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

Phenylephrine precontraction increases the sensitivity of rabbit femoral artery to serotonin by enabling 5-HT1-like receptors

We used selective receptor antagonists to identify the receptors mediating the isometric contractile response to serotonin in control and phenylephrine (PHE)-precontracted rabbit femoral artery rings. Serotonin, in the absence of PHE, elicited monophasic concentration-response curves (CRCs) early, but biphasic CRCs late in the course of the study. In the monophasic curves, the threshold and maximal concentrations were 10 and 1,000 microM, respectively. In biphasic CRCs, the threshold and maximal concentrations of the first phase were 0.03 and 3 microM, respectively. The respective values for the second phase were 10 and 1,000 microM. Prazosin, 0.1 microM, a selective alpha1-adrenoceptor antagonist, inhibited the monophasic curves, but only the second phase of the biphasic curves. Ritanserin, 0.01 microM, a selective 5-HT2A-receptor antagonist, shifted the first phase of the biphasic serotonin CRCs to the right but had little effect on the second phase. PHE increased the sensitivity of rabbit femoral artery response to serotonin. This amplified response to serotonin was antagonized by 0.01 microM GR 127935T, a selective 5-HT1B-receptor antagonist. The selective 5-HT1 agonist, sumatriptan, had no effect in control femoral arteries, but caused a concentration-dependent contraction after PHE precontraction. These results suggest that 5-HT1-like receptors are normally inactive or "silent" in the absence of PHE. However, in the presence of PHE, these receptors become enabled and mediate the amplified response to serotonin. The evidence also suggests that, in the absence of PHE, alpha1-adrenoceptors mediated the contractile response to serotonin in the monophasic CRCs. In the biphasic curves observed late in the study, the first phase was mediated by 5-HT2A receptors, and the second, by the alpha1-adrenergic receptors.

Further evidence that 5-HT-induced relaxation of pig pulmonary artery is mediated by endothelial 5-HT(2B) receptors

The endothelial 5-HT receptor mediating relaxation of pig pulmonary artery has been characterized using the selective 5-HT(2B) receptor agonist BW 723C86 and a variety of structurally diverse 5-HT receptor antagonists. If arterial rings with intact endothelium were precontracted with prostaglandin F(2alpha) (3 microM), BW 723C86 caused concentration-dependent relaxation with a pEC(50)=8.21+/-0.03 and E(max)=89+/-4% relative to 5-HT. The relaxant responses to BW 723C86 were inhibited by the 5-HT(2B) receptor antagonist SB 204741, the 5-HT(2B/2C) receptor antagonist SB 206553 and the antimigraine drug pizotifen, yielding pA(2) values of 6.68, 7.20 and 8.32, respectively. The pA(2) values against BW 723C86 were similar to those determined against 5-HT. The relaxant effect of 5-HT was antagonized by a variety of 22 compounds of diverse chemical structures. Based on the calculated mean pA(2) values the order of the most potent antagonists was ritanserin (9.38) > methysergide (8. 86) > pizotifen (8.47) >/= methiothepin (8.32) > LY 53857 (7.84) >/= amoxapine (7.80) >/= loxapine (7.73) >/= metergoline (7.64) >/= mianserin (7.51) >/= rauwolscine (7.39). Compounds with weak blocking potency were yohimbine (6.37), spiperone (5.88) and ketanserin (5.85). Correlation analysis between the affinities of the antagonists in pig pulmonary artery and those from radioligand binding studies at human and rat 5-HT(2B) receptors showed a highly significant correlation (r=0.95 and 0.84, P<0.002 and <0.005). Correlation with 5-HT(2C) receptors was much lower (r=0.57, P=0.035), and no correlations were obtained with 5-ht(6) and 5-HT(7) receptors. It is concluded that the 5-HT receptor mediating endothelium-dependent relaxation of pig pulmonary artery is of the 5-HT(2B) subtype.

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.

Characterisation of a 5-HT(7) binding site in mouse ileum

The aim of the present study was to identify 5-hydroxytryptamine(7) (5-HT(7)) binding sites in the mouse ileum, where the presence of mRNA for the receptor has been reported. Studies were performed using [3H]mesulergine, an antagonist with high affinity at 5-HT(7) receptors. In the presence of a combination of masking drugs to inhibit the binding of the radioligand to other receptors at which it has affinity, such as 5-HT(2A), 5-HT(2C) and dopamine D(2) receptors as well as alpha(1)/alpha(2)-adrenoceptors, [3H]mesulergine labelled two sites with pK(D) values of 9.7+/-0.7 and 7.4+/-0.4 and B(max) values of 37.2+/-21.4 and 247.8+/-62.1 fmol mg protein(-1), respectively. Displacement studies also indicated the presence of non-homogenous binding sites, which showed a significant correlation (Pearson correlation factors of 0.91 and 0. 85) with the 5-HT(2C) and 5-HT(7) receptors, respectively. Total binding to the 5-HT(2C) receptor was minimal; <30% of the total specific receptor binding. The antagonist order of affinity at the greater proportion of receptors was: risperidone (pK(i)pindolol (5. 6). This receptor also showed a high affinity for 5-carboxamidotryptamine (5-CT; 10.6) and moderate affinity for (+/-)-2-dipropyl-amino-8-hydroxy-1,2,3,4,-tetrahydronaphthalene (8-OH-DPAT; 7.2), which is typical of the 5-HT(7) receptor profile.

5-HT modifiers as a potential treatment of asthma

Increased levels of free 5-HT have been shown to be present in the plasma of symptomatic asthmatic patients compared with levels in asymptomatic patients. In addition, free 5-HT has been shown to correlate positively with clinical status and negatively with pulmonary function. These findings suggest that 5-HT might play a role in the pathophysiology of acute asthma. Accordingly, modifiers of the 5-HT transmitter system such as compounds that affect the 5-HT transporter, prejunctional 5-HT receptors or postsynaptic 5-HT receptors might represent a novel treatment of asthma.

Pyrroloquinoxaline derivatives as high-affinity and selective 5-HT(3) receptor agonists: synthesis, further structure-activity relationships, and biological studies

The synthesis, pharmacological evaluation, and structure-activity relationships (SARs) of a series of novel pyrroloquinoxalines and heteroaromatic-related derivatives are described. The new pyrroloquinoxaline-related ligands were tested in rat cortex, a tissue expressing high density of 5-HT(3) receptors, and on NG108-15 cells and exhibited IC(50) values in the low nanomolar or subnanomolar range, as measured by the inhibition of [(3)H]zacopride binding. The SAR studies detailed herein delineated a number of structural features required for improving affinity. Some of the ligands were employed as "molecular yardsticks" to probe the spatial dimensions of the lipophilic pockets L1, L2, and L3 in the 5-HT(3) receptor cleft, while the 7-OH pyrroloquinoxaline analogue was designed to investigate hydrogen bonding with a putative receptor site H1 possibly interacting with the serotonin hydroxy group. The most active pyrroloquinoxaline derivatives showed subnanomolar affinity for the 5-HT(3) receptor. In functional studies ([(14)C]guanidinium accumulation test in NG108-15 hybrid cells, in vitro) most of the tested compounds showed clear-cut 5-HT(3) agonist properties, while some others were found to be partial agonists. Several heteroaromatic systems, bearing N-substituted piperazine moieties, have been explored with respect to 5-HT(3) affinity, and novel structural leads for the development of potent and selective central 5-HT(3) receptor agonists have been identified. Preliminary pharmacokinetic studies indicate that these compounds easily cross the blood-brain barrier (BBB) after systemic administration with a brain/plasma ratio between 2 and 20, unless they bear a highly hydrophilic group on the piperazine ring. None of the tested compounds showed in vivo anxiolytic-like activity, but potential analgesic-like properties have been possibly disclosed for this new class of 5-HT(3) receptor agonists.

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.

5-Hydroxytryptamine inhibits the tachycardia induced by selective preganglionic sympathetic stimulation in pithed rats

It has been shown in several species that serotonin (5-hydroxytryptamine; 5-HT) is able to inhibit the responses produced by sympathetic stimulation in a wide variety of blood vessels and other organs, including the heart. However, in pithed rats, the analysis of potential sympatho-inhibitory actions of 5-HT is hampered by the fact that 5-HT (given as i.v. bolus injections) produces tachycardia per se. Moreover, most studies have investigated 5-HT-induced sympatho-inhibition at only one frequency of stimulation. Thus, the present study set out to find the experimental conditions to overcome these problems. In this regard, we analyzed the potential ability of 5-HT, administered as i.v. continuous infusions, to inhibit the tachycardia caused by stimulation of the preganglionic (C7-T1) sympathetic outflow in pithed rats. Sympathetic cardiostimulation (0.01-3 Hz) resulted in frequency-dependent increases in heart rate; these responses were potentiated after desipramine (50 microg/kg, i.v.). During continuous infusions of 5-HT (3.1-10 microg/kg.min, i.v.), but not saline, the sympathetically-induced tachycardia was dose-dependently inhibited in both control and desipramine-pretreated rats. This inhibitory effect of 5-HT was significantly more pronounced at lower frequencies of stimulation. In contrast, the above infusions of 5-HT did not inhibit the tachycardia induced by i.v. bolus injections of noradrenaline in both control and desipramine-pretreated rats. Taken together, the above findings confirm that 5-HT induces inhibition of the sympathetic chronotropic outflow in the rat by acting at receptors located prejunctionally, without evoking tachycardia, over a wide range of stimulation frequencies.

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

1. It has previously been shown that the antimigraine drug sumatriptan constricts porcine carotid arteriovenous anastomoses via 5-HT1-like receptors, identical to 5-H1B/1D receptors. The recent availability of silent antagonists selective for the 5-HT1B (SB224289) and 5-HT1D (BRL15572) receptor led us to further analyse the nature of receptors involved. 2. In pentobarbitone-anaesthetized, bilaterally vagosympathectomized pigs, sumatriptan (30, 100 and 300 microg kg(-1), i.v.) dose-dependently decreased carotid arteriovenous anastomotic conductance by up to 70+/-5%. 3. The dose-related decreases in carotid arteriovenous anastomotic conductance by sumatriptan (30, 100 and 300 microg kg(-1), i.v.) remained unchanged in animals treated (i.v.) with 1 mg kg(-1) of BRL15572 (maximum decrease: 72+/-3%), but were significantly attenuated by 1 mg kg(-1) (maximum decrease: 30+/-11%) and abolished by 3 mg kg(-1) (maximum decrease: 3+/-7%) of SB224289. The highest dose of SB224289 did not attenuate the hypertension, tachycardia or increases in carotid blood flow induced by bolus injections of noradrenaline (0.1-3 microg kg(-1), i.v.). 4. The results indicate that sumatriptan constricts porcine carotid arteriovenous anastomoses primarily via 5-HT1B, but not via 5-HT1D receptors.

Pindolol-insensitive [3H]-5-hydroxytryptamine binding in the rat hypothalamus; identity with 5-hydroxytryptamine7 receptors

Pindolol-insensitive [3H]-5-hydroxytryptamine ([3H]-5-HT) binding to rat hypothalamic membranes was pharmacologically and functionally characterized to resolve whether this procedure selectively labels 5-HT7 receptors. Consistent with a previous report, 3 microM and not 100 nM pindolol was required to occupy fully 5-HT1A and 5-HT1B receptors. Remaining [3H]-5-HT binding was saturable (KD, 1.59+/-0.21 nM; Bmax, 53.8+/-3.1 fmol x mg protein(-1)). Displacement of [3H]-5-HT with metergoline and 5-CT revealed shallow Hill slopes (<0.5) but seven other compounds had slopes >0.8 and pKi values and the rank order of affinity were significantly correlated (r = 0.81 and 0.93, respectively) with published [3H]-5-HT binding to rat recombinant 5-HT7 receptors. In the presence of pindolol, 5-HT-enhanced accumulation of [32P]-cyclic AMP was unaffected by the 5-HT4 antagonist RS39604 (0.1 microM) or the 5-ht6 antagonist Ro 04-6790 (1 microM) but significantly attenuated by mesulergine (250 nM), ritanserin (450 nM) or methiothepin (200 nM) which have high affinity for the 5-HT7 receptor. Intracerebroventricular pretreatment with the serotonergic neurotoxin 5,7-dihydroxytryptamine, 5,7-DHT, elevated the [3H]-5-HT Bmax 2 fold, indicating that the hypothalamic 5-HT7 receptor is post-synaptic to 5-HT nerve terminals and regulated by synaptic 5-HT levels. These results suggest that, in the presence of 3 microM pindolol, [3H]-5-HT selectively labels hypothalamic binding sites consistent with functional 5-HT7 receptors.

Cycloalkanecarboxylic esters derived from lysergol, dihydrolysergol-I, and elymoclavine as partial agonists and antagonists at rat 5-HT2A receptors: pharmacological evidence that the indolo[4,3-fg]quinoline system of the ergolines is responsible for high 5-HT2A receptor affinity

Three series of cycloalkanecarboxylic esters derived from the naturally occurring clavine alkaloids lysergol, dihydrolysergol-I, and elymoclavine were synthesized to study their interaction with 5-HT2A receptors and alpha1-adrenoceptors in rat tail artery and aorta, respectively. Especially cycloalkanecarboxylic esters derived from lysergol showed complex behavior as partial agonists and antagonists of the contractile effect of 5-HT. Within this group, partial 5-HT2A receptor agonist activity was most potent for cyclopropanecarboxylic ester 6a (pKP = 7.67, alpha = 0.21) and decreased as the volume requirement of the alicyclic ring increased. This tendency was echoed in experiments where the compounds were used as antagonists of the contractile effect of 5-HT. From the structure-activity study, the N-1-isopropyl homologue of 6a, compound 6b, emerged as the ligand with the highest affinity for rat 5-HT2A receptors (pA2 = 8.74). For cycloalkanecarboxylic esters derived from dihydrolysergol-I and elymoclavine, no clear structure-affinity relationship could be deduced, although those compounds that had smaller cycloalkyl rings in the acyl portion and an isopropyl substituent at N-1 showed the highest 5-HT2A receptor affinity. On the other hand, cycloalkanecarboxylic esters derived from lysergol, dihydrolysergol-I, and elymoclavine displayed low or marginal affinity at alpha1-adrenoceptors. A further aim of the study was to examine to what extent the complete removal of the acyl portion of the esters would affect 5-HT2A receptor affinity. The parent alcohols of the three series of N-1-isopropyl homologues, 1-isopropyllysergol (1b), 1-isopropyldihydrolysergol-I (2b), and 1-isopropylelymoclavine (3b), displayed higher affinity for 5-HT2A receptors (pA2 = 9.15, 8.50, 9.14) than the corresponding esters. Compounds 1b-3b had no contractile effects by themselves and displayed low affinity at guinea-pig 5-HT1B receptors and rat alpha1-adrenoceptors. The high affinity for rat 5-HT2A receptors was retained when clavines even more simple in structure than 1b-3b, compounds 4b and 5b, were examined as 5-HT2A receptor antagonists. The nanomolar antagonist activity of simple clavines (1b-5b) in the rat suggests that the indolo[4,3-fg]quinoline system of the ergolines is the molecular fragment that is responsible for 5-HT2A receptor affinity, and not the substituent at position C-8.

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.

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.