Role of the 5-HT7 receptor in the central nervous system: from current status to future perspectives

Neuroprotective and Neurite Outgrowth Stimulating Effects of New Low-Basicity 5-HT7 Receptor Agonists: In Vitro Study in Human Neuroblastoma SH-SY5Y Cells

There is some evidence that the serotonin receptor subtype 7 (5-HT7) could be new therapeutic target for neuroprotection. The aim of this study was to compare the neuroprotective and neurite outgrowth potential of new 5-HT7 receptor agonists (AH-494, AGH-238, AGH-194) with 5-CT (5-carboxyamidotryptamine) in human neuroblastoma SH-SY5Y cells. The results revealed that 5-HT7 mRNA expression was significantly higher in retinoic acid (RA)-differentiated cells when compared to undifferentiated ones and it was higher in cell cultured in neuroblastoma experimental medium (DMEM) compared to those placed in neuronal (NB) medium. Furthermore, the safety profile of compounds was favorable for all tested compounds at concentration used for neuroprotection evaluation (up to 1 μM), whereas at higher concentrations (above 10 μM) the one of the tested compounds, AGH-194 appeared to be cytotoxic. While we observed relatively modest protective effects of 5-CT and AH-494 in UN-SH-SY5Y cells cultured in DMEM, in UN-SH-SY5Y cells cultured in NB medium we found a significant reduction of H2O2-evoked cell damage by all tested 5-HT7 agonists. However, 5-HT7-mediated neuroprotection was not associated with inhibition of caspase-3 activity and was not observed in RA-SH-SY5Y cells exposed to H2O2. Furthermore, none of the tested 5-HT7 agonists altered the damage induced by 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenylpyridinium ion (MPP +) and doxorubicin (Dox) in UN- and RA-SH-SY5Y cells cultured in NB. Finally we showed a stimulating effect of AH-494 and AGH-194 on neurite outgrowth. The obtained results provide insight into neuroprotective and neurite outgrowth potential of new 5-HT7 agonists.

DSP-6745, a novel 5-hydroxytryptamine modulator with rapid antidepressant, anxiolytic, antipsychotic and procognitive effects

BACKGROUND

Current treatment of major depressive disorder is facing challenges, including a low remission rate, late onset of efficacy, and worsening severity due to comorbid symptoms such as psychosis and cognitive dysfunction. Serotonin (5-HT) neurotransmission is involved in a wide variety of psychiatric diseases and its potential as a drug target continues to attract attention.

OBJECTIVES

The present study elucidates the effects of a novel 5-HT modulator, DSP-6745, on depression and its comorbid symptoms.

RESULTS

In vitro radioligand binding and functional assays showed that DSP-6745 is a potent inhibitor of 5-HT transporter and 5-HT2A, 5-HT2C, and 5-HT7 receptors. In vivo, DSP-6745 (6.4 and 19.1 mg/kg as free base, p.o.) increased the release of not only 5-HT, norepinephrine, and dopamine, but also glutamate in the medial prefrontal cortex. The results of in vivo mouse phenotypic screening by SmartCube® suggested that DSP-6745 has a behavioral signature combined with antidepressant-, anxiolytic-, and antipsychotic-like signals. A single oral dose of DSP-6745 (6.4 and 19.1 mg/kg) showed rapid antidepressant-like efficacy in the rat forced swim test, even at 24 h post-dosing, and anxiolytic activity in the rat social interaction test. Moreover, DSP-6745 (12.7 mg/kg, p.o.) led to an improvement in the apomorphine-induced prepulse inhibition deficit in rats. In the marmoset object retrieval with detour task, which is used to assess cognitive functions such as attention and behavioral inhibition, DSP-6745 (7.8 mg/kg, p.o.) enhanced cognition.

CONCLUSIONS

These data suggest that DSP-6745 is a multimodal 5-HT receptor antagonist and a 5-HT transporter inhibitor and has the potential to be a rapid acting antidepressant with efficacies in mitigating the comorbid symptoms of depression.

Identification of an Antagonist Targeting G Protein and β-Arrestin Signaling Pathways of 5-HT7R

In consideration of the limited number of FDA-approved drugs for autism spectrum disorder (ASD), significant efforts have been devoted to identifying novel drug candidates. Among these, 5-HT7R modulators have garnered considerable attention due to their potential in alleviating autism-like behaviors in ASD animal models. In this study, we designed and synthesized biphenyl-3-ylmethylpyrrolidines 3 and biphenyl-3-yl-dihydroimidazoles 4 as 5-HT7R modulators. Through extensive biological tests of 3 and 4 in G protein and β-arrestin signaling pathways of 5-HT7R, it was determined that 2-(2'-methoxy-[1,1'-biphenyl]-3-yl)-4,5-dihydro-1H-imidazole 4h acted as a 5-HT7R antagonist in both signaling pathways. In in vivo study with Shank3-/- transgenic (TG) mice, the self-grooming behavior test was performed with 4h, resulting in a significant reduction in the duration of self-grooming. In addition, an immunohistochemical experiment with 4h restored reduced neurogenesis in Shank3-/- TG mice, which is confirmed by the quantification of doublecortin (DCX) positive neurons, suggesting the promising therapeutic potential of 4h.

SSRIs in the Treatment of Depression: A Pharmacological CUL-DE-SAC?

The widespread adoption of selective serotonin reuptake inhibitors (SSRIs) as first-line pharmacological treatments in the management of clinical depression transformed the landscape of drug therapy for this condition. SSRIs are safer and better tolerated than the tricyclic antidepressants (TCAs) that they replaced. However, they have limitations that may have placed a ceiling on the expectations of first-line pharmacological treatment. Notable problems with SSRIs include induction of anxiety on treatment initiation, delayed onset of significant therapeutic effect, sexual dysfunction, sleep disturbance and overall modest efficacy. The latter is linked with an inability of SSRIs to effectively treat syndromes of anhedonia and cognitive impairment. Combined serotonin and noradrenaline reuptake inhibitors (SNRIs), such as venlafaxine, have produced some limited improvements over SSRIs in efficacy, at the cost of a greater side-effect burden. Attempts to supplement serotonin reuptake activity with actions at serotonin receptor sub-types have not yet yielded substantial benefits; however, vortioxetine may provide more utility in the management of cognitive impairment. Future advances might come from the development of SNRIs, which more closely mimic the actions of effective TCAs. There may also be possible benefits to be derived from combining SSRIs with 5-HT4 receptor agonists and 5-HT7 receptor antagonists.

Functional Dimerization of Serotonin Receptors: Role in Health and Depressive Disorders

Understanding the neurobiological underpinnings of depressive disorder constitutes a pressing challenge in the fields of psychiatry and neurobiology. Depression represents one of the most prevalent forms of mental and behavioral disorders globally. Alterations in dimerization capacity can influence the functional characteristics of serotonin receptors and may constitute a contributing factor to the onset of depressive disorders. The objective of this review is to consolidate the current understanding of interactions within the 5-HT receptor family and between 5-HT receptors and members of other receptor families. Furthermore, it aims to elucidate the role of such complexes in depressive disorders and delineate the mechanisms through which antidepressants exert their effects.

Therapeutic Potential and Limitation of Serotonin Type 7 Receptor Modulation

Although a number of mood-stabilising atypical antipsychotics and antidepressants modulate serotonin type 7 receptor (5-HT7), the detailed contributions of 5-HT7 function to clinical efficacy and pathophysiology have not been fully understood. The mood-stabilising antipsychotic agent, lurasidone, and the serotonin partial agonist reuptake inhibitor, vortioxetine, exhibit higher binding affinity to 5-HT7 than other conventional antipsychotics and antidepressants. To date, the initially expected rapid onset of antidepressant effects-in comparison with conventional antidepressants or mood-stabilising antipsychotics-due to 5-HT7 inhibition has not been observed with lurasidone and vortioxetine; however, several clinical studies suggest that 5-HT7 inhibition likely contributes to quality of life of patients with schizophrenia and mood disorders via the improvement of cognition. Furthermore, recent preclinical studies reported that 5-HT7 inhibition might mitigate antipsychotic-induced weight gain and metabolic complication by blocking other monoamine receptors. Further preclinical studies for the development of 5-HT7 modulation against neurodevelopmental disorders and neurodegenerative diseases have been ongoing. To date, various findings from various preclinical studies indicate the possibility that 5-HT7 modifications can provide two independent strategies. The first is that 5-HT7 inhibition ameliorates the dysfunction of inter-neuronal transmission in mature networks. The other is that activation of 5-HT7 can improve transmission dysfunction due to microstructure abnormality in the neurotransmission network-which could be unaffected by conventional therapeutic agents-via modulating intracellular signalling during the neurodevelopmental stage or via loss of neural networks with aging. This review attempts to describe the current and novel clinical applications of 5-HT7 modulation based on preclinical findings.

Identification and Pharmacological Characterization of Two Serotonin Type 7 Receptor Isoforms from Mythimna separata

Serotonin (5-hydroxytryptamine, 5-HT) is an important neuroactive molecule, as neurotransmitters regulate various biological functions in vertebrates and invertebrates by binding and activating specific 5-HT receptors. The pharmacology and tissue distribution of 5-HT receptors have been investigated in several model insects, and these receptors are recognized as potential insecticide targets. However, little is known about the pharmacological characterization of the 5-HT receptors in important agricultural pests. In this study, we investigated the sequence, pharmacology, and tissue distribution of 5-HT7 receptors from oriental armyworm Mythimna separata (Walker) (Lepidoptera: Noctuidae), an important migratory and polyphagous pest species. We found that the 5-HT7 receptor gene encodes two molecularly distinct transcripts, Msep5-HT7L and Msep5-HT7S, by the mechanism of alternative splicing in M. separata. Msep5-HT7S differs from Msep5-HT7L based on the deletion of 95 amino acids within the third intracellular loop. Two Msep5-HT7 receptor isoforms were activated by 5-HT and synthetic agonists α-methylserotonin, 8-hydroxy-DPAT, and 5-methoxytryptamine, resulting in increased intracellular cAMP levels in a dose-dependent manner, although these agonists showed much poorer potency and efficacy than 5-HT. The maximum efficacy of 5-HT compared to the two 5-HT isoforms was equivalent, but 5-HT exhibited 2.63-fold higher potency against the Msep5-HT7S than the Msep5-HT7L receptor. These two isoforms were also blocked by the non-selective antagonist methiothepin and the selective antagonists WAY-100635, ketanserin, SB-258719, and SB-269970. Moreover, two distinct mRNA transcripts were expressed preferentially in the brain and chemosensory organs of M. separata adults, as determined by qPCR assay. This study is the first comprehensive characterization of two splicing isoforms of 5-HT7 receptors in M. separata, and the first to demonstrate that alternative splicing is also the mechanism for producing multiple 5-HT7 isoforms in insects. Pharmacological and gene expression profiles offer important information that could facilitate further exploration of their function in the central nervous system and peripheral chemosensory organs, and may even contribute to the development of new selective pesticides.

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.

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.

5-HT7 receptors expressed in the mouse parafacial region are not required for respiratory chemosensitivity

Abstract

A brainstem homeostatic system senses CO₂/H⁺ to regulate ventilation, blood gases and acid–base balance. Neurons of the retrotrapezoid nucleus (RTN) and medullary raphe are both implicated in this mechanism as respiratory chemosensors, but recent pharmacological work suggested that the CO₂/H⁺ sensitivity of RTN neurons is mediated indirectly, by raphe‐derived serotonin acting on 5‐HT7 receptors. To investigate this further, we characterized Htr7 transcript expression in phenotypically identified RTN neurons using multiplex single cell qRT‐PCR and RNAscope. Although present in multiple neurons in the parafacial region of the ventrolateral medulla, Htr7 expression was undetectable in most RTN neurons (Nmb⁺/Phox2b⁺) concentrated in the densely packed cell group ventrolateral to the facial nucleus. Where detected, Htr7 expression was modest and often associated with RTN neurons that extend dorsolaterally to partially encircle the facial nucleus. These dorsolateral Nmb⁺/Htr7⁺ neurons tended to express Nmb at high levels and the intrinsic RTN proton detectors Gpr4 and Kcnk5 at low levels. In mouse brainstem slices, CO₂‐stimulated firing in RTN neurons was mostly unaffected by a 5‐HT7 receptor antagonist, SB269970 (n = 11/13). At the whole animal level, microinjection of SB269970 into the RTN of conscious mice blocked respiratory stimulation by co‐injected LP‐44, a 5‐HT7 receptor agonist, but had no effect on CO₂‐stimulated breathing in those same mice. We conclude that Htr7 is expressed by a minor subset of RTN neurons with a molecular profile distinct from the established chemoreceptors and that 5‐HT7 receptors have negligible effects on CO₂‐evoked firing activity in RTN neurons or on CO₂‐stimulated breathing in mice.

Key points

Neurons of the retrotrapezoid nucleus (RTN) are intrinsic CO₂/H⁺ chemosensors and serve as an integrative excitatory hub for control of breathing.

Serotonin can activate RTN neurons, in part via 5‐HT7 receptors, and those effects have been implicated in conferring an indirect CO₂ sensitivity.

Multiple single cell molecular approaches revealed low levels of 5‐HT7 receptor transcript expression restricted to a limited population of RTN neurons.

Pharmacological experiments showed that 5‐HT7 receptors in RTN are not required for CO₂/H⁺‐stimulation of RTN neuronal activity or CO₂‐stimulated breathing.

These data do not support a role for 5‐HT7 receptors in respiratory chemosensitivity mediated by RTN neurons.

Benzoxazolone-arylpiperazinyl scaffold-based PET ligand for 5-HT7 : Synthesis and biological evaluation

Efforts are underway to improve the diagnosis and treatment for neurological disorders like depression, anxiety, epilepsy, and schizophrenia. The G-protein-coupled receptors (GPCRs) 5-HT₇   receptor, the most recently identified member of 5-HT receptor family dysregulation has an association with various central nervous system (CNS) disorders and its ligands have an edge as potential therapeutics. Here, we report the synthesis, characterization, and biological evaluation of diversely substituted methoxy derivatives of 2-benzoxazolone arylpiperazine for targeting 5-HT₇  receptors. Out of all derivatives, only C-2 substituted derivative, 3-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)benzoxazol-2(3H)-one/ABO demonstrate a high affinity for human 5-HT₇ receptors. [¹¹ C]ABO was obtained by O-methylation of desmethyl-precursor using [¹¹ C]CH₃ OTf in the presence of NaOH giving a high radiochemical yield of 25 ± 12% (decay-corrected, n = 7) with stability up to 1.5 h postradiolabeling. In vitro autoradiography displays binding of [¹¹ C]ABO in accordance with 5-HT₇ distribution with a decrease of approximately 80% and 40% activity in the hippocampus and cerebellum brain region when administered with 10 µM cold ligand. Prefatory positron emission tomography scan results in Sprague-Dawley (SD) rat brain revealed fast and high radioactivity build-up in 5-HT₇ receptor-rich regions, namely, the hippocampus (2.75 ± 0.16 SUV) and the cerebral cortex (2.27 ± 0.02 SUV) establishing selective targeting of [¹¹ C]ABO. In summary, these pieces of data designate [¹¹ C]ABO as a promising 5-HT₇  receptor ligand that can have possible roles in clinics after its further optimization on different animal models.

5-HT Receptors and Temperature Homeostasis

The present review summarizes the data concerning the influence of serotonin (5-HT) receptors on body temperature in warm-blooded animals and on processes associated with its maintenance. This review includes the most important part of investigations from the first studies to the latest ones. The established results on the pharmacological activation of 5-HT1A, 5-HT3, 5-HT7 and 5-HT2 receptor types are discussed. Such activation of the first 3 type of receptors causes a decrease in body temperature, whereas the 5-HT2 activation causes its increase. Physiological mechanisms leading to changes in body temperature as a result of 5-HT receptors' activation are discussed. In case of 5-HT1A receptor, they include an inhibition of shivering and non-shivering thermogenesis, as well simultaneous increase of peripheral blood flow, i.e., the processes of heat production and heat loss. The physiological processes mediated by 5-HT2 receptor are opposite to those of the 5-HT1A receptor. Mechanisms of 5-HT3 and 5-HT7 receptor participation in these processes are yet to be studied in more detail. Some facts indicating that in natural conditions, without pharmacological impact, these 5-HT receptors are important links in the system of temperature homeostasis, are also discussed.

Discovery of G Protein-Biased Antagonists against 5-HT7R

5-HT₇R belongs to a family of G protein-coupled receptors and is associated with a variety of physiological processes in the central nervous system via the activation of the neurotransmitter serotonin (5-HT). To develop selective and biased 5-HT₇R ligands, we designed and synthesized a series of pyrazolyl-diazepanes 2 and pyrazolyl-piperazines 3, which were evaluated for binding affinities to 5-HTR subtypes and functional selectivity for G protein and β-arrestin signaling pathways of 5-HT₇R. Among them, 1-(3-(3-chlorophenyl)-1H-pyrazol-4-yl)-1,4-diazepane 2c showed the best binding affinity for 5-HT₇R and selectivity over other 5-HTR subtypes. It was also revealed as a G protein-biased antagonist. The self-grooming behavior test was performed with 2c in vivo with Shank3^-/- transgenic (TG) mice, wherein 2c significantly reduced self-grooming duration time to the level of wild-type mice. The results suggest that 5-HT₇R could be a potential therapeutic target for treating autism spectrum disorder stereotypy.

Serotonin and beyond-a tribute to Manfred Göthert (1939-2019)

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

Social interaction masking contributes to changes in the activity of the suprachiasmatic nucleus and impacts on circadian rhythms

Light is the most powerful temporal cue that entrains physiology and behavior through modulation of the suprachiasmatic nucleus (SCN) of the hypothalamus. However, on a daily basis, individuals face a combination of light and several non-photic cues, such as social interaction. In order to investigate whether SCN activity and SCN-driven rhythms are altered by social interaction, adult male C57BLJ/6 mice were maintained in groups of 3-4 animals per cage or 1 animal per cage (socially isolated) under 12:12 h / light:dark (LD) cycles or constant darkness (DD). Analysis of the two anatomical subdivisions (ventral, v and dorsal, d) of the medial SCN revealed an effect of housing conditions on the d-SCN but not on the v-SCN on the number of c-Fos immunoreactive (ir) neurons. As such, 2 h after the light-phase onset d-SCN c-Fos-ir number was lower in single-housed mice under LD. Importantly, under DD there were no effect of housing conditions in the number of c-Fos-ir SCN neurons. Social isolation increased the amplitude and strength of SCN-driven rhythm of body temperature (Tc) entrained to LD and it advanced its onset, uncoupling with spontaneous locomotor activity (SLA) rhythm, without altering endogenous Tc and SLA rhythms expressed under DD. Associated with reduced Tc in the light phase, single-housed mice showed reduced body weight. However, these phenotypes were not accompanied by changes in the number of c-Fos-ir neurons in the preoptic area (POA), which are known to regulate energy metabolism and Tc. Altogether, these results imply that the social interaction masking effect on the d-SCN is added to that of light stimulus, in order to achieve full c-Fos expression in the SCN, which, in turn seems to be required to maintain daily-phase coherence between the photo-entrained rhythms of Tc and SLA. There might be an inter-relationship between masking (social interaction) and entrainment stimulus (light) that impacts the circadian parameters of the photo-entrained Tc rhythm. As such, in the absence of social interactions a more robust Tc rhythm is shown. This inter-relationship seems to occur in the dorsal subdivision of the SCN but not in the POA.

Synthesis and Biological Evaluation of 99mTc-Labeled Phenylpiperazine Derivatives as Selective Serotonin-7 Receptor Ligands for Brain Tumor Imaging

With a poor prognosis, glioblastoma multiforme is the most aggressive tumor of the central nervous system in humans. The aim of this study was to develop novel tracers for the tumor targeting and imaging of overexpressed serotonin-7 receptors (5-HT₇Rs) in U-87 MG glioma xenografted nude mice. Two phenylpiperazine derivatives named as PHH and MPHH were designed, and the corresponding radiotracers ^99mTc-PHH and ^99mTc-MPHH were synthesized in high radiochemical purity (>95%). ^99mTc-MPHH showed a higher affinity to 5-HT₇Rs on U-87 MG cells compared to ^99mTc-PHH. In biodistribution studies, the radiocomplexes showed good brain uptake at 15 min combined with good radioactivity retention in the brain for 240 min. Regional rabbit brain studies indicated a higher radioactivity concentration in the hippocampus and diencephalon than in the cerebellum. Compared to ^99mTc-MPHH, the ^99mTc-PHH exhibited a significantly increased tumor uptake at 15 and 60 min, but the rapid blood clearance of ^99mTc-MPHH led to enhanced tumor-to-muscle ratios at 240 min. A significant reduction in tumor uptake 60 min after an injection of pimozide (5-HT₇ receptor antagonist) confirms the tumor uptake was receptor-mediated specifically. The tumor-to-contralateral muscle tissue ratio of ^99mTc-PHH and ^99mTc-MPHH in nude mice with U-87 MG xenograft was measured (5.25 and 4.65) at 60 min as well as (6.25 and 6.76) at 240 min, respectively.

Discovery of Nonracemic Amisulpride to Maximize Benefit/Risk of 5-HT7 and D2 Receptor Antagonism for the Treatment of Mood Disorders

In contrast to the dose‐occupancy relationship in the treatment of schizophrenia, the minimal effective level of dopamine receptor 2 (D2R) blockade for antipsychotics in the treatment of bipolar depression is unknown. Lower doses aimed at reducing extrapyramidal side effects must be balanced against the need to retain the therapeutic benefit of D2R blockade on emergent cycling, mixed, manic, anxiety, and/or psychotic symptoms. Dose‐reductions intended to lower D2R blockade, however, could also decrease concomitant serotonin receptor antagonism and its potential benefit on depressive symptoms. Here, we uncoupled the potential antidepressant activity in amisulpride, driven by 5‐HT7 receptor (5‐HT7R) antagonism, from the D2R‐mediated antipsychotic activity by discovering that each enantiomer favors a different receptor. Aramisulpride was more potent at 5‐HT7R relative to esamisulpride (Ki 47 vs. 1,900 nM, respectively), whereas esamisulpride was more potent at D2R (4.0 vs. 140 nM). We hypothesized that a nonracemic ratio might achieve greater 5‐HT7R‐mediated antidepressant effects at a lower level of D2R blockade. The dose‐occupancy relationship of esamisulpride at D2R was determined by positron emission tomography (PET) imaging in human volunteers. Separately the dose‐relationship of aramisulpride was established in humans using suppression of rapid eye movement (REM) sleep as a marker of 5‐HT7R antagonism. These results led to the discovery of an 85:15 ratio of aramisulpride to esamisulpride (SEP‐4199) that maximizes the potential for antidepressant benefit of aramisulpride via 5‐HT7R and reduces esamisulpride to minimize D2R‐related extrapyramidal side effects while still retaining D2R‐mediated effects predicted to provide benefit in bipolar depression. The antidepressant efficacy of SEP‐4199 was recently confirmed in a proof‐of‐concept trial for the treatment of bipolar depression (NCT03543410).

HTR7 promotes laryngeal cancer growth through PI3K/AKT pathway activation

BACKGROUND

Laryngeal cancer is a common malignancy of the head and neck, it's important to find novel targets for its therapy. The 5-hydroxytryptamine receptor 7 (HTR7) belongs to the G protein-coupled receptors (GPCRs) family which are easily druggable in diseases; however, its role in laryngeal cancer remains unknown.

METHODS

Colony formation assay, Soft agar growth assay, BrdU incorporation assay and MTT assay were used to analyze the effect of HTR7 on laryngeal cancer cell proliferation. Xenograft tumors in nude mice was used to analyze the effect of HTR7 on laryngeal cancer growth. Luciferase reporter assay was used to analyze the effect of HTR7 on phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/protein kinase B (AKT) pathway activity.

RESULTS

We found that HTR7 was significantly upregulated in laryngeal cancer tissues and cells, and patients with high HTR7 expression had shorter survival time than those with low HTR7 expression. Univariate and multivariate Cox regression models showed that HTR7 was an independent predictive factor for the prognosis of patients with laryngeal cancer. Cell proliferation assays and an animal model showed that HTR7 overexpression promoted laryngeal cancer proliferation and growth, while HTR7 knockdown inhibited laryngeal cancer proliferation and growth. Further analysis showed HTR7 activated the PI3K/AKT pathway, characterized by increased phosphorylation of AKT, luciferase reporter activity of forkhead box O (FOXO) factors, and target expression. Inhibition of the PI3K/AKT pathway in HTR7-overexpressing cells suppressed proliferation and growth, suggesting that HTR7 promotes laryngeal cancer proliferation and growth by activating the PI3K/AKT pathway.

CONCLUSIONS

HTR7 is not only a target for laryngeal cancer therapy but also a prognostic factor for the prognosis of patients with laryngeal cancer.

Effects of Subchronic Administrations of Vortioxetine, Lurasidone, and Escitalopram on Thalamocortical Glutamatergic Transmission Associated with Serotonin 5-HT7 Receptor

The functional suppression of serotonin (5-HT) type 7 receptor (5-HT7R) is forming a basis for scientific discussion in psychopharmacology due to its rapid-acting antidepressant-like action. A novel mood-stabilizing atypical antipsychotic agent, lurasidone, exhibits a unique receptor-binding profile, including a high affinity for 5-HT7R antagonism. A member of a novel class of antidepressants, vortioxetine, which is a serotonin partial agonist reuptake inhibitor (SPARI), also exhibits a higher affinity for serotonin transporter, serotonin receptors type 1A (5-HT1AR) and type 3 (5-HT3R), and 5-HT7R. However, the effects of chronic administration of lurasidone, vortioxetine, and the selective serotonin reuptake inhibitor (SSRI), escitalopram, on 5-HT7R function remained to be clarified. Thus, to explore the mechanisms underlying the clinical effects of vortioxetine, escitalopram, and lurasidone, the present study determined the effects of these agents on thalamocortical glutamatergic transmission, which contributes to emotional/mood perception, using multiprobe microdialysis and 5-HT7R expression using capillary immunoblotting. Acute local administration of a 5-HT7R agonist and antagonist into the mediodorsal thalamic nucleus (MDTN) enhanced and reduced thalamocortical glutamatergic transmission, induced by N-methyl-D-aspartate (NMDA)/glutamate receptor inhibition in the reticular thalamic nucleus (RTN). Acute local administration of a relevant therapeutic concentration of vortioxetine and lurasidone into the MDTN suppressed the thalamocortical glutamatergic transmission via 5-HT7R inhibition, whereas that of escitalopram activated 5-HT7R. Subchronic administration of effective doses of vortioxetine and lurasidone (for 7 days) reduced the thalamocortical glutamatergic transmission, but escitalopram did not affect it, whereas subchronic administration of these three agents attenuated the stimulatory effects of the 5-HT7R agonist on thalamocortical glutamatergic transmission. Subchronic administration of effective doses of vortioxetine, lurasidone, and escitalopram downregulated the 5-HT7R expression of the plasma membrane in the MDTN; the 5-HT7R downregulation induced by vortioxetine and lurasidone was observed at 3 days, but that induced by escitalopram required a longer duration of 7 days. These results indicate that chronic administration of vortioxetine, escitalopram, and lurasidone generate downregulation of 5-HT7R in the thalamus; however, the direct inhibition of 5-HT7R associated with vortioxetine and lurasidone generates more rapid downregulation than the indirect elevation of the extracellular serotonin level via serotonin transporter inhibition by escitalopram.

Current Limitations and Candidate Potential of 5-HT7 Receptor Antagonism in Psychiatric Pharmacotherapy

Several mood-stabilizing atypical antipsychotics and antidepressants weakly block serotonin (5-HT) receptor type-7 (5-HT7R); however, the contributions of 5-HT7R antagonism to clinical efficacy and pathophysiology are yet to be clarified. A novel mood-stabilizing antipsychotic agent, lurasidone exhibits predominant binding affinity to 5-HT7R when compared with other monoamine receptors. To date, we have failed to discover the superior clinical efficacy of lurasidone on schizophrenia, mood, or anxiety disorders when compared with conventional mood-stabilizing atypical antipsychotics; however, numerous preclinical findings have indicated the possible potential of 5-HT7R antagonism against several neuropsychiatric disorders, as well as the generation of novel therapeutic options that could not be expected with conventional atypical antipsychotics. Traditional experimental techniques, electrophysiology, and microdialysis have demonstrated that the effects of 5-HT receptor type-1A (5-HT1AR) and 5-HT7R on neurotransmission are in contrast, but the effect of 5-HT1AR is more predominant than that of 5-HT7R, resulting in an insufficient understanding of the 5-HT7R function in the field of psychopharmacology. Accumulating knowledge regarding the pharmacodynamic profiles of 5-HT7R suggests that 5-HT7R is one of the key players in the establishment and remodeling of neural development and cytoarchitecture during the early developmental stage to the mature brain, and dysfunction or modulation of 5-HT7R is linked to the pathogenesis/pathophysiology of neuropsychiatric and neurodevelopmental disorders. In this review, to explore candidate novel applications for the treatment of several neuropsychiatric disorders, including mood disorders, schizophrenia, and other cognitive disturbance disorders, we discuss perspectives of psychopharmacology regarding the effects of 5-HT7R antagonism on transmission and intracellular signaling systems, based on preclinical findings.

The retrotrapezoid nucleus and the neuromodulation of breathing

Breathing is regulated by a host of arousal and sleep-wake state-dependent neuromodulators to maintain respiratory homeostasis. Modulators such as acetylcholine, norepinephrine, histamine, serotonin (5-HT), adenosine triphosphate (ATP), substance P, somatostatin, bombesin, orexin, and leptin can serve complementary or off-setting functions depending on the target cell type and signaling mechanisms engaged. Abnormalities in any of these modulatory mechanisms can destabilize breathing, suggesting that modulatory mechanisms are not overly redundant but rather work in concert to maintain stable respiratory output. The present review focuses on the modulation of a specific cluster of neurons located in the ventral medullary surface, named retrotrapezoid nucleus, that are activated by changes in tissue CO₂/H⁺ and regulate several aspects of breathing, including inspiration and active expiration.

Investigation of the Role of Stimulation and Blockade of 5-HT7 Receptors in Ketamine Anesthesia

Although several pieces of evidence have indicated the ability of the serotonin-7 receptor (5-HTR₇) to modulate N-methyl-_D-aspartate receptor (NMDAR) activation, the possible impact on ketamine anesthesia has not been examined directly. The purpose of the present study is thus to investigate the possible role of the 5-HTR₇ in ketamine anesthesia using a 5-HTR₇ agonist and/or antagonist. The influence of a 5-HTR₇ agonist/antagonist on ketamine anesthesia for behavioral impact was assessed by testing potential anesthetic parameters. Its functional impact was assessed by mRNA expression with real-time PCR and immunostaining in the hippocampus and prefrontal cortex of mice. Two different doses of ketamine-high and low-were administered to induce anesthesia. In the high-dose ketamine-applied group in particular, the administration of both the 5-HTR₇ agonist and antagonist intensified the anesthetic effect of ketamine. The reflection of the change in anesthesia parameters to 5-HTR₇ expression was observed as an increase in the hippocampus and a decrease in the prefrontal cortex in the anesthetized groups by stimulation of 5-HTR₇. It is noteworthy that the results of NMDAR expressions are parallel to the results of the 5-HTR₇ expressions of both the hippocampus and the prefrontal cortex. The 5-HTR₇ may play a role in ketamine anesthesia. It may act through NMDAR in ketamine anesthesia, depending on the parallelism between both receptors.

Effects of new generation triptans - frovatriptan and almotriptan - on hemodynamic parameters in intact male and female rats

The introduction of the second generation triptans in clinical and experimental practice was a major progress in the pharmacotherapy of migraine. Frovatriptan is a second generation triptan with strong 5-HT1B/1D serotonergic agonism and low 5-HT1A/7 receptor affinity, while almotriptan possesses not only the typical 5-HT1B/1D receptor agonist activity, but shows an affinity to the 5-HT1F receptor. The aim of our study was to assess the impact of frovatriptan and almotriptan on hemodynamics in male and female rats. We used a non-invasive "tail-cuff" method to measure the arterial blood pressure. Female and male Wistar rats were treated separately with high and low dosages of frovatriptan and almotriptan. Male and female rats showed reduction in all hemodynamic parameters, but only male rats showed an increase in the heart rate. In general, we could say that both almotriptan and frovatriptan potentiate cardiovascular safety.

Role of 5-HT receptors in neuropathic pain: potential therapeutic implications

5-HT plays a crucial role in the progress and adjustment of pain both centrally and peripherally. The therapeutic action of the 5-HT receptors` agonist and antagonist in neuropathic pain have been widely reported in many studies. However, the specific roles of 5-HT subtype receptors have not been reviewed comprehensively. Therefore, we summarized the recent findings on multiple subtypes of 5-HT receptors in both central and peripheral nervous system in neuropathic pain, particularly, 5-HT1, 5-HT2, 5-HT3 and 5-HT7 receptors. In addition, 5-HT4, 5-HT5 and 5-HT6 receptors were also reviewed. Most of studies focused on the function of 5-HT subtype receptors in spinal level compared to brain areas. Based on these evidences, the pain process can be facilitated or inhibited that depending on the specific subtypes and the distribution of 5-HT receptors. Therefore, this review may provide potential therapeutic implications in treatment of neuropathic pain.

Radiosynthesis and preclinical evaluation of [11 C]Cimbi-701 - Towards the imaging of cerebral 5-HT7 receptors

The serotonin 7 (5-HT7 ) receptor is suggested to be involved in a broad variety of CNS disorders, but very few in vivo tools exist to study this important target. Molecular imaging with positron emission tomography (PET) would enable an in vivo characterization of the 5-HT7 receptor. However, no clinical PET radiotracer exists for this receptor, and thus we aimed to develop such a tracer. In this study, we present the preclinical evaluation of [11 C]Cimbi-701. Cimbi-701 was synthesized in a one-step procedure starting from SB-269970. Its selectivity profile was determined using an academic screening platform (NIMH Psychoactive Drug Screening Program). Successful radiolabeling of [11 C]Cimbi-701 and subsequent in vivo evaluation was conducted in rats, pigs and baboon. In vivo specificity was investigated by 5-HT7 and σ receptor blocking studies. P-gp efflux transporter dependency was investigated using elacridar. [11 C]Cimbi-701 could successfully be synthesized. Selectivity profiling revealed high affinity for the 5-HT7 (Ki = 18 nM), σ-1 (Ki = 9.2 nM) and σ-2 (Ki = 1.6 nM) receptors. In rats, [11 C]Cimbi-701 acted as a strong P-gp substrate. After P-gp inhibition, rat brain uptake could specifically be blocked by 5-HT7 and σ receptor ligands. In pig, high brain uptake and specific 5-HT7 and σ-receptor binding was found for [11 C]Cimbi-701 without P-gp inhibition. Finally, low brain uptake was found in baboons. Both the specific σ-receptor binding and the low brain uptake of [11 C]Cimbi-701 displayed in baboon discouraged further translation to humans. Instead, we suggest exploration of this structural class as results indicate that selective 5-HT7 receptor imaging might be possible when more selective non-P-gp substrates are identified.

Role of the Serotonin Receptor 7 in Brain Plasticity: From Development to Disease

Our knowledge on the plastic functions of the serotonin (5-HT) receptor subtype 7 (5-HT7R) in the brain physiology and pathology have advanced considerably in recent years. A wealth of data show that 5-HT7R is a key player in the establishment and remodeling of neuronal cytoarchitecture during development and in the mature brain, and its dysfunction is linked to neuropsychiatric and neurodevelopmental diseases. The involvement of this receptor in synaptic plasticity is further demonstrated by data showing that its activation allows the rescue of long-term potentiation (LTP) and long-term depression (LTD) deficits in various animal models of neurodevelopmental diseases. In addition, it is becoming clear that the 5-HT7R is involved in inflammatory intestinal diseases, modulates the function of immune cells, and is likely to play a role in the gut-brain axis. In this review, we will mainly focus on recent findings on this receptor’s role in the structural and synaptic plasticity of the mammalian brain, although we will also illustrate novel aspects highlighted in gastrointestinal (GI) tract and immune system.

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.

The Role of Descending Pain Modulation in Chronic Primary Pain: Potential Application of Drugs Targeting Serotonergic System

Chronic primary pain (CPP) is a group of diseases with long-term pain and functional disorders but without structural or specific tissue pathologies. CPP is becoming a serious health problem in clinical practice due to the unknown cause of intractable pain and high cost of health care yet has not been satisfactorily addressed. During the past decades, a significant role for the descending pain modulation and alterations due to specific diseases of CPP has been emphasized. It has been widely established that central sensitization and alterations in neuroplasticity induced by the enhancement of descending pain facilitation and/or the impairment of descending pain inhibition can explain many chronic pain states including CPP. The descending serotonergic neurons in the raphe nuclei target receptors along the descending pain circuits and exert either pro- or antinociceptive effects in different pain conditions. In this review, we summarize the possible underlying descending pain regulation mechanisms in CPP and the role of serotonin, thus providing evidence for potential application of analgesic medications based on the serotonergic system in CPP patients.

Towards selective CNS PET imaging of the 5-HT7 receptor system: Past, present and future

Since its discovery in 1993, the serotonin receptor subtype 7 (5-HT₇) has attracted significant attention as a potential drug target; due to its elucidated roles in conditions such as insomnia, schizophrenia, and more. Therefore, it is unsurprising that there has been relatively early efforts undertaken to develop a positron emission tomography (PET) imaging agent for said receptor system. PET can be clinically used to probe receptor systems in vivo, permitting information such as a drug's occupancy against this system to be investigated. This review focuses on the efforts towards the development of a 5-HT₇R selective PET CNS tracer over the last 20 years, critically reflecting on applied strategies and commonly employed chemical frameworks and suggests future considerations that are needed to successfully develop a PET tracer for this clinically relevant target. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Enantioseparation of 5-chloro-2-{2-[3,4-dihydroisoquinoline-2(1H)-yl]ethyl}-2-methyl-2,3-dihydro-1H-inden-1-one (SYA 40247), a high-affinity 5-HT7 receptor ligand, by HPLC-PDA using amylose tris-(3, 5- dimethylphenylcarbamate) as a chiral stationary phase

In previous structure-activity relationship studies to identify new and selective 5-HT₇ receptor (5-HT₇ R) ligands, we identified the chiral compound, 5-chloro-2-{2-[3,4-dihydroisoquinoline-2(1H)-yl]ethyl}-2-methyl-2,3-dihydro-1H-inden-1-one (SYA 40247), with high-affinity binding to the 5-HT₇ R. Thus, it was of interest to separate the enantiomers in order to evaluate their affinity at the 5-HT₇ R. To achieve this separation, a normal-phase analytical method using HPLC-PDA and a 4.6 × 250 mm Chiralpak AD-H column was developed. Optimized isocratic conditions of 1.00 mL/min 95:5:0.1 v/v/v hexane-ethanol-diethylamine and a 254 nm analysis wavelength yielded a 6.07 min baseline separation. The method was scaled up to a 10 × 250 mm Chiralpak AD-H column, allowing 3 mg of racemate to be separated with a single injection, and 6 mg for an overlapping double injection in the same run. The separated enantiomers were reinjected into the analytical HPLC system, peak identities confirmed by retention time and PDA UV spectra, and the enantiomeric purities determined to be 100% for peak 1 and 100% for peak 2. A Jasco P-1020 polarimeter was used to determine the specific rotation [α] of the enantiomers of peaks 1 and 2, which were -86.2 and +93.3 (deg mL)/(g dm) respectively. No racemization was observed, and the enantiomeric purity remained at 100% for each peak.

The microRNA-29a Modulates Serotonin 5-HT7 Receptor Expression and Its Effects on Hippocampal Neuronal Morphology

miRNAs are master regulators of gene expression in diverse biological processes, including the modulation of neuronal cytoarchitecture. The identification of their physiological target genes remains one of the outstanding challenges. Recently, it has been demonstrated that the activation of serotonin receptor 7 (5-HT7R) plays a key role in regulating the neuronal structure, synaptogenesis, and synaptic plasticity during embryonic and early postnatal development of the central nervous system (CNS). In order to identify putative miRNAs targeting the 3'UTR of 5-HT7R mouse transcript, we used a computational prediction tool and detected the miR-29 family members as the only candidates. Thus, since miR-29a is more expressed than other members in the brain, we investigated its possible involvement in the regulation of neuronal morphology mediated by 5-HT7R. By luciferase assay, we show that miR-29a can act as a post-transcriptional regulator of 5-HT7R mRNA. Indeed, it downregulates 5-HT7R gene expression in cultured hippocampal neurons, while the expression of other serotonin receptors is not affected. From a functional point of view, miR-29a overexpression in hippocampal primary cultures impairs the 5HT7R-dependent neurite elongation and remodeling through the inhibition of the ERK intracellular signaling pathway. In vivo, the upregulation of miR-29a in the developing hippocampus parallels with the downregulation of 5-HT7R expression, supporting the hypothesis that this miRNA is a physiological modulator of 5-HT7R expression in the CNS.

Changes in brain white matter structure are associated with urine proteins in urologic chronic pelvic pain syndrome (UCPPS): A MAPP Network study

The Multidisciplinary Approach to the Study of Chronic Pelvic Pain (MAPP) Research Network has yielded neuroimaging and urinary biomarker findings that highlight unique alterations in brain structure and in urinary proteins related to tissue remodeling and vascular structure in patients with Urological Chronic Pelvic Pain Syndrome (UCPPS). We hypothesized that localized changes in diffusion tensor imaging (DTI) measurements might be associated with corresponding changes in urinary protein levels in UCPPS. To test this hypothesis, we created statistical parameter maps depicting the linear correlation between DTI measurements (fractional anisotropy (FA) and apparent diffusion coefficient (ADC)) and urinary protein quantification (MMP2, MMP9, NGAL, MMP9/NGAL complex, and VEGF) in 30 UCPPS patients from the MAPP Research Network, after accounting for clinical covariates. Results identified a brainstem region that showed a strong correlation between both ADC (R² = 0.49, P<0.0001) and FA (R² = 0.39, P = 0.0002) with urinary MMP9 levels as well as a correlation between both ADC (R² = 0.42, P = 0.0001) and FA (R² = 0.29, P = 0.0020) and urinary MMP9/NGAL complex. Results also identified significant correlations between FA and urinary MMP9 in white matter adjacent to sensorimotor regions (R² = 0.30, P = 0.002; R² = 0.36, P = 0.0005, respectively), as well as a correlation in similar sensorimotor regions when examining ADC and urinary MMP2 levels (R² = 0.42, P<0.0001) as well as FA and urinary MMP9/NGAL complex (R² = 0.33, P = 0.0008). A large, diffuse cluster of white matter was identified as having a strong correlation between both ADC (R² = 0.35, P = 0.0006) and FA (R² = 0.43, P<0.0001) with urinary NGAL levels. In contrast, no significant association between DTI measurements and VEGF was observed. Results suggest that elevated MMP9 or MMP9/NGAL in UCPPS may be related to degenerative neuronal changes in brainstem nuclei through excitotoxicity, while also facilitating synaptic plasticity in sensorimotor regions.

Rescue of prepulse inhibition deficit and brain mitochondrial dysfunction by pharmacological stimulation of the central serotonin receptor 7 in a mouse model of CDKL5 Deficiency Disorder

Mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene cause CDKL5 Deficiency Disorder (CDD), a rare neurodevelopmental syndrome characterized by severe behavioural and physiological symptoms. No cure is available for CDD. CDKL5 is a kinase that is abundantly expressed in the brain and plays a critical role in neurodevelopmental processes, such as neuronal morphogenesis and plasticity. This study provides the first characterization of the neurobehavioural phenotype of 1 year old Cdkl5-null mice and demonstrates that stimulation of the serotonin receptor 7 (5-HT₇R) with the agonist molecule LP-211 (0.25 mg/kg once/day for 7 days) partially rescues the abnormal phenotype and brain molecular alterations in Cdkl5-null male mice. In particular, LP-211 treatment completely normalizes the prepulse inhibition defects observed in Cdkl5-null mice and, at a molecular level, restores the abnormal cortical phosphorylation of rpS6, a downstream target of mTOR and S6 kinase, which plays a direct role in regulating protein synthesis. Moreover, we demonstrate for the first time that mitochondria show prominent functional abnormalities in Cdkl5-null mouse brains that can be restored by pharmacological stimulation of brain 5-HT₇R.

Activation of Serotonin 5-HT7 Receptors Modulates Hippocampal Synaptic Plasticity by Stimulation of Adenylate Cyclases and Rescues Learning and Behavior in a Mouse Model of Fragile X Syndrome

We have previously demonstrated that activation of serotonin 5-HT₇ receptors (5-HT₇R) reverses metabotropic glutamate receptor-mediated long term depression (mGluR-LTD) in the hippocampus of wild-type (WT) and Fmr1 Knockout (KO) mice, a model of Fragile X Syndrome (FXS) in which mGluR-LTD is abnormally enhanced. Here, we have investigated intracellular mechanisms underlying the effect of 5-HT₇R activation using patch clamp on hippocampal slices. Furthermore, we have tested whether in vivo administration of LP-211, a selective 5-HT₇R agonist, can rescue learning and behavior in Fmr1 KO mice. In the presence of an adenylate cyclase blocker, mGluR-LTD was slightly enhanced in WT and therefore the difference between mGluR-LTD in WT and Fmr1 KO slices was no longer present. Conversely, activation of adenylate cyclase by either forskolin or Pituitary Adenylate Cyclase Activating Polypeptide (PACAP) completely reversed mGluR-LTD in WT and Fmr1 KO. 5-HT₇R activation reversed mGluR-LTD in WT and corrected exaggerated mGluR-LTD in Fmr1 KO; this effect was abolished by blockade of either adenylate cyclase or protein kinase A (PKA). Exposure of hippocampal slices to LP-211 caused an increased phosphorylation of extracellular signal regulated kinase (ERK), an intracellular effector involved in mGluR-LTD, in WT mice. Conversely, this effect was barely detectable in Fmr1 KO mice, suggesting that 5-HT₇R-mediated reversal of mGluR-LTD does not require ERK stimulation. Finally, an acute in vivo administration of LP-211 improved novel object recognition (NOR) performance in WT and Fmr1 KO mice and reduced stereotyped behavior in Fmr1 KO mice. Our results indicate that mGluR-LTD in WT and Fmr1 KO slices is bidirectionally modulated in conditions of either reduced or enhanced cAMP formation. Activation of 5-HT₇ receptors reverses mGluR-LTD by activation of the cAMP/PKA intracellular pathway. Importantly, a systemic administration of a 5-HT₇R agonist to Fmr1 KO mice corrected learning deficits and repetitive behavior. We suggest that selective 5-HT₇R agonists might become novel pharmacological tools for FXS therapy.

Review: 5-HT1, 5-HT2, 5-HT3 and 5-HT7 Receptors and their Role in the Modulation of Pain Response in the Central Nervous System

BACKGROUND

The aim of this review was to identify the mechanisms by which serotonin receptors involved at the central level are able to modulate the nociceptive response. Pain is a defense mechanism of the body that entails physiological, anatomical, neurochemical, and psychological changes, and is defined as an unpleasant sensory and emotional experience with potential risk of tissue damage, comprising the leading cause of appointments with Physicians worldwide. Treatment for this symptom has generated several neuropharmacological lines of research, due to the different types of pain and the various drugs employed to treat this condition. Serotonin [5- HydroxyTryptamine (5-HT)] is a neurotransmitter with seven families (5-HT1-5-HT7) and approximately 15 receptor subtypes. Serotonin modulates neuronal activity; however, this neurotransmitter is related with a number of physiological processes, such as cardiovascular function, gastric motility, renal function, etc. On the other hand, several researches reported that serotonin modulates nociceptive response through 5-HT1, 5-HT2, 5-HT3, and 5-HT7 receptors in the Central Nervous System (CNS).

METHOD

In this review, a search was conducted on PubMed, ProQuest, EBSCO, and the Science Citation Index for studies evaluating the effects of 5-HT1, 5-HT2, 5-HT3, and 5-HT7 receptors in the CNS on the modulation of different types of pain.

CONCLUSION

We concluded that 5-HT1, 5-HT2, 5-HT3, and 5-HT7 receptors in the CNS modulate the pain, but this depends on the distribution of the receptors, dose of agonists or antagonists, administration route, pain type and duration in order to inhibit, excite, or even maintain the nociceptive response.

Amino Acid Hot Spots of Halogen Bonding: A Combined Theoretical and Experimental Case Study of the 5-HT7 Receptor

A computational approach combining a structure-activity relationship library of halogenated and the corresponding unsubstituted ligands (called XSAR) with QM-based molecular docking and binding free energy calculations was used to search for amino acids frequently targeted by halogen bonding (hot spots) in a 5-HT₇R as a case study. The procedure identified two sets of hot spots, extracellular (D2.65, T2.64, and E7.35) and transmembrane (C3.36, T5.39, and S5.42), which were further verified by a synthesized library of halogenated arylsulfonamide derivatives of (aryloxy)ethylpiperidines. It was found that a halogen bond formed between T5.39 and a bromine atom at 3-position of the aryloxy fragment caused the most remarkable, 35-fold increase in binding affinity for 5-HT₇R when compared to the nonhalogenated analog. The proposed paradigm of halogen bonding hot spots was additionally verified on D₄ dopamine receptor showing that it can be used in rational drug design/optimization for any protein target.

Downregulation of 5-hydroxytryptamine7 receptor in the medial prefrontal cortex ameliorates impulsive actions in animal models of schizophrenia

R mRNA in the mPFC was significantly decreased to 5-Hydroxytryptamine₇ (5-HT₇) receptors in the medial prefrontal cortex (mPFC) play a critical role in complex cognitive impairment in schizophrenia. The mouse model of schizophrenia was established through the neonatal administration of phencyclidine (nPCP). Recombinant adeno-associated virus-mediated gene knockdown was used to investigate the role of mPFC 5-HT₇ receptor in the schizophrenia-like symptoms in mice. Under baseline conditions in the 5-choice serial reaction time task (5-CSRTT), nPCP produced a significant attentional impairment that was exacerbated when mice were tested under LITI. Premature and perseverative responding in nPCP mice were both increased, thus suggesting deficits in inhibitory response control. The deficits in attentional performance and premature responding of nPCP mice were improved or fully rescued by 5-HT₇ receptor downregulation under heavy perceptual load. Downregulation of the 5-HT₇ receptor in the mPFC ameliorated spatial working memory and had no effects on nPCP-induced impairments in recognition memory and MA-induced hyperlocomotion. These results suggest that 5-HT₇ receptor is involved in the cognitive outcomes of schizophrenia-like symptoms similar to humans. Downregulation of the 5-HT₇ receptor in the mPFC exert complex effects in a mouse model of schizophrenia and may be of benefit in treating schizophrenia-related impulsive actions.

Synaptic Remodeling Depends on Signaling between Serotonin Receptors and the Extracellular Matrix

Rewiring of synaptic circuitry pertinent to memory formation has been associated with morphological changes in dendritic spines and with extracellular matrix (ECM) remodeling. Here, we mechanistically link these processes by uncovering a signaling pathway involving the serotonin 5-HT7 receptor (5-HT7R), matrix metalloproteinase 9 (MMP-9), the hyaluronan receptor CD44, and the small GTPase Cdc42. We highlight a physical interaction between 5-HT7R and CD44 (identified as an MMP-9 substrate in neurons) and find that 5-HT7R stimulation increases local MMP-9 activity, triggering dendritic spine remodeling, synaptic pruning, and impairment of long-term potentiation (LTP). The underlying molecular machinery involves 5-HT7R-mediated activation of MMP-9, which leads to CD44 cleavage followed by Cdc42 activation. One important physiological consequence of this interaction includes an increase in neuronal outgrowth and elongation of dendritic spines, which might have a positive effect on complex neuronal processes (e.g., reversal learning and neuronal regeneration).

Genetic contributions to precocity traits in racing Thoroughbreds

Adaptation to early training and racing (i.e. precocity), which is highly variable in racing Thoroughbreds, has implications for the selection and training of horses. We hypothesised that precocity in Thoroughbred racehorses is heritable. Age at first sprint training session (work day), age at first race and age at best race were used as phenotypes to quantify precocity. Using high-density SNP array data, additive SNP heritability (hSNP2) was estimated to be 0.17, 0.14 and 0.17 for the three traits respectively. In genome-wide association studies (GWAS) for age at first race and age at best race, a 1.98-Mb region on equine chromosome 18 (ECA18) was identified. The most significant association was with the myostatin (MSTN) g.66493737C>T SNP (P = 5.46 × 10^-12 and P = 1.89 × 10^-14 respectively). In addition, two SNPs on ECA1 (g.37770220G>A and g.37770305T>C) within the first intron of the serotonin receptor gene HTR7 were significantly associated with age at first race and age at best race. Although no significant associations were identified for age at first work day, the MSTN:g.66493737C>T SNP was among the top 20 SNPs in the GWAS (P = 3.98 × 10^-5 ). Here we have identified variants with potential roles in early adaptation to training. Although there was an overlap in genes associated with precocity and distance aptitude (i.e. MSTN), the HTR7 variants were more strongly associated with precocity than with distance. Because HTR7 is closely related to the HTR1A gene, previously implicated in tractability in young Thoroughbreds, this suggests that behavioural traits may influence precocity.

Effects of medial prefrontal cortex 5-HT7 receptor knockdown on cognitive control after acute heroin administration

Heroin abuse is linked to a deleterious effect on cognitive functioning in the individual. Recent evidences suggest that the serotonin₇ receptor (5-HT₇R) is engaged in the regulation of cognitive control and the drug use-associated behaviors. However, the role of 5-HT₇R in the cognitive control after acute heroin administration has not been studied. The present study aims to investigate whether the knockdown of the 5-HT₇R by virus-mediated gene silencing in the medial prefrontal cortex (mPFC) could ameliorate the acute heroin-induced cognitive impairments. The attentional function, impulsivity and compulsivity were assessed by the 5-choice serial reaction time task (5-CSRTT) in mice. The memory ability and locomotor activity were examined by the novel objects recognition (NOR), Y-maze and open-field test (OFT). Acute heroin administration at 5 mg/kg produced robust disruptions in attention, impulsivity and motivation in mice. 5-HT₇R knockdown in the mPFC did not affect the 5-CSRTT baseline performance, spatial working memory, visual episodic memory and locomotion. However, mPFC 5-HT₇R knockdown selectively ameliorated acute heroin-induced increase in omissions and premature responses under conditions of increased perceptual load. In addition, mPFC 5-HT₇R knockdown induced increases in perseverative responding observed across both saline and heroin-treated animals. Moreover, 5-HT₇R knockdown prevented the heroin-induced decrease in NR1/CaMKII phosphorylation in mPFC, thus suggesting that 5-HT₇R and N-methyl-d-aspartic acid (NMDA) receptor signaling may be involved in the cognitive outcomes of acute heroin administration. Altogether, these observations suggest modest and restricted effects of mPFC 5-HT7R knockdown on cognitive behaviors, both in the presence or absence of acute heroin treatment.

Time of day but not aging regulates 5-HT7 receptor binding sites in the hamster hippocampus

Activation of 5-HT₇ receptors influences memory as well as circadian rhythms and other processes. This study investigated the regulation of the 5-HT₇ receptors in the hippocampus, a likely substrate for the effects of 5-HT₇ receptor compounds on memory. Because endogenous serotonin release is higher during the active phase, and chronic treatment with a serotonin-selective reuptake inhibitor down-regulates 5-HT₇ receptors, we hypothesized that 5-HT₇ receptors exhibit 24-h variations. We also hypothesized that aging decreases 5-HT₇ receptors in the hippocampus, as it does in the dorsal raphe nucleus, a brain site for serotonergic resetting of circadian rhythms. Male hamsters (young, 3-5 mos; old, 17-21 mos) exposed to a light:dark cycle were euthanized at 4 times of day (zeitgeber times [ZT]1, 6, 13, & 19; ZT12=time of lights:off). 5-HT₇ receptor autoradiography was conducted on hippocampal sections using [³H]8-OH-DPAT [2nM] as the radioligand and SB-269970 [1μM] to define nonspecific binding. Slide-mounted sections and radioactive standards were apposed to X-ray films; the resultant autoradiograms were assessed by computer-assisted microdensitometry. Specific 5-HT₇ receptor binding was robustly expressed in the dentate gyrus (DG) and CA1 but not in the CA2 or CA3. In the CA1 and DG, specific 5-HT₇ receptor binding exhibited 24-h rhythms with troughs at night (P<0.005; P<0.05, respectively). Aging did not significantly affect specific 5-HT₇ receptor binding in these regions, nor were significant time and age interactions observed. These findings suggest that the therapeutic effectiveness of 5-HT₇ drugs may vary with time of day of administration but not with the age of the recipient.

Improvising 5-HT7R homology model for design of high affinity ligands: model validation with docking, embrace minimization, MM-GBSA, and molecular dynamic simulations

The subtype, 5-HT₇R has been implicated in neurological disorders and presents itself as a promising target for antidepressant drugs. Design of targeted selective ligands, require a sound knowledge of 3D-receptor structure. In absence of receptor structure, structure-based design of targeted ligands relies on generation of 5-HT₇R homology model. In this study, the impact of template choice, alignment, and model building methods on the homology model of 5-HT₇R is addressed. The compactness and model quality due to the presence of cholesterol (lipidic receptor) have also been observed. The results suggest good stereochemical quality of the final model. Ramachandran Plot Analysis indicated more than 97.5% amino acid residues in the favorable region. The overall quality factor was 91.8% using ERRAT. The Z-score for backbone confirmation and packing quality were -1.248 and -1.427, respectively, using WHATCHECK. The RMS Z-score for side chain planarity was .711. Other validation results for the final model include binding site analysis in which Asp162, Val163, Phe343, Phe344, Arg350, Arg367, and Leu370 conserved residues were found in the active site, correlation coefficient of .82 in ligand-based screening and .85 in embrace minimization. Further, the model showed good correlation for agonist and antagonist in docking ([Formula: see text] ≈ .76, [Formula: see text] ≈ .82), embrace minimization ([Formula: see text] ≈ .73, [Formula: see text] ≈ .72), and MM-GBSA ([Formula: see text] ≈ .69, [Formula: see text] ≈ .75) studies. The model was subjected to Molecular Dynamics (MD) simulation of 20 ns both in ligand-free and ligand-bound receptor (agonist and antagonist) system in order to assess its stability.

Low-basicity 5-HT7 Receptor Agonists Synthesized Using the van Leusen Multicomponent Protocol

A series of 5-aryl-1-alkylimidazole derivatives was synthesized using the van Leusen multicomponent reaction. The chemotype is the first example of low-basicity scaffolds exhibiting high affinity for 5-HT₇ receptor together with agonist function. The chosen lead compounds 3-(1-ethyl-1H-imidazol-5-yl)-5-iodo-1H-indole (AGH-107, 1o, K_{i 5-HT7} = 6 nM, EC₅₀ = 19 nM, 176-fold selectivity over 5-HT_1AR) and 1e (5-methoxy analogue, K_{i 5-HT7} = 30 nM, EC₅₀ = 60 nM) exhibited high selectivity over related CNS targets, high metabolic stability and low toxicity in HEK-293 and HepG2 cell cultures. A rapid absorption to the blood, high blood-brain barrier permeation and a very high peak concentration in the brain (C_max = 2723 ng/g) were found for 1o after i.p. (5 mg/kg) administration in mice. The compound was found active in novel object recognition test in mice, at 0.5, 1 and 5 mg/kg. Docking to 5-HT₇R homology models indicated a plausible binding mode which explain the unusually high selectivity over the related CNS targets. Halogen bond formation between the most potent derivatives and the receptor is consistent with both the docking results and SAR. 5-Chlorine, bromine and iodine substitution resulted in a 13, 27 and 89-fold increase in binding affinities, respectively, and in enhanced 5-HT_1AR selectivity.