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The serotonin (5-HT) 6 receptor still raises particular interest given its unique spatio-temporal pattern of expression among the serotonin receptor subtypes. It is the only serotonin receptor specifically expressed in the central nervous system, where it is detected very early in embryonic life and modulates key neurodevelopmental processes, from neuronal migration to brain circuit refinement. Its predominant localization in the primary cilium of neurons and astrocytes is also unique among the serotonin receptor subtypes. Consistent with the high expression levels of the 5-HT 6 receptor in brain regions involved in the control of cognitive processes, it is now well-established that the pharmacological inhibition of the receptor induces pro-cognitive effects in several paradigms of cognitive impairment in rodents, including models of neurodevelopmental psychiatric disorders and neurodegenerative diseases. The 5-HT 6 receptor can engage several signaling pathways in addition to the canonical Gs signaling, but there is still uncertainty surrounding the signaling pathways that underly its modulation of cognition, as well as how the receptor's coupling is dependent on its cellular compartmentation. Here, we describe recent findings showing how the proper subcellular localization of the receptor is achieved, how this peculiar localization determines signaling pathways engaged by the receptor, and their pathophysiological influence.

In line with recent clinical trials demonstrating that ondansetron, a 5-HT 3 receptor (5-HT 3 R) antagonist, ameliorates cognitive deficits of schizophrenia and the known procognitive effects of 5-HT 6 receptor (5-HT 6 R) antagonists, we applied the hybridization strategy to design dual-acting 5-HT 3 /5-HT 6 R antagonists. We identified the first-in-class compound FPPQ , which behaves as a 5-HT 3 R antagonist and a neutral antagonist 5-HT 6 R 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-HT 6 R inverse agonist SB399885 nor neutral 5-HT 6 R antagonist CPPQ reversed (PCP)-induced hyperactivity. Thus, combination of 5-HT 3 R antagonism and 5-HT 6 R 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-HT 3 /5-HT 6 receptors and encourage further studies on dual-acting 5-HT 3 /5-HT 6 R antagonists for the treatment of psychiatric disorders.

Serotonin type 6 receptor (5-HT 6 R) 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 1 H -pyrrolo[3,2- c ]quinoline scaffold to provide the 2-phenyl-1 H -pyrrole-3-carboxamide, which is devoid of canonical indole-like skeleton and retains recognition of 5-HT 6 R. This modification has changed the compound's activity at 5-HT 6 R-operated signaling pathways from neutral antagonism to inverse agonism. The study identified compound 27 that behaves as an inverse agonist of the 5-HT 6 R 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-1 H -pyrrole-3-carboxamide might be used as a template for designing 5-HT 6 R inverse agonists.

Chronic neuropathic pain is a highly disabling syndrome that is poorly controlled by currently available analgesics. Here, we show that painful symptoms and associated cognitive deficits induced by spinal nerve ligation in the rat are prevented by the administration of serotonin 5-HT 6 receptor inverse agonists or by the mTOR inhibitor rapamycin. In contrast, they are not alleviated by the administration of 5-HT 6 receptor neutral antagonists. Likewise, activation of mTOR by constitutively active 5-HT 6 receptors mediates allodynia in oxaliplatin-induced peripheral neuropathy in rats but not mechanical nociception in healthy rats. Furthermore, both painful and co-morbid cognitive symptoms in neuropathic rats are strongly reduced by intrathecal delivery of a cell-penetrating peptide that disrupts 5-HT 6 receptor/mTOR physical interaction. Collectively, these findings demonstrate a deleterious influence of non-physiological mTOR activation by constitutively active spinal 5-HT 6 receptors upon painful and cognitive symptoms in neuropathic pains of different etiologies. They suggest that targeting the constitutive activity of 5-HT 6 receptors with inverse agonists or disrupting the 5-HT 6 receptor/mTOR interaction might be valuable strategies for the alleviation of neuropathic pain and cognitive co-morbidities., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

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-HT 6 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-HT 6 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-HT 6 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-HT 6 receptors engage the mTOR pathway to compromise cognition in neurodevelopmental models of schizophrenia, and a role of the 5-HT 6 -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-HT 6 receptors. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'., Competing Interests: Declaration of competing interest The authors declare no competing interest with the present article., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The serotonin (5-hydroxytrypatmine) receptor 5-HT 6 (5-HT 6 R) has emerged as a promising target to alleviate the cognitive symptoms of neurodevelopmental diseases. We previously demonstrated that 5-HT 6 R 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-HT 6 R 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-HT 6 R-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., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

A small library of novel 3H-imidazo[4,5-b]pyridine and 1H-imidazo[4,5-c]pyridine derivatives was designed and synthesized as non-sulfonamide 5-HT 6 receptor ligands. In vitro evaluation allowed to identify compound 17 (2-ethyl-3-(3-fluorobenzyl)-7-(piperazin-1-yl)-3H-imidazo[4,5-b]pyridine) as potent 5-HT 6 receptor partial inverse agonist in G s signaling (K i  = 6 nM, IC 50  = 17.6 nM). Compound 17 displayed high metabolic stability, favorable cytochrome P450 isoenzyme (2D6, 3A4) profile, did not affect PgP-protein binding, without evoking mutagenic effects. It was orally bioavailable and brain penetrant. In contrast to intepirdine (SB-742457), which prevented 5-HT 6 R-elicited neurite growth and behaved as an inverse agonist of cyclin-dependent kinase 5 (Cdk5), compound 17 has no influence on neuronal differentiation. Compound 17 exerted significant pro-cognitive properties in novel object recognition (NOR) task in rats reversing both phencyclidine- and scopolamine-induced memory deficits (MED = 1 and 0.3 mg/kg, p.o, respectively). These effects were similar to those produced by intepirdine. Additionally, combination of inactive doses of compound 17 (0.1 mg/kg) and donepezil (0.3 mg/kg) produced synergistic effect to reverse scopolamine-induced memory deficits. Accordingly, investigating putative divergence between inverse agonists and neutral antagonists as cognitive enhancers in neurodegenerative and psychiatric disorders is certainly of utmost interest., (Copyright © 2017 Elsevier Masson SAS. All rights reserved.)

Active G protein-coupled receptor (GPCR) conformations not only are promoted by agonists but also occur in their absence, leading to constitutive activity. Association of GPCRs with intracellular protein partners might be one of the mechanisms underlying GPCR constitutive activity. Here, we show that serotonin 5 hydroxytryptamine 6 (5-HT 6 ) receptor constitutively activates the Gs/adenylyl cyclase pathway in various cell types, including neurons. Constitutive activity is strongly reduced by silencing expression of the Ras-GTPase activating protein (Ras-GAP) neurofibromin, a 5-HT 6 receptor partner. Neurofibromin is a multidomain protein encoded by the NF1 gene, the mutation of which causes Neurofibromatosis type 1 (NF1), a genetic disorder characterized by multiple benign and malignant nervous system tumors and cognitive deficits. Disrupting association of 5-HT 6 receptor with neurofibromin Pleckstrin Homology (PH) domain also inhibits receptor constitutive activity, and PH domain expression rescues 5-HT 6 receptor-operated cAMP signaling in neurofibromin-deficient cells. Furthermore, PH domains carrying mutations identified in NF1 patients that prevent interaction with the 5-HT 6 receptor fail to rescue receptor constitutive activity in neurofibromin-depleted cells. Further supporting a role of neurofibromin in agonist-independent Gs signaling elicited by native receptors, the phosphorylation of cAMP-responsive element-binding protein (CREB) is strongly decreased in prefrontal cortex of Nf1 +/- mice compared with WT mice. Moreover, systemic administration of a 5-HT 6 receptor inverse agonist reduces CREB phosphorylation in prefrontal cortex of WT mice but not Nf1 +/- mice. Collectively, these findings suggest that disrupting 5-HT 6 receptor-neurofibromin interaction prevents agonist-independent 5-HT 6 receptor-operated cAMP signaling in prefrontal cortex, an effect that might underlie neuronal abnormalities in NF1 patients., Competing Interests: The authors declare no conflict of interest.

5-HT6 receptor (5-HT6R) is a G protein-coupled receptor that has recently emerged as a new regulator of neural development. In addition to the canonical Gs adenylyl cyclase pathway, recent proteomics approaches reveal that 5-HT6R is able to engage key developmental signaling pathways controlling neuronal circuit formation, neuronal connectivity, and psychiatric-relevant behaviors. For example, at early stages of neuronal development, expression of 5-HT6R constitutively regulates the activity of the cyclin-dependent kinase (Cdk)5 and, through this mechanism, controls cellular processes involved in circuit formation, including neuronal migration and neurite outgrowth. In addition to the Cdk5 pathway, 5-HT6R modulates a variety of key developmental targets such as Fyn, Jab1, and mammalian target of rapamycin (mTOR). Engagement of developmental pathways through 5-HT6R pharmacological manipulation has led to interesting new therapeutic perspectives in the field of psychiatric-related disorders. Indeed, 5-HT6R blockade can rescue a pathological overactivation of the mTOR pathway induced by early life insults in rodents and normalizes the associated social and episodic memory deficits. Here, we review recent evidence supporting the notion that 5-HT6R is at the interface of key developmental signaling pathways and a novel actor in the orchestration of neural circuit formation.

The formation of a laminar structure such as the mammalian neocortex relies on the coordinated migration of different subtypes of excitatory pyramidal neurons in specific layers. Cyclin-dependent kinase 5 (Cdk5) is a master regulator of pyramidal neuron migration. Recently, we have shown that Cdk5 binds to the serotonin 6 receptor (5-HT6R), a G protein-coupled receptor (GPCR). Here, we investigated the role of 5-HT6R in the positioning and migration of pyramidal neurons during mouse corticogenesis. We report that constitutive expression of 5-HT6R controls pyramidal neuron migration through an agonist-independent mechanism that requires Cdk5 activity. These data provide the first in vivo evidence of a role for constitutive activity at a GPCR in neocortical radial migration., (© 2014. Published by The Company of Biologists Ltd.)

The serotonin6 receptor (5-HT6R) is a promising target for treating cognitive deficits of schizophrenia often linked to alterations of neuronal development. This receptor controls neurodevelopmental processes, but the signaling mechanisms involved remain poorly understood. Using a proteomic strategy, we show that 5-HT6Rs constitutively interact with cyclin-dependent kinase 5 (Cdk5). Expression of 5-HT6Rs in NG108-15 cells induced neurite growth and expression of voltage-gated Ca(2+) channels, two hallmarks of neuronal differentiation. 5-HT6R-elicited neurite growth was agonist independent and prevented by the 5-HT6R antagonist SB258585, which behaved as an inverse agonist. Moreover, it required receptor phosphorylation at Ser350 by Cdk5 and Cdc42 activity. Supporting a role of native 5-HT6Rs in neuronal differentiation, neurite growth of primary neurons was reduced by SB258585, by silencing 5-HT6R expression or by mutating Ser350 into alanine. These results reveal a functional interplay between Cdk5 and a G protein-coupled receptor to control neuronal differentiation.

Cognitive deficits in schizophrenia severely compromise quality of life and are poorly controlled by current antipsychotics. While 5-HT(6) receptor blockade holds special promise, molecular substrates underlying their control of cognition remain unclear. Using a proteomic strategy, we show that 5-HT(6) receptors physically interact with several proteins of the mammalian target of rapamycin (mTOR) pathway, including mTOR. Further, 5-HT(6) receptor activation increased mTOR signalling in rodent prefrontal cortex (PFC). Linking this signalling event to cognitive impairment, the mTOR inhibitor rapamycin prevented deficits in social cognition and novel object discrimination induced by 5-HT(6) agonists. In two developmental models of schizophrenia, specifically neonatal phencyclidine treatment and post-weaning isolation rearing, the activity of mTOR was enhanced in the PFC, and rapamycin, like 5-HT(6) antagonists, reversed these cognitive deficits. These observations suggest that recruitment of mTOR by prefrontal 5-HT(6) receptors contributes to the perturbed cognition in schizophrenia, offering new vistas for its therapeutic control., (Copyrights © 2012 The Authors. Published by John Wiley and Sons, Ltd on behalf of EMBO.)