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5‐HT4 receptors are G protein‐coupled receptors that link to the stimulatory protein Gs which activates adenylate cyclase to increase intracellular cyclic AMP which then activates protein kinase A (PKA). 5‐HT4 receptors are expressed by neurons in the central and peripheral nervous systems especially the enteric nervous system (ENS). In general, 5‐HT4 receptors are stimulatory and their activation in the ENS enhances neurotransmitter release and propulsive motility patterns. 5‐HT4 receptors are expressed by enterochromaffin (EC) cells, Goblet cells, and most enteric neurons. The study by Konen and colleagues in this issue of Neurogastroenterology and Motility features two novel 5‐HT4 receptor agonists (5‐HT4‐LA1 and 5‐HT4‐LA‐2) that are not absorbed from the gastrointestinal tract of mice and act locally in the colonic mucosa to stimulate propulsive motility. The authors show that 5‐HT4‐LA1 and 5‐HT4‐LA2 were not absorbed from the colon and that both drugs stimulated colonic transit when administered by gavage. Both agonists stimulated colonic glass bead expulsion, and 5‐HT4LA1 activation stimulated fecal output and increased fecal water content. These effects were detected in young and aged mice. 5‐HT4 receptors were also localized to the epithelium of the human duodenum, ileum, and colon. These studies highlight novel 5‐HT4 receptor agonists that have prokinetic actions on the GI tract. These drugs are not absorbed and act locally in the gut mucosa to stimulate propulsive motility while minimizing access to systemic 5‐HT4 receptors and avoiding potential unwanted side effects. [ABSTRACT FROM AUTHOR]

Background: The 5-hydroxytryptamine (5-HT) neurotransmitter system and lateral habenula (LHb) are involved in the regulation of depression, while the mechanisms remain to be clarified. Objectives: The effects and possible mecha-nism underlying activation or blockade of 5-HT4 receptors (5-HT4Rs) in the LHb in depression were investigated by behavioral and neurochemical methods based on a Parkinson's disease (PD) rat model. Method: 6-Hydroxydopamine (6-OHDA) was injected unilaterally into the substantia nigra pars compacta to establish the PD rat model. The depressive-like behaviors were measured by the forced swimming test (FST) and sucrose preference test (SPT). The concentrations of dopamine (DA), noradrenaline (NA) and 5-HT in the related brain regions were measured by a neurochemical method. Results: The 6-OHDA lesions increased the immobility time in the FST and decreased the sucrose consumption in the SPT, suggesting the induction of depressive-like behaviors. Intra-LHb injection of BIMU-8 (5-HT4R agonist) or GR113808 (5-HT4R antagonist) produced antidepressant effects in the lesioned rats. Intra-LHb injection of BIMU-8 significantly increased the DA levels in the medial prefrontal cortex (mPFC) and ventral hippocampus (vHip), increased the 5-HT level in the mPFC and decreased the NA level in the vHip only in the lesioned rats, while intra-LHb injection of GR113808 changed DA, NA and 5-HT levels in the mPFC, LHb and vHip in both sham and the lesioned rats. Conclusions: All these results suggest that activation or blockade of the LHb 5-HT4Rs produce antidepressant effects in the 6-OHDA-lesioned rats, which are related to the changes of monoamines in the limbic and limbic-related regions. [ABSTRACT FROM AUTHOR]

Abstract Objective Tegaserod is a 5-hydroxytryptamine type 4 (5-HT4) receptor agonist, formerly used in treating constipation predominant irritable bowel syndrome, which desensitizes 5-HT4 receptors in rat oesophagus and colon in vitro. Desensitization of 5-HT4 receptors is regulated by G-protein coupled receptor kinases. This study was designed to assess the effect of 5-HT4 receptor activation on the expression of GRK2 and GRK6 in the rat oesophagus and distal colon by acute administration of tegaserod. Results Rats were treated with a single dose of tegaserod (5 mg/kg) and tissue samples of the oesophagus and distal colon were prepared and level of GRK2 and GRK6 protein expression was determined using western blotting. The immunodensity of GRK2 and GRK6 was normalized against the loading control β-actin and compared with control animals. Acute administration of tegaserod for 1, 2, 3, 4, 6, and 8 h did not change significantly the immunodensity of GRK2 or GRK6 in the oesophagus or GRK2 in the distal colon when compared with control animals. This may indicate that the basal level of GRK2 and GRK6 expression is sufficient to regulate the desensitization of 5-HT4 receptors in acute drug treatment.

This work describes the conception, synthesis, in vitro and in vivo biological evaluation of novel Multi-Target Directed Ligands (MTDL) able to both activate 5-HT4 receptors, block 5-HT6 receptors and inhibit acetylcholinesterase activity (AChE), in order to exert a synergistic anti-amnesic effect, potentially useful in the treatment of Alzheimer’s disease (AD). Indeed, both activation of 5-HT4 and blockage of 5-HT6 receptors led to an enhanced acetylcholine release, suggesting it could lead to efficiently restoring the cholinergic neurotransmission deficit observed in AD. Furthermore, 5-HT4 receptor agonists are able to promote the non-amyloidogenic cleavage of the amyloid precursor protein (APP) and to favor the production of the neurotrophic protein sAPPα. Finally, we identified a pleiotropic compound, [1-(4-amino-5-chloro-2-methoxyphenyl)-3-(1-(3-methylbenzyl)piperidin-4-yl)propan-1-one fumaric acid salt (10)], which displayed in vivo an anti-amnesic effect in a model of scopolamine-induced deficit of working memory at a dose of 0.3 mg/kg.

This work describes the conception, synthesis, in vitro and in vivo biological evaluation of novel Multi-Target Directed Ligands (MTDL) able to both activate 5-HT4 receptors, block 5-HT6 receptors and inhibit acetylcholinesterase activity (AChE), in order to exert a synergistic anti-amnesic effect, potentially useful in the treatment of Alzheimer's disease (AD). Indeed, both activation of 5-HT4 and blockage of 5-HT6 receptors led to an enhanced acetylcholine release, suggesting it could lead to efficiently restoring the cholinergic neurotransmission deficit observed in AD. Furthermore, 5-HT4 receptor agonists are able to promote the non-amyloidogenic cleavage of the amyloid precursor protein (APP) and to favor the production of the neurotrophic protein sAPPα. Finally, we identified a pleiotropic compound, [1-(4-amino-5-chloro-2-methoxyphenyl)-3-(1-(3-methylbenzyl)piperidin-4-yl)propan-1-one fumaric acid salt (10)], which displayed in vivo an anti-amnesic effect in a model of scopolamine-induced deficit of working memory at a dose of 0.3 mg/kg. [ABSTRACT FROM AUTHOR]

Secondary peristalsis contributes to the clearance of the refluxate from the esophagus. Acute administration of 5‐hydroxytryptamine 4 (5‐HT4) receptors agonist, mosapride or esophageal infusion of hydrochloric acid (HCl) facilitates secondary peristalsis. The aim of this study was to determine whether esophageal acid infusion and administration of mosapride had different effects on secondary peristalsis. Secondary peristalsis was performed with esophageal distension with rapid and slow air injections in 16 healthy subjects. We performed two separate sessions with HCl (0.1 N) and 40 mg oral mosapride to compare their influence on secondary peristaltic parameters. The threshold volume of secondary peristalsis was significantly lower with HCl infusion than mosapride (P = 0.01) by slow air injections. The threshold volume to generate secondary peristalsis was significantly lower with HCl infusion than mosapride (P = 0.002) by rapid air injections. More secondary peristalsis was trigged by rapid air injections after HCl infusion than mosapride (P = 0.003). Infusion of HCl or mosapride administration has similar effects on peristaltic wave amplitude and duration of primary and secondary peristalsis. Acute esophageal acid infusion can induce greater mechanosensitivity to distension‐induced secondary peristalsis than 5‐HT4 receptors agonist mosapride. The data suggest that acid‐sensitive afferents are more likely to contribute to sensory modulation of esophageal secondary peristalsis; however, the motility aspects of secondary peristalsis are comparable between acute esophageal acidification and 5‐HT4 receptors activation via mosapride. [ABSTRACT FROM AUTHOR]

Alzheimer’s disease (AD) is a multifactorial neurodegenerative disease which is still poorly understood. The drugs currently used against AD, mainly acetylcholinesterase inhibitors (AChEI), are considered clinically insufficient and are responsible for deleterious side effects. AChE is, however, currently receiving renewed interest through the discovery of a chaperone role played in the pathogenesis of AD. But AChE could also serve as an activating protein for pleiotropic prodrugs. Indeed, inhibiting central AChE with brain-penetrating designed carbamates which are able to covalently bind to the enzyme and to concomitantly liberate active metabolites in the brain could constitute a clinically more efficient approach which, additionally, is less likely to cause peripheral side effects. We aim in this article to pave the road of this new avenue with an in vitro and in vivo study of pleiotropic prodrugs targeting both the 5-HT4 receptor and AChE, in order to display a neuroprotective activity associated with a sustained restoration of the cholinergic neurotransmission and without the usual peripheral side effects associated with classic AChEI. This plural activity could bring to AD patients effective, relatively safe, symptomatic and disease-modifying therapeutic benefits.

Functional interaction between cannabinoid and serotonin neuronal systems have been reported in different tasks related to memory assessment. The present study investigated the effect of serotonin 5-HT4 agents into the dorsal hippocampus (the CA1 region) on spatial and object novelty detection deficits induced by activation of cannabinoid CB1 receptors (CB1Rs) using arachidonylcyclopropylamide (ACPA) in a non-associative behavioral task designed to forecast the ability of rodents to encode spatial and non-spatial relationships between distinct stimuli. Post-training, intra-CA1 microinjection of 5-HT4 receptor agonist RS67333 or 5-HT4 receptor antagonist RS23597 both at the dose of 0.016 μg/mouse impaired spatial memory, while cannabinoid CB1R antagonist AM251 (0.1 μg/mouse) facilitated object novelty memory. Also, post-training, intraperitoneal administration of CB1R agonist ACPA (0.005-0.05 mg/kg) impaired both memories. However, a subthreshold dose of RS67333 restored ACPA response on both memories. Moreover, a subthreshold dose of RS23597 potentiated ACPA (0.01 mg/kg) and reversed ACPA (0.05 mg/kg) responses on spatial memory, while it potentiated ACPA response at the dose of 0.005 or 0.05 mg/kg on object novelty memory. Furthermore, effective dose of AM251 restored ACPA response at the higher dose. AM251 blocked response induced by combination of RS67333 or RS23597 and the higher dose of ACPA on both memories. Our results highlight that hippocampal 5-HT4 receptors differently affect cannabinoid signaling in spatial and object novelty memories. The inactivation of CB1 receptors blocks the effect of 5-HT4 agents into the CA1 region on memory deficits induced by activation of CB1Rs via ACPA. [ABSTRACT FROM AUTHOR]

Chronic exposure to cannabinoids during adolescence results in long-lasting behavioral deficits that match some symptomatologic aspects of schizophrenia. The aim of this study was to investigate the reversibility of the emotional and the cognitive effects of chronic exposure to cannabinoids during adolescence, via subsequent modulation of the serotoninergic 5-HT4 and dopaminergic D3 receptors. RS67333 as a 5-HT4 agonist and U-99194A as a D3 antagonist were administered separately at 1 mg/kg and 20 mg/kg, and in combination at 0.5 mg/kg and 10 mg/kg to adult animals undergoing chronic treatment with the synthetic cannabinoid receptor agonist WIN55,212-2 (1 mg/kg) during adolescence. Animals were tested for anxiety-like behavior and episodic-like memory in the open field and novel object recognition tests respectively 30 minutes after the last drug administration. Chronic WIN55,212-2 treated animals exhibited a lasting disruption of episodic memory and increased anxiety levels. The effect on episodic-like memory were partially restored by acute administration of RS67333 and U-99194A and completely by administration of both drugs in combination at lower doses. However, only RS67333 (20 mg/kg) improved the anxiogenic-like effect of WIN55,212-2. These findings give further support that chronic exposure to cannabinoids during adolescence may be used as an animal model for schizophrenia, and highlight D3 and 5-HT4 receptors as potential targets for an enhanced treatment of the cognitive aspect of this disease. [ABSTRACT FROM AUTHOR]

Cognitive dysfunction is a common symptom in Parkinson's disease (PD). Serotonin4 (5-HT 4) receptors are richly expressed in the dorsal hippocampus (dHIPP) and play an important role in cognitive activities. However, the mechanism underlying the role of dHIPP 5-HT 4 receptors in PD-related cognitive dysfunction remains unclear. Here we found that unilateral 6-hydroxydopamine lesions of the medial forebrain bundle increased the protein expression of 5-HT 4 receptors in the dHIPP, decreased hippocampal theta rhythm, and impaired working memory and hippocampus-dependent memory in the T-maze and hole-board test, respectively. Both activation and blockade of dHIPP 5-HT 4 receptors (agonist BIMU8 and antagonist GR113808) improved working memory and hippocampus-dependent memory in the lesioned rats, but not in sham rats. Activation of dHIPP 5-HT 4 receptors increased hippocampal theta rhythm in the lesioned rats. The neurochemical studies showed that injection of BIMU8, GR113808 or GR113808/BIMU8 in the dHIPP increased the levels of dopamine in the medial prefrontal cortex (mPFC), dHIPP and amygdala, and the level of 5-HT in the amygdala in the lesioned rats, but not in sham rats. Injection of GR113808 or GR113808/BIMU8 into the dHIPP also increased the levels of noradrenaline in the mPFC, dHIPP and amygdala only in the lesioned rats. These results suggest that activation or blockade of dHIPP 5-HT 4 receptors may improve the cognitive impairments in parkinsonian rats, which may be due to the increase of hippocampal theta rhythm, up-regulated expressions of 5-HT 4 receptors in the dHIPP and the changes in the levels of monoamines in the relative brain areas. • Activation and blockade dHIPP 5-HT 4 Rs improved memory in sham and the lesioned rats. • The lesion increased the protein expression of 5-HT 4 R in the dHIPP. • Activation dHIPP 5-HT 4 R increased hippocampal theta rhythm in the lesioned rats. • Activation or blockade dHIPP 5-HT 4 R changed monoamines levels in some brain regions. [ABSTRACT FROM AUTHOR]

The roles of 5-HT3 and 5-HT4 receptors in the modulation of intestinal propulsion by luminal application of 5-HT and augmentation of endogenous 5-HT effects were studied in segments of guinea-pig ileum in vitro. Persistent propulsive contractions evoked by saline distension were examined using a modified Trendelenburg method. When 5-HT (30 nM), fluoxetine (selective serotonin reuptake inhibitor; 1 nM), 2-methyl-5-HT (5-HT3 receptor agonist; 1 mM) or RS 67506 (5-HT4 receptor agonist, 1 µM) was infused into the lumen, the pressure needed to initiate persistent propulsive activity fell significantly. A specific 5-HT4 receptor antagonist, SB 207266 (10 nM in lumen), abolished the effects of 5-HT, fluoxetine, and RS 67506, but not those of 2-methyl-5-HT. Granisetron (5-HT3 receptor antagonist; 1 µM in lumen) abolished the effect of 5-HT, fluoxetine, RS 67506 and 2-methyl-5-HT. The NK3 receptor antagonist SR 142801 (100 nM in lumen) blocked the effects of 5-HT, fluoxetine and 2-methyl-5-HT. SB 207266, granisetron and SR 142801 had no effect by themselves. Higher concentrations of fluoxetine (100 nM and 300 nM) and RS 67506 (3 µM and 10 µM) had no effect on the distension threshold for propulsive contractions. These results indicate that luminal application of exogenous 5-HT, or increased release of endogenous mucosal 5-HT above basal levels, acts to lower the threshold for propulsive contractions in the guinea-pig ileum via activation of 5-HT3 and 5-HT4 receptors and the release of tachykinins. The results further indicate that basal release of 5-HT is insufficient to alter the threshold for propulsive motor activity.

We demonstrate here that acute third ventricle injections of GR 113808, a highly selective 5-HT4 antagonist, decrease water intake induced by a previous salt load while potentiating drinking elicited by hypovolemia induced by previous subcutaneous administration of polyethylene glycol in male Wistar rats (200 ± 20 g). At the dose of 160 nmol/rat, third ventricle injections of GR 113808 induced a significant reduction of water intake in salt-loaded animals after 120 min as compared to salt-loaded animals receiving third ventricle injections of saline (salt load + GR = 3.44 ± 0.41 ml, N = 12; salt load + saline = 5.74 ± 0.40 ml, N = 9). At the dose of 80 nmol/rat, GR 113808 significantly enhanced water intake in hypovolemic animals after 120 min as compared to hypovolemic animals receiving third ventricle injections of saline (hypovol + GR = 4.01 ± 0.27 ml, N = 8; hypovol + saline = 2.41 ± 0.23 ml, N = 12). We suggest that central 5-HT4 receptors may exert a positive drive on water intake due to hyperosmolarity and a negative input on drinking provoked by hypovolemia.

Background Adverse effects of previously developed 5- HT4 receptor agonists to treat functional constipation ( FC) and constipation IBS ( IBS-C) patients have limited their use but have given rise to new and more selective 5- HT4 receptor agonists. This work was aimed to evaluate the influence of YKP10811, a new potent 5- HT4 receptor partial agonist, on rat models of colorectal hypersensitivity to distension. Methods Male and female rats were submitted to colorectal distension ( CRD) before and after trinitrobenzene sulfonic acid ( TNBS) infusion, acute ( PRS) or chronic (water avoidance -10 days - WAS) stress. Electromyographic ( EMG) response of abdominal muscles to CRD (15-60 mmHg) was used to measure pain. Changes of colonic tone were also evaluated. The influence of YKP10811 was compared to that of tegaserod with or without exposure of rats to a 5- HT4 receptor antagonist in TNBS treated rats and to both tegaserod and CP-154,526, a corticotropine releasing factor-R1 antagonist in WAS. We tested a possible pharmacological tachyphylaxis of YKP10811 in TNBS-induced hypersensitivity. Key Results YKP10811 (30 mg/kg) had no effect on basal sensitivity and tone in male and female rats but suppressed TNBS-induced hypersensitivity, an effect blocked by the 5- HT4 receptor antagonist GR113808 (10 mg/kg, SC). YKP10811 attenuated acute PRS-induced but not chronic WAS-induced colonic hypersensitivity. In addition, YKP10811 but not tegaserod reduced TNBS-induced colorectal hypersensitivity after 7 days of treatment. Conclusions & Inferences YKP10811exhibits antinociceptive activity in inflammation and acute stress-induced colonic hypersensitivity through 5- HT4 receptors but unlike tegaserod, YKP10811 maintains its activity after repeated administrations and may represent a new candidate to treat IBS-C patients. [ABSTRACT FROM AUTHOR]

Abstract: Introduction: A new radiotracer for imaging the serotonin 4 receptors (5-HT4) in brain, [18F]MNI-698, was recently developed by our group. Evaluation in nonhuman primates indicates the novel radiotracer holds promise as an imaging agent of 5-HT4 in brain. This paper aims to describe the whole-body biodistribution and dosimetry estimates of [18F]MNI-698. Methods: Whole-body positron emission tomography (PET) images were acquired over 240minutes after intravenous bolus injection of [18F]MNI-698 in adult rhesus monkeys. Different models were investigated for quantification of radiation absorbed and effective doses using OLINDA/EXM 1.0 software. Results: The radiotracer main elimination route was found to be urinary and the critical organ was the urinary bladder. Modeling of the urinary bladder voiding interval had a considerable effect on the estimated effective dose. Normalization of rhesus monkeys’ organs and whole-body masses to human equivalent reduced the calculated dosimetry values. The effective dose ranged between 0.017 and 0.027mSv/MBq. Conclusion: The dosimetry estimates, obtained when normalizing organ and whole-body weights and applying the urinary bladder model, indicate that the radiation doses from [18F]MNI-698 comply with limits and guidelines recommended by key regulatory authorities that govern the translation of radiotracers to human clinical trials. The timing of urinary bladder emptying should be considered when designing future clinical protocols with [18F]MNI-698, in order to minimize the subject absorbed doses. [Copyright &y& Elsevier]

Abstract: The 5-HT4 receptor agonist tegaserod (TEG) has been reported to modulate visceral pain. However, the underlying mechanism remains unknown. The objective of the present study was to examine the analgesic mechanism and site of action of TEG. In male rats, visceral pain was assessed by measuring visceromotor response (VMR) to colorectal distension (CRD). Inflammation was induced by intracolonic injection of tri-nitrobenzene sulfonic acid (TNBS). The effect of TEG on the VMR was tested by injecting intraperitoneal (i.p.), intrathecal (i.t.), intracerebroventricular (i.c.v) or in the rostroventral medulla (RVM). The effect of the drug was also tested on responses of CRD-sensitive pelvic nerve afferents (PNA) and lumbo-sacral (LS) spinal neurons. Systemic injection of TEG attenuated VMR in naive and TNBS-treated rats. Similarly, supraspinal, but not spinal, injection of TEG attenuated the VMR. While GR113808, (selective 5-HT4 antagonist) blocked the effect, naloxone (NLX) an opioid receptor antagonist reversed the effect of TEG. Although i.t. NLX did not block the inhibitory effect of TEG in VMR study, i.t. injection of α2-adrenergic receptor antagonist yohimbine blocked the effect of TEG when given systemically. While TEG had no effect on the responses of CRD-sensitive PNA, it inhibited the responses of CRD-sensitive LS neurons in spinal intact condition. This inhibition was blocked by GR113808, NLX and β-funaltrexamine (β-FNA) when injected into the RVM. Results indicate that TEG produces analgesia via activation of supraspinal 5-HT4 receptors which triggers the release of opioids at supraspinal site, which activates descending noradrenergic pathways to the spinal cord to produce analgesia. [Copyright &y& Elsevier]

We explored a novel effect of 5-hydroxytryptamine 4 receptor (5-HT4R) agonists in vivo to reconstruct the enteric neural circuitry that mediates a fundamental distal gut reflex. The neural circuit insult was performed in guinea pigs and rats by rectal transection and anastomosis. A 5-HT4R-agonist, mosapride citrate (MOS) applied orally and locally at the anastomotic site for 2 weeks promoted the regeneration of the impaired neural circuit or the recovery of the distal gut reflex. MOS generated neurofilament-, 5-HT4R- and 5-bromo-2'-deoxyuridine-positive cells and formed neural network in the granulation tissue at the anastomosis. Possible neural stem cell markers increased during the same time period. These novel actions by MOS were inhibited by specific 5-HT4R-antagonist such as GR113808 (GR) or SB-207266. The activation of enteric neural 5-HT4R promotes reconstruction of an enteric neural circuit that involves possibly neural stem cells. We also succeeded in forming dense enteric neural networks by MOS in a gut differentiated from mouse embryonic stem cells. GR abolished the formation of enteric neural networks. MOS up-regulated the expression of mRNA of 5-HT4R, and GR abolished this upregulation, suggesting MOS differentiated enteric neural networks, mediated via activation of 5-HT4R. In the small intestine in H-line: Thy1 promoter green fluorescent protein (GFP) mice, we obtained clear 3-dimensional imaging of enteric neurons that were newly generated by oral application of MOS after gut transection and anastomosis. All findings indicate that treatment with 5-HT4R-agonists could be a novel therapy for generating new enteric neurons to rescue aganglionic disorders in the whole gut. [ABSTRACT FROM AUTHOR]

Segmentation is an important process in nutrient mixing and absorption; however, the mechanisms underlying this motility pattern are poorly understood. Segmentation can be induced by luminal perfusion of fatty acid in guinea pig small intestine in vitro and mimicked by the serotonin (5-HT) reuptake inhibitor fluoxetine (300 nM) and by cholecystokinin (CCK). Serotonergic and CCK-related mechanisms underlying nutrient-induced segmentation were investigated using selective 5-HT and CCK receptor antagonists on isolated segments of small intestine luminally perfused with 1 mM decanoic acid. Motility patterns were analyzed using video imaging and spatiotemporal maps. Segmenting activity mediated by decanoic acid was depressed following luminal application of the 5-HT receptor antagonists granisetron (5-HT3, 1 μM) and SB-207266 (5-HT4, 10 nM) and the CCK receptor antagonists devazepide (CCK-1, 300 nM) and L-365260 (CCK-2, 300 nM), but these antagonists did not further depress segmentation when combined. The P2 receptor antagonist pyridoxal phosphate-6-azophenyl-2=,4=-disulfonate (10M) had no effect on activity. Serosal application of 5-HT antagonists had little effect on segmentation in the duodenum but reduced activity in the jejunum when granisetron and SB-207266 were applied together. These results reveal that 5-HT3 and 5-HT4 receptors, as well as CCK-1 and CCK-2 receptors, are critical in regulating decanoic acid-induced segmentation. Computational simulation indicated that these data are consistent with decanoic acid activating two pathways in the mucosa that converge within the enteric neural circuitry, while contraction-induced release of 5-HT from the mucosa provides feedback into the neural circuit to set the time course of the overall contractile activity. [ABSTRACT FROM AUTHOR]

Abstract: This study in pig colon descendens circular muscle investigated the possible role of phosphodiesterases (PDEs) (1) in the control of smooth muscle activity and (2) in the signal transduction of the 5-HT4 receptors located on the cholinergic neurons. Submaximal cholinergic contractions were electrically induced in colonic circular muscle strips and the influence of the non-selective PDE inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) and selective inhibitors for the 5 classic PDE families (1–5) vinpocetine (PDE1), EHNA (PDE2), cilostamide (PDE3), rolipram (PDE4) and zaprinast (PDE5) was evaluated. IBMX and cilostamide concentration-dependently reduced the amplitude of the cholinergic contractions, as good as abolishing them at 30 and 0.3μM respectively. EHNA only reduced the contractions significantly at the highest concentration tested (30μM). IBMX and cilostamide also concentration-dependently inhibited submaximal cholinergic contractions induced with the muscarinic receptor agonist carbachol. The 5-HT4 receptor agonist prucalopride (1μM) significantly enhanced the electrically induced cholinergic contractions. IBMX, vinpocetine and EHNA did not influence the facilitating effect of prucalopride but rolipram tended to enhance it. When rolipram was added after prucalopride, the facilitating effect of prucalopride was significantly enhanced. These results suggest that PDE3 is the main regulator of circular smooth muscle activity and that the signal transduction of 5-HT4 receptors on the cholinergic nerves towards the circular muscle layer is regulated by PDE4 in pig colon descendens. [Copyright &y& Elsevier]

Background Colonic dysfunction occurs after pelvic autonomic nerve damage. The enteric nervous system can compensate. We investigated the role of mucosal serotonin receptors, 5-HT3 and 5-HT4, in the colonic motility restoration over 2 weeks after parasympathetic pelvic nerve transection in a rat model. Methods Male Sprague-Dawley rats underwent pelvic nerve transection or sham operation. Colonic transit was expressed as the geometric center of 51Cr distribution. Mucosal 5-HT3 and 5-HT4 receptor expression was evaluated by Western blot. Intraluminal pressure increase was measured after 5-HT3 (ondansetron) or 5-HT4 receptor antagonist (GR125487) administration in vitro in sham and denervated distal colons. Key Results At 2 weeks, colonic transit in the denervated group was 30% slower compared to the sham group ( P < 0.01). At 1 and 2 weeks, 5-HT3 receptor expression was increased two-fold in the denervated group, compared to shams ( P < 0.05). A three-fold smaller dose of ondansetron was required in denervated tissues to inhibit intraluminal pressure rise than in sham colons ( P < 0.01). There was no difference in the expression of 5-HT4 receptor or the response to GR125487 in denervated vs sham colons. Conclusions & Inferences Colonic motility was restored to approximately 70% normal over 1 week without further improvement at 2 weeks. Enteric nervous system compensated by upregulating mucosal 5-HT3, but not 5-HT4, receptors. [ABSTRACT FROM AUTHOR]

Abstract: Aims: Besides acting as gastrointestinal prokinetic agents, 5-hydroxytryptamine4 (5-HT4) receptor agonists can induce positive inotropism in human isolated atrium, but not in ventricles. We pharmacologically evaluated the gastroprokinetic 5-HT4 receptor agonists tegaserod, prucalopride, R199715, cisapride, the cisapride metabolite norcisapride, and the 5-HT3 receptor agonist MKC773 on human isolated myocardial trabeculae, and compared their effects with those induced by 5-HT and 5-methoxytryptamine (5-MeOT). Main methods: Atrial and ventricular trabeculae were paced and changes in contractile force were studied in the absence or presence of the 5-HT4 receptor antagonist GR113808. Partial agonism was assessed using 5-HT4 receptor agonists as antagonists against 5-HT. To test the contribution of L-type calcium channels, the inotropic responses to 5-HT and 5-MeOT were studied in the absence or presence of verapamil. Key findings: Like 5-HT and 5-MeOT, cisapride and tegaserod, but not prucalopride, R19971 and MKC­733, induced concentration-dependent positive inotropic responses on atrial trabeculae, which were abolished by GR113808. The L-type calcium channel blocker verapamil attenuated inotropic responses to 5-HT and 5-MeOT. None of the agonists affected the contraction of left ventricular trabeculae. Concentration response curves to 5-HT were shifted to the right in the presence of prucalopride, cisapride, tegaserod and R199715, but not MKC-773. Significance: We conclude that (i) inotropic responses to 5-HT and 5-MeOT seem to depend on L-type calcium channels, (ii) tegaserod and cisapride behave as partial 5-HT4 receptor agonists, while prucalopride, norcisapride and MKC-733 cause no significant effects on human atrial trabeculae, (iii) R199715 seems to behave as a 5-HT4 receptor antagonist. [Copyright &y& Elsevier]

Abstract: In normal aging, or pathological brain diseases in humans, implicit memory (or procedural memory in rats) is spared while explicit memory (or reference memory in rats) is deeply impaired. Selective activation of 5-HT4 receptors by a partial 5-HT4 receptor agonist (SL65.0155) improved memory performance in an olfactory associative discrimination task in aged rats. Detailed analysis of subcategories of long-term memory using a hippocampal-dependent olfactory associative discrimination task revealed a substantial benefit on reference memory. This agent could be used to treat early mnesic deficits observed in normal aging or in neurodegenerative disorders like Alzheimer disease. [Copyright &y& Elsevier]

Abstract: Desensitization of 5-HT4 receptors is regulated by G-protein coupled receptor kinases (GRKs). However, the specific GRK(s) that regulates the desensitization of 5-HT4 receptors in the in vivo setting is unknown. We investigated the in situ expression of 5-HT4 receptors and the GRKs in the rat gastrointestinal tract using immunohistochemistry and their interaction using coimmunoprecipitation. 5-HT4 receptors were expressed in the tunica muscularis mucosae of the oesophagus, longitudinal muscle, myenteric plexus, circular muscle, submucosal plexus and muscularis mucosae of both the proximal and distal colon. GRK2 was expressed in longitudinal muscle and occasionally in myenteric plexus whilst GRK5 showed limited expression in the nerve endings of the myenteric plexus and submucosal plexus of the colon. GRK3 was expressed in the tunica muscularis mucosae of the oesophagus, circular muscle, submucosal plexus and muscularis mucosae of the colon. GRK6 was expressed in the tunica muscularis mucosae of the oesophagus, longitudinal muscle, circular muscle, and muscularis mucosae of the colon. Stimulation of tunica muscularis mucosae of the oesophagus and distal colon using the 5-HT4 receptor agonist, tegaserod, followed by analysis of the 5-HT4 receptor antibody immunoprecipitate, revealed the coimmunoprecipitation of GRK6 with 5-HT4 receptors in the tunica muscularis mucosae of oesophagus while GRK2 and GRK6 were coimmunoprecipitated with 5-HT4 receptors in the distal colon. This study indicates that GRK6 may be involved in the regulation of the desensitization of 5-HT4 receptors in the rat oesophagus whilst GRK2 and GRK6 may be involved in regulation of the desensitization of 5-HT4 receptors in the distal colon. [ABSTRACT FROM AUTHOR]

Background Constipation affects up to 50% of the elderly; this study evaluates the efficacy, safety, and tolerability of the selective 5-HT4 agonist prucalopride in chronically constipated elderly patients. Methods Three hundred chronic constipation patients aged ≥65 years were randomized to prucalopride (1, 2, or 4 mg once daily) or placebo for 4 weeks. The primary endpoint was the percentage of patients with ≥3 spontaneous complete bowel movements (SCBM) per week. Secondary endpoints included the percentage with an increase of ≥1 SCBM per week, BM frequency, constipation-related symptoms, quality of life (QoL), safety, and tolerability. Key Results More patients achieved ≥3 SCBM per week with prucalopride than with placebo. This difference was largest and significant during the first week of 4 mg prucalopride ( P ≤ 0.05). Significantly more patients in each prucalopride group achieved an increase of ≥1 SCBM per week from baseline vs placebo (e.g. 60% with 1 mg prucalopride vs 34% with placebo at week 4; P ≤ 0.05). More patients had improvement in PAC-QOL satisfaction score of ≥1 with 1 mg prucalopride than with placebo ( P ≤ 0.05); the same was true for PAC-SYM stool symptoms (1 and 4 mg prucalopride; P ≤ 0.05). Treatment-emergent adverse events were similar between groups: the most frequently reported with prucalopride were headache and gastrointestinal events. There were no clinically significant differences between prucalopride and placebo for vital signs, laboratory assessments, or ECG variables. Conclusions & Inferences Prucalopride, in the dose-range tested (1–4 mg once daily), has beneficial effects on bowel movements, symptoms, and QoL, and is safe and well-tolerated in elderly patients with chronic constipation. [ABSTRACT FROM AUTHOR]

Abstract: This study was undertaken to investigate the potential antidepressant-like properties of SL65.0155, a serotonin 5-HT4 receptor partial agonist, in male rats of the Wistar strain tested in the forced swim test (FST), an experimental model widely used to assess antidepressant-like activity. The expression of hippocampal neurotrophic factors, such as the brain-derived neurotrophic factor (BDNF), the phosphorilated cAMP response element-binding protein (p-CREB), the B cell lymphoma-2 (Bcl-2), the Bax and the vascular endothelium growth factor (VEGF) were also evaluated by Western Blot analysis. Different groups of rats received intraperitoneally (i.p.) injections of SL65.0155 (0.1, 0.5 and 1 mg/kg), clomipramine (50 mg/kg), citalopram (15 mg/kg) or vehicle, respectively, 24, 5 and 1 h prior to the FST. Compared to the control group, SL65.0155 (0.5 and 1 mg/kg), clomipramine or citalopram injected animals showed an increased swimming and climbing behavior and reduced immobility time in the FST. Interestingly, this effect was not due to changes in the locomotor activity since all treated groups failed to show any change in motor ability as assessed in the open field test. Western blot analysis of hippocampal homogenates showed an enhancement of p-CREB, BDNF Bcl-2 and VEGF protein levels in SL65.0155 treated groups, but not in citalopram or clomipramine treated groups, used here as positive control. No change was found in Bax expression in any treated group. These findings give further support to the hypothesis that the stimulation of serotonin 5-HT4 receptors may be a therapeutic target for depression. [Copyright &y& Elsevier]

The mode of action of antidepressant drugs may be related to mechanisms of monoamines receptor adaptation, including serotonin 5-HT4 receptor subtypes. Here we investigated the effects of repeated treatment with the selective serotonin reuptake inhibitor fluoxetine for 21 days (5 and 10 mg/kg, p.o., once daily) on the sensitivity of 5-HT4 receptors by using receptor autoradiography, adenylate cyclase assays and extracellular recording techniques in rat brain. Fluoxetine treatment decreased the density of 5-HT4 receptor binding in the CA1 field of hippocampus as well as in several areas of the striatum over the doses of 5–10 mg/kg. In a similar way, we found a significant lower response to zacopride-stimulated adenylate cyclase activity in the fluoxetine 10 mg/kg/day treated group. Furthermore, post-synaptic 5-HT4 receptor activity in hippocampus-measured as the excitatory action of zacopride in the pyramidal cells of CA1 evoked by Schaffer collateral stimulation was attenuated in rats treated with both doses of fluoxetine. Taken together, these results support the concept that a net decrease in the signalization pathway of 5-HT4 receptors occurs after chronic selective serotonin reuptake inhibitor treatment: this effect may underlie the therapeutic efficacy of these drugs. [ABSTRACT FROM AUTHOR]

Background and Purpose: Atrial inotropic responses to 5-HT mediated through 5-HT(4) receptors fade, presumably through phosphodiesterase (PDE) activity. We investigated the influence of a selective inhibitor of PDE3 (cilostamide) or of PDE4 (rolipram) on the fade of 5-HT responses in atrial muscle.Experimental Approach: 5-HT responses were compared, ex vivo, on sinoatrial beating rate of newborn piglets, porcine atrial and ventricular force, and human atrial force. cAMP levels were assessed in piglet atrium.Key Results: 5-HT-evoked sinoatrial tachycardia did not fade and was not potentiated by cilostamide (300 nmol.L(-1)) or rolipram (1 micromol.L(-1)). Inotropic responses to 5-HT faded in atria from piglets, adolescent pigs and humans. Cilostamide reduced atrial fade of 5-HT responses in adolescent pigs and humans but not in newborn piglets. Cilostamide disclosed 5-HT ventricular responses in newborn, but not adolescent pigs. Rolipram reduced fade of atrial 5-HT responses in newborn and adolescent pigs but not in humans. Concurrent cilostamide + rolipram abolished fade of 5-HT responses in porcine left atria and facilitated ventricular 5-HT responses, but did not reduce residual fade in human atrium in the presence of cilostamide. 5-HT-evoked increases in cAMP faded; fade was abolished by concurrent cilostamide + rolipram.Conclusions and Implications: PDE3-induced control of porcine 5-HT responses differed in atrium and ventricle and changed with age. PDE3 and PDE4 jointly prevented fade of inotropic and cAMP responses to 5-HT in porcine atrium. Unlike porcine atria, only PDE3 induced fade of 5-HT responses in human atria. [ABSTRACT FROM AUTHOR]

• Fenfluramine is anticonvulsant in the DBA/1 mouse model of SUDEP. • The 5-HT receptor(s) that mediate this action of fenfluramine were examined. • We evaluated if selective 5-HT receptor antagonists could block fenfluramine's action. • The action of fenfluramine was mediated primarily by 5-HT 4 receptors in these mice. • A 5-HT 4 receptor agonist was also anticonvulsant in this SUDEP model. Our previous study showed that the recently approved anticonvulsant drug, fenfluramine, which enhances the release of serotonin (5-hydroxytryptamine, 5-HT) in the brain, prevents seizure-induced respiratory arrest (S-IRA) in the DBA/1 mouse model of sudden unexpected death in epilepsy (SUDEP). The present study examined the role of 5-HT receptor subtypes in mediating the effect of this agent by combined administration of fenfluramine with selective 5-HT receptor antagonists prior to seizure in DBA/1 mice. Fenfluramine (15 mg/kg, i.p.) was administered to primed DBA/1 mice, and audiogenic seizure (Sz) was induced 16 h later. Thirty min prior to Sz induction a selective antagonist acting on 5-HT 1A , 5-HT 2 , 5-HT 3 5-HT 4 , 5-HT 5A , 5-HT 6 or 5-HT 7 receptors at a sub-toxic dose was administered, and changes in seizure-induced behaviors were evaluated. Follow-up studies examined the effect of administration of a 5-HT 4 receptor agonist, BIMU 8, as well as the effect of co-administration of ineffective doses of fenfluramine and BIMU-8 on Sz behaviors. The 5-HT 4 antagonist (GR125487) was the only 5-HT receptor antagonist that was able to reverse the action of fenfluramine to block Sz and S-IRA. Treatment with the 5-HT 4 receptor agonist (BIMU-8), or co-administration of ineffective doses of BIMU-8 and fenfluramine significantly reduced the incidence of S-IRA and tonic Sz in DBA/1 mice. The antagonists for 5-HT 3 , 5-HT 5A 5-HT 6 , and 5-HT 7 receptors did not significantly affect the action of fenfluramine. However, the 5-HT 1A and the 5-HT 2 antagonists enhanced the anticonvulsant effects of fenfluramine. These findings suggest that the action of fenfluramine to prevent seizure-induced sudden death in DBA/1 mice is mediated primarily by activation of 5-HT 4 receptors. These studies are the first to indicate the therapeutic potential of 5-HT 4 receptor agonists either alone or in combination with fenfluramine for preventing SUDEP. Enhancement of the anticonvulsant effect of fenfluramine by 5-HT 1A and 5-HT 2 antagonists may involve presynaptic actions of these antagonists. Thus, the Sz and S-IRA blocking actions of fenfluramine involve complex interactions with several 5-HT receptor subtypes. These data also provide further support for the serotonin hypothesis of SUDEP. [ABSTRACT FROM AUTHOR]

Endogenous ghrelin causes interdigestive contractions of the stomach in rats. In contrast, previous studies showed that 5-HT3 and 5-HT4 receptors were involved in regulating intestinal interdigestive contractions. We studied the possible role of endogenous ghrelin and 5-HT regulating interdigestive gastrointestinal (GI) contractions in rats. Four strain gauge transducers were implanted on the antrum, duodenum, and proximal and distal jejunum. After an overnight fast, GI contractions were recorded in freely moving conscious rats and ghrelin receptor antagonists [(D-lys3)GHRP6; 1 µmol/kg], 5-HT3 antagonists (Ondansetron; 0.5 mg/kg) and 5-HT4 antagonists (GR 125,487; 1 mg/kg) were administered (bolus iv). To evaluate the relationship between the luminal concentrations of 5-HT and phase III-like contractions of the duodenum, duodenal juice was collected via the intraduodenal catheter. 5-HT content of the duodenal juice was measured by HPLC. (D-lys3)GHRP6 significantly attenuated the occurrence and amplitude of phase Ill-like contractions of the antrum, but not the duodenum and jejunum. 5-HT4 antagonists significantly reduced spontaneous phase Ill-like contractions of the jejunum, without affecting those of the antrum and duodenum. In contrast, 5-HT3 antagonists did not affect phase Ill-like contractions in GI tract. Luminal concentration of 5-HT at the phase Ill-like contraction (36.0 ± 13.3 ng/ml, n = 9) was significantly higher than that at the phase I-like contractions of the duodenum (4.9 ± 1.6 ng/ml, n = 9, P < 0.05). It is suggested that released ghrelin from the gastric mucosa mediates gastric phase Ill-like contractions, whereas 5-HT released from enterochromaffin cells of the duodenal mucosa mediates intestinal phase Ill-like contractions via 5-HT4 receptors. [ABSTRACT FROM AUTHOR]

Abstract: Bilateral intradentate injections of 3.0μg of colchicine induced a substantial loss of granule cells and damage to the overlying pyramidal cell layer in region CA1 in adult male Long-Evans rats. All rats with such lesions showed a significant associative learning deficit in an olfactory discrimination task, while being unimpaired in the procedural component of this task. Injection of a partial selective 5-HT4 agonist (SL65.0155; 0.01mg/kg, i.p., vs. saline) before the third of six training sessions enabled complete recovery of associative learning performance in the lesioned rats. Activation of 5-HT4 receptors by a selective agonist such as SL65.0155 might therefore provide an opportunity to reduce learning and memory deficits associated with temporal lobe damage, and could be useful for the symptomatic treatment of memory dysfunctions related to pathological aging such as Alzheimer’s disease. [Copyright &y& Elsevier]

The 5-HT4 partial agonist tegaserod is effective in the treatment of chronic constipation and constipation predominant irritable bowel syndrome. 5-HT4 receptors are located on presynaptic terminals in the enteric nervous system. Stimulation of 5-HT4 receptors enhances the release of acetylcholine and calcitonin gene related peptide from stimulated nerve terminals. This action strengthens neurotransmission in prokinetic pathways, enhancing gastrointestinal motility. The knockout of 5-HT4 receptors in mice not only slows gastrointestinal activity but also, after 1 month of age, increases the age-related loss of enteric neurons and decreases the size of neurons that survive. 5-HT4 receptor agonists, tegaserod and RS67506, increase numbers of enteric neurons developing from precursor cells and/or surviving in culture; they also increase neurite outgrowth and decrease apoptosis. The 5-HT4 receptor antagonist, GR113808, blocks all of these effects, which are thus specific and 5-HT4-mediated. 5-HT4 receptor agonists, therefore, are neuroprotective and neurotrophic for enteric neurons. Because the age-related decline in numbers of enteric neurons may contribute to the dysmotilities of the elderly, the possibility that the neuroprotective actions of 5-HT agonists can be utilized to prevent the occurrence or worsening of these conditions should be investigated. [ABSTRACT FROM AUTHOR]

Abstract: 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 pK B 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 pK B value of 7.19 (5-HT4 block with 1 μM 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-HT3Ashort and 5-HT3B subunits were co-expressed at similar levels, but the 5-HT3Along 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. [Copyright &y& Elsevier]

Tegaserod is a 5-HT4 receptor partial agonist approved for the treatment of irritable bowel syndrome in women with constipation and in both men and women with chronic constipation. The efficacy of tegaserod is based on the importance of 5-HT4 receptors regulating intestinal peristalsis and secretion, and possibly visceral sensory pathways. Our aim was to investigate the effect of tegaserod on colorectal sensitivity using models of normal and exaggerated responsiveness to colorectal distension (CRD). The visceromotor responses (VMR) to CRD at graded pressures (0–60 mmHg) were measured by the number of reflex abdominal contractions. Acute colorectal hypersensitivity was induced by intracolonic infusion of dilute acetic acid. Chronic hypersensitivity was observed in rats following spontaneous resolution of trinitrobenzenesulfonic acid-induced colitis. Rats with normosensitive colons served as controls. Tegaserod (0.1–10 mg kg−1) caused dose-dependent reduction of the VMR to CRD in control rats and in those with colonic hypersensitivity. 5-HT4 antagonists reversed the effects of tegaserod in rats with normosensitive colons, and partially inhibited effects in rats with colonic hypersensitivity. Central administration of tegaserod had no inhibitory effect. These results support the assumption that colonic hypersensitivity could be normalized by tegaserod acting, at least in part, through peripheral 5-HT4 receptors. [ABSTRACT FROM AUTHOR]

Recent research has provided new information about drugs that could be used to treat functional motility disorders. Promotility drugs accelerate gastric emptying or colonic transit and these properties may contribute to their efficacy in treating symptoms associated with gastroparesis, functional dyspepsia or constipation. 5-Hydroxytryptamine4 receptors are targets for drugs (tegaserod, renzapride) that treat symptoms in constipated irritable bowel syndrome patients and in gastroparesis. Drugs acting at motilin (erythromycin) and cholecystokinin-1 (dexloxiglumide) receptors accelerate gastric emptying. Dexloxiglumide might be useful in the treatment of functional dyspepsia particularly that associated with lipid intake. Alvimopan is a μ-opioid receptor antagonist that does not cross the blood brain barrier. Alvimopan is effective in treating postsurgical ileus and perhaps opiate-induced bowel dysfunction. Successes and failures of recent efforts to develop promotility agents revealed opportunities and challenges for developing new promotility drugs. The pharmacological properties of partial agonists might be exploited to develop effective promotility drugs. However, opposing actions of promotility agents on motility (increased contraction vs decreased accommodation) limit the clinical efficacy of drugs with these opposing actions. Selection of appropriate patient populations for evaluation of new drugs is also critical. [ABSTRACT FROM AUTHOR]

Background: We recently identified a facilitory control exerted by serotonin4 (5-HT4) receptors on the in vivo firing activity of dorsal raphé nucleus (DRN) serotonergic (5-HT) neurons. However, these findings were based on acute administrations of 5-HT4 receptor agonists and antagonists, which were active only in a subpopulation of 5-HT neurons. We had no evidence that this influence was significant when considering the entire DRN, nor if it was persistent after chronic treatments. In addition, the poor distribution of 5-HT4 receptors within the DRN raised the question of the neuroanatomical bases underlying this control. Methods and results: Here we show that the subacute intraperitoneal (IP) injection of the 5-HT4 receptor agonists prucalopride (2.5 mg/kg) and RS 67333 (1.5 mg/kg) 30 minutes before the beginning of recordings augment the mean firing rate of DRN neurons by 40% and 66%, respectively. These increases remain stable when the compounds are administered continuously during 3 and 21 days; the effects of the 3-day treatment are blocked by the 5-HT4 receptor antagonist GR 125487 (1000 μg/kg, intravenous [i.v.]). In addition, stereotaxic microinjections of herpes simplex viruses, transformed to overexpress 5-HT4 receptors, increase DRN 5-HT neuronal mean activity when performed in the medial prefrontal cortex (mPFC) but not in the striatum or in the hippocampus. Conclusions: This finding suggests the existence of a 5-HT4-dependent activation of DRN that may involve the mPFC, unveiling the 5-HT4 receptor as a putative player in the physiopathology of several disorders related to central 5-HT dysfunction [Copyright &y& Elsevier]

Firstly, olfactory association learning was used to determine the modulating effect of 5-HT4 receptor involvement in learning and long-term memory. Secondly, the effects of systemic injections of a 5-HT4 partial agonist and an antagonist on long-term potentiation (LTP) and depotentiation in the dentate gyrus (DG) were tested in freely moving rats. The modulating role of the 5-HT4 receptors was studied by using a potent, 5-HT4 partial agonist RS 67333 [1-(4-amino-5-chloro-2-methoxyphenyl)-3-(1-n-butyl-4-piperidinyl)-1-propanone] and a selective 5-HT4 receptor antagonist RS 67532 [1-(4-amino-5-chloro-2-(3,5-dimethoxybenzyloxyphenyl)-5-(1-piperidinyl)-1-propanone]. Agonist or antagonist systemic chronic injections prior to five training sessions yielded a facilitatory effect on procedural memory during the first session only with the antagonist. Systemic injection of the antagonist only before the first training session improved procedural memory during the first session and associative memory during the second session. Similar injection with the 5-HT4 partial agonist had an opposite effect.The systemic injection of the 5-HT4 partial agonist prior to the induction of LTP in the dentate gyrus by high-frequency stimulation was followed by a population spike increase, while the systemic injection of the antagonist accelerated the depotentiation 48 h later. The behavioural and physiological results pointed out the involvement of 5-HT4 receptors in processing related to the long-term hippocampal-dependent memory system, and suggest that specific 5-HT4 agonists could be used to treat amnesic patients with a dysfunction in this particular system. [Copyright &y& Elsevier]

Serotonin (5-hydroxytryptamine, 5-HT) increases contractile force and elicits arrhythmias through 5-HT4 receptors in porcine and human atrium, but its ventricular effects are unknown. We now report functional 5-HT4 receptors in porcine and human ventricle. 5-HT4 mRNA levels were determined in porcine and human ventricles and contractility studied in ventricular trabeculae. Cyclic AMP-dependent protein kinase (PKA) activity was measured in porcine ventricle. Porcine and human ventricles expressed 5-HT4 receptor mRNA. Ventricular 5-HT4(b) mRNA was increased by four times in 20 failing human hearts compared with five donor hearts. 5-HT increased contractile force maximally by 16% (EC50=890 nM) and PKA activity by 20% of the effects of (-)-isoproterenol (200 µM) in ventricular trabeculae from new-born piglets in the presence of the phosphodiesterase-inhibitor 3-isobutyl-1-methylxanthine. In ventricular trabeculae from adult pigs (3-isobutyl-1-methylxanthine present) 5-HT increased force by 32% (EC50=60 nM) and PKA activity by 39% of (-)-isoproterenol. In right and left ventricular trabeculae from failing hearts, exposed to modified Krebs solution, 5-HT produced variable increases in contractile force in right ventricular trabeculae from 4 out of 6 hearts and in left ventricular trabeculae from 3 out of 3 hearts- range 1-39% of (-)-isoproterenol, average 8%. In 11 left ventricular trabeculae from the failing hearts of four ß-blocker-treated patients, pre-exposed to a relaxant solution with 0.5 mM Ca2+ and 1.2 mM Mg2+ followed by a switch to 2.5 mM Ca2+ and 1 mM Mg2+, 5-HT (1-100 µM, 3-isobutyl-1-methylxanthine present) consistently increased contractile force and hastened relaxation by 46% and 25% of (-)-isoproterenol respectively. 5-HT caused arrhythmias in three trabeculae from 3 out of 11 patients. In the absence of phosphodiesterase inhibitor, 5-HT increased force in two trabeculae, but not in another six trabeculae from 4 patients. All 5-HT responses were blocked by 5-HT4 receptor antagonists. We conclude that phosphodiesterase inhibition uncovers functional ventricular 5-HT4 receptors, coupled to a PKA pathway, through which 5-HT enhances contractility, hastens relaxation and can potentially cause arrhythmias. [ABSTRACT FROM AUTHOR]

Serotonin 5-HT2A and 5-HT4 binding parameters and their second messengers 1,4,5-inositol triphosphate (IP3) and cyclic adenosyl monophosphate (cAMP) were studied in the frontal cortex, hippocampus, caudate nucleus and amygdala of 19 control subjects and 19 antidepressant-free, violent suicide victims. A significantly higher number of 5-HT4 receptors and higher second messenger cAMP concentrations were found in the frontal cortex and caudate nucleus of the depressed suicide victims as compared with the control group. Furthermore, significantly increased 5-HT2A binding sites and IP3 concentrations were noted in the caudate nucleus of the suicide victims, together with a significantly reduced number of 5-HT2A binding sites, higher binding affinity and increased 1P3 concentrations in the hippocampus. No significant alterations in 5-HT4 and cAMP or in 5-HT2A and IP3 concentrations were observed in the amygdala. The caudate nucleus of depressed suicide victims seems to be the brain region with the highest alteration of the serotonergic system, and hence with the most diagnostic sensitivity. Further studies on suicidality and depression should focus on the functionality of the caudate nucleus. [ABSTRACT FROM AUTHOR]

Dopamine neurons of the substantia nigra pars compacta receive a prominent serotonin (5-HT) projection from the dorsal raphe nucleus and important functional interactions between the serotonergic and the dopaminergic system have been postulated. In the present report we examined the role of 5-HT in the modulation of the metabotropic glutamate receptor-mediated inhibitory postsynaptic current (mGluR-IPSC) in midbrain dopamine neurons, and we found a reversible depression of this synaptic response at concentrations of 5-HT ranging from 100 n m to 30 µ m (EC50 1.06 µ m). This resulted in a shift towards excitation of the overall dopamine neuron response to glutamatergic synaptic input. This effect was not because of a direct modulation of the Ca2+-sensitive K+ conductances underlying the mGluR-IPSC, but was associated with a decrease in the intracellular calcium signal triggered by mGluR stimulation. Similar results were obtained with α-methyl-5-hydroxytryptamine and 5-methoxytryptamine, but not with 5-carboxamidotryptamine or 1-(3-chlorophenyl) piperazine. No significant depression of the mGluR-IPSC by 5-HT was observed in the presence of the 5-HT2 antagonist cinanserin or the 5-HT4 receptor antagonist RS 23597–190, whereas the 5-HT2C antagonist RS 102221 was ineffective. Our results demonstrate a powerful inhibition of the mGluR-IPSC by 5-HT in midbrain dopamine neurons, most probably through stimulation of 5-HT2A and 5-HT4 receptors. [ABSTRACT FROM AUTHOR]

The autoradiographic distribution of the 5-HT4 receptor was described using human postmortem brain sections and the selective radioligand [125I]SB 207710 [(1-n-butyl-4-piperidinyl)methyl-8-amino-7-[125I]iodo-1,4-benzodioxane-5-carboxylate]. The specific binding was highest in regions of the basal ganglia (caudate nucleus, putamen, nucleus accumbens, globus pallidus and substantia nigra) and the hippocampal formation (CA1 and subiculum). In the neocortex, the binding showed a distinct lamination pattern with high levels in superficial layers and a band displaying lower levels in deep cortical layers. The results confirm previous studies on the distribution of 5-HT4 receptors in the human brain in vitro and provide high-resolution correlates for in vivo imaging studies using the radioligand recently developed for single photon emission tomography (SPET), [123I]SB 207710. [Copyright &y& Elsevier]

Cytotoxic drug-induced nausea and vomiting are the side effects most feared by cancer patients. Emesis is an instinctive defense reaction caused by the somatoautonomic nerve reflex, which is integrated in the medulla oblongata. Emesis caused by cytotoxic drugs such as cisplatin is associated with an increase in the concentration of 5-hydroxytryptamine (5-HT) in the intestine and the brainstem. It is proposed that cytotoxic drugs evoke 5-HT release from the enterochromaffin (EC) cells in the intestinal mucosa and that the released 5-HT stimulates the 5-HT receptors on the adjacent vagal afferent nerves. The depolarization of the vagal afferent nerves stimulates the vomiting center in the brainstem and eventually induces a vomiting reflex. 5-HT released from EC cells seems to mediate the cisplatin-induced emesis sensitive to 5-HT3 receptor antagonists. The release of 5-HT from the EC cells, however, is regulated by polymodal mechanisms on autoreceptors or heteroreceptors. The precise role of 5-HT on the occurrence of vomiting has not been fully elucidated. The present review aims to describe the role of 5-HT in anticancer drug-induced emesis from the viewpoint of 5-HT release and afferent vagus nerve activity. Various methods for predicting emesis are also evaluated. [Copyright &y& Elsevier]

Using extracellular ex vivo recording we studied changes in the reactivity of rat frontal cortical neurons to the 5-HT1A, 5-HT2 and 5-HT4 receptor agonists (±)-2-dipropyloamino-8-hydroxy-1,2,3,4-tetrahydronaphtalene hydrobromide (8-OH-DPAT), (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) and zacopride, respectively, induced by a repeated treatment with imipramine or citalopram. Rats were treated with imipramine or citalopram for 14 days (10 mg/kg p.o.) twice daily. Frontal cortical slices were prepared 2 days after the last drug administration. Spontaneous epileptiform discharges were induced in slices by perfusion with a medium devoid of Mg2+ ions and with added picrotoxin (30 μM). While the application of 2 μM 8-OH-DPAT resulted in a reversible decrease of the discharge frequency, in the presence of DOI (1 μM) or zacopride (5 μM), the discharge frequency was increased. Both repeated imipramine and citalopram enhanced the effect of the activation of 5-HT1A receptor and attenuated the effect related to 5-HT2 receptor activation, while the effect of the activation of 5-HT4 receptor remained unchanged. Moreover, imipramine, but not citalopram, induced a reduction of epileptiform discharge frequency and an increase of the time of occurrence of epileptiform activity. These data indicate that antidepressants enhance the 5-HT-mediated inhibition in neuronal circuitry of the frontal cortex. [ABSTRACT FROM AUTHOR]

The effects of serotonin (5-HT) as well as 5-HT1A and 5-HT4 receptor agonists, (±)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT) and zacopride, respectively, on population spikes evoked by electrical stimulation and on spontaneous epileptiform activity were investigated in CA1 area of hippocampal slices. Spontaneous epileptiform activity was recorded from slice in a nominally Mg2+-free medium. While 5-HT application resulted in a decrease of population spikes evoked in standard incubation conditions, in accordance with earlier studies, it exerted two opposite effects on epileptiform activity. The early inhibitory effect was mimicked by 8-OH-DPAT while the later, excitatory, by zacopride. The application of 8-OH-DPAT decreased, and that of zacopride increased, the amplitude of population spikes. A comparison of the dose-dependence of the excitatory and inhibitory effects of serotonergic agonists on the amplitude of the population spike and on the frequency of epileptiform discharges indicated that the latter is a more sensitive measure of the activation of 5-HT1A and 5-HT4 receptors than the former. Thus, spontaneous epileptiform activity recorded in a nominally Mg2+-free slice medium represents a convenient model for investigation of hippocampal neuronal reactivity to the activation of various 5-HT receptor subtypes. [ABSTRACT FROM AUTHOR]

In vivo microdialysis and single-cell extracellular recordings were used to assess the involvement of serotonin4 (5-HT4) receptors in the effects induced by morphine, amphetamine and cocaine on nigrostriatal and mesoaccumbal dopaminergic (DA) pathway activity.The increase in striatal DA release induced by morphine (2.5 mg/kg, s.c.) was significantly reduced by the selective 5-HT4 antagonists GR 125487 (0.1 and 1 mg/kg, i.p.) or SB 204070 (1 mg/kg, i.p.), and potentiated by the 5-HT4 agonist prucalopride (5 mg/kg, i.p.). Neither of these compounds affected morphine-stimulated DA release in the nucleus accumbens. In both regions, amphetamine (2 mg/kg, i.p.) and cocaine (15 mg/kg, i.p.) induced DA release was affected neither by GR 125487 nor by prucalopride. None of the 5-HT agents used modified basal DA release in either brain region. Finally, GR 125487 (445 μg/kg, i.v.), whilst not affecting basal firing of DA neurons within either the substantia nigra pars compacta nor the ventral tegmental area, significantly reduced morphine (0.1–10 mg/kg, i.v.) stimulated firing of nigrostriatal DA neurons only.These results confirm that 5-HT4 receptors exert a state-dependent facilitatory control restricted to the nigrostriatal DA pathway, and indicate that 5-HT4 receptors selectively modulate DA exocytosis associated with increased DA neuron firing rate. [Copyright &y& Elsevier]

The characteristics of endogenous serotonin (5-HT)-mediated synaptic transmission were investigated in the hippocampal CA1 and CA3 fields of anaesthetized rats. Electrophysiological approaches were used to elucidate the effects of a selective 5-HT reuptake inhibitor, fluvoxamine, on synaptic transmission by determining the population spike amplitude (PSA). Fluvoxamine (10 or 30 mg/kg i.p.) increased the PSA in the CA1 and CA3 fields concentration dependently, whereby this facilitatory response was greater in the CA3 than in the CA1 field. Fluvoxamine (30 mg/kg i.p.)-induced increases in the PSA were augmented by the 5-HT1A receptor antagonist NAN 190 (0.5 mg/kg i.p.) and prevented by the 5-HT4 receptor antagonist GR 113808 (20 µg/rat i.c.v.) or by the 5-HT7 receptor antagonist DR 4004 (10 µg/rat i.c.v.) in the CA1 field. Thus, fluvoxamine-induced facilitatory effects appear to be mediated via 5-HT1A and 5-HT4/5-HT7 receptors in an inhibitory and a stimulatory manner, respectively. In the CA3 field, excitability produced by fluvoxamine was abolished by either NAN 190 or DR 4004, but not by GR 113808, suggesting that 5-HT1A and 5-HT7 receptors contribute to this facilitation. These findings were supported in part by the results obtained by exogenously applied 5-HT receptor agonists: the 5-HT1A receptor agonist tandospirone (10 mg/kg i.p.) decreased PSA in the CA1 field, whereas the 5-HT4 receptor agonist SC 53116 (10 µg/rat i.c.v.) increased it. In contrast, in the CA3 field, tandospirone increased PSA, whereas SC 53116 had no effect. Taken together, the present study revealed that characteristics of synaptic transmission mediated via 5-HT receptors differ between the CA1 and CA3 fields: in the CA1 field, three 5-HT receptors, 5-HT1A, 5-HT4 and 5-HT7, are associated with the endogenous 5-HT-induced facilitation in an opposite and independent manner. In the CA3 field, at least two 5-HT receptors, i.e. 5-HT1A and 5-HT7, are involved in this mediation in a facilitatory manner. 5-HT neurons may modulate pyramidal neuron responses to incoming stimuli by complex mechanisms involving these 5-HT receptors. In other words, the net effects resulting from the differential characteristics in the CA1 and CA3 fields may play an important role in the physiological functions of the hippocampus. [ABSTRACT FROM AUTHOR]

In this study, we evaluated the role of central 5-HT4 receptors in the control of motor behaviors related to change of nigrostriatal dopamine (DA) transmission, namely, stereotyped behavior and catalepsy in rats. Indeed, given that 5-HT4 receptors indirectly modulate nigrostriatal DA neuron activity, we hypothesized that these receptors would regulate nigrostriatal DA transmission in the basal ganglia, and consequently, associated motor responses. Stereotypy was induced either by an acute administration of apomorphine (0.3 and 1.5 mg/kg sc), or by a single morphine administration (15 mg/kg sc) in chronically morphine-treated (15 mg/kg sc, twice daily for 10 days) rats. Catalepsy was induced by the typical neuroleptic haloperidol (HAL; 1 mg/kg sc). The selective 5-HT4 antagonist, GR 125487 (1 mg/kg ip), modified neither apomorphine- nor morphine-induced stereotypy. HAL-induced catalepsy, while reduced by the systemic administration of the 5-HT1A agonist 8-OH-DPAT (0.1 mg/kg sc), was insensitive to GR 125487, systemically (1, 3, 10 mg/kg ip) or locally (20 and 40 nmol/20 μl) administered into the third ventricle. Also, HAL-induced catalepsy was not affected by the selective 5-HT4 antagonist GR 113808 (3 mg/kg ip). The obtained results indicate that 5-HT4 receptor antagonism does not modulate motor behaviors related to change of striatal DA transmission. [Copyright &y& Elsevier]