Quantitative mRNA analysis of serotonin 5-HT4 receptor isoforms, calcium handling proteins and ion channels in human atrial fibrillation

Serotonin 5-HT(4) receptors are present in human atrial myocytes and have been proposed to contribute to the generation of atrial fibrillation (AF). Here, we quantified 5-HT(4) receptors as well as other key genes involved in cardiac rhythm and contraction in right atrial appendages of patients with chronic AF (CAF) and acute AF (AAF). Right atrial appendages were obtained from eleven patients in sinus rhythm (SR), five with AAF and six with CAF (>12 months). TaqMan real time quantitative RT-PCR was performed on total RNA. Results were normalised to the average of three housekeeping genes, cyclophilin, GADPH and RL-19. The rank order of expression of h5-HT(4) receptors variants was (b)>(a)>(g)>(c) in the group of patients in SR. In AAF, we found a strong decrease in h5-HT(4(b)), h5-HT(4(c),) and h5-HT(4(g)) transcripts. In CAF patients, the mRNA expression level of the h5-HT(4(b)) isoform significantly increased two fold versus SR. A similar increase was reported for beta(1)-adrenergic receptor, connexin 43 and the L-type Ca(2+) channel CaCNA1C subunit. Interestingly, CAF was associated with a strong increase in the expression of Na(+)/Ca(2+) exchanger and the voltage-dependent Na(+) channel SCN5A subunit. Our results indicate that h5-HT(4(b)) is the dominant cardiac isoform of human 5-HT(4) receptors and its expression is increased in CAF. These data support the involvement of 5-HT(4) receptors in atrial arrhythmia.

5-hydroxytryptamine 4 receptor activation of the extracellular signal-regulated kinase pathway depends on Src activation but not on G protein or beta-arrestin signaling

The 5-hydroxytryptamine(4) (5-HT(4)) receptors have recently emerged as key modulators of learning, memory, and cognitive processes. In neurons, 5-hydroxytryptamine(4) receptors (5-HT(4)Rs) activate cAMP production and protein kinase A (PKA); however, nothing is known about their ability to activate another key signaling pathway involved in learning and memory: the extracellular signal-regulated kinase (ERK) pathway. Here, we show that 5-HT(4)R stimulation, in primary neurons, produced a potent but transient activation of the ERK pathway. Surprisingly, this activation was mostly PKA independent. Similarly, using pharmacological, genetic, and molecular tools, we observed that 5-HT(4)Rs in human embryonic kidney 293 cells, activated the ERK pathway in a G(s)/cAMP/PKA-independent manner. We also demonstrated that other classical G proteins (G(q)/G(i)/G(o)) and associated downstream messengers were not implicated in the 5-HT(4)R-activated ERK pathway. The 5-HT(4)R-mediated ERK activation seemed to be dependent on Src tyrosine kinase and yet totally independent of beta-arrestin. Immunocytofluorescence revealed that ERK activation by 5-HT(4)R was restrained to the plasma membrane, whereas p-Src colocalized with the receptor and carried on even after endocytosis. This phenomenon may result from a tight interaction between 5-HT(4)R and p-Src detected by coimmunoprecipitation. Finally, we confirmed that the main route by which 5-HT(4)Rs activate ERKs in neurons was Src dependent. Thus, in addition to classical cAMP/PKA signaling pathways, 5-HT(4)Rs may use ERK pathways to control memory process.

A comparison of the pharmacological properties of guinea-pig and human recombinant 5-HT4 receptors

BACKGROUND AND PURPOSE

5-HT(4) receptor agonists are used therapeutically to treat disorders of reduced gastrointestinal motility. Since such compounds are evaluated in guinea-pigs, we cloned, expressed and pharmacologically characterized the guinea-pig 5-HT(4) and human 5-HT(4(b)) splice variant, which share 95% homology. The functional properties of guinea-pig 5-HT(4(b)) receptors were compared with native receptors in guinea-pig colon.

EXPERIMENTAL APPROACH

Membrane radioligand binding and whole cell cAMP accumulation assays were used to determine the affinities, potencies and intrinsic activities (IA). Contraction of the guinea-pig distal colon longitudinal muscle myenteric plexus preparation (LMMP) was monitored to evaluate functional activity.

KEY RESULTS

pK(i) values for guinea-pig and human recombinant receptors, and guinea-pig striatum 5-HT(4) receptors, were in agreement, as were the potency and IA values for guinea-pig and human 5-HT(4) receptors expressed at a similar density ( approximately 0.2 pmol mg(-1) protein). Tegaserod was a potent (pEC(50)=8.4 and 8.7, respectively), full agonist at both guinea-pig and human 5-HT(4) receptors. In contrast, in the LMMP preparation, tegaserod was a potent, partial agonist (pEC(50)=8.2; IA=66%).

CONCLUSIONS AND IMPLICATIONS

Close agreement between the pharmacological properties of guinea-pig and human 5-HT(4) receptors support the use of guinea-pig model systems for the identification of 5-HT(4) receptor therapeutics. However, the mechanisms underlying the different agonist properties of tegaserod in recombinant and isolated tissue preparations, and the extent to which these impact the clinical efficacy of tegaserod as a prokinetic agent, remain to be determined.

Estradiol induces expression of 5-hydroxytryptamine (5-HT) 4, 5-HT5, and 5-HT6 receptor messenger ribonucleic acid in rat anterior pituitary cell aggregates and allows prolactin release via the 5-HT4 receptor

Serotonin [5-hydroxytryptamine (5-HT)] is known to control prolactin (PRL) release at a hypothalamic level, but a pituitary site of action remains poorly studied. The present study explores the acute effect of 5-HT on PRL release in rat anterior pituitary aggregate cell cultures, the influence of steroid and thyroid hormones, and the 5-HT receptor (5-HTR) subtype(s) involved. 5-HT elicited a prompt increase in basal PRL release, an effect strongly potentiated by estradiol (E(2)) in the culture medium (dose response 1-100 nm). In E(2) condition, the PRL response was not affected by the nonselective 5-HTR antagonists methysergide and methiothepin nor by 5-HTR1, 5-HTR2, 5-HTR3, 5-HTR6, and 5-HTR7/5 antagonists, but was fully blocked by the 5-HTR4 antagonist GR 113808. Among various agonist analogs, only the 5-HTR4 agonist cisapride and the 5-HTR2 agonist alpha-methyl-5-HT evoked PRL release. The effect of alpha-methyl-5-HT also required E(2) during culture and was abolished by GR 113808 but not by combined 5-HTR2A, B, and C blockade. In E(2)-treated aggregates, 5-HT caused a 5-fold increase in cAMP levels. The intact anterior pituitary expressed mRNA of all known members of the 5-HTR family. In aggregates, 5-HTR4, 5-HTR5, and 5-HTR6 mRNA expression required E(2) during culture. The effect of 5-HT on PRL release was not affected by blocking the serotonin transporter or the vesicular monoamine transporter. The present data suggest a widespread expression of 5-HTRs in the rat anterior pituitary, several of which are up-regulated by estrogen, and that, in the presence of estrogen, one of these, the 5-HTR4, mediates acute PRL release.

Comparison of 5-HT4 and 5-HT7 receptor expression and function in the circular muscle of the human colon

Serotonin receptors are potential targets for treating functional bowel disorders. This study investigated the functional roles and expression of the 5-HT4 and the 5-HT7 receptor, which coexist in human colon circular smooth muscle. 5-HT3 receptor expression was also investigated. Part of the relaxant response to 5-HT was due to activation of 5-HT4 receptors as the apparent pKB value of the selective 5-HT4 antagonist, GR 113808, was 9.36. 5-HT4 mRNA levels were low in five tissues and undetectable in four others, but all responded to 5-HT with an EC50 value of 102.54+/-19.32 nM. The contribution of 5-HT7 receptors to the response was not readily demonstrated using the selective 5-HT7 antagonist, SB-269970, as its apparent pKB value of 7.19 (5-HT4 block with 1 microM GR 113808) was lower than the value obtained using the 5-HT7 guinea pig ileum assay (8.62). Nevertheless, the 5-HT7 receptor was expressed more consistently than the 5-HT4, but at similar levels. The 5-HT(3Ashort) and 5-HT(3B) subunits were co-expressed at similar levels, but the 5-HT(3Along) subunit was detected in only five of the nine samples tested. The findings show that 5-HT4-induced relaxation occurs at low to undetectable levels of tissue mRNA, as measured by qPCR. Although 5-HT7 receptor mRNA is detected at low, but consistent levels, the functional activity of this receptor is not readily identified given the currently available drugs.

Activation of 5-HT4 receptors inhibits secretion of β-amyloid peptides and increases neuronal survival

Activation of 5-HT4 receptors has been shown to improve memory processes in preclinical cognition models, suggesting potential utility of 5-HT4 agonists for the symptomatic treatment of Alzheimer's disease (AD). Recent studies have shown that 5-HT4 agonists also increase the secretion of the non-amyloidogenic soluble amyloid precursor protein-alpha (sAPPalpha). In the present study, we demonstrated that a selective 5-HT4 partial agonist, RS67333, inhibited the generation of beta-amyloid peptide (Abeta) in primary cortical cultures of Tg2576 transgenic mice expressing human APP(K670N/M671L). Furthermore, treatments with RS67333 selectively increased the survival of transgenic neurons in a dose-dependent manner, which was inhibited by 5-HT4 antagonists. These and previous data collectively suggest that the 5-HT4 receptor may be an effective therapeutic target for AD, providing both symptomatic improvements and neuroprotection.

The role of central 5-hydroxytryptamine (5-HT, serotonin) receptors in the control of micturition

At present the most investigated 5-HT receptor that has been shown to play a role in the control of micturition is the 5-HT(1A) receptor followed by 5-HT(7), 5-HT(2) and 5-HT(3) receptors. Most experiments focus on the control these receptors have on the parasympathetic outflow to the bladder and the somatic outflow to the external urethral sphincter (EUS) in the rat. Furthermore, 5-HT(1A) and 5-HT(7) receptors have been identified as having an excitatory physiological role in the control of bladder function. 5-HT(1A) receptors act, at least in the rat, at both a spinal (probably a heteroreceptor) and supraspinal (probably an autoreceptor) level, while 5-HT(7) receptors only act at a supraspinal level. Additionally, in the rat, 5-HT administered at a spinal or supraspinal site has an excitatory action, although earlier experiments have shown that activating 5-HT-containing brain areas causes inhibition of the bladder. Recent experiments have also indicated that blockade of the 5-HT(1A) receptor pathway shows rapid tolerance. However, no data exist for the development of tolerance for the 5-HT(7) receptor pathway. Neither receptor seems to play a role in the control of the urethra. Regarding 5-HT(2) receptors, activation of this receptor subtype inhibits micturition, and this inhibitory action may occur at a spinal, supraspinal or both levels. Although no physiological role for 5-HT(2C) receptors can yet be identified, 5-HT(2C) receptors have been implicated in the proposed supraspinal tonically active 5-HT(1A) autoreceptor (negative feedback) pathway. This proposition reconciles the data that central 5-HT-containing pathways are inhibitory to micturition, while 5-HT(1A) receptors, although inhibitory to adenylyl cyclase, have an excitatory function. This is because activation of 5-HT(1A) autoreceptors reduces the release of 5-HT thus reducing the activation of the 5-HT(2C) receptors, which are inhibitory in the control of micturition (disinhibition). Furthermore, 5-HT(2A) receptors in the rat and 5-HT(2C) receptors in the guinea pig cause activation of the EUS. In this respect, 5-ht(5A) receptors have also been identified in Onuf's nucleus, the site of somatic motoneurones controlling this sphincter. In the cat there is very little evidence to indicate that 5-HT receptors are involved in micturition except under pathological conditions in which activation of 5-HT(1A) receptors causes inhibition of micturition. Interestingly, under such conditions 5-HT(1A) receptors cause excitation of the EUS. Nevertheless, spinal 5HT(3) receptors have been implicated in the physiological control of micturition in the cat, but not yet in the rat. Overall, the data support the view that 5-HT receptors are important in the control of micturition. However, many more studies are required to fully understand these roles and why there are such species differences.

Nonantiarrhythmic drug therapy for atrial fibrillation

Recent studies have begun to elucidate the molecular mechanisms that promote the generation and progressive nature of atrial fibrillation. Evidence from both experimental and clinical investigations has implicated an important role for the renin-angiotensin-aldosterone system, inflammation, and oxidative stress, with data that suggest a potential beneficial effect for angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, aldosterone receptor antagonists, antiinflammatory agents, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins), and omega-3 polyunsaturated fatty acids. In addition, compounds that increase gap junctional conductance or that block 5-hydroxytryptamine-4 receptors have also shown promise in the experimental setting. Large-scale, prospective clinical trials will clarify the utility of these new therapeutic approaches to prevent atrial fibrillation in specific clinical settings.

Wood creosote prevents CRF-induced motility via 5-HT3 receptors in proximal and 5-HT4 receptors in distal colon in rats

Wood creosote has been used as an herbal medicine against acute diarrhea caused by food poisoning and has an inhibitory effect on colonic motility and enterotoxin-induced ion secretion. Since no previous studies have examined the effects of wood creosote on stress-induced alteration of colonic motility, we examined the effects on the colonic motility altered by intracerebroventricular (i.c.v.) injection of corticotropin-releasing factor (CRF), which is a key mediator in responses to stress. We recorded motor activity in proximal and distal colon of unrestrained conscious rats via two manometory catheters. The frequencies of phase III-like contraction and the % motor indices in both proximal and distal colon were measured. At the same time the number of fecal pellets excreted was counted. I.c.v. injection of CRF increased the motor activity in both proximal and distal colon, and these effects were completely antagonized by i.c.v. injection of a selective CRF type 1 antagonist but not by a CRF type 2 antagonist. Changes in colonic motility induced by CRF were reversed by intravenously administered wood creosote. Intraluminal administration of the 5-HT(3) receptor antagonist granisetron, or the 5-HT(4) receptor antagonist SB 204070 blocked the increase in colonic motility induced by i.c.v. injection of CRF. Wood creosote prevented the increase in colonic motility induced by the 5-HT(3) receptor agonist SR57227A in the proximal colon, while it prevented the increase in colonic motility induced by the 5-HT(4) receptor agonist RS67506 in the distal colon. These results indicate that wood creosote prevents the increase in colonic motility induced by CRF via 5-HT(3) receptors in the proximal colon, and via 5-HT(4) receptors in the distal colon, suggesting that wood creosote might be useful to treat stress-induced diarrhea.

Epac- and Rap- independent ERK1/2 phosphorylation induced by Gs-coupled receptor stimulation in HEK293 cells

Serotonin activates Ras and Ras-dependent ERK1/2 phosphorylation in HEK293 cells expressing G(s)-coupled 5-HT(4) or 5-HT(7) serotonin receptors through unknown mechanisms. Both Epac/Rap-dependent and -independent pathways for Ras-dependent ERK1/2 activation have been suggested. Epac overexpression or Epac-specific 8-CPT-2'-O-Me-cAMP did not cause ERK1/2 phosphorylation, despite Rap activation. The data did not support a role for PLCepsilon or DAG-dependent Ras GEFs of the Ras-GRP family in Ras-dependent ERK1/2 phosphorylation. However, serotonin stimulated phosphorylation of endogenous and recombinant Ras-GRF1, increased [Ca(2+)](i) and caused Ca(2+)- and calmodulin-dependent ERK1/2 phosphorylation. Different signalling pathways seem to be utilised by G(s)-coupled receptors in various isolates of HEK293 cells.

5-Hydroxytryptamine-4 receptor messenger ribonucleic acid levels and densities in gastrointestinal muscle layers from healthy dairy cows1

Serotonin (5-hydroxytryptamine, 5-HT) is involved in gastrointestinal tract (GIT) motor functions through binding to specific receptors located in the GIT walls. The objectives of the current study were to compare mRNA levels and binding sites of 5-HT(4) receptors (5-HTR(4)) in smooth muscle layers from the fundus abomasi, pylorus, ileum, cecum, proximal loop of the ascending colon (PLAC), and external loop of the spiral colon (ELSC) of healthy dairy cows, and to verify whether mRNA and protein expression were correlated. Smooth muscle samples were prepared by scraping the mucosa and submucosa from full-thickness intestinal wall samples. The mRNA levels of 5-HTR(4) were measured by real-time PCR and expressed relative to those of the housekeeping gene glyceraldehyde phosphate dehydrogenase. Binding studies were performed using the 5-HTR(4) antagonist [(3)H]GR113808. The mRNA levels of 5-HTR(4) were affected (P < 0.05) by location along the GIT. The mRNA levels of 5-HTR(4) in the ELSC and the ileum were greater than in the PLAC (P = 0.05 and P = 0.07, respectively) but similar to those of all other locations. The competitive binding of [(3)H]GR113808 to suspended membranes from the fundus abomasi, pylorus, cecum, and ELSC was best fit by a 2-site receptor model, whereas it was best fit by a 1-site receptor model in the ileum and PLAC. The mRNA levels and numbers of 5-HTR(4) were not correlated (r = 0.14; P = 0.71). In conclusion, mRNA and binding sites for 5-HTR(4) are present in the smooth muscle layer of the entire GIT of dairy cows and may play a role with respect to motility. The effects of activation of this receptor subtype may be different among GIT locations due to differences in the amount of high- relative to low-affinity binding sites.

Differential functional effects of two 5-HT4 receptor isoforms in adult cardiomyocytes

Serotonin 5-HT4 receptors are present in human atrial myocytes and have been proposed to contribute to the generation of atrial fibrillation. However, 5-HT4 receptors have so far been only found in human and pig atria and are absent from the heart of small laboratory animals, such as rat, guinea pig, rabbit and frog, which limits the experimental settings for studying their functional properties. In this study, we developed an adenovirus expression system to examine the properties of two human 5-HT4 receptor splice variants, h5-HT4(b) and h5-HT4(d), expressed in adult cardiomyocytes devoid of native 5-HT4 receptors. When expressed in the HL-1 murine cell line of atrial origin, both receptors caused specific binding of the 5-HT4 selective antagonist GR113808 and activated adenylyl cyclase in the presence of serotonin (5-HT, 1 microM). When expressed in freshly isolated adult rat ventricular cardiomyocytes, a stimulation of the L-type Ca2+ current (ICa,L) by 5-HT (100 nM) was revealed. Both effects were blocked by GR113808. In HL-1 cells, the h5-HT4(d) receptor was found to be more efficiently coupled to adenylyl cyclase than the h5-HT4(b). Pertussis toxin treatment (250 ng/ml for 5 h) potentiated the stimulatory effect of 5-HT on ICa,L in rat myocytes expressing the h5-HT4(b) but not the h5-HT4(d) receptor, indicating a likely coupling of the (b) isoform to both Gs and Gi/o proteins. Adenoviral expression of h5-HT4 receptor isoforms in adult cardiac myocytes provides a valuable means for the exploration of the receptor signaling cascades in normal and pathological situations.

5‐HT4 receptor activation induces long‐lasting EPSP‐spike potentiation in CA1 pyramidal neurons

Recent studies implicated involvement of the 5-hydroxytryptamine4 (5-HT4) receptor in cognitive and emotional processes. The highest 5-HT4 receptor densities in the brain are found in the limbic system including the hippocampus. Here we used the selective 5-HT4 receptor full agonist, N-pentyl-N'-aminoguanidine carbazimidamide (SDZ-216454) to characterize effects of 5-HT4 receptor activation in whole-cell and field recordings in the area CA1 in hippocampal slices prepared from 3 to 4- and 6 to 9-week-old rats, respectively. Extracellular recordings showed that transient 5-HT4 receptor activation by 10-20 min application of SDZ-216454 induces field excitatory postsynaptic potential (fEPSP)-population spike potentiation (ESP(5-HT4)), which persisted for as long as we held the recordings (> 2 h). ESP(5-HT4) displayed characteristics different from EPSP-spike potentiation that accompanies long-term potentiation; it developed without an associated increase in synaptic transmission, was independent on afferent input, activity of postsynaptic neurons and N-methyl-d-aspartate receptor activation; and was expressed in the presence of GABA receptor antagonists. ESP(5-HT4) was also induced by transient application of the natural neurotransmitter, 5-HT. The increase in the evoked population spike (PS) induced by SDZ-216454 was not prevented by blockers of hyperpolarization-activated cation current (Ih), Cs+ and ZD-7288, but was mimicked and occluded by 150 microm Ba2+. Whole-cell voltage-clamp recordings from pyramidal neurons demonstrated that SDZ-216454 application increases membrane resistance with a concomitant decrease in a Ba2+-sensitive inwardly rectifying K+ current and the Ba2+-insensitive K+ current underlying slow afterhyperpolarization (I(sAHP)). We conclude that 5-HT4 receptor activation may cause a long-lasting excitability increase in CA1 pyramidal neurons by inhibition of a Ba2+-sensitive inwardly rectifying K+ current.

Adaptive changes in serotonin neurons of the raphe nuclei in 5-HT4receptor knock-out mouse

Decreased serotonin (5-HT) transmission is thought to underlie several mental diseases, including depression and feeding disorders. However, whether deficits in genes encoding G protein-coupled receptors may down-regulate the activity of 5-HT neurons is unknown currently. Based on recent evidence that stress-induced anorexia may involve 5-HT(4)receptors (5-HT(4)R), we measured various aspects of 5-HT function in 5-HT(4)R knock-out (KO) mice. When compared to dorsal raphe nucleus (DRN) 5-HT neurons from wild-type mice, those from 5-HT(4)R KO mice exhibited reduced spontaneous electrical activity. This reduced activity was associated with diminished tissue levels of 5-HT and its main metabolite, 5-hydroxyindole acetic acid (5-HIAA). Cumulative, systemic doses of the 5-HT uptake blocker citalopram, that reduced 5-HT cell firing by 30% in wild-type animals, completely inhibited 5-HT neuron firing in the KO mice. This effect was reversed by administration of the 5-HT(1A) receptor (5-HT(1A)R) antagonist, WAY100635, in mice of both genotypes. Other changes in DRN of the KO mice included increases in the levels of 5-HT plasma membrane transporter sites and mRNA, as well as a decrease in the density of 5-HT(1A)R sites without any change in 5-HT(1A) mRNA content. With the exception of increased 5-HT turnover index in the hypothalamus and nucleus accumbens and a decreased density of 5-HT(1A)R sites in the dorsal hippocampus (CA1) and septum, no major changes were detected in 5-HT territories of projection, suggesting region-specific adaptive changes. The mechanisms whereby 5-HT(4)R mediate a tonic positive influence on the firing activity of DRN 5-HT neurons and 5-HT content remain to be determined.

Electro-acupuncture attenuates stress-induced defecation in rats with chronic visceral hypersensitivity via serotonergic pathway

Acupuncture has long been used for patients with irritable bowel syndrome. However, it has remained unclear. The aim of this study was to testify the effect of electro-acupuncture(EA) on (1) visceral hypersensitivity induced by the mechanical colorectal irritation during postnatal development of rats, and (2) stress-induced colonic motility changes on rats with chronic visceral hypersensitivity. The abdominal withdrawal reflex (pain threshold and score) for visceral hypersensitivity and fecal pellet output for motor dysfunction were selected as two indexes for measurement. In addition, the effect of EA on 5-HT(4a) receptor and serotonin transporter (SERT) expression in the colon mucosa was analyzed semi-quantitatively through immunohistochemistry and 5-HT concentration in the colon tissue was observed through spectro-photo-fluorimeter detection, respectively. Our results showed that EA significantly elevated pain threshold, decreased the scores and also decreased fecal pellet output during water avoid stress. Furthermore, EA decreased 5-HT concentration in colon in rats with CVH and CVH rats with water avoidance stress, and increased the 5-HT(4a) and SERT expression in rats with CVH. Thus, it can be concluded that EA attenuates behavioral hyperalgesia and stress-induced colonic motor dysfunction in CVH rats via serotonergic pathway.

Identification of neurons that express 5-hydroxytryptamine4 receptors in intestine

5-Hydroxytryptamine (5-HT) is an endogenous stimulant of intestinal propulsive reflexes. It exerts its effects partly through 5-HT4 receptors; 5-HT4 receptor agonists that are stimulants of intestinal transit are in clinical use. Both pharmacological and recent immunohistochemical studies indicate that 5-HT4 receptors are present on enteric neurons but the specific neurons that express the receptors have not been determined. In the present work, we describe the characterization of an anti-5-HT4 receptor antiserum that reveals immunoreactivity for enteric neurons and other cell types in the gastrointestinal tract. With this antiserum, 5-HT4 receptor immunoreactivity has been found in the muscularis mucosae of the rat oesophagus, a standard assay tissue for 5-HT4 receptors. It is also present in the muscularis mucosae of the guinea-pig and mouse oesophagus. In guinea-pig small intestine and rat and mouse colon, 5-HT4 receptor immunoreactivity occurs in subpopulations of enteric neurons, including prominent large neurons. Double-staining has shown that these large neurons in the guinea-pig small intestine are also immunoreactive for two markers of intrinsic primary afferent neurons, cytoplasmic NeuN and calbindin. Some muscle motor neurons in the myenteric ganglia are immunoreactive for this receptor, whereas it is rarely expressed by secretomotor neurons. Immunoreactivity also occurs in the interstitial cells of Cajal but is faint in the external muscle. Expression of the protein and mRNA has been confirmed in extracts containing enteric neurons. The observations suggest that one site of action of 5-HT4 receptor agonists is the intrinsic primary afferent neurons.

Design and synthesis of specific probes for human 5-HT4 receptor dimerization studies

Recently, human 5-HT4 receptors have been demonstrated to form constitutive dimers in living cells. To evaluate the role of dimerization on the 5-HT4 receptor function, we investigated the conception and the synthesis of bivalent molecules able to influence the dimerization process. Their conception is based on a model of the 5-HT4 receptor dimer derived from protein/protein docking experiments. These bivalent ligands are constituted by two ML10302 units, a specific 5-HT4 ligand, linked through a spacer of different sizes and natures. These synthesized bivalent ligands were evaluated in binding assays and cyclic AMP production on the 5-HT4(e/g) receptor isoform stably transfected in C6 glial cells. Our data showed that bivalent ligands conserved a similar affinity compared to the basal ML10302 unit. Nevertheless, according to the nature and the size of the spacer, the pharmacological profile of ML10302 is more or less conserved. In view of the interest of bivalent ligands for investigating the GPCR dimerization process, these 5-HT4 specific bivalent ligands constitute valuable pharmacological tools for the study of 5-HT4 receptor dimerization.

Optimization of a pharmacophore model for 5-HT4 agonists using CoMFA and receptor based alignment

Twenty two 5-HT4 agonists obtained from our laboratory and the recent literature were used to develop a CoMFA model to predict 5-HT4 agonist activity. Two models were produced and compared for predictivity, the first by alignments based on atom overlapping (model A) and the second by adding agonist binding site interacting points of the 5-HT4 receptor (model B). Comparison of the two models showed that the q2 value for model A was 0.564 vs. 0.582 for model B. Model B indicated that the predictive power model stems from far lower steric contributions, 0.270 compared to model A's 0.502. The dominant defining features were the electrostatic contributions for model B, 0.664 up from 0.477 in model A. The contributions from the LogP factor were minimal, 0.085 in both models. The synthesized compounds showed agonist activity at mumol level.

Internalization of Human 5-HT4a and 5-HT4b Receptors is Splice Variant Dependent

The family of 5-HT4 receptors comprises 16 putative splice variants. We have previously shown that there are differences in signal transduction of the h5-HT4a and h5-HT4b receptors. In the present study, the internalization of these two splice variants following receptor stimulation was investigated with confocal microscopy on living cells. Chimeric receptors, h5-HT4a-GFP and h5-HT4b-GFP were generated by fusing the coding sequence of the 5-HT4 receptor with the coding sequence of the GFP. The agonist stimulation of fluorescent receptors resulted in a time-dependent internalization of the h5-HT4b-GFP receptor, but not of the h5-HT4a-GFP receptor. The h5-HT4b receptor displays a dual coupling to Gαi,o and Gαs proteins, in contrast to the h5-HT4a receptor, which couples to Gαs proteins only. We investigated whether the difference in internalization of the two splice variant receptors was related to their differential coupling. Therefore, we performed agonist-stimulation of the receptor following inhibition of the Gαi,o protein coupling using PTX. The h5-HT4b receptor internalization is PTX insensitive. We co-transfected the fluorescent chimeric receptors with other wild-type variants, which did not produce an alteration of the receptor trafficking. These findings provide the first evidence of differential internalization between the two splice variants, 5-HT4a and 5-HT4b receptors.

Prucalopride is a partial agonist through human and porcine atrial 5-HT4 receptors: comparison with recombinant human 5-HT4 splice variants

Prucalopride is a gastrointestinal prokinetic drug that acts through 5-HT4 receptors, but its potential effects on cardiac atrial function are unknown. We investigated the effects of prucalopride on human right atrium, piglet left atrium, and piglet sinoatrial node. The effects of prucalopride on 5-HT4 receptor splice variants a, b, g and i, known to be expressed in human atrium, were studied for comparison. Prucalopride was an inotropic partial agonist, compared with 5-HT, on paced human atrial trabeculae (-logEC50M=7.4) and porcine left atria (-logEC50M=7.2), with intrinsic activity of 0.77 and 0.63 respectively. Prucalopride (1 microM) surmountably antagonized the positive inotropic effects of 5-HT on human (pK(P)=7.2) and porcine (pK(P)=7.1) atrium. Prucalopride was also a chronotropic partial agonist (-logEC50M=7.4, intrinsic activity=0.72 with respect to 5-HT) on spontaneously beating piglet atria. The cardiostimulant effects of prucalopride were prevented by GR113808 (1 microM), consistent with mediation through 5-HT4 receptors. Prucalopride bound to recombinant 5-HT4(a), 5-HT4(b), 5-HT4(g), and 5-HT4(i) receptors, labeled by [3H]GR113808, with pKi values of 7.6, 7.5, 7.4, and 7.8 respectively. Prucalopride stimulated adenylyl cyclase as a partial agonist on 5-HT4(a), 5-HT4(b), and 5-HT4(i) receptors with intrinsic activities of 0.82, 0.86, and 0.78 and -logEC50 values of 7.2, 7.3, and 7.2 respectively. At the 5-HT4(g) receptor prucalopride acted as a full agonist (-logEC50M=8.0) compared with 5-HT in the cell line tested, which was probably due to high receptor expression levels. We conclude that prucalopride is a cardiostimulatory partial agonist through human and porcine 5-HT4 receptors. Since prucalopride acts similarly through 5-HT4(a), 5-HT4(b), 5-HT4(g), and 5-HT4(i) receptors, any of these variants could be involved in the mediation of cardiostimulation.

Molecular Characterization of a Purified 5-HT4 Receptor

Serotonin 5-HT(4(a)) receptor, a G-protein-coupled receptor (GPCR), was produced as a functional isolated protein using Escherichia coli as an expression system. The isolated receptor was characterized at the molecular level by circular dichroism (CD) and steady-state fluorescence. A specific change in the near-UV CD band associated with the GPCR disulfide bond connecting the third transmembrane domain to the second extracellular loop (e2) was observed upon agonist binding to the purified receptor. This is a direct experimental evidence for a change in the conformation of the e2 loop upon receptor activation. Different variations were obtained depending whether the ligand was an agonist (partial or full) or an inverse agonist. In contrast, antagonist binding did not induce any variation. These observations provide a first direct evidence for the fact that free (or antagonist-occupied), active (partial- or full agonist-occupied) and silent (inverse agonist-occupied) states of the receptor involve different arrangements of the e2 loop. Finally, ligand-induced changes in the fluorescence emission profile of the purified receptor confirmed that the partial agonist stabilized a single, well-defined, conformational state and not a mixture of different states. This result is of particular interest in a pharmacological perspective since it directly demonstrates that the efficacy of a drug is likely due to the stabilization of a ligand-specific state rather than selection of a mixture of different conformational states of the receptor.

Modifications of 5-HT4 receptor expression in rat brain during memory consolidation

Pharmacological evidence indicates a specific role of 5-HT(4) receptors on memory function. These receptors are members of G-protein-coupled 7-transmembrane domain receptor superfamily, are positively coupled to adenylyl cyclase, and are heterogeneously located in some structures important for memory, such as the hippocampus and cortical regions. To further clarify 5-HT(4) receptors' role in memory, the expression of these receptors in passive (P3) untrained and autoshaping (A3) trained (3 sessions) adult (3 months) and old (P9 or A9; 9 months) male rats was determined by autoradiography. Adult trained (A3) rats showed a better memory respect to old trained (A9). Using [(3)H] GR113808 as ligand (0.2 nM specific activity 81 Ci/mmol) for 5-HT(4) receptor expression, 29 brain areas were analyzed, 16 areas of A3 and 17 of A9 animals displayed significant changes. The medial mammillary nucleus of A3 group showed diminished 5-HT(4) receptor expression, and in other 15 brain areas of A3 or 10 of A9 animals, 5-HT(4) receptors were increased. Thus, for A3 rats, 5-HT(4) receptors were augmented in olfactory lobule, caudate putamen, fundus striatum, CA2, retrosplenial, frontal, temporal, occipital, and cingulate cortex. Also, 5-HT(4) receptors were increased in olfactory tubercule, hippocampal CA1, parietal, piriform, and cingulate cortex of A9. However, hippocampal CA2 and CA3 areas, and frontal, parietal, and temporal cortex of A9 rats, expressed less 5-HT(4) receptors. These findings suggest that serotonergic activity, via 5-HT(4) receptors in hippocampal, striatum, and cortical areas, mediates memory function and provides further evidence for a complex and regionally specific regulation over 5-HT receptor expression during memory formation.

Serotonin inhibits Na+/H+ exchange activity via 5-HT4 receptors and activation of PKC alpha in human intestinal epithelial cells

BACKGROUND & AIMS

Increased serotonin levels have been implicated in the pathophysiology of diarrhea associated with celiac and inflammatory diseases. However, the effects of serotonin on Na+ /H+ exchange (NHE) activity in the human intestine have not been investigated fully. The present studies examined the acute effects of 5-hydroxytryptamine (5-HT) on NHE activity using Caco-2 cells as an in vitro model.

METHODS

Caco-2 cells were treated with 5-HT (.1 micromol/L, 1 h) and NHE activity was measured as ethyl-isopropyl-amiloride (EIPA)-sensitive 22Na uptake. The effect of 5-HT receptor-specific agonists and antagonists was examined. The role of signaling intermediates in 5-HT-mediated effects on NHE activity was elucidated using pharmacologic inhibitors and immunoblotting.

RESULTS

NHE activity was inhibited significantly (approximately 50%-75%, P < .05) by .1 micromol/L 5-HT via inhibition of maximal velocity (Vmax) without any changes in apparent affinity (Km) for the substrate Na+ . NHE inhibition involved a decrease of both NHE2 and NHE3 activities. Studies using specific inhibitors and agonists showed that the effects of 5-HT were mediated by 5-HT4 receptors. 5-HT-mediated inhibition of NHE activity was dependent on phosphorylation of phospholipase C gamma 1 (PLC gamma 1) via activation of src-kinases. Signaling pathways downstream of PLC gamma 1 involved increase of intracellular Ca 2+ levels and subsequent activation of protein kinase C alpha (PKC alpha). The effects of 5-HT on NHE activity were not cell-line specific because T84 cells also showed NHE inhibition.

CONCLUSIONS

A better understanding of the regulation of Na+ absorption by 5-HT offers the potential for providing insights into molecular and cellular mechanisms involved in various diarrheal and inflammatory disorders.

Serotonin 5-HT4receptors and their mRNAs in rat and guinea pig brain: Distribution and effects of neurotoxic lesions

Serotonin 5-HT4 receptors are widely distributed in the periphery and in brain, where they modulate the release of various neurotransmitters and have been implicated in learning and memory. Nine C-terminal splice variants of this receptor have been cloned in mammalian species. In the rat, three such variants have been described: 5-HT4(a), 5-HT4(b), and 5-HT4(e). In the present study, we have examined several aspects of the distribution of these receptors in brain. First, we provide, in rat and guinea pig, a detailed comparison of the distribution of 5-HT4 receptors labeled by the antagonist [125I]-SB 207710 with the distribution of their encoding mRNA visualized by in situ hybridization histochemistry (ISHH). The results suggest that, in several projection systems (striato-nigral and striato-pallidal pathways, projection from dentate granule cells to field CA3, habenulo-interpeduncular pathway), 5-HT4 receptors are located both somatodendritically and axonally. Second, we have analyzed the distribution of mRNA for the three known rat splice variants by reverse transcription-polymerase chain reaction (RT-PCR) and by ISHH. RT-PCR indicates that all three variants are widely distributed, with 5-HT4(b) mRNA being present in all regions examined (olfactory tubercle, striatum, hippocampus, inferior colliculus, substantia nigra, parietal cortex) and 5-HT4(a) and 5-HT4(e) showing a somewhat more restricted distribution. In other regions (periaqueductal gray, reticular formation, medial septum, diagonal band), faint ISHH signals are observed for 5-HT4(a)+4(e) mRNAs, whereas 5-HT4(b) mRNA signals are almost undetectable. Finally, neurotoxic lesions of basal ganglia components in guinea pig also indicate a location of these receptors on terminals of striatal projection neurons.

Pharmacological modulation of the hyperpolarization-activated current (If) in human atrial myocytes: focus on G protein-coupled receptors

AIMS

In human atrial myocytes (HuAM) two beta-adrenergic receptors (beta-AR) and four splicing-variants of the serotonin 5-HT(4) receptor are present. Multiple coupling with G stimulatory (G(s)) and G inhibitory (G(i)) proteins has been proposed for both beta(2)-AR and 5-HT((4b)) subtypes, but no functional data exist in HuAM. Serotonin (5-HT) and catecholamines are able to trigger arrhythmias in human atrium, but the underlying cellular mechanisms are not completely understood. The pacemaker current (I(f)) is an inward Na(+)/K(+) current, constitutively present in HuAM and directly modulated by cAMP; I(f) could play a role in triggering human atrial arrhythmias. This study evaluated the different G protein coupling of beta(1)-AR, beta(2)-AR and 5-HT(4) receptors by assessing the modulation of I(f) by selective stimuli.

METHODS

HuAM were isolated from right atrial appendages and utilized for patch-clamp recording. The coupling of receptor subtypes with G(i) proteins was tested by incubating HuAM in pertussis toxin (PTX).

RESULTS

Beta(1)-AR stimulation (Isoprenaline [ISO] + ICI 118,551), and 5-HT caused a concentration-dependent significant shift of the half activation potential of I(f) activation curve (DeltaV(h)), P < 0.01. beta(2)-AR stimulation (ISO 1 microM + CGP 20712A) also significantly shifted V(h) (P < 0.0001), but with DeltaV(h)[beta(2)-AR] significantly smaller than the effect caused by 1 microM beta(1)-AR stimulation (P < 0.05). Pre-treatment of HuAM with PTX did not alter the effect of beta(1)-AR stimulation (both 0.1 and 1 microM) and 1 microM 5-HT on I(f), but significantly increased the effect in response to beta(2)-AR stimulation and 0.1 microM 5-HT (P < 0.05 for both), thus suggesting a G(i) protein coupling of these receptors.

CONCLUSIONS

Our results provide the first functional evidence of the different G protein coupling of beta(1)-AR, beta(2)-AR and 5-HT(4) receptors in HuAM. Further they support the view that I(f) current might play an important role in triggering catecholamines and serotonin-induced atrial arrhythmias.