5-HT(6) receptor and cognition

5-HT6 receptor neutral antagonists protect astrocytes: A lesson from 2-phenylpyrrole derivatives

The serotonin type 6 receptor (5-HT6R) displays a strong constitutive activity, suggesting it participates largely in the physiological and pathological processes controlled by the receptor. The active states of 5-HT6R engage particular signal transduction pathways that lead to different biological responses. In this study, we present the development of 5-HT6R neutral antagonists at Gs signaling built upon the 2-phenylpyrrole scaffold. Using molecular dynamics simulations, we outline the relationship between the exposure of the basic center of the molecules and their ability to target the agonist-activated state of the receptor. Our study identifies compound 30 as a potent and selective neutral antagonist at 5-HT6R-operated Gs signaling. Furthermore, we demonstrate the cytoprotective effects of 30 and structurally diverse 5-HT6R neutral antagonists at Gs signaling in C8-D1A cells and human astrocytes exposed to rotenone. This effect is not observed for 5-HT6R agonists or inverse agonists. In light of these findings, we propose compound 30 as a valuable molecular probe to study the biological effects associated with the agonist-activated state of 5-HT6R and provide insight into the glioprotective properties of 5-HT6R neutral antagonists at Gs signaling.

Regiodivergent synthesis of sulfone-tethered lactam-lactones bearing four contiguous stereocenters

Sulfone-tethered lactones/amides/amines display a diverse spectrum of biological activities, including anti-psychotic and anti-hypertensive. Sulfones are also widely present in functional materials and fragrances. We therefore reasoned that a regiodivergent and stereocontrolled strategy that merges the sulfone, lactone, and lactam motifs would likely lead to the discovery of new pharmacophores and functional materials. Here, we report mild conditions for the sulfonyllactonization of γ-lactam-tethered 5-aryl-4(E)-pentenoic acids. The annulation is highly modular, chemoselective, and diastereoselective. With respect to regioselectivity, trisubstituted alkenoic acids display a preference for 5-exo-trig cyclization whereas disubstituted alkenoic acids undergo exclusive 6-endo-trig cyclization. The lactam-fused sulfonyllactones bear angular quaternary as well as four contiguous stereocenters. The products are post-modifiable, especially through a newly developed Co-catalyzed reductive cross-coupling protocol.

Novel thienocycloalkylpyridazinones as useful scaffolds for acetylcholinesterase inhibition and serotonin 5-HT6 receptor interaction

A library of eighteen thienocycloalkylpyridazinones was synthesized for human acetylcholinesterase (hAChE) and butyrylcholinesterase (hBChE) inhibition and serotonin 5-HT₆ receptor subtype interaction by following a multitarget-directed ligand approach (MTDL), as a suitable strategy for treatment of Alzheimer's disease (AD). The novel compounds featured a tricyclic scaffold, namely thieno[3,2-h]cinnolinone, thienocyclopentapyridazinone and thienocycloheptapyridazinone, connected through alkyl chains of variable length to proper amine moieties, most often represented by N-benzylpiperazine or 1-(phenylsulfonyl)-4-(piperazin-1-ylmethyl)-1H-indole as structural elements addressing AChE and 5-HT₆ interaction, respectively. Our study highlighted the versatility of thienocycloalkylpyridazinones as useful architectures for AChE interaction, with several N-benzylpiperazine-based analogues emerging as potent and selective hAChE inhibitors with IC₅₀ in the 0.17-1.23 μM range, exhibiting low to poor activity for hBChE (IC₅₀ = 4.13-9.70 μM). The introduction of 5-HT₆ structural moiety phenylsulfonylindole in place of N-benzylpiperazine, in tandem with a pentamethylene linker, gave potent 5-HT₆ thieno[3,2-h]cinnolinone and thienocyclopentapyridazinone-based ligands both displaying hAChE inhibition in the low micromolar range and unappreciable activity towards hBChE. While docking studies provided a rational structural explanation for AChE/BChE enzyme and 5-HT₆ receptor interaction, in silico prediction of ADME properties of tested compounds suggested further optimization for development of such compounds in the field of MTDL for AD.

Impact of the Substitution Pattern at the Basic Center and Geometry of the Amine Fragment on 5-HT6 and D3R Affinity in the 1H-Pyrrolo[3,2-c]quinoline Series

Salt bridge (SB, double-charge-assisted hydrogen bonds) formation is one of the strongest molecular non-covalent interactions in biological systems, including ligand-receptor complexes. In the case of G-protein-coupled receptors, such an interaction is formed by the conserved aspartic acid (D3.32) residue and the basic moiety of the aminergic ligand. This study aims to determine the influence of the substitution pattern at the basic nitrogen atom and the geometry of the amine moiety at position 4 of 1H-pyrrolo[3,2-c]quinoline on the quality of the salt bridge formed in the 5-HT₆ receptor and D₃ receptor. To reach this goal, we synthetized and biologically evaluated a new series of 1H-pyrrolo[3,2-c]quinoline derivatives modified with various amines. The selected compounds displayed a significantly higher 5-HT₆R affinity and more potent 5-HT₆R antagonist properties when compared with the previously identified compound PZ-1643, a dual-acting 5-HT₆R/D₃R antagonist; nevertheless, the proposed modifications did not improve the activity at D₃R. As demonstrated by the in silico experiments, including molecular dynamics simulations, the applied structural modifications were highly beneficial for the formation and quality of the SB formation at the 5-HT₆R binding site; however, they are unfavorable for such interactions at D₃R.

Spatiotemporal dynamics of 5-HT6 receptor ciliary localization during mouse brain development

The serotonin 5-HT₆ receptor (5-HT₆R) is a promising target to improve cognitive symptoms of psychiatric diseases of neurodevelopmental origin, such as autism spectrum disorders and schizophrenia. However, its expression and localization at different stages of brain development remain largely unknown, due to the lack of specific antibodies to detect endogenous 5-HT₆R. Here, we used transgenic mice expressing a GFP-tagged 5-HT₆R under the control of its endogenous promoter (Knock-in) as well as embryonic stem cells expressing the GFP-tagged receptor to extensively characterize its expression at cellular and subcellular levels during development. We show that the receptor is already expressed at E13.5 in the cortex, the striatum, the ventricular zone, and to a lesser extent the subventricular zone. In adulthood, it is preferentially found in projection neurons of the hippocampus and cerebral cortex, in striatal medium-sized spiny neurons, as well as in a large proportion of astrocytes, while it is expressed in a minor population of interneurons. Whereas the receptor is almost exclusively detected in the primary cilia of neurons at embryonic and adult stages and in differentiated stem cells, it is located in the somatodendritic compartment of neurons from some brain regions at the neonatal stage and in the soma of undifferentiated stem cells. Finally, knocking-out the receptor induces a shortening of the primary cilium, suggesting that it plays a role in its function. This study provides the first global picture of 5-HT₆R expression pattern in the mouse brain at different developmental stages. It reveals dynamic changes in receptor localization in neurons at the neonatal stage, which might underlie its key role in neuronal differentiation and psychiatric disorders of neurodevelopmental origin.

Pharmacological targeting of cognitive impairment in depression: recent developments and challenges in human clinical research

Impaired cognition is often overlooked in the clinical management of depression, despite its association with poor psychosocial functioning and reduced clinical engagement. There is an outstanding need for new treatments to address this unmet clinical need, highlighted by our consultations with individuals with lived experience of depression. Here we consider the evidence to support different pharmacological approaches for the treatment of impaired cognition in individuals with depression, including treatments that influence primary neurotransmission directly as well as novel targets such as neurosteroid modulation. We also consider potential methodological challenges in establishing a strong evidence base in this area, including the need to disentangle direct effects of treatment on cognition from more generalised symptomatic improvement and the identification of sensitive, reliable and objective measures of cognition.

Participation of Hippocampal 5-HT5A, 5-HT6 and 5-HT7 Serotonin Receptors on the Consolidation of Social Recognition Memory

Social recognition is the ability of animals to identify and recognize a conspecific. The consolidation of social stimuli in long-term memory is crucial for the establishment and maintenance of social groups, reproduction and species survival. Despite its importance, little is known about the circuitry and molecular mechanisms involved in the social recognition memory (SRM). Serotonin (5-hydroxytryptamine, 5-HT) is acknowledged as a major neuromodulator, which plays a key role in learning and memory. Focusing on the more recently described 5-HT receptors, we investigated in the CA1 region of the dorsal hippocampus the participation of 5-HT_5A, 5-HT₆ and 5-HT₇ receptors in the consolidation of SRM. Male Wistar rats cannulated in CA1 were subjected to a social discrimination task. In the sample phase the animals were exposed to a juvenile conspecific for 1 h. Immediately after, they received different pharmacological treatments. Twenty-four hours later, they were submitted to a 5 min retention test in the presence of the previously presented juvenile (familiar) and a novel juvenile. The animals that received infusions of 5-HT_5A receptor antagonist SB-699551 (10 µg/µL), 5-HT₆ receptor agonist WAY-208466 (0.63 µg/µL) or 5-HT₇ receptor agonist AS-19 (5 µg/µL) intra-CA1 were unable to recognize the familiar juvenile. This effect was blocked by the coinfusion of WAY-208466 plus 5-HT₆ receptor antagonist SB-271046 (10 µg/µL) or AS-19 plus 5-HT₇ receptor antagonist SB-269970 (5 µg/µL). The present study helps to clarify the neurobiological functions of the 5-HT receptors more recently described and extends our knowledge about mechanisms underlying the SRM.

In Silico Screening of Natural Compounds for Candidates 5HT6 Receptor Antagonists against Alzheimer's Disease

Alzheimer's disease (AD), a devastating neurodegenerative disease, is the focus of pharmacological research. One of the targets that attract the most attention for the potential therapy of AD is the serotonin 5HT6 receptor, which is the receptor situated exclusively in CNS on glutamatergic and GABAergic neurons. The neurochemical impact of this receptor supports the hypothesis about its role in cognitive, learning, and memory systems, which are of critical importance for AD. Natural products are a promising source of novel bioactive compounds with potential therapeutic potential as a 5HT6 receptor antagonist in the treatment of AD dementia. The ZINC-natural product database was in silico screened in order to find the candidate antagonists of 5-HT6 receptor against AD. A virtual screening protocol that includes both short-and long-range interactions between interacting molecules was employed. First, the EIIP/AQVN filter was applied for in silico screening of the ZINC database followed by 3D QSAR and molecular docking. Ten best candidate compounds were selected from the ZINC Natural Product database as potential 5HT6 Receptor antagonists and were proposed for further evaluation. The best candidate was evaluated by molecular dynamics simulations and free energy calculations.

Systematic Analysis of Neurotransmitter Receptors in Human Breast Cancer Reveals a Strong Association With Outcome and Uncovers HTR6 as a Survival-Associated Gene Potentially Regulating the Immune Microenvironment

Many epidemiological reports have indicated an increase in the incidence of breast cancer among psychotic patients, suggesting that the targets of antipsychotics, neurotransmitter receptors, may have a role in tumorigenesis. However, the functions of neurotransmitter receptors in cancer are barely known. Here, we analyzed 44 neurotransmitter receptors in breast cancer and revealed that the expression of 34 receptors was positively correlated with relapse-free survival rates (RFS) of patients using the public database (n = 3951). Among all these receptors, we revealed decreased expression of HTR6 in human advanced breast cancer versus tumors in situ using our original data (n = 44). After a pan-cancer analysis including 22 cancers (n = 11262), we disclosed that HTR6 was expressed in 12 tumors and uncovered its influence on survival in seven tumors. Using multi-omics datasets from Linkedomics, we revealed a potential regulatory role of HTR6 in MAPK, JUN, and leukocyte-differentiation pathways through enriching 294 co-expressed phosphorylated proteins of HTR6. Furthermore, we proclaimed a close association of HTR6 expression with the immune microenvironment. Finally, we uncovered two possible reasons for HTR6 down-regulation in breast cancer, including deep deletion in the genome and the up-regulation of FOXA1 in breast cancer, which was a potential negatively regulatory transcription factor of HTR6. Taken together, we revealed a new function of neurotransmitter receptors in breast cancer and identified HTR6 as a survival-related gene potentially regulating the immune microenvironment. The findings in our study would improve our understanding of the pathogenesis of breast cancer and provided a theoretical basis for personalized medication in psychotic patients.

Alzheimer's disease and sleep disorders: Insights into the possible disease connections and the potential therapeutic targets

One of the comorbid conditions in an individual with Alzheimer's disease is a sleep disorder. Clinical features of sleep disorders involve various sleep disturbances such as Obstructive Sleep Apnea (OSAS), Excessive Daytime Sleepiness (EDS), Rapid Eye Movement (REM), Breathing Disorders, Periodic limb movements in sleep (PLMS), etc. The primary tools used for the identification of such disturbances are Polysomnography (PSG) and Wrist actigraphy. This review will highlight and explains the different approaches used in the treatment of sleep disorders. Non-pharmacological treatments include Peter Hauri rules, sleep education program, and light therapy which play a key role in the regulation of sleep-wake cycles. Pharmacological therapy described in this article may be useful in treating sleep destruction in patients with Alzheimer's disease. Along with the Non-pharmacological and pharmacological treatment, here we discuss five commonly recognized plant-based nutraceuticals with hypothesized impact on sleep disorders: caffeine, chamomile, cherries, L-tryptophan, and valerian by the proper emphasis on the known mechanism of their action.

Electrochemical Sulfonylation-Induced Lactonization of Alkenes: Synthesis of Sulfonyl Phthalides

An electrochemical cascade sulfonylation and lactonization process of alkenes and a most widely used arylsulfonylation reagent─sulfonyl hydrazines─was developed for the first time. This electrochemical sulfonyl lactonization avoided the use of toxic metal catalysts or stoichiometric oxidants and was carried out under mild conditions. The target product γ-sulfonylated phthalides with broad and excellent substrate tolerance were achieved.

Development and crystallography-aided SAR studies of multifunctional BuChE inhibitors and 5-HT6R antagonists with β-amyloid anti-aggregation properties

The lack of an effective treatment makes Alzheimer's disease a serious healthcare problem and a challenge for medicinal chemists. Herein we report interdisciplinary research on novel multifunctional ligands targeting proteins and processes involved in the development of the disease: BuChE, 5-HT₆ receptors and β-amyloid aggregation. Structure-activity relationship analyses supported by crystallography and docking studies led to the identification of a fused-type multifunctional ligand 50, with remarkable and balanced potencies against BuChE (IC₅₀ = 90 nM) and 5-HT₆R (K_i = 4.8 nM), and inhibitory activity against Aβ aggregation (53% at 10 μM). In in vitro ADME-Tox and in vivo pharmacokinetic studies compound 50 showed good stability in the mouse liver microsomes, favourable safety profile and brain permeability with the brain to plasma ratio of 6.79 after p.o. administration in mice, thus being a promising candidate for in vivo pharmacology studies and a solid foundation for further research on effective anti-AD therapies.

The Phenoxyalkyltriazine Antagonists for 5-HT6 Receptor with Promising Procognitive and Pharmacokinetic Properties In Vivo in Search for a Novel Therapeutic Approach to Dementia Diseases

Among the serotonin receptors, one of the most recently discovered 5-HT₆ subtype is an important protein target and its ligands may play a key role in the innovative treatment of cognitive disorders. However, none of its selective ligands have reached the pharmaceutical market yet. Recently, a new chemical class of potent 5-HT₆ receptor agents, the 1,3,5-triazine-piperazine derivatives, has been synthesized. Three members, the ortho and meta dichloro- (1,2) and the unsubstituted phenyl (3) derivatives, proved to be of special interest due to their high affinities (1,2) and selectivity (3) toward 5-HT₆ receptor. Thus, a broader pharmacological profile for 1–3, including comprehensive screening of the receptor selectivity and drug-like parameters in vitro as well as both, pharmacokinetic and pharmacodynamic properties in vivo, have been investigated within this study. A comprehensive analysis of the obtained results indicated significant procognitive-like activity together with beneficial drug-likeness in vitro and pharmacokinetics in vivo profiles for both, (RS)-4-[1-(2,3-dichlorophenoxy)propyl]-6-(4-methylpiperazin-1-yl)-1,3,5-triazin-2-amine (2) and (RS)-4-(4-methylpiperazin-1-yl)-6-(1-phenoxypropyl)-1,3,5-triazin-2-amine (3), but insensibly predominant for compound 2. Nevertheless, both compounds (2 and 3) seem to be good Central Nervous System drug candidates in search for novel therapeutic approach to dementia diseases, based on the 5-HT₆ receptor target.

Structure-Based Design and Optimization of FPPQ, a Dual-Acting 5-HT3 and 5-HT6 Receptor Antagonist with Antipsychotic and Procognitive Properties

In line with recent clinical trials demonstrating that ondansetron, a 5-HT₃ receptor (5-HT₃R) antagonist, ameliorates cognitive deficits of schizophrenia and the known procognitive effects of 5-HT₆ receptor (5-HT₆R) antagonists, we applied the hybridization strategy to design dual-acting 5-HT₃/5-HT₆R antagonists. We identified the first-in-class compound FPPQ, which behaves as a 5-HT₃R antagonist and a neutral antagonist 5-HT₆R of the Gs pathway. FPPQ shows selectivity over 87 targets and decent brain penetration. Likewise, FPPQ inhibits phencyclidine (PCP)-induced hyperactivity and displays procognitive properties in the novel object recognition task. In contrast to FPPQ, neither 5-HT₆R inverse agonist SB399885 nor neutral 5-HT₆R antagonist CPPQ reversed (PCP)-induced hyperactivity. Thus, combination of 5-HT₃R antagonism and 5-HT₆R antagonism, exemplified by FPPQ, contributes to alleviating the positive-like symptoms. Present findings reveal critical structural features useful in a rational polypharmacological approach to target 5-HT₃/5-HT₆ receptors and encourage further studies on dual-acting 5-HT₃/5-HT₆R antagonists for the treatment of psychiatric disorders.

Blockade of Serotonin 5-HT6 Receptor Constitutive Activity Alleviates Cognitive Deficits in a Preclinical Model of Neurofibromatosis Type 1

Neurofibromatosis type 1 (NF1) is a common inherited disorder caused by mutations of the NF1 gene that encodes the Ras-GTPase activating protein neurofibromin, leading to overactivation of Ras-dependent signaling pathways such as the mTOR pathway. It is often characterized by a broad range of cognitive symptoms that are currently untreated. The serotonin 5-HT₆ receptor is a potentially relevant target in view of its ability to associate with neurofibromin and to engage the mTOR pathway to compromise cognition in several cognitive impairment paradigms. Here, we show that constitutively active 5-HT₆ receptors contribute to increased mTOR activity in the brain of Nf1^+/− mice, a preclinical model recapitulating some behavioral alterations of NF1. Correspondingly, peripheral administration of SB258585, a 5-HT₆ receptor inverse agonist, or rapamycin, abolished deficits in long-term social and associative memories in Nf1^+/− mice, whereas administration of CPPQ, a neutral antagonist, did not produce cognitive improvement. These results show a key influence of mTOR activation by constitutively active 5-HT₆ receptors in NF1 cognitive symptoms. They provide a proof of concept that 5-HT₆ receptor inverse agonists already in clinical development as symptomatic treatments to reduce cognitive decline in dementia and psychoses, might be repurposed as therapies alleviating cognitive deficits in NF1 patients.

Chemical update on the potential for serotonin 5-HT6 and 5-HT7 receptor agents in the treatment of Alzheimer's disease

Despite the better understanding of the mechanisms underlying Alzheimer's Disease (AD) and launched clinical trials, no AD-modifying treatment based on a synthetic drug has been introduced for almost twenty years. The serotonin 5-HT₆ and 5-HT₇ receptors turned out to be promising biological targets for modulation of central nervous system dysfunctions including cognitive impairment. Within this paper, we evaluate the pharmacological potency of both, 5-HT₆R and 5-HT₇R, agents in search for novel AD treatment. An overview of chemical structures of the 5-HTRs ligands with simultaneous procognitive action which have undergone preclinical and clinical studies within the last 10 years has been performed.

Electrochemical Radical δ-H Sulfonylation Reaction for the Synthesis of 4-((Aryl,Arylsul fonyl)methylene)-2,5-Cyclohexadiene Derivatives

A novel and efficient electrochemical radical δ-H sulfonylation reaction of para-quinone methides (p-QMs) and sodium sulfinates has been achieved under common laboratory conditions. In this strategy, a new C(sp²)-S bond was constructed for the synthesis of 4-((aryl,arylsulfonyl)methylene)-2,5-cyclohexadiene derivatives with a broad substrate scope, good functional group tolerance, and mild conditions. Further studies showed that the reaction had an excellent regional selectivity.

Discovery of a new class of multi-target heterocycle piperidine derivatives as potential antipsychotics with pro-cognitive effect

A series of benzoisoxazoleylpiperidine derivatives were synthesized by using the multi-target strategies and their potent affinities for dopamine (DA), serotonin (5-HT) and human histamine H₃ receptors have been evaluated. Of these compounds, the promising candidate 4w displayed high affinities for D₂, D₃, 5-HT_1A, 5-HT_2A and H₃, a moderate affinity for 5-HT₆, negligible effects on the human ether-a-go-go-related gene (hERG) channel, low affinities for off-target receptors (5-HT_2C, adrenergic α₁ and H₁). In addition, the animal behavioral study revealed that, compared to risperidone, compound 4w significantly inhibited apomorphine-induced climbing and MK-801-induced movement behaviors with a high threshold for catalepsy and low liabilities for weight gain and hyperprolactinemia. Results from the conditioned avoidance response test and novel object recognition task demonstrated that 4w had pro-cognitive effects. Thus, the antipsychotic drug-like activities of 4w indicate that it may be a potential polypharmacological antipsychotic candidate drug.

2-Phenyl-1H-pyrrole-3-carboxamide as a New Scaffold for Developing 5-HT6 Receptor Inverse Agonists with Cognition-Enhancing Activity

Serotonin type 6 receptor (5-HT₆R) has gained particular interest as a promising target for treating cognitive deficits, given the positive effects of its antagonists in a wide range of memory impairment paradigms. Herein, we report on degradation of the 1H-pyrrolo[3,2-c]quinoline scaffold to provide the 2-phenyl-1H-pyrrole-3-carboxamide, which is devoid of canonical indole-like skeleton and retains recognition of 5-HT₆R. This modification has changed the compound’s activity at 5-HT₆R-operated signaling pathways from neutral antagonism to inverse agonism. The study identified compound 27 that behaves as an inverse agonist of the 5-HT₆R at the Gs and Cdk5 signaling pathways. Compound 27 showed high selectivity and metabolic stability and was brain penetrant. Finally, 27 reversed scopolamine-induced memory decline in the novel object recognition test and exhibited procognitive properties in the attentional set-shifting task in rats. In light of these findings, 27 might be considered for further evaluation as a new cognition-enhancing agent, while 2-phenyl-1H-pyrrole-3-carboxamide might be used as a template for designing 5-HT₆R inverse agonists.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

A dual-acting 5-HT6 receptor inverse agonist/MAO-B inhibitor displays glioprotective and pro-cognitive properties

The complex etiology of Alzheimer's disease has initiated a quest for multi-target ligands to address the multifactorial causes of this neurodegenerative disorder. In this context, we designed dual-acting 5-HT₆ receptor (5-HT₆R) antagonists/MAO-B inhibitors using pharmacophore hybridization strategy. Our approach involved linking priviliged scaffolds of 5-HT₆R with aryloxy fragments derived from reversible and irreversible MAO-B inhibitors. The study identified compound 48 that acts as an inverse agonist of 5-HT₆R at G_s signaling and an irreversible MAO-B inhibitor. Compound 48 showed moderate metabolic stability in rat microsomal assay, artificial membrane permeability, no hepatotoxicity, and it was well distributed to the brain. Additionally, 48 showed glioprotective properties in a model of cultured astrocytes using 6-OHDA as the cytotoxic agent. Finally, compound 48 (MED = 1 mg/kg, p.o.) fully reversed memory deficits in the NOR task induced by scopolamine in rats. A better understanding of effects exerted by dual-acting 5-HT₆R/MAO-B modulators may impact the future development of neurodegenerative-directed treatment strategies.

Dual-acting agents for improving cognition and real-world function in Alzheimer's disease: Focus on 5-HT6 and D3 receptors as hubs

To date, there are no interventions that impede the inexorable progression of Alzheimer's disease (AD), and currently-available drugs cholinesterase (AChE) inhibitors and the N-Methyl-d-Aspartate receptor antagonist, memantine, offer only modest symptomatic benefit. Moreover, a range of mechanistically-diverse agents (glutamatergic, histaminergic, monoaminergic, cholinergic) have disappointed in clinical trials, alone and/or in association with AChE inhibitors. This includes serotonin (5-HT) receptor-6 antagonists, despite compelling preclinical observations in rodents and primates suggesting a positive influence on cognition. The emphasis has so far been on high selectivity. However, for a multi-factorial disorder like idiopathic AD, 5-HT6 antagonists possessing additional pharmacological actions might be more effective, by analogy to "multi-target" antipsychotics. Based on this notion, drug discovery programmes have coupled 5-HT6 blockade to 5-HT4 agonism and inhibition of AchE. Further, combined 5-HT6/dopamine D3 receptor (D3) antagonists are of especial interest since D3 blockade mirrors 5-HT6 antagonism in exerting broad-based pro-cognitive properties in animals. Moreover, 5-HT6 and dopamine D3 antagonists promote neurocognition and social cognition via both distinctive and convergent actions expressed mainly in frontal cortex, including suppression of mTOR over-activation and reinforcement of cholinergic and glutamatergic transmission. In addition, 5-HT6 blockade affords potential anti-anxiety, anti-depressive and anti-epileptic properties, and antagonising 5-HT6 receptors may be associated with neuroprotective ("disease-modifying") properties. Finally D3 antagonism may counter psychotic episodes and D3 receptors themselves offer a promising hub for multi-target agents. The present article reviews the status of "R and D" into multi-target 5-HT6 and D3 ligands for improved treatment of AD and other neurodegenerative disorders of aging. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Early 5-HT6 receptor blockade prevents symptom onset in a model of adolescent cannabis abuse

Cannabis abuse during adolescence confers an increased risk for developing later in life cognitive deficits reminiscent of those observed in schizophrenia, suggesting common pathological mechanisms that remain poorly characterized. In line with previous findings that revealed a role of 5-HT6 receptor-operated mTOR activation in cognitive deficits of rodent developmental models of schizophrenia, we show that chronic administration of ∆9-tetrahydrocannabinol (THC) to mice during adolescence induces a long-lasting activation of mTOR in prefrontal cortex (PFC), alterations of excitatory/inhibitory balance, intrinsic properties of layer V pyramidal neurons, and long-term depression, as well as cognitive deficits in adulthood. All are prevented by administrating a 5-HT6 receptor antagonist or rapamycin, during adolescence. In contrast, they are still present 2 weeks after the same treatments delivered at the adult stage. Collectively, these findings suggest a role of 5-HT6 receptor-operated mTOR signaling in abnormalities of cortical network wiring elicited by THC at a critical period of PFC maturation and highlight the potential of 5-HT6 receptor antagonists as early therapy to prevent cognitive symptom onset in adolescent cannabis abusers.

G-Protein-Coupled Receptors in CNS: A Potential Therapeutic Target for Intervention in Neurodegenerative Disorders and Associated Cognitive Deficits

Neurodegenerative diseases are a large group of neurological disorders with diverse etiological and pathological phenomena. However, current therapeutics rely mostly on symptomatic relief while failing to target the underlying disease pathobiology. G-protein-coupled receptors (GPCRs) are one of the most frequently targeted receptors for developing novel therapeutics for central nervous system (CNS) disorders. Many currently available antipsychotic therapeutics also act as either antagonists or agonists of different GPCRs. Therefore, GPCR-based drug development is spreading widely to regulate neurodegeneration and associated cognitive deficits through the modulation of canonical and noncanonical signals. Here, GPCRs’ role in the pathophysiology of different neurodegenerative disease progressions and cognitive deficits has been highlighted, and an emphasis has been placed on the current pharmacological developments with GPCRs to provide an insight into a potential therapeutic target in the treatment of neurodegeneration.

p-TsOH-mediated synthesis of substituted 2,4-diaryl-3-sulfonylquinolines from functionalized 2-aminobenzophenones and aromatic β-ketosulfones under microwave irradiation

This study describes an efficient protocol for the preparation of substituted 2,4-diaryl-3-sulfonylquinolines from functionalized 2-aminobenzophenones and aromatic β-ketosulfones by using p-toluenesulfonic acid monohydrate under microwave irradiation. In this atom-economical synthetic route, a series of pharmaceutically active 3-arylsulfonylquinolines with good functional group tolerance are prepared in good to excellent yields. Some structures are confirmed by single-crystal X-ray diffraction analysis.

Dynamic interactions of the 5-HT6 receptor with protein partners control dendritic tree morphogenesis

The serotonin (5-hydroxytrypatmine) receptor 5-HT₆ (5-HT₆R) has emerged as a promising target to alleviate the cognitive symptoms of neurodevelopmental diseases. We previously demonstrated that 5-HT₆R finely controls key neurodevelopmental steps, including neuronal migration and the initiation of neurite growth, through its interaction with cyclin-dependent kinase 5 (Cdk5). Here, we showed that 5-HT₆R recruited G protein-regulated inducer of neurite outgrowth 1 (GPRIN1) through a G_s-dependent mechanism. Interactions between the receptor and either Cdk5 or GPRIN1 occurred sequentially during neuronal differentiation. The 5-HT₆R-GPRIN1 interaction enhanced agonist-independent, receptor-stimulated cAMP production without altering the agonist-dependent response in NG108-15 neuroblastoma cells. This interaction also promoted neurite extension and branching in NG108-15 cells and primary mouse striatal neurons through a cAMP-dependent protein kinase A (PKA)-dependent mechanism. This study highlights the complex allosteric modulation of GPCRs by protein partners and demonstrates how dynamic interactions between GPCRs and their protein partners can control the different steps of highly coordinated cellular processes, such as dendritic tree morphogenesis.

Synthesis of 3-sulfonylquinolines by visible-light promoted metal-free cascade cycloaddition involving N-propargylanilines and sodium sulfinates

A visible-light promoted radical cascade reaction of N-propargylanilines with sodium sulfinates as sulfonyl radical precursors was developed under metal-free conditions.

Serotonin Type 6 and 7 Receptors as a Novel Therapeutic Target for the Treatment of Schizophrenia

Schizophrenia is a serious disease of the central nervous system that affects a person's ability to think, feel and behave clearly. Even though the pathophysiological hypothesis of the disease is not clearly understood, dysfunction of dopamine, glutamate, serotonin and other neurotransmitters is widely believed to be involved. Serotonin within the synaptic vesicles functions as neurotransmitter and neurohormone in regulation of emotion, learning, memory, hormone release, cognition and motor function. Dysfunction of normal brain activity of serotonin is associated with schizophrenia. The role of serotonin 6 and 7 receptors in schizophrenia, interaction with neurotransmitters and the effect of drugs on those receptors in schizophrenia are the goal of this review. The aim of this review was to provide information for researchers and other scholars to identify the possible intervention points in the management of schizophrenia. The serotonin 6 and 7 receptors are associated with schizophrenia via modulating cyclic adenosine monophosphate, regulation of Fyn kinase and induction of structural plasticity. The above modulatory effects affect cholinergic, dopaminergic, glutamatergic, adrenergic and GABAergic systems. Recently, diverse numbers of selective agonist and antagonist ligands were developed for both receptors. SGS-518, ABT-354, Lu AE58054, SB-742,457, S-518, AVN-211, AVN-322, SYN-114 and SYN-120 are serotonin 6 receptor antagonists and aripiprazole-controlled release serotonin 7 receptor agonists under clinical trial for schizophrenia. Thus, research on novel drugs that act on serotonin 6 and 7 receptors likely facilitates the intervention into schizophrenia patients seeking better quality of life in the future.

Copper-Catalyzed Electrophilic Cyclization of N-Propargylamines with Sodium Sulfinate for the Synthesis of 3-Sulfonated Quinolines

A convenient and effective protocol for the synthesis of 3-sulfonated quinolines via copper-catalyzed electrophilic cyclization of N-propargylamines has been developed, in which cheap and stable sodium sulfinates were utilized as green sulfonylation reagents. This cascade transformation involves radical addition, cyclization and dehydrogenative aromatization processes in a one-pot reaction under mild conditions.

The 5-HT6 receptor interactome: New insight in receptor signaling and its impact on brain physiology and pathologies

The serotonin (5-HT)₆ receptor is a Gs-coupled receptor exclusively expressed in the central nervous system. Highest receptor densities are found in brain regions implicated in mnemonic functions where the receptor is primarily but not exclusively located in the primary cilium of neurons. The 5-HT₆ receptor continues to raise particular interest for neuropharmacologists, given the pro-cognitive effects of antagonists in a wide range of cognitive impairment paradigms in rodents and human. The 5-HT₆ receptor also finely controls key neuro-developmental processes including neuron migration and differentiation. However, its influence upon neurodevelopment and cognition is not solely mediated by its coupling to the Gs-adenylyl cyclase pathway, suggesting alternative signal transduction mechanisms. This prompted studies aimed at characterizing the receptor interactome that identified 125 candidate receptor partners, making the 5-HT₆ receptor one of the G protein-coupled receptors with the most extensively characterized interactome. These studies showed that the receptor localization at the plasma membrane and, consequently, its signal transduction, are finely modulated by several receptor partners. They demonstrated that prefrontal 5-HT₆ receptors engage the mTOR pathway to compromise cognition in neurodevelopmental models of schizophrenia, and a role of the 5-HT₆-mTOR pathway in temporal epilepsy. Finally, they revealed that the receptor activates Cdk5 signaling in an agonist-independent manner through a mechanism involving receptor phosphorylation by the associated Cdk5 and highlighted its key role in the migration of neurons and neurite growth. These new receptor-operated signaling mechanisms should be considered in the future development of drugs acting on 5-HT₆ receptors. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

5-HT6 receptor agonist and antagonist improve memory impairments and hippocampal BDNF signaling alterations induced by MK-801

The aim of the present study was to investigate and compare the effects of acute and chronic (21-day) administration of agonist (WAY-181187) and antagonist (SB-742457) of the 5-hydroxytryptamine 6 receptor (5-HT₆R) on MK-801-induced memory impairments in novel object recognition (NORT) and Y-maze continuous spontaneous alternation tests (Y-CAT). Further, the expression of the brain-derived neurotrophic factor (BDNF) in rat hippocampus was measured after 21-day administration to investigate BDNF participation in the pro-cognitive effects of 5-HT₆R ligands. We found that acute administration of WAY-181187, as well as SB-742457, reversed the effects of MK-801 in NORT and Y-CAT, and that this influence persisted after prolonged application in NORT but not in Y-CAT. Both 5-HT₆R ligands increased hippocampal BDNF protein expression, but WAY-181187 was much more potent than SB-742457 and alleviated the MK-801-induced inhibition of BDNF signaling pathways better, which seems to translate into a stronger WAY-181187 effect in behavioral tests. Collectively, both the 5-HT₆R agonist and the antagonist, administered acutely and chronically, prevent memory impairments and alterations in BDNF signaling induced by MK-801 in rats. The present results confirm the pro-cognitive properties of both types of 5-HT₆R ligands and suggest that BDNF pathways may be involved in their mechanism of action.

Multi-target-directed-ligands acting as enzyme inhibitors and receptor ligands

In this review, we present the latest advances in the field of multi-target-directed ligand (MTDL) design for the treatment of various complex pathologies of multifactorial origin. In particular, latest findings in the field of MTDL design targeting both an enzyme and a receptor are presented for different diseases such as Alzheimer's disease (AD), depression, addiction, glaucoma, non-alcoholic steatohepatitis and pain and inflammation. The ethology of the diseases is briefly described, with special emphasis on how the MTDL can evolve into novel therapies that replace the classic pharmacological dogma "one target one disease". Considering the current needs for therapy adherence improvement, it is exposed as from the medicinal chemistry, different molecular scaffolds are studied. With the use of structure activity relationship studies and molecular optimization, new hybrid molecules are generated with improved biological properties acting at two biologically very distinct targets.

Pharmacological Management of Dementia with Lewy Bodies

Dementia with Lewy bodies (DLB) is a complex disease that involves a variety of cognitive, behavioral and neurological symptoms, including progressive memory loss, visual hallucinations, parkinsonism, cognitive fluctuations and rapid eye movement sleep behavior disorder (RBD). These symptoms may appear in varying combinations and levels of severity in each patient who is seen in the clinic, making diagnosis and treatment a challenge. DLB is the third most common of all the neurodegenerative diseases behind both Alzheimer’s disease and Parkinson’s disease (PD). The median age of onset for DLB (76.3 years) is younger than that seen in PD dementia (81.4 years). New pathological studies have shown that most DLB patients have variable amounts of Alzheimer’s changes in their brains, explaining the wide variability in this disease’s clinical presentation and clinical course. This review discusses the three cholinesterase inhibitors that have been shown to be effective in managing the cognitive and behavioral symptoms of DLB: rivastigmine, galantamine and donepezil. Memantine is able to improve clinical global impression of change in those with mild to moderate DLB. Levodopa can treat the parkinsonism of some DLB patients, but the dose is often limited due to the fact that it can cause agitation or worsening of visual hallucinations. A recent phase 2 clinical trial showed the benefit of zonisamide when it is added as an adjunct to levodopa for treating DLB parkinsonism. While atypical antipsychotic drugs may not always be helpful as monotherapy in managing the agitation associated with DLB, low doses of valproic acid can be effective when added as an adjunct to drugs like quetiapine. Pimavanserin may prove to be a useful treatment for psychosis in DLB patients, but like other antipsychotic drugs that are used in dementia patients, there is a small increased risk of mortality. RBD, which is a common core clinical feature of DLB, can be managed with either melatonin or clonazepam. Two agents targeting alpha-synuclein (NPT200-11 and ambroxol) currently hold promise as disease-modifying therapies for DLB, but they are yet to be tested in clinical trials. An agent (E2027) that offers hope of neuroprotection by increasing central cyclic guanosine monophosphate (cGMP) levels is currently being examined in clinical trials in DLB patients.

Serotonin type 6 receptor antagonist attenuates the impairment of long-term potentiation and memory induced by Abeta

Alzheimer's disease (AD) is the most common cause of dementia, characterized by memory impairment and synaptic loss. Long-term potentiation (LTP), a type of synaptic plasticity, is impaired during AD. Serotonin type 6 receptor (5-HT6R) inactivation is proposed as a therapeutic target for AD. This study examined the effects of chronic administration of the 5-HT6R antagonist, SB-258585, on cognitive, memory, and hippocampal plasticity in a rat model of AD. Abeta neurotoxicity was induced in rats using Aβ (1.35 pmol intracerebroventricular [ICV] injection). The following groups were formed: control sustained surgery and saline-treated, Aβ+saline (1 μL ICV for 30 days), and Aβ+SB-258585 (0.024 mg/kg, ICV for 30 days). The learning and memory were tested using the novel object recognition and passive avoidance tests. Next, anesthetized rats were placed in a stereotaxic apparatus. The population spike (PS) amplitude and the slope of the excitatory postsynaptic potentials (fEPSPs) of the LTP were measured following high-frequency stimulation in the dentate gyrus. The Aβ injection reduced step-through latency in the passive avoidance test and decreased the discrimination index in the novel object test. Aβ diminished both the amplitude of hippocampal neuron population spikes and the slope of excitatory postsynaptic potentials, compared to the control group. The administration of SB-258585 in rats receiving Aβ attenuated the Aβ-induced deficits in cognition, memory, and LTP in comparison with the Aβ group. It can be concluded that chronic treatment with SB-258585 antagonist can prevent Aβ-related deficiencies in learning and memory performance by improving neuronal plasticity. SB-258585 can prevent the progression of AD.

Physical and Functional Interaction between 5-HT6 Receptor and Nova-1

5-HT₆ receptor (5-HT₆R) is implicated in cognitive dysfunction, mood disorder, psychosis, and eating disorders. However, despite its significant role in regulating the brain functions, regulation of 5-HT₆R at the molecular level is poorly understood. Here, using yeast two-hybrid assay, we found that human 5-HT₆R directly binds to neuro-oncological ventral antigen 1 (Nova-1), a brain-enriched splicing regulator. The interaction between 5-HT₆R and Nova-1 was confirmed using GST pull-down and co-immunoprecipitation assays in cell lines and rat brain. The splicing activity of Nova-1 was decreased upon overexpression of 5-HT₆R, which was examined by detecting the spliced intermediates of gonadotropin-releasing hormone (GnRH), a known pre-mRNA target of Nova-1, using RT-PCR. In addition, overexpression of 5-HT₆R induced the translocation of Nova-1 from the nucleus to cytoplasm, resulting in the reduced splicing activity of Nova-1. In contrast, overexpression of Nova-1 reduced the activity and the total protein levels of 5-HT₆R. Taken together, these results indicate that when the expression levels of 5-HT₆R or Nova-1 protein are not properly regulated, it may also deteriorate the function of the other.

Visible-light-induced multicomponent cascade cycloaddition involving N-propargyl aromatic amines, diaryliodonium salts and sulfur dioxide: rapid access to 3-arylsulfonylquinolines

A visible-light-induced, Eosin Y catalyzed three-component synthesis of 3-arylsulfonylquinoline derivatives through N-propargyl aromatic amines, diaryliodonium salts and sulfur dioxide has been discovered. This transformation represents an efficient and attractive method for the straightforward synthesis of 3-arylsulfonylquinoline derivatives via the formation of C-S bonds and quinolines in one step. In addition, it exhibits good substrate scope and functional group tolerance. The use of easy-to-handle diaryliodonium salts, sulfur dioxide sources and the cheap photocatalyst Eosin Y together with facile operation at room temperature makes this protocol very practical.

AVN-101: A Multi-Target Drug Candidate for the Treatment of CNS Disorders

Lack of efficacy of many new highly selective and specific drug candidates in treating diseases with poorly understood or complex etiology, as are many of central nervous system (CNS) diseases, encouraged an idea of developing multi-modal (multi-targeted) drugs. In this manuscript, we describe molecular pharmacology, in vitro ADME, pharmacokinetics in animals and humans (part of the Phase I clinical studies), bio-distribution, bioavailability, in vivo efficacy, and safety profile of the multimodal drug candidate, AVN-101. We have carried out development of a next generation drug candidate with a multi-targeted mechanism of action, to treat CNS disorders. AVN-101 is a very potent 5-HT₇ receptor antagonist (Ki = 153 pM), with slightly lesser potency toward 5-HT₆, 5-HT_2A, and 5HT-_2C receptors (Ki = 1.2–2.0 nM). AVN-101 also exhibits a rather high affinity toward histamine H1 (Ki = 0.58 nM) and adrenergic α2A, α2B, and α2C (Ki = 0.41–3.6 nM) receptors. AVN-101 shows a good oral bioavailability and facilitated brain-blood barrier permeability, low toxicity, and reasonable efficacy in animal models of CNS diseases. The Phase I clinical study indicates the AVN-101 to be well tolerated when taken orally at doses of up to 20 mg daily. It does not dramatically influence plasma and urine biochemistry, nor does it prolong QT ECG interval, thus indicating low safety concerns. The primary therapeutic area for AVN-101 to be tested in clinical trials would be Alzheimer’s disease. However, due to its anxiolytic and anti-depressive activities, there is a strong rational for it to also be studied in such diseases as general anxiety disorders, depression, schizophrenia, and multiple sclerosis.

Spiro[pyrrolidine-3,3'-oxindoles] and Their Indoline Analogues as New 5-HT6 Receptor Chemotypes

Synthetic derivatives of spiro[pyrrolidinyl-3,3′-oxindole] alkaloids (coerulescine analogues) were investigated as new ligands for aminergic G-protein coupled receptors (GPCRs). The chemical starting point 2′-phenylspiro[indoline-3,3′-pyrrolidin]-2-one scaffold was identified by virtual fragment screening utilizing ligand- and structure based methods. As a part of the hit-to-lead optimization a structure-activity relationship analysis was performed to explore the differently substituted 2′-phenyl-derivatives, introducing the phenylsulphonyl pharmacophore and examining the corresponding reduced spiro[pyrrolidine-3,3′-indoline] scaffold. The optimization process led to ligands with submicromolar affinities towards the 5-HT₆ receptor that might serve as viable leads for further optimization.

Novel 3-(1,2,3,6-Tetrahydropyridin-4-yl)-1H-indole-Based Multifunctional Ligands with Antipsychotic-Like, Mood-Modulating, and Procognitive Activity

The most troublesome aspects of behavioral and psychological symptoms of dementia (BPSD) are nowadays addressed by antidepressant, anxiolytic, and antipsychotic drugs, often administered off-label. Considering their modest effectiveness in dementia patients, the increased risk of adverse events and cognitive decline, there is an unmet need for well-tolerated and effective therapy of BPSD. We designed and synthesized multifunctional ligands characterized in vitro as high-affinity partial agonists of D₂R, antagonists of 5-HT₆R, and blockers of SERT. Moreover, the molecules activated 5-HT_1AR and blocked 5-HT₇R while having no relevant affinity for off-target M₁R and hERG channel. Compound 16 (N-{2-[4-(5-chloro-1H-indol-3-yl)-1,2,3,6-tetrahydropyridin-1-yl]ethyl}-3-methylbenzene-1-sulfonamide) exhibited a broad antipsychotic-, antidepressant-, and anxiolytic-like activity, not eliciting motor impairments in mice. Most importantly, 16 showed memory-enhancing properties and it ameliorated memory deficits induced by scopolamine. The molecule outperformed most important comparators in selected tests, indicating its potential in the treatment of both cognitive and noncognitive (behavioral and psychological) symptoms of dementia.