Mediation of 5-HT-induced external carotid vasodilatation in GR 127935-pretreated vagosympathectomized dogs by the putative 5-HT7 receptor

Migraine signaling pathways: amino acid metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a common, debilitating disorder for which attacks typically result in a throbbing, pulsating headache. Although much is known about migraine, its complexity renders understanding the complete etiology currently out of reach. However, two important facts are clear, the brain and the metabolism of the migraineur differ from that of the non-migraineur. This review centers on the altered amino acid metabolism in migraineurs and how it helps define the pathology of migraine.

Stimulation of the Serotonin Receptor 7 Restores Brain Histone H3 Acetylation and MeCP2 Corepressor Protein Levels in a Female Mouse Model of Rett Syndrome

Rett syndrome (RTT) is a rare neurological disorder caused by mutations in the X-linked MECP2 gene, characterized by severe behavioral and physiological impairments for which no cure is available. The stimulation of serotonin receptor 7 (5-HT7R) with its selective agonist LP-211 (0.25 mg/kg/day for 7 days) was proved to rescue neurobehavioral alterations in a mouse model of RTT. In the present study, we aimed at gaining insight into the mechanisms underpinning the efficacy of 5-HT7R pharmacological stimulation by investigating its epigenetic outcomes in the brain of RTT female mice bearing a truncating MeCP2 mutation. Treatment with LP-211 normalized the reduced histone H3 acetylation and HDAC3/NCoR levels, and increased HDAC1/Sin3a expression in RTT mouse cortex. Repeated 5-HT7R stimulation also appeared to strengthen the association between NCoR and MeCP2 in the same brain region. A different profile was found in RTT hippocampus, where LP-211 rescued H3 hyperacetylation and increased HDAC3 levels. Overall, the present data highlight a new scenario on the relationship between histone acetylation and serotoninergic pathways. 5-HT7R is confirmed as a pivotal therapeutic target for the recovery of neuronal function supporting the translational value of this promising pharmacological approach for RTT.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Serotonin 5-HT7 receptor agonist, LP-211, exacerbates Na(+), K(+)-ATPase/Mg(2+)-ATPase imbalances in spinal cord-injured male rats

BACKGROUND

The observed controversy that N-(4-cyanophenylmethyl)-4-(2-diphenyl)-1-piperazinehexanamide (LP-211), a selective serotonin (5-HT7) receptor agonist, may either modify or exacerbate imbalances in serum electrolyte concentrations and renal tissue of spinal cord trauma cases has not been reported yet. The aim of this study was to better understand the effects of a new 5-HT7 receptor agonist, LP-211, on serum electrolyte changes in spinal cord injured- (SCI) rats.

METHODS

Sixty male rats were assigned to the following groups: A) Intact (saline as vehicle, 1 ml/kg, i.p.), B) Intact [LP-211, (0.003-0.3 mg/kg, i.p.)], C) Sham-operated [laminectomy + vehicle (1 ml/kg, i.p.)], D) Sham-operated [laminectomy + LP-211 (0.003-0.3 mg/kg, i.p.)], E) Treatment [laminectomy + spinal trauma (SCI) + vehicle (1 ml/kg, i.p.)], F) Treatment [laminectomy + spinal trauma + LP-211 (0.003-0.3 mg/kg, i.p.)]. SCI was performed by placing an aneurysm clip, extradurally at the level of T10. After two weeks, LP-211 was administered cumulatively and each dose was injected (i.p.) with 20 min interval. At the end of the experiment, blood samples were collected for biochemical evaluations of the electrolytes employing standard commercial kits.

RESULTS

The present results indicate elevated serum levels of Na(+), K(+), and Mg(2+) in SCI rats and significant differences demonstrated between the groups [P < 0.001, F(5, 35) = 23.92], [P < 0.001, F(5, 35) = 67.63], [P < 0.001, F(5, 35) = 71.144], respectively. So that, in groups B, D and F, there was a significant increase in K(+) and Mg(2+) serum levels compared to the groups A, C, and E (P < 0.001). Furthermore, Na(+) serum levels in SCI (LP-211), laminectomy (LP-211), and intact (LP-211) groups tended to be statistically lower than SCI (saline), laminectomy (saline) and intact (saline) groups. Infact, hyponatremia, hyperkalemia and hypermagnesemia was obtained in group F. Nevertheless, in the remaining measured serum electrolytes such as calcium (Ca(2+)), iron (Fe(2+)) and phosphorus (P(3-)), chlorine (Cl(-)), copper (Cu(+)), and zinc (Zu(+)), no significant changes were observed.

CONCLUSION

It was shown that acute additive LP-211 treatments in the SCI group led to hyponatremia, hyperkalemia and hypermagnesemia, it may be stated that LP-211 treatment as a promising candidate for treating SCI complications in some systems especially urinary tract might take into consideration and further studies would be needed to clarify its benefits or drawbacks. The observed discrepancies, nevertheless; will also pose new questions. Altogether, this will ultimately contribute to further understanding the pathophysiological role regarding 5-HT7 receptor activation.

Activation of 5-hydroxytryptamine1B/1D/1F receptors as a mechanism of action of antimigraine drugs

INTRODUCTION

The introduction of the triptans (5-hydroxytryptamine (5-HT)1B/1D receptor agonists) was a great improvement in the acute treatment of migraine. However, shortcomings of the triptans have prompted research on novel serotonergic targets for the treatment of migraine.

AREAS COVERED

In this review the different types of antimigraine drugs acting at 5-HT receptors, their discovery and development are discussed. The first specific antimigraine drugs were the ergot alkaloids, consisting of ergotamine, dihydroergotamine and methysergide, which are agonists at 5-HT receptors, but can also bind α-adrenoceptors and dopamine receptors. In the 1990s, the triptans became available on the market. They are 5-HT1B/1D receptor agonists, showing fewer side effects due to their receptor specificity. In the last years, compounds that bind specifically to 5-HT1D, 5-HT1F and 5-HT7 receptors have been explored for their antimigraine potential. Furthermore, the serotonergic system seems to act in tight connection with the glutamatergic as well as the CGRP-ergic systems, which may open novel therapeutic avenues.

EXPERT OPINION

Although the triptans are very effective in treating migraine attacks, their shortcomings have stimulated the search for novel drugs. Currently, the focus is on 5-HT1F receptor agonists, which seem devoid of vascular side effects. Moreover, novel compounds that affect multiple transmitter and/or neuropeptide systems that are involved in migraine could be of therapeutic relevance.

Blood pressure modulation following activation of mast cells by cationic cell penetrating peptides

Short cell penetrating peptides (CPP) are widely used in vitro to transduce agents into cells. But their systemic effect has not been yet studied in detail. We studied the systemic effect of the cell penetrating peptides, penetratin, transportan and pro-rich, on rat hemodynamic functions. Intra-arterial monitoring of blood pressure showed that injection of the positively charged penetratin and transportan in a wide range of concentrations (2.5-320 μg/kg) caused highly significant transient decrease in the systolic and diastolic blood pressure in a dose dependent manner (p<0.01). Pretreatment with histamine receptors blockers or with cromolyn, a mast cell stabilizing agent, significantly attenuated this effect. Furthermore, in vitro incubation of these both peptides with mast cells line, LAD2, caused a massive mast cell degranulation. In vitro studies showed that these CPP in a wide range of concentrations were not cytotoxic without any effect on the survival of LAD2 mast cell line. In contrast, the less positively charged and proline-rich CPP, pro-rich, had no systemic effects with no effect on mast cell degranulation. Our results indicate that intravenously administrated positively charged CPP may have deleterious consequences due to their induced BP drop, mediated by mast cell activation. Therefore, the major effect of mast cell activation on BP should be considered in developing possible future drug therapies based on the injection of membrane-permeable and positively charged CPP. Nevertheless, lower levels of such CPP may be considered as a treatment of systemic high BP through moderate systemic mast cell activation.

Black cohosh (Cimicifuga racemosa) relaxes the isolated rat thoracic aorta through endothelium-dependent and -independent mechanisms

AIM OF THE STUDY

The rhizome of the Cimicifuga racemosa (commonly known as black cohosh) has been used in treatment of climacteric complaints for decades in North America and Europe. A number of studies investigated the estrogenic potential of black cohosh, but its effectiveness is still controversial. Recently, it was reported that the extract of black cohosh acted as an agonist at the serotonin (5-HT) receptor and 5-HT derivative was isolated out of the black cohosh extract. Because it is well known that the 5-HT elicited the various cardiovascular effects including vasorelaxation, we investigated the vasorelaxant effects of the extract of black cohosh and its possible mechanisms of action.

MATERIALS AND METHODS

The extract of black cohosh (BcEx) was examined for its vasorelaxant effects in isolated rat aorta. The aortic rings were equilibrated under resting tension and induced reproducible contraction in organ bath. The control contraction was produced by 300 nM NE, and then BcEx were added. In experiments where specific inhibitors were used, they were added 20 min before NE contraction.

RESULTS

BcEx elicited two phases of relaxation in rat aorta pre-contracted with norepinephrine. The first, a rapid relaxation, which occurred within seconds of BcEx administration, was eliminated by pretreatment with N(G)-nitro-l-arginine (l-NNA) or methylene blue. The endogenous NO synthase substrate l-Arg markedly reversed the action of l-NNA, indicating that BcEx elicited the vasorelaxant effect via the NO/cGMP pathway. The second, slowly developing relaxation was not affected by the endothelium denudation. BcEx-induced endothelium-independent vasorelaxation appears to involve the inhibition of calcium influx mediated by the opening of inward rectifier potassium channels.

CONCLUSIONS

BcEx elicits the vasorelaxant effect via endothelium-dependent and -independent mechanisms and may contribute to a better understanding of a potential link between the use of black cohosh and its beneficial effects on vascular health.

Role of the 5-HT7 receptor in the central nervous system: from current status to future perspectives

Pharmacological and genetic tools targeting the 5-hydroxytryptamine (5-HT)7 receptor in preclinical animal models have implicated this receptor in diverse (patho)physiological processes of the central nervous system (CNS). Some data obtained with 5-HT7 receptor knockout mice, selective antagonists, and, to a lesser extent, agonists, however, are quite contradictory. In this review, we not only discuss in detail the role of the 5-HT7 receptor in the CNS but also propose some hypothetical models, which could explain the observed inconsistencies. These models are based on two novel concepts within the field of G protein-coupled receptors (GPCR), namely biphasic signaling and G protein-independent signaling, which both have been shown to be mediated by GPCR dimerization. This led us to suggest that the 5-HT7 receptor could reside in different dimeric contexts and initiate different signaling pathways, depending on the neuronal circuitry and/or brain region. In conclusion, we highlight GPCR dimerization and G protein-independent signaling as two promising future directions in 5-HT7 receptor research, which ultimately might lead to the development of more efficient dimer- and/or pathway-specific therapeutics.

Serotonin 5-HT7 receptor agents: Structure-activity relationships and potential therapeutic applications in central nervous system disorders

Since its discovery in the 1940s in serum, the mammalian intestinal mucosa, and in the central nervous system, serotonin (5-HT) has been shown to be involved in virtually all cognitive and behavioral human functions, and alterations in its neurochemistry have been implicated in the etiology of a plethora of neuropsychiatric disorders. The cloning of 5-HT receptor subtypes has been of importance in enabling them to be classified as specific protein molecules encoded by specific genes. The 5-HT(7) receptor is the most recently classified member of the serotonin receptor family. Since its identification, it has been the subject of intense research efforts driven by its presence in functionally relevant regions of the brain. The availability of some selective antagonists and agonists, in combination with genetically modified mice lacking the 5-HT(7) receptor, has allowed for a better understanding of the pathophysiological role of this receptor. This paper reviews data on localization and pharmacological properties of the 5-HT(7) receptor, and summarizes the results of structure-activity relationship studies aimed at the discovery of selective 5-HT(7) receptor ligands. Additionally, an overview of the potential therapeutic applications of 5-HT(7) receptor agonists and antagonists in central nervous system disorders is presented.

Mast cell activation by fibrinogen-related homologous c-terminal peptides (haptides) modulates systemic blood pressure

BACKGROUND

Haptides are a family of short peptides homologous to C-termini sequences of fibrinogen chains β and γ (haptides Cβ and preCγ, respectively) which were previously shown to penetrate and bind cells.

OBJECTIVES

This work investigates the systemic effect of the haptides with possible clinical implications.

METHODS

Intra-arterial monitoring in rats recorded the haptides' effects on systemic blood pressure. In parallel, their effect was also tested in vitro on isolated rat peritoneal mast cells and on human mast cells.

RESULTS

Intra-arterial monitoring in rats showed that intravenous administration of low haptides concentrations (35-560 μg/kg rat) caused a shocklike behavior with transient decrease in the systolic and diastolic blood pressure by up to 55% (P < .05) in a dose-dependent manner and a minor increase in their heart rate. Randomly scrambled sequences of the haptides had no such effect, suggesting a specific interaction with receptors. Intravenous administration of blockers to histamine receptors H1 and H2 before haptides administration attenuated this effect. Furthermore, in vitro incubation of human LAD2 mast cell line or isolated rat peritoneal mast cells with the haptides caused degranulation of the mast cells. We found that the haptides Cβ and preCγ activated mast cells causing histamine release, resulting in a steep decrease in blood pressure, comparable to anaphylactic shock.

CONCLUSION

In treating vascular occlusive diseases, massive fibrinolysis is induced, and haptide-containing sequences are released. We suggest that treatment with histamine receptor blockers or with mast cell stabilizing agents in such pathological conditions may overcome this effect.

5-HT(7) receptor modulators: a medicinal chemistry survey of recent patent literature (2004 - 2009)

IMPORTANCE OF THE FIELD

The 5-HT(7) receptors are discretely localized within the CNS (thalamus, hypothalamus, limbic and cortical regions). The 5-HT(7) receptors are also present in smooth muscle cells from blood vessels and have been reported in gastrointestinal tract as well as in rat lumbar dorsal root and sympathetic ganglia. The 5-HT(7) receptors have been implicated in depression, disorders related to circadian rhythms, pain and migraine. Thus, there is a great interest in developing potent and selective 5-HT(7) receptor modulators.

AREAS COVERED IN THIS REVIEW

This review article highlights the research advances published in the patent literature between January 2004 and December 2009, giving emphasis to the medicinal chemist's standpoint.

WHAT THE READER WILL GAIN

Readers will rapidly gain an overview of the various 5-HT(7) receptor modulators reported in the patent literature in the past 6 years. Furthermore, the readers will learn which structure type can interact with 5-HT(7) receptor and also the different companies that are the main players in the field.

TAKE HOME MESSAGE

Although no 5-HT(7) modulator has entered clinical trials, the development and future use of different agonists and antagonists suitable for use in vivo seem very promising.

Studies on 1-arylpiperazine derivatives with affinity for rat 5-HT7 and 5-HT1A receptors

Several 1-aryl-4-(2-arylethyl)piperazine derivatives were synthesized and tested in-vitro for their binding affinity for 5-HT7 and 5-HT1A receptors. These compounds displayed 5-HT7 receptor affinity ranging between Ki = 474 nm and Ki = 8.2 nm, besides high affinity for the 5-HT1A receptor. Intrinsic activity of the most potent compounds was assessed. 4-[2-(3-Methoxyphenyl)ethyl]-1-(2-methoxyphenyl)piperazine (16) and 1-(1,2-benzisoxazol-3-yl)-4-[2-(3-methoxyphenyl)ethyl]piperazine (20) (Ki = 24.5 and 8.2 nm, respectively) behaved as partial agonist and full agonist, respectively, when tested for 5-HT7 receptor-mediated relaxation of substance P-induced guinea-pig ileum contraction.

The 5-HT7 receptor and disorders of the nervous system: an overview

RATIONALE

The 5-HT(7) receptor is a more recently discovered G-protein-coupled receptor for serotonin. The functions and possible clinical relevance of this receptor are not yet fully understood.

OBJECTIVE

The present paper reviews to what extent the use of animal models of human psychiatric and neurological disorders have implicated the 5-HT(7) receptor in such disorders. The studies have used a combination of pharmacological and genetic tools targeting the receptor to evaluate effects on behavior.

RESULTS

Models of anxiety and schizophrenia have yielded mixed results with no clear role for the 5-HT(7) receptor described in these disorders. Some data are available for epilepsy, migraine, and pain but it is still very early to draw any definitive conclusions. There is a considerable amount of evidence supporting a role for the 5-HT(7) receptor in depression. Both blockade and inactivation of the receptor have resulted in an antidepressant-like profile in models of depression. Supporting evidence has also been obtained in sleep studies. Especially interesting are the augmented effects achieved by combining antidepressants and 5-HT(7) receptor antagonists. The antidepressant effect of amisulpride has been shown to most likely be mediated by the 5-HT(7) receptor.

CONCLUSIONS

The use of pharmacological and genetic tools in preclinical animal models strongly supports a role for the 5-HT(7) receptor in depression. Indirect evidence exists showing that 5-HT(7) receptor antagonism is clinically useful in the treatment of depression. Available data also indicate a possible involvement of the 5-HT(7) receptor in anxiety, epilepsy, pain, and schizophrenia.

Effect of central serotonin depletion on 5-HT receptor-mediated vasomotor responses in the middle meningeal artery of anaesthetized rats

1 It has been hypothesized that craniovascular 5-HT receptors mediating dilatation of cranial vessels undergo sensitization on decreased serotonergic transmission in migraine. This study analysed the effect of chemical lesion of the 5-HT system in the brain with 5,7-dihydroxytryptamine (5,7-DHT) on 5-HT receptor-mediated dilator responses to 5-carboxamidotryptamine (5-CT) in the middle meningeal artery of anaesthetized rats. 5-CT has recently been shown to elicit dilator responses in this cranial vessel via 5-HT(7) receptors and, to a much lesser extent, 5-HT(1B/1D) receptors. 2 Pretreatment with 5,7-DHT produced a drastic and selective decrease of 5-HT levels in the brain (78 +/- 6% and 94 +/- 2% in dorsal raphe and hypothalamic paraventricular nuclei, respectively) compared with controls (1% ascorbic acid). 3 Topical application of 5-CT (1-1000 microm) to exposed dura mater encephali produced concentration-dependent decreases in diastolic blood pressure and dilator responses in the middle meningeal artery that were similar in vehicle- and 5,7-DHT-pretreaed animals. 4 Hypotensive and meningeal dilator responses to 5-CT were unaltered by the 5-HT(1B/1D) receptor antagonist, GR-127935 (1 mg kg(-1), i.v.), but were strongly inhibited by the 5-HT(7) receptor antagonist, SB-269970 (1 mg kg(-1), i.v.), with similar efficacy, in both groups of animals. Treatment with GR-127935 + SB-269970 (1 mg kg(-1), i.v. each), produced a stronger inhibitory effect than individual treatments on hypotensive but not on meningeal responses to 5-CT. Meningeal 5-HT(7) receptor-mediated responses (i.e. in GR-127935-pretreated animals) were unchanged by 5,7-DHT pretreatment. 5 Results suggest that the sensitivity of craniovascular 5-HT(7) receptors mediating dilatation is unaffected by a decrease of 5-HT levels in the brain. A neuronal involvement of 5-HT in migraine seems more likely, therefore.

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.

Potential role of female sex hormones in the pathophysiology of migraine

Clinical evidence indicates that female sex steroids may contribute to the high prevalence of migraine in women, as well as changes in the frequency or severity of migraine attacks that are in tandem with various reproductive milestones in women's life. While female sex steroids do not seem to be involved in the pathogenesis of migraine per se, they may modulate several mediators and/or receptor systems via both genomic and non-genomic mechanisms; these actions may be perpetuated at the central nervous system, as well as at the peripheral (neuro)vascular level. For example, female sex steroids have been shown to enhance: (i) neuronal excitability by elevating Ca(2+) and decreasing Mg(2+) concentrations, an action that may occur with other mechanisms triggering migraine; (ii) the synthesis and release of nitric oxide (NO) and neuropeptides, such as calcitonin gene-related peptide CGRP, a mechanism that reinforces vasodilatation and activates trigeminal sensory afferents with a subsequent stimulation of pain centres; and (iii) the function of receptors mediating vasodilatation, while the responses of receptors inducing vasoconstriction are attenuated. The serotonergic, adrenergic and gamma-aminobutyric acid (GABA)-ergic systems are also modulated by sex steroids, albeit to a varying degree and with potentially contrasting effects on migraine outcome. Taken together, female sex steroids seem to be involved in an array of components implicated in migraine pathogenesis. Future studies will further delineate the extent and the clinical relevance of each of these mechanisms, and will thus expand the knowledge on the femininity of migraine.

5-HT7 receptor-mediated dilatation in the middle meningeal artery of anesthetized rats

Topical administration of 5-carboxamidotryptamine (5-CT; 0.01-1000 microM) to the exposed dura mater encephali of anesthetized rats produced decreases in blood pressure and dilatation in the middle meningeal artery. Pretreatment with 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 (GR-127935; 1 mg/kg, i.v.), unmasked meningeal dilator responses to lower concentrations of 5-CT, and attenuated those to higher concentrations; GR-127935 also inhibited 5-CT-induced hypotension. The 5-HT7 receptor antagonist, (R)-1-{(3-hydroxyphenyl)sulfonyl}-2-{2-(2-(4-methyl-1-piperidinyl) ethyl} pyrrolidine (SB-269970; 1 mg/kg, i.v.), strongly inhibited dilator and hypotensive responses to 5-CT; the combination of GR-127935+SB-269970 (1 mg/kg, i.v., each) further inhibited meningeal and hypotensive responses. Thus, 5-CT may produce dilatation in the middle meningeal artery via 5-HT7 receptors; complex effects appear to involve 5-HT(1B/1D) receptors.

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.

Characterization of 5-HT7-receptor-mediated gastric relaxation in conscious dogs

We aimed to evaluate the gastric relaxant capacity of the 5-HT(1/7)-receptor agonist 5-carboxamidotryptamine (5-CT) in conscious dogs and to clarify the mechanism of action by use of selective antagonists, vagotomy, and in vitro experiments. A barostat enabled us to monitor the intragastric volume in response to different treatments (intravenously administered) before and after supradiaphragmatic vagotomy [results presented as the maximum volume change after treatment (mean; n = 5-11)]. In vitro experiments were performed with isolated muscle strips cut from four different stomach regions of the vagotomized dogs [results were fitted to the operational model of agonism to determine the efficacy parameter tau (n = 5)]. 5-CT (0.5-10 microg/kg) caused a dose-dependent gastric relaxation (29-267 ml) that was completely blocked by the selective 5-HT(7)-receptor antagonist SB-269970 (50 microg/kg). After vagotomy, the relaxation to 10 microg/kg 5-CT was significantly less pronounced (73 vs. 267 ml; P < 0.05) but still blocked by SB-269970, whereas the response to the nitric oxide donor nitroprusside was similar to that before vagotomy (178 vs. 218 ml). In vitro, 5-CT concentration dependently inhibited the PGF(2alpha)-contracted muscle strips before and after vagotomy. Although before and after vagotomy the response in every region was mediated by 5-HT(7) receptors (apparent affinity dissociation constant: SB-269970, 8.2-8.6 vs. 8.3-8.6, respectively), the response after vagotomy was less efficacious (log tau: 1.9 to 0.5 vs. 1.4 to -0.1). The results indicate that the 5-CT-induced proximal stomach relaxation in conscious dogs before and after vagotomy is mediated via 5-HT(7) receptors. The decreased efficacy of 5-CT in vitro after vagotomy is probably related to vagotomy-induced changes in receptor density or coupling efficiency and provides a possible explanation for the decreased in vivo response to 5-CT after vagotomy.

Pharmacological profile of the vascular responses to dopamine in the canine external carotid circulation

The present study investigated the effects of dopamine on the canine external carotid circulation. One min. intracarotid artery (i.c.) infusions of dopamine (10-310 microg min.-1) produced dose-dependent decreases in the canine external carotid conductance without affecting blood pressure or heart rate. This effect was mimicked by the D1/2-like receptor agonist apomorphine (1-310 microg min-1), but not by the D2-like receptor agonist, bromocriptine (31-310 microg min.-1). In contrast, fenoldopam (1-310 microg min.-1, intracarotid), a D1-like receptor agonist, produced dose-dependent increases in external carotid conductance. The vasoconstrictor response to dopamine was abolished after intravenous administration of the antagonists, phentolamine (alpha1/2; 2000 microg kg-1) or rauwolscine (alpha2; 100 microg kg-1), but remained unaffected after prazosin (alpha1; 100 microg kg-1) or haloperidol (D2-like; 1000 microg kg-1). Interestingly, after phentolamine not only were the vasoconstrictor responses to dopamine abolished, but even a dose-dependent vasodilator component was unmasked. These vasodilator responses to dopamine remained unchanged after intravenous haloperidol or propranolol (1000 microg kg-1 each). On the other hand, the vasodilator responses to fenoldopam, which remained unchanged after intravenous saline (0.1 ml kg-1), propranolol (1000 microg kg-1) or vagosympathectomy, were abolished by the D1-like receptor antagonist, SCH-23390 (10 microg kg-1). Lastly, the responses to dopamine and fenoldopam were not significantly altered after intraperitoneal pretreatment with reserpine (5 mg kg-1; -24 hr). The above results suggest that the canine external carotid vasoconstrictor responses to dopamine: (i) are mainly mediated by alpha2-adrenoceptors; and (ii) overshadow a vasodilator component, which involves vascular D1-like receptors.

Molecular cloning and tissue distribution of mRNA encoding porcine 5-HT7 receptor and its comparison with the structure of other species

The effects of 5-hydroxytriptamine (5-HT, serotonin) are mediated via five main receptor types of which the 5-HT7 receptor is the most recently characterised member. The 5-HT7 receptor has been shown to participate in mediating cranial blood vessels dilatation that may result in migraine headache. We report here the cDNA cloning, sequencing and tissue distribution of porcine 5-HT7 receptor and illustrate its comparison with corresponding receptor of known species. Employing a combination of reverse transcriptase and inverse polymerase chain reaction we amplified and sequenced a full length cDNA from the porcine cerebral cortex. The deduced amino acid sequence comparison confirmed that the cloned porcine receptor belongs to 5-HT7 receptor as described for human and other species and showing overall homology of 92-96%. The expression of 5-HT7 receptor mRNA was observed in porcine central (cerebral cortex, trigeminal ganglion and cerebellum) as well as in peripheral (pulmonary and coronary arteries, superior vena cava and saphenous vein) tissues. The established cDNA sequence and tissue distribution of porcine 5-HT7 receptor will be helpful in exploring the role of this receptor in pathophysiological processes and to predict as a potential therapeutic target for antimigraine drug development.

Is the 5-HT(7) receptor involved in the pathogenesis and prophylactic treatment of migraine?

The mechanisms underlying the pathogenesis of migraine and their possible association with serotonin (5-hydroxytryptamine; 5-HT) have not yet been elucidated. One of the major obstacles in achieving this goal is the lack of information on the mechanisms by which the monoamine could possibly trigger and/or modulate the basic pathophysiological features of the condition, that is, cranial vasodilatation and neurogenic inflammation. This information should provide a useful theoretical framework to insight the nature of the postulated fundamental triggering mechanism in the brain that ultimately results in head pain. Novel avenues for research and drug development may be envisaged upon the recent observations showing that 5-HT is actually able to produce vasodilatation of intra- and extra-cranial blood vessels through a mechanism pharmacologically resembling the 5-HT(7) receptor type, and that the messenger RNA (mRNA) encoding for this receptor is highly expressed in cranial vessels. Other lines of evidence have suggested that the 5-HT(7) receptor may play an excitatory role in neuronal systems and that it may be involved in hyperalgesic pain and neurogenic inflammation. On the basis of these observations, it is proposed that the 5-HT(7) receptor may well represent a link between the abnormal phenomena of 5-HT processing and neurotransmission that are observed in migraine patients, and the vascular and neurogenic alterations that account for migraine headache. This view is supported by the fact that most of the migraine prophylactic 5-HT receptor antagonists display relatively high affinity for the 5-HT(7) receptor, which significantly correlates with their pharmaceutically active oral doses.

SB-258741: a 5-HT7 receptor antagonist of potential clinical interest

Recently, a series of 5-HT7 receptor antagonists have been developed (24,29,36,68). Among them SB-258741, R-(+)-1-(toluene-3-sulfonyl)-2-[2-(4-methylpiperidin-1-yl)ethyl]-pyrrolidine, (compound "13" in 36,37) was one of the most potent and specific compounds. Due to a lack of specific ligands the pharmacology of 5-HT7 receptor antagonists is still relatively unexplored. It has been suggested, however, that 5-HT7 receptor ligands could be useful in the therapy of various disorders such as sleep disorders, schizophrenia, depression, migraine, epilepsy, pain, or memory impairment. Many of these conceivable indications are not supported by pharmacological data. It is, therefore, of particular interest to review the data generated from studies of one of these most potent and specific 5-HT7 receptor antagonists, SB-258741, with a goal of testing the validity of the proposed clinical indications. In this review, the author describes pharmacology of this compound in order to define its potential clinical use. The available safety pharmacology data are discussed in an attempt to predict potential side effects of specific 5-HT7 receptor antagonists.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Evidence for 5-HT(1B/1D) and 5-HT(2A) receptors mediating constriction of the canine internal carotid circulation

The present study has investigated the preliminary pharmacological profile of the receptors mediating vasoconstriction to 5-hydroxytryptamine (5-HT) in the internal carotid bed of vagosympathectomised dogs. One minute intracarotid infusions of the agonists 5-HT (0.1 - 10 microg min(-1)), sumatriptan (0.3 - 10 microg min(-1); 5-HT(1B/1D)), 5-methoxytryptamine (1 - 100 microg min(-1); 5-HT(1), 5-HT(2), 5-HT(4), 5-ht(6) and 5-HT(7)) or DOI (0.31 - 10 microg min(-1); 5-HT(2)), but not 5-carboxamidotryptamine (0.01 - 0.3 microg min(-1); 5-HT(1), 5-ht(5A) and 5-HT(7)), 1-(m-chlorophenyl)-biguanide (mCPBG; 1 - 1000 microg min(-1); 5-HT(3)) or cisapride (1 - 1000 microg min(-1); 5-HT(4)), resulted in dose-dependent decreases in internal carotid blood flow, without changing blood pressure or heart rate. The vasoconstrictor responses to 5-HT, which remained unaffected after saline, were resistant to blockade by i.v. administration of the antagonists ritanserin (100 microg kg(-1); 5-HT(2A/2B/2C)) in combination with tropisetron (3000 microg kg(-1); 5-HT(3/4)) or the cyclo-oxygenase inhibitor, indomethacin (5000 microg kg(-1)), but were abolished by the 5-HT(1B/1D) receptor antagonist, GR127935 (30 microg kg(-1)). Interestingly, after administration of GR127935, the subsequent administration of ritanserin unmasked a dose-dependent vasodilator component. GR127935 or saline did not practically modify the vasoconstrictor effects of 5-MeO-T. In animals receiving GR127935, the subsequent administration of ritanserin abolished the vasoconstrictor responses to 5-MeO-T unmasking a dose-dependent vasodilator component. The vasoconstriction induced by sumatriptan was antagonized by GR127935, but not by ritanserin. Furthermore, ritanserin (100 microg kg(-1)) or ketanserin (100 microg kg(-1); 5-HT(2A)), but not GR127935, abolished DOI-induced vasoconstrictor responses. The above results suggest that 5-HT-induced internal carotid vasoconstriction is predominantly mediated by 5-HT(1B/1D) and 5-HT(2A) receptors.

The GR127935-sensitive 5-HT(1) receptors mediating canine internal carotid vasoconstriction: resemblance to the 5-HT(1B), but not to the 5-HT(1D) or 5-ht(1F), receptor subtype

This study has further investigated the pharmacological profile of the GR127935-sensitive 5-HT(1) receptors mediating vasoconstriction in the internal carotid bed of anaesthetized vagosympathectomized dogs. One-minute intracarotid infusions of the agonists 5-hydroxytryptamine (5-HT; 0.1 - 10 microg min(-1); endogenous ligand) and sumatriptan (0.3 - 10 microg min(-1); 5-HT(1B/1D)), but not PNU-142633 (1 - 1000 microg min(-1); 5-HT(1D)) or LY344864 (1 - 1000 microg min(-1); 5-ht(1F)), produced dose-dependent decreases in internal carotid blood flow without changing blood pressure or heart rate. The responses to 5-HT were apparently resistant to blockade by i.v. administration of the antagonists SB224289 (300 microg kg(-1); 5-HT(1B)), BRL15572 (300 microg kg(-1); 5-HT(1D)) or ritanserin (100 microg kg(-1); 5-HT(2)). In contrast, the responses to sumatriptan were antagonized by SB224289, but not by BRL15572. In the animals receiving SB224289, but not those receiving BRL15572, the subsequent administration of ritanserin abolished the 5-HT-induced vasoconstriction and unmasked a vasodilator component. Similarly, in ritanserin-treated animals, the subsequent administration of SB224289, but not BRL15572, completely blocked the 5-HT-induced vasoconstriction, revealing vasodilatation. In animals receiving initially BRL15572, the subsequent administration of SB224289 did not affect (except at 10 microg min(-1)) the vasoconstrictor responses to 5-HT. Notably, in animals pretreated with 1000 microg kg(-1) of mesulergine, a 5-HT(2/7) receptor antagonist, 5-HT produced a dose-dependent vasoconstriction, which was practically abolished by SB224289. After BRL15572, no further blockade was produced and the subsequent administration of ritanserin was similarly inactive. These results suggest that the GR127935-sensitive 5-HT(1) receptors mediating canine internal carotid vasoconstriction resemble the 5-HT(1B) but not the 5-HT(1D) or 5-ht(1F), receptor subtype.

In vivo analysis of adrenergic and serotoninergic constrictions of the rabbit saphenous vein

We aimed to develop a model to study in vivo the rabbit saphenous vein pharmacology and to investigate constrictions mediated by adrenoceptor and 5-HT receptor subtypes. We used the technique of high precision ultrasonic echo-tracking for direct measurement of saphenous vein diameters in pentobarbital anesthetized rabbits. Saphenous vein constrictions induced in rabbits by the alpha(1)-adrenoceptor agonist L-phenylephrine and the 5-HT(1B) receptor agonist sumatriptan were comparable with those induced in dogs but those induced by the 5-HT(1B) and 5-HT(7) receptor agonist 5-carboxamidotryptamine failed to appear in dogs. Dose-related constrictions of rabbit veins were obtained with L-phenylephrine and the alpha(2)-adrenoceptor agonist dexmedetomidine. Frequency-related constrictions of rabbit veins induced by nerve stimulation were partially inhibited by an alpha(1)-adrenoceptor or a postsynaptic alpha(2)-adrenoceptor antagonist (prazosin and SKF 104,078) but not affected by the pre- and post-synaptic alpha(2)-adrenoceptor antagonists BRL 44408 or rauwolscine. Constrictions of rabbit veins to sumatriptan and 5-CT were inhibited by GR 127935 and those induced by quipazine, a 5-HT(2) receptor agonist were prevented by ritanserin. The initial constrictions induced by 5-CT were followed by dilatations which were inhibited by the 5-HT(7) receptor antagonist mesulergine. These data indicate that rabbit saphenous veins, in vivo and at rest, respond to activation of 5-HT(1B) and 5-HT(2) receptors, alpha(1)- and alpha(2)-adrenoceptors and nerve stimulation; the dilator effect mediated by 5-HT(7) receptor activation was also detected. The data validate a new animal model to study superficial vein reactivity and its pharmacological sensitivity.

Serotonin 5-HT(7) receptors mediate relaxation of porcine pial veins

Isolated porcine pial veins in the presence of active muscle tone have been shown to exhibit rhythmic contractions (RC) that are inhibited by serotonin (5-HT) in a concentration-dependent manner. The 5-HT inhibition of RC is mediated by an as yet unidentified 5-HT receptor subtype located on the vascular smooth muscle. 5-carboxamidotryptamine, which is a potent but nonselective agonist at 5-HT(7) receptors, has been shown to be the most potent inhibitor of RC in porcine pial veins. Therefore, the present study was designed to determine if the 5-HT-mediated inhibition of RC in pial veins is mediated by 5-HT(7) receptors and if 5-HT(7) receptor mRNA is expressed in endothelium-denuded pial veins; the study was done with the use of an in vitro tissue bath and RT-PCR techniques. Our findings indicated that 5-HT inhibition of RC in porcine pial veins was prevented by 5-HT(7)-receptor antagonists (clozapine, pimozide, and LY-215840) in a concentration-dependent manner. Furthermore, a strong PCR signal for the 5-HT(7) receptor was consistently detected in endothelium-denuded pial veins. Sequence analysis of the amplified products confirmed their high degree of homology with the porcine and/or human 5-HT(7)-receptor gene. Taken together, these data suggest that the 5-HT-induced inhibition of RC in porcine pial veins is at least in part mediated by 5-HT(7) receptors located on the venous smooth muscle.

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.

Pharmacological evidence for the 5-HT7 receptor mediating smooth muscle relaxation in canine cerebral arteries

1. We investigated in the present study whether 5-HT is able to exert direct relaxant responses in canine basilar and middle cerebral arteries via the 5-HT7 receptor. 2. In arterial rings deprived of endothelium and pre-contracted with prostaglandin F2 alpha (2 microM), 5-HT, 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine, sumatriptan or alpha-methyl-5-HT produced further increase in tone and/or slight relaxation. Blockade of 5-HT1B 1D and 5-HT2A receptors with GR127935 (1 microM) and ketanserin (0.1 microM), respectively, antagonized the vasoconstrictor component of the response and unmasked a concentration-dependent relaxation to 5-HT, 5-CT and 5-methoxytryptamine; sumatriptan and alpha-methyl-5-HT remained inactive as relaxant agonists. The rank order of agonist potency in both arteries was 5-CT > 5-HT > 5-methoxytryptamine >> sumatriptan > or = alpha-methyl-5-HT. 3. In dog basilar artery, pre-incubated with GR127935 (1 microM) and ketanserin (0.1 microM) and precontracted with prostaglandin F2 alpha (2 microM), the 5-HT7 ligands, clozapine (1 microM), mesulergine (0.3 microM), methiothepin (3 nM), risperidone (3 nM), spiperone (1 microM) and LY215840 (10-100 nM), produced significant rightward shifts of the concentration-response curves for 5-HT and 5-CT. Only methiothepin and risperidone reduced significantly the maximum relaxant response (Emax), whilst the other drugs behaved as competitive antagonists with affinity values (pKB) that significantly correlated with their binding affinity (pKi) at recombinant 5-HT7 receptors. 4. These data disclosing the involvement of the 5-HT7 receptor in cerebrovascular relaxation may be strongly relevant in the light of: (1) the involvement of 5-HT in migraine; (2) the putative linkage between cephalovascular vasodilatation and migraine headache; and (3) the relatively high 5-HT7 receptor affinity of migraine prophylactic 5-HT antagonists.

Canine external carotid vasoconstriction to methysergide, ergotamine and dihydroergotamine: role of 5-HT1B/1D receptors and alpha2-adrenoceptors

The antimigraine drugs methysergide, ergotamine and dihydroergotamine (DHE) produce selective vasoconstriction in the external carotid bed of vagosympathectomized dogs anaesthetized with pentobarbital and artificially respired, but the receptors involved have not yet been completely characterized. Since the above drugs display affinity for several binding sites, including alpha-adrenoceptors and several 5-HT1 and 5-HT2 receptor subtypes, this study has analysed the mechanisms involved in the above responses. Intracarotid (i.c.) infusions during 1 min of methysergide (31-310 microg min(-1)), ergotamine (0.56-5.6 microg min(-1)) or DHE (5.6-31 microg min(-1)) dose-dependently reduced external carotid blood flow (ECBF) by up to 46+/-4, 37+/-4 and 49+/-5%, respectively. Blood pressure and heart rate remained unchanged. The reductions in ECBF by methysergide were abolished and even reversed to increases in animals pre-treated with GR127935 (10 microg kg(-1), i.v.). The reductions in ECBF by ergotamine and DHE remained unchanged in animals pre-treated (i.v.) with prazosin (300 microg kg(-1)), but were partly antagonized in animals pre-treated with either GR127935 (10 or 30 microg kg(-1)) or yohimbine (1000 microg kg(-1)). Pre-treatment with a combination of GR127935 (30 microg kg(-1)) and yohimbine (1000 microg kg(-1)) abolished the responses to both ergotamine and DHE. The above doses of antagonists were shown to produce selective antagonism at their respective receptors. These results suggest that the external carotid vasoconstrictor responses to methysergide primarily involve 5-HT1B/1D receptors, whereas those to ergotamine and DHE are mediated by 5-HT1B/1D receptors as well as alpha2-adrenoceptors.

The canine external carotid vasoconstrictor 5-HT1 receptor: blockade by 5-HT1B (SB224289), but not by 5-HT1D (BRL15572) receptor antagonists

In vagosympathectomised dogs pre-treated intravenously (i.v.) with mesulergine (300 microg/kg), 1-min intracarotid (i.c.) infusions of 5-hydroxytryptamine (5-HT; 0.3-30 microg/min) and sumatriptan (1-30 microg/min) dose-dependently decreased external carotid blood flow, without affecting mean blood pressure or heart rate. Treatment with the selective 5-HT1B receptor antagonist SB224289 (2,3,6,7-tetrahydro-1'-methyl-5-[2'-methyl-4'(5-methyl-1,2,4-oxadiazo l-3-yl) biphenyl-4-carbonyl]furo[2,3f]indole-3-spiro-4'-piperidine hydrochloride; 30-300 microg/kg, i.v.) produced a potent, specific and dose-dependent blockade of this response, whereas the selective 5-HT1D receptor antagonist BRL15572 (1-(3-chlorophenyl)-4-[3,3-diphenyl(2-(S,R) hydroxypropanyl)piperazine]hydrochloride; 30-300 microg/kg, i.v.) was ineffective. It is concluded that mainly 5-HT1B, but not 5-HT1D receptors mediate the canine external carotid vasoconstriction by 5-HT and sumatriptan.

External carotid effects of 2-(2-aminoethyl)-quinoline (D-1997) in vagosympathectomized dogs

Serotonin (5-hydroxytryptamine; 5-HT) elicits external carotid vasoconstriction in vagosympathectomized dogs via 5-HT1B/1D receptors and a mechanism unrelated to the 5-HT1, 5-HT2, 5-HT3 and 5-HT4 types. In order to further explore the nature of this novel mechanism, the canine external carotid effects of 2-(2-aminoethyl)-quinoline (D-1997), a novel 5-HT1 receptor agonist, were analyzed and compared with those of 5-HT and sumatriptan. Intracarotid (i.c.) infusions of 5-HT, D-1997 and sumatriptan to vagosympathectomized dogs dose-dependently decreased external carotid conductance, the rank order of agonist potency being 5-HT > sumatriptan > D-1997. The effects to D-1997 were resistant to intravenous (i.v.) pretreatment with 5-HT2 and 5-HT3/5-HT4 receptor antagonists. Remarkably, the effects induced by lower (10-100 microg/min), but not higher (300-1000 microg/min), doses of D-1997 were blocked by high doses of methiothepin (1 and 3 mg/kg, i.v.), as previously shown with 5-HT. In addition, GR-127935 (1-10 microg/kg, i.v.), partially and dose-dependently antagonized D-1997-induced responses. However, the effects of D-1997 remained unaltered after blockade of alpha- and beta-adrenoceptors, muscarinic, nicotinic, histamine and dopamine receptors, or inhibition of 5-HT-uptake or cyclo-oxygenase, depletion of biogenic amines or blockade of Ca2+ channels. These results may support our previous contention that lower doses of 5-HT elicit external carotid vasoconstriction in vagosympathectomized dogs by activation of 5-HT1B/1D receptors, whilst higher doses of 5-HT stimulate a novel vasoconstrictor mechanism.

5-HT1-like receptors: a time to bid goodbye

It is exactly half a century ago that 5-hydroxytryptamine was discovered and over four decades ago two types of 5-HT receptors were described. In this article, Pramod Saxena, Peter De Vries and Carlos Villalón trace the development of the modern classification and nomenclature of 5-HT receptors, which now include more than a dozen subtypes. In doing so, they advocate that the so-called '5-HT1-like' receptors, having been shown to be a heterogeneous population of 5-HT1B, 5-HT1D and 5-HT7 receptors, are now redundant.

The 5-HT1-like receptors mediating inhibition of sympathetic vasopressor outflow in the pithed rat: operational correlation with the 5-HT1A, 5-HT1B and 5-HT1D subtypes

1. It has been suggested that the inhibition of sympathetically-induced vasopressor responses produced by 5-hydroxytryptamine (5-HT) in pithed rats is mediated by 5-HT1-like receptors. The present study has re-analysed this suggestion with regard to the classification schemes recently proposed by the NC-IUPHAR subcommittee on 5-HT receptors. 2. Intravenous (i.v.) continuous infusions of 5-HT and the 5-HT1 receptor agonists, 8-OH-DPAT (5-HT1A), indorenate (5-HT1A), CP 93,129 (5-HT1B) and sumatriptan (5-HT(1B/1D)), resulted in a dose-dependent inhibition of sympathetically-induced vasopressor responses. 3. The sympatho-inhibitory responses induced by 5-HT, 8-OH-DPAT, indorenate, CP 93,129 or sumatriptan were analysed before and after i.v. treatment with blocking doses of the putative 5-HT receptor antagonists, WAY 100635 (5-HT1A), cyanopindolol (5-HT(1A/1B)) or GR 127935 (5-HT(1B/1D)). Thus, after WAY 100635, the responses to 5-HT and indorenate, but not to 8-OH-DPAT, CP 93,129 and sumatriptan, were blocked. After cyanopindolol, the responses to 5-HT, indorenate and CP 93,129 were abolished, whilst those to 8-OH-DPAT and sumatriptan (except at the lowest frequency of stimulation) remained unaltered. In contrast, after GR 127935, the responses to 5-HT, CP 93,129 and sumatriptan, but not to 8-OH-DPAT and indorenate, were abolished. 4. In additional experiments, the inhibition induced by 5-HT was not modified after 5-HT7 receptor blocking doses of mesulergine. 5. The above results suggest that the 5-HT1-like receptors, which inhibit the sympathetic vasopressor outflow in pithed rats, display the pharmacological profile of the 5-HT1A, 5-HT1B and 5-HT1D, but not that of 5-HT7, receptors.

Characterization of 5-HT receptors mediating constriction of porcine carotid arteriovenous anastomoses; involvement of 5-HT1B/1D and novel receptors

1. It was previously shown that porcine cranial arteriovenous anastomoses (AVAs) constrict to 5-hydroxytryptamine (5-HT), ergotamine, dihydroergotamine, as well as sumatriptan and that sumatriptan acts exclusively via 5-HT1B/1D receptors. The present study was devoted to establish the contribution of 5-HT1B/1D receptors in the constriction of AVAs elicited by 5-HT (in presence of 0.5 mg kg(-1) ketanserin), ergotamine and dihydroergotamine in anaesthetized pigs. 2. Intracarotid infusion of 5-HT (2 microg kg(-1) min(-1)) and intravenous doses of ergotamine (2.5-20 microg kg(-1)) and dihydroergotamine (3-100 microg kg(-1)) reduced AVA and increased nutrient blood flows and vascular conductances. The vasodilator response to 5-HT, observed mainly in the skin and ear, was much more prominent than that of the ergot alkaloids. 3. Treatment with the 5-HT1B/1D receptor antagonist GR127935 (0.5 mg kg(-1), i.v.) significantly attenuated both ergot-induced AVA constriction and arteriolar dilatation, whereas GR127935 only slightly affected the carotid vascular effects of 5-HT. 4. The results suggest that 5-HT constricts carotid AVAs primarily via receptors, which seem to differ from those (5-HT1B/1D) stimulated by sumatriptan. The ergot alkaloids produce AVA constriction for a substantial part via 5-HT1B/1D receptors, but also stimulate unidentified receptors. Both these non-5-HT1B/1D receptors may be targets for the development of novel antimigraine drugs. 5. The moderate vasodilator response to the ergot derivatives seems to be mediated, at least in part, by 5-HT1B/1D receptors, whereas the arteriolar dilatation caused by 5-HT may be mediated by other, possibly 5-HT7 receptors.

Pharmacological analysis of the haemodynamic effects of 5-HT1B/D receptor agonists in the normotensive rat

1 The receptors involved in mediating the haemodynamic effects of three 5-HT1B/D receptor agonists were investigated in pentobarbitone anaesthetized rats (n = 6-17 per group). 2 Cumulative intravenous (i.v.) infusions of rizatriptan and sumatriptan (from 0.63 to 2500 microg kg(-1); each dose over 5 min) induced dose-dependent and marked hypotension (-42+/-6 and -34+/-4 mmHg at the highest dose, respectively; both P<0.05 vs vehicle: +5+/-3 mmHg) and bradycardia (-85+/-16 and -44+/-12 beats min(-1) at the highest dose, respectively; both P<0.05 vs vehicle: +16+/-6 beats min(-1)). Zolmitriptan evoked only moderate hypotension at the highest dose (-19+/-9 mmHg; P<0.05 vs vehicle). 3 A high dose of the 5-HT1B/D receptor antagonist, GR 127935 (0.63 mg kg(-1), i.v.), failed to antagonize the hypotension and bradycardia evoked by sumatriptan (-35+/-6 mmHg and -52+/-19 beats min(-1), respectively; both not significant vs sumatriptan in untreated rats), but moderately reduced the hypotension and bradycardia evoked by rizatriptan (-20+/-5 mmHg and -30+/-17 beats min(-1), respectively; both P<0.05 vs vehicle and vs rizatriptan in untreated rats). 4 The selective 5-HT1A receptor antagonist, WAY 100635 (0.16 and 0.63 mg kg(-1), i.v.), dose-dependently attenuated the haemodynamic responses evoked by rizatriptan and sumatriptan, which were almost abolished by the higher dose of WAY 100635 (-4+/-3 mmHg and -15+/-8 beats min(-1); both not significant vs vehicle and P<0.05 vs rizatriptan in untreated rats). A slight but statistically significant reduction in mean arterial pressure (MAP) persisted at the highest dose of sumatriptan (-13+/-4 mmHg following the higher dose of WAY 100635; P<0.05 vs vehicle). 5 In pithed rats with MAP normalized by angiotensin II, rizatriptan failed to induce hypotension or bradycardia (+5+/-4 mmHg and -6+/-16 beats min(-1), respectively; both NS vs vehicle and P<0.05 vs rizatriptan in untreated rats). Similarly, sumatriptan failed to induce bradycardia in pithed rats (+5+/-6 beats min(-1); not significant vs vehicle and P<0.05 vs sumatriptan in untreated rats), whereas a slight but statistically significant reduction in MAP, compared to controls, occurred at the highest dose (-9+/-9 mmHg; P<0.05 vs both vehicle and sumatriptan in untreated rats). 6 In bilaterally vagotomized and atropine-treated (1 mg kg(-1), i.v.) rats, the reductions in MAP and heart rate evoked by rizatriptan (-31+/-4 mmHg and -64+/-9 beats min(-1), respectively; both P<0.05 vs vehicle and not significant vs rizatriptan in controls) and sumatriptan (-47+/-8 mmHg and -56+/-10 beats min(-1), respectively; both P<0.05 vs vehicle and not significant vs sumatriptan in controls) were not statistically significantly different from those observed in controls. 7 In conclusion, the 5-HT1B/D receptor agonists, rizatriptan and sumatriptan, elicit hypotension and bradycardia in the normotensive anaesthetized rat predominantly via activation of central 5-HT1A receptors, and a consequent reduction in sympathetic outflow.

Characterization of putative 5-HT7 receptors mediating tachycardia in the cat

1. It has been suggested that the tachycardic response to 5-hydroxytryptamine (5-HT) in the spinal-transected cat is mediated by '5-HT1-like' receptors since this effect, being mimicked by 5-carboxamidotryptamine (5-CT), is not modified by ketanserin or MDL 72222, but it is blocked by methiothepin, methysergide or mesulergine. The present study was set out to reanalyse this suggestion in terms of the IUPHAR 5-HT receptor classification schemes proposed in 1994 and 1996. 2. Intravenous (i.v.) bolus injections of the tryptamine derivatives, 5-CT (0.01, 0.03, 0.1, 0.3, 1, 3, 10 and 30 microg kg(-1)), 5-HT (3, 10 and 30 microg kg(-1)) and 5-methoxytryptamine (3, 10 and 30 microg kg(-1)) as well as the atypical antipsychotic drug, clozapine (1000 and 3000 microg kg(-1)) resulted in dose-dependent increases in heart rate, with a rank order of agonist potency of 5-CT >> 5-HT > 5-methoxytryptamine >> clozapine. 3. The tachycardic effects of 5-HT and 5-methoxytryptamine were dose-dependently antagonized by i.v. administration of lisuride (30 and 100 microg kg(-1)), ergotamine (100 and 300 microg kg(-1)) or mesulergine (100, 300 and 1000 microg kg(-1)); the highest doses of these antagonists used also blocked the tachycardic effects of 5-CT. Clozapine (1000 and 3000 microg kg(-1)) did not affect the 5-HT-induced tachycardia, but attenuated, with its highest dose, the responses to 5-methoxytryptamine and 5-CT. However, these doses of clozapine as well as the high doses of ergotamine (300 microg kg(-1)) and mesulergine (300 and 1000 microg kg(-1)) also attenuated the tachycardic effects of isoprenaline. In contrast, 5-HT-, 5-methoxytryptamine- and 5-CT-induced tachycardia were not significantly modified after i.v. administration of physiological saline (0.1 and 0.3 ml kg(-1)), the 5-HT(1B/1D) receptor antagonist, GR127935 (500 microg kg(-1)) or the 5-HT(3/4) receptor antagonist, tropisetron (3000 microg kg(-1)). 4. Intravenous injections of the 5-HT1 receptor agonists, sumatriptan (30, 100 and 300 microg kg(-1)) and indorenate (300 and 1000 microg kg(-1)) or the 5-HT4 receptor (partial) agonist cisapride (300 and 1000 microg kg(-1)) were devoid of effects on feline heart rate per se and failed to modify significantly 5-HT-induced tachycardic responses. 5. Based upon the above rank order of agonist potency, the failure of sumatriptan, indorenate or cisapride to produce cardioacceleration and the blockade by a series of drugs showing high affinity for the cloned 5-ht7 receptor, the present results indicate that the 5-HT receptor mediating tachycardia in the cat is operationally similar to other putative 5-HT7 receptors mediating vascular and non-vascular responses (e.g. relaxation of the rabbit femoral vein, canine external carotid and coronary arteries, rat systemic vasculature and guinea-pig ileum). Since these responses represent functional correlates of the 5-ht7 gene product, the 5-HT7 receptor appellation is reinforced. Therefore, the present experimental model, which is not complicated by the presence of other 5-HT receptors, can be utilized to characterize and develop new drugs with potential agonist and antagonist properties at functional 5-HT7 receptors.