Agonists and antagonists bind to an A-A interface in the heteromeric 5-HT3AB receptor

5-HT3 Receptors on Mitochondria Influence Mitochondrial Function

The 5-hydroxytryptamine 3 (5-HT₃) receptor belongs to the pentameric ligand-gated cation channel superfamily. Humans have five different 5-HT₃ receptor subunits: A to E. The 5-HT₃ receptors are located on the cell membrane, but a previous study suggested that mitochondria could also contain A subunits. In this article, we explored the distribution of 5-HT₃ receptor subunits in intracellular and cell-free mitochondria. Organelle prediction software supported the localization of the A and E subunits on the inner membrane of the mitochondria. We transiently transfected HEK293T cells that do not natively express the 5-HT₃ receptor with an epitope and fluorescent protein-tagged 5HT3A and 5HT3E subunits. Fluorescence microscopy and cell fractionation indicated that both subunits, A and E, localized to the mitochondria, while transmission electron microscopy revealed the location of the subunits on the mitochondrial inner membrane, where they could form heteromeric complexes. Cell-free mitochondria isolated from cell culture media colocalized with the fluorescent signal for A subunits. The presence of A and E subunits influenced changes in the membrane potential and mitochondrial oxygen consumption rates upon exposure to serotonin; this was inhibited by pre-treatment with ondansetron. Therefore, it is likely that the 5-HT₃ receptors present on mitochondria directly impact mitochondrial function and that this may have therapeutic implications.

Phylogenetic analyses of 5-hydroxytryptamine 3 (5-HT3) receptors in Metazoa

The 5-hydroxytrptamine 3 (5-HT3) receptor is a member of the 'Cys-loop' family and the only pentameric ligand gated ion channel among the serotonin receptors. 5-HT3 receptors play an important role in controlling growth, development, and behaviour in animals. Several 5-HT3 receptor antagonists are used to treat diseases (e.g., irritable bowel syndrome, nausea and emesis). Humans express five different subunits (A-E) enabling a variety of heteromeric receptors to form but all contain 5HT3A subunits. However, the information available about the 5-HT3 receptor subunit occurrence among the metazoan lineages is minimal. In the present article we searched for 5-HT3 receptor subunit homologs from different phyla in Metazoa. We identified more than 1000 5-HT3 receptor subunits in Metazoa in different phyla and undertook simultaneous phylogenetic analysis of 526 5HT3A, 358 5HT3B, 239 5HT3C, 70 5HT3D, and 173 5HT3E sequences. 5-HT3 receptor subunits were present in species belonging to 11 phyla: Annelida, Arthropoda, Chordata, Cnidaria, Echinodermata, Mollusca, Nematoda, Orthonectida, Platyhelminthes, Rotifera and Tardigrada. All subunits were most often identified in Chordata phylum which was strongly represented in searches. Using multiple sequence alignment, we investigated variations in the ligand binding region of the 5HT3A subunit protein sequences in the metazoan lineage. Several critical amino acid residues important for ligand binding (common structural features) are commonly present in species from Nematoda and Platyhelminth gut parasites through to Chordata. Collectively, this better understanding of the 5-HT3 receptor evolutionary patterns raises possibilities of future pharmacological challenges facing Metazoa including effects on parasitic and other species in ecosystems that contain 5-HT3 receptor ligands.

Molecular Mingling: Multimodal Predictions of Ligand Promiscuity in Pentameric Ligand-Gated Ion Channels

Background: Human pentameric ligand-gated ion channels (pLGICs) comprise nicotinic acetylcholine receptors (nAChRs), 5-hydroxytryptamine type 3 receptors (5-HT3Rs), zinc-activated channels (ZAC), γ-aminobutyric acid type A receptors (GABAARs) and glycine receptors (GlyRs). They are recognized therapeutic targets of some of the most prescribed drugs like general anesthetics, anxiolytics, smoking cessation aids, antiemetics and many more. Currently, approximately 100 experimental structures of pLGICs with ligands bound exist in the protein data bank (PDB). These atomic-level 3D structures enable the generation of a comprehensive binding site inventory for the superfamily and the in silico prediction of binding site properties. Methods: A panel of high throughput in silico methods including pharmacophore screening, conformation analysis and descriptor calculation was applied to a selection of allosteric binding sites for which in vitro screens are lacking. Variant abundance near binding site forming regions and computational docking complement the approach. Results: The structural data reflects known and novel binding sites, some of which may be unique to individual receptors, while others are broadly conserved. The membrane spanning domain, comprising four highly conserved segments, contains ligand interaction sites for which in vitro assays suitable for high throughput screenings are critically lacking. This is also the case for structurally more variable novel sites in the extracellular domain. Our computational results suggest that the phytocannabinoid Δ9-tetrahydrocannabinol (Δ9-THC) can utilize multiple pockets which are likely to exist on most superfamily members. Conclusion: With this study, we explore the potential for polypharmacology among pLGICs. Our data suggest that ligands can display two forms of promiscuity to an extent greater than what has been realized: 1) Ligands can interact with homologous sites in many members of the superfamily, which bears toxicological relevance. 2) Multiple pockets in distinct localizations of individual receptor subtypes share common ligands, which counteracts efforts to develop selective agents. Moreover, conformational states need to be considered for in silico drug screening, as certain binding sites display considerable flexibility. In total, this work contributes to a better understanding of polypharmacology across pLGICs and provides a basis for improved structure guided in silico drug development and drug derisking.

Tapping into 5-HT3 Receptors to Modify Metabolic and Immune Responses

5-hydroxytryptamine type 3 (5-HT₃) receptors are ligand gated ion channels, which clearly distinguish their mode of action from the other G-protein coupled 5-HT or serotonin receptors. 5-HT₃ receptors are well established targets for emesis and gastrointestinal mobility and are used as adjunct targets in treating schizophrenia. However, the distribution of these receptors is wider than the nervous system and there is potential that these additional sites can be targeted to modulate inflammatory and/or metabolic conditions. Recent progress in structural biology and pharmacology of 5-HT₃ receptors have provided profound insights into mechanisms of their action. These advances, combined with insights into clinical relevance of mutations in genes encoding 5-HT₃ subunits and increasing understanding of their implications in patient's predisposition to diseases and response to the treatment, open new avenues for personalized precision medicine. In this review, we recap on the current status of 5-HT₃ receptor-based therapies using a biochemical and physiological perspective. We assess the potential for targeting 5-HT₃ receptors in conditions involving metabolic or inflammatory disorders based on recent findings, underscoring the challenges and limitations of this approach.

A Single Mutation in the Outer Lipid-Facing Helix of a Pentameric Ligand-Gated Ion Channel Affects Channel Function Through a Radially-Propagating Mechanism

Pentameric ligand-gated ion channels (pLGICs) mediate fast synaptic transmission and are crucial drug targets. Their gating mechanism is triggered by ligand binding in the extracellular domain that culminates in the opening of a hydrophobic gate in the transmembrane domain. This domain is made of four α-helices (M1 to M4). Recently the outer lipid-facing helix (M4) has been shown to be key to receptor function, however its role in channel opening is still poorly understood. It could act through its neighboring helices (M1/M3), or via the M4 tip interacting with the pivotal Cys-loop in the extracellular domain. Mutation of a single M4 tyrosine (Y441) to alanine renders one pLGIC-the 5-HT3A receptor-unable to function despite robust ligand binding. Using Y441A as a proxy for M4 function, we here predict likely paths of Y441 action using molecular dynamics, and test these predictions with functional assays of mutant receptors in HEK cells and Xenopus oocytes using fluorescent membrane potential sensitive dye and two-electrode voltage clamp respectively. We show that Y441 does not act via the M4 tip or Cys-loop, but instead connects radially through M1 to a residue near the ion channel hydrophobic gate on the pore-lining helix M2. This demonstrates the active role of the M4 helix in channel opening.

Differential Regulation of Human Serotonin Receptor Type 3A by Chanoclavine and Ergonovine

Irritable bowel syndrome (IBS) is a chronic disease that causes abdominal pain and an imbalance of defecation patterns due to gastrointestinal dysfunction. The cause of IBS remains unclear, but intestinal-brain axis problems and neurotransmitters have been suggested as factors. In this study, chanoclavine, which has a ring structure similar to 5-hydroxytryptamine (5-HT), showed an interaction with the 5-HT_3A receptor to regulate IBS. Although its derivatives are known to be involved in neurotransmitter receptors, the molecular physiological mechanism of the interaction between chanoclavine and the 5-HT_3A receptor is unknown. Electrophysiological experiments were conducted using a two-electrode voltage-clamp analysis to observe the inhibitory effects of chanoclavine on Xenopus oocytes in which the h5-HT_3A receptor was expressed. The co-application of chanoclavine and 5-HT resulted in concentration-dependent, reversible, voltage-independent, and competitive inhibition. The 5-HT_3A response induced by 5-HT was blocked by chanoclavine with half-maximal inhibitory response concentration (IC₅₀) values of 107.2 µM. Docking studies suggested that chanoclavine was positioned close F130 and N138 in the 5-HT_3A receptor-binding site. The double mutation of F130A and N138A significantly attenuated the interaction of chanoclavine compared to a single mutation or the wild type. These data suggest that F130 and N138 are important sites for ligand binding and activity. Chanoclavine and ergonovine have different effects. Asparagine, the 130th amino acid sequence of the 5-HT_3A receptor, and phenylalanine, the 138th, are important in the role of binding chanoclavine, but ergonovine has no interaction with any amino acid sequence of the 5-HT_3A receptor. The results of the electrophysiological studies and of in silico simulation showed that chanoclavine has the potential to inhibit the hypergastric stimulation of the gut by inhibiting the stimulation of signal transduction through 5-HT_3A receptor stimulation. These findings suggest chanoclavine as a potential antiemetic agent for excessive gut stimulation and offer insight into the mechanisms of 5-HT_3A receptor inhibition.

Structure, Function and Physiology of 5-Hydroxytryptamine Receptors Subtype 3

5-hydroxytryptamine receptor subtype 3 (5-HT₃R) is a pentameric ligand-gated ion channel (pLGIC) involved in neuronal signaling. It is best known for its prominent role in gut-CNS signaling though there is growing interest in its other functions, particularly in modulating non-serotonergic synaptic activity. Recent advances in structural biology have provided mechanistic understanding of 5-HT₃R function and present new opportunities for the field. This chapter gives a broad overview of 5-HT₃R from a physiological and structural perspective and then discusses the specific details of ion permeation, ligand binding and allosteric coupling between these two events. Biochemical evidence is summarized and placed within a physiological context. This perspective underscores the progress that has been made as well as outstanding challenges and opportunities for future 5-HT₃R research.

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.

High-resolution structures of multiple 5-HT3AR-setron complexes reveal a novel mechanism of competitive inhibition

Serotonin receptors (5-HT3AR) play a crucial role in regulating gut movement, and are the principal target of setrons, a class of high-affinity competitive antagonists, used in the management of nausea and vomiting associated with radiation and chemotherapies. Structural insights into setron-binding poses and their inhibitory mechanisms are just beginning to emerge. Here, we present high-resolution cryo-EM structures of full-length 5-HT3AR in complex with palonosetron, ondansetron, and alosetron. Molecular dynamic simulations of these structures embedded in a fully-hydrated lipid environment assessed the stability of ligand-binding poses and drug-target interactions over time. Together with simulation results of apo- and serotonin-bound 5-HT3AR, the study reveals a distinct interaction fingerprint between the various setrons and binding-pocket residues that may underlie their diverse affinities. In addition, varying degrees of conformational change in the setron-5-HT3AR structures, throughout the channel and particularly along the channel activation pathway, suggests a novel mechanism of competitive inhibition.

Evaluation of serotonin receptors (5HTR2A and 5HTR3A) mRNA expression changes in tumor of breast cancer patients

Background: Several studies have proven the pattern of neurotransmitters, especially serotonin, in carcinogenesis and tumor development. Several studies have also shown that changes in serotonin receptors, especially 5HTR2A and 5HTR3A, can play an important role in incidence of cancers. This study was conducted to investigate changes in mRNA expression of 5HTR2A and 5HTR3A receptors in the breast tumor tissue compared to their marginal zone.

Methods: In this study, tissue samples were obtained from 40 female patients with breast cancer. Entire RNA was obtained from the tissues and cDNA synthesis was performed. Finally, real ime PCR technique was performed to investigate the gene expression variation of both 5HTR2A and 5HTR3A. To analyze the results of real time PCR, both ΔΔCt and 2-ΔΔCt equations were used. All statistical analyses were performed using the SPSS 18 software and R-Studio 1.0.136. P values less than 0.05 (p<0.05) and 0.001 (p<0.001) were considered statistically significant.

Results: The results showed increased expression of 5HTR2A and 5HTR3A genes in tumoral tissues of patients with breast cancer compared to their marginal tissues, where the 5HTR2A and 5HTR3A genes expression in tumor tissue was 3.12 and 3.24 times more than that of the marginal zone, respectively.

Conclusion: The results indicated an increase in the mRNA expression of serotonin receptors (5HTR2A and 5HTR3A) in the tumor tissue compared to the marginal zone, which due to the mitogenic nature of these receptors, is likely to induce more proliferation of cancer cells.

Bupropion Inhibits Serotonin Type 3AB Heteromeric Channels at Clinically Relevant Concentrations

Bupropion, a Food and Drug Administration–approved antidepressant and smoking cessation aid, blocks dopamine and norepinephrine reuptake transporters and noncompetitively inhibits nicotinic acetylcholine and serotonin (5-HT) type 3A receptors (5-HT_3ARs). 5-HT₃ receptors are pentameric ligand-gated ion channels that regulate synaptic activity in the central and peripheral nervous system, presynaptically and postsynaptically. In the present study, we examined and compared the effect of bupropion and its active metabolite hydroxybupropion on mouse homomeric 5-HT_3A and heteromeric 5-HT_3AB receptors expressed in Xenopus laevis oocytes using two-electrode voltage clamp experiments. Coapplication of bupropion or hydroxybupropion with 5-HT dose dependently inhibited 5-HT–induced currents in heteromeric 5-HT type 3AB receptors (5-HT_3ABRs) (IC₅₀ = 840 and 526 μM, respectively). The corresponding IC₅₀s for bupropion and hydroxybupropion for homomeric 5-HT_3ARs were 10- and 5-fold lower, respectively (87 and 113 μM). The inhibition of 5-HT_3ARs and 5-HT_3ABRs was non–use dependent and voltage independent, suggesting bupropion is not an open channel blocker. The inhibition by bupropion was reversible and time-dependent. Of note, preincubation with a low concentration of bupropion that mimics therapeutic drug conditions inhibits 5-HT–induced currents in 5-HT_3A and 5-HT_3AB receptors considerably. In summary, we demonstrate that bupropion inhibits heteromeric 5-HT_3ABRs as well as homomeric 5-HT_3ARs. This inhibition occurs at clinically relevant concentrations and may contribute to bupropion’s clinical effects.

Characterization of Residues in the 5-HT3 Receptor M4 Region That Contribute to Function

5-HT₃ receptors are members of the family of pentameric ligand gated ion channels (pLGICs). Each subunit has four transmembrane α-helices (M1-M4), with M4 being most distant from the central pore. Residues in this α-helix interact with adjacent lipids and the neighboring M1 and M3 helices, contributing to both receptor assembly and channel function. This study probes the role of each M4 receptor residue in the 5-HT₃A receptor using mutagenesis and subsequent expression in HEK293 cells, probing functional parameters using fluorescence membrane potential sensitive dye. The data show that only one residue in M4 (Y441) and two flanking residues (D434 and W459) result in nonfunctional receptors when substituted with Ala: D434A and W459A-containing receptors ablate expression, while Y441A-containing receptor do not, suggesting the latter is involved in channel gating. Most other altered residues have wild-type-like properties, which is inconsistent with data from other pLGICs. Substitution of Y441 and W459 with other aromatics restores function, suggesting the π ring is important. Further substitutions indicate interactions of Y441 with D238 in M1, W459 with F144 in the Cys loop, and D434 with R251 in M2, data consistent with recently published structures. These regions are critical for transducing binding into gating, and thus interactions of these residues can explain their importance in the function of the 5-HT₃ receptor. We also conclude that the small number of critical M4 residues compared to related receptors supports the hypothesis that M4 does not behave identically in all pLGICs.

Peripheral 5-HT3 Receptors Are Involved in the Antinociceptive Effect of Bunodosine 391

Bunodosine 391 (BDS 391), a low molecular weight compound isolated from the sea anemone Bunodosoma cangicum, increases the nociceptive threshold and inhibits inflammatory hyperalgesia. Serotonin receptors are involved in those effects. In this study, we have expanded the characterization of the antinociceptive effect of BDS 391 demonstrating that, in rats: (a) the compound inhibits (1.2–12 ng/paw) overt pain, in the formalin test, and mechanical hyperalgesia (0.6–6.0 ng/paw) detected in a model of neuropathic pain; (b) intraplantar administration of ondansetron, a selective 5-HT3 receptor antagonist, blocks the effect of BDS 391, whereas ketanserin, a 5-HT2 receptor antagonist, partially reversed this effect, indicating the involvement of peripheral 5-HT2 and 5-HT3 receptors in BDS 391 antinociception; and (c) in binding assay studies, BDS 391 was not able to displace the selective 5-HT receptor antagonists, suggesting that this compound does not directly bind to these receptors. The effect of biguanide, a selective 5-HT3 receptor agonist, was also evaluated. The agonist inhibited the formalin’s nociceptive response, supporting an antinociceptive role for 5-HT3 receptors. Our study is the first one to show that a non-peptidic low molecular weight compound obtained from a sea anemone is able to induce antinociception and that activation of peripheral 5-HT3 receptors contributes to this effect.

Stimulation of contractions in pregnant human myometrium is associated with 5-HT3 receptors

BACKGROUND

Serotonin (5-HT) is known to play an important role in regulating uterine contractions. However, the specific receptors involved have not been well characterized. We evaluated whether 5-HT3 receptors exist in human myometrium, and their effects on myometrial contractility when stimulated by a 5-HT3 agonist.

METHODS

Four tissue samples taken from patients undergoing hysterectomy (n=2) and elective cesarean delivery (n=2) were used to detect expression of 5-HT3 receptors on the myometrium using western blotting. For isometric tension measurement, another 12 myometrial strips obtained from patients undergoing elective cesarean delivery were randomly divided into a control group (Group CON) and an RS56812 group (Group RG). In increasing doses from 10-7M to 10-4M, RS56812, a 5-HT3 receptor agonist, was used to investigate the contractile effects after bonding to the 5-HT3 receptor, following which the effects of granisetron were assessed. Amplitude, interval and duration of myometrial contractions were recorded.

RESULTS

Proteins with a molecular mass of 55kDa, consistent with 5-HT3 receptors, were detected both on non-pregnant and late-pregnant human uteri. RS56812 increased the contractile amplitude at concentrations of 10-6M, 10-5M and 10-4M, achieving maximum effect at 10-5M. A prolonged contractile interval was detected at the concentration of 10-4M. However, RS56812 showed no significant effect on contraction duration. Granisetron did not reverse the contractile effects induced by RS56812.

CONCLUSION

5-HT3 receptors are expressed on non-pregnant and pregnant uteri. RS56812 enhanced myometrial contractions, but this was not affected by granisetron, the mechanism of which requires further investigation.

Novel mechanism of modulation at a ligand-gated ion channel; action of 5-Cl-indole at the 5-HT3 A receptor

BACKGROUND AND PURPOSE

The 5-HT₃ receptor is a prototypical member of the Cys-loop ligand-gated ion channel (LGIC) superfamily and an established therapeutic target. In addition to activation via the orthosteric site, receptor function can be modulated by allosteric ligands. We have investigated the pharmacological action of Cl-indole upon the 5-HT₃ A receptor and identified that this positive allosteric modulator possesses a novel mechanism of action for LGICs.

EXPERIMENTAL APPROACH

The impact of Cl-indole upon the 5-HT₃ receptor was assessed using single cell electrophysiological recordings and [³ H]-granisetron binding in HEK293 cells stably expressing the 5-HT₃ receptor.

KEY RESULTS

Cl-indole failed to evoke 5-HT₃ A receptor-mediated responses (up to 30 μM) or display affinity for the [³ H]-granisetron binding site. However, in the presence of Cl-indole, termination of 5-HT application revealed tail currents mediated via the 5-HT₃ A receptor that were independent of the preceding 5-HT concentration but were antagonized by the 5-HT₃ receptor antagonist, ondansetron. These tail currents were absent in the 5-HT₃ AB receptor. Furthermore, the presence of 5-HT revealed a concentration-dependent increase in the affinity of Cl-indole for the orthosteric binding site of the human 5-HT₃ A receptor.

CONCLUSIONS AND IMPLICATIONS

Cl-indole acts as both an orthosteric agonist and an allosteric modulator, but the presence of an orthosteric agonist (e.g. 5-HT) is a prerequisite to reveal both actions. Precedent for ago-allosteric action is available, yet the essential additional presence of an orthosteric agonist is now reported for the first time. This widening of the pharmacological mechanisms to modulate LGICs may offer further therapeutic opportunities.

Ion channels gated by acetylcholine and serotonin: structures, biology, and drug discovery

The nicotinic acetylcholine receptors (nAChRs) and the 5-HT3 receptors (5-HT3Rs) are cation-selective members of the pentameric ligand-gated ion channels (pLGICs), which are oligomeric protein assemblies that convert a chemical signal into an ion flux through postsynaptic membrane. They are critical components for synaptic transmission in the nervous system, and their dysfunction contributes to many neurological disorders. The diverse subunit compositions of pLGICs give rise to complex mechanisms of ligand recognition, channel gating, and ion-selective permeability, which have been demonstrated in numerous electrophysiological and molecular biological studies, and unraveled by progress in studying the structural biology of this protein family. In this review, we discuss recent insights into the structural and functional basis of two cation-selective pLGICs, the nAChR and the 5-HT3R, including their subunit compositions, ligand binding, and channel gating mechanisms. We also discuss their relevant pharmacology and drug discovery for treating various neurological disorders. Finally, we review a model of two alternative ion conducting pathways based on the latest 5-HT3A crystal structure.

Allosteric activation of the 5-HT3AB receptor by mCPBG

The 5-HT3AB receptor contains three A and two B subunits in an A-A-B-A-B order. However, serotonin function at the 5-HT3AB receptor has been shown to depend solely on the A-A interface present in the homomeric receptor. Using mutations at sites on both the primary (E122) and complementary (Y146) faces of the B subunit, we demonstrate that meta-chlorophenyl biguanide (mCPBG), a 5-HT3 selective agonist, is capable of binding to and activating the 5-HT3AB receptor at all five subunit interfaces of the heteromer. Further, mCPBG is capable of allosterically modulating the activity of serotonin from these sites. While these five binding sites are similar enough that they conform to a monophasic dose - response relationship, we uncover subtle differences in the heteromeric binding sites. We also find that the A-A interface appears to contribute disproportionately to the efficacy of 5-HT3AB receptor activation.

Characterizing new fluorescent tools for studying 5-HT₃ receptor pharmacology

The pharmacological characterization of ligands depends upon the ability to accurately measure their binding properties. Fluorescence provides an alternative to more traditional approaches such as radioligand binding. Here we describe the binding and spectroscopic properties of eight fluorescent 5-HT3 receptor ligands. These were tested on purified receptors, expressed receptors on live cells, or in vivo. All compounds had nanomolar affinities with fluorescent properties extending from blue to near infra-red emission. A fluorescein-derivative had the highest affinity as measured by fluorescence polarization (FP; 1.14 nM), flow cytometry (FC; 3.23 nM) and radioligand binding (RB; 1.90 nM). Competition binding with unlabeled 5-HT3 receptor agonists (5-HT, mCPBG, quipazine) and antagonists (granisetron, palonosetron, tropisetron) yielded similar affinities in all three assays. When cysteine substitutions were introduced into the 5-HT3 receptor binding site the same changes in binding affinity were seen for both granisetron and the fluorescein-derivative, suggesting that they both adopt orientations that are consistent with co-crystal structures of granisetron with a homologous protein (5HTBP). As expected, in vivo live imaging in anaesthetized mice revealed staining in the abdominal cavity in intestines, but also in salivary glands. The unexpected presence of 5-HT3 receptors in mouse salivary glands was confirmed by Western blots. Overall, these results demonstrate the wide utility of our new high-affinity fluorescently-labeled 5-HT3 receptor probes, ranging from in vitro receptor pharmacology, including FC and FP ligand competition, to live imaging of 5-HT3 expressing tissues.

Discriminating between 5-HT₃A and 5-HT₃AB receptors

The 5-HT3B subunit was first cloned in 1999, and co-expression with the 5-HT3A subunit results in heteromeric 5-HT₃AB receptors that are functionally distinct from homomeric 5-HT₃A receptors. The affinities of competitive ligands at the two receptor subtypes are usually similar, but those of non-competitive antagonists that bind in the pore often differ. A competitive ligand and allosteric modulator that distinguishes 5-HT₃A from 5-HT₃AB receptors has recently been described, and the number of non-competitive antagonists identified with this ability has increased in recent years. In this review, we discuss the differences between 5-HT₃A and 5-HT₃AB receptors and describe the possible sites of action of compounds that can distinguish between them.

The Concise Guide to PHARMACOLOGY 2013/14: ligand-gated ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full.

Ligand-gated ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets.

It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

A single channel mutation alters agonist efficacy at 5-HT3A and 5-HT3AB receptors

BACKGROUND AND PURPOSE

5-HT₃ receptors are composed of 5-HT₃A subunits (homomeric receptors), or combinations of 5-HT₃A and other 5-HT₃ receptor subunits (heteromeric receptors, the best studied of which are 5-HT₃AB receptors). Here we explore the effects of partial agonists at 5-HT₃A and 5-HT₃AB receptors, and the importance of a channel-lining residue in determining the efficacy of activation.

EXPERIMENTAL APPROACH

Wild type and mutant 5-HT₃A and 5-HT₃AB receptors were expressed in Xenopus oocytes and examined using two-electrode voltage-clamp, or expressed in HEK293 cells and examined using [³H]granisetron binding.

KEY RESULTS

Dopamine, quipazine and VUF10166 were partial agonists at wild type 5-HT₃A and 5-HT₃AB receptors, with quipazine and VUF10166 causing a long-lived (>20 min) inhibition of subsequent agonist responses. At 5-HT₃A receptors, mCPBG was a partial agonist, but was a superagonist at 5-HT₃AB receptors, as it produced a response 2.6× greater than that of 5-HT. A T6'S substitution in the 5-HT₃A subunit decreased EC₅₀ and increased R_max of dopamine and quipazine at both homomeric and heteromeric receptors. The greatest changes were seen with VUF10166 at 5-HT₃A_T6'SB receptors, where it became a full agonist (EC₅₀ = 7 nM) with an EC₅₀ 58-fold less than 5-HT (EC₅₀ = 0.4 μM) and no longer caused inhibition of subsequent agonist responses.

CONCLUSIONS AND IMPLICATIONS

These results indicate that a mutation in the pore lining domain in both 5-HT₃A and 5-HT₃AB receptors alters the relative efficacy of a series of agonists, changing some (e.g. quipazine) from apparent antagonists to potent and efficacious agonists.

The 5-HT3AB receptor shows an A3B2 stoichiometry at the plasma membrane

The 5-HT3AB receptor is the best-characterized heteropentameric 5-HT3 receptor. Under conditions of heterologous expression, the 5-HT3AB receptor shows a single functionally resolvable population, suggesting the presence of a unique subunit stoichiometry; however, conflicting previous reports have suggested two different possible stoichiometries. Here we isolate plasma membrane sheets containing assembled receptors from individual HEK293T cells. We then determine the stoichiometry of 5-HT3AB receptors on the plasma membrane by fluorescence methods, employing meCFP- and meYFP-labeled A and B subunits. Over a wide range of cDNA transfection ratios, fluorescence intensity ratios are closest to values that correspond to a subunit ratio of A3B2. Förster resonance energy transfer (family FRET) efficiencies provide minor corrections (3-6%) to the subunit ratios and provide independent support for a predominantly A3B2 stoichiometry on the plasma membrane sheets. Twin FRET efficiencies support these data, also suggesting that the two B subunits are nonadjacent in most of the heteropentamers. The high-frequency variant HTR3B p.Y129S (c.386A>C, rs11767445), linked to psychiatric disease, also forms A3B2 receptors on the plasma membrane. The 5-HT3B Y129S, subunit incorporates in a slightly (11-14%) more efficient manner than the common variant. In general, most of the subunits reside within the cell. In contrast to the findings for the plasma membrane, the relative abundances and FRET characteristics of intracellular subunits depend strongly on the transfection ratio. The straightforward and unambiguous combination of plasma membrane-sheet isolation, fluorescence intensity ratios, and FRET is a generally promising procedure for determining stoichiometry of proteins on the plasma membrane.

Exploring a potential palonosetron allosteric binding site in the 5-HT(3) receptor

Palonosetron (Aloxi) is a potent second generation 5-HT(3) receptor antagonist whose mechanism of action is not yet fully understood. Palonosetron acts at the 5-HT(3) receptor binding site but recent computational studies indicated other possible sites of action in the extracellular domain. To test this hypothesis we mutated a series of residues in the 5-HT3A receptor subunit (Tyr(73), Phe(130), Ser(163), and Asp(165)) and in the 5-HT3B receptor subunit (His(73), Phe(130), Glu(170), and Tyr(143)) that were previously predicted by in silico docking studies to interact with palonosetron. Homomeric (5-HT(3)A) and heteromeric (5-HT(3)AB) receptors were then expressed in HEK293 cells to determine the potency of palonosetron using both fluorimetric and radioligand methods to test function and ligand binding, respectively. The data show that the substitutions have little or no effect on palonosetron inhibition of 5-HT-evoked responses or binding. In contrast, substitutions in the orthosteric binding site abolish palonosetron binding. Overall, the data support a binding site for palonosetron at the classic orthosteric binding pocket between two 5-HT3A receptor subunits but not at allosteric sites previously identified by in silico modelling and docking.

Association, interaction, and replication analysis of genes encoding serotonin transporter and 5-HT3 receptor subunits A and B in alcohol dependence

On the basis of the converging evidence showing regulation of drinking behavior by 5-HT3AB receptors and the serotonin transporter, we hypothesized that the interactive effects of genetic variations in the genes HTR3A, HTR3B, and SLC6A4 confer greater susceptibility to alcohol dependence (AD) than do their effects individually. We examined the associations of AD with 22 SNPs across HTR3A, HTR3B, and two functional variants in SLC6A4 in 500 AD and 280 healthy control individuals of European descent. We found that the alleles of the low-frequency SNPs rs33940208:T in HTR3A and rs2276305:A in HTR3B were inversely and nominally significantly associated with AD with odds ratio (OR) and 95 % confidence interval of 0.212 and 0.073, 0.616 (P = 0.004) and 0.261 and 0.088, 0.777 (P = 0.016), respectively. Further, our gene-by-gene interaction analysis revealed that two four-variant models that differed by only one SNP carried a risk for AD (empirical P < 1 × 10(-6) for prediction accuracy of the two models based on 10(6) permutations). Subsequent analysis of these two interaction models revealed an OR of 2.71 and 2.80, respectively, for AD (P < 0.001) in carriers of genotype combinations 5'-HTTLPR:LL/LS(SLC6A4)-rs1042173:TT/TG(SLC6A4)-rs1176744:AC(HTR3B)-rs3782025:AG(HTR3B) and 5'-HTTLPR:LL/LS(SLC6A4)-rs10160548:GT/TT(HTR3A)-rs1176744:AC(HTR3B)-rs3782025:AG(HTR3B). Combining all five genotypes resulted in an OR of 3.095 (P = 2.0 × 10(-4)) for AD. Inspired by these findings, we conducted the analysis in an independent sample, OZ-ALC-GWAS (N = 6699), obtained from the NIH dbGAP database, which confirmed the findings, not only for all three risk genotype combinations (Z = 4.384, P = 1.0 × 10(-5); Z = 3.155, P = 1.6 × 10(-3); and Z = 3.389, P = 7.0 × 10(-4), respectively), but also protective effects for rs33940208:T (χ (2) = 3.316, P = 0.0686) and rs2276305:A (χ (2) = 7.224, P = 0.007). These findings reveal significant interactive effects among variants in SLC6A4-HTR3A-HTR3B affecting AD. Further studies are needed to confirm these findings and characterize the molecular mechanisms underlying these effects.

Approaching the 5-HT₃ receptor heterogeneity by computational studies of the transmembrane and intracellular domains

5-hydroxytryptamine type-3 receptor (5-HT₃), an important target of many neuroactive drugs, is a cation selective transmembrane pentamer whose functional stoichiometries and subunit arrangements are still debated, due to the extreme complexity of the system. The three dimensional structure of the 5-HT₃R subunits has not been solved so far. Moreover, most of the available structural and functional data is related to the extracellular ligand-binding domain, whereas the transmembrane and the intracellular receptor domains are far less characterised, although they are crucial for receptor function. Here, for the first time, 3D homology models of the transmembrane and the intracellular receptor domains of all the known human 5-HT₃ subunits have been built and assembled into homopentameric (5-HT(3A)R, 5-HT(3B)R, 5-HT(3C)R, 5-HT(3D)R and 5-HT(3E)R) and heteropentameric receptors (5-HT(3AB), 5-HT(3AC), 5-HT(3AD) and 5-HT(3AE)), on the basis of the known three-dimensional structures of the nicotinic-acetylcholine receptor and of the ligand gated channel from Erwinia chrysanthemi. The comparative analyses of sequences, modelled structures, and computed electrostatic properties of the single subunits and of the assembled pentamers shed new light both on the stoichiometric composition and on the physicochemical requirements of the functional receptors. In particular, it emerges that a favourable environment for the crossing of the pore at the transmembrane and intracellular C terminus domain levels by Ca²⁺ ions is granted by the maximum presence of two B subunits in the 5-HT₃ pentamer.

Agonists and antagonists induce different palonosetron dissociation rates in 5-HT₃A and 5-HT₃AB receptors

Palonosetron is a potent 5-HT₃ receptor antagonist with a unique structure and some unusual properties. Here we explore the properties of palonosetron at heterologously expressed 5-HT₃A and 5-HT₃AB receptors. We used receptors expressed in HEK293 cells, and functionally analysed them using a membrane potential sensitive dye in a Flexstation, which revealed IC₅₀s of 0.24 nM and 0.18 nM for 5-HT₃A and 5-HT₃AB receptors respectively. Radioligand binding studies with [³H]palonosetron revealed similar K_ds: 0.34 nM for 5-HT₃A and 0.15 nM for 5-HT₃AB receptors. Kinetic studies showed palonosetron association and dissociation rates were slightly faster in 5-HT₃AB than 5-HT₃A receptors, and for both subtypes dissociation rates were ligand-dependent, with antagonists causing more rapid dissociation than agonists. Similar ligand effects were not observed for [³H]granisetron dissociation studies. These data support previous studies which show palonosetron has actions distinct to other 5-HT₃ receptor antagonists, and the slow rates observed for agonist induced dissociation (t_1/2 > 10 h) could at least partly explain the long duration of palonosetron effects in vivo.

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Palonosetron IC₅₀s and K_ds for 5-HT₃A and 5-HT₃AB receptors in HEK293 cells are similar.

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Palonosetron association and dissociation rates are slower at 5-HT₃A than 5-HT₃AB receptors.

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Agonist-induced palonosetron dissociation rates are slower than those for antagonists in both 5-HT₃A and 5-HT₃AB receptors.

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Agonist- and antagonist-induced granisetron dissociation rates are similar in both 5-HT₃A and 5-HT₃AB receptors.

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Palonosetron and granisetron have distinct actions.

Subunit-dependent inhibition and potentiation of 5-HT3 receptor by the anticancer drug, topotecan

The 5-hydroxytryptamine (serotonin, 5-HT) type 3 (5-HT3) receptor belongs to the superfamily of Cys-loop ligand-gated ion channels, and can be either homopentameric (5-HT3A) or heteropentameric (5-HT3AB) receptor. Several modulators are known, which either inhibit or potentiate this channel, but few have any appreciable selectivity between the two subtypes or can modulate one receptor differently to the other. In this study, we show that the anticancer drug, topotecan, bidirectionally modulates the 5-HT3 receptor using a two-electrode voltage clamp technique. Topotecan inhibited 5-HT-gated current through homomeric 5-HT3A receptors. Interestingly, however, additional expression of the 5-HT3B subunit changed the response to topotecan dramatically from an inhibitory to a potentiatory one. This effect was dependent on the level of 5-HT3B subunit expression. Moreover, the effect was reduced in the receptors containing the 5-HT3B(Y129S) polymorphic variant. These finding could explain individual differences in the sensitivity to topotecan-induced nausea and vomiting.

5-HT(3) receptors

5-Hydroxytryptamine type 3 (5-HT(3)) receptors are cation-selective Cys loop receptors found in both the central and peripheral nervous systems. There are five 5-HT(3) receptor subunits (A-E), and all functional receptors require at least one A subunit. Regions from noncontiguous parts of the subunit sequence contribute to the agonist-binding site, and the roles of a range of amino acid residues that form the binding pocket have been identified. Drugs that selectively antagonize 5-HT(3) receptors (the "setrons") are the current gold standard for treatment of chemotherapy-induced and postoperative nausea and vomiting and have potential for the treatment of a range of other conditions.

A first step towards the understanding of the 5-HT3 receptor subunit heterogeneity from a computational point of view

The functional serotonin type-3 receptor (5-HT(3)-R), which is the target of many neuroactive drugs, is known to be a homopentamer made of five identical subunits A (5-HT(3A)-R) or a binary heteropentamer made of subunits A and B (5-HT(3A/B)-R) with a still debated arrangement and stoichiometry. This complex picture has been recently further complicated by the discovery of additional 5-HT(3)-R subunits, C, D, and E, which, similarly to the B subunit, are apparently able to form functional receptors only if co-expressed with subunit A. Being the binding site for both serotonin and antagonists (i.e. drugs) located at the extracellular interface between two adjacent subunits, the large variability of the 5-HT(3)-R composition becomes a crucial issue, since it can originate many different interfaces providing non-equivalent ligand binding sites and complicating the pharmacological modulation. Here, the different 5-HT(3)-R interfaces are analysed, on the bases of the structural conformations of previously built 3D homology models and of the known subunit sequences, by addressing their physicochemical characterization. The results confirm the presence of an aromatic cluster located in the core of the A-A interface as a key determinant for having an interface both stable and functional. This is used as a discriminant to make hypotheses about the capability of all the other possible interfaces constituted by the known 5-HT(3)-R sequences A, B, C, D, and E to build active receptors.

Stoichiometry and subunit arrangement of α1β glycine receptors as determined by atomic force microscopy

The glycine receptor is an anion-permeable member of the Cys-loop ion channel receptor family. Synaptic glycine receptors predominantly comprise pentameric α1β subunit heteromers. To date, attempts to define the subunit stoichiometry and arrangement of these receptors have not yielded consistent results. Here we introduced FLAG and six-His epitopes into α1 and β subunits, respectively, and imaged single antibody-bound α1β receptors using atomic force microscopy. This permitted us to infer the number and relative locations of the respective subunits in functional pentamers. Our results indicate an invariant 2α1:3β stoichiometry with a β-α-β-α-β subunit arrangement.

Discovery of a novel allosteric modulator of 5-HT3 receptors: inhibition and potentiation of Cys-loop receptor signaling through a conserved transmembrane intersubunit site

The ligand-gated ion channels in the Cys-loop receptor superfamily mediate the effects of neurotransmitters acetylcholine, serotonin, GABA, and glycine. Cys-loop receptor signaling is susceptible to modulation by ligands acting through numerous allosteric sites. Here we report the discovery of a novel class of negative allosteric modulators of the 5-HT(3) receptors (5-HT(3)Rs). PU02 (6-[(1-naphthylmethyl)thio]-9H-purine) is a potent and selective antagonist displaying IC(50) values of ~1 μM at 5-HT(3)Rs and substantially lower activities at other Cys-loop receptors. In an elaborate mutagenesis study of the 5-HT(3)A receptor guided by a homology model, PU02 is demonstrated to act through a transmembrane intersubunit site situated in the upper three helical turns of TM2 and TM3 in the (+)-subunit and TM1 and TM2 in the (-)-subunit. The Ser(248), Leu(288), Ile(290), Thr(294), and Gly(306) residues are identified as important molecular determinants of PU02 activity with minor contributions from Ser(292) and Val(310), and we propose that the naphthalene group of PU02 docks into the hydrophobic cavity formed by these. Interestingly, specific mutations of Ser(248), Thr(294), and Gly(306) convert PU02 into a complex modulator, potentiating and inhibiting 5-HT-evoked signaling through these mutants at low and high concentrations, respectively. The PU02 binding site in the 5-HT(3)R corresponds to allosteric sites in anionic Cys-loop receptors, which emphasizes the uniform nature of the molecular events underlying signaling through the receptors. Moreover, the dramatic changes in the functional properties of PU02 induced by subtle changes in its binding site bear witness to the delicate structural discrimination between allosteric inhibition and potentiation of Cys-loop receptors.

The 5-HT3B subunit affects high-potency inhibition of 5-HT3 receptors by morphine

BACKGROUND AND PURPOSE

Morphine is an antagonist at 5-HT(3) A receptors. 5-HT(3) and opioid receptors are expressed in many of the same neuronal pathways where they modulate gut motility, pain and reinforcement. There is increasing interest in the 5-HT3B subunit, which confers altered pharmacology to 5-HT(3) receptors. We investigated the mechanisms of inhibition by morphine of 5-HT(3) receptors and the influence of the 5-HT3B subunit.

EXPERIMENTAL APPROACH

5-HT-evoked currents were recorded from voltage-clamped HEK293 cells expressing human 5-HT3A subunits alone or in combination with 5-HT3B subunits. The affinity of morphine for the orthosteric site of 5-HT(3) A or 5-HT(3) AB receptors was assessed using radioligand binding with the antagonist [(3) H]GR65630.

KEY RESULTS

When pre-applied, morphine potently inhibited 5-HT-evoked currents mediated by 5-HT(3) A receptors. The 5-HT3B subunit reduced the potency of morphine fourfold and increased the rates of inhibition and recovery. Inhibition by pre-applied morphine was insurmountable by 5-HT, was voltage-independent and occurred through a site outside the second membrane-spanning domain. When applied simultaneously with 5-HT, morphine caused a lower potency, surmountable inhibition of 5-HT(3) A and 5-HT(3) AB receptors. Morphine also fully displaced [(3) H]GR65630 from 5-HT(3) A and 5-HT(3) AB receptors with similar potency.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that morphine has two sites of action, a low-affinity, competitive site and a high-affinity, non-competitive site that is not available when the channel is activated. The affinity of morphine for the latter is reduced by the 5-HT3B subunit. Our results reveal that morphine causes a high-affinity, insurmountable and subunit-dependent inhibition of human 5-HT(3) receptors.

VUF10166, a novel compound with differing activities at 5-HT₃A and 5-HT₃AB receptors

The actions of a novel, potent 5-HT₃ receptor ligand, [2-chloro-(4-methylpiperazine-1-yl)quinoxaline (VUF10166)], were examined at heterologously expressed human 5-HT₃A and 5-HT₃AB receptors. VUF10166 displaced [³H]granisetron binding to 5-HT₃A receptors expressed in human embryonic kidney cells with high affinity (K(i) = 0.04 nM) but was less potent at 5-HT₃AB receptors (K(i) = 22 nM). Dissociation of [³H]granisetron in the presence of VUF10166 was best fit with a single time constant (t(1/2) = 53 min) at 5-HT₃A receptors, but with two time constants (t(1/2) = 55 and 2.4 min) at 5-HT₃AB receptors. Electrophysiological studies in oocytes revealed that VUF10166 inhibited 5-HT-induced responses at 5-HT₃A receptors at nanomolar concentrations, but inhibition and recovery were too slow to determine an IC₅₀. At 5-HT₃AB receptors, inhibition and recovery were faster, yielding an IC₅₀ of 40 nM. Cysteine substitutions in the complementary (-), but not the principal (+), face of the 5-HT₃B subunit produced heteromeric receptors in which the actions of VUF10166 resembled those at homomeric receptors. At 5-HT₃A receptors, VUF10166 at higher concentrations also behaved as a partial agonist (EC₅₀ = 5.2 μM; R(max) = 0.24) but did not elicit significant responses at 5-HT₃AB receptors at ≤100 μM. Thus, we propose that VUF10166 binds to the common A+A- site of both receptor types and to a second A+B- modulatory site in the heteromeric receptor. The ability of VUF10166 to distinguish between 5-HT₃A and 5-HT₃AB receptors could help evaluate differences between these receptor types and has potential therapeutic value.

High-affinity fluorescent ligands for the 5-HT(3) receptor

The synthesis, photophysical and biological characterization of a small library of fluorescent 5-HT(3) receptor ligands is described. Several of these novel granisetron conjugates have high quantum yields and show high affinity for the human 5-HT(3)AR.

Binding sites for bilobalide, diltiazem, ginkgolide, and picrotoxinin at the 5-HT3 receptor

Bilobalide (BB), ginkgolide B (GB), diltiazem (DTZ), and picrotoxinin (PXN) are 5-hydroxytryptamine type 3 (5-HT(3)) receptor antagonists in which the principal sites of action are in the channel. To probe their exact binding locations, 5-HT(3) receptors with substitutions in their pore lining residues were constructed (N-4'Q, E-1'D, S2'A, T6'S, L7'T, L9'V, S12'A, I16'V, D20'E), expressed in Xenopus laevis oocytes, and the effects of the compounds on 5-HT-induced currents were examined. EC(50) values at mutant receptors were less than 6-fold different from those of wild type, indicating that the mutations were well tolerated. BB, GB, DTZ, and PXN had pIC(50) values of 3.33, 3.14, 4.67, and 4.97, respectively. Inhibition by BB and GB was abolished in mutant receptors containing T6'S and S12'A substitutions, but their potencies were enhanced (42- and 125-fold, respectively) in S2'A mutant receptors. S2'A substitution also caused GB ligand trap. PXN potency was modestly enhanced (5-fold) in S2'A, abolished in T6'S, and reduced in L9'V (40-fold) and S12'A (7-fold) receptors. DTZ potency was reduced in L7'T and S12'A receptors (5-fold), and DTZ also displaced [(3)H]granisetron binding, indicating mixed competitive/noncompetitive inhibition. We conclude that regions close to the hydrophobic gate of M2 are important for the inhibitory effects of BB, GB, DTZ, and PXN at the 5-HT(3) receptor; for BB, GB, and PXN, the data show that the 6' channel lining residue is their major site of action, with minor roles for 2', 9', and 12' residues, whereas for DTZ, the 7' and 12' sites are important.

Cysteine modification reveals which subunits form the ligand binding site in human heteromeric 5-HT3AB receptors

The ligand binding site of Cys-loop receptors is formed by residues on the principal (+) and complementary (-) faces of adjacent subunits, but the subunits that constitute the binding pocket in many heteromeric receptors are not yet clear. To probe the subunits involved in ligand binding in heteromeric human 5-HT(3)AB receptors, we made cysteine substitutions to the + and - faces of A and B subunits, and measured their functional consequences in receptors expressed in Xenopus oocytes. All A subunit mutations altered or eliminated function. The same pattern of changes was seen at homomeric and heteromeric receptors containing cysteine substitutions at A(R92) (- face), A(L126)(+), A(N128)(+), A(I139)(-), A(Q151)(-) and A(T181)(+), and these receptors displayed further changes when the sulphydryl modifying reagent methanethiosulfonate-ethylammonium (MTSEA) was applied. Modifications of A(R92C)(-)- and A(T181C)(+)-containing receptors were protected by the presence of agonist (5-HT) or antagonist (d-tubocurarine). In contrast modifications of the equivalent B subunit residues did not alter heteromeric receptor function. In addition a double mutant, A(S206C)(-)(/E229C)(+), only responded to 5-HT following DTT treatment in both homomeric and heteromeric receptors, indicating receptor function was inhibited by a disulphide bond between an A+ and an A- interface in both receptor types. Our results are consistent with binding to an A+A- interface at both homomeric and heteromeric human 5-HT(3) receptors, and explain why the competitive pharmacologies of these two receptors are identical.

5-HT(3) receptors: potential of individual isoforms for personalised therapy

Serotonin type 3 (5-HT(3)) receptors are ligand-gated ion channels built by five subunits of diverse composition. In humans, five subunits exist (5-HT3A-E) which are encoded by the genes HTR3A-E and are expressed in various isoforms. Recently, the importance of factors influencing receptor expression became clear, such as chaperones and microRNAs. Owing to their expression profile and physiological functions, 5-HT(3) receptors have been implicated in irritable bowel syndrome (IBS) and psychiatric disorders. Interestingly, HTR3 variants have now been shown to be associated with these conditions. This underlines the potential of 5-HT(3) receptors as therapeutic targets and may enable personalised therapies in the future.

The structural basis of function in Cys-loop receptors

Cys-loop receptors are membrane-spanning neurotransmitter-gated ion channels that are responsible for fast excitatory and inhibitory transmission in the peripheral and central nervous systems. The best studied members of the Cys-loop family are nACh, 5-HT3, GABAA and glycine receptors. All these receptors share a common structure of five subunits, pseudo-symmetrically arranged to form a rosette with a central ion-conducting pore. Some are cation selective (e.g. nACh and 5-HT3) and some are anion selective (e.g. GABAA and glycine). Each receptor has an extracellular domain (ECD) that contains the ligand-binding sites, a transmembrane domain (TMD) that allows ions to pass across the membrane, and an intracellular domain (ICD) that plays a role in channel conductance and receptor modulation. Cys-loop receptors are the targets for many currently used clinically relevant drugs (e.g. benzodiazepines and anaesthetics). Understanding the molecular mechanisms of these receptors could therefore provide the catalyst for further development in this field, as well as promoting the development of experimental techniques for other areas of neuroscience.In this review, we present our current understanding of Cys-loop receptor structure and function. The ECD has been extensively studied. Research in this area has been stimulated in recent years by the publication of high-resolution structures of nACh receptors and related proteins, which have permitted the creation of many Cys loop receptor homology models of this region. Here, using the 5-HT3 receptor as a typical member of the family, we describe how homology modelling and ligand docking can provide useful but not definitive information about ligand interactions. We briefly consider some of the many Cys-loop receptors modulators. We discuss the current understanding of the structure of the TMD, and how this links to the ECD to allow channel gating, and consider the roles of the ICD, whose structure is poorly understood. We also describe some of the current methods that are beginning to reveal the differences between different receptor states, and may ultimately show structural details of transitions between them.

5-HT(3) receptors: role in disease and target of drugs

Serotonin type 3 (5-HT(3)) receptors are pentameric ion channels belonging to the superfamily of Cys-loop receptors. Receptor activation either leads to fast excitatory responses or modulation of neurotransmitter release depending on their neuronal localisation. 5-HT(3) receptors are known to be expressed in the central nervous system in regions involved in the vomiting reflex, processing of pain, the reward system, cognition and anxiety control. In the periphery they are present on a variety of neurons and immune cells. 5-HT(3) receptors are known to be involved in emesis, pain disorders, drug addiction, psychiatric and GI disorders. Progress in molecular genetics gives direction to personalised medical strategies for treating complex diseases such as psychiatric and functional GI disorders and unravelling individual drug responses in pharmacogenetic approaches. Here we discuss the molecular basis of 5-HT(3) receptor diversity at the DNA and protein level, of which our knowledge has greatly extended in the last decade. We also evaluate their role in health and disease and describe specific case-control studies addressing the involvement of polymorphisms of 5-HT3 subunit genes in complex disorders and responses to drugs. Furthermore, we focus on the actual state of the pharmacological knowledge concerning not only classical 5-HT(3) antagonists--the setrons--but also compounds of various substance classes targeting 5-HT(3) receptors such as anaesthetics, opioids, cannabinoids, steroids, antidepressants and antipsychotics as well as natural compounds derived from plants. This shall point to alternative treatment options modulating the 5-HT(3) receptor system and open new possibilities for drug development in the future.