Evidence for expression of heteromeric serotonin 5-HT(3) receptors in rodents

Stereological investigation of 5-HT3 receptors in the substantia nigra and dorsal raphe nucleus in the rat

Serotonin (5-HT) is a common neurotransmitter in mammals, playing a central role in the regulation of various processes such as sleep, perception, cognitive and autonomic functions in the nervous system. Previous studies have demonstrated that 5-HT type 3 (5-HT₃) receptors are expressed in either or both the substantia nigra (SN) and the dorsal raphe nucleus (DRN) in humans, marmosets, rats and Syrian hamsters. Here, we quantify the distribution of 5-HT₃ receptors across these regions in the adult rat. Fluorescent immunohistochemistry was performed on sections of rat brain covering the entire rostro-caudal extent of the SN and DRN with antibodies specific to the 5-HT_3A receptor subunit, as well as others targeting the monoaminergic markers tyrosine hydroxylase (TH) and the 5-HT transporter (SERT). The number of 5-HT_3A receptor-positive, TH-positive (n = 28,428 ± 888, Gundersen's m = 1 coefficient of error [CE] = 0.05) and SERT-positive (n = 12,852 ± 462, CE = 0.06) cells were estimated in both the SN and the DRN using stereology. We found that 5-HT_3A receptor-positive cells are present in the SNr (n = 1250 ± 64, CE = 0.24), but they did not co-localise with TH-positive cells, nor were they present in the SNc. In contrast, no 5-HT_3A receptor-positive cells were found in the DRN. These results support the presence of 5-HT₃ receptors in the SN, but not in the DRN, and do not support their expression on monoaminergic cells within these two brain areas.

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.

Novel Striatal GABAergic Interneuron Populations Labeled in the 5HT3a(EGFP) Mouse

Histological and morphological studies indicate that approximately 5% of striatal neurons are cholinergic or γ-aminobutyric acidergic (GABAergic) interneurons (gINs). However, the number of striatal neurons expressing known interneuron markers is too small to account for the entire interneuron population. We therefore studied the serotonin (5HT) receptor 3a-enhanced green fluorescent protein (5HT3a(EGFP)) mouse, in which we found that a large number of striatal gINs are labeled. Roughly 20% of 5HT3a(EGFP)-positive cells co-express parvalbumin and exhibit fast-spiking (FS) electrophysiological properties. However, the majority of labeled neurons do not overlap with known molecular interneuron markers. Intrinsic electrical properties reveal at least 2 distinct novel subtypes: a late-spiking (LS) neuropeptide-Y (NPY)-negative neurogliaform (NGF) interneuron, and a large heterogeneous population with several features resembling low-threshold-spiking (LTS) interneurons that do not express somatostatin, NPY, or neuronal nitric oxide synthase. Although the 5HT3a(EGFP) NGF and LTS-like interneurons have electrophysiological properties similar to previously described populations, they are pharmacologically distinct. In direct contrast to previously described NPY(+) LTS and NGF cells, LTS-like 5HT3a(EGFP) cells show robust responses to nicotine administration, while the 5HT3a(EGFP) NGF cell type shows little or no response. By constructing a molecular map of the overlap between these novel populations and existing interneuron populations, we are able to reconcile the morphological and molecular estimates of striatal interneuron numbers.

Elevated serotonergic signaling amplifies synaptic noise and facilitates the emergence of epileptiform network oscillations

Serotonin fibers densely innervate the cortical sheath to regulate neuronal excitability, but its role in shaping network dynamics remains undetermined. We show that serotonin provides an excitatory tone to cortical neurons in the form of spontaneous synaptic noise through 5-HT3 receptors, which is persistent and can be augmented using fluoxetine, a selective serotonin re-uptake inhibitor. Augmented serotonin signaling also increases cortical network activity by enhancing synaptic excitation through activation of 5-HT2 receptors. This in turn facilitates the emergence of epileptiform network oscillations (10-16 Hz) known as fast runs. A computational model of cortical dynamics demonstrates that these two combined mechanisms, increased background synaptic noise and enhanced synaptic excitation, are sufficient to replicate the emergence fast runs and their statistics. Consistent with these findings, we show that blocking 5-HT2 receptors in vivo significantly raises the threshold for convulsant-induced seizures.

The minimum M3-M4 loop length of neurotransmitter-activated pentameric receptors is critical for the structural integrity of cytoplasmic portals

The 5-HT3A receptor homology model, based on the partial structure of the nicotinic acetylcholine receptor from Torpedo marmorata, reveals an asymmetric ion channel with five portals framed by adjacent helical amphipathic (HA) stretches within the 114-residue loop between the M3 and M4 membrane-spanning domains. The positive charge of Arg-436, located within the HA stretch, is a rate-limiting determinant of single channel conductance (γ). Further analysis reveals that positive charge and volume of residue 436 are determinants of 5-HT3A receptor inward rectification, exposing an additional role for portals. A structurally unresolved stretch of 85 residues constitutes the bulk of the M3-M4 loop, leaving a >45-Å gap in the model between M3 and the HA stretch. There are no additional structural data for this loop, which is vestigial in bacterial pentameric ligand-gated ion channels and was largely removed for crystallization of the Caenorhabditis elegans glutamate-activated pentameric ligand-gated ion channels. We created 5-HT3A subunit loop truncation mutants, in which sequences framing the putative portals were retained, to determine the minimum number of residues required to maintain their functional integrity. Truncation to between 90 and 75 amino acids produced 5-HT3A receptors with unaltered rectification. Truncation to 70 residues abolished rectification and increased γ. These findings reveal a critical M3-M4 loop length required for functions attributable to cytoplasmic portals. Examination of all 44 subunits of the human neurotransmitter-activated Cys-loop receptors reveals that, despite considerable variability in their sequences and lengths, all M3-M4 loops exceed 70 residues, suggesting a fundamental requirement for portal integrity.

Serotonin circuits and anxiety: what can invertebrates teach us?

Fear, a reaction to a threatening situation, is a broadly adaptive feature crucial to the survival and reproductive fitness of individual organisms. By contrast, anxiety is an inappropriate behavioral response often to a perceived, not real, threat. Functional imaging, biochemical analysis, and lesion studies with humans have identified the HPA axis and the amygdala as key neuroanatomical regions driving both fear and anxiety. Abnormalities in these biological systems lead to misregulated fear and anxiety behaviors such as panic attacks and post-traumatic stress disorders. These behaviors are often treated by increasing serotonin levels at synapses, suggesting a role for serotonin signaling in ameliorating both fear and anxiety. Interestingly, serotonin signaling is highly conserved between mammals and invertebrates. We propose that genetically tractable invertebrate models organisms, such as Drosophila melanogaster and Caenorhabditis elegans, are ideally suited to unravel the complexity of the serotonin signaling pathways. These model systems possess well-defined neuroanatomies and robust serotonin-mediated behavior and should reveal insights into how serotonin can modulate human cognitive functions.

Differential expression of the 5-HT(3A) and 5-HT(3B) receptor in differentiated NG108-15 cells

Previous work from this laboratory has shown that the serotonin (5-HT) induced response is significantly augmented in differentiated NG108-15 (NG) cells treated with dibutyryl cAMP (Bt(2)cAMP) due to qualitative and quantitative changes in the expression of the 5-HT(3) receptor as demonstrated by specific [(3)H] LY-278584 (a selective 5HT(3) receptor antagonist) binding. In this study, we investigated whether there is any change in the relative expression of the 5-HT(3A) and 5-HT(3B) subunits in NG cells differentiated following Bt(2)cAMP treatment cells. The major findings of this study were that the relative amount of 5-HT(3B) subunit mRNA in Bt(2)cAMP-treated NG cells 5 days following Bt(2)cAMP-treatment was greater than that in the untreated cells. In contrast, the relative expression of the 5-HT(3B) subunit protein in the Bt(2)cAMP-treated NG cells was much less than in the untreated cells, but the relative expression of the 5-HT(3A) subunit in the Bt(2)cAMP-treated NG cells was similar to the untreated cells. Therefore, no relationship between mRNA and protein expression for 5-HT(3A) and 5-HT(3B) subunits in Bt(2)cAMP treated and untreated NG cells were observed. It was also found that fluorescent intensity for the 5-HT(3B) subunit in the cell body of the Bt(2)cAMP treated and untreated NG cells gradually decreased from the day 1-5 after Bt(2)cAMP treatment. However, in specific areas such as the varicosity and nerve endings of the Bt(2)cAMP treated cells, staining intensity for the 5-HT(3B) subunits was stronger than in the untreated cells at the all time points, peaking at day 5 post-treatment. These results suggest that the augmented response induced by 5-HT acting via 5-HT(3) receptors in differentiated NG cells may be due to changes in the relative amount of the 5-HT(3B) subunit, particularly the ratio and distribution of the 5-HT(3A) to (3B) subunits.

The role of serotonin in axon and dendrite growth

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) plays multiple roles in the enteric, peripheral, and central nervous systems (CNS). Although its most prominent biological function is as a signal transmission messenger from pre- to postsynaptic neurons, other roles such as shaping brain development and regulating neurite growth have also been described. Here, we review the less well-studied role of 5-HT as a modulator of neurite growth. 5-HT has been shown to regulate neurite growth in multiple systems and species, including in the mammalian CNS. 5-HT predominantly appears to suppress neurite growth, but depending on the model system and 5-HT receptor subtype, in rare cases, it may promote neurite outgrowth and elongation. Failure of axon regeneration in the adult mammalian CNS is a major problem in multiple diseases, and understanding how 5-HT receptors signal opposing effects on neurite growth may lead to novel neuroregenerative therapies, by targeting either 5-HT receptors or their downstream signaling pathways.

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.

Mapping spatial relationships between residues in the ligand-binding domain of the 5-HT3 receptor using a molecular ruler

The serotonin 5-HT(3) receptor (5-HT(3)R) is a member of the Cys-loop ligand-gated ion channel family. We used a combination of site-directed mutagenesis, homology modeling, and ligand-docking simulations to analyze antagonist-receptor interactions. Mutation of E236, which is near loop C of the binding site, to aspartate prevents expression of the receptor on the cell surface, and no specific ligand binding can be detected. On the other hand, mutation to glutamine, asparagine, or alanine produces receptors that are expressed on the cell surface, but decreases receptor affinity for the competitive antagonist d-tubocurarine (dTC) 5-35-fold. The results of a double-mutant cycle analysis employing a panel of dTC analogs to identify specific points of interactions between the dTC analogs and E236 are consistent with E236 making a direct physical interaction with the 12 -OH of dTC. dTC is a rigid molecule of known three-dimensional structure. Together with previous studies linking other regions of dTC to specific residues in the binding site, these data allow us to define the relative spatial arrangement of three different residues in the ligand-binding site: R92 (loop D), N128 (loop A), and E236 (near loop C). Molecular modeling employing these distance constraints followed by molecular-dynamics simulations produced a dTC/receptor complex consistent with the experimental data. The use of the rigid ligands as molecular rulers in conjunction with double-mutant cycle analysis provides a means of mapping the relative positions of various residues in the ligand-binding site of any ligand-receptor complex, and thus is a useful tool for delineating the architecture of the binding site.

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.

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.

A conserved cysteine residue in the third transmembrane domain is essential for homomeric 5-HT3 receptor function

The cysteine (Cys) residue at position 312 in the third transmembrane domain (M3) is conserved among 5-hydroxytryptamine type 3 (5-HT(3)) receptor subunits and many other subunits of the nicotinic acetylcholine (nACh) related Cys-loop receptor family, including most of the gamma-aminobutyric acid type A (GABA(A)) and glycine receptor subunits. To elucidate a possible role for the Cys-312 in human 5-HT(3)A receptors, we replaced it with alanine and expressed the 5-HT(3)A(C312A) mutant in HEK293 cells. The mutation resulted in an absence of 5-HT-induced whole-cell current without reducing homopentamer formation, surface expression or 5-HT binding. The 5-HT(3)A(C312A) mutant, when co-expressed with the wild-type 5-HT(3)A subunit, did not affect functional expression of receptors, suggesting that the mutant is not dominant negative. Interestingly, co-expression of 5-HT(3)A(C312A) with 5-HT(3)B led to surface expression of heteropentamers that mediated small 5-HT responses. This suggests that the Cys-312 is essential for homomeric but not heteromeric receptor gating. To further investigate the relationship between residue 312 and gating we replaced it with amino acids located at the equivalent position within other Cys-loop subunits that are either capable or incapable of forming functional homopentamers. Replacement of 5-HT(3)A Cys-312 by Gly or Leu (equivalent residues in the nACh receptor delta and gamma subunits) abolished and severely attenuated function, respectively, whereas replacement by Thr or Ser (equivalent residues in nACh receptor alpha7 and GABA(A) subunits) supported robust function. Thus, 5-HT(3)A residue 312 and equivalent polar residues in the M3 of other Cys-loop subunits are essential determinants of homopentameric gating.

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.

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.

Characteristics for enhanced response of serotonin-evoked ion dynamics in differentiated NG108-15 cells

Characteristics for the up-regulated response in the concentration of intracellular calcium ion ([Ca(2+)]( i )) and in the sodium ion (Na(+)) current by serotonin (5-HT) were investigated in differentiated neuroblastoma x glioma hybrid NG108-15 (NG) cells. The results for the changes in [Ca(2+)]( i ) by 5-HT were as follows, (1) The 5-HT-induced Ca(2+) response was inhibited by 3 x 10(-9) M tropisetron (a 5-HT(3) receptor blocker), but not by other types of 5-HT receptor blockers; (2) The 5-HT-induced Ca(2+) response was mainly inhibited by calciseptine (a L-type Ca(2+) blocker), but not by other types of Ca(2+) channel blockers or 10(-7) M TTX (a voltage-sensitive Na(+) channel blocker); (3) When the extracellular Na(+) was removed by exchange with choline chloride or N-methyl-D-glucamine, the 5-HT-induced Ca(2+) response was extremely inhibited. The results for the 5-HT-induced Na(+) current by the whole cell patch-clamp technique were as follows, (1) The 5-HT-induced Na(+) current in differentiated cells was significantly larger than that in undifferentiated cells; (2) The ED(50) value for 5-HT-induced Na(+) current in undifferentiated and differentiated cells was almost the same, about 4 x 10(-6) M each other; (3) The 5-HT-induced Na(+) current was completely blocked by 3 x 10(-9) M tropisetron, but not by other 5-HT receptor antagonists and 10(-7) M TTX. These results suggested that 5-HT-induced Ca(2+) response in differentiated NG cells was mainly due to L-type voltage-gated Ca(2+) channels allowing extracellular Na(+) to enter via 5-HT(3) receptors, but not through voltage-gated Na(+) channels.

On the voltage-dependent Ca2+ block of serotonin 5-HT3 receptors: a critical role of intracellular phosphates

Natively expressed serotonin 5-HT(3) receptors typically possess a negative-slope conductance region in their I-V curve, due to a voltage-dependent block by external Ca(2+) ions. However, in almost all studies performed with heterologously expressed 5-HT(3) receptors, this feature was not observed. Here we show that mere addition of ATP to the pipette solution is sufficient to reliably observe a voltage-dependent block in homomeric (h5-HT(3A)) and heteromeric (h5-HT(3AB)) receptors expressed in HEK293 cells. A similar block was observed with a plethora of molecules containing a phosphate moiety, thus excluding a role of phosphorylation. A substitution of three arginines in the intracellular vestibule of 5-HT(3A) with their counterpart residues from the 5-HT(3B) subunit (RRR-QDA) was previously shown to dramatically increase single channel conductance. We find this mutant to have a linear I-V curve that is unaffected by the presence of ATP, with a fractional Ca(2+) current (Pf%) that is reduced (1.8 +/- 0.2%) compared to that of the homomeric receptor (4.1 +/- 0.2%), and similar to that of the heteromeric form (2.0 +/- 0.3%). Moreover, whereas ATP decreased the Pf% of the homomeric receptor, this was not observed with the RRR-QDA mutant. Finally, ATP was found to be critical for voltage-dependent channel block also in hippocampal interneurons that natively express 5-HT(3) receptors. Taken together, our results indicate a novel mechanism by which ATP, and similar molecules, modulate 5-HT(3) receptors via interactions with the intracellular vestibule of the receptor.

Activation of the serotonin 5-HT3 receptor in the dorsal raphe nucleus suppresses REM sleep in the rat

The effects of the selective 5-HT(3) receptor agonist and antagonist m-chlorophenylbiguanide (m-CPBG) and ondansetron, respectively, were studied in adult male Wistar rats implanted for chronic sleep recordings. Microinjection of m-CPBG (2.0 and 4.0 mM) into the dorsal raphe nucleus (DRN) decreased rapid-eye-movement sleep (REMS) and the number of REM periods during the first, second, and third 2-h recording period. On the other hand, direct infusion of ondansetron (0.5-1.0 mM) into the DRN induced no significant changes in sleep variables over the 6 h of recording. Pretreatment with ondansetron (0.5 mM) antagonized the m-CPBG (2.0 mM)-induced reduction of REMS and of the number of REM periods. The data are consistent with the hypothesis that the 5-HT(3) receptor is involved in the effect of DRN serotonergic neurons on brainstem structures that act to promote and induce REMS. It is suggested that the suppression of REMS after the microinjection of m-CPBG into the DRN is related, at least in part, to the stimulation of glutamatergic interneurons that express 5-HT(3) receptors. Activation of these receptors facilitates the release of glutamate, which, in turn, acts on postsynaptic N-methyl-d-aspartate and non-N-methyl-d-aspartate receptors expressed by serotonergic neurons of the DRN and increases the release of 5-HT at postsynaptic sites.

Molecular biology of 5-HT receptors

Serotonin (5-hydroxytryptamine; 5-HT) is a monoamine neurotransmitter whose effects are mediated by at least 13 distinct G protein-coupled receptors (GPCRs) of the type A family which includes the monoamine receptors and a combination of ligand-gated ion channels (5-HT3) of the Cys loop family which constitutes heteropentamers. 5-HT receptors are currently divided into seven classes (5-HT1 to 5-HT7), based on structural, transductional and operational features. While this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity is supported by the existence of a great number of splice and editing variants for several 5-HT receptors, their possible modulation by accessory proteins and chaperones, as well as their potential to form homo or heteromers both at the GPCR and at the ligand-gated channel level.

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.

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.

Dynamic modification of a mutant cytoplasmic cysteine residue modulates the conductance of the human 5-HT3A receptor

Structural models suggest that Arg(436) lies within five cytoplasmic portals of the 5-HT(3A) receptor. We tested both the accessibility of residue 436 and the influence of its charge on single channel conductance (gamma) by substituting Arg(436) with Cys and examining the effect of methanethiosulfonate (MTS) reagents on gamma. Inclusion of positively charged 2-aminoethyl-MTS (MTSEA) within the electrode solution reduced gamma of 5-HT(3A)(R436C) receptors in outside-out patches from 7.8 +/- 0.5 to 5.0 +/- 0.5 picosiemens (pS). To increase gamma, we substituted Arg(436) by Cys in the 5-HT(3A)(R432Q,R440A) mutant, yielding the 5-HT(3A)(QCA) construct with a gamma of 17.7 +/- 0.4 pS. Modification of 5-HT(3A)(QCA) receptors by MTSEA or 2-(trimethylammonium) ethyl-MTS reduced gamma to 8.7 +/- 0.5 and 6.7 +/- 0.4 pS, respectively, both significantly below that of channels exposed to nonpolar propyl-MTS. Extracellular MTSEA, but not 2-(trimethylammonium) ethyl-MTS, crossed the membrane and in so doing slowly (tau = 94 s) reduced gamma. MTSEA more rapidly (tau = 15 s) reduced the gamma of 5-HT(3A)(QCA) receptors in inside-out patches, an effect reversed by the reducing agent dithiothreitol. Cys(436) modification by negatively charged 2-carboxyethyl-MTS and 2-sulfonatoethyl-MTS increasedgamma to 23 +/- 1.0 and 26 +/- 0.7 pS, respectively. MTS reagents did not affect gamma values for 5-HT(3A)(QDA) constructs with Cys substituted for Lys(431) predicted to be outside the entrance to the portals. Collectively, the data demonstrate that the dynamic modification of the charge of a cytoplasmic residue regulates gamma, consistent with the existence of cytoplasmic portals that impose a rate-limiting barrier to ion conduction in Cys loop receptors.

Characterization of the Caenorhabditis elegans G protein-coupled serotonin receptors

Serotonin (5-HT) regulates a wide range of behaviors in Caenorhabditis elegans, including egg laying, male mating, locomotion and pharyngeal pumping. So far, four serotonin receptors have been described in the nematode C. elegans, three of which are G protein-coupled receptors (GPCR), (SER-1, SER-4 and SER-7), and one is an ion channel (MOD-1). By searching the C. elegans genome for additional 5-HT GPCR genes, we identified five further genes which encode putative 5-HT receptors, based on sequence similarities to 5-HT receptors from other species. Using loss-of-function mutants and RNAi, we performed a systematic study of the role of the eight GPCR genes in serotonin-modulated behaviors of C. elegans (F59C12.2, Y22D7AR.13, K02F2.6, C09B7.1, M03F4.3, F16D3.7, T02E9.3, C24A8.1). We also examined their expression patterns. Finally, we tested whether the most likely candidate receptors were able to modulate adenylate cyclase activity in transfected cells in a 5-HT-dependent manner. This paper is the first comprehensive study of G protein-coupled serotonin receptors of C. elegans. It provides a direct comparison of the expression patterns and functional roles for 5-HT receptors in C. elegans.

Serotonin induces the increase in intracellular Ca2+ that enhances neurite outgrowth in PC12 cells via activation of 5-HT3 receptors and voltage-gated calcium channels

As a neurotransmitter and neuromodulator, serotonin (5-HT) influences neuronal outgrowth in the nervous systems of several species. In PC12 cells, 5-HT is known to have neuritogenic effects, although the signal transduction pathway responsible for these effects is not understood. In this study, we hypothesized that a 5-HT-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) could be involved in mediating the effects of 5-HT. Application of 5-HT to PC12 cells enhanced nerve growth factor (NGF)-induced neurite outgrowth in a dose-dependent manner, and the sensitivity of this neuritogenic effect was increased in differentiated PC12 cells. In accordance, an increase in [Ca(2+)](i) was observed following application of 5-HT in differentiated PC12 cells. This increase was amplified by further NGF treatment. 5-HT-induced increases in [Ca(2+)](i) were inhibited by MDL 72222, a selective 5-HT(3) receptor antagonist, and nifedipine, an L-type calcium channel blocker, but not by ketanserin, a 5-HT(2) receptor antagonist, or thapsigargin, a specific inhibitor of endoplasmic reticulum Ca(2+)-ATPase. These pharmacological tests indicated that 5-HT-induced increases in [Ca(2+)](i) are mediated by activation of voltage-gated calcium channels via 5-HT(3) receptors and that 5-HT-induced increases in [Ca(2+)](i) are likely to be independent of activation of 5-HT(2) receptors in PC12 cells. Furthermore, the neuritogenic effect of 5-HT was suppressed by MDL 72222, nifedipine, calmodulin (CaM) inhibitor, and calcineurin inhibitors. Taken together, our results indicate that 5-HT-induced increases in [Ca(2+)](i), which are mediated via 5-HT(3) receptors and L-type calcium channels in PC12 cells, and subsequent activation of CaM and calcineurin enhance NGF-induced neurite outgrowth.

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.

Induction by Cocaine of the Serotonergic 5-HT3 Receptor in Rat Cerebellum

The expression of the 5-HT(3) receptor, a member of the serotonin receptor family, was examined in rat cerebellum of saline- or cocaine-treated rats. Both immunohistochemistry and Western blot analysis were used. We found that the expression of this serotonin receptor subtype was increased in the cerebellum of rats injected either acutely or repeatedly (1 injection/day for 10 days) with cocaine. The stimulation was more pronounced after a single injection than after a series of 10 injections. Surprisingly, the expression of the 5-HT(3) receptor was mainly localized in Bergmann glial cells, as assessed from the co-localization of the receptor with the glial cell marker glial fibrillary acidic protein (GFAP). Our data emphasize the importance of the 5-HT(3) receptor induction in the cerebellum as part of the neuroadaptations taking place in rat brain in response to the psychostimulant cocaine.

Mapping residues in the ligand-binding domain of the 5-HT(3) receptor onto d-tubocurarine structure

The serotonin 5-HT(3) receptor (5-HT(3)R) is a member of the cys-loop ligand-gated ion channel family. We have used the combination of site-directed mutagenesis, homology modeling of the 5-HT(3)R extracellular domain, and ligand docking simulations as a way to map the architecture of the 5-HT(3)R ligand binding domain. Mutation of Phe226 in loop C of the binding site to tyrosine (F226Y) has no effect on the apparent affinity of the competitive antagonist d-tubocurarine (dTC) for the receptor. On the other hand, replacement of Asn128 in loop A of the binding site with alanine (N128A) increases the apparent affinity of dTC by approximately 10-fold. Double-mutant cycle analysis employing a panel of dTC analogs with substitutions at various positions to identify specific points of interactions between the dTC analogs and Asn128 suggests that Asn128 makes a direct interaction with the 2'N of dTC. Molecular modeling of the 5-HT(3)R extracellular domain using the antagonist-bound conformation of the Aplysia californica acetylcholine binding protein as a template followed by ligand docking simulations produces two classes of structures of the 5-HT(3)R/dTC complex; only one of these has the 2'N of dTC positioned at Asn128 and thus is consistent with the data from this study and previously published data. The use of the rigid dTC analogs as "molecular rulers" in conjunction with double-mutant cycle analysis of mutant receptors can allow the spatial mapping of the position of various residues in the ligand-binding site.

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

BACKGROUND

The 5-HT3 receptor is a member of a neurotransmitter-gated ion channel family which includes nicotinic acetylcholine, GABAA, and glycine receptors. While antibodies specific for the 5-HT3A receptor subunit are plentiful, and have revealed a wealth of structural and functional information, few antisera exist for the detection of 5-HT3B receptor subunits. Here we describe the generation and characterisation of a rabbit polyclonal antiserum that specifically recognises 5-HT3B receptor subunits

RESULTS

Immunization of a rabbit with a 20-mer peptide, corresponding to the N-terminus of the human 5-HT3B receptor subunit, generated serum with polyclonal antibodies from which an IgG fraction was purified, yielding pAb77. The antibodies were shown to label 5-HT3B receptor subunits in transfected human embryonic kidney cells and rodent tissues using Western blots. Immunocytochemistry using pAb77 on these cells showed that 5-HT3B receptor subunits do not reach the plasma membrane in the absence of 5-HT3A receptor subunits. Immunohistochemical analysis of rat brain sections showed pAb77 immunoreactivity in distinct populations of cells in the hippocampus.

CONCLUSION

We have demonstrated that pAb77 antibodies specifically label native and recombinant 5-HT3B receptor subunits with high affinity and specificity. The antibody was shown to be useful for the determination of both receptor trafficking and also mapping 5-HT3B receptor subunit expression in the CNS.

Transmembrane signaling in the brain by serotonin, a key regulator of physiology and emotion

Serotonin (5-HT) is an ancient chemical that plays a crucial functional role in almost every living organism. It regulates platelet aggregation, activation of immune cells, and contraction of stomach and intestinal muscles. In addition, serotonin acts as a neurotransmitter in the brain and the peripheral nervous system. These activities are initiated by the binding of serotonin to 15 or more receptors that are pharmacologically classified into seven groups, 5-HT1 through 5-HT7. Each group is further divided into subgroups of receptors that are homologous but are encoded by discrete genes. With the exception of the 5-HT3 receptor--a cation channel--all of the others are G protein-coupled receptors that potentially activate or inhibit a large number of biochemical cascades. This review will endeavor to compare and contrast such signaling pathways with special attention to their tissue-specific occurrence, their possible role in immediate effects on covalent modification of other proteins, and relatively slower effects on gene expression, physiology and behavior.

Molecular determinants of single-channel conductance and ion selectivity in the Cys-loop family: insights from the 5-HT3 receptor

The molecular determinants of the ionic selectivity and single-channel conductance of the Cys-loop family of transmitter-gated ion channels are beginning to be understood with increasing precision, in part, as a result of the recent availability of refined ultrastructural information for the archetype of the family, the nicotinic acetylcholine receptor (nAChR). Studies of another member of this family, the 5-HT(3) receptor, have now provided insight into the structure of its channel pore, the location of its gate and mechanisms of ion selectivity and translocation. The anomaly of the extremely low single-channel conductance of the homo-oligomeric 5-HT(3A) receptor has recently been solved, revealing that an intracellular domain of the protein is an important determinant of single-channel conductance. Such data are interpreted, in this article, in light of the most recent developments in structural characterization of the nAChR.

General Anesthetic-Induced Channel Gating Enhancement of 5-Hydroxytryptamine Type 3 Receptors Depends on Receptor Subunit Composition

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

Tyrosine residues that control binding and gating in the 5-hydroxytryptamine3 receptor revealed by unnatural amino acid mutagenesis

The mechanism by which agonist binding triggers pore opening in ligand-gated ion channels is poorly understood. Here, we used unnatural amino acid mutagenesis to introduce subtle changes to the side chains of tyrosine residues (Tyr141, Tyr143, Tyr153, and Tyr234), which dominate the 5-HT3 receptor binding site. Heterologous expression in oocytes, combined with radioligand binding data and a model of 5-HT (serotonin) computationally docked into the binding site, has allowed us to determine which of these residues are responsible for binding and/or gating. We have shown that Tyr 143 forms a hydrogen bond that is essential for receptor gating but does not affect binding, whereas a hydrogen bond formed by Tyr153 is involved in both binding and gating of the receptor. The aromatic group of Tyr234 is essential for binding and gating, whereas its hydroxyl does not affect binding but plays a steric role in receptor gating. Tyr141 is not involved in agonist binding or receptor gating but is important for antagonist interactions. These data, combined with a new model of the nonliganded 5-HT3 receptor, lead to a mechanistic explanation of the interactions that initiate the conformational change leading to channel opening. Thus, we suggest that agonist entry into the binding pocket may displace Tyr143 and Tyr153 and results in their forming new hydrogen bonds. These bonds may form part of the network of bond rearrangements that trigger the conformational change leading to channel opening. Similar rearrangements may initiate gating in all Cys-loop receptors.

Three putative N-glycosylation sites within the murine 5-HT3A receptor sequence affect plasma membrane targeting, ligand binding, and calcium influx in heterologous mammalian cells

The serotonin type 3(A) receptor (5-HT3(A)R) is a ligand-gated ion channel (LGIC) that modulates a diverse set of cognitive and physiological functions. The 5-HT3(A)R, as with other LGICs, is a pentameric ion channel comprising five glycoprotein subunits. Although the N-terminal of the 5-HT3(A)R contains three putative N-linked glycosylation sites, the importance of each glycosylation site has not yet been established. To address this question, we used tunicamycin treatment and site-directed mutagenesis to inhibit selectively N-linked glycosylation at each site and then examined the effects of these treatments on receptor expression and function in transiently transfected heterologous cells. We show that the murine 5-HT3(A)R is glycosylated and that each N-linked glycosylation site plays a role in receptor regulation. Our findings suggest that N109 is necessary for receptor assembly, whereas N174 and N190 are important for plasma membrane targeting and ligand binding. Furthermore, we demonstrate that each site is necessary for 5-HT3(A)R-mediated Ca(2+) influx. We conclude that N-glycosylation is a critical step in the maturation, trafficking, and function of the murine 5-HT3(A)R.

The 5-HT3B subunit confers reduced sensitivity to picrotoxin when co-expressed with the 5-HT3A receptor

There are currently no known agents that display selectivity between homomeric 5-hydroxytryptamine type 3A (5-HT3A) and heteromeric 5-HT3A/3B receptors. In the present study, we show that the CNS convulsant picrotoxin selectively interacts with 5-HT3A receptors. In whole-cell patch clamp recordings, the inhibitory effect of PTX was reduced 100-fold in heteromeric mouse 5-HT3A/3B receptors, compared to homomeric 5-HT3A receptors. Picrotoxin should prove to be a useful probe for determining the presence of homomeric vs. heteromeric 5-HT3 receptors in both native tissue and recombinant receptor preparations.

α-Thujone reduces 5-HT3 receptor activity by an effect on the agonist-induced desensitization

The convulsant effects of alpha-thujone, the psychotropic component of absinthe, were attributed to inhibitory actions at the GABAA receptor. Here, we investigated for the first time the 5-HT3 receptor as a potential site of the psychotropic actions of alpha-thujone. This cation permeable ligand-gated ion channel shows considerable homology to the GABAA receptor. We previously demonstrated that in homomeric assemblies of cloned human 5-HT,A receptor subunits. the endogenous agonist 5-HT induced desensitization via channel blockade. When the 5-HT3 B receptor subunit was co-expressed, the resulting heteromeric assemblies desensitized independent from channel blockade. In the present study, patch-clamp experiments revealed an inhibitory action of alpha-thujone on both homomeric and heteromeric 5-HT3 receptors. This inhibitory action was mediated via channel blockade. However, it was not alpha-thujone itself which blocked the channel. The present experiments suggested that, in homomeric receptors, alpha-thujone enhanced the inherent channel-blocking potency of the natural ligand. 5-HT. In heteromeric receptors, alpha-thujonerecruited an additional channel-blocking component of the agonist. By means of kinetic modeling, we simulated possible mechanisms by which alpha-thuljone decreased the 5-HT-induced responses. It is suggested that alpha-thujone reduced 5-HT3 receptor activity by an effect on mechanisms involved in receptor desensitization, which depend on receptor subunit composition. It remains to be shown if this inhibitory action on serotonergic responses contributes to behavioral effects of alpha-thujone.

A cluster of novel serotonin receptor 3-like genes on human chromosome 3

The ligand-gated ion channel family includes receptors for serotonin (5-hydroxytryptamine, 5-HT), acetylcholine, GABA, and glutamate. Drugs targeting subtypes of these receptors have proven useful for the treatment of various neuropsychiatric and neurological disorders. To identify new ligand-gated ion channels as potential therapeutic targets, drafts of human genome sequence were interrogated. Portions of four novel genes homologous to 5-HT(3A) and 5-HT(3B) receptors were identified within human sequence databases. We named the genes 5-HT(3C1)-5-HT(3C4). Radiation hybrid (RH) mapping localized these genes to chromosome 3q27-28. All four genes shared similar intron-exon organizations and predicted protein secondary structure with 5-HT(3A) and 5-HT(3B). Orthologous genes were detected by Southern blotting in several species including dog, cow, and chicken, but not in rodents, suggesting that these novel genes are not present in rodents or are very poorly conserved. Two of the novel genes are predicted to be pseudogenes, but two other genes are transcribed and spliced to form appropriate open reading frames. The 5-HT(3C1) transcript is expressed almost exclusively in small intestine and colon, suggesting a possible role in the serotonin-responsiveness of the gut.

Modulation of 5-HT3 receptor-mediated response and trafficking by activation of protein kinase C

Modulation of neurotransmitter-gated membrane ion channels by protein kinase C (PKC) has been the subject of a number of studies. However, less is known about PKC modulation of the serotonin type 3 (5-HT3) receptor, a ligand-gated membrane ion channel that can mediate fast synaptic transmission in the central and peripheral nervous system. Here, we show that PKC potentiated 5-HT3 receptor-mediated current in Xenopus oocytes expressing 5-HT3A receptors and mouse N1E-115 neuroblastoma cells. In addition, using a specific antibody directed to the extracellular N-terminal domain of the 5-HT3A receptor, treatment with the PKC activator, 4 beta-phorbol 12-myristate 13-acetate (PMA), significantly increased surface immunofluorescence. PKC also increased the amount of 5-HT3A receptor protein in the cell membrane without affecting the amount receptor protein in the total cell extract. The magnitude of PMA potentiation of 5-HT3A receptor-mediated responses is correlated with the magnitude of PMA enhancement of the receptor abundance in the cell surface membrane. PMA potentiation is unlikely to occur via direct phosphorylation of the 5-HT3A receptor protein since the potentiation was not affected by point mutation of each of the putative sites for PKC phosphorylation. However, preapplication of phalloidin, which stabilizes the actin polymerization, significantly inhibited PMA potentiation of 5-HT-activated responses in both N1E-115 cells and oocytes expressing 5-HT3A receptors. On the other hand, latrunculin-A, which destabilizes actin cytoskeleton, enhanced the PMA potentiation of 5-HT3A receptors. The observations suggest that PKC can modulate 5-HT3A receptor function and trafficking through an F-actin-dependent mechanism.

A cytoplasmic region determines single-channel conductance in 5-HT3 receptors

5-hydroxytryptamine type 3 (5-HT3) receptors are cation-selective transmitter-gated ion channels of the Cys-loop superfamily. The single-channel conductance of human recombinant 5-HT3 receptors assembled as homomers of 5-HT3A subunits, or heteromers of 5-HT3A and 5-HT3B subunits, are markedly different, being 0.4 pS (refs 6, 9) and 16 pS (ref. 7), respectively. Paradoxically, the channel-lining M2 domain of the 5-HT3A subunit would be predicted to promote cation conduction, whereas that of the 5-HT3B subunit would not. Here we describe a determinant of single-channel conductance that can explain these observations. By constructing chimaeric 5-HT3A and 5-HT3B subunits we identified a region (the 'HA-stretch') within the large cytoplasmic loop of the receptor that markedly influences channel conductance. Replacement of three arginine residues unique to the HA-stretch of the 5-HT3A subunit by their 5-HT3B subunit counterparts increased single-channel conductance 28-fold. Significantly, ultrastructural studies of the Torpedo nicotinic acetylcholine receptor indicate that the key residues might frame narrow openings that contribute to the permeation pathway. Our findings solve the conundrum of the anomalously low conductance of homomeric 5-HT3A receptors and indicate an important function for the HA-stretch in Cys-loop transmitter-gated ion channels.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

5-HT3 receptors in the CNS: 3B or not 3B?

5-HT(3) receptors are widely distributed in the CNS and PNS where they participate in a variety of physiological processes. Native 5-HT(3) receptors in the CNS display functional and pharmacological heterogeneity, suggesting the existence of multiple receptor subunits. However, recent evidence suggests that of the two known subunits only the 5-HT(3A) receptor subunit (and not the 5-HT(3B) receptor subunit) is functionally present in the CNS. The molecular makeup of the 5-HT(3) receptor therefore remains an open question.

Co-expression of the 5-HT3B serotonin receptor subunit alters the biophysics of the 5-HT3 receptor

Homomeric complexes of 5-HT(3A) receptor subunits form a ligand-gated ion channel. This assembly does not fully reproduce the biophysical and pharmacological properties of native 5-HT(3) receptors which might contain the recently cloned 5-HT(3B) receptor subunit. In the present study, heteromeric assemblies containing human 5-HT(3A) and 5-HT(3B) subunits were expressed in HEK 293 cells to detail the functional diversity of 5-HT(3) receptors. We designed patch-clamp experiments with homomeric (5-HT(3A)) and heteromeric (5-HT(3AB)) receptors to emphasize the kinetics of channel activation and desensitization. Co-expression of the 5-HT(3B) receptor subunit reduced the sensitivity for 5-HT (5-HT(3A) receptor: EC(50) 3 micro M, Hill coefficient 1.8; 5-HT(3AB) receptor: EC(50) 25 micro M, Hill coefficient 0.9) and markedly altered receptor desensitization. Kinetic modeling suggested that homomeric receptors, but not heteromeric receptors, desensitize via an agonist-induced open-channel block. Furthermore, heteromeric 5-HT(3AB) receptor assemblies recovered much faster from desensitization than homomeric 5-HT(3A) receptor assemblies. Unexpectedly, the specific 5-HT(3) receptor agonist mCPBG induced an open-channel block at both homomeric and heteromeric receptors. Because receptor desensitization and resensitization massively affect amplitude, duration, and frequency of synaptic signaling, these findings are evidence in favor of a pivotal role of subunit composition of 5-HT(3) receptors in serotonergic transmission.

Interaction of d-tubocurarine analogs with mutant 5-HT(3) receptors

d-Tubocurarine is a potent competitive antagonist of both the muscle-type nicotinic acetylcholine receptor (AChR) and the serotonin type-3 receptor (5HT(3)R). We have previously used a series of structural analogs of d-tubocurarine to demonstrate that the ligand-binding domains of both receptors share common structural features. We have now extended these studies to examine the interaction of a series of d-tubocurarine analogs with 5HT(3)Rs containing mutations at either of two residues within the ligand-binding domain of the receptor (W90F and R92A). The W90F mutation results in an approximately 2-4-fold decrease in the affinity of the analogs relative to wild-type receptors, while the R92A results in an approximately 8-10-fold increase in affinity. However, since the effect of a given mutation is more or less equivalent for all analogs, neither residue W90 nor R92 is likely to make a specific interaction with d-tubocurarine itself. Rather, these two residues are likely to play a role in determining both the geometry of the binding site, as well as the overall environment that a ligand encounters in the binding site.