5-HT4 receptor stimulation facilitates acetylcholine release in rat frontal cortex

The Serotonin 4 Receptor Subtype: A Target of Particular Interest, Especially for Brain Disorders

In recent years, particular attention has been paid to the serotonin 4 receptor, which is well expressed in the brain, but also peripherally in various organs. The cerebral distribution of this receptor is well conserved across species, with high densities in the basal ganglia, where they are expressed by GABAergic neurons. The 5-HT4 receptor is also present in the cerebral cortex, hippocampus, and amygdala, where they are carried by glutamatergic or cholinergic neurons. Outside the central nervous system, the 5-HT4 receptor is notably expressed in the gastrointestinal tract. The wide distribution of the 5-HT4 receptor undoubtedly contributes to its involvement in a plethora of functions. In addition, the modulation of this receptor influences the release of serotonin, but also the release of other neurotransmitters such as acetylcholine and dopamine. This is a considerable asset, as the modulation of the 5-HT4 receptor can therefore play a direct or indirect beneficial role in various disorders. One of the main advantages of this receptor is that it mediates a much faster antidepressant and anxiolytic action than classical selective serotonin reuptake inhibitors. Another major benefit of the 5-HT4 receptor is that its activation enhances cognitive performance, probably via the release of acetylcholine. The expression of the 5-HT4 receptor is also altered in various eating disorders, and its activation by the 5-HT4 agonist negatively regulates food intake. Additionally, although the cerebral expression of this receptor is modified in certain movement-related disorders, it is still yet to be determined whether this receptor plays a key role in their pathophysiology. Finally, there is no longer any need to demonstrate the value of 5-HT4 receptor agonists in the pharmacological management of gastrointestinal disorders.

5-HT4 Receptor Agonist Effects on Functional Connectivity in the Human Brain: Implications for Procognitive Action

BACKGROUND

Cognitive deficits are often comorbid with mood disorders and can cause significant functional impairment even after resolution of the primary mood symptoms. We do not currently have pharmacological treatments that adequately address these deficits. 5-HT4 receptor agonists show promise as potential procognitive agents in animal and early human translational studies. Optimal cognitive performance in humans is directly associated with appropriate functional connectivity between specific resting-state neural networks. However, so far the effect of 5-HT4 receptor agonism on resting-state functional connectivity (rsFC) in the brain in humans is unknown.

METHODS

We collected resting-state functional magnetic resonance imaging scans from 50 healthy volunteers, of whom 25 received 6 days × 1 mg prucalopride (a highly selective 5-HT4 receptor agonist) and 25 received placebo in a randomized double-blind design.

RESULTS

Network analyses identified that participants in the prucalopride group had enhanced rsFC between the central executive network and the posterior/anterior cingulate cortex. Seed analyses also showed greater rsFC between the left and right rostral anterior cingulate cortex and the left lateral occipital cortex, and reduced rsFC between the hippocampus and other default mode network regions.

CONCLUSIONS

Similar to other potentially procognitive medications, low-dose prucalopride in healthy volunteers appeared to enhance rsFC between regions involved in cognitive networks and reduce rsFC within the default mode network. This suggests a mechanism for the behavioral cognitive enhancement previously seen with 5-HT4 receptor agonists in humans and supports the potential for 5-HT4 receptor agonists to be used in clinical psychiatric populations.

Stress induced exacerbation of Alzheimer's disease brain pathology is thwarted by co-administration of nanowired cerebrolysin and monoclonal amyloid beta peptide antibodies with serotonin 5-HT6 receptor antagonist SB-399885

Alzheimer's disease is one of the devastating neurodegenerative diseases affecting mankind worldwide with advancing age mainly above 65 years and above causing great misery of life. About more than 7 millions are affected with Alzheimer's disease in America in 2023 resulting in huge burden on health care system and care givers and support for the family. However, no suitable therapeutic measures are available at the moment to enhance quality of life to these patients. Development of Alzheimer's disease may reflect the stress burden of whole life inculcating the disease processes of these neurodegenerative disorders of the central nervous system. Thus, new strategies using nanodelivery of suitable drug therapy including antibodies are needed in exploring neuroprotection in Alzheimer's disease brain pathology. In this chapter role of stress in exacerbating Alzheimer's disease brain pathology is explored and treatment strategies are examined using nanotechnology based on our own investigation. Our observations clearly show that restraint stress significantly exacerbate Alzheimer's disease brain pathology and nanodelivery of a multimodal drug cerebrolysin together with monoclonal antibodies (mAb) to amyloid beta peptide (AβP) together with a serotonin 5-HT6 receptor antagonist SB399885 significantly thwarted Alzheimer's disease brain pathology exacerbated by restraint stress, not reported earlier. The possible mechanisms and future clinical significance is discussed.

Usmarapride (SUVN-D4010), a 5-HT4 receptor partial agonist for the potential treatment of Alzheimer's disease: Behavioural, neurochemical and pharmacological profiling

Alzheimer's disease (AD) is a progressive neurodegenerative disorder which affects cognitive functions with negative impact on day to day activities and an ultimate loss of independent living. Current standard of care (SOC) for AD, viz. donepezil, rivastigmine, galantamine, memantine etc. either alone or in combination show modest efficacy without changing the course of the disease. On prolonged treatment, side effects are more common with an eventual loss of efficacy. Aducanumab, a monoclonal antibody is a disease modifying therapeutic agent targeting the toxic amyloid beta (Aβ) proteins for its clearance. However, it is found to have only modest efficacy in AD patients and its approval by FDA is controversial. Alternate, effective and safe therapeutics are need of the hour, as AD cases are expected to be doubled by 2050. Recently, 5-HT₄ receptors have been envisioned as target for alleviating AD associated cognitive impairment with potential disease modifying ability impacting disease progression. Usmarapride is a 5-HT₄ receptor partial agonist, being developed for the possible treatment of AD with symptomatic and disease modifying potential. Usmarapride demonstrated promising effects in ameliorating cognitive deficits in diverse animal models of episodic, working, social, and emotional memories. Usmarapride produced elevation in cortical acetylcholine in rats. Furthermore, usmarapride increased levels of soluble amyloid precursor protein alpha, a potential mechanism to reverse toxic Aβ peptide pathology. Usmarapride also potentiated the pharmacological effects of donepezil in animal models. To conclude, usmarapride may be a promising intervention for alleviating the cognitive dysfunction in AD patients with disease modifying potential.

Activation of uroepithelial 5-HT4R inhibits mechanosensory activity of murine bladder afferent nerves

Serotonin (5-HT) is known to act via multiple 5-HT receptors at spinal and supraspinal levels to regulate micturition. However, the contribution of peripheral 5-HT and its receptors in bladder physiology and pathology is not very well understood, despite evidence showing expression of multiple 5-HT receptors in the bladder wall and 5-HT may activate bladder afferent nerves. The current study was designed to investigate the possible role of 5-HT4R in modulation of the sensitivity of bladder afferents to bladder filling. Immunofluorescent staining showed abundant 5-HT4R immunoreactivity largely confined to the uroepithelium in wild type (WT) but not 5-HT4R−/− mice. In the ex vivo bladder-pelvic nerve preparation, intravesical application of the 5-HT4R agonist RS67333 (1–30 μm) caused concentration-dependent decreases of the pelvic nerve response to bladder filling. Such effect was not observed in the presence of 5-HT4R antagonist GR125487 or in 5-HT4R−/− preparations. A cohort of 5-HT4R−/− and WT control mice were treated with intraperitoneal injections of cyclophosphamide (CYP) (75 mg/kg, three times at 2 days interval) to induce chronic cystitis. Void spot analysis showed that CYP-treated 5-HT4R−/− mice urinated more frequently than their CYP-treated WT counterparts. Concomitantly, bladder afferents of CYP-treated 5-HT4R−/− mice displayed exaggerated sensitivity to bladder filling in comparison with the CYP-treated WT controls. These data suggest that 5-HT4R expressed on uroepithelial cells plays an inhibitory role in mechanosensory transduction in the bladder. Loss of 5-HT4R-mediated inhibition may enhance bladder afferent sensitivity and exacerbate bladder overactivity in pathological conditions. We propose that 5-HT4R agonists might be exploited for the treatment of overactive and painful bladder symptoms.

Cognition and serotonin in Parkinson's disease

Cognitive impairment affects up to 80% of patients with Parkinson's disease (PD) and is associated with poor quality of life. PD cognitive dysfunction includes poor working memory, impairments in executive function and difficulty in set-shifting. The pathophysiology underlying cognitive impairment in PD is still poorly understood, but there is evidence to support involvements of the cholinergic, dopaminergic, and noradrenergic systems. Only rivastigmine, an acetyl- and butyrylcholinesterase inhibitor, is efficacious for the treatment of PD dementia, which limits management of cognitive impairment in PD. Whereas the role of the serotonergic system in PD cognition is less understood, through its interactions with other neurotransmitters systems, namely, the cholinergic system, it may be implicated in cognitive processes. In this chapter, we provide an overview of the pharmacological, clinical and pathological evidence that implicates the serotonergic system in mediating cognition in PD.

The Effect of the 5-HT4 Agonist, Prucalopride, on a Functional Magnetic Resonance Imaging Faces Task in the Healthy Human Brain

Depression is a common and often recurrent illness with significant negative impact on a global scale. Current antidepressants are ineffective for up to one third of people with depression, many of whom experience persistent symptomatology. 5-HT₄ receptor agonists show promise in both animal models of depression and cognitive deficit. We therefore studied the effect of the 5-HT₄ partial agonist prucalopride (1 mg daily for 6 days) on the neural processing of emotional faces in 43 healthy participants using a randomised placebo-controlled design. Participants receiving prucalopride were more accurate at identifying the gender of emotional faces. In whole brain analyses, prucalopride was also associated with reduced activation in a network of regions corresponding to the default mode network. However, there was no evidence that prucalopride treatment produced a positive bias in the neural processing of emotional faces. Our study provides further support for a pro-cognitive effect of 5-HT₄ receptor agonism in humans. While our current behavioural and neural investigations do not suggest an antidepressant-like profile of prucalopride in humans, it will be important to study a wider dose range in future studies.

Role of Receptors in Relation to Plaques and Tangles in Alzheimer's Disease Pathology

Despite the identification of Aβ plaques and NFTs as biomarkers for Alzheimer’s disease (AD) pathology, therapeutic interventions remain elusive, with neither an absolute prophylactic nor a curative medication available to impede the progression of AD presently available. Current approaches focus on symptomatic treatments to maintain AD patients’ mental stability and behavioral symptoms by decreasing neuronal degeneration; however, the complexity of AD pathology requires a wide range of therapeutic approaches for both preventive and curative treatments. In this regard, this review summarizes the role of receptors as a potential target for treating AD and focuses on the path of major receptors which are responsible for AD progression. This review gives an overall idea centering on major receptors, their agonist and antagonist and future prospects of viral mimicry in AD pathology. This article aims to provide researchers and developers a comprehensive idea about the different receptors involved in AD pathogenesis that may lead to finding a new therapeutic strategy to treat AD.

5-HT4 Receptors Are Not Involved in the Effects of Fluoxetine in the Corticosterone Model of Depression

Clinical and preclinical studies report the implication of 5-hydroxytryptamine 4 receptors (5-HT₄Rs) in depression and anxiety. Here, we tested whether the absence of 5-HT4Rs influences the response to the antidepressant fluoxetine in mice subjected to chronic corticosterone administration, an animal model of depression and anxiety. Therefore, the effects of chronic administration of fluoxetine in corticosterone-treated wild-type (WT) and 5-HT₄R knockout (KO) mice were evaluated in the open-field and novelty suppressed feeding tests. As 5-HT_1A receptor (5-HT_1AR) and brain-derived neurotrophic factor (BDNF) are critically involved in depression and anxiety, we further evaluated 5-HT_1A receptor functionality by [³⁵S]GTPγS autoradiography and BDNF mRNA expression by in situ hybridization techniques. We found that 5-HT₄R KO and WT mice displayed anxiety- and depressive-like behavior following chronic administration of corticosterone, as evidenced in the open-field and novelty suppressed feeding tests. In the open-field, a decreased central activity was observed in naïve and corticosterone-treated mice of both genotypes following chronic fluoxetine administration. In the novelty suppressed feeding test, a predictive paradigm of antidepressant activity, chronic treatment with fluoxetine reverted the latency to eat in both genotypes. The antidepressant also potentiated the corticosterone-induced desensitization of the 5-HT_1AR in the dorsal raphe nucleus. Further, chronic fluoxetine increased BDNF mRNA expression in the dentate gyrus of the hippocampus in corticosterone-treated mice of both genotypes. Therefore, our findings indicate that the behavioral effects of fluoxetine in the corticosterone model of depression and anxiety appear not to be dependent on 5-HT₄Rs.

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.

Serotonin modulation of hippocampal functions: From anatomy to neurotherapeutics

The hippocampal region receives a dense serotoninergic innervation originating from both medial and dorsal raphe nuclei. This innervation regulates hippocampal activity through the activation of distinct receptor families that are expressed in excitatory and inhibitory neurons, terminals of several afferent neurotransmitter systems, and glial cells. Preclinical and clinical studies indicate that hippocampal dysfunctions are involved in learning and memory deficits, dementia, Alzheimer's disease, epilepsy and mood disorders such as anxiety, depression and post-traumatic syndrome disorder, whereas the hippocampus participates also in the therapeutic mechanisms of numerous medicines. Not surprisingly, several drugs acting via 5-HT mechanisms are efficacious to some extent in some diseases and the link between 5-HT and the hippocampus although clear remains difficult to untangle. For this reason, we review reported data concerning the distribution and the functional roles of the 5-HT receptors in the hippocampal region in health and disease. The impact of the 5-HT systems on the hippocampal function is such that the research of new 5-HT mechanisms and drugs is still very active. It concerns notably drugs acting at the 5-HT_1A,_2A,_2C,_4,6 receptor subtypes, in addition to the already existing drugs including the selective serotonin reuptake inhibitors.

Alterations and interactions of subcortical modulatory systems in Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is not fully understood. Here we summarize current knowledge on the involvement of the serotonergic, noradrenergic, dopaminergic, cholinergic, and opioid systems in AD, emphasizing the importance of interactions between the serotonergic and the other subcortical modulatory systems during the progression of AD. In physiological conditions, all neurotransmitter systems function in concert and are interdependent at both the neuroanatomical and molecular levels. Through their early involvement in AD, cognitive and behavioral abilities that rely on their interactions also become disrupted. Considering that serotonin (5HT) regulates the release of noradrenaline (NA), dopamine (DA) and acetylcholine (ACh), any alteration in 5HT levels leads to disturbance of NA, DA, and ACh homeostasis in the brain. One of the earliest pathological changes during the prodromal phase of AD is a decrease of serotonergic transmission throughout the brain, with serotonergic receptors being also affected. Additionally, serotonergic and noradrenergic as well as serotonergic and dopaminergic nuclei are reciprocally interconnected. As the serotonergic dorsal raphe nucleus (DRN) is affected by pathological changes early in AD, and the noradrenergic locus coeruleus (LC) and dopaminergic ventral tegmental area (VTA) exhibit AD-related pathological changes, their connectivity also becomes altered in AD. Such disrupted interactions among neurotransmitter systems in AD can be used in the development of multi-target drugs. Some of the potential AD therapeutics (such as ASS234, RS67333, tropisetron) target multiple neurotransmitter systems to achieve the best possible improvement of cognitive and behavioral deficits observed in AD. Here, we review how serotonergic system interacts with other subcortical modulatory systems (noradrenergic, dopaminergic, cholinergic, and opioid systems) during AD.

A role for 5-HT4 receptors in human learning and memory

BACKGROUND

5-HT4 receptor stimulation has pro-cognitive and antidepressant-like effects in animal experimental studies; however, this pharmacological approach has not yet been tested in humans. Here we used the 5-HT4 receptor partial agonist prucalopride to assess the translatability of these effects and characterise, for the first time, the consequences of 5-HT4 receptor activation on human cognition and emotion.

METHODS

Forty one healthy volunteers were randomised, double-blind, to a single dose of prucalopride (1 mg) or placebo in a parallel group design. They completed a battery of cognitive tests measuring learning and memory, emotional processing and reward sensitivity.

RESULTS

Prucalopride increased recall of words in a verbal learning task, increased the accuracy of recall and recognition of words in an incidental emotional memory task and increased the probability of choosing a symbol associated with a high likelihood of reward or absence of loss in a probabilistic instrumental learning task. Thus acute prucalopride produced pro-cognitive effects in healthy volunteers across three separate tasks.

CONCLUSIONS

These findings are a translation of the memory enhancing effects of 5-HT4 receptor agonism seen in animal studies, and lend weight to the idea that the 5-HT4 receptor could be an innovative target for the treatment of cognitive deficits associated with depression and other neuropsychiatric disorders. Contrary to the effects reported in animal models, prucalopride did not reveal an antidepressant profile in human measures of emotional processing.

Rational design of novel benzisoxazole derivatives with acetylcholinesterase inhibitory and serotoninergic 5-HT4 receptors activities for the treatment of Alzheimer's disease

A rigidification strategy was applied to the preclinical candidate donecopride, an acetylcholinesterase inhibitor possessing 5-HT₄R agonist activity. Inspired by promising bioactive benzisoxazole compounds, we have conducted a pharmacomodulation study to generate a novel series of multitarget directed ligands. The chemical synthesis of the ligand was optimized and compounds were evaluated in vitro against each target and in cellulo. Structure-activity relationship was supported by docking analysis in human acetylcholinesterase binding site. Among the synthesized compounds, we have identified a novel hybrid 32a (3-[2-[1-(cyclohexylmethyl)-4-piperidyl]ethyl]-4-methoxy-1,2-benzoxazole) able to display nanomolar acetylcholinesterase inhibitory effects and nanomolar Ki for 5-HT₄R.

Therapeutic potential of serotonin 4 receptor for chronic depression and its associated comorbidity in the gut

The latest estimates from world health organization suggest that more than 450 million people are suffering from depression and other psychiatric conditions. Of these, 50-60% have been reported to have progression of gut diseases. In the last two decades, researchers introduced incipient physiological roles for serotonin (5-HT) receptors (5-HTRs), suggesting their importance as a potential pharmacological target in various psychiatric and gut diseases. A growing body of evidence suggests that 5-HT systems affect the brain-gut axis in depressive patients, which leads to gut comorbidity. Recently, preclinical trials of 5-HT4R agonists and antagonists were promising as antipsychotic and prokinetic agents. In the current review, we address the possible pharmacological role and contribution of 5-HT4R in the pathophysiology of chronic depression and associated gut abnormalities. Physiologically, during depression episodes, centers of the sympathetic and parasympathetic nervous system couple together with neuroendocrine systems to alter the function of hypothalamic-pituitary-adrenal (HPA) axis and enteric nervous system (ENS), which in turn leads to onset of gastrointestinal tract (GIT) disorders. Consecutively, the ENS governs a broad spectrum of physiological activities of gut, such as visceral pain and motility. During the stages of emotional stress, hyperactivity of the HPA axis alters the ENS response to physiological and noxious stimuli. Consecutively, stress-induced flare, swelling, hyperalgesia and altered reflexes in gut eventually lead to GIT disorders. In summary, the current review provides prospective information about the role and mechanism of 5-HT4R-based therapeutics for the treatment of depressive disorder and possible consequences for the gut via brain-gut axis interactions. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Effects of chronic antidepressant use on neurophysiological responses to tDCS post-stroke

BACKGROUND

Transcranial direct current stimulation (tDCS) induces neuroplastic changes in the motor cortex of healthy individuals and has become a candidate intervention to promote recovery post-stroke. However, neurophysiological effects of tDCS in stroke are poorly understood. Antidepressant medications, which are commonly prescribed post-stroke, have the potential to significantly affect cortical excitability and alter responsiveness to tDCS interventions, yet these effects have not previously been examined.

OBJECTIVE/HYPOTHESIS

To examine the effects of chronic antidepressant use, tDCS, and the interaction of the two on motor cortical excitability in people with chronic stroke. Based on previous literature in nondisabled adults, we hypothesized that post-stroke, antidepressant-takers would show decreased baseline motor cortical excitability but enhanced responsiveness to anodal tDCS.

METHODS

Twenty-six participants with chronic stroke (17 control, 9 antidepressant) received real and sham anodal tDCS during separate sessions at least a week apart. Motor cortical excitability was measured before and after tDCS was applied to the lesioned hemisphere primary motor cortex. We compared baseline cortical excitability and neurophysiological responses to tDCS between groups and sessions.

RESULTS

Baseline motor cortical excitability was not different between control and antidepressant groups. Following anodal tDCS over the ipsilesional primary motor cortex, cortical excitability in the non-lesioned hemisphere decreased in controls, but, surprisingly, increased in antidepressant-takers.

CONCLUSIONS

Chronic antidepressant use may not affect motor cortical excitability post-stroke, however it appears to reverse some of the expected effects of tDCS. Therefore future utilization of tDCS in post-stroke neurorehabilitation research should take antidepressant medication status into account.

A Novel in vivo Anti-amnesic Agent, Specially Designed to Express Both Acetylcholinesterase (AChE) Inhibitory, Serotonergic Subtype 4 Receptor (5-HT4R) Agonist and Serotonergic Subtype 6 Receptor (5-HT6R) Inverse Agonist Activities, With a Potential Interest Against Alzheimer's Disease

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.

Effects of separate or combined exposure of nonylphenol and octylphenol on central 5-HT system and related learning and memory in the rats

This study evaluated toxic effects of nonylphenol (NP) and octylphenol (OP) on central 5-hydroxytryptamine (5-HT) system and related learning and memory in the rats. Male Sprague-Dawley rats were exposed to NP (30, 90, or 270 mg/kg), OP (40, 120, or 360 mg/kg), or a mixture of NP and OP [(mixed with the corresponding NP, OP alone exposed low, medium and high dose according to the natural environment exists NP:OP = 4:1; NOL (24 mg/kg NP+8 mg/kg OP), NOM (72 mg/kg NP+24 mg/kg OP), NOH (216 mg/kg NP+72 mg/kg OP)] by gavage every other day for 30 d. Learning and memory were assessed using a passive-avoidance test. Levels of estrogen receptor β (ERβ), 5-HT, tryptophan hydroxylase 2 (TPH2), monoamine oxidase (MAOA) enzyme, serotonin transporter (SERT), the vesicular monoamine transporter 2 (VMAT2), 5-hydroxytryptamine 1 A (5-HT_1A), 5-hydroxytryptamine 3 A (5-HT_3A), 5-hydroxytryptamine 3B (5-HT_3B), 5-hydroxytryptamine 4 A (5-HT_4A) and 5-hydroxytryptamine 6 A (5-HT_6A) were measured using ELISA kits. Levels of ERβ, MAOA, SERT, VMAT2, 5-HT_1A, 5-HT_3A, 5-HT_3B, 5-HT_4A and 5-HT_6A in rat hippocampal reduced by a high dose of NP and/or OP. Levels of TPH2 in rat midbrain and 5-HT in rat hippocampal increased by a high dose of NP and/or OP. In addition, latency was significantly shorter and errors were significantly greater in the high dose NP and NP+OP (NO) groups. Taken together, these results suggest that NP and/or OP may affect learning and memory in rats by inhibiting levels of ERβ, which could then lead to decreases in levels of 5-HT_1A, 5-HT_3A, 5-HT_3B, 5-HT_4A, and 5-HT_6A in the rat hippocampus. These findings suggested that separate and combined exposure to NP and OP could produce toxic effects on central 5-HT system and related learning and memory in the rats.

Alterations of Expression of the Serotonin 5-HT4 Receptor in Brain Disorders

The serotonin 4 receptor, 5-HT₄R, represents one of seven different serotonin receptor families and is implicated in a variety of physiological functions and their pathophysiological variants, such as mood and depression or anxiety, food intake and obesity or anorexia, or memory and memory loss in Alzheimer's disease. Its central nervous system expression pattern in the forebrain, in particular in caudate putamen, the hippocampus and to lesser extent in the cortex, predispose it for a role in executive function and reward-related actions. In rodents, regional overexpression or knockdown in the prefrontal cortex or the nucleus accumbens of 5-HT₄R was shown to impact mood and depression-like phenotypes, food intake and hypophagia; however, whether expression changes are causally involved in the etiology of such disorders is not clear. In this context, more data are emerging, especially based on PET technology and the use of ligand tracers that demonstrate altered 5-HT₄R expression in brain disorders in humans, confirming data stemming from post-mortem tissue and preclinical animal models. In this review, we would like to present the current knowledge of 5-HT₄R expression in brain regions relevant to mood/depression, reward and executive function with a focus on 5-HT₄R expression changes in brain disorders or caused by drug treatment, at both the transcript and protein levels.

Sargassum swartzii extracts ameliorate memory functions by neurochemical modulation in a rat model

Recently, considerable attention has been paid to drug exploration from natural sources for treating memory loss, a major manifestation of various neurodegenerative diseases. Increasing evidences implicate brain serotonin metabolism in learning and memory, supporting the notion that targeting 5-HT (5-hydroxytryptamine) and its receptors would be beneficial in the treatment of cognitive disorders. In the present study, behavioral and neurochemical effects were examined following administration of Sargassum swartzii extracts in albino Wistar rats. Increase in spatial working memory and recognition memory was exhibited by the seaweed-treated rats as compared to controls. Plasma tryptophan, brain 5-HT, and 5-hydroxyindoleacetic acid levels were measured using HPLC-ECD, and a significant increase in brain 5-HT metabolism was observed in the seaweed-treated rats. The increase in memory functions following repeated administration of S. swartzii extracts is suggested to be due to the increased serotonergic neurotransmission in the brain of seaweed-treated rats.

Modulation of cannabinoid signaling by hippocampal 5-HT4 serotonergic system in fear conditioning

Behavioral studies have suggested a key role for the cannabinoid system in the modulation of conditioned fear memory. Likewise, much of the literature has revealed that the serotonergic system affects Pavlovian fear conditioning and extinction. A high level of functional overlap between the serotonin and cannabinoid systems has also been reported. To clarify the interaction between the hippocampal serotonin (5-HT4) receptor and the cannabinoid CB1 receptor in the acquisition of fear memory, the effects of 5-HT4 agents, arachidonylcyclopropylamide (ACPA; CB1 receptor agonist), and the combined use of these drugs on fear learning were studied in a fear conditioning task in adult male NMRI mice. Pre-training intraperitoneal administration of ACPA (0.1 mg/kg) decreased the percentage of freezing time in both context- and tone-dependent fear conditions, suggesting impairment of the acquisition of fear memory. Pre-training, intra-hippocampal (CA1) microinjection of RS67333, a 5-HT4 receptor agonist, at doses of 0.1 and 0.2 or 0.2 µg/mouse impaired contextual and tone fear memory, respectively. A subthreshold dose of RS67333 (0.005 µg/mouse) did not alter the ACPA response in either condition. Moreover, intra-CA1 microinjection of RS23597 as a 5-HT4 receptor antagonist did not alter context-dependent fear memory acquisition, but it did impair tone-dependent fear memory acquisition. However, a subthreshold dose of the RS23597 (0.01 µg/mouse) potentiated ACPA-induced fear memory impairment in both conditions. Therefore, we suggest that the blockade of hippocampal 5-HT4 serotonergic system modulates cannabinoid signaling induced by the activation of CB1 receptors in conditioned fear.

The serotonergic 5-HT4 receptor: A unique modulator of hippocampal synaptic information processing and cognition

Serotonin (5-hydroxytryptamine, 5-HT) contributes in multifarious ways to the regulation of brain function, spanning key aspects such as the sleep-wake cycle, appetite, mood and mental health. The 5-HT receptors comprise seven receptor families (5-HT_1-7) that are further subdivided into 14 receptor subtypes. The role of the 5-HT receptor in the modulation of neuronal excitability has been well documented. Recently, however, it has become apparent that the 5-HT₄ receptor may contribute significantly to cognition and regulates less ostensible aspects of brain function: it engages in metaplastic regulation of synaptic responsiveness in key brain structures such as the hippocampus, thereby specifically promoting persistent forms of synaptic plasticity, and influences the direction of change in synaptic strength in selected hippocampal subfields. This highly specific neuromodulatory control by the 5-HT₄ receptor may in turn explain the reported role for this receptor in hippocampus-dependent cognition. In this review article, we describe the role of the 5-HT₄ receptor in hippocampal function, and describe how this receptor plays a unique and highly specialised role in synaptic information storage and cognition.

The serotonergic system and cognitive function

Symptoms of cognitive dysfunction like memory loss, poor concentration, impaired learning and executive functions are characteristic features of both schizophrenia and Alzheimer’s disease (AD). The neurobiological mechanisms underlying cognition in healthy subjects and neuropsychiatric patients are not completely understood. Studies have focused on serotonin (5-hydroxytryptamine, 5-HT) as one of the possible cognitionrelated biomarkers. The aim of this review is to provide a summary of the current literature on the role of the serotonergic (5-HTergic) system in cognitive function, particularly in AD and schizophrenia.

The role of the 5-HTergic system in cognition is modulated by the activity and function of 5-HT receptors (5-HTR) classified into seven groups, which differ in structure, action, and localization. Many 5-HTR are located in the regions linked to various cognitive processes. Preclinical studies using animal models of learning and memory, as well as clinical in vivo (neuroimaging) and in vitro (post-mortem) studies in humans have shown that alterations in 5-HTR activity influence cognitive performance.

The current evidence implies that reduced 5-HT neurotransmission negatively influences cognitive functions and that normalization of 5-HT activity may have beneficial effects, suggesting that 5-HT and 5-HTR represent important pharmacological targets for cognition enhancement and restoration of impaired cognitive performance in neuropsychiatric disorders.

Chronic Enhancement of Serotonin Facilitates Excitatory Transcranial Direct Current Stimulation-Induced Neuroplasticity

Serotonin affects memory formation via modulating long-term potentiation (LTP) and depression (LTD). Accordingly, acute selective serotonin reuptake inhibitor (SSRI) administration enhanced LTP-like plasticity induced by transcranial direct current stimulation (tDCS) in humans. However, it usually takes some time for SSRI to reduce clinical symptoms such as anxiety, negative mood, and related symptoms of depression and anxiety disorders. This might be related to an at least partially different effect of chronic serotonergic enhancement on plasticity, as compared with single-dose medication. Here we explored the impact of chronic application of the SSRI citalopram (CIT) on plasticity induced by tDCS in healthy humans in a partially double-blinded, placebo (PLC)-controlled, randomized crossover study. Furthermore, we explored the dependency of plasticity induction from the glutamatergic system via N-methyl-D-aspartate receptor antagonism. Twelve healthy subjects received PLC medication, combined with anodal or cathodal tDCS of the primary motor cortex. Afterwards, the same subjects took CIT (20 mg/day) consecutively for 35 days. During this period, four additional interventions were performed (CIT and PLC medication with anodal/cathodal tDCS, CIT and dextromethorphan (150 mg) with anodal/cathodal tDCS). Plasticity was monitored by motor-evoked potential amplitudes elicited by transcranial magnetic stimulation. Chronic application of CIT increased and prolonged the LTP-like plasticity induced by anodal tDCS for over 24 h, and converted cathodal tDCS-induced LTD-like plasticity into facilitation. These effects were abolished by dextromethorphan. Chronic serotonergic enhancement results in a strengthening of LTP-like glutamatergic plasticity, which might partially explain the therapeutic impact of SSRIs in depression and other neuropsychiatric diseases.

Serotonergic modulation of the activity of mesencephalic dopaminergic systems: Therapeutic implications

Since their discovery in the mammalian brain, it has been apparent that serotonin (5-HT) and dopamine (DA) interactions play a key role in normal and abnormal behavior. Therefore, disclosure of this interaction could reveal important insights into the pathogenesis of various neuropsychiatric diseases including schizophrenia, depression and drug addiction or neurological conditions such as Parkinson's disease and Tourette's syndrome. Unfortunately, this interaction remains difficult to study for many reasons, including the rich and widespread innervations of 5-HT and DA in the brain, the plethora of 5-HT receptors and the release of co-transmitters by 5-HT and DA neurons. The purpose of this review is to present electrophysiological and biochemical data showing that endogenous 5-HT and pharmacological 5-HT ligands modify the mesencephalic DA systems' activity. 5-HT receptors may control DA neuron activity in a state-dependent and region-dependent manner. 5-HT controls the activity of DA neurons in a phasic and excitatory manner, except for the control exerted by 5-HT_2C receptors which appears to also be tonically and/or constitutively inhibitory. The functional interaction between the two monoamines will also be discussed in view of the mechanism of action of antidepressants, antipsychotics, anti-Parkinsonians and drugs of abuse.

Chronic activation of 5-HT4 receptors or blockade of 5-HT6 receptors improve memory performances

5-HT4 and 5-HT6 serotonergic receptors are located in brain structures involved in memory processes. Neurochemical and behavioural studies have demonstrated that acute activation of 5-HT4 receptors (5-HT4R) or blockade of 5-HT6 receptors (5-HT6R) improves memory. To evaluate the potential of these two receptors as targets in the treatment of memory disorders encountered in several situations (ageing, Alzheimer's disease, schizophrenia, etc.), it is necessary to assess whether their beneficial effects occur after chronic administration, and if such treatment induces adverse effects. The goal of this study was to assess the effects of chronic 5-HT4R or 5-HT6R modulation on recognition memory, and to observe the possible manifestation of side effects (modification of weight gain, locomotor activity or exploratory behaviour, etc.). Mice were treated for 14 days with a 5-HT4R partial agonist (RS-67333) or a 5-HT6R antagonist (SB-271046) at increasing doses. Memory performances, locomotor activity, and exploration were assessed. Both chronic 5-HT4R activation and 5-HT6R blockade extended memory traces in an object recognition test, and were not associated with any adverse effects in the parameters assessed. Chronic modulation of one or both of these receptors thus seems promising as a potential strategy for the treatment memory deficits.

Design of donecopride, a dual serotonin subtype 4 receptor agonist/acetylcholinesterase inhibitor with potential interest for Alzheimer's disease treatment

RS67333 is a partial serotonin subtype 4 receptor (5-HT4R) agonist that has been widely studied for its procognitive effect. More recently, it has been shown that its ability to promote the nonamyloidogenic cleavage of the precursor of the neurotoxic amyloid-β peptide leads to the secretion of the neurotrophic protein sAPPα. This effect has generated great interest in RS67333 as a potential treatment for Alzheimer's disease (AD). We show herein that RS67333 is also a submicromolar acetylcholinesterase (AChE) inhibitor and therefore, could contribute, through this effect, to the restoration of the cholinergic neurotransmission that becomes altered in AD. We planned to pharmacomodulate RS67333 to enhance its AChE inhibitory activity to take advantage of this pleiotropic pharmacological profile in the design of a novel multitarget-directed ligand that is able to exert not only a symptomatic but also, a disease-modifying effect against AD. These efforts allowed us to select donecopride as a valuable dual (h)5-HT4R partial agonist (Ki = 10.4 nM; 48.3% of control agonist response)/(h)AChEI (IC50 = 16 nM) that further promotes sAPPα release (EC50 = 11.3 nM). Donecopride, as a druggable lead, was assessed for its in vivo procognitive effects (0.1, 0.3, 1, and 3 mg/kg) with an improvement of memory performances observed at 0.3 and 1 mg/kg on the object recognition test. On the basis of these in vitro and in vivo activities, donecopride seems to be a promising drug candidate for AD treatment.

Dual histamine H3R/serotonin 5-HT4R ligands with antiamnesic properties: pharmacophore-based virtual screening and polypharmacology

In recent years, preclinical and clinical studies have generated considerable interest in the development of histamine H3 receptor (H3R) antagonists as novel treatment for degenerative disorders associated with impaired cholinergic function. To identify novel scaffolds for H3R antagonism, a common feature-based pharmacophore model was developed and used to screen the 17,194 compounds of the CERMN (Centre d'Etudes et de Recherche sur le Médicament de Normandie) chemical library. Out of 268 virtual hits which have been gathered in 34 clusters, we were particularly interested in tricyclic derivatives also exhibiting a potent 5HT4R affinity. Benzo[h][1,6]naphthyridine derivatives showed the highest H3R affinity, and compound 17 (H3R Ki = 41.6 nM; 5-HT4R Ki = 208 nM) completely reversed the amnesiant effect of scopolamine at 3 mg/kg in a spatial working memory experiment. For the first time we demonstrated the feasibility to combine H3R and 5-HT4R activities in a single molecule, raising the exciting possibility that dual H3R antagonist/5HT4R agonist have potential for the treatment of neurodegenerative diseases such as Alzheimer's disease.

SSP-002392, a new 5-HT4 receptor agonist, dose-dependently reverses scopolamine-induced learning and memory impairments in C57Bl/6 mice

5-HT4 receptors (5-HT4R) are suggested to affect learning and memory processes. Earlier studies have shown that animals treated with 5-HT4R agonists, often with limited selectivity, show improved learning and memory with retention memory often being assessed immediately after or within 24 h after the last training session. In this study, we characterized the effect of pre-training treatment with the selective 5-HT4R agonist SSP-002392 on memory acquisition and the associated long-term memory retrieval in animal models of impaired cognition. Pre-training treatment with SSP-002392 (0.3 mg/kg, 1.5 mg/kg and 7.5 mg/kg p.o.) dose-dependently inhibited the cognitive deficits induced by scopolamine (0.5 mg/kg s.c.) in two different behavioral tasks: passive avoidance and Morris water maze. In the Morris water maze, spatial learning was significantly improved after treatment with SSP-002392 translating in an accelerated and more efficient localization of the hidden platform compared to scopolamine-treated controls. Moreover, retention memory was assessed 24 h (passive avoidance) and 72 h (Morris water maze) after the last training session of cognitive-impaired animals and this was significantly improved in animals treated with SSP-002392 prior to the training sessions. Furthermore, the effects of SSP-002392 were comparable to galanthamine hydrobromide. We conclude that SSP-002392 has potential as a memory-enhancing compound.

Visceral analgesic effect of 5-HT(4) receptor agonist in rats involves the rostroventral medulla (RVM)

The 5-HT(4) 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-HT(4) 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-HT(4) receptors which triggers the release of opioids at supraspinal site, which activates descending noradrenergic pathways to the spinal cord to produce analgesia.

Regulation of amyloid precursor protein processing by serotonin signaling

Proteolytic processing of the amyloid precursor protein (APP) by the β- and γ-secretases releases the amyloid-β peptide (Aβ), which deposits in senile plaques and contributes to the etiology of Alzheimer's disease (AD). The α-secretase cleaves APP in the Aβ peptide sequence to generate soluble APPα (sAPPα). Upregulation of α-secretase activity through the 5-hydroxytryptamine 4 (5-HT₄) receptor has been shown to reduce Aβ production, amyloid plaque load and to improve cognitive impairment in transgenic mouse models of AD. Consequently, activation of 5-HT₄ receptors following agonist stimulation is considered to be a therapeutic strategy for AD treatment; however, the signaling cascade involved in 5-HT₄ receptor-stimulated proteolysis of APP remains to be determined. Here we used chemical and siRNA inhibition to identify the proteins which mediate 5-HT_4d receptor-stimulated α-secretase activity in the SH-SY5Y human neuronal cell line. We show that G protein and Src dependent activation of phospholipase C are required for α-secretase activity, while, unexpectedly, adenylyl cyclase and cAMP are not involved. Further elucidation of the signaling pathway indicates that inositol triphosphate phosphorylation and casein kinase 2 activation is also a prerequisite for α-secretase activity. Our findings provide a novel route to explore the treatment of AD through 5-HT₄ receptor-induced α-secretase activation.

The role of serotonin in memory: interactions with neurotransmitters and downstream signaling

Serotonin, or 5-hydroxytryptamine (5-HT), is found to be involved in many physiological or pathophysiological processes including cognitive function. Seven distinct receptors (5-HT1-7), each with several subpopulations, have been identified for serotonin, which are different in terms of localization and downstream signaling. Because of the development of selective agonists and antagonists for these receptors as well as transgenic animal models of cognitive disorders, our understanding of the role of serotonergic transmission in learning and memory has improved in recent years. A large body of evidence indicates the interplay between serotonergic transmission and other neurotransmitters including acetylcholine, dopamine, γ-aminobutyric acid (GABA) and glutamate, in the neurobiological control of learning and memory. In addition, there has been an alteration in the density of serotonergic receptors in aging and Alzheimer's disease, and serotonin modulators are found to alter the process of amyloidogenesis and exert cognitive-enhancing properties. Here, we discuss the serotonin-induced modulation of various systems involved in mnesic function including cholinergic, dopaminergic, GABAergic, glutamatergic transmissions as well as amyloidogenesis and intracellular pathways.

Early administration of RS 67333, a specific 5-HT4 receptor agonist, prevents amyloidogenesis and behavioral deficits in the 5XFAD mouse model of Alzheimer's disease

Amyloid β (Aβ) accumulation is considered the main culprit in the pathogenesis of Alzheimer’s disease (AD). Recent studies suggest that decreasing Aβ production at very early stages of AD could be a promising strategy to slow down disease progression. Serotonin 5-HT₄ receptor activation stimulates α-cleavage of the amyloid precursor protein (APP), leading to the release of the soluble and neurotrophic sAPPα fragment and thus precluding Aβ formation. Using the 5XFAD mouse model of AD that shows accelerated Aβ deposition, we investigated the effect of chronic treatments (treatment onset at different ages and different durations) with the 5-HT₄ receptor agonist RS 67333 during the asymptomatic phase of the disease. Chronic administration of RS 67333 decreased concomitantly the number of amyloid plaques and the level of Aβ species. Reduction of Aβ levels was accompanied by a striking decrease in hippocampal astrogliosis and microgliosis. RS 67333 also transiently increased sAPPα concentration in the cerebrospinal fluid and brain. Moreover, a specific 5-HT₄ receptor antagonist (RS 39604) prevented the RS 67333-mediated reduction of the amyloid pathology. Finally, the novel object recognition test deficits of 5XFAD mice were reversed by chronic treatment with RS 67333. Collectively, these results strongly highlight this 5-HT₄ receptor agonist as a promising disease modifying-agent for AD.

Role of synaptic activity in the regulation of amyloid beta levels in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia. Accumulation of amyloid-beta (Aβ) peptides is regarded as the critical component associated with AD pathogenesis, which is derived from the amyloid precursor protein (APP) cleavage. Recent studies suggest that synaptic activity is one of the most important factors that regulate Aβ levels. It has been found that synaptic activity facilitates APP internalization and influences APP cleavage. Glutamatergic, cholinergic, serotonergic, leptin, adrenergic, orexin, and gamma-amino butyric acid receptors, as well as the activity-regulated cytoskeleton-associated protein (Arc) are all involved in these processes. The present review summarizes the evidence for synaptic activity-modulated Aβ levels and the mechanisms underlying this regulation. Interestingly, the immediate early gene product Arc may also be the downstream signaling molecule of several receptors in the synaptic activity-modulated Aβ levels. Elucidating how Aβ levels are regulated by synaptic activity may provide new insights in both the understanding of the pathogenesis of AD and in the development of therapies to slow down the progression of AD.

Effect of serotonin on paired associative stimulation-induced plasticity in the human motor cortex

Serotonin modulates diverse brain functions. Beyond its clinical antidepressant effects, it improves motor performance, learning and memory formation. These effects might at least be partially caused by the impact of serotonin on neuroplasticity, which is thought to be an important foundation of the respective functions. In principal accordance, selective serotonin reuptake inhibitors enhance long-term potentiation-like plasticity induced by transcranial direct current stimulation (tDCS) in humans. As other neuromodulators have discernable effects on different kinds of plasticity in humans, here we were interested to explore the impact of serotonin on paired associative stimulation (PAS)-induced plasticity, which induces a more focal kind of plasticity, as compared with tDCS, shares some features with spike timing-dependent plasticity, and is thought to be relative closely related to learning processes. In this single-blinded, placebo-controlled, randomized crossover study, we administered a single dose of 20 mg citalopram or placebo medication and applied facilitatory- and excitability-diminishing PAS to the left motor cortex of 14 healthy subjects. Cortico-spinal excitability was explored via single-pulse transcranial magnetic stimulation-elicited MEP amplitudes up to the next evening after plasticity induction. After citalopram administration, inhibitory PAS-induced after-effects were abolished and excitatory PAS-induced after-effects were enhanced trendwise, as compared with the respective placebo conditions. These results show that serotonin modulates PAS-induced neuroplasticity by shifting it into the direction of facilitation, which might help to explain mechanism of positive therapeutic effects of serotonin in learning and medical conditions characterized by enhanced inhibitory or reduced facilitatory plasticity, including depression and stroke.

Chronic 5-HT4 receptor activation decreases Aβ production and deposition in hAPP/PS1 mice

Lowering the production and accumulation of Aβ has been explored as treatment for Alzheimer's disease (AD), because Aβ is postulated to play an important role in the pathogenesis of AD. 5-HT4 receptors are an interesting drug target in this regard, as their activation might stimulate α-secretase processing, which increases sAPPα and reduces Aβ, at least according to the central dogma in APP processing. Here we describe a novel high-affinity 5-HT4 receptor agonist SSP-002392 that, in cultured human neuroblastoma cells, potently increases the levels of cAMP and sAPPα at 100-fold lower concentrations than the effective concentrations of prucalopride, a known selective 5-HT4 receptor agonist. Chronic administration of this compound in a hAPP/PS1 mouse model of Alzheimer's disease decreased soluble and insoluble Aβ in hippocampus, but the potential mechanisms underlying these observations seem to be complex. We found no evidence for direct α-secretase stimulation in the brain in vivo, but observed decreased APP and BACE-1 expression and elevated astroglia and microglia responses. Taken together these results provide support for a potential disease-modifying aspect when stimulating central 5-HT4 receptors; however, the complexity of the phenomena warrants further research.

In-vivo rat striatal 5-HT4 receptor occupancy using non-radiolabelled SB207145

Objectives

The objective of the current investigation was to develop a simple, rapid method for determining in-vivo 5-hydroxytryptamine type 4 receptor (5-HT4R) occupancy in rat brain using non-radiolabelled SB207145 as a tracer for accelerating the drug discovery process.

Methods

In-vivo tracer optimization studies for tracer dose, survival intervals and brain distribution profile were carried out in rats. The tracer was pharmacologically validated using potent well-characterized 5-HT4R ligands. The brain regional concentrations of tracer (SB207145); plasma and brain concentrations of 5-HT4R ligands were quantified using high-performance liquid chromatography coupled with a tandem mass spectrometric detector (LC-MS/MS).

Key findings

SB207145 showed a higher specific binding in striatum (1.96 ng/g) and lower binding in cerebellum (0.66 ng/g), which is consistent with findings of other published 5-HT4R expression studies. Pretreatment with potent 5-HT4 ligands dose-dependently reduced striatal SB207145 concentration and the effective dose to achieve 50% receptor occupancy (ED50) values were 4.8, 2.0, 7.4, 9.9, 3.8 and 0.02 mg/kg for GR113808, piboserod, prucalopride, RS67333, TD8954 and PF04995274, respectively.

Conclusions

Results from the mass spectrometry approach to determine 5-HT4R occupancy in rat brain are comparable with those reported using radiolabelled scintillation spectroscopy methods. In conclusion, the LC-MS/MS characterization permits use of tracer at a preclinical stage in high-throughput fashion as well as characterization of target expression.

Identification of multiple 5-HT₄ partial agonist clinical candidates for the treatment of Alzheimer's disease

The cognitive impairments observed in Alzheimer's disease (AD) are in part a consequence of reduced acetylcholine (ACh) levels resulting from a loss of cholinergic neurons. Preclinically, serotonin 4 receptor (5-HT(4)) agonists are reported to modulate cholinergic function and therefore may provide a new mechanistic approach for treating cognitive deficits associated with AD. Herein we communicate the design and synthesis of potent, selective, and brain penetrant 5-HT(4) agonists. The overall goal of the medicinal chemistry strategy was identification of structurally diverse clinical candidates with varying intrinsic activities. The exposure-response relationships between binding affinity, intrinsic activity, receptor occupancy, drug exposure, and pharmacodynamic activity in relevant preclinical models of AD were utilized as key selection criteria for advancing compounds. On the basis of their excellent balance of pharmacokinetic attributes and safety, two lead 5-HT(4) partial agonist candidates 2d and 3 were chosen for clinical development.

The 5-hydroxytryptamine4 receptor agonists prucalopride and PRX-03140 increase acetylcholine and histamine levels in the rat prefrontal cortex and the power of stimulated hippocampal θ oscillations

5-Hydroxytryptamine (5-HT)(4) receptor agonists reportedly stimulate brain acetylcholine (ACh) release, a property that might provide a new pharmacological approach for treating cognitive deficits associated with Alzheimer's disease. The purpose of this study was to compare the binding affinities, functional activities, and effects on neuropharmacological responses associated with cognition of two highly selective 5-HT(4) receptor agonists, prucalopride and 6,7-dihydro-4-hydroxy-7-isopropyl-6-oxo-N-[3-(piperidin-1-yl)propyl]thieno[2,3-b]pyridine-5-carboxamide (PRX-03140). In vitro, prucalopride and PRX-03140 bound to native rat brain 5-HT(4) receptors with K(i) values of 30 nM and 110 nM, respectively, and increased cAMP production in human embryonic kidney-293 cells expressing recombinant rat 5-HT(4) receptors. In vivo receptor occupancy studies established that prucalopride and PRX-03140 were able to penetrate the brain and bound to 5-HT(4) receptors in rat brain, achieving 50% receptor occupancy at free brain exposures of 330 nM and 130 nM, respectively. Rat microdialysis studies revealed that prucalopride maximally increased ACh and histamine levels in the prefrontal cortex at 5 and 10 mg/kg, whereas PRX-03140 significantly increased cortical histamine levels at 50 mg/kg, failing to affect ACh release at doses lower than 150 mg/kg. In combination studies, donepezil-induced increases in cortical ACh levels were potentiated by prucalopride and PRX-03140. Electrophysiological studies in rats demonstrated that both compounds increased the power of brainstem-stimulated hippocampal θ oscillations at 5.6 mg/kg. These findings show for the first time that the 5-HT(4) receptor agonists prucalopride and PRX-03140 can increase cortical ACh and histamine levels, augment donepezil-induced ACh increases, and increase stimulated-hippocampal θ power, all neuropharmacological parameters consistent with potential positive effects on cognitive processes.

Synergistic effect of acetylcholinesterase inhibition (donepezil) and 5-HT(4) receptor activation (RS67333) on object recognition in mice

Facing inefficiency of current treatments to cure Alzheimer disease (AD), a pharmacological approach is now emerging on the assumption that a single compound may be able to hit multiple targets, namely Multi-Target-Directed Ligands (MTDLs). Displaying numerous advantages, several MTDL for AD have been recently described but none associating an inhibition of AChE and an activation of 5-HT(4)R. The aim of this study was to validate the concept of a synergistic action of these two targets on episodic-like memory performances in mice. Among potent molecules, RS67333, a reference 5-HT(4)R agonist and donepezil (DNPZ), a reference acetylcholinesterase inhibitor, have been particularly chosen because of their close chemical structure. Administered separately, RS67333 (0.3 and 1mg/kg) and DNPZ (1mg/kg) improved recognition performances compared to saline treated animals but not with lower doses. Co-administration of subactive doses of RS67333 (0.1mg/kg) and DNPZ (0.3mg/kg) improved memory, moreover, this improvement is prevented if a 5-HT(4)R antagonist (GR125487, 10mg/kg) is also administered. Activation of 5-HT(4)R combined with inhibition of AChE with subactive doses of RS67333 and of DNPZ has synergistic effects on memory performances in mice. These molecules having close chemical structures, the synergistic effect of their combination affords new hope to chemist for the synthesis of MTDL.