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

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.

Pharmacology and Therapeutic Potential of the 5-HT7 Receptor

It is well-documented that serotonin (5-HT) exerts its pharmacological effects through a series of 5-HT receptors. The most recently identified member of this family, 5-HT₇, was first identified in 1993. Over the course of the last 25 years, this receptor has been the subject of intense investigation, and it has been demonstrated that 5-HT₇ plays an important role in a wide range of pharmacological processes. As a result of these findings, modulation of 5-HT₇ activity has been the focus of numerous drug discovery and development programs. This review provides an overview of the roles of 5-HT₇ in normal physiology and the therapeutic potential of this interesting drug target.

Gi-protein-coupled 5-HT1B/D receptor agonist sumatriptan induces type I hyperalgesic priming

We have recently described a novel form of hyperalgesic priming (type II) induced by agonists at two clinically important Gi-protein-coupled receptors (Gi-GPCRs), mu-opioid and A1-adenosine. Like mu-opioids, the antimigraine triptans, which act at 5-HT1B/D Gi-GPCRs, have been implicated in pain chronification. We determined whether sumatriptan, a prototypical 5-HT1B/D agonist, produces type II priming. Characteristic of hyperalgesic priming, intradermal injection of sumatriptan (10 ng) induced a change in nociceptor function such that a subsequent injection of prostaglandin-E2 (PGE2) induces prolonged mechanical hyperalgesia. However, onset to priming was delayed 3 days, characteristic of type I priming. Also characteristic of type I priming, a protein kinase Cε, but not a protein kinase A inhibitor attenuated the prolongation phase of PGE2 hyperalgesia. The prolongation of PGE2 hyperalgesia was also permanently reversed by intradermal injection of cordycepin, a protein translation inhibitor. Also, hyperalgesic priming did not occur in animals pretreated with pertussis toxin or isolectin B4-positive nociceptor toxin, IB4-saporin. Finally, as observed for other agonists that induce type I priming, sumatriptan did not induce priming in female rats. The prolongation of PGE2 hyperalgesia induced by sumatriptan was partially prevented by coinjection of antagonists for the 5-HT1B and 5-HT1D, but not 5-HT7, serotonin receptors and completely prevented by coadministration of a combination of the 5-HT1B and 5-HT1D antagonists. Moreover, the injection of selective agonists, for 5-HT1B and 5-HT1D receptors, also induced hyperalgesic priming. Our results suggest that sumatriptan, which signals through Gi-GPCRs, induces type I hyperalgesic priming, unlike agonists at other Gi-GPCRs, which induce type II priming.

Sustained Hypoxia Elicits Competing Spinal Mechanisms of Phrenic Motor Facilitation

Acute intermittent hypoxia (AIH) induces phrenic long-term facilitation (pLTF), a form of spinal motor plasticity. Competing mechanisms give rise to phrenic motor facilitation (pMF; a general term including pLTF) depending on the severity of hypoxia within episodes. In contrast, moderate acute sustained hypoxia (mASH) does not elicit pMF. By varying the severity of ASH and targeting competing mechanisms of pMF, we sought to illustrate why moderate AIH (mAIH) elicits pMF but mASH does not. Although mAIH elicits serotonin-dependent pLTF, mASH does not; thus, mAIH-induced pLTF is pattern sensitive. In contrast, severe AIH (sAIH) elicits pLTF through adenosine-dependent mechanisms, likely from greater extracellular adenosine accumulation. Because serotonin- and adenosine-dependent pMF interact via cross talk inhibition, we hypothesized that pMF is obscured because the competing mechanisms of pMF are balanced and offsetting during mASH. Here, we demonstrate the following: (1) blocking spinal A2A receptors with MSX-3 reveals mASH-induced pMF; and (2) sASH elicits A2A-dependent pMF. In anesthetized rats pretreated with intrathecal A2A receptor antagonist injections before mASH (PaO2 = 40-54 mmHg) or sASH (PaO2 = 25-36 mmHg), (1) mASH induced a serotonin-dependent pMF and (2) sASH induced an adenosine-dependent pMF, which was enhanced by spinal serotonin receptor inhibition. Thus, competing adenosine- and serotonin-dependent mechanisms contribute differentially to pMF depending on the pattern/severity of hypoxia. Understanding interactions between these mechanisms has clinical relevance as we develop therapies to treat severe neuromuscular disorders that compromise somatic motor behaviors, including breathing. Moreover, these results demonstrate how competing mechanisms of plasticity can give rise to pattern sensitivity in pLTF.

SIGNIFICANCE STATEMENT

Intermittent hypoxia elicits pattern-sensitive spinal plasticity and improves motor function after spinal injury or during neuromuscular disease. Specific mechanisms of pattern sensitivity in this form of plasticity are unknown. We provide evidence that competing mechanisms of phrenic motor facilitation mediated by adenosine 2A and serotonin 2 receptors are differentially expressed, depending on the pattern/severity of hypoxia. Understanding how these distinct mechanisms interact during hypoxic exposures differing in severity and duration will help explain interesting properties of plasticity, such as pattern sensitivity, and may help optimize therapies to restore motor function in patients with neuromuscular disorders that compromise movement.

Multi-generational effects of propranolol on Daphnia magna at different environmental concentrations

To evaluate the effects of propranolol on Daphnia magna (D. magna), we employed a multi-generational exposure period for eight generations and an environmentally relevant low concentration with 1.5 ng/L, 0.2 μg/L and 26 μg/L to reflect a realistic exposure scenario. Physiological endpoints were checked, including growth, number of neonates, heart rate, frequency of abdominal appendage movement and malformation rate of neonates. In the results, growth and abdominal appendage movement were affected by environmental concentration during several generations, and the responses showed consistent tendencies of response increase with concentration increase. Heart rate was the only endpoint affected throughout all exposure generations. Inhibitory and acceleratory effects on heart rate, growth and abdominal appendage movement suggest that it is necessary to cover sub-lethal endpoints of non-targeted organisms in eco-toxicity study because the physiological responses were detected at much lower concentrations than the results of traditional toxicity tests, including environmental concentration.

3D QSAR based design of novel oxindole derivative as 5HT7 inhibitors

To understand the structural requirements of 5-hydroxytryptamine (5HT7) receptor inhibitors and to design new ligands against 5HT7 receptor with enhanced inhibitory potency, a three-dimensional quantitative structure-activity relationship study with comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) for a data set of 56 molecules consisting of oxindole, tetrahydronaphthalene, aryl ketone substituted arylpiperazinealkylamide derivatives was performed. Derived model showed good statistical reliability in terms of predicting 5HT7 inhibitory activity of the molecules, based on molecular property fields like steric, electrostatic, hydrophobic, hydrogen bond donor and hydrogen bond acceptor fields. This is evident from statistical parameters like conventional r2 and a cross validated (q2) values of 0.985, 0.743 for CoMFA and 0.970, 0.608 for CoMSIA, respectively. Predictive ability of the models to determine 5HT7 antagonistic activity is validated using a test set of 16 molecules that were not included in the training set. Predictive r2 obtained for the test set was 0.560 and 0.619 for CoMFA and CoMSIA, respectively. Steric, electrostatic fields majorly contributed toward activity which forms the basis for design of new molecules. Absorption, distribution, metabolism and elimination (ADME) calculation using QikProp 2.5 (Schrodinger 2010, Portland, OR) reveals that the molecules confer to Lipinski's rule of five in majority of the cases.

Serotonin and blood pressure regulation

5-Hydroxytryptamine (5-HT; serotonin) was discovered more than 60 years ago as a substance isolated from blood. The neural effects of 5-HT have been well investigated and understood, thanks in part to the pharmacological tools available to dissect the serotonergic system and the development of the frequently prescribed selective serotonin-reuptake inhibitors. By contrast, our understanding of the role of 5-HT in the control and modification of blood pressure pales in comparison. Here we focus on the role of 5-HT in systemic blood pressure control. This review provides an in-depth study of the function and pharmacology of 5-HT in those tissues that can modify blood pressure (blood, vasculature, heart, adrenal gland, kidney, brain), with a focus on the autonomic nervous system that includes mechanisms of action and pharmacology of 5-HT within each system. We compare the change in blood pressure produced in different species by short- and long-term administration of 5-HT or selective serotonin receptor agonists. To further our understanding of the mechanisms through which 5-HT modifies blood pressure, we also describe the blood pressure effects of commonly used drugs that modify the actions of 5-HT. The pharmacology and physiological actions of 5-HT in modifying blood pressure are important, given its involvement in circulatory shock, orthostatic hypotension, serotonin syndrome and hypertension.

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.

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.

Recent advances in understanding serotonin regulation of cardiovascular function

Serotonin is an important neurohormonal factor that has been implicated in cardiovascular function. It can regulate vascular tone, act directly on cardiomyocytes and stimulate chemosensitive nerves in the heart. Cardiovascular dysfunction is observed when serotonin signaling is altered or when variation in serotonin concentration occurs. Recent studies have provided evidence that, in the absence of peripheral serotonin synthesis, blood serotonin (which is almost exclusively stored in platelets) is markedly reduced, and that this drop leads to heart failure. This implies that the level of circulating serotonin is a key factor in maintaining normal cardiovascular activity. These findings offer new prospects for the use of serotonin in therapies for cardiovascular diseases.

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.

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.

5-HT in systemic hypertension: foe, friend or fantasy?

Since its discovery by Erspamer in the 1930s and identification by Page in the 1950s, 5-HT (5-hydroxytryptamine; serotonin) has been an elusive candidate as a substance that plays a role in the disease of high blood pressure, also known as hypertension. In both animal and human hypertension, arterial contraction to 5-HT is profoundly enhanced. Additionally, 5-HT is a vascular smooth muscle cell mitogen. Because both increased arterial contractility and smooth muscle growth contribute to the disease of hypertension, it is logical to believe that 5-HT is a potential cause of disease, and thus a foe. However, decades of research have produced conflicting results as to the potential role of 5-HT in hypertension. This review will discuss historical findings which both support and refute the involvement of 5-HT in hypertension, and pose some new questions that may reveal novel ways for 5-HT to modify vascular control of blood pressure.

The atypical 5-HT2 receptor mediating tachycardia in pithed rats: pharmacological correlation with the 5-HT2A receptor subtype

1. In pithed rats, 5-HT mediates tachycardia both directly (by 5-HT(2) receptors) and indirectly (by a tyramine-like effect). The receptor mediating tachycardia directly has been classified as an 'atypical' 5-HT(2) receptor since it was 'weakly' blocked by ketanserin. Moreover, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT(2) agonist, failed to mimic 5-HT-induced tachycardia. Since 5-HT(2) receptors consist of 5-HT(2A), 5-HT(2B) and 5-HT(2C) subtypes, this study investigated if these subtypes mediate the above response. 2. In pithed rats, intraperitoneally (i.p.) pre-treated with reserpine (5 mg kg(-1)), intravenous (i.v.) administration of 5-HT, 5-methoxytryptamine (5-MeO-T), 1-(3-chlorophenyl) piperazine (mCPP) and 5-carboxamidotryptamine (5-CT) (10, 30, 100 and 300 microg kg(-1) each), produced dose-dependent tachycardic responses. Interestingly, DOI (10 - 1000 microg kg(-1), i.v.) induced only slight, dose-unrelated, tachycardic responses, whilst the 5-HT(2C) agonist, Ro 60-0175 (10 - 1000 microg kg(-1), i.v.), produced a slight tachycardia only at 300 and 1000 microg kg(-1). In contrast, sumatriptan and 1-(m-trifluoromethylphenyl)- piperazine (TFMPP) were inactive. The rank order of potency was: 5-HT > or =5-MeO-T> mCPP > or =5-CT > or =DOI > Ro 60-0175. 3. The tachycardic responses to 5-HT, which remained unaffected after i.v. saline (0.3 and 1 ml kg(-1)) or propranolol (3 mg kg(-1)), were selectively blocked by the 5-HT(2A) antagonists ketanserin (30 and 100 microg kg(-1)) or spiperone (10 and 30 microg kg(-1)) as well as by the non-selective 5-HT(2) antagonists, ritanserin (10 and 30 microg kg(-1)) or mesulergine (100 microg kg(-1)). Remarkably, these responses were unaffected by the antagonists rauwolscine (5-HT(2B)), SB204741 (5-HT(2B/2C)) or Ro 04-6790 (5-ht(6)) (300 and 1000 microg kg(-1) each). 4. These results suggest that the 'atypical' 5-HT(2) receptors mediating tachycardia in reserpinized pithed rats are pharmacologically similar to the 5-HT(2A) receptor subtype.

Molecular cloning, pharmacological properties and tissue distribution of the porcine 5-HT(1B) receptor

Using a combination of RT - PCR and inverse-PCR techniques, we amplified, cloned and sequenced a full-length porcine 5-HT(1B) receptor cDNA derived from porcine cerebral cortex. Sequence analysis revealed 1170 bp encoding an open reading frame of 390 amino acids showing a 95% similarity with the human 5-HT(1B) receptor. The recombinant porcine 5-HT(1B) cDNA was expressed in monkey Cos-7 cells and its pharmacological profile was determined by radioligand binding assay using [(3)H]-GR125743. The affinities of several agonists (L694247>ergotamine > or =5-carboxamidotryptamine=dihydroergotamine=5-HT>CP122638=zolmitriptan>sumatriptan) and putative antagonists (GR127935>methiothepin>SB224289>>ritanserin>ketanserin > or =BRL15572) correlated highly with those described for the recombinant human 5-HT(1B) receptor. In membranes obtained from cells co-expressing the porcine 5-HT(1B) receptor and a mutant G(alphao)Cys(351)Ile protein, 5-HT and zolmitriptan increased, while the 5-HT(1B) receptor antagonist SB224289 decreased basal [(35)S]-GTPgammaS binding, thus showing inverse agonism. The potency of zolmitriptan in the [(35)S]-GTPgammaS binding assay (pEC(50): 7.64+/-0.04) agreed with its affinity in displacing the antagonist [(3)H]-GR125743 (pK(i): 7.36+/-0.07). The 5-HT(1B) receptor mRNA was observed by RT-PCR in several blood vessels, cerebral cortex, cerebellum and trigeminal ganglion. In situ hybridization performed in frontal cerebral cortex sections revealed the expression of 5-HT(1B) receptor mRNA in pyramidal cells. In conclusion, we have cloned and established the amino acid sequence, ligand binding profile and location of the porcine 5-HT(1B) receptor. This information may be useful in exploring the role of 5-HT(1B) receptor in pathophysiological processes relevant for novel drug discovery in diseases such as migraine.

No evidence for functional involvement of 5-HT2B receptors in serotonin-induced vasodilatation in the human forearm

The receptor involved in the serotonin (5-hydroxytryptamine [5-HT])-induced vasodilatation in the human forearm has not yet been identified. Experimental data point to the 5-HT2B receptor located on the endothelium. RS-127445 (2-amino-4-(4-fluoronaphthyl-1-yl)-6-isopropylpyrimidine) is a novel potent and selective 5-HT2B receptor antagonist. The effect of oral RS-127445 (500 mg) on 5-HT-induced vasodilatation was studied in a double-blind, randomized, placebo-controlled, crossover study in six healthy volunteers. On each study day 5-HT (0.5 ng/kg/min) was infused into the brachial artery for 8 min, before drug administration and at intervals of 20, 65, 110, 230, and 470 min after oral ingestion. At each infusion, plasma samples for study drug assay were taken and forearm blood flow was assessed using venous occlusion plethysmography. Although (log) drug concentrations exceeded pKi, there was no correlation between RS-127445 concentrations and 5-HT-induced vasodilatation. 5-HT-induced vasodilatation did not differ between treatments and time points. It appears that there is no functional involvement of 5-HT2B receptors in 5-HT-mediated vasodilatation in the human forearm.

Methysergide delays the decompensatory responses to severe hemorrhage by activating 5-HT(1A) receptors

Central administration of the serotonin receptor ligand methysergide delays the decompensatory response to hypotensive hemorrhage. This study was performed to determine the receptor subtype that mediates this effect. Lateral ventricular (LV) injection of methysergide (40 microg) delayed the hypotensive, bradycardic, and sympathoinhibitory responses to blood withdrawal (1.26 ml/min) in conscious rats. The response was quantified, in part, as the blood volume withdrawal that produced a 40-mmHg fall in blood pressure. The delayed hypotensive response produced by methysergide (8.2 +/- 0.2 vs. 5.6 +/- 0.2 ml, P < 0.01) was reversed by the 5-hydroxytryptamine (HT)(1A) antagonist WAY-100635 (30 microg iv: 6.7 +/- 0.4 ml, P < 0. 01; 100 microg iv: 5.6 +/- 0.1 ml, P < 0.01). LV injection of the 5-HT(1A) agonist (+)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) also delayed the hypotensive (10 microg: 8.6 +/- 0.3, P < 0.01; 20 microg: 9.2 +/- 0.3 ml, P < 0.01), bradycardic, and sympathoinhibitory responses to hemorrhage. WAY-100635 (10 microg iv) completely reversed the effects of 8-OH-DPAT (20 microg: 5.4 +/- 0.3 ml). Neither selective blockade of 5-HT(2) receptors nor stimulation of 5-HT(1B/1D) receptors had any effect on hemorrhage responses. These data indicate that methysergide stimulates 5-HT(1A) receptors to delay the decompensatory responses to hemorrhage.

Identification of the adenylyl cyclase-activating 5-hydroxytryptamine receptor subtypes expressed in the rat submandibular gland

1. Serotonin (5-hydroxytryptamine, 5-HT) has been shown to increase cyclic AMP production in dispersed cell aggregates from the major salivary glands of the rat. The goal of the present study was to identify the 5-HT receptor subtypes that mediate these effects in rat submandibular glands (SMG). 2. Among the 5-HT receptor subtypes identified in the rat, 5-HT(4(a,b)), 5-HT(6) and 5-HT(7(a,b,c)) activate adenylyl cyclase (AC). We used subtype specific primers to screen rat SMG by reverse transcription-PCR. Results indicate the presence of mRNA for 5-HT(4(b)) and 5-HT(7(a)) but not for 5-HT(4(a)), 5-HT(6) and 5-HT(7(b,c)). 3. In dispersed SMG cells, 5-carboxyamidotryptamine (5-CT), a 5-HT(7) receptor agonist, stimulated cyclic AMP synthesis with higher potency (EC(50)=27+/-5 nM) but lower efficacy than 5-HT, suggesting a 5-HT(7) component and an additional component in the response to 5-HT. The 5-HT(7) contribution was further supported by antagonism of the 5-CT effect by metergoline, a 5-HT(7) antagonist, which exhibited an affinity (K(i)=50 nM) similar to that obtained at the cloned 5-HT(7) receptor. 4. In the presence of a maximally effective concentration of 5-CT, 5-HT produced an additional increase in cyclic AMP production that was inhibited by the 5-HT(4) receptor antagonist, GR113808, suggesting that the second component of cyclic AMP production is mediated by 5-HT(4) receptors. 5. These findings indicate the presence in rat SMG of both 5-HT(4(b)) and 5-HT(7(a)) receptors positively coupled to AC.

5-HT7 receptors: current knowledge and future prospects

Identification of three splice variants of the 5-HT7 receptor suggests a possible diversity in 5-HT7 receptor action. Indeed, 5-HT7 receptors have been implicated in the pathophysiology of several disorders; they play a role in smooth muscle relaxation within the vasculature and in the gastrointestinal tract. However, most of these assignments are derived from receptor localization studies and investigations using nonselective ligands, and are therefore mainly suggestive. The development of selective 5-HT7 receptor antagonists will be of utmost importance in determining the actual physiological and pharmacological roles of this receptor. Major challenges of 5-HT7 receptor research are determination of the transcriptional regulation of the gene encoding the 5-HT7 receptor and elucidation of the differences in regulation and signalling of its four gene products.

Evidence for 5-HT7 receptors mediating relaxation of human colonic circular smooth muscle

5-HT4 receptors mediate relaxation of human colon circular muscle. However, after 5-HT4 receptor blockade (SB 204070 10 nM), 5-HT still induced a relaxation (pEC50 6.3). 5-HT4 receptors were sufficiently blocked, as the curves to 5-HT obtained in the presence of 10 and 100 nM SB 204070 were indistinguishable. This 5-HT-induced relaxation was tetrodotoxin-insensitive, indicative of a smooth muscle relaxant 5-HT receptor. This, and the rank order of potency (5-CT=5-MeOT=5-HT) suggested involvement of 5-HT1 or 5-HT7 receptors. Mesulergine, a 5-HT7 receptor antagonist at nanomolar concentrations, and a 5-HT1 receptor antagonist at micromolar concentrations, competitively antagonized the 5-HT-induced relaxation (pKB 8.3) and antagonized the relaxation to 5-CT. Methysergide antagonized the 5-HT-induced relaxation (pA2 7.6). It is concluded that the profile of the smooth muscle inhibitory 5-HT receptor resembles that of the 5-HT7 receptor. These data provide the first evidence for functional human 5-HT7 receptors.

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.

Characterisation of 5-HT receptors in human coronary arteries by molecular and pharmacological techniques

5-Hydroxytryptamine (5-HT) can produce both vasoconstrictor and vasorelaxant effects in human coronary arteries and the response to 5-HT can be influenced by the presence of disease. The aim of the present study was to elucidate the 5-HT receptor subtypes responsible for mediating 5-HT-evoked contraction of human coronary arteries using pharmacological, molecular and immunocytochemical approaches. Normal human coronary arteries, with intact endothelium, were mounted in tissue baths, and the vascular responses to 5-HT and 5-HT receptor agonists were studied. The effects of 5-HT1 and 5-HT2 receptor antagonists on these responses were also studied. Expression of messenger ribonucleic acid (mRNA) encoding different 5-HT receptors in human coronary arteries, atrium, ventricle wall and epicardium was determined using reverse transcriptase-polymerase chain reaction (RT-PCR) and Southern blot analysis. The expression of 5-HT1B or 5-HT1D receptor protein was studied using subtype selective antibodies and standard immunocytochemical techniques. The rank order of 5-HT receptor agonist potency in causing vasoconstriction was 5-carboxamido tryptamine, (5-CT) > zolmitriptan = BW183C91 (N10-desmethyl zolmitriptan) = alpha-methyl-5-hydroxytryptamine (alpha-CH3-5-HT) = 5-HT = sumatriptan > 2-methyl-5-hydroxytryptamine (2-CH3-5-HT) = 8-hydroxy-DPAT (8-OH-DPAT). Alpha-CH3-5-HT, 5-CT, 5-HT, zolmitriptan and BW 183C91 were significantly more potent (approximately 3-fold) than sumatriptan and 2-CH3-5-HT, which in turn were more potent than 8-OH-DPAT. Ketanserin and methiothepin (5-HT2 and 5-HT1 receptor antagonists, respectively) caused parallel rightward shifts of the concentration-effect curves to alpha-CH3-5-HT or 5-CT, respectively, without changing the maximum contractile response. In human coronary arteries, atrium. ventricle and epicardium. RT-PCR products corresponding to the human 5-HT2A, 5-HT1B and 5-HT1F receptors were expressed in high levels, mRNAs coding for 5-HT7, 5-HT1A and 5-HT1D receptors were only weakly expressed. No 5-HT1F receptor mRNA was detected. In coronary arteries there was a differential expression of 5-HT1B versus 5-HT1D receptor mRNAs, with 5-HT1B mRNAs being found in greater abundance. Dense 5-HT1B-immunoreactivity was detected on smooth muscle layer within coronary artery, however, 5-HT1D-immunoreactivity was not detected. It is concluded that 5-HT-evoked contraction of human coronary arteries is most probably mediated via the activation of both 5-HT1B and 5-HT2A receptors.

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.

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.