Modulation of human 5-hydroxytryptamine type 3AB receptors by volatile anesthetics and n-alcohols

Functional 5-hydroxytryptamine type 3 (5-HT3) receptors can be formed by 5-HT3A subunits alone or in combination with the 5-HT3B subunit, but only the 5-HT3A receptor has been previously studied with respect to the modulation by volatile anesthetics and n-alcohols. Using two-electrode voltage-clamp, we show for the first time the modulation of heteromeric human (h)5-HT3AB receptors, expressed in Xenopus oocytes, by a series of n-alcohols and halogenated volatile anesthetics. At twice their anesthetic concentration, compounds having a molecular volume of less than 110 A3 enhanced submaximal 5-HT-evoked current. Compounds larger than 110 A3 inhibited submaximal 5-HT-evoked current. In experiments examining 5-HT concentration-response relationships, chloroform and butanol caused a slight decrease in the 5-HT EC50. Sevoflurane and octanol inhibited 5-HT-evoked current at all 5-HT concentrations tested but had no effect upon the 5-HT EC50. Compared with previous data on homomeric h5-HT3A receptors, the presence of the h5-HT3B subunit reduces the enhancement of h5-HT3 receptors by smaller halogenated volatile anesthetics and n-alcohols. In summary, these results suggest that heteromeric h5-HT3AB receptors are modulated by halogenated volatile anesthetics at clinically relevant concentrations, in addition to n-alcohols, suggesting that these receptors may be another physiological target for these compounds. The modulation is dependent upon the molecular volume of the compound, further supporting the concept of an anesthetic binding pocket of limited volume common on other Cys-loop ligand-gated ion channels. Incorporation of the 5-HT3B subunit alters either the anesthetic binding site or the allosteric interactions between anesthetic binding and channel opening.     

Electrocardiographic and cardiovascular effects of the 5-hydroxytryptamine3 receptor antagonists

OBJECTIVE: To review the electrocardiographic (ECG) and cardiovascular effects of 5-hydroxytryptamine(3) (5-HT(3)) receptor antagonists preclinically, in healthy volunteers, and in patients undergoing chemotherapy or surgery. DATA SOURCES: A MEDLINE search was performed of clinical trials and preclinical data published between 1963 and December 2002 assessing the ECG and cardiovascular effects of 5-HT(3) receptor antagonists, supplemented with reviews and secondary sources. STUDY SELECTION AND DATA EXTRACTION: All of the articles identified were evaluated and all information deemed relevant was included in this review. DATA SYNTHESIS: There are no clinically relevant differences in efficacy and safety among the available 5-HT(3) receptor antagonists for prevention and treatment of chemotherapy-induced and postoperative nausea and vomiting. As a class, they have well-defined electrophysiologic activity. Changes in ECG parameters (PR, QRS, QT, QTc, JT intervals) are small, reversible, clinically insignificant, and independent of the patient population studied, and patients are asymptomatic during these changes. ECG changes are most prominent 1-2 hours after a dose of dolasetron, ondansetron, and granisetron and return to baseline within 24 hours. Clinically important adverse cardiovascular events associated with these changes are rare. No serious cardiac events (including torsade de pointes) arising from ECG interval changes have been attributed to 5-HT(3) receptor antagonist use. CONCLUSIONS: Clinical data demonstrate that ECG interval changes are a class effect of the 5-HT(3) receptor antagonists. Theoretical concern regarding cardiovascular adverse events with these agents is not supported by clinical experience. The significant benefits of these agents outweigh the theoretical small risk of meaningful cardiovascular events.     

General anesthetic-induced channel gating enhancement of 5-hydroxytryptamine type 3 receptors depends on receptor subunit composition

5-Hydroxytryptamine (serotonin) (5-HT) type 3 (5-HT(3)) receptors are members of an anesthetic-sensitive superfamily of Cys-loop ligand-gated ion channels that can be formed as homomeric 5-HT(3A) or heteromeric 5-HT(3AB) receptors. When the efficacious agonist 5-HT is used, the inhaled anesthetics halothane and chloroform (at clinically relevant concentrations) significantly reduce the agonist EC(50) for 5-HT(3A) receptors but not for 5-HT(3AB) receptors. In the present study, we used dopamine (DA), a highly inefficacious agonist for 5-HT(3) receptors, to determine whether the difference in sensitivity between 5-HT(3A) and 5-HT(3AB) receptors to the potentiating effects of halothane and chloroform is due to differential modulation of agonist affinity, channel gating, or both. Using the two-electrode voltage-clamp technique with 5-HT(3A) and 5-HT(3AB) receptors expressed in Xenopus oocytes, we found that chloroform and halothane enhanced currents evoked by receptor-saturating concentrations of DA for both receptor subtypes in a concentration-dependent manner but that the magnitude of enhancement was substantially greater for 5-HT(3A) receptors than for 5-HT(3AB) receptors. Isoflurane induced only a small enhancement of currents evoked by receptor-saturating concentrations of DA for 5-HT(3A) receptors and no enhancement for 5-HT(3AB) receptors. For both receptor subtypes, none of the three test anesthetics significantly altered the agonist EC(50) for DA, implying that these anesthetics do not affect agonist binding affinity. Our results show that chloroform, halothane, and (to a much lesser degree) isoflurane enhance channel gating for 5-HT(3A) receptors and that the incorporation of 5-HT(3B) subunits to produce heteromeric 5-HT(3AB) receptors markedly attenuates the ability of these anesthetics to enhance channel gating.     

Varenicline is a potent agonist of the human 5-hydroxytryptamine3 receptor

Varenicline, a widely used and successful smoking cessation agent, acts as a partial agonist at nicotinic acetylcholine receptors. Here, we explore the effects of varenicline at human and mouse 5-Hydroxytryptamine(3) (5-HT(3)) receptors. Application of varenicline to human 5-HT(3) receptors expressed in Xenopus laevis oocytes reveal it is almost a full agonist (R(max) = 80%) with an EC(50) (5.9 μM) 3-fold higher than 5-HT. At mouse 5-HT(3) receptors varenicline is a partial agonist (R(max) = 35%) with an EC(50) (18 μM) 20-fold higher than 5-HT. Displacement of the competitive 5-HT(3) receptor antagonist [(3)H]granisetron reveals similar IC(50) values for varenicline at mouse and human receptors expressed in human embryonic kidney 293 cells, although studies in these cells using a membrane potential-sensitive dye show that again varenicline is a 4- or 35-fold less potent agonist than 5-HT in human and mouse receptors, respectively. Thus the data suggest that the efficacy, but not the affinity, of varenicline is greater at human 5-HT(3) receptors compared with mouse. Docking studies provide a possible explanation for this difference, because they suggest distinct orientations of the ligand in the mouse versus human 5-HT(3) agonist binding sites. Additional binding selectivity studies in a broad panel of recombinant receptors and enzymes confirmed an interaction with 5-HT(3) receptors but revealed no additional interactions of varenicline. Therefore, activation of human 5-HT(3) receptors may be responsible for some of the side effects that preclude use of higher doses during varenicline treatment.     

Molecular determinants of picrotoxin inhibition of 5-hydroxytryptamine type 3 receptors

Previously, we reported that the GABA(A) receptor antagonist picrotoxin also antagonizes serotonin (5-HT)3 receptors and that its effects are subunit-dependent. Here, we sought to identify amino acids involved in picrotoxin inhibition of 5-HT3 receptors. Mutation of serine to alanine at the transmembrane domain 2 (TM2) 2' position did not affect picrotoxin (PTX) sensitivity in murine 5-HT3A receptors. However, mutation of the 6' TM2 threonine to phenylalanine dramatically reduced PTX sensitivity. Mutation of 6' asparagine to threonine in the 5-HT3B subunit enhanced PTX sensitivity in heteromeric 5-HT3A/3B receptors. Introduction of serine (native to the human 3B subunit) at the 6' position also increased PTX sensitivity, suggesting a species-specific effect. Mutation of the 7' leucine to threonine in 5-HT3A receptors increased PTX sensitivity roughly 10-fold, comparable with that observed in GABA(A) receptors, and also conferred distinct gating kinetics. The equivalent mutation in the 3B subunit (i.e., 7' valine to threonine) had no impact on PTX sensitivity in 5-HT3A/3B receptors. Interestingly, [3H]ethynylbicycloorthobenzoate ([3H]EBOB), a high-affinity ligand to the convulsant site in GABA(A) receptors, did not exhibit specific binding in 5-HT3A receptors. The structurally related compound, tert-butylbicyclophosphorothionate (TBPS), which potently inhibits GABA(A) receptors, did not inhibit 5-HT3 currents. Our results indicate that the TM2 6' residue is a common determinant of PTX inhibition of both 5-HT3 and GABA(A) receptors and demonstrate a role of the 7' residue in PTX inhibition. However, lack of effects of EBOB and TBPS in 5-HT3A receptors suggests that the functional domains in the two receptors are not equivalent and underscores the complexity of PTX modulation of LGICs.     

Emesis and defecations induced by the 5-hydroxytryptamine (5-HT3) receptor antagonist zacopride in the ferret

Three antiemetic compounds (zacopride, batanopride, granisetron [BRL43694]) were evaluated for the production of gastrointestinal side effects in the conscious ferret after i.v. or p.o. administration. Zacopride evoked multiple emetic and defecatory responses at clinically relevant doses (0.003-0.3 mg/kg) and in a dose-dependent manner. The oral route was the more potent one for eliciting emesis (ED50 0.033 mg/kg). At 0.3 mg/kg p.o., zacopride reliably evoked an 80 to 100% incidence of emesis and a 40 to 80% incidence of defecation. In contrast, batanopride and BRL43694 i.v. evoked a small (10%) incidence of these side effects, but only at 0.1 to 10 mg/kg doses. When given p.o. (0.00003-10 mg/kg), these latter compounds never evoked emesis and significantly reduced (P less than .05) the incidence of defecation below that of vehicle. Responses to zacopride (0.3 mg/kg p.o.) were challenged by i.p. pretreatment with the 5-hydroxytryptamine receptor agonist 2-methyl serotonin, the 5-hydroxytryptamine receptor antagonist BRL43694, the quaternary atropine derivative glycopyrrolate, the dopamine receptor antagonist domperidone or selective abdominal vagotomies. All compounds and either bilateral or dorsal vagotomy significantly reduced the incidence of emesis, but did not abolish it. Latency to first emesis was delayed by BRL43694, domperidone or dorsal vagotomy. The data suggest that the emetic response to p.o. zacopride is mediated in part by 5-hydroxytryptamine receptors residing on either enteric neurons or vagal afferents. However, the underlying pharmacology of the response may also include activation of cholinergic and dopaminergic pathways.     

Pharmacological characterization of a neuronal receptor for 5-hydroxytryptamine in guinea pig ileum with properties similar to the 5-hydroxytryptamine receptor

Experiments were undertaken to characterize pharmacologically a neuronal receptor to 5-HT in guinea pig ileum. Segments of longitudinal muscle myenteric plexus preparations were treated with phenoxybenzamine and exposed to submaximal electrical field stimulation to evoke the cholinergically mediated "twitch" response. The ability of 5-HT to enhance the submaximal twitch response was investigated. Results using several antagonists (metergoline, spiperone, cyanopindolol, N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide, N-hexanoyl-5-hydroxytryptophyl-5-hydroxytryptophan amide, ICS 205-930, GR 38032F, MDL 72222 and cocaine) indicate that 5-HT (3 X 10(-10) to 1 x 10(-7) M) agonizes a novel 5-HT receptor site distinct from the 5-HT1, 5-HT2, 5-HT3 and 5-HT1P subtypes as well as the M receptor. The receptor site is located neuronally and is characterized positively by a low affinity for ICS 205-930 (pA2 = 6.5 vs. 5-HT) and by the following order of agonist potency: 5-HT greater than 5-methoxytryptamine greater than BRL 24924 greater than alpha-methyl-5-hydroxytryptamine greater than zacopride = cisapride = 5-carboxamidotryptamine. Agonist-independent pA2 estimates for ICS 205-930 (6.3-6.6) suggest a single site of agonism. 2-Methyl-5-hydroxytryptamine and 5-hydroxyindalpine were inactive at 1 x 10(-5) M either as agonists or antagonists. Thus, the receptor site exhibits a pharmacological profile similar to that characterizing the recently described 5-HT4 [corrected] receptor. Unlike Gaddum's M receptor, which equates with the 5-HT3 [corrected] receptor, the putative 5-HT4 [corrected] receptor site exhibits a higher sensitivity to agonism by 5-HT and is resistant to antagonism by cocaine.     

Effects of 5-hydroxytryptamine and 5-hydroxytryptamine receptor agonists on ion transport across mammalian airway epithelia.

1. 5-Hydroxytryptamine has been suggested as a candidate for an endogenous inhibitor of airway sodium transport. Amiloride, an inhibitor of epithelial sodium channels, has therapeutic potential in disorders of airway ion transport such as cystic fibrosis, but its duration of action in vivo is short. 5-Hydroxytryptamine and related compounds have been studied to investigate whether any might be a useful alternative to amiloride for clinical use, and to further assess the possible physiological role of 5-hydroxytryptamine in the regulation of airway ion transport. 2. Sheep tracheal epithelium was mounted in Ussing chambers under short-circuit conditions. Mucosal application of 5-hydroxytryptamine resulted in an immediate, reversible, concentration-related decrease in the short-circuit current, maximal with 38% inhibition of the short-circuit current at 25 mmol/l. This response was completely inhibited by pretreatment of tissues with mucosal amiloride (100 mumol/l). These features are consistent with a direct effect of 5-hydroxytryptamine on amiloride-sensitive sodium channels. Similar results were obtained in a limited number of studies using human bronchial epithelium. 3. The effects of mucosal addition of a range of 5-hydroxytryptamine agonists were studied to determine if any was a more potent blocker of amiloride-sensitive sodium transport than 5-hydroxytryptamine. The 5-HT3 agonist 2-methyl-5-hydroxytryptamine had no effect on the short-circuit current at concentrations of up to 5 mmol/l. The 5-HT1D agonist sumatriptan had no effect at concentrations below 5 mmol/l and at 5 mmol/l had only a transient effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ligand dependency of 5-hydroxytryptamine 2C receptor internalization

Agonist-induced internalization of G protein-coupled receptors (GPCRs) is a well characterized phenomenon believed to contribute to receptor desensitization. The 5-hydroxytryptamine (5-HT)2C subtype of serotonin receptor is a GPCR that we have shown to internalize upon agonist incubation. In this study, we have examined the effects of 5-HT2C receptor agonists serotonin, Ro 60-0175 [(S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine], and WAY-161503 [(4aR)-8,9-dichloro-2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5(6H)-one]; partial agonists mCPP [1-(m-chlorophenyl)piperazine] and DOI [(+)-1-(2,5-dimethoxy-4-iodophenyl)-2-amino-propane]; inverse agonists SB-206553 [N-3-pyridinyl-3,5-dihydro-5-methylbenzo(1,2-b:4,5-b')dipyrrole-1(2H)carboxamide] and mianserin; and neutral antagonists SB-242084 [6-chloro-5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-indoline] and 5-methoxygramine on the internalization of a C-terminal green fluorescent protein (GFP)-tagged 5-HT2C receptor (VSV isoform) expressed in transiently transfected human embryonic kidney cells. We detected internalization with an automated, cell-based fluorescence-imaging system (Arrayscan) and monitored function with intracellular Ca2+ measurements (flourometric imaging plate reader). The 5-HT2C-GFP construct exhibited appropriate pharmacology, and we observed that although all three agonists resulted in similar magnitudes of dose-dependent internalization, the partial agonists resulted in approximately 50% less internalization, and the inverse agonists and neutral antagonists failed to induce internalization. These results were confirmed by confocal microscopy. They demonstrate that the 5-HT2C receptor is internalized by incubation with agonists and partial agonists but not with inverse agonists or neutral antagonists.     

Differential agonist-mediated internalization of the human 5-hydroxytryptamine 7 receptor isoforms

The human 5-hydroxytryptamine 7 (5-HT(7)) serotonin receptor is a class A G-protein coupled receptor that has three isoforms, 5-HT(7(a)), 5-HT(7(b)), and 5-HT(7(d)), which are produced by alternative splicing. The 5-HT(7) receptors are expressed in discrete areas of the brain and in both vascular and gastrointestinal smooth muscle. Central nervous system 5-HT(7) receptors may play a role in mood and sleep disorders. 5-HT(7) receptors show high affinity for a number of antidepressants and typical and atypical antipsychotics. We report here that the human 5-HT(7(d)) isoform expressed in human embryonic kidney (HEK) 293 cells exhibits a pattern of receptor trafficking in response to agonist that differ from 5-HT(7(a)) or 5-HT(7(b)) isoforms. We employed a modification of a live cell-labeling technique to demonstrate that surface 5-HT(7(d)) receptors are constitutively internalized in the absence of agonist. This is in contrast to 5-HT(7(a)) and 5-HT(7(b)) isoforms, which do not show this profound agonist-independent internalization. Indeed, the 5-HT(7(d)) isoform displays this internalization in the presence of a 5-HT(7) -specific antagonist. In addition, the human 5-HT(7) isoform shows a diminished efficacy in stimulation of cAMP-responsive reporter gene activity in transfected cells compared with 5-HT(7(a)) or 5-HT(7(b)) receptors expressed at comparable levels. Thus, the carboxy-terminal tail of 5-HT(7(d)), which is the longest among known human 5-HT(7) isoforms, may contain a motif that interacts with cellular transport mechanisms that is distinct from 5-HT(7(a)) and 5-HT(7(b)).     

Selective 5-hydroxytryptamine type 3 receptor antagonism with ondansetron as treatment for diarrhea-predominant irritable bowel syndrome: a pilot study.

Serotoninergic innervation may contribute to the control of colonic motility and to visceral sensation from the large bowel. Indeed, ondansetron hydrochloride, a selective 5-hydroxytryptamine type 3 receptor antagonist, has been shown to slow colonic transit in healthy volunteers. Thus, we wished to determine whether 5-hydroxytryptamine type 3 receptor blockade slows colonic and small bowel transit in patients with diarrhea-predominant irritable bowel syndrome (IBS) and whether symptoms would be ameliorated with drug therapy. Of 14 patients with well-established IBS who entered a randomized, double-blind, placebo-controlled crossover pilot trial of 4 weeks of treatment with ondansetron, 16 mg three times daily, 11 completed the study. A minimal "washout period" of 4 weeks (median, 7 weeks) separated the two phases of the trial because patients were required to have similar symptoms before both periods of the study. Colonic transit tended to be longer during drug therapy than during the placebo trial, but this difference was not significant. Small intestinal transit and orocecal transit were unchanged by the drug. The integrated and peak postprandial increases in neurotensin, peptide YY, and human pancreatic polypeptide in serum were not significantly different in the drug and placebo periods. After treatment with ondansetron, stool consistency improved significantly; however, stool frequency, stool weight, abdominal pain, and the symptom criteria for IBS were not significantly altered by the drug. The results of this pilot study suggest that the motor effects expected with 5-hydroxytryptamine type 3 receptor blockade (namely, slowed colonic transit) may be diminished in some patients with IBS. The subjective improvement in stool consistency may reflect changes in the perception of defecation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inward currents in neurons from newborn guinea pig intestine: mediation by 5-hydroxytryptamine type 3 receptors.

The whole-cell patch-clamp technique was used to analyze the effects of 5-hydroxytryptamine (5-HT) and alosetron on cultured myenteric neurons from newborn guinea pigs. All neurons responded to 5-HT (EC(50) approximately 38.7 microM) with a concentration-dependent inward current (reversal potential = 7.1 +/- 1.7 mV) with a short latency and rapid decay. Because the 5-HT-induced inward current was mimicked by 2-methyl-5-hydroxytryptamine (50 microM) and blocked by ondansetron (5.0 microM) and MDL 72222 (0.05 microM), it was 5-HT(3)-mediated. Alosetron blocked (IC(50) approximately 0.05 microM; Hill coefficient approximately 1.24) the 5-HT- and 2-methyl-5-hydroxytryptamine-induced inward currents. This effect was independent of membrane potential and was not seen when alosetron was delivered to the inside of cells. Alosetron-sensitive sites are, thus, accessible only on the ectodomain of the plasmalemma. The effect of alosetron was reversible, but not surmountable. Although nicotine (100 microM) mimicked the 5-HT-induced inward current, the response was antagonized by hexamethonium (100 microM), but not by alosetron, implying its potential to be a selective 5-HT(3) antagonist. Hexamethonium did not affect responses to 5-HT. Most neurons in the cultures were 5-HT-immunoreactive and immunostained with an antibody raised against 5-HT(3) receptors. The 5-HT-selective uptake inhibitor, fluoxetine (30 microM), gradually reduced the amplitude of the current induced by 5-HT; the residual response was abolished by alosetron (0.2 microM). The effect of fluoxetine could have been caused by either the desensitization of 5-HT(3) receptors or by a nonspecific 5-HT(3) antagonistic effect of fluoxetine. It is concluded that alosetron is a potent and noncompetitive 5-HT(3) antagonist on myenteric neurons.     

5-hydroxytryptamine2A receptor inverse agonists as antipsychotics

We have used a cell-based functional assay to define the pharmacological profiles of a wide range of central nervous system active compounds as agonists, competitive antagonists, and inverse agonists at almost all known monoaminergic G-protein-coupled receptor (GPCR) subtypes. Detailed profiling of 40 antipsychotics confirmed that as expected, most of these agents are potent competitive antagonists of the dopamine D2 receptor. Surprisingly, this analysis also revealed that most are potent and fully efficacious 5-hydroxytryptamine (5-HT)2A receptor inverse agonists. No other molecular property was shared as universally by this class of compounds. Furthermore, comparisons of receptor potencies revealed that antipsychotics with the highest extrapyramidal side effects (EPS) liability are significantly more potent at D2 receptors, the EPS-sparing atypical agents had relatively higher potencies at 5-HT2A receptors, while three were significantly more potent at 5-HT2A receptors. Functional high-throughput screening of a diverse chemical library identified 530 ligands with inverse agonist activity at 5-HT2A receptors, including several series of compounds related to known antipsychotics, as well as a number of novel chemistries. An analog of one of the novel chemical series, AC-90179, was pharmacologically profiled against the remaining monoaminergic GPCRs and found to be a highly selective 5-HT2A receptor inverse agonist. The behavioral pharmacology of AC-90179 is characteristic of an atypical antipsychotic agent.     

Effects of 5-hydroxytryptamine 3 receptor antagonists on gastrointestinal motor activity in conscious dogs.

We studied the effect of 5-hydroxytryptamine 3 (5-HT3) receptor antagonists on gastrointestinal (GI) motor activity in conscious dogs with force transducers implanted chronically. During the digestive state, GR38032F, BRL 43694 and ICS 205-930 did not affect GI motor activity at all at doses up to 1 mg/kg, whereas BRL 24924 at 1.0 mg/kg i.v. significantly stimulated GI motor activity from the stomach to the colon. When GR38032F, BRL 43694 or ICS 205-930 was given i.v. at doses of 0.1 to 1.0 mg/kg during the phase I period, no direct effect was observed on GI motor activity, but the treatment inhibited the occurrence of the subsequent phase III activity in the stomach completely and inhibited it partially in the duodenum without affecting the plasma motilin concentration. GR38032F, BRL 43694 and ICS 205-930 did not influence the caudal migration of phase III activity in the small intestine. In contrast, BRL 24924 at doses of 0.1 to 1.0 mg/kg i.v. significantly stimulated GI motor activity during the phase I period. On the other hand, when GR38032F, BRL 43694 or ICS 205-930 was given during natural or motilin-induced phase III activity, the total contractions in the stomach and partial contractions in the duodenum were inhibited but the caudal propagation of phase III activity in the small intestine was not affected.(ABSTRACT TRUNCATED AT 250 WORDS)     

Toward selective drug development for the human 5-hydroxytryptamine 1E receptor: a comparison of 5-hydroxytryptamine 1E and 1F receptor structure-affinity relationships

The 5-hydroxytryptamine (5-HT) 1E receptor is highly expressed in the human frontal cortex and hippocampus, and this distribution suggests the function of 5-HT(1E) receptors might be linked to memory. To test this hypothesis, behavioral experiments are needed. Because rats and mice lack a 5-HT(1E) receptor gene, knockout strategies cannot be used to elucidate this receptor's functions. Thus, selective pharmacological tools must be developed. The tryptamine-related agonist BRL54443 [5-hydroxy-3-(1-methylpiperidin-4-yl)-1H-indole] is one of the few agents that binds 5-HT(1E) receptors with high affinity and some selectively; unfortunately, it binds equally well to 5-HT(1F) receptors (K(i) ≈ 1 nM). The differences between tryptamine binding requirements of these two receptor populations have never been extensively explored; this must be done to guide the design of analogs with greater selectivity for 5-HT(1E) receptors versus 5-HT(1F) receptors. Previously, we determined the receptor binding affinities of a large series of tryptamine analogs at the 5-HT(1E) receptor; we now examine the affinities of this same series of compounds at 5-HT(1F) receptors. The affinities of these compounds at 5-HT(1E) and 5-HT(1F) receptors were found to be highly correlated (r = 0.81). All high-affinity compounds were full agonists at both receptor populations. We identified 5-N-butyryloxy-N,N-dimethyltryptamine as a novel 5-HT(1F) receptor agonist with >60-fold selectivity versus 5-HT(1E) receptors. There is significant overlap between 5-HT(1E) and 5-HT(1F) receptor orthosteric binding properties; thus, identification of 5-HT(1E)-selective orthosteric ligands will be difficult. The insights generated from this study will inform future drug development and molecular modeling studies for both 5-HT(1E) and 5-HT(1F) receptors.     

Antagonists of the type 3 serotonin receptor (5 -HT3) in IBS

Irritable bowel syndrome (IBS) is a common chronic gastrointestinal (GI) disorder, but its pathophysiology remains unknown. 5-hydroxytryptamine (5-HT, serotonin) is an important neurotransmitter involved in the brain-gut connection. Alosetron, a 5-HT3 receptor antagonist, has been demonstrated in randomized, placebo-controlled trials (RCT) to be effective in diarrhea-predominant IBS(IBS-D). Constipation is the most common adverse event. Alosetron improved abdominal pain and discomfort and stool consistency in both female and male patients, but it did not improve other symptoms (sense of urgency, stool frequency and bloating) in male patients. Although less is known about the gender differences in therapeutic benefit, a new 5-HT3 antagonist, cilansetron, has demonstrated effectiveness in male and female IBS-D patients and is currently under clinical trials.     

Regulation of 5-hydroxytryptamine1A receptor function in rat hippocampus by short- and long-term administration of 5-hydroxytryptamine1A agonist and antidepressants.

Single s.c. injections of the 5-hydroxytryptamine (5-HT)1A receptor agonists buspirone at 4 mg/kg, 8-hydroxy-2-(di-n-propylamino)tetralin at 1 or 4 mg/kg or ipsapirone at 10 mg/kg did not affect 5-HT inhibition of forskolin-stimulated adenylate cyclase activity in rat hippocampal membranes. However, a single injection of buspirone at 8 mg/kg, and daily injections of each of the agonists for 8 days, resulted in a reduction in the degree of enzyme inhibition by 5-HT. Chronic administration of the antidepressants fluoxetine, zimelidine and maprotiline by i.p. injections at 15 mg/kg for 3 weeks also resulted in a decreased degree of enzyme inhibition. Chronic iprindole at the same dose had no effect. It is concluded that the antidepressant-like properties of 5-HT1A receptor agonists may be mediated partly by a postsynaptic action at the level of serotonergic second messenger transduction in the hippocampus.     

Unsurmountable antagonism to 5-hydroxytryptamine in rat kidney results from pseudoirreversible inhibition rather than multiple receptors or allosteric receptor modulation

Experiments were conducted in the isolated perfused rat kidney to determine the mechanism of unsurmountable antagonism by metergoline toward the vasoconstrictor response to 5-hydroxytryptamine (5-HT). Three possible mechanisms were investigated: 1) multiple receptors for 5-HT; 2) pseudoirreversible inhibition; and 3) allosteric modulation of 5-HT receptors. Metergoline and six other 5-HT antagonists acted as potent unsurmountable antagonists, only (-)-propranolol acted competitively (pA2 = 6.5). Multiple receptors for 5-HT were ruled out as the mechanism of unsurmountable antagonism because 5-HT and four 5-HT analogs gave similar pA2 values with (-)-propranolol. In addition, the KA value for 5-HT (185 nM) and the order and relativity of agonist potency suggest the presence of a single 5-HT2 receptor. Pseudoirreversible inhibition, over allosteric modulation of the 5-HT2 receptor, is suggested from experiments in which reversal of metergoline-induced inhibition by (-)-propranolol (studied in the presence of metergoline) was matched, in magnitude, by wash-out of metergoline. (-)-Propranolol is suggested to mimic wash-out by occupying and protecting receptors from which metergoline has dissociated. This is accomplished by virtue of its faster kinetics, on-off the 5-HT receptor, allowing restoration of 5-HT responses via a competitive interaction. Pseudoirreversible antagonism, over allosteric receptor modulation, offers an explanation that does not involve the postulation of allosteric effector sites or changes in the state of the 5-HT2 receptor.