5-HT4 Receptor Agonist Research Articles

Practical management approach to gastroparesis.

Gastroparesis is a syndrome characterised by delayed gastric emptying in the absence of mechanical obstruction. Symptoms can include early satiety, abdominal pain, bloating, vomiting and regurgitation which cause significant morbidity in addition to nutritional deficits. There is a higher prevalence in diabetics and females, but the incidence in the Australian population has not been well studied. Management of gastroparesis involves investigating and correcting nutritional deficits, optimising glycaemic control and improving gastrointestinal motility. Symptom control in gastroparesis can be challenging. Nutritional deficits should be addressed initially through dietary modification. Enteral feeding is a second-line option when oral intake is insufficient. Home parenteral nutrition is rarely used, and only accessible through specialised clinics in the outpatient setting. Prokinetic medication classes that have been used include dopamine receptor antagonists, motilin receptor agonists, 5-HT4 receptor agonists and ghrelin receptor agonists. Anti-emetic agents are often used for symptom control. Interventional treatments include gastric electrical stimulation, gastric per-oral endoscopic myotomy, feeding jejunostomy and gastrostomy/jejunstomy for gastric venting and enteral feeding. In this article we propose a framework to manage gastroparesis in Australia based on current evidence and available therapies.

Zacopride Exerts an Antiarrhythmic Effect by Specifically Stimulating the Cardiac Inward Rectifier Potassium Current in Rabbits: Exploration of a New Antiarrhythmic Strategy.

Zacopride, a potent antagonist of 5-HT3 receptors and an agonist of 5-HT4 receptors, is a gastrointestinal prokinetic agent. In a previous study, we discovered that zacopride selectively stimulated the inward rectifier potassium current (IK1) in the rat and that agonizing IK1 prevented or eliminated aconitine-induced arrhythmias in rats. Our aims were to confirm that the antiarrhythmic effects of zacopride are mediated by selectively enhancing IK1 in rabbits. The effects of zacopride on the function of the main ion channels were investigated using a whole-cell patch-clamp technique in rabbits. Effects of zacopride on cardiac arrhythmias were also explored experimentally both in vivo and in vitro. Zacopride moderately enhanced cardiac IK1 but had no apparent action on voltage-gated sodium current (INa), L- type calcium current (ICa-L), sodium-calcium exchange current (INa/Ca), transient outward potassium current (Ito), or delayed rectifier potassium current (IK) in rabbits. Zacopride also had a marked antiarrhythmic effect in vivo and in vitro. We proved that the resting membrane potential (RMP) was hyperpolarized in the presence of 1 μmol/L zacopride, and the action potential duration (APD) at 90% repolarization (APD90) was shortened by zacopride (0.1-10 μmol/L) in a concentration- dependent manner. Furthermore, zacopride at 1 μmol/L significantly decreased the incidence of drug-induced early afterdepolarization (EAD) in rabbit ventricular myocytes. Zacopride is a selective agonist of rabbit cardiac IK1 and that IK1 enhancement exerts potential antiarrhythmic effects.

Molecular mechanistic vision on binding interaction of triptan drug, a serotonin (5-HT1) agonist with human serum albumin through multispectral and computational assessments

The triptan drug such as eletriptan in combination with hydrochloride (ETP) is a 5-HT1 receptor agonist used to treat the migraine headache. Human serum albumin (HSA), the fundamental serum protein, executes various functions, that includes transporting and binding of many ligands. HSA binding interaction with ETP is elucidated from molecular docking in composite with fluorescence (emission, 3D and synchronous), UV-vis and FT-IR spectroscopy at 296, 304 and 312 K (pH = 7.40). ETP after interaction modified the HSA secondary structure and its micro-environments. Energy transfer and thermodynamic parameters were evaluated. Various quenching and binding constants were computed for formed ETP-HSA complex. The dominant interactive forces for ETP and HSA binding are hydrogen bonds join up with van der Waals extent possibly at site III (IB). The presence of Ca2+, Co2+, Na+, Mg2+ and Fe3+ ions significantly affected binding ability of ETP towards HSA. The essentialness of this investigation is beneficial in life sciences, medicinal chemistry, pharmaceutical industry and clinical medicine.

132-OR: Peripheral Serotonin Regulates Dietary Fat Absorption and Intestinal Chylomicron Production

Introduction: Diabetic dyslipidemia is a key feature of insulin resistance and is characterized by overproduction of chylomicron particles by the intestine. Chylomicron output can be regulated by hormones secreted from the intestine. Enterochromaffin cells are specialized enteroendocrine cells that secrete serotonin (5-HT). Gut-derived 5-HT is emerging as a regulator of systemic energy homeostasis by regulating adipose and liver lipid metabolism. Currently, the role of 5-HT on intestinal fat absorption and chylomicron production remains unknown. We hypothesize that 5-HT regulates postprandial intestinal fat absorption and chylomicron production. Methods/Results: Syrian golden hamsters underwent jugular cannulation surgery. The next day, hamsters received an oral fat load (200μl olive oil) followed by intravenous infusion of Triton WR-1339 to prevent lipoprotein clearance. Blood samples were collected prior to and up to 6 hours post fat load. Triglyceride-rich lipoproteins (TRL) which includes chylomicrons were isolated from plasma by density gradient ultracentrifugation and triglyceride (TG) content in this fraction was measured using an enzyme based colorimetric assay. Intravenous administration of 5-HT (1mg/kg) 5 minutes prior to oral fat load increased the rate of plasma TG and TRL-TG accumulation while higher doses of 5-HT (5 and 10mg/kg) produced a transient decrease in plasma TG and TRL-TG accumulation. Treatment with 5-HT4 receptor agonist, Bimu-8 (1mg/kg), increased plasma TG and TRL-TG accumulation. Endogenous 5-HT depletion by para-chloro-phenylalanine (PCPA), an inhibitor of 5-HT synthesis, reduced the rate of TG and TRL-TG accumulation compared to vehicle controls. Furthermore, enhancing 5-HT action by inhibiting 5-HT reuptake with fluoxetine increased the rate of TG and TRL-TG accumulation. Our results demonstrate that 5-HT is a key regulator of postprandial lipoprotein metabolism and could have important implications for diabetic dyslipidemia. Disclosure F. Raka: None. K. Adeli: None. Funding Canadian Institutes of Health Research (967131)

Thermosensitive mucoadhesive in situ gel for intranasal delivery of Almotriptan malate: Formulation, characterization, and evaluation

The present investigation deals with development, characterization, and evaluation of thermosensitive in-situ nasal gel of Almotriptan malate (selective serotonin 5-HT1 receptor agonist), serving as a substitute for drug delivery to the brain. Gels were prepared by cold technique varying the concentration of Pluronics (PF127 and PF68) and carboxymethyl chitosan as thermoreversible and mucoadhesive polymers respectively. Drug content before and after gelation was ranged between 96.42-98.92% and 95.8–98.66% respectively. The pH value was observed in the range of 5.56–6.31. Notably, the results obtained were in agreement with the phase transition and formulations exhibited pseudoplastic rheology. Mucoadhesive strengths of all the formulations increased with the chitosan concentration and were in the range of 3300–5193.65 dyn/cm2. The in-vitro drug diffusion data of the optimized formulation i.e. 17% w/v PF127, 2% w/v PF68, 0.1% w/v carboxymethyl chitosan, revealed 89.36% release at the end of 6 h. Histopathological studies confirmed the non-toxic effect of optimized formulation on the microscopic structure of nasal mucosa during ex-vivo permeation studies. Short term stability study exhibited 4±1 °C as an appropriate storage condition for the formulations. In nutshell, a thermosensitive in-situ gel of Almotriptan malate can be intended as an effective approach of migraine treatment.

649 DE NOVO ENTERIC NEUROGENESIS IN POST-EMBRYONIC ZEBRAFISH FROM SCHWANN CELL PRECURSORS RATHER THAN RESIDENT CELL TYPES

The enteric nervous system (ENS) is essential for normal gastrointestinal function. Although the embryonic origin of enteric neurons from the neural crest is well established, conflicting evidence exists regarding postnatal enteric neurogenesis. Here, we address this by examining the origin of de novo neurogenesis in the post-embryonic zebrafish ENS. Although new neurons are added during growth and after injury, the larval intestine appears to lack resident neurogenic precursors or classical glia marked by sox10, plp1a, gfap or s100. Rather, lineage tracing with lipophilic dye or inducible Sox10-Cre suggests that post-embryonic enteric neurons arise from trunk neural crest-derived Schwann cell precursors that migrate from the spinal cord into the intestine. Furthermore, the 5-HT₄ receptor agonist prucalopride increases enteric neurogenesis in normal development and after injury. Taken together, the results suggest that despite the lack of resident progenitors in the gut, post-embryonic enteric neurogenesis occurs via gut-extrinsic Schwann cell precursors during development and injury, and is promoted by serotonin receptor agonists. The absence of classical glia in the ENS further suggests that neural crest-derived enteric glia might have evolved after the teleost lineage.

Blocking properties of terguride at the 5-HT2 receptor subtypes mediating cardiovascular responses in the rat.

In vitro studies have suggested that terguride blocks the contractile and relaxant responses produced by 5-hydroxytryptamine (5-HT) via 5-HT2A/2B receptors. This study has now investigated terguride's blocking properties on central/peripheral 5-HT2 receptors in anaesthetized or pithed rats. Male Wistar anaesthetized/pithed rats were cannulated for recording blood pressure and heart rate and for i.v. administration of several compounds. In both groups of rats, i.v. bolus injections of 5-HT or (±)-DOI (a 5-HT2 receptor agonist; 1-1000 μg/kg) produced dose-dependent increases in diastolic blood pressure and heart rate. These responses were dose-dependently antagonized by terguride (10-3000 μg/kg). In anaesthetized rats, i.v. bolus injections of BW723C86 (a 5-HT2B receptor agonist; 1-1000 μg/kg) produced dose-dependent increases in diastolic blood pressure and not dose-dependent increases in heart rate, while in pithed rats, these responses were attenuated. The vasopressor responses elicited by BW723C86 in anaesthetized rats were dose-dependently blocked by terguride (10-300 μg/kg), whereas its the tachycardic responses were dose-independently blocked. These results, taken together, suggest that terguride behaved as an antagonist at the 5-HT2 receptors located in the central nervous system and (or) the systemic vasculature. This is the first evidence demonstrating that terguride can block central/peripheral 5-HT2 receptors mediating cardiovascular responses in anaesthetized or pithed rats.

Translating the promise of 5HT4 receptor agonists for the treatment of depression.

Animal experimental studies suggest that 5-HT4 receptor activation holds promise as a novel target for the treatment of depression and cognitive impairment. 5-HT4 receptors are post-synaptic receptors that are located in striatal and limbic areas known to be involved in cognition and mood. Consistent with this, 5-HT4 receptor agonists produce rapid antidepressant effects in a number of animal models of depression, and pro-cognitive effects in tasks of learning and memory. These effects are accompanied by molecular changes, such as the increased expression of neuroplasticity-related proteins that are typical of clinically useful antidepressant drugs. Intriguingly, these antidepressant-like effects have a fast onset of their action, raising the possibility that 5-HT4 receptor agonists may be a particularly useful augmentation strategy in the early stages of SSRI treatment. Until recently, the translation of these effects to humans has been challenging. Here, we review the evidence from animal studies that the 5-HT4 receptor is a promising target for the treatment of depression and cognitive disorders, and outline a potential pathway for the efficient and cost-effective translation of these effects into humans and, ultimately, to the clinic.

Role of Mast Cells in the Control of Aldosterone Secretion.

Mast cells are immune cells present in adrenals from various species. Proliferation and activation of adrenal mast cells seem to be influenced by environment, since they increase during summer and in response to sodium restriction in frogs and mouse, respectively. Although the physiological factors regulating adrenal mast cell activity have not been identified, they might involve neurotransmitters and the renin-angiotensin system. Some data indicate that adrenal mast cells stimulate proliferation of steroidogenic cells in the zona glomerulosa and activate the mineralocorticoid production. In human, mast cell degranulation stimulates aldosterone synthesis through the release of serotonin (5-HT) and activation of 5-HT4 receptors. Increase in mast cell population and upregulation of the 5-HT signaling pathway occur in aldosterone-producing adenomas. In particular, aldosterone-producing adenoma cells overexpress 5-HT4 receptors and are hyper-responsive to 5-HT4 receptor agonists. These data suggest that the intra-adrenal serotonergic regulatory system represents a potential target for development of both adrenal imaging methods to evaluate the lateralization of aldosterone production, and pharmacological treatments of primary aldosteronism.

Effects of 5-hydroxytryptamine Receptor on the Reaction of the Lower Esophageal Sphincter Under the Electrical Field Stimulation

In the first and second parts of this study, 5-hydroxytryptamine (5HT) receptors, including 5-HT3 and 5-HT4 with the highest expression level, were found in clasp and sling fibres of the lower esophageal sphincter (LES). Specific 5-HT3 and 5-HT4 receptor agonists can induce the contraction effect of clasp and sling fibres of the LES while specific 5-HT7 receptor agonists showed no effects. In the study of this part, the in-vitro muscle tension measurement technology and EFS methods were used to detect the effect of the selective 5-HT receptor antagonist on the clasp and sling fibres of the in-vitro LES under the electrical field stimulation (EFS), and further to ensure the effect of 5-HT receptor in the LES neuroregulatory pathway, and deeply explore the effect of 5-HT receptor in the systolic and diastolic function regulation of the LES.

Efficacy and Safety of 5-HT4 Receptor Agonist Minesapride for Irritable Bowel Syndrome with Constipation in a Randomized Controlled Trial

Treatment options for irritable bowel syndrome with constipation (IBS-C) are limited-new prokinetic drugs are needed. We evaluated the efficacy and safety of minesapride (DSP-6952), a partial agonist with high affinity for 5-HT4 receptors, in patients with IBS-C in Japan. We performed a double-blind phase 2 study of 171 patients with Rome III-defined IBS-C at 33 centers in Japan, from December 2012 through August 2013. Patients were randomly assigned to groups given minesapride (1, 4, 12, or 40 mg) or placebo once daily for 4 weeks. The primary outcome was efficacy, defined as improvement in the weekly frequency of complete spontaneous bowel movements (CSBMs), abdominal symptoms, and IBS-C symptoms (according to the Japanese version of the IBS severity index score). For evaluation of safety, adverse events (AEs) were recorded. The least squares mean change from baseline in the weekly frequency of CSBMs was greater in all minesapride groups than in the placebo group at week 4 (40 mg vs placebo, P = .040). The abdominal symptoms score improved in minesapride 40 mg group. The overall IBS severity index score decreased from baseline to week 4 in all treatment groups-especially in the 12 mg and 40 mg groups (P = .048 and <.001 vs placebo, respectively). The proportions of patients with treatment-emergent AEs in the pooled minesapride and placebo groups were 55.0% and 60.0%, respectively. The most common treatment-emergent AE was diarrhea (in 42.9% and 37.1% of patients in the pooled minesapride and placebo groups, respectively). In a phase 2 trial of patients with IBS-C in Japan, minesapride increased stool frequency (measured by CSBMs), reduced abdominal and overall IBS-C symptoms, and was well tolerated. Japan Pharmaceutical Information Center trial no: JapicCTI-122041.

New insights into the earlymechanisms of epileptogenesis in a zebrafish model of Dravet syndrome.

To pinpoint the earliest cellular defects underlying seizure onset (epileptogenic period) during perinatal brain development in a new zebrafish model of Dravet syndrome (DS) and to investigate potential disease-modifying activity of the 5HT2 receptor agonist fenfluramine. We used CRISPR/Cas9 mutagenesis to introduce a missense mutation, designed to perturb ion transport function in all channel isoforms, into scn1lab, the zebrafish orthologue of SCN1A (encoding voltage-gated sodium channel alpha subunit 1). We performed behavioral analysis and electroencephalographic recordings to measure convulsions and epileptiform discharges, followed by single-cell RNA-Seq, morphometric analysis of transgenic reporter-labeled γ-aminobutyric acidergic (GABAergic) neurons, and pharmacological profiling of mutant larvae. Homozygous mutant (scn1labmut/mut ) larvae displayed spontaneous seizures with interictal, preictal, and ictal discharges (mean=7.5 per 20-minute recording; P<.0001; one-way analysis of variance). Drop-Seq analysis revealed a 2:1 shift in the ratio of glutamatergic to GABAergic neurons in scn1labmut/mut larval brains versus wild type (WT), with dynamic changes in neuronal, glial, and progenitor cell populations. To explore disease pathophysiology further, we quantified dendritic arborization in GABAergic neurons and observed a 40% reduction in arbor number compared to WT (P<.001; n=15 mutant, n=16 WT). We postulate that the significant reduction in inhibitory arbors causes an inhibitory to excitatory neurotransmitter imbalance that contributes to seizures and enhanced electrical brain activity in scn1labmut/mut larvae (high-frequency range), with subsequent GABAergic neuronal loss and astrogliosis. Chronic fenfluramine administration completely restored dendritic arbor numbers to normal in scn1labmut/mut larvae, whereas similar treatment with the benzodiazepine diazepam attenuated seizures, but was ineffective in restoring neuronal cytoarchitecture. BrdU labeling revealed cell overproliferation in scn1labmut/mut larval brains that were rescued by fenfluramine but not diazepam. Our findings provide novel insights into early mechanisms of DS pathogenesis, describe dynamic cell population changes in the scn1labmut/mut brain, and present first-time evidence for potential disease modification by fenfluramine.

Thorough QT/QTc Study Shows That a Novel 5-HT4 Receptor Partial Agonist Minesapride Has No Effect on QT Prolongation.

Minesapride (drug code: DSP-6952) is a potential gastrointestinal prokinetic agent with high selectivity for 5-hydroxytryptamine 4 (5-HT4 ) receptor that acts as a partial agonist. Although 5-HT4 receptor agonists are expected to show efficacy in patients with irritable bowel syndrome with constipation, only tegaserod is available for female patients, with limitations, in the United States. Previously, another 5-HT4 receptor agonist, cisapride, was widely used for the treatment of upper gastrointestinal diseases, but was withdrawn from the market because of arrhythmia with QT prolongation. Chemically, benzamide is one of the most common structures among 5-HT4 receptor agonists. Some benzamide derivatives, such as cisapride, are responsible for QT prolongation, while some, such as mosapride, are not. Thus, we planned a thorough QT/QTc study to investigate the effects of minesapride, a newly designed benzamide derivative, on the QT/QTc. This was a randomized, placebo-controlled, 4-arm, 4-period, crossover study conducted in healthy adults, with administration of single oral doses of minesapride (40 mg and 120 mg), placebo, and moxifloxacin in the fasted state. Minesapride and placebo were administered in a double-blind fashion, while the positive control moxifloxacin was administered in an open-label fashion. Japanese subjects (48 total: 24 males and 24 females) were randomized, and 47subjects completed all treatment periods. A review of other electrocardiographic data revealed that neither therapeutic (40mg) nor supratherapeutic (120 mg) doses of minesapride were associated with increased risk of prolonged QT interval.

Spinal AMP kinase activity differentially regulates phrenic motor plasticity.

Acute intermittent hypoxia (AIH) elicits phrenic motor plasticity via multiple distinct cellular mechanisms. With moderate AIH, phrenic motor facilitation (pMF) requires Gq protein-coupled serotonin type 2 receptor activation, ERK MAP kinase activity, and new synthesis of brain-derived neurotrophic factor. In contrast, severe AIH elicits pMF by an adenosine-dependent mechanism that requires exchange protein activated by cAMP, Akt, and mammalian target of rapamycin (mTOR) activity, followed by new tyrosine receptor kinase B protein synthesis; this same pathway is also initiated by Gs protein-coupled serotonin 7 receptors (5-HT7). Because the metabolic sensor AMP-activated protein kinase (AMPK) inhibits mTOR-dependent protein synthesis, and mTOR signaling is necessary for 5-HT7 but not 5-HT2 receptor-induced pMF, we hypothesized that spinal AMPK activity differentially regulates pMF elicited by these distinct receptor subtypes. Serotonin type 2A receptor [5-HT2A; (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride] or 5-HT7 (AS-19) receptor agonists were administered intrathecally at C4 (3 injections, 5-min intervals) while recording integrated phrenic nerve activity in anesthetized, vagotomized, paralyzed, and ventilated rats. Consistent with our hypothesis, spinal AMPK activation with 2-deoxyglucose or metformin blocked 5-HT7, but not 5-HT2A receptor-induced pMF; in both cases, pMF inhibition was reversed by spinal administration of the AMPK inhibitor compound C. Thus, AMPK differentially regulates cellular mechanisms of serotonin-induced phrenic motor plasticity.NEW & NOTEWORTHY Spinal AMP-activated protein kinase (AMPK) overactivity, induced by local 2-deoxyglucose or metformin administration, constrains serotonin 7 (5-HT7) receptor-induced (but not serotonin type 2A receptor-induced) respiratory motor facilitation, indicating that metabolic challenges might regulate specific forms of respiratory motor plasticity. Pharmacological blockade of spinal AMPK activity restores 5-HT7 receptor-induced respiratory motor facilitation in the presence of either 2-deoxyglucose or metformin, showing that AMPK is an important regulator of 5-HT7 receptor-induced respiratory motor plasticity.

Genotoxicity and carcinogenicity risk assessment of prucalopride, a selective 5-hydroxytryptamine 4 receptor agonist

Prucalopride, a high affinity, selective serotonin type 4 (5-HT4) receptor agonist, was associated with increased neoplasia incidence (in endocrine tissues and liver) in 2-year rodent bioassays, without evidence of a genotoxic mechanism of action. Proposed mechanisms of action involve prolactin and the constitutive androstane receptor (CAR). Epigenetic mechanisms and their relevance to humans are discussed. Data from in vitro and in vivo rodent studies demonstrated that prucalopride-related stimulation of prolactin secretion (via dopamine receptor D2 antagonism at high doses) is a rodent-specific, non-genotoxic mechanism for inducing hyperplasia and neoplasia in prolactin receptor-expressing endocrine tissues. Additional data demonstrated that CAR-mediated liver enzyme induction underlies the observed hepatocellular adenomas and thyroid follicular adenomas in rodents. A 12-month neonatal mouse carcinogenicity study confirmed the lack of a genotoxic mechanism of action. Furthermore, tumors were observed only at very high exposures (200 and 63 fold higher in mice and rats, respectively, than human exposure after a daily therapeutic dose of 2 mg). The studies indicate that non-genotoxic, rodent-specific, epigenetic mechanisms that are considered clinically irrelevant are responsible for the increased incidence of neoplasias associated with very high exposure to prucalopride in rodents, and that prucalopride does not pose a carcinogenic safety risk to humans.

De novo enteric neurogenesis in post-embryonic zebrafish from Schwann cell precursors rather than resident cell types.

The enteric nervous system (ENS) is essential for normal gastrointestinal function. Although the embryonic origin of enteric neurons from the neural crest is well established, conflicting evidence exists regarding postnatal enteric neurogenesis. Here, we address this by examining the origin of de novo neurogenesis in the post-embryonic zebrafish ENS. Although new neurons are added during growth and after injury, the larval intestine appears to lack resident neurogenic precursors or classical glia marked by sox10, plp1a, gfap or s100 Rather, lineage tracing with lipophilic dye or inducible Sox10-Cre suggests that post-embryonic enteric neurons arise from trunk neural crest-derived Schwann cell precursors that migrate from the spinal cord into the intestine. Furthermore, the 5-HT4 receptor agonist prucalopride increases enteric neurogenesis in normal development and after injury. Taken together, the results suggest that despite the lack of resident progenitors in the gut, post-embryonic enteric neurogenesis occurs via gut-extrinsic Schwann cell precursors during development and injury, and is promoted by serotonin receptor agonists. The absence of classical glia in the ENS further suggests that neural crest-derived enteric glia might have evolved after the teleost lineage.This article has an associated 'The people behind the papers' interview.

Role of Hippocampal 5-HT6 Receptors in Glucocorticoid-Induced Enhancement of Memory Consolidation in Rats.

Introduction:of the study: Post-training administration of glucocorticoids enhance memory consolidation of inhibitory avoidance learning. Given the involvement of 5-HT6 receptors in memory processing and the interaction of glucocorticoids with the brain serotonergic system in modulating memory processing, we investigated whether the effect of glucocorticoids on the consolidation of emotionally arousing training depends on hippocampal 5-HT6 receptors.Methods:Rats were trained in an inhibitory avoidance task and immediately received the systemic injections of corticosterone (CORT) as well as the intra-hippocampal injections of 5-HT receptors agonist or antagonist. The memory retention test was done 48 hours after training and immediately after the behavioral test, the animals were sacrificed and the hippocampi (left and right) rapidly dissected out for molecular studies.Results:Post-training injections of different doses of CORT (1.25, 2.5, 5, and 10 mg/kg) enhanced memory retention in a dose-dependent manner. The CORT-induced enhancement of memory consolidation was blocked by bilateral intra-hippocampal injections of 5-HT6 receptor antagonist SB271046 (5 or 10 ng/per side), but not agonist EMD386088 (5 or 10 ng/per side). Furthermore, systemic CORT reduced 5-HT6 receptor mRNA and protein expression in the hippocampus. Both doses of 5-HT6 receptor agonist and antagonist significantly enhanced and reduced the expression of the 5-HT6 receptor, respectively, and both ligands at the higher dose (10 ng) enhanced memory consolidation. Moreover, CORT injection attenuated and enhanced, respectively, the effects of agonist and antagonist on 5-HT6 receptor expression.Conclusion:These behavioral and molecular findings indicated an interaction between glucocorticoids and hippocampal 5-HT6 receptors in the consolidation of emotionally arousing experiences.

Blockade of 5-HT2 receptors with sarpogrelate uncovers 5-HT7 receptors inhibiting the tachycardic sympathetic drive in pithed rats.

Our group has previously shown in pithed rats that the cardiac sympathetic drive, which produces tachycardic responses, is inhibited by 5-HT via the activation of prejunctional 5-HT1B/1D/5 receptors. Interestingly, when 5-HT2 receptors are chronically blocked with sarpogrelate, the additional role of cardiac sympatho-inhibitory 5-HT1F receptors is unmasked. Although 5-HT2 receptors mediate tachycardia in rats, and the chronic blockade of 5-HT2 receptors unmasked 5-HT7 receptors mediating cardiac vagal inhibition, the role of 5-HT7 receptors in the modulation of the cardiac sympathetic tone remains virtually unexplored. On this basis, male Wistar rats were pretreated during 14days with sarpogrelate (a 5-HT2 receptor antagonist) in drinking water (30mg/kg/day; sarpogrelate-pretreated group) or equivalent volumes of drinking water (control group). Subsequently, the rats were pithed to produce increases in heart rate by either electrical preganglionic spinal (C7 -T1 ) stimulation of the cardiac sympathetic drive or iv administration of exogenous noradrenaline. The iv continuous infusion of AS-19 (a 5-HT7 receptor agonist; 10µg/kg/min) (i) inhibited the tachycardic responses to sympathetic stimulation, but not those to exogenous noradrenaline only in sarpogrelate-pretreated rats. This inhibition was completely reversed by SB258719 (a selective 5-HT7 receptor antagonist; 1mg/kg, iv) or glibenclamide (an ATP-sensitive K+ channel blocker; 20mg/kg, iv). These results suggest that chronic 5-HT2 receptor blockade uncovers a cardiac sympatho-inhibitory mechanism mediated by 5-HT7 receptors, involving a hyperpolarization due to the opening of ATP-sensitive K+ channels. Thus, these findings support the role of 5-HT7 receptors in the modulation of the cardiac sympathetic neurotransmission.

Citalopram reduces glutamatergic synaptic transmission in the auditory cortex via activation of 5-HT1A receptors.

Serotonin modulates cognitive processes and is related to various psychiatric disorders, including major depression. Administration of citalopram reduces the amplitude of auditory evoked potentials in depressed people and animal models, suggesting that 5-HT has an inhibitory role. Here, we characterize the modulation of excitatory post-synaptic currents by application of either 5-HT or agonists of 5-HT1A and 5-HT2 receptors, or by endogenous 5-HT evoked by citalopram on pyramidal neurons from layer II/III of rat auditory cortex. We found that application of 5-HT concentration-dependently reduces excitatory post-synaptic currents amplitude without changing the paired-pulse ratio, suggesting a post-synaptic modulation. We observed that selective agonists of 5-HT1A and 5-HT2 receptors [8-OH-DPAT (10 µM) and DOI (10 µM), respectively] mimic the effect of 5-HT on the excitatory post-synaptic currents. Effect of 5-HT was entirely blocked by co-application of the antagonists NAN-190 (1 µM) and ritanserin (200 nM). Similarly, citalopram application (1 μM) reduced the amplitude of the evoked excitatory post-synaptic currents. Reduction in the magnitude of the excitatory post-synaptic currents by endogenous 5-HT was interpolated in the dose-response curve elicited by exogenous 5-HT, yielding that citalopram raised the extracellular 5-HT concentration to 823 nM. Effect of citalopram was blocked by the previous application of NAN-190 but not ritanserin, indicating that citalopram reduces glutamatergic synaptic transmission via 5-HT1A receptors in layer II/III of the auditory cortex. These results suggest that the local activity of 5-HT contributes to decrease in the basal excitability of the auditory cortex for enhancing the detection of external relevant acoustic signals.

Serotonin 5-HT4 Receptor Agonists Improve Facilitation of Contextual Fear Extinction in an MPTP-Induced Mouse Model of Parkinson's Disease.

Previously, we found that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinson’s disease (PD) model mice (PD mice) showed facilitation of hippocampal memory extinction via reduced cyclic adenosine monophosphate (cAMP)/cAMP-dependent response element-binding protein (CREB) signaling, which may cause cognitive impairment in PD. Serotonergic neurons in the median raphe nucleus (MnRN) project to the hippocampus, and functional abnormalities have been reported. In the present study, we investigated the effects of the serotonin 5-HT4 receptor (5-HT4R) agonists prucalopride and velusetrag on the facilitation of memory extinction observed in PD mice. Both 5-HT4R agonists restored facilitation of contextual fear extinction in PD mice by stimulating the cAMP/CREB pathway in the dentate gyrus of the hippocampus. A retrograde fluorogold-tracer study showed that γ-aminobutyric acid-ergic (GABAergic) neurons in the reticular part of the substantia nigra (SNr), but not dopaminergic (DAergic) neurons in the substantia nigra pars compacta (SNpc), projected to serotonergic neurons in the MnRN, which are known to project their nerve terminals to the hippocampus. It is possible that the degeneration of the SNpc DAergic neurons in PD mice affects the SNr GABAergic neurons, and thereafter, the serotonergic neurons in the MnRN, resulting in hippocampal dysfunction. These findings suggest that 5HT4R agonists could be potentially useful as therapeutic drugs for treating cognitive deficits in PD.