flow probe was placed around the common carotid artery, with ligation of the internal carotid and occipital branches, and connected to an ultrasonic flowmeter. The thyroid artery was cannulated for infusion of agonists. KEY RESULTS: Intracarotid infusions of capsaicin, alpha-CGRP and acetylcholine dose-dependently increased blood flow through the carotid artery. These responses remained unaffected after intravenous (i.v.) infusions of sumatriptan, PNU-142633, PNU-109291 or physiological saline; in contrast, donitriptan significantly attenuated the vasodilator responses to capsaicin, but not those to alpha-CGRP or acetylcholine. Only sumatriptan and donitriptan dose-dependently decreased the carotid blood flow. Interestingly, i.v. administration of the antagonist, SB224289 (5-HT(1B)), but not of BRL15572 (5-HT(1D)), abolished the inhibition by donitriptan. CONCLUSIONS AND IMPLICATIONS: Our results suggest that the inhibition produced by donitriptan of capsaicin-induced external carotid vasodilatation is mainly mediated by 5-HT(1B), rather than 5-HT(1D), receptors, probably by a central mechanism.

Experimental migraine models and their relevance in migraine therapy.

Although the understanding of migraine pathophysiology is incomplete, it is now well accepted that this neurovascular syndrome is mainly due to a cranial vasodilation with activation of the trigeminal system. Several experimental migraine models, based on vascular and neuronal involvement, have been developed. Obviously, the migraine models do not entail all facets of this clinically heterogeneous disorder, but their contribution at several levels (molecular, in vitro, in vivo) has been crucial in the development of novel antimigraine drugs and in the understanding of migraine pathophysiology. One important vascular in vivo model, based on an assumption that migraine headache involves cranial vasodilation, determines porcine arteriovenous anastomotic blood flow. Other models utilize electrical stimulation of the trigeminal ganglion/nerve to study neurogenic dural inflammation, while the superior sagittal sinus stimulation model takes into account the transmission of trigeminal nociceptive input in the brainstem. More recently, the introduction of integrated models, namely electrical stimulation of the trigeminal ganglion or systemic administration of capsaicin, allows studying the activation of the trigeminal system and its effect on the cranial vasculature. Studies using in vitro models have contributed enormously during the preclinical stage to characterizing the receptors in cranial blood vessels and to studying the effects of several putative antimigraine agents. The aforementioned migraine models have advantages as well as some limitations. The present review is devoted to discussing various migraine models and their relevance to antimigraine therapy.

Lack of effect of the adenosine A1 receptor agonist, GR79236, on capsaicin-induced CGRP release in anaesthetized pigs.

Migraine is a common neurological disorder that is associated with an increase in plasma calcitonin gene-related peptide (CGRP) levels. CGRP, a potent vasodilator released from the activated trigeminal sensory nerves, dilates intracranial blood vessels and transmits vascular nociception. Hence, inhibition of trigeminal CGRP release may prevent neurotransmission and, thereby, ameliorate migraine headache. Therefore, the present study in anaesthetized pigs investigates the effects of a selective adenosine A(1) receptor agonist, GR79236 (3, 10 and 30 microg/kg, i.v.) on capsaicin-induced carotid haemodynamic changes and on plasma CGRP release. Intracarotid (i.c.) infusion of capsaicin (10 microg/kg/min, i.c.) increased the total carotid blood flow and conductance as well as carotid pulsations, but decreased the difference between arterial and jugular venous oxygen saturations. These responses to capsaicin were dose-dependently attenuated by GR79236. However, the increases in the plasma CGRP concentrations by capsaicin remained essentially unmodified after GR79236 treatment. The above results suggest that GR79236 may have an antimigraine potential due to its postjunctional effects (carotid vasoconstriction) rather than to prejunctional inhibition of trigeminal CGRP release.

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 microg/kg, i.v.) on capsaicin-induced carotid haemodynamic changes and on CGRP release. Intracarotid (i.c.) infusions of capsaicin (10 microg/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 microg/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.

Assessment of anti-migraine potential of a novel alpha-adrenoceptor agonist S19014: effects on porcine carotid and regional haemodynamics and human coronary artery.

Taking into account the drawbacks associated with the use of triptans, attempts are being made to explore other avenues for the treatment of migraine. Recently, it has been shown that both alpha1- and alpha2-adrenoceptors mediate the constriction of porcine carotid arteriovenous anastomoses, which has effectively served as an experimental model predictive of anti-migraine activity. The present study investigated the effects of a novel alpha-adrenoceptor agonist S19014 (spiro[(1,3-diazacyclopent-1-ene)-5 : 2'-(4',5'-dimethylindane)] fumarate) on carotid and systemic haemodynamics in anaesthetized pigs, and on human isolated coronary arteries. Increasing doses of S19014 (1-30 micro g/kg, i.v.) produced a dose-dependent initial short-lasting vasopressor response and a decrease of total carotid blood flow and conductance. The carotid blood flow and conductance changes were exclusively due to constriction of carotid arteriovenous anastomoses (capillary blood flow increased) and were accompanied by an increase in arterio-jugular venous oxygen saturation difference. Whereas prazosin (100 micro g/kg, i.v.) was ineffective, rauwolscine (300 micro g/kg, i.v.) attenuated the responses to S19014. The compound did not much affect the distribution of cardiac output to peripheral organs when compared with the vehicle group. Furthermore, S19014 only slightly contracted the human isolated coronary artery and its contractions, contrary to those of sumatriptan, were not increased in blood vessels pre-contracted with U46619. These results suggest that (i) the systemic and carotid vascular effects of S19014 are mainly mediated by alpha2-adrenoceptors, and (ii) S19014 could be effective in the treatment of migraine with an improved cardiovascular tolerance.

Possible role of alpha-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 alpha-adrenoceptors and their subtypes has been examined only relatively recently. Using experimental animal models and alpha1- and alpha2-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 alpha1- (alpha1A, alpha1B, alpha1D) and alpha2- (alpha2A, alpha2B, alpha2C) adrenoceptor subtypes revealed that specific receptors mediate the carotid haemodynamic responses in these animals. From these observations, together with the potential limited role of alpha1B- and alpha2C-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.

An introduction to migraine: from ancient treatment to functional pharmacology and antimigraine therapy.

Migraine treatment has evolved from the realms of the supernatural into the scientific arena, but it seems still controversial whether migraine is primarily a vascular or a neurological dysfunction. Irrespective of this controversy, the levels of serotonin (5-hydroxytryptamine; 5-HT), a vasoconstrictor and a central neurotransmitter, seem to decrease during migraine (with associated carotid vasodilatation) whereas an i.v. infusion of 5-HT can abort migraine. In fact, 5-HT as well as ergotamine, dihydroergotamine and other antimigraine agents invariably produce vasoconstriction in the external carotid circulation. The last decade has witnessed the advent of sumatriptan and second generation triptans (e.g. zolmitriptan, rizatriptan, naratriptan), which belong to a new class of drugs, now known as 5-HT1B/1D/1F receptor agonists. Compared to sumatriptan, the second-generation triptans have a higher oral bioavailability and longer plasma half-life. In line with the vascular and neurogenic theories of migraine, all triptans produce selective carotid vasoconstriction (via 5-HT1B receptors) and presynaptic inhibition of the trigeminovascular inflammatory responses implicated in migraine (via 5-HT1D/5-ht1F receptors). Moreover, selective agonists at 5-HT1D (PNU-142633) and 5-ht1F (LY344864) receptors inhibit the trigeminovascular system without producing vasoconstriction. Nevertheless, PNU-142633 proved to be ineffective in the acute treatment of migraine, whilst LY344864 did show some efficacy when used in doses which interact with 5-HT1B receptors. Finally, although the triptans are effective antimigraine agents producing selective cranial vasoconstriction, efforts are being made to develop other effective antimigraine alternatives acting via the direct blockade of vasodilator mechanisms (e.g. antagonists at CGRP receptors, antagonists at 5-HT7 receptors, inhibitors of nitric oxide biosynthesis, etc). These alternatives will hopefully lead to fewer side-effects.

Pharmacological profile of the mechanisms involved in the external carotid vascular effects of the antimigraine agent isometheptene in anaesthetised dogs.

The present study set out to investigate the external carotid vascular effects of isometheptene in vagosympathectomised dogs, anaesthetised with pentobarbital. One-minute intracarotid (intra-arterial; i.a.) infusions of isometheptene (10, 30, 100 and 300 microg/min) produced dose-dependent decreases in external carotid blood flow, without affecting blood pressure or heart rate. The vasoconstrictor responses to 100 microg/min and 300 microg/min of isometheptene were clearly attenuated in animals pretreated with reserpine (5,000 microg/kg). Moreover, after prazosin (an alpha1-adrenoceptor antagonist; 100 microg/kg), the responses to isometheptene remained unaltered in untreated as well as reserpine-pretreated dogs. In contrast, the responses to isometheptene were attenuated by rauwolscine (an alpha2-adrenoceptor antagonist; 300 microg/kg) in untreated animals, and were practically abolished in reserpine-pretreated dogs. Further investigation into the specific alpha2-adrenoceptor subtypes, using selective antagonists, showed that BRL44408 (alpha2A) and MK912 (alpha2C) markedly attenuated this response, while imiloxan (alpha2B) was ineffective. The involvement of 5-HT1B and 5-HT1D receptors seems highly unlikely since antagonists at 5-HT1B (SB224289) and 5-HT1D (BRL15572) receptors (both at 300 microg/kg) were ineffective. On this basis, it is concluded that isometheptene-induced canine external carotid vasoconstriction is mediated by both indirect (a tyramine-like action) and direct (acting at receptors) mechanisms, which mainly involve alphaA- and alpha2C-adrenoceptors, while the involvement of alpha1-adrenoceptors seems rather limited.

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.

Alpha1-adrenoceptor subtypes mediating vasoconstriction in the carotid circulation of anaesthetized pigs: possible avenues for antimigraine drug development.

It has recently been shown that the alpha-adrenoceptors mediating vasoconstriction of porcine carotid arteriovenous anastomoses resemble both alpha1- and alpha2-adrenoceptors, but no attempt was made to identify the specific subtypes (alpha1A, alpha1B and alpha1D) involved. Therefore, the present study was designed to elucidate the specific subtype(s) of alpha1-adrenoceptors involved in the above response, using the alpha1-adrenoceptor agonist phenylephrine and alpha1-adrenoceptor antagonists 5-methylurapidil (alpha1A), L-765 314 (alpha1B) and BMY 7378 (alpha1D). Ten-minute intracarotid infusions of phenylephrine (1, 3 and 10 microgkg-1.min-1) induced a dose-dependent decrease in total carotid and arteriovenous anastomotic conductance, accompanied by a small tachycardia. These carotid vascular effects were abolished by L-765 314 (1000 microgkg-1; i.v.), while these responses were only attenuated by 5-methylurapidil (1000 microgkg-1; i.v.), and BMY 7378 (1000 microgkg-1; i.v.). Furthermore, intravenous bolus injections of phenylephrine (3 and 10 microgkg-1) produced a dose-dependent vasopressor response, which was only affected by 1000 microgkg-1 of 5-methylurapidil, while the other antagonists were ineffective. These results, coupled to the binding affinities of the above antagonists at the different alpha1-adrenoceptors, suggest that both alpha1A- and alpha1B-adrenoceptors mediate constriction of carotid arteriovenous anastomoses in anaesthetized pigs. In view of the less ubiquitous nature of alpha1B- compared to alpha1A-adrenoceptors, the development of potent and selective alpha1B-adrenoceptor agonists may prove to be important for the treatment of migraine.

A61603-induced vasoconstriction in porcine carotid vasculature: involvement of a non-adrenergic mechanism.

It has recently been shown that the pharmacological profile of alpha(1)-adrenoceptors mediating constriction of porcine carotid arteriovenous anastomoses resembles that of alpha(1A)- and alpha(1B)-adrenoceptor subtypes. In an attempt to verify the involvement of alpha(1A)-adrenoceptors, we used the potent alpha(1A)-adrenoceptor agonist N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydro-naphthalen-1-yl]methane sulphonamide (A61603) and found that intracarotid (i.c.) administration of A61603 (0.3-10 microg kg(-1)) dose-dependently decreased porcine carotid blood flow and vascular conductance. This decrease was exclusively due to a constriction of carotid arteriovenous anastomoses; the capillary blood flow and conductance remained unchanged. Surprisingly, the responses to A61603 were little modified by prior i.v. treatment with 5-methylurapidil (1000 microg kg(-1)), prazosin (100 microg kg(-1)) or a combination of prazosin and rauwolscine (100 and 300 microg kg(-1), respectively). The 5-HT(1B/1D) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'(5-methyl-1,2,4-oxadiazol-3-yl)[1,1,-biphenyl]-4-carboxamide hydrochloride monohydrate (GR127935; 500 microg kg(-1)) and ketanserin (500 microg kg(-1)) also failed to modify carotid vascular responses to A61603, but, interestingly, methiothepin (3000 microg kg(-1)) proved to be an effective antagonist. Taken together, the present results show that A61603 is a relatively poor agonist at the alpha(1A)-adrenoceptor in anaesthetised pigs and that the carotid vasoconstriction produced by A61603 is mediated by a novel non-adrenergic mechanism.