Detection of human and rodent 5-HT3B receptor subunits by anti-peptide polyclonal antibodies

The roles of aromatic residues in the glycine receptor transmembrane domain

Background

Cys-loop receptors play important roles in fast neuronal signal transmission. Functional receptors are pentamers, with each subunit having an extracellular, transmembrane (TM) and intracellular domain. Each TM domain contains 4 α-helices (M1–M4) joined by loops of varying lengths. Many of the amino acid residues that constitute these α-helices are hydrophobic, and there has been particular interest in aromatic residues, especially those in M4, which have the potential to contribute to the assembly and function of the receptor via a range of interactions with nearby residues.

Results

Here we show that many aromatic residues in the M1, M3 and M4 α-helices of the glycine receptor are involved in the function of the receptor. The residues were explored by creating a range of mutant receptors, characterising them using two electrode voltage clamp in Xenopus oocytes, and interpreting changes in receptor parameters using currently available structural information on the open and closed states of the receptor. For 7 residues function was ablated with an Ala substitution: 3 Tyr residues at the extracellular end of M1, 2 Trp residues located towards the centers of M1 and M3, and a Phe and a Tyr residue in M4. For many of these an alternative aromatic residue restored wild-type-like function indicating the importance of the π ring. EC₅₀s were increased with Ala substitution of 8 other aromatic residues, with those in M1 and M4 also having reduced currents, indicating a role in receptor assembly. The structure shows many potential interactions with nearby residues, especially between those that form the M1/M3/M4 interface, and we identify those that are supported by the functional data.

Conclusion

The data reveal the importance and interactions of aromatic residues in the GlyR M1, M3 and M4 α-helices, many of which are essential for receptor function.

Electronic supplementary material

The online version of this article (10.1186/s12868-018-0454-8) contains supplementary material, which is available to authorized users.

Multiple regions in the extracellular domain of the glycine receptor determine receptor activity

Glycine receptors (GlyRs) are Cys-loop receptors that mediate fast synaptic inhibition in the brain stem and spinal cord. They are involved in the generation of motor rhythm, reflex circuit coordination, and sensory signal processing and therefore represent targets for therapeutic interventions. The extracellular domains (ECDs) of Cys-loop receptors typically contain many aromatic amino acids, but only those in the receptor binding pocket have been extensively studied. Here, we show that many Phe residues in the ECD that are not located in the binding pocket are also involved in GlyR function. We examined these Phe residues by creating several GlyR variants, characterizing these variants with the two-electrode voltage clamp technique in Xenopus oocytes, and interpreting changes in receptor parameters by using currently available structural information on the open and closed states of the GlyR. Substitution of six of the eight Phe residues in the ECD with Ala resulted in loss of function or significantly increased the EC₅₀ and also altered the maximal response to the partial GlyR agonist taurine compared with glycine in those receptor variants that were functional. Substitutions with other amino acids, combined with examination of nearby residues that could potentially interact with these Phe residues, suggested interactions that could be important for GlyR function, and possibly similar interactions could contribute to the function of other members of the Cys-loop receptor family. Overall, our results suggest that many ECD regions are important for GlyR function and that these regions could inform the design of therapeutic agents targeting GlyR activity.

Human-derived gut microbiota modulates colonic secretion in mice by regulating 5-HT3 receptor expression via acetate production

Serotonin [5-hydroxytryptamine (5-HT)], an important neurotransmitter and a paracrine messenger in the gastrointestinal tract, regulates intestinal secretion by its action primarily on 5-HT₃ and 5-HT₄ receptors. Recent studies highlight the role of gut microbiota in 5-HT biosynthesis. In this study, we determine whether human-derived gut microbiota affects host secretory response to 5-HT and 5-HT receptor expression. We used proximal colonic mucosa-submucosa preparation from age-matched Swiss Webster germ-free (GF) and humanized (HM; ex-GF colonized with human gut microbiota) mice. 5-HT evoked a significantly greater increase in short-circuit current (ΔI_sc) in GF compared with HM mice. Additionally, 5-HT₃ receptor mRNA and protein expression was significantly higher in GF compared with HM mice. Ondansetron, a 5-HT₃ receptor antagonist, inhibited 5-HT-evoked ΔI_sc in GF mice but not in HM mice. Furthermore, a 5-HT₃ receptor-selective agonist, 2-methyl-5-hydroxytryptamine hydrochloride, evoked a significantly higher ΔI_sc in GF compared with HM mice. Immunohistochemistry in 5-HT_3A-green fluorescent protein mice localized 5-HT₃ receptor expression to enterochromaffin cells in addition to nerve fibers. The significant difference in 5-HT-evoked ΔI_sc between GF and HM mice persisted in the presence of tetrodotoxin (TTX) but was lost after ondansetron application in the presence of TTX. Application of acetate (10 mM) significantly lowered 5-HT₃ receptor mRNA in GF mouse colonoids. We conclude that host secretory response to 5-HT may be modulated by gut microbiota regulation of 5-HT₃ receptor expression via acetate production. Epithelial 5-HT₃ receptor may function as a mediator of gut microbiota-driven change in intestinal secretion.NEW & NOTEWORTHY We found that gut microbiota alters serotonin (5-HT)-evoked intestinal secretion in a 5-HT₃ receptor-dependent mechanism and gut microbiota metabolite acetate alters 5-HT₃ receptor expression in colonoids.View this article's corresponding video summary at https://www.youtube.com/watch?v=aOMYJMuLTcw&feature=youtu.be.

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.

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.

Lack of association of the 5-HT(3A) receptor with schizophrenia

The serotonin type 3 (5-HT(3) ) receptor (R) belongs to the family of ligand-gated ion channels. It is supposed to play an important role in the pathogenesis of schizophrenia and might also represent an interesting target for the pharmacological treatment of this disorder. In this study, we searched for variations within the 5-HT(3A) receptor gene which might be specifically associated to schizophrenia. Twenty-nine single nucleotide polymorphisms (SNPs) of 943 schizophrenic patients compared to 2,343 healthy individuals were analyzed. SNPs were selected taking into account previous results on a 5-HT(3A) receptor domain involved in neuroleptic binding. Dominant logistic and linear regression models were calculated for the phenotypes number of hospitalizations, duration of hospitalization, age at onset and case-control. The data did not show significant associations of any SNP under investigation specific for schizophrenic patients. In conclusion, our study does not support the hypothesis that the 5-HT(3A) receptor plays a major role in the pathogenesis of schizophrenia.

Therapeutics of 5-HT3 receptor antagonists: current uses and future directions

The 5-Hydroxytryptamine3 (5-HT3) receptor is a member of the cys-loop family of ligand gated ion channels, of which the nicotinic acetylcholine receptor is the prototype. All other 5-HT receptors identified to date are metabotropic receptors. The 5-HT3 receptor is present in the central and peripheral nervous systems, as well as a number of non-nervous tissues. As an ion channel that is permeable to the cations, Na(+), K(+), and Ca(2+), the 5-HT3 receptor mediates fast depolarizing responses in pre- and post-synaptic neurons. As such, 5-HT3 receptor antagonists that are used clinically block afferent and efferent synaptic transmission. The most well established physiological roles of the 5-HT3 receptor are to coordinate emesis and regulate gastrointestinal motility. Currently marketed 5-HT3 receptor antagonists are indicated for the treatment of chemotherapy, radiation, and anesthesia-induced nausea and vomiting, as well as irritable bowel syndrome. Other therapeutic uses that have been explored include pain and drug addiction. The 5-HT3 receptor is one of a number of receptors that play a role in mediating nausea and vomiting, and as such, 5-HT3 receptor antagonists demonstrate the greatest anti-emetic efficacy when administered in combination with other drug classes.

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

The 5-HT3 receptor is a member of the Cys-loop family of transmitter receptors. It can function as a homopentamer (5-HT3A-only subunits) or as a heteropentamer. The 5-HT3AB receptor is the best characterized heteropentamer. This receptor differs from a homopentamer in its kinetics, voltage dependence, and single-channel conductance, but its pharmacology is similar. To understand the contribution of the 5-HT3B subunit to the binding site, we created homology models of 5-HT3AB receptors and docked 5-HT and granisetron into AB, BA, and BB interfaces. To test whether ligands bind in any or all of these interfaces, we mutated amino acids that are important for agonist and antagonist binding in the 5-HT3A subunit to their corresponding residues in the 5-HT3B subunit and vice versa. Changes in [3H]granisetron binding affinity (Kd) and 5-HT EC50 were determined using receptors expressed in HEK-293 cells and Xenopus oocytes, respectively. For all A-to-B mutant receptors, except T181N, antagonist binding was altered or eliminated. Functional studies revealed that either the receptors were nonfunctional or the EC50 values were increased. In B-to-A mutant receptors there were no changes in Kd, although EC50 values and Hill slopes, except for N170T mutant receptors, were similar to those for 5-HT3A receptors. Thus, the experimental data do not support a contribution of the 5-HT3B subunit to the binding pocket, and we conclude that both 5-HT and granisetron bind to an AA binding site in the heteromeric 5-HT3AB receptor.

Impact of lipid raft integrity on 5-HT3 receptor function and its modulation by antidepressants

Because of the biochemical colocalization of the 5-HT(3) receptor and antidepressants within raft-like domains and their antagonistic effects at this ligand-gated ion channel, we investigated the impact of lipid raft integrity for 5-HT(3) receptor function and its modulation by antidepressants. Treatment with methyl-beta-cyclodextrine (MbetaCD) markedly reduced membrane cholesterol levels and caused a more diffuse membrane distribution of the lipid raft marker protein flotillin-1 indicating lipid raft impairment. Both amplitude and charge of serotonin evoked cation currents were diminished following cholesterol depletion by either MbetaCD or simvastatin (Sim), whereas the functional antagonistic properties of the antidepressants desipramine (DMI) and fluoxetine (Fluox) at the 5-HT(3) receptor were retained. Although both the 5-HT(3) receptor and flotillin-1 were predominantly found in raft-like domains in western blots following sucrose density gradient centrifugation, immunocytochemistry revealed only a coincidental degree of colocalization of these two proteins. These findings and the persistence of the antagonistic effects of DMI and Fluox against 5-HT(3) receptors after lipid raft impairment indicate that their modulatory effects are likely mediated through non-raft 5-HT(3) receptors, which are not sufficiently detected by means of sucrose density gradient centrifugation. In conclusion, lipid raft integrity appears to be important for 5-HT(3) receptor function in general, whereas it is not a prerequisite for the antagonistic properties of antidepressants such as DMI and Fluox at this ligand-gated ion channel.

Identification of a domain which affects kinetics and antagonistic potency of clozapine at 5-HT3 receptors

The widely used atypical antipsychotic clozapine is a potent competitive antagonist at 5-HT₃ receptors which may contribute to its unique psychopharmacological profile. Clozapine binds to 5-HT₃ receptors of various species. However, the structural requirements of the respective binding site for clozapine remain to be determined. Differences in the primary sequences within the 5-HT_3A receptor gene in schizophrenic patients may result in an alteration of the antipsychotic potency and/or the side effect profile of clozapine. To determine these structural requirements we constructed chimeras with different 5-HT_3A receptor sequences of murine and human origin and expressed these mutants in human embryonic kidney (HEK) 293 cells. Clozapine antagonises recombinant mouse 5-HT_3A receptors with higher potency compared to recombinant human 5-HT_3A receptors. 5-HT activation curves and clozapine inhibition curves yielded the parameters EC₅₀ and IC₅₀ for all receptors tested in the range of 0.6–2.7 µM and 1.5–83.3 nM, respectively. The use of the Cheng-Prusoff equation to calculate the dissociation constant K_b values for clozapine revealed that an extracellular sequence (length 86 aa) close to the transmembrane domain M1 strongly determines the binding affinity of clozapine. K_b values of clozapine were significantly lower (0.3–1.1 nM) for receptors containing the murine sequence and higher when compared with receptors containing the respective human sequence (5.8–13.4 nM). Thus, individual differences in the primary sequence of 5-HT₃ receptors may be crucial for the antipsychotic potency and/or the side effect profile of clozapine.

3B but which 3B and that's just one of the questions: the heterogeneity of human 5-HT3 receptors

The 5-hydroxytryptamine 3 (5-HT3) receptor is expressed widely in the central and peripheral nervous systems, where it mediates or modulates a wide range of physiological processes. The receptor is targeted by drugs administered for nausea and/or emesis and irritable bowel syndrome and has been proposed as a potential drug target in various psychiatric disorders. The 5-HT3 receptor is a pentameric ligand-gated ion channel and belongs to the Cys-loop receptor family. In contrast to the immense heterogeneity characterizing other Cysloop receptors, native 5-HT3 receptors historically have been considered a much more homogenous receptor population. However, the recent discovery of additional 5-HT3 subunits and the dawning realization that central and peripheral 5-HT3 receptor populations might comprise several subtypes characterized by distinct functional properties has emphasized the complexity of human 5-HT3 receptor signaling. In this review potential implications of these findings and of the entirely new layer of interindividual diversity introduced to the 5-HT3 receptor system by genetic variations will be outlined.

Direct subunit-dependent multimodal 5-hydroxytryptamine3 receptor antagonism by methadone

Homomeric 5-hydroxytryptamine (5-HT)(3A) and heteromeric 5-HT(3AB) receptors mediate rapid excitatory responses to serotonin in the central and peripheral nervous systems. The alkaloid morphine, in addition to being a mu-opioid receptor agonist, is a potent competitive inhibitor of 5-HT(3) receptors. We examined whether methadone, an opioid often used to treat morphine dependence, also exhibited 5-HT(3) receptor antagonist properties. Racemic (R/S)-methadone inhibited currents mediated by human homomeric 5-HT(3A) receptors (IC(50) = 14.1 +/- 2.5 microM). Incorporation of the 5-HT(3B) subunit into heteromeric 5-HT(3AB) receptors reduced the potency of inhibition by (R/S)-methadone (IC(50) = 41.1 +/- 0.9 microM). (R/S)-Methadone also increased apparent desensitization of both 5-HT(3) receptor subtypes. The inhibition of the 5-HT(3A) receptor was competitive; however, incorporation of the 5-HT(3B) subunit caused the appearance of inhibition that was insurmountable by 5-HT. In the absence of rapid desensitization, when dopamine was used as an agonist of 5-HT(3AB) receptors, the inhibition by (R/S)-methadone was voltage-dependent. The antagonist and desensitization-enhancing effects of (R/S)-methadone were shared by pure (R)- and (S)-methadone enantiomers, which had similar actions on 5-HT-evoked currents mediated by 5-HT(3) receptors. However, (R)-methadone exhibited a larger voltage-dependent inhibition of dopamine-evoked currents mediated by 5-HT(3AB) receptors than did (S)-methadone. Inhibition of 5-HT(3A) receptors by (R/S)-methadone was not influenced by voltage. Thus, methadone displays multimodal subunit-dependent antagonism of 5-HT(3) receptors.

The 5-HT3 receptor--the relationship between structure and function

The 5-hydroxytryptamine type-3 (5-HT3) receptor is a cation-selective ion channel of the Cys-loop superfamily. 5-HT3 receptor activation in the central and peripheral nervous systems evokes neuronal excitation and neurotransmitter release. Here, we review the relationship between the structure and the function of the 5-HT3 receptor. 5-HT3A and 5-HT3B subunits are well established components of 5-HT3 receptors but additional HTR3C, HTR3D and HTR3E genes expand the potential for molecular diversity within the family. Studies upon the relationship between subunit structure and the ionic selectivity and single channel conductances of 5-HT3 receptors have identified a novel domain (the intracellular MA-stretch) that contributes to ion permeation and selectivity. Conventional and unnatural amino acid mutagenesis of the extracellular domain of the receptor has revealed residues, within the principle (A-C) and complementary (D-F) loops, which are crucial to ligand binding. An area requiring much further investigation is the subunit composition of 5-HT3 receptors that are endogenous to neurones, and their regional expression within the central nervous system. We conclude by describing recent studies that have identified numerous HTR3A and HTR3B gene polymorphisms that impact upon 5-HT3 receptor function, or expression, and consider their relevance to (patho)physiology.

High affinity binding of epibatidine to serotonin type 3 receptors

Epibatidine and mecamylamine are ligands used widely in the study of nicotinic acetylcholine receptors (nAChRs) in the central and peripheral nervous systems. In the present study, we find that nicotine blocks only 75% of (125)I-epibatidine binding to rat brain membranes, whereas ligands specific for serotonin type 3 receptors (5-HT(3)Rs) block the remaining 25%. (125)I-Epibatidine binds with a high affinity to native 5-HT(3)Rs of N1E-115 cells and to receptors composed of only 5-HT(3A) subunits expressed in HEK cells. In these cells, serotonin, the 5-HT(3)R-specific antagonist MDL72222, and the 5-HT(3)R agonist chlorophenylbiguanide readily competed with (125)I-epibatidine binding to 5-HT(3)Rs. Nicotine was a poor competitor for (125)I-epibatidine binding to 5-HT(3)Rs. However, the noncompetitive nAChR antagonist mecamylamine acted as a potent competitive inhibitor of (125)I-epibatidine binding to 5-HT(3)Rs. Epibatidine inhibited serotonin-induced currents mediated by endogenous 5-HT(3)Rs in neuroblastoma cell lines and 5-HT(3A)Rs expressed in HEK cells in a competitive manner. Our results demonstrate that 5-HT(3)Rs are previously uncharacterized high affinity epibatidine binding sites in the brain and indicate that epibatidine and mecamylamine act as 5-HT(3)R antagonists. Previous studies that depended on epibatidine and mecamylamine as nAChR-specific ligands, in particular studies of analgesic properties of epibatidine, may need to be reinterpreted with respect to the potential role of 5-HT(3)Rs.

Differential Subcellular Localization of RIC-3 Isoforms and Their Role in Determining 5-HT3 Receptor Composition

RIC-3 has been identified as a chaperone molecule involved in promoting the functional expression of nicotinic acetylcholine and 5-HT(3) receptors in mammalian cells. In this study, we examined the effects of RIC-3a (isoform a) and a truncated isoform (isoform d) on RIC-3 localization, mobility, and aggregation and its effect on 5-HT3 receptor composition in mammalian cells. Human RIC-3a possesses an amino-terminal signal sequence that targets it to the endoplasmic reticulum where it is distributed within the reticular network, often forming large diffuse "slicks" and bright "halo" structures. RIC-3a is highly mobile within and between these compartments. Despite the propensity for RIC-3a to aggregate, its expression enhances the level of surface 5-HT3A (homomeric) receptors. In contrast, RIC-3a exerts an inhibitory action on the surface expression of heteromeric 5-HT3A/B receptors. RIC-3d exhibits an altered subcellular distribution, being localized to the endoplasmic reticulum, large diffuse slicks, tubulo-vesicular structures, and the Golgi. Bidirectional trafficking between the endoplasmic reticulum and Golgi suggests that RIC-3d constitutively cycles between these two compartments. In support of the large coiled-coil domain of RIC-3a being responsible for protein aggregation, RIC-3d, lacking this cytoplasmic domain, does not aggregate or induce the formation of bright aggregates. Regardless of these differences, isoform d is still capable of enhancing homomeric, and inhibiting heteromeric, 5-HT3 receptor expression. Thus, both isoforms of RIC-3 play a role in determining 5-HT3 receptor composition.

MD-354: what is it good for?

MD-354 (meta-chlorophenylguanidine) has been identified as a member of a novel class of 5-HT(3) serotonin receptor agonists. MD-354 is a 5-HT(3) receptor partial agonist that has been shown to behave as an agonist in some assays, and as an antagonist in others. MD-354 also binds at alpha-adrenoceptors (ARs) and displays an affinity for alpha(2B)-ARs comparable to its affinity for 5-HT(3) receptors. Although devoid of antinociceptive actions following systemic administration alone, MD-354 markedly enhances the antinociceptive actions of clonidine in the mouse tail-flick assay without potentiating the sedative side effects of clonidine. Although studies with MD-354 are still in progress, some pharmacological findings are described here. MD-354-related agents may represent drug adjuvants for the relief of severe pain.

Immunolabelling of the 5-HT3B receptor subunit in the central and peripheral nervous systems in rodents

The 5-HT(3) receptor is a member of the superfamily of neurotransmitter-gated ion channels involved in fast synaptic signalling and in modulation of neurotransmitter release. As for many other channel receptors, the electrophysiological properties and the functions of the 5-HT(3) receptor are determined by subunit composition of the pentameric channel. Because in situ hybridization did not allow the detection of mRNA encoding the 5-HT(3B) subunit in the rodent central nervous system, or in nearly half of the neurons expressing the 5-HT(3A) subunit in peripheral ganglia, it has been suggested that subunit composition could define at least two 5-HT(3) receptor-expressing neuronal populations. In order to challenge this hypothesis, we have developed polyclonal antibodies directed against a portion of the second intracytoplasmic loop of the mouse 5-HT(3B) subunit. Immunohistochemical analysis in the mouse and the rat revealed that immunolabelling was most prominent in peripheral ganglia, particularly in trigeminal ganglia (TG). In rats, transection or ligature of the infraorbital nerve resulted in a pronounced accumulation of immunoreactive material at the proximal side of the lesioned nerve, and an up-regulation of both subunits in 5-HT(3) receptor-expressing TG neurons. Surprisingly, nearly 100% of neurons expressing 5-HT(3A) subunits were also labelled by anti-5-HT(3B) antibodies. We also detected 5-HT(3B) immunoreactivity in the rat hippocampal CA1 layer and in scattered cortical neurons, indicating that detection of 5-HT(3) subunit mRNA by in situ hybridization might not provide really complete mapping of heteromeric 5-HT(3A/B) vs. homomeric 5-HT(3A) receptors in the peripheral and central nervous systems in rodents.

Inhibition of human 5-HT(3A) and 5-HT(3AB) receptors by etomidate, propofol and pentobarbital

The actions of intravenous anaesthetics on 5-HT(3AB) receptors have not been studied. Using oocyte electrophysiology, the effects of etomidate, propofol, and pentobarbital on human 5-HT(3A) and 5-HT(3AB) receptors were studied and compared. Inhibition of peak currents by all three compounds in both receptor subtypes was anaesthetic concentration-dependant and non-competitive. Because the half-maximal inhibitory concentrations for etomidate, propofol and pentobarbital in 5-HT(3A) and 5-HT(3AB) receptors were all above their respective anaesthetic concentrations, the results of our study suggest that neither 5-HT(3) receptor subtype contributes to the anaesthetic actions of etomidate, propofol or pentobarbital.

Naturally occurring variations in the human 5-HT3A gene profoundly impact 5-HT3 receptor function and expression

BACKGROUND

The serotonin [5-hydroxytryptamine (5-HT)]-gated ion channel 5-HT3 is involved in the mediation of postoperative and radiotherapy/chemotherapy-induced nausea/emesis and in irritable bowel syndrome. It has also been suggested to play a role in various psychiatric diseases. Five naturally occurring single nucleotide polymorphisms leading to amino acid changes have been identified in the human 5-HT3A gene.

METHODS AND RESULTS

We investigated the functional effects of these polymorphisms on the 5-HT3A receptor using fluorescence-based cellular assays. Notably, variants A33T, S253N, and M257I displayed 5-HT-induced maximal responses of 3-64% of the wild-type response, whereas R344H and P391R exhibited wild-type-like function. All variants displayed wild-type-like potencies of 5-HT and three 5-HT3 antagonists. Furthermore, all variants displayed Kd values similar to that of the wild-type receptor in a [H]GR65630-binding assay. The surface expression of A33T, M257I, and R344H was reduced 2-4-fold compared with the wild-type, despite similar total expression levels. Finally, coexpression of wild-type 5-HT3A or 5-HT3B subunits with 5-HT3A variants A33T, S253N, or M257I resulted in mixed or heteromeric receptors, characterized by significantly reduced maximal responses to 5-HT compared with the wild-type receptors.

CONCLUSIONS

Three polymorphisms of the 5-HT3A gene gave rise to functionally impaired receptors whose function could not be rescued by either wild-type 5-HT3A or 5-HT3B. Three of the variant receptors were surface-expressed at reduced levels in spite of total expression levels similar to wild-type, indicating that these variants affect receptor biogenesis and/or trafficking. These severe single nucleotide polymorphism effects hold promise for identification of new 5-HT3A gene-disease causalities.

Identification of 5-HT3A and 5-HT3B receptor subunits in human hippocampus

The pentameric 5-HT(3) receptor complex is a ligand-gated ion channel that mediates fast synaptic transmission in the brain. Expression of two subunits (5-HT(3A) and 5-HT(3B) subunits) gives rise to at least two receptor isoforms (homomeric 5-HT(3A) and heteromeric 5-HT(3A/3B) receptors), which differ in their biophysical characteristics, although expression of these proteins has not been investigated in human brain. The expression of h5-HT(3A) and 5-HT(3B) subunits in the human hippocampus was investigated using selective polyclonal antibodies (SDS-PAGE/Western blotting, immunohistochemistry), with expression of each subunit verified by PCR detection of subunit transcripts. 5-HT(3A) and 5-HT(3B) subunit immunoreactivity was identified within the human hippocampus. The cellular pattern of expression for each subunit was similar, with predominant immunoreactivity associated with pyramidal neurones in CA fields 2 and 3, and also the relatively large neurones within the hilus (CA4 field). Transcripts for each subunit were also identified in human hippocampal tissue. These findings indicate that human hippocampal neurones are capable of forming at least two, functionally different, isoforms of the 5-HT(3) receptor. Furthermore the expression pattern of 5-HT(3A) and 5-HT(3B) subunits in human hippocampus appears to differ with the rodent counterpart, which may underlie the differences in some of the behavioural effects of 5-HT(3) receptor antagonists between these species.

Subunit-dependent modulation of the 5-hydroxytryptamine type 3 receptor open-close equilibrium by n-alcohols

5-Hydroxytryptamine (5-HT, serotonin) type 3 (5-HT(3)) receptors belong to the alcohol-sensitive superfamily of Cys-loop ligand-gated ion channels, and they are thought to play an important role in alcoholism. Alcohols with small molecular volumes increase the amplitude of currents evoked by low 5-HT concentrations and shift the 5-HT concentration-response curve for 5-HT(3) receptor activation leftward, indicative of increased receptor sensitivity to agonist. This action is significantly smaller when currents are mediated by heteromeric 5-HT(3AB) receptors compared with homomeric 5-HT(3A) receptors. In this study, we used the highly inefficacious 5-HT(3) receptor agonist dopamine to determine whether this difference between 5-HT(3A) and 5-HT(3AB) receptors reflects differential alcohol modulation of agonist binding affinity or channel gating efficacy. Human recombinant 5-HT(3A) and 5-HT(3AB) receptors were expressed in Xenopus oocytes, and currents were measured in the absence and presence of alcohols using the two-electrode voltage-clamp technique. Modulation by alcohols of peak currents elicited by maximally activating concentrations of dopamine was alcohol concentration-dependent. Potentiation by smaller alcohols was consistently significantly greater in 5-HT(3A) than in 5-HT(3AB) receptors, whereas inhibition by larger alcohols was not. A representative small (butanol) and large (octanol) alcohol failed to alter the EC(50) value for channel activation by dopamine. We conclude that the presence of the 5-HT(3B) subunit in 5-HT(3AB) receptors significantly reduces the enhancement of gating efficacy by small alcohols without altering the inhibitory actions of large alcohols.

Tissue-specific alternative promoters of the serotonin receptor gene HTR3B in human brain and intestine

The serotonin receptor type 3 is a pentameric ligand-gated ion channel regulating intestinal motility, nausea, and vomiting in humans. The HTR3B gene codes for the subunit B of this receptor. The HTR3B transcription start site is not unequivocally identified. In the present study we used transcription start site analyses, transcript-specific RT-PCR, and functional promoter analyses to identify the 5' structure of the HTR3B gene. According to these experiments, two alternative promoters control the expression of different HTR3B transcripts in the peripheral and central nervous system. The transcription start sites observed in the intestine corresponded to the current human genome annotation (NCBI Build 36.1, March 2006). The transcription start sites in the brain, however, were localized in a region about 4000 bp downstream. The brain transcripts lacked the coding first exon of the HTR3B structure published earlier but had an upstream-extended exon 2 containing a new potential translational start site. Reporter gene analyses showed significant promoter activity of the genomic region located 1560 bp upstream to 93 bp downstream of the brain-specific transcription start sites. This data suggests a different transcriptional regulation of the HTR3B gene in the peripheral and the central nervous system that leads to the expression of transcripts with variations in the 5' coding sequence. Further studies on the expression, structure and function of therefore expected tissue-specific 5-HT(3B) isoforms are required.