Role of endothelin in mediating neurogenic plasma extravasation in rat dura mater

Role of endothelin in the pathophysiology of migraine: A new view on an old player

There is cumulating evidence that endothelin-1 (ET-1) may play a role in migraine, however controversial findings still impede a conclusion to be drawn. Herein we tested the hypothesis that endothelin ETB receptors are major contributors to migraine-like responses. ET-1, IRL-1620 (selective ETB receptor agonist) or CGRP were injected into the trigeminal ganglion (TG) of female Wistar rats, and the development of periorbital mechanical allodynia was assessed hourly with von Frey hairs. Twenty-four hours later, rats were exposed to an aversive light for 1 h, after which the reactivation of periorbital mechanical allodynia (indicating photic sensitivity) was assessed up to 4 h. Moreover, the effect of systemic Bosentan (ETA/ETB receptors antagonist) or the selective antagonists of ETA (BQ-123) and ETB (BQ-788) receptors injected into the TG were evaluated against CGRP-induced responses. ET-1 and IRL-1620 injection into the TG induced periorbital mechanical allodynia and photic sensitivity. Bosentan attenuated periorbital mechanical allodynia but failed to affect photic sensitivity induced by CGRP. Selective blockade of ETB receptors in the TG fully prevented the development of periorbital mechanical allodynia and photic sensitivity induced by CGRP, but ETA receptor blockade caused only a slight reduction of periorbital mechanical allodynia without affecting photic sensitivity. ETB receptor-operated mechanisms in the TG may contribute to migraine-like responses in female rats.

Neurovascular contributions to migraine: Moving beyond vasodilation

Migraine is the third most common disease worldwide, the most common neurological disorder, and one of the most common pain conditions. Despite its prevalence, the basic physiology and underlying mechanisms contributing to the development of migraine are still poorly understood and development of new therapeutic targets is long overdue. Until recently, the major contributing pathophysiological event thought to initiate migraine was cerebral and meningeal arterial vasodilation. However, the role of vasodilation in migraine is unclear and recent findings challenge its necessity. While vasodilation itself may not contribute to migraine, it remains possible that vessels play a role in migraine pathophysiology in the absence of vasodilation. Blood vessels consist of a variety of cell types that both release and respond to numerous mediators including growth factors, cytokines, adenosine triphosphate (ATP), and nitric oxide (NO). Many of these mediators have actions on neurons that can contribute to migraine. Conversely, neurons release factors such as norepinephrine and calcitonin gene-related peptide (CGRP) that act on cells native to blood vessels. Both normal and pathological events occurring within and between vascular cells could thus mediate bi-directional communication between vessels and the nervous system, without the need for changes in vascular tone. This review will discuss the potential contribution of the vasculature, specifically endothelial cells, to current neuronal mechanisms hypothesized to play a role in migraine. Hypothalamic activity, cortical spreading depression (CSD), and dural afferent input from the cranial meninges will be reviewed with a focus on how these mechanisms can influence or be impacted by blood vessels. Together, the data discussed will provide a framework by which vessels can be viewed as important potential contributors to migraine pathophysiology, even in light of the current uncertainty over the role of vasodilation in this disorder.

Understanding migraine: Potential role of neurogenic inflammation

Neurogenic inflammation, a well-defined pathophysiologial process is characterized by the release of potent vasoactive neuropeptides, predominantly calcitonin gene-related peptide (CGRP), substance P (SP), and neurokinin A from activated peripheral nociceptive sensory nerve terminals (usually C and A delta-fibers). These peptides lead to a cascade of inflammatory tissue responses including arteriolar vasodilation, plasma protein extravasation, and degranulation of mast cells in their peripheral target tissue. Neurogenic inflammatory processes have long been implicated as a possible mechanism involved in the pathophysiology of various human diseases of the nervous system, respiratory system, gastrointestinal tract, urogenital tract, and skin. The recent development of several innovative experimental migraine models has provided evidence suggestive of the involvement of neuropeptides (SP, neurokinin A, and CGRP) in migraine headache. Antidromic stimulation of nociceptive fibers of the trigeminal nerve resulted in a neurogenic inflammatory response with marked increase in plasma protein extravasation from dural blood vessels by the release of various sensory neuropeptides. Several clinically effective abortive antimigraine medications, such as ergots and triptans, have been shown to attenuate the release of neuropeptide and neurogenic plasma protein extravasation. These findings provide support for the validity of using animal models to investigate mechanisms of neurogenic inflammation in migraine. These also further strengthen the notion of migraine being a neuroinflammatory disease. In the clinical context, there is a paucity of knowledge and awareness among physicians regarding the role of neurogenic inflammation in migraine. Improved understanding of the molecular biology, pharmacology, and pathophysiology of neurogenic inflammation may provide the practitioner the context-specific feedback to identify the novel and most effective therapeutic approach to treatment. With this objective, the present review summarizes the evidence supporting the involvement of neurogenic inflammation and neuropeptides in the pathophysiology and pharmacology of migraine headache as well as its potential significance in better tailoring therapeutic interventions in migraine or other neurological disorders. In addition, we have briefly highlighted the pathophysiological role of neurogenic inflammation in various other neurological disorders.

Animal migraine models for drug development: status and future perspectives

Migraine is number seven in WHO's list of all diseases causing disability and the third most costly neurological disorder in Europe. Acute attacks are treatable by highly selective drugs such as the triptans but there is still a huge unmet therapeutic need. Unfortunately, drug development for headache has almost come to a standstill partly because of a lack of valid animal models. Here we review previous models with emphasis on optimal characteristics of a future model. In addition to selection of animal species, the method of induction of migraine-like changes and the method of recording responses elicited by such measures are crucial. The most naturalistic way of inducing attacks is by infusion of endogenous signaling molecules that are known to cause migraine in patients. The most valid response is recording of neural activity in the trigeminal system. The most useful headache related responses are likely to be behavioral, allowing multiple experiments in each individual animal. Distinction is made between acute and prophylactic models and how to validate each of them. Modern insight into neurobiological mechanisms of migraine is so good that it is only a question of resources and efforts that determine when valid models with ability to predict efficacy in migraine will be available.

The dura and migraine

This article concerns the possible role of dural changes in migraine pain. The hypothesis that changes in the dura are important in migraine pain has become widely accepted among migraine scientists. A critical examination of the evidence for and against dural involvement in migraine pain shows that in spite of the extensive research that has been carried out on dural physiology and pathophysiology, there is no hard evidence that dural changes actually occur in humans during a migraine. On the contrary, the available evidence appears to indicate that it is unlikely that dural changes are part of the migraine pain process.

Endothelin and neonatal capsaicin regulate gastric resistance to injury in BDL rats

AIM: To investigate the relationship between primary afferent neurons, endothelin (ET) and the role of its receptors on ethanol-induced gastric damage in cirrhotic rats.

METHODS: Cirrhosis and portal hypertension were induced in rats by bile duct ligation (BDL) while controls had a sham operation. The association between ET and afferent neurons on the gastric mucosa was evaluated by capsaicin treatment in newborn rats, the use of ET agonists or antagonists, gastric ET-1 and -3 mRNA and synthetic capacity. Ethanol-induced damage was assessed using ex vivo gastric chamber experiments. Gastric blood flow was measured by laser-Doppler flowmetry.

RESULTS: ET-3 and an ET_B receptor antagonist significantly reduced the extent of ethanol-induced gastric damage in BDL rats. Gastric ET-1 and -3 levels were 30% higher in BDL rats compared to control rats. Capsaicin treatment restored the gastric resistance and blood flow responses to topical application of ethanol in BDL rats and ET-1 and -3 production to levels observed in controls.

CONCLUSION: Our results suggest that the reduced resistance of the gastric mucosa of cirrhotic rats to ethanol-induced injury is a phenomenon modulated by ET through the ET_B receptor and by sensory afferent neurons.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Verisimilitude (or "truthlikeness") as an alternative to pro and cons: migraine and cluster headache mechanisms

Calculating verisimilitude (or "truthlikeness") ad modum Popper is a quantitative alternative to the usual pros and cons in migraine and cluster headache mechanisms. The following items were evaluated: dilation of large cranial arteries during migraine; CGRP increase during migraine; migraine as a brain disorder; aura and migraine headache; brain stem activation during migraine; rCBF in migraine without aura; NO and pathophysiology of migraine; neurogenic inflammation and migraine; aura in cluster headache; and hypothalamic activation in cluster headache. It is concluded that verisimilitude calculations can be helpful when judging pathophysiological problems in migraine and cluster headache.

Endothelins as pronociceptive mediators of the rat trigeminal system: role of ETA and ETB receptors

The trigeminal nerve is comprised of three main divisions, ophthalmic, maxillary and mandibular, each providing somatosensory innervation to distinct regions of the head, face and oral cavity. Recently, a role for endothelins in nociceptive signaling in the trigeminal system has been proposed. The present study aimed to gain better insight into the participation of the endothelin system in trigeminal nociceptive transmission. Herein ET-1 and ET-3 mRNA was detected in the rats' trigeminal ganglion (TG). Fluorescent labeling of TG neurons revealed that ET(A) and ET(B) receptors are distributed along the entire TG, but ET(A) receptor expression slightly predominated within the three divisions. TRPV1 receptors were also detected throughout the entire TG, and a significant proportion of TRPV1-positive neurons (approximately 30%) co-expressed either ET(A) or ET(B) receptors. Our behavioral data showed that ET-1 (3 to 30 pmol/site) induced overt nociceptive responses after injection into the upper lip or temporomandibular joint (TMJ) and hyperalgesic actions when applied to the eye, while ET-3 and the selective ET(B) receptor agonist IRL-1620 (each at 3 to 30 pmol/site) showed only the first two effects. Injection of BQ-123, but not BQ-788 (ET(A) and ET(B) receptor antagonists, respectively, 10 nmol/site each, 30 min beforehand), into the ipsilateral upper lip abolished ET-1 induced facial grooming, but both antagonists markedly reduced the nociceptive responses induced by ET-1 injected into the TMJ. Taken together, these findings suggest that endothelins, acting through ET(A) and/or ET(B) receptors, may play an important role in mediating pain resulting from activation of most trigeminal nerve branches.

Mechanisms operated by endothelin ETA and ETB receptors in the trigeminal ganglion contribute to orofacial thermal hyperalgesia induced by infraorbital nerve constriction in rats

Endothelins, acting through specific endothelin ET(A) and/or ET(B) receptors, participate in nociceptive processing in models of cancer, inflammatory and neuropathic pain. The present study investigated which cell types express endothelin receptors in the trigeminal ganglion, and the contribution of mechanisms mediated by endothelin ET(A) and ET(B) receptors to orofacial heat hyperalgesia induced by unilateral constriction of the infraorbital nerve (CION). Both receptor types were identified by immunohistochemistry in the trigeminal ganglion, ET(A) receptors on small-sized non-myelinated and myelinated A-fibers and ET(B) receptors on both satellite glial cells and small-sized non-myelinated neuronal cells. CION promoted ipsilateral orofacial heat hyperalgesia which lasted from Day 2 until Day 10 after surgery. Ongoing CION-induced heat hyperalgesia (on Day 4) was reduced transiently, but significantly, by systemic or local treatment with antagonists of endothelin ET(A) receptors (atrasentan, 10 mg/kg, i.v.; or BQ-123, 10 nmol/lip), endothelin ET(B) receptors (A-192621, 20 mg/kg, i.v.; or BQ-788, 10 nmol/ lip), or of both ET(A)/ET(B) receptors (bosentan, 10 mg/kg, i.v.; or BQ-123 plus BQ-788, each at 10 nmol/lip). On the other hand, CION-induced heat hyperalgesia was transiently abolished over the first 90 min following i.p. injection of morphine hydrochloride (2.5 mg/kg), but fully resistant to reversal by indomethacin (4 mg/kg, i.p.) or celecoxib (10 mg/kg, i.p.). Thus, heat hyperalgesia induced by CION is maintained, in part, by peripheral signaling mechanisms operated by ET(A) and ET(B) receptors. Endothelin receptors might represent promising therapeutic targets for the control of trigeminal neuropathic pain.

Endothelin-converting enzyme 1 promotes re-sensitization of neurokinin 1 receptor-dependent neurogenic inflammation

BACKGROUND AND PURPOSE

The metalloendopeptidase endothelin-converting enzyme 1 (ECE-1) is prominently expressed in the endothelium where it converts big endothelin to endothelin-1, a vasoconstrictor peptide. Although ECE-1 is found in endosomes in endothelial cells, the role of endosomal ECE-1 is unclear. ECE-1 degrades the pro-inflammatory neuropeptide substance P (SP) in endosomes to promote recycling and re-sensitization of its neurokinin 1 (NK(1)) receptor. We investigated whether ECE-1 regulates NK(1) receptor re-sensitization and the pro-inflammatory effects of SP in the endothelium.

EXPERIMENTAL APPROACH

We examined ECE-1 expression, SP trafficking and NK(1) receptor re-sensitization in human microvascular endothelial cells (HMEC-1), and investigated re-sensitization of SP-induced plasma extravasation in rats.

KEY RESULTS

HMEC-1 expressed all four ECE-1 isoforms (a-d), and fluorescent SP trafficked to early endosomes containing ECE-1b/d. The ECE-1 inhibitor SM-19712 prevented re-sensitization of SP-induced Ca2+ signals in HMEC-1 cells. Immunoreactive ECE-1 and NK(1) receptors co-localized in microvascular endothelial cells in the rat. SP-induced extravasation of Evans blue in the urinary bladder, skin and ears of the rat desensitized when the interval between two SP injections was 10 min, and re-sensitized after 480 min. SM-19712 inhibited this re-sensitization.

CONCLUSIONS AND IMPLICATIONS

By degrading endocytosed SP, ECE-1 promotes the recycling and re-sensitization of NK(1) receptors in endothelial cells, and thereby induces re-sensitization of the pro-inflammatory effects of SP. Thus, ECE-1 inhibitors may ameliorate the pro-inflammatory actions of SP.

Cardiovascular responses produced by 5-hydroxytriptamine:a pharmacological update on the receptors/mechanisms involved and therapeutic implications

The complexity of cardiovascular responses produced by 5-hydroxytryptamine (5-HT, serotonin), including bradycardia or tachycardia, hypotension or hypertension, and vasodilatation or vasoconstriction, has been explained by the capability of this monoamine to interact with different receptors in the central nervous system (CNS), on the autonomic ganglia and postganglionic nerve endings, on vascular smooth muscle and endothelium, and on the cardiac tissue. Depending, among other factors, on the species, the vascular bed under study, and the experimental conditions, these responses are mainly mediated by 5-HT(1), 5-HT(2), 5-HT(3), 5-HT(4), 5-ht(5A/5B), and 5-HT(7) receptors as well as by a tyramine-like action or unidentified mechanisms. It is noteworthy that 5-HT(6) receptors do not seem to be involved in the cardiovascular responses to 5-HT. Regarding heart rate, intravenous (i.v.) administration of 5-HT usually lowers this variable by eliciting a von Bezold-Jarisch-like reflex via 5-HT(3) receptors located on sensory vagal nerve endings in the heart. Other bradycardic mechanisms include cardiac sympatho-inhibition by prejunctional 5-HT(1B/1D) receptors and, in the case of the rat, an additional 5-ht(5A/5B) receptor component. Moreover, i.v. 5-HT can increase heart rate in different species (after vagotomy) by a variety of mechanisms/receptors including activation of: (1) myocardial 5-HT(2A) (rat), 5-HT(3) (dog), 5-HT(4) (pig, human), and 5-HT(7) (cat) receptors; (2) adrenomedullary 5-HT(2) (dog) and prejunctional sympatho-excitatory 5-HT(3) (rabbit) receptors associated with a release of catecholamines; (3) a tyramine-like action mechanism (guinea pig); and (4) unidentified mechanisms (certain lamellibranch and gastropod species). Furthermore, central administration of 5-HT can cause, in general, bradycardia and/or tachycardia mediated by activation of, respectively, 5-HT(1A) and 5-HT(2) receptors. On the other hand, the blood pressure response to i.v. administration of 5-HT is usually triphasic and consists of an initial short-lasting vasodepressor response due to a reflex bradycardia (mediated by 5-HT(3) receptors located on vagal afferents, via the von Bezold-Jarisch-like reflex), a middle vasopressor phase, and a late, longer-lasting, vasodepressor response. The vasopressor response is a consequence of vasoconstriction mainly mediated by 5-HT(2A) receptors; however, vasoconstriction in the canine saphenous vein and external carotid bed as well as in the porcine cephalic arteries and arteriovenous anastomoses is due to activation of 5-HT(1B) receptors. The late vasodepressor response may involve three different mechanisms: (1) direct vasorelaxation by activation of 5-HT(7) receptors located on vascular smooth muscle; (2) inhibition of the vasopressor sympathetic outflow by sympatho-inhibitory 5-HT(1A/1B/1D) receptors; and (3) release of endothelium-derived relaxing factor (nitric oxide) by 5-HT(2B) and/or 5-HT(1B/1D) receptors. Furthermore, central administration of 5-HT can cause both hypotension (mainly mediated by 5-HT(1A) receptors) and hypertension (mainly mediated by 5-HT(2) receptors). The increasing availability of new compounds with high affinity and selectivity for the different 5-HT receptor subtypes makes it possible to develop drugs with potential therapeutic usefulness in the treatment of some cardiovascular illnesses including hypertension, migraine, some peripheral vascular diseases, and heart failure.

Neurosensory changes in a human model of endothelin-1 induced pain: a behavioral study

Although pain is a frequent feature in patients with cancer, its etiology is still poorly understood. In recent years, endothelin-1 (ET-1) has become a major target molecule in the etiology of cancer pain. In this randomised, double-blind study the effects of intradermal injection of ET-1 on spontaneous pain, temperature perception and sensation of punctate stimulation were evaluated. Thirty-five subjects were randomised to receive either placebo or one of four concentrations of ET-1 (ranging from 10(-10) to 10(-6)M). Besides assessment of spontaneous pain, three neurosensory testings were performed: (1) cold and warm sensation, (2) cold and heat pain, and (3) punctate stimulation using a von Frey monofilament. ET-1 produced a dose-dependent flare zone that was absent after placebo injection. Subjects reported a short-lasting spontaneous pain upon administration of the highest concentrations of ET-1. Injection of ET-1 induced a long-lasting and dose-dependent punctate hyperalgesia in an area around the injection site (secondary hyperalgesia). Thermal testing revealed a short period of hypoesthesia to non-noxious warm and cold stimuli after some doses of ET-1. In addition to the mechanical hyperalgesia, intradermal injection of ET-1 almost instantaneously induced a state of cold hyperalgesia outlasting the study period (120 min). No development of heat hyperalgesia was observed. The observed psychophysical characteristics of this new model of ET-1 induced nociception indicate its potential as a human experimental model for cancer pain.

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.

Orofacial cold hyperalgesia due to infraorbital nerve constriction injury in rats: reversal by endothelin receptor antagonists but not non-steroidal anti-inflammatory drugs

The susceptibility of changes in responsiveness to noxious cold stimulation of rats submitted to chronic constriction of the infraorbital nerve (CION) or carrageenan to drug inhibition was compared. Nocifensive responses were measured as total time rats engaged in bilateral facial grooming with both forepaws over the first 2 min following tetrafluoroethane spray application to the snout. Carrageenan (50 microg, s.c. into upper lip) caused short-lived ipsilateral cold hyperalgesia (peak at 3 h: vehicle 8.4+/-1.3, carrageenan 21.2+/-3.0 s) which was markedly suppressed by i.p. indomethacin (4 mg/kg), celecoxib (10mg/kg) or s.c. dexamethasone (0.5 mg/kg), endothelin ET(A) or ET(B) receptor antagonists (BQ-123 and BQ-788, respectively; 10 nmol/lip). CION caused ipsilateral cold hyperalgesia between Days 2 and 12, which peaked on Days 4 (sham 15.3+/-1.8, CION 32.4+/-5.3s) to 6. Established peak CION-induced cold hyperalgesia was unaffected by indomethacin and celecoxib, whereas dexamethasone, BQ-123, BQ-788, and i.v. injections of selective antagonists of ET(A) (atrasentan, 3-10 mg/kg) or ET(B) (A-192621, 5-20 mg/kg) receptors caused significant inhibitions lasting 1-2.5h (peaks approximately 65-90%). Bosentan (dual ET(A)/ET(B) receptor antagonist, 10 mg/kg, i.v.) abolished CION-induced cold hyperalgesia for up to 6h. Thus, once established, CION-induced orofacial hyperalgesia to cold stimuli appears to lack an inflammatory component, but is alleviated by endothelin ET(A) and/or ET(B) receptor antagonists. If this CION injury model bears predictive value to trigeminal neuralgia (i.e., paroxysmal orofacial pain triggered by various stimuli), endothelin receptors might constitute new targets for treatment of this disorder.

The pharmacology of bosentan

This article describes the pharmacological properties and the overall preclinical and clinical profiling of bosentan (Ro 47-0203), a non-peptide endothelin receptor antagonist with oral activity. Bosentan is a combined and competitive antagonist of both ETA and ETB receptors that is selective for the endothelin system. In vitro and in vivo, bosentan potently antagonises the vascular response elicited by the endothelins. Preclinical efficacy is demonstrated in a variety of pathological models including pulmonary and essential hypertension, renal failure of ischaemic and nephrotic origin and cerebral vasospasm following subarachnoid haemorrhage. Effects are particularly marked in experimental models of heart failure (HF) where bosentan acts as a potent vasodilator that improves overall left ventricular performance. After chronic treatment, bosentan also improves survival in rats with HF. As a result of the first encouraging clinical results that show pulmonary and systemic vasodilation, long-term studies are ongoing in the treatment of congestive heart failure (CHF).

Endothelin antagonism with bosentan: current status and future perspectives

Endothelin receptor antagonists have been proposed for the treatment of a variety of disorders in which the endothelins may act as pathogenic mediators, such as hypertension, congestive heart failure, and cerebral vasospasm. Bosentan is a nonpeptide competitive antagonist, which can be a good tool for studying the endothelin system. It is specific for the endothelin system and blocks the actions of endothelin at both mammalian receptors (A and B). Bosentan has recently moved into Phase III clinical trial. This review will attempt to overview the experimental and clinical effects of bosentan.

Bosentan: a dual endothelin receptor antagonist

The peptide endothelin plays a significant role in a wide array of pathological conditions, including primary pulmonary hypertension and pulmonary arterial hypertension associated with collagen vascular disease. These are life-threatening conditions that can severely compromise the function of the lungs and heart. Inhibiting the actions of endothelin by blockade of its receptors provides a new and effective approach to therapy for patients with these conditions. Bosentan (Tracleer ) is the first orally-active dual endothelin receptor antagonist and has recently been approved in the US, Canada, Switzerland and the EU for the treatment of pulmonary arterial hypertension. Bosentan significantly improves exercise capacity, symptoms and functional status in patients with this disease and also slows clinical deterioration, which may be indicative of a delay of disease progression. Results from large-scale studies of bosentan in patients with pulmonary arterial hypertension and chronic heart failure have established its long-term safety and tolerability profiles. The introduction of the dual endothelin receptor antagonist bosentan has provided an essential treatment for pulmonary arterial hypertension and ongoing trials are evaluating its potential role in the management of other endothelin-mediated disease states.

Endothelin-1 plasma levels in cluster headache

BACKGROUND AND OBJECTIVE

A role for endothelin-1, a potent vasoconstrictor peptide, in some cerebrovascular diseases has been proposed. To obtain preliminary data about peripheral concentrations of endothelin-1 in acute cluster headache, we measured the plasma endothelin-1 secretory pattern in 10 men with cluster during and independent of a headache attack.

METHODS

We collected blood samples for plasma endothelin-1 determinations at 0, 15, 30, 45, 60, 90, and 120 minutes during a cluster attack and closely monitored blood pressures. We repeated the same sampling during an asymptomatic period.

RESULTS

The mean values of plasma endothelin-1 (before a cluster headache, 3.3 +/- 0.3 pg/mL) significantly increased (F = 2.578, P < .05) during an attack, reaching their peak at 30 minutes (5.0 +/- 0.5 pg/mL, P < .05). We found no significant variations in mean arterial pressure.

CONCLUSION

Endothelin-1 may play a role in the pathophysiology of cluster attacks. The increase in plasma observed during cluster attacks may be linked to alterations in systemic hemodynamics and vascular tone.

Expression and localization of endothelin receptors: implications for the involvement of peripheral glia in nociception

The endothelins (ETs) are peptides that have a diverse array of functions mediated by two receptor subtypes, the endothelin A receptor (ET(A)R) and the endothelin B receptor (ET(B)R). Pharmacological studies have suggested that in peripheral tissues, ET(A)R expression may play a role in signaling acute or neuropathic pain, whereas ET(B)R expression may be involved in the transmission of chronic inflammatory pain. To begin to define the mechanisms by which ET can drive nociceptive signaling, autoradiography and immunohistochemistry were used to examine the distribution of ET(A)R and ET(B)R in dorsal root ganglia (DRG) and peripheral nerve of the rat, rabbit, and monkey. In DRG and peripheral nerve, ET(A)R-immunoreactivity was present in a subset of small-sized peptidergic and nonpeptidergic sensory neurons and their axons and to a lesser extent in a subset of medium-sized sensory neurons. However, ET(B)R-immunoreactivity was not seen in DRG neurons or axons but rather in DRG satellite cells and nonmyelinating ensheathing Schwann cells. Thus, when ETs are released in peripheral tissues, they could act directly on ET(A)R-expressing sensory neurons and on ET(B)R-expressing DRG satellite cells or nonmyelinating Schwann cells. These data indicate that ETs can have direct, nociceptive effects on the peripheral sensory nervous system and that peripheral glia may be directly involved in signaling nociceptive events in peripheral tissues.

Pharmacological opportunities and pitfalls in the therapy of migraine

The mild vasoconstrictor effects of modern antimigraine drugs, such as serotonin (5-HT; 5-hydroxytryptamine)1B/D agonists, have led to a search for nonvasoconstrictor approaches to therapy. Such approaches have included substance P (neurokinin I) antagonists, endothelin antagonists and highly specific 5HT1D agonists. All of these substances are effective in animal models and have no significant vasoconstrictive effects. However, all of them failed to demonstrate any antimigraine effects. Current clinical and experimental evidence therefore supports the view that isolated peripheral trigeminal nerve inhibition is insufficient to relieve acute migraine.

Neurogenic inflammation in the context of migraine

Despite considerable research into the pathogenesis of idiopathic headaches, such as migraine, the pathophysiological mechanisms underlying them remain poorly understood. Although it is well established that the trigeminal nerve becomes activated during migraine, the consequences of this activation remain controversial. One theory, based on preclinical observations, is that activation of trigeminal sensory fibers leads to a painful neurogenic inflammation within the meningeal (dural) vasculature mediated by neuropeptide release from trigeminal sensory fibres and characterized by plasma protein extravasation, vasodilation, and mast cell degranulation. Effective antimigraine agents such as ergots, triptans, opioids, and valproate inhibit preclinical neurogenic dural extravasation, suggesting that this activity may be a predictor of potential clinical efficacy of novel agents. However, several clinical trials with other agents that inhibit this process preclinically have failed to show efficacy in the acute treatment of migraine in man. Alternatively, it has been proposed that painful neurogenic vasodilation of meningeal blood vessels could be a key component of the inflammatory process during migraine headache. This view is supported by the observation that jugular plasma levels of the potent vasodilator, calcitonin gene-related peptide (CGRP) are elevated during the headache and normalized by successful sumatriptan treatment. Preclinically, activation of trigeminal sensory fibers evokes a CGRP-mediated neurogenic dural vasodilation, which is blocked by dihydroergotamine, triptans, and opioids but unaffected by NK1 receptor antagonists that failed in clinical trials. These observations suggest that CGRP release with associated neurogenic dural vasodilation may be important in the generation of migraine pain, a theory that would ultimately be tested by the clinical testing of a CGRP receptor antagonist.

Substance P receptor antagonists in the therapy of migraine

Clinical observations, the vascular component of migraine pain, its pulsating or throbbing pain character, have focused attention on the trigeminal innervation of pain-sensitive intracranial structures, such as the dura mater and large vessels. These intracranial structures are innervated by the ophthalmic branch of the trigeminal nerve, which is marked by the presence of vasoactive peptides, such as substance P and calcitonin gene-related peptide. Substance P is a mediator of the sterile inflammation of the dura mater, which has been considered to be the source of migraine pain. Modern antimigraine drugs, such as 5-HT(1B/D) agonists (triptans), block this dural neurogenic inflammation dose-dependently in an animal model but their vasoconstrictor effects have led to a search for non-vasoconstrictor approaches. One such approach has been substance P (neurokinin-1) antagonists. These are highly effective in animal models of dural inflammation and have no significant vasoconstrictive effect. However, several NK(1) antagonists failed to demonstrate any effect in acute migraine. Current clinical and experimental evidence therefore supports the view that NK(1) receptor antagonists may have no significant antimigraine properties.

Pharmacologic characterization of the novel, orally available endothelin-A--selective antagonist SB 247083

Competition radioligand binding with [125I]ET-1 at human cloned ETA and ETB receptors demonstrated ET-A selective affinity by SB 247083 (Ki 0.41 and 467 nM, respectively). Accordingly, similar competitive, functional ETA receptor antagonism was observed. In vitro, SB 247083 exhibited a Kb of 3.5 +/- 0.3 nM (ET-1--induced rat aortic contraction). SB 247083 was significantly less potent as a functional ETB antagonist (Kb 0.34 +/- 0.01 microM; S6c-induced rabbit pulmonary artery contraction). In contrast to ETB-selective and mixed ETA/B antagonists, and consistent with its ETA-selective profile, in vivo administration of SB 247083 was not associated with an elevation in plasma ET-1 levels. Pharmacodynamic and pharmacokinetic studies revealed that SB 247083 was effectively absorbed from the gastrointestinal tract. A single bolus dose inhibited the hemodynamic actions of ET-1 for up to 8 h, consistent with a molecule shown to be 46% bioavailable. Therefore, the present study demonstrates that SB 247083, a unique chemical entity, represents a potent class of nonpeptide, orally active ETA-selective antagonists.

Acute treatments: some blind alleys

The introduction of sumatriptan, a selective 5-HT(1B/1D) agonist, for the treatment of migraine sparked a new era of drug research in this field. Many novel targets have since been developed, and tested in the clinic. The promise of these approaches is to deliver an anti-migraine compound with the optimal efficacy and safety profile. In this chapter, blind alleys in anti-migraine development are discussed. The failing soldiers have included the NK-1 antagonists, some second-generation 5-HT(1B/1D) agonists, CP-122,288, 4991W93, the neurosteroid ganaxolone, selective 5-HT(1F) (LY334370) and 5-HT(1D) agonists (PNU-142,633), and the endothelin-1 antagonist bosentan. Some of these promising targets failed to demonstrate clinical efficacy, while others were stopped for preclinical toxicity.

The pharmacology of headache

Headache is a common problem which besets most of us at some time or the other. The pharmacology of headache is complex in an overall sense but can be understood in terms of the anatomy and physiology of the pain-producing structures. Migraine can be used as a template to understand the activation of nociceptive systems in the head and thus their neurotransmitter mediation and modulation. In recent years, the role of serotonin (5-HT) in headache pharmacology has been unravelled in the context of both understanding its role in the nociceptive systems related to headache and by exploiting its 5-HT1 receptor subtypes in headache therapeutics. The pharmacology of the head pain systems, as they are known and as they might evolve, are explored in the context of both, the anatomy and physiology of trigeminovascular nociception and in the context of clinical questions, such as those of efficacy, headache recurrence and adverse events.

Endothelin-1-induced ET(A) receptor-mediated nociception, hyperalgesia and oedema in the mouse hind-paw: modulation by simultaneous ET(B) receptor activation

Endothelin-1 causes ET(A) receptor-mediated enhancement of capsaicin-induced nociception in mice. We have assessed if this hyperalgesic effect of endothelin-1 is also accompanied by other pro-inflammatory effects, namely nociception and oedema, and characterized the endothelin ET receptors involved. Intraplantar (i. pl.) hind-paw injection of endothelin-1 (0.3 - 30 pmol) induced graded nociceptive responses (accumulated licking time: vehicle, 20. 5+/-3.3 s; endothelin-1 at 30 pmol, 78.1+/-9.8 s), largely confined to the first 15 min. Endothelin-1 (1 - 10 pmol) potentiated ipsilateral capsaicin-induced (0.1 microgram, i.pl.; at 30 min) nociception (vehicle, 40.2+/-2.6 s; endothelin-1 at 10 pmol, 98.4+/-5.8 s, but 30 pmol was inactive), and caused oedema (increase in paw weight 5 min after capsaicin: vehicle, 46.3+/-2.3 mg; endothelin-1 at 30 pmol, 100.3+/-6.1 mg). Selective ET(B) receptor agonists sarafotoxin S6c (up to 30 pmol) and IRL 1620 (up to 100 pmol) were inactive, whereas endothelin-3 (up to 30 pmol) induced only modest oedema. ET(A) receptor antagonists BQ-123 (1 nmol, i.pl. ) or A-127722-5 (6 micromol kg(-1), i.v.) prevented all effects of endothelin-1 (10 pmol), but the ET(B) receptor antagonist BQ-788 (1 or 10 nmol, i.pl.) was ineffective. BQ-788 (10 nmol, i.pl.) unveiled hyperalgesic effects of 30 pmol endothelin-1 and endothelin-3. Sarafotoxin S6c (30 pmol, i.pl.) did not modify endothelin-1-induced (10 pmol) nociception or oedema, but abolished hyperalgesia. Thus, endothelin-1 triggers ET(A) receptor-mediated nociception, hyperalgesia and oedema in the mouse hind-paw. Simultaneous activation of ET(B) receptors by endothelin-1 or selective agonists can limit the hyperalgesic, but not the nociceptive or oedematogenic, effects of the peptide.

Simultaneous depletion of neurokinin A, substance P and calcitonin gene-related peptide from the caudal trigeminal nucleus of the rat during electrical stimulation of the trigeminal ganglion

The central terminals of the primary sensory trigeminal ganglion (TG) neurons projecting into the caudal trigeminal nucleus (CTN) of the rat exhibit neurokinin A (NKA)-, substance P (SP)-, and calcitonin gene-related peptide (CGRP)-immunoreactivities (IRs). We stimulated the TG in the rat to induce some of the alterations which might occur during migraine (neurogenic inflammation). Under a stereotaxic apparatus and by means of a bipolar electrode, one-side TG of the animals were electrically stimulated (7.5 Hz, 5 ms, 0.8-1. 4 mA) with square pulses for 5 min. Then, using immunohistochemical methods, the lower medulla of each rat was studied for NKA-, SP- and CGRP-IRs. Light microscopic examination of brain-stem sequencial sections revealed a simultaneous decrease in the immunoreactivities of all neuropeptides (NKA, SP and CGRP) in the CTN ipsilateral to TG stimulation in comparison with the other (not stimulated) side CTN. It is suggested that this decrease in immunoreactivity would be due to the co-release of neuropeptides following noxious stimuli and that NKA, SP and CGRP might therefore act as co-transmitters or co-modulators at the first central synapses of the trigeminal sensory pathway.

Pharmacological aspects of experimental headache models in relation to acute antimigraine therapy

The last decade has witnessed a tremendous progress in the acute therapy of migraine, with sumatriptan, belonging to a new class of drugs, now known as 5-HT(1B/1D/1F) receptor agonists, leading the way. The undoubted success of sumatriptan stimulated the development of new triptans as well as other suitable pharmacological tools and experimental models to probe into complex migraine mechanisms. In this review, we discuss the main experimental models for migraine, against the background of the disease pathophysiology and 5-HT receptors considered most important for migraine therapy. We believe that the use of these migraine models will provide even better treatment for migraine patients in the next millennium.

Trigeminal nerve ganglion stimulation-induced neurovascular reflexes in the anaesthetized cat: role of endothelin(B) receptors in carotid vasodilatation

1. The effects of intravenous administration of endothelin (ET) receptor antagonists SB-209670 (0.001-10.0 mg kg(-1)), SB-217242, SB-234551 (0.01-10.0 mg kg(-1)) and BQ-788 (0.001-1.0 mg kg(-1)) were investigated on trigeminal nerve ganglion stimulation-induced neurovascular reflexes in the carotid vasculature of the anaesthetized cat. Comparisons were made with sumatriptan (0.003-3.0 mg kg(-1)) and alpha-CGRP8-37 (0.001-0.1 mg kg(-1)). 2. Trigeminal nerve ganglion stimulation produced frequency related increases in carotid blood flow, reductions in carotid vascular resistance and non-frequency related increases in blood pressure. Guanethidine (3 mg kg(-1), i.v.) blocked trigeminal nerve ganglion-induced increases in blood pressure but had no effect on changes in carotid flow or resistance. Maximal reductions in carotid vascular resistance was observed at 10 Hz, and this frequency was selected to investigate the effects of drugs on trigeminal nerve ganglion stimulation-induced responses in guanethidine treated cats. 3. Saline, alpha-CGRP8-37 SB-209670 and BQ-788 had little or no effect on resting haemodynamic parameters. SB-217242 (10 mg kg(-1), n=3) produced a 56% reduction in arterial blood pressure whereas SB-233451 (10 mg kg(-1), n=3) produced a 30% reduction in carotid vascular resistance. Sumatriptan produced dose-related reductions in resting carotid flow and increases (max. 104% at 0.3 mg kg(-1), n = 5) in vascular resistance. 4. SB-209670 (n=6-7), SB-217242 (n=3) and BQ-788 (n=3) produced inhibition of trigeminal nerve ganglion stimulation-induced reductions in carotid vascular resistance. Saline, SB-234551, alpha-CGRP8-37 and sumatriptan had no effect. 5. These data demonstrate ET(B) receptor blockade attenuates the vasodilator effects of trigeminal nerve ganglion stimulation in the carotid vascular bed of guanethidine pretreated anaesthetized cats.

Substance P blockade with the potent and centrally acting antagonist GR205171 does not effect central trigeminal activity with superior sagittal sinus stimulation

The development and use of serotonin-1B/1D agonists to treat the acute attack of migraine has been a significant advance, but their vasoconstrictor effects have lead to a search for non-vasoconstrictor approaches to the management of the acute attack of migraine. One such suggested approach has been substance P (neurokinin-1) antagonists, since substance P is involved in mediating neurogenic plasma protein extravasation and has long been held to have a role in pain transmission. In this study, one such candidate compound, GR205171, a highly lipophilic potent neurokinin-1 antagonist, has been tested in a model of trigeminovascular nociception with considerable predictive value for anti-migraine activity. The superior sagittal sinus was isolated in the alpha-chloralose (60 mg/kg, i.p., and 20 mg/kg, i.v., supplemented every 2 h)-anaesthetized cat. The sinus was stimulated electrically (100 V, 250 micros duration, 0.3 Hz) and neurons in the dorsal C2 spinal cord monitored using electrophysiological methods. In separate experiments, the animals were prepared for stimulation and then maintained for 24 h before stimulation and perfusion for Fos immunohistochemistry. Stimulation of the superior sagittal sinus resulted in activation of cells in the dorsal horn of C2. Cells fired with a probability of 0.7 +/- 0.1 at a latency of 10.7 +/- 0.2 ms. Administration of GR205171 (100 microg/kg, i.v.) had no effect on probability of firing or latency. Stimulation of the sinus in separate cats resulted in increased expression over control levels in the superficial laminae of the trigeminal nucleus caudalis and C1/2 dorsal horns. GR205171 in the same dose had no effect upon Fos expression. Inhibition of substance P by the potent, selective and brain penetrant neurokinin-1 antagonist GR205171 had no effect upon either cell firing or Fos expression in the central trigeminal cells activated by stimulation of the superior sagittal sinus. These data and the published clinical data for other compounds suggest that neurokinin-1 blockade alone will not be an effective anti-migraine strategy. Further data will be required to assess whether neurokinin-1 antagonists will have any more general value in pain.

Endothelin in migraine patients

Endothelins (ETs) are the most potent vasoconstrictors known, and may be the mediators of the vasoconstrictive phase in migraine attacks. We studied 31 previously selected migraine patients with (9) and without (22) aura ictally and interictally to determine their plasma ET-1 values. The mean interictal and ictal values were 5.3 pg/ml (SD 1.8) and 6.4 pg/ml (SD 3.9), respectively. The ictal values were markedly elevated at the beginning of the migraine attack (<2 h) and declined to interictal or even lower level later (4 to 6 h) in the course of an attack. The local vasoconstriction at the beginning of a migraine attack might be ET-mediated secondarily to serotonin activation.

BMS-181885, a 5-HT1B/1D receptor ligand, in experimental models predictive of antimigraine activity and coronary side-effect potential

Many acutely acting antimigraine drugs have the ability to constrict porcine arteriovenous anastomoses as well as the human isolated coronary artery. These two experimental models seem to serve as indicators, respectively, for the therapeutic and coronary side-effect potential of the compounds. Using these two models, we have investigated the effects of BMS-181885 (3-[3-[4-(5-methoxy-4-pyrimidyl)-1-piperazinyl]propyl]-5-(1,2-dioxo-4-me thyl-3-cyclobuten-3-yl)amino-1H-indole), a 5-HT1B/1D receptor ligand. In anaesthetised pigs, BMS-181885 (10, 30, 100 and 300 microg kg(-1)) decreased the total carotid blood flow and conduction, exclusively at the expense of the arteriovenous anastomotic fraction as the capillary fraction did in fact increase. The highest dose (300 microg kg(-1)) produced a reduction of 52+/-6% from the baseline arteriovenous anastomotic flow. When carotid haemodynamic changes after a single 100 microg kg(-1)dose of BMS-181885 or sumatriptan were studied at different time-points, BMS-188185 had a longer duration of action. Both BMS-181885 (pD2:7.9+/-0.1; Emax:9+/-3% of the contraction to 100 mM K+) and sumatriptan (pD2:6.3+/-0.1; Emax:28+/-8% of the contraction to 100 mM K+) contracted the human isolated coronary artery. The above results suggest that (i) the longer-lasting vasoconstrictor action of BMS-181885 on porcine carotid arteriovenous anastomoses may be related to its reported slow dissociation from 5-HT1B/1D receptor, and (ii) BMS-181885 should be able to abort migraine headaches in patients. It will be interesting to find out whether these properties are clinically important so that the drug exhibits less headache recurrence and coronary side-effects than sumatriptan.

CGRP and nitric oxide of neuronal origin and their involvement in neurogenic vasodilatation in rat skin microvasculature

1. Sensory nerves are important for the initiation of neurogenic inflammation and tissue repair. Both calcitonin gene-related peptide (CGRP) and nitric oxide (NO) have been implicated in neurogenic vasodilatation and inflammatory responses. 2. A blister model in the rat hind footpad was used as a site to induce neurogenic vasodilatation in response to antidromic electrical stimulation of the sciatic nerve. Blood flux was monitored with a laser Doppler flow monitor. 3. The quantitative contributions of CGRP and NO to vasodilatation were examined by use of the CGRP receptor antagonist CGRP8-37 and NO synthase inhibitors 7-nitroindazole (7-NI), 3-bromo 7-NI and N(G)-nitro L-arginine methyl ester (L-NAME). The potential modulatory role of endothelin was examined by use of the ET(A) receptor antagonist BQ-123. 4. CGRP8-37 (10 microM) was perfused over the blister base before nerve stimulation and continuously throughout the post-stimulation period, resulting in a significant reduction (41%) in the blood flux vascular response. 5. Pretreatment with the specific neuronal NO synthase inhibitors, 7-NI and 3-bromo 7-NI (10 mg kg(-1), i.v.), and of the non-specific L-NAME (100 microM), resulted in significant inhibition of the blood flux response (36%, 72% and 57% decrease, respectively). In contrast, 7-NI treatment in young rats pretreated with capsaicin had no further effect on the vascular response, suggesting that the source of NO is the sensory nerves. 6. BQ-123 (10 microM) significantly enhanced the stimulation-induced blood flux response (61% increase). When 7-NI was co-administered with either CGRP8-37 or BQ-123, the drug actions were additive, suggesting that there was no interaction between NO and CGRP or endothelin. 7. These data suggest that both NO and CGRP participate in neurogenic vasodilatation in rat skin microvasculature and that this response is modulated by endogenous endothelin.

Endothelin B receptor-mediated increase of cerebral blood flow in experimental pneumococcal meningitis

Study investigates the role of endothelin (ET) receptors in mediating early changes in cerebral blood flow--as measured by laser Doppler flowmetry (CBFLDF)--during experimental pneumococcal meningitis. Meningitis was induced with heat-killed pneumococci and confirmed by a significant increase in CBFLDF (baseline 100%; 225.3 +/- 21.8% after 6 hours; mean +/- SD), intracranial pressure (ICP), brain water content, and white blood cell count in the CSF. Intravenous administration of the selective endothelin B (ETB) receptor antagonist BQ-788 immediately before pneumococcal challenge (but not 4 hours afterward) significantly attenuated these pathophysiologic alterations (e.g., CBFLDF 6 hours after pneumococcal challenge: 116.7 +/- 17.4%). Pretreatment with BQ-123, a selective endothelin A receptor antagonist, had no significant effect on ICP and brain water content, but augmented the increase in CBFLDF and CSF white blood cell count. Since ET is known to trigger the release of nitric oxide (NO) by ETB receptor activation, we examined specific ET-NO interactions in primary rat cerebromicrovascular endothelial cells after stimulation with heat-killed pneumococci. Pneumococci induced a significant increase in both ET and NO concentrations in endothelial cell culture medium. Treatment with phosphoramidon, an inhibitor of the endothelin-converting enzyme, prevented the production of endothelin and markedly reduced NO generation. Our data provide evidence that ET is involved as a mediator in early pneumococcal meningitis in the rat and contributes to the increase in CBFLDF, ICP, brain water content, and CSF pleocytosis, presumably through ETB receptor-mediated NO production.

Control of vascular tone by endogenous endothelin-1 in human pial arteries

BACKGROUND AND PURPOSE

Endothelin-1 (ET) has been shown to be involved in human pathological conditions, but its physiological function remains to be elucidated. The aim of this work was to assess whether endothelium-derived ET was involved in the overall responsiveness of freshly isolated human pial arteries.

METHODS

Samples of cerebral cortex, otherwise discarded, were obtained during tumor or epileptic lesion resections (n = 10 donors). Arterial segments were isolated and mounted on a microvessel myograph.

RESULTS

Inhibition of nitric oxide (NO) formation with N omega-nitro-L-arginine (L-NA, 100 micromol/L) increased basal tone by 7+/-1% Emax (n=5). This increase in tone was fully abolished in the presence of BQ123 (1 micromol/L; ET(A) receptor antagonist, P<.05) but potentiated by a subthreshold concentration of exogenous ET (1 nmol/L; 33+/-8% Emax; P<.05). In the presence of L-NA, serotonin (10 micromol/L)-induced tone was doubled compared with the control response (P<.05) but reduced by 90% in the presence of BQ123 (P<.05). In the absence of L-NA, BQ123 prevented serotonin-induced tone (n=3). Oxymetazoline, a selective alpha2-adrenergic receptor agonist, induced an endothelium-dependent relaxation of preconstricted human pial arteries. The relaxation was partially sensitive to NO synthase inhibition and fully prevented by the addition of ET, whereas substance P-induced relaxation was preserved. Glibenclamide (1 micromol/L), an inhibitor of ATP-sensitive K+ channels and tetraethylammonium (1 mmol/L), an inhibitor of Ca2+-activated K+ channels had no effect on oxymetazoline-induced relaxation.

CONCLUSIONS

The results of this study suggest first that ET is involved in the tonic response induced by NO synthase inhibition; second, part of the contractile response induced by serotonin is endothelium-dependent and sensitive to BQ123; and third, the data suggest that activation of alpha2-adrenergic receptors generated an endothelium-dependent relaxation that was selectively inhibited by exogenous ET.

Effects of the 5-HT1 receptor agonists, sumatriptan and CP 93,129, on dural arterial flow in the rat

The blood flow in and around the medial meningeal artery (dural arterial flow) was recorded in the exposed parietal dura mater encephali of the anesthetized rat using laser Doppler flowmetry. Local electrical stimulation of the dura mater (pulses of 0.5 ms delivered at 7.5-17.5 V and 5 or 10 Hz for 30 s) caused temporary increases in dural arterial flow. The effects of the 5-HT1 receptor agonists sumatriptan and CP 93,129 on the basal flow and the electrically evoked increases in flow were examined. Topical administration of undiluted sumatriptan (12 mg/ml) lowered the basal and the evoked flow by 20% on average. Systemic (i.v.) administration of sumatriptan (0.24, 0.72 and 3.6 mumol/kg) caused a short-lasting reduction of the evoked flow increases only at the higher doses while the basal flow was not significantly altered. Systemic administration of CP 93, 129 (0.46 and 4.6 mumol/kg) caused no significant changes of the basal and the evoked flow. At a dose of 23 mumol/kg CP 93,129 lowered the basal flow by 20% and the evoked flow by 30% for 20 min. The systemic arterial pressure was not significantly altered by sumatriptan and CP 93,129 within the whole range of doses. It is suggested that sumatriptan and CP 93,129 at high doses exert inhibitory effects on those fine afferent nerve fibers which release the calcitonin gene-related peptide, since this neuropeptide mediates the evoked increases in dural arterial flow.

Current concepts of the pathophysiology of migraine

An understanding of the basic anatomy and physiology of the cranial circulation facilitates the assessment and management of patients with headache, particularly vascular-type headaches, such as migraine. At the very least, all pain is perceived and processed in the brain. With migraine it is likely that the fundamental problem and its clinical expression are driven by the CNS; thus study of the brain regarding headache is warranted. As therapy evolves during the 1900s, such an understanding will be necessary, as new and highly specific receptor-targeted compounds allow treatment and improvement of headache in many patients. Since writing this article, the nomenclature for serotonin (5HT) receptors has changed so that any reference herein to 5HTID alpha is now 5HTID and 5HTID beta is now 5HTI beta. This change in nomenclature is discussed in Martig PR, Hoyer D, Humphrey PPA, et al: Alignment of receptor nomenclature with the human genome: Classification of 5HT-1 beta and 5HT-1D receptor subtypes. Trends in the Pharmacological Sciences 17:103, 1996.

Endothelin antagonist bosentan blocks neurogenic inflammation, but is not effective in aborting migraine attacks

Bosentan, a specific mixed antagonist of endothelin receptors with no vasoconstrictor activity, inhibits neurogenic plasma extravasation (NPE) within rat dura mater. This would predict efficacy in aborting migraine attacks, without causing cardiovascular side-effects. We investigated the efficacy of 250 mg i.v. bosentan in a randomized, double-blind, placebo-controlled, clinical trial. Improvement from moderate/severe to mild/no headache at 2 h (primary efficacy measure) occurred in 5/23 (22%) of bosentan-treated and in 9/25 (36%) of placebo-treated patients (effect difference -14%; 95% CI -52%, 24%). Thus, inhibition of NPE may not predict clinical efficacy of experimental antimigraine drugs. Vasoconstrictor action may be needed.

[Animal models and their results in relation to the therapy of migraine]

Until now, our understanding of migraine pathophysiology has been fairly incomplete. So far no animal model has allowed an explanation of all facets of the clinically heterogeneous condition migraine. However, it is now generally accepted that the migraine headache is due to activation of the trigeminal system. The model of neurogenic inflammation after stimulation of the trigeminal ganglion or systemic administration of capsaicin allows study of the inhibitory interactions between antimigraine compounds and peripheral trigeminal fibre terminals that sustain a sterile meningeal inflammation through release of allogenic and vasoactive neuropeptides, such as substance P and calcitonin gene-related peptide. Studies with the model of superior sagittal sinus stimulation have revealed central actions of antimigraine agents such as ergotamine and sumatriptan, but also acetylsalicylic acid on neurotransmission of trigeminal nociceptive input in the brainstem. A likely explanation for the slowly progressing neurological deficits is cortical spreading depression (CSD), which can easily be elicited in many species. However, CSD has not been observed in vivo in humans. The described models strongly influenced the development of new medications for migraine treatment and have improved our understanding of migraine pathophysiology.

The endothelin system and its potential as a therapeutic target in cardiovascular disease

Endothelin (ET)-1, an endothelium-derived peptide, is the most potent vasoconstrictor agent described to date. ET-1 also has positive inotropic and chronotropic effects in the heart and is a co-mitogen in both cardiac and vascular myocytes. The major elements of the system involved in formation of ET-1 and its isopeptides, as well as the receptors mediating their effects, have been cloned and characterised. Antagonists of the ET receptors are now available, and selective inhibitors of the ET-converting enzymes are being developed. Early studies using receptor antagonists support the involvement of ET-1 in the pathophysiology of several cardiovascular diseases. The relative merits of ET-converting enzyme inhibitors and receptor antagonists for the treatment of cardiovascular disease are discussed.