Cloning, expression and pharmacology of a truncated splice variant of the human 5-HT7 receptor (h5-HT7b)

"Selective" serotonin 5-HT2A receptor antagonists

Blockade of the serotonin 5-HT_2A G protein-coupled receptor (5-HT_2AR) is a fundamental pharmacological characteristic of numerous antipsychotic medications, which are FDA-approved to treat schizophrenia, bipolar disorder, and as adjunctive therapies in major depressive disorder. Meanwhile, activation of the 5-HT_2AR by serotonergic psychedelics may be useful in treating neuropsychiatric indications, including major depressive and substance use disorders. Serotonergic psychedelics and other 5-HT_2AR agonists, however, often bind other receptors, and standard 5-HT_2AR antagonists lack sufficient selectivity to make well-founded mechanistic conclusions about the 5-HT_2AR-dependent effects of these compounds and the general neurobiological function of 5-HT_2ARs. This review discusses the limitations and strengths of currently available "selective" 5-HT_2AR antagonists, the molecular determinants of antagonist selectivity at 5-HT_2ARs, and the utility of molecular pharmacological and computational methods in guiding the discovery of novel unambiguously selective 5-HT_2AR antagonists.

Alternative Pre-mRNA Splicing of the Mu Opioid Receptor Gene, OPRM1: Insight into Complex Mu Opioid Actions

Most opioid analgesics used clinically, including morphine and fentanyl, as well as the recreational drug heroin, act primarily through the mu opioid receptor, a class A Rhodopsin-like G protein-coupled receptor (GPCR). The single-copy mu opioid receptor gene, OPRM1, undergoes extensive alternative splicing, creating multiple splice variants or isoforms via a variety of alternative splicing events. These OPRM1 splice variants can be categorized into three major types based on the receptor structure: (1) full-length 7 transmembrane (TM) C-terminal variants; (2) truncated 6TM variants; and (3) single TM variants. Increasing evidence suggests that these OPRM1 splice variants are pharmacologically important in mediating the distinct actions of various mu opioids. More importantly, the OPRM1 variants can be targeted for development of novel opioid analgesics that are potent against multiple types of pain, but devoid of many side-effects associated with traditional opiates. In this review, we provide an overview of OPRM1 alternative splicing and its functional relevance in opioid pharmacology.

Differential localization of serotoninergic system elements in human amniotic epithelial cells†

Serotonin or 5-hydroxytryptamine (5-HT) is a biogenic amine involved in regulating several functions, including development. However, its impact on human embryo development has been poorly studied. The present work investigated the expression and distribution of the main components of the serotoninergic system in human amniotic tissue and human amniotic epithelial cells (hAEC) in vitro, as an alternative model of early human embryo development. Amniotic membranes from full-term healthy pregnancies were used. Human amnion tissue or hAEC isolated from the amnion was processed for reverse transcription-polymerase chain reaction and immunofluorescence analyses of the main components of the serotoninergic system. We found the expression of tryptophan hydroxylase type 1 (TPH1), type 2 (TPH2), serotonin transporter (SERT), monoamine oxidase-A (MAOA), as well as HTR1D and HTR7 receptors at mRNA level in amnion tissue as well in hAEC. Interestingly, we found the presence of 5-HT in the nucleus of the cells in amnion tissue, whereas it was located in the cytoplasm of isolated hAEC. We detected TPH1, TPH2, and HTR1D receptor in both the nucleus and cytoplasm. SERT, MAOA, and HTR7 receptor were only observed in the cytoplasm. The results presented herein show, for the first time, the presence of the serotoninergic system in human amnion in vivo and in vitro.

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.

An inactive receptor-G protein complex maintains the dynamic range of agonist-induced signaling

G protein-coupled receptors (GPCRs) are targeted by a large fraction of approved drugs and regulate many important cellular processes. Conventional signaling by GPCRs is triggered when agonist-activated receptors associate with heterotrimeric G proteins. We found that serotonin 5-HT₇ receptors couple to G_s proteins in an unconventional manner, in which agonist binding instead promotes dissociation of preexisting inactive 5-HT₇–G_s complexes. Therefore, agonists can initiate signaling via two distinct mechanisms, by promoting the association of active receptors and G proteins and by promoting dissociation of inactive receptors and G proteins.

Agonist binding promotes activation of G protein-coupled receptors (GPCRs) and association of active receptors with G protein heterotrimers. The resulting active-state ternary complex is the basis for conventional stimulus-response coupling. Although GPCRs can also associate with G proteins before agonist binding, the impact of such preassociated complexes on agonist-induced signaling is poorly understood. Here we show that preassociation of 5-HT₇ serotonin receptors with G_s heterotrimers is necessary for agonist-induced signaling. 5-HT₇ receptors in their inactive state associate with G_s, as these complexes are stabilized by inverse agonists and receptor mutations that favor the inactive state. Inactive-state 5-HT₇–G_s complexes dissociate in response to agonists, allowing the formation of conventional agonist–5-HT₇–G_s ternary complexes and subsequent G_s activation. Inactive-state 5-HT₇–G_s complexes are required for the full dynamic range of agonist-induced signaling, as 5-HT₇ receptors spontaneously activate G_s variants that cannot form inactive-state complexes. Therefore, agonist-induced signaling in this system involves two distinct receptor-G protein complexes, a conventional ternary complex that activates G proteins and an inverse-coupled binary complex that maintains the inactive state when agonist is not present.

Role of 5-HT7 receptors in the immune system in health and disease

In mammalians, serotonin (5-HT) has critical roles in the central nervous system (CNS), including mood stability, pain tolerance, or sleep patterns. However, the vast majority of serotonin is produced by intestinal enterochromaffin cells of the gastrointestinal tract and circulating blood platelets, also acting outside of the CNS. Serotonin effects are mediated through its interaction with 5-HT receptors (5-HTRs), a superfamily with a repertoire of at least fourteen well-characterized members. 5-HT7 receptors are the last 5-HTR member to be identified, with well-defined functions in the nervous, gastrointestinal, and vascular systems. The effects of serotonin on the immune response are less well understood. Mast cells are known to produce serotonin, while T cells, dendritic cells, monocytes, macrophages and microglia express 5-HT7 receptor. Here, we review the known roles of 5-HT7 receptors in the immune system, as well as their potential therapeutic implication in inflammatory and immune-mediated disorders.

Whole genome sequencing of a dizygotic twin suggests a role for the serotonin receptor HTR7 in autism spectrum disorder

Whole genome sequencing of a severely affected dizygotic twin with an autism spectrum disorder and intellectual disability revealed a compound heterozygous mutation in the HTR7 gene as the only variation not detected in control databases. Each parent carries one allele of the mutation, which is not present in an unaffected stepsister. The HTR7 gene encodes the 5-HT₇ serotonin receptor that is involved in brain development, synaptic transmission, and plasticity. The paternally inherited p.W60C variant is situated at an evolutionary conserved nucleotide and predicted damaging by Polyphen2. A mutation akin to the maternally inherited pV286I mutation has been reported to significantly affect the binding characteristics of the receptor. Therefore, the observed sequence alterations provide a first suggestive link between a genetic abnormality in the HTR7 gene and a neurodevelopmental disorder. © 2016 Wiley Periodicals, Inc.

Long-Term Citalopram Treatment Alters the Stress Responses of the Cortical Dopamine and Noradrenaline Systems: the Role of Cortical 5-HT1A Receptors

BACKGROUND

Cortical dopamine and noradrenaline are involved in the stress response. Citalopram, a selective serotonin reuptake inhibitor, has direct and indirect effects on the serotonergic system. Furthermore, long-term treatment with citalopram affects the dopamine and noradrenaline systems, which could contribute to the therapeutic action of antidepressants.

METHODS

The effects of long-term treatment with citalopram on the responses of the dopamine and noradrenaline systems in the rat prefrontal cortex to acute handling stress were evaluated using in vivo microdialysis.

RESULTS

Acute handling stress increased dopamine and noradrenaline levels in the prefrontal cortex. The dopamine and noradrenaline responses were suppressed by local infusion of a 5-HT1A receptor agonist, 7-(Dipropylamino)-5,6,7,8-tetrahydronaphthalen-1-ol;hydrobromide, into the prefrontal cortex. The dopamine response was abolished by long-term treatment with citalopram, and the abolished dopamine response was reversed by local infusion of a 5-HT1A receptor antagonist, (Z)-but-2-enedioic acid;N-[2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]-N-pyridin-2-ylcyclohexanecarboxamide into the prefrontal cortex. On the other hand, long-term treatment with citalopram reduced the basal noradrenaline levels (approximately 40% of the controls), but not the basal dopamine levels. The noradrenaline response was maintained despite the low basal noradrenaline levels. Signaling from the 5-HT1A receptors and α2-adrenoceptors was not involved in the decrease in the basal noradrenaline levels but partially affected the noradrenaline response.

CONCLUSIONS

Chronic citalopram treatment differentially suppresses the dopamine and noradrenaline systems in the prefrontal cortex, and the dopamine stress response was preferentially controlled by upregulating 5-HT1A receptor signaling. Our findings provide insight into how antidepressants modulate the dopamine and noradrenaline systems to overcome acute stress.

Alternative Splicing of G Protein-Coupled Receptors: Relevance to Pain Management

Drugs that target G protein-coupled receptors (GPCRs) represent the primary treatment strategy for patients with acute and chronic pain; however, there is substantial individual variability in both the efficacy and adverse effects associated with these drugs. Variability in drug responses is due, in part, to individuals' diversity in alternative splicing of pain-relevant GPCRs. G protein-coupled receptor alternative splice variants often exhibit distinct tissue distribution patterns, drug-binding properties, and signaling characteristics that may impact disease pathology as well as the extent and direction of analgesic effects. We review the importance of GPCRs and their known splice variants to the management of pain.

Serotonergic regulation of excitability of principal cells of the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is one of the first stations within the central auditory pathway where the basic computations underlying sound localization are initiated and heightened activity in the DCN may underlie central tinnitus. The neurotransmitter serotonin (5-hydroxytryptamine; 5-HT), is associated with many distinct behavioral or cognitive states, and serotonergic fibers are concentrated in the DCN. However, it remains unclear what is the function of this dense input. Using a combination of in vitro electrophysiology and optogenetics in mouse brain slices, we found that 5-HT directly enhances the excitability of fusiform principal cells via activation of two distinct 5-HT receptor subfamilies, 5-HT2A/2CR (5-HT2A/2C receptor) and 5-HT7R (5-HT7 receptor). This excitatory effect results from an augmentation of hyperpolarization-activated cyclic nucleotide-gated channels (Ih or HCN channels). The serotonergic regulation of excitability is G-protein-dependent and involves cAMP and Src kinase signaling pathways. Moreover, optogenetic activation of serotonergic axon terminals increased excitability of fusiform cells. Our findings reveal that 5-HT exerts a potent influence on fusiform cells by altering their intrinsic properties, which may enhance the sensitivity of the DCN to sensory input.

Molecular underpinnings of ventral surface chemoreceptor function: focus on KCNQ channels

Central chemoreception is the mechanism by which CO₂/H(+) -sensitive neurons (i.e. chemoreceptors) regulate breathing in response to changes in tissue CO₂/H(+) . Neurons in the retrotrapezoid nucleus (RTN) directly regulate breathing in response to changes in tissue CO₂/H(+) and function as a key locus of respiratory control by integrating information from several respiratory centres, including the medullary raphe. Therefore, chemosensitive RTN neurons appear to be critically important for maintaining breathing, thus understanding molecular mechanisms that regulate RTN chemoreceptor function may identify therapeutic targets for the treatment of respiratory control disorders. We have recently shown that KCNQ (Kv7) channels in the RTN are essential determinants of spontaneous activity ex vivo, and downstream effectors for serotonergic modulation of breathing. Considering that loss of function mutations in KCNQ channels can cause certain types of epilepsy including those associated with sudden unexplained death in epilepsy (SUDEP), we propose that dysfunctions of KCNQ channels may be one cause for epilepsy and respiratory problems associated with SUDEP. In this review, we will summarize the role of KCNQ channels in the regulation of RTN chemoreceptor function, and suggest that these channels represent useful therapeutic targets for the treatment of respiratory control disorders.

Design, synthesis and biological evaluation of small molecule-based PET radioligands for the 5-hydroxytryptamine 7 receptor

A new prospective approach for PET imaging of 5-HT₇ by a small molecule ligand.

HCN channels contribute to serotonergic modulation of ventral surface chemosensitive neurons and respiratory activity

Chemosensitive neurons in the retrotrapezoid nucleus (RTN) provide a CO2/H(+)-dependent drive to breathe and function as an integration center for the respiratory network, including serotonergic raphe neurons. We recently showed that serotonergic modulation of RTN chemoreceptors involved inhibition of KCNQ channels and activation of an unknown inward current. Hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels are the molecular correlate of the hyperpolarization-activated inward current (Ih) and have a high propensity for modulation by serotonin. To investigate whether HCN channels contribute to basal activity and serotonergic modulation of RTN chemoreceptors, we characterize resting activity and the effects of serotonin on RTN chemoreceptors in vitro and on respiratory activity of anesthetized rats in the presence or absence of blockers of KCNQ (XE991) and/or HCN (ZD7288, Cs(+)) channels. We found in vivo that bilateral RTN injections of ZD7288 increased respiratory activity and in vitro HCN channel blockade increased activity of RTN chemoreceptors under control conditions, but this was blunted by KCNQ channel inhibition. Furthermore, in vivo unilateral RTN injection of XE991 plus ZD7288 eliminated the serotonin response, and in vitro serotonin sensitivity was eliminated by application of XE991 and ZD7288 or SQ22536 (adenylate cyclase blocker). Serotonin-mediated activation of RTN chemoreceptors was blocked by a 5-HT7-receptor blocker and mimicked by a 5-HT7-receptor agonist. In addition, serotonin caused a depolarizing shift in the voltage-dependent activation of Ih. These results suggest that HCN channels contribute to resting chemoreceptor activity and that serotonin activates RTN chemoreceptors and breathing in part by a 5-HT7 receptor-dependent mechanism and downstream activation of Ih.

Serotonin and sensory nerves: meeting in the cardiovascular system

Blood pressure regulation by 5-HT has proven to be a complex story to unravel. The work by Cuesta et al., in this issue of Vascular Pharmacology adds another layer of complexity by providing sound in vivo data that 5-HT, through the 5-HT7 receptor, can inhibit the vasodepressor actions of the sensory nervous system and thereby promote blood pressure maintenance. This interaction of 5-HT with the sensory nervous system is inhibitory, whereas 5-HT is understood to be stimulatory in other systems. Moreover, activation of the 5-HT7 receptor has been linked to both reduction and elevation of blood pressure. These interactions are discussed in this mini-review, as are potential steps forward in understanding the interplay of 5-HT, the sensory nervous system and blood pressure.

The arylpiperazine derivatives N-(4-cyanophenylmethyl)-4-(2-diphenyl)-1-piperazinehexanamide and N-benzyl-4-(2-diphenyl)-1-piperazinehexanamide exert a long-lasting inhibition of human serotonin 5-HT7 receptor binding and cAMP signaling

We performed a detailed in vitro pharmacological characterization of two arylpiperazine derivatives, compound N-(4-cyanophenylmethyl)-4-(2-diphenyl)-1-piperazinehexanamide (LP-211) previously identified as a high-affinity brain penetrant ligand for 5-hydroxytryptamine (serotonin) type 7 (5-HT₇) receptors, and its analog N-benzyl-4-(2-diphenyl)-1-piperazinehexanamide (MEL-9). Both ligands exhibited competitive displacement of [³H]-(2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine ([³H]-SB-269970) radioligand binding and insurmountable antagonism of 5-carboxamidotryptamine (5-CT)-stimulated cyclic adenosine monophosphate (cAMP) signaling in human embryonic kidney (HEK293) cells stably expressing human 5-HT₇ receptors. They also inhibited forskolin-stimulated adenylate cyclase activity in 5-HT₇-expressing HEK293 cells but not in the parental cell line. The compounds elicited long-lasting (at least 24 h) concentration-dependent inhibition of radioligand binding at 5-HT₇-binding sites in whole-cell radioligand binding assays, after pretreatment of the cells with the compounds and subsequent compound removal. In cAMP assays, pretreatment of cells with the compounds rendered 5-HT₇ receptors unresponsive to 5-CT and also rendered 5-HT₇-expressing HEK293 cells unresponsive to forskolin. Compound 1-(2-biphenyl)piperazine (RA-7), a known active metabolite of LP-211 present in vivo, was able to partially inhibit 5-HT₇ radioligand binding in a long-lasting irreversible manner. Hence, LP-211 and MEL-9 were identified as high-affinity long-acting inhibitors of human 5-HT₇ receptor binding and function in cell lines. The detailed in vitro characterization of these two pharmacological tools targeting 5-HT₇ receptors may benefit the study of 5-HT₇ receptor function and it may lead to the development of novel selective pharmacological tools with defined functional properties at 5-HT₇ receptors.

Development of 3,4-dihydroisoquinolin-1(2H)-one derivatives for the Positron Emission Tomography (PET) imaging of σ₂ receptors

σ₂ Receptors are promising biomarkers for cancer diagnosis given the relationship between the proliferative status of tumors and their density. With the aim of contributing to the research of σ₂ receptor Positron Emission Tomography (PET) probes, we developed 2-[3-[6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl]propyl]-3,4-dihydroisoquinolin-1(2H)-one (3), with optimal σ₂ pharmacological properties and appropriate lipophilicity. Hence, 3 served as the lead compound for the development of a series of dihydroisoquinolinones amenable to radiolabeling. Radiosynthesis for compound 26, which displayed the most appropriate σ₂ profile, was developed and σ₂ specific binding for the corresponding [(18)F]-26 was confirmed by in vitro autoradiography on rat brain slices. Despite the excellent in vitro properties, [(18)F]-26 could not successfully image σ₂ receptors in the rat brain in vivo, maybe because of its interaction with P-gp. Nevertheless, [(18)F]-26 may still be worthy of further investigation for the imaging of σ₂ receptors in peripheral tumors devoid of P-gp overexpression.

Serotonergic mechanisms are necessary for central respiratory chemoresponsiveness in situ

Evidence from in vivo and in vitro experiments conclude that serotonin (5-HT) neurons are involved in and play an important role in central respiratory CO2/H(+) chemosensitivity. This study was designed to assess the importance of 5-HT neurons and 5-HT receptor activation in the frequency and amplitude components of the hypercapnic response of the respiratory network in the unanesthetized perfused in situ juvenile rat brainstem preparation that exhibits patterns of phrenic nerve discharge similar to breathing in vivo. Exposure to a hypercapnic perfusate increased phrenic burst frequency and/or amplitude, the neural correlates of breathing frequency and tidal volume in vivo. Hypercapnic responses were also assessed during exposure to ketanserin (5-HT2 receptor antagonist), and 8-OH-DPAT (inhibiting 5-HT neurons via 5-HT1A autoreceptors). Neither of these drugs substantially altered baseline activity, however, both abolished hypercapnic responses of the respiratory network. These data illustrate that 5-HT neurons and 5-HT receptor activation are not required for respiratory rhythm generation per se, but are critical for CO2 responses in situ, supporting the hypothesis that 5-HT neurons play an important role in central ventilatory chemosensitivity in vivo.

Serotonin (5-HT7) receptor-stimulated activation of cAMP-PKA pathway in bovine corneal epithelial and endothelial cells

BACKGROUND

5-Hydroxytryptamine (5-HT; serotonin) is a major neurotransmitter, and its receptors are found throughout the whole body. The 5-HT7 receptor subtype was detected in human corneal epithelial and endothelial cells and found to be functionally active in a corneal epithelial cell line. The aim of the present study was to demonstrate that native bovine corneal epithelial and endothelial cells express a functional 5-HT7 receptor positively coupled to adenylyl cyclase and protein kinase A (PKA) formation.

METHODS

5-HT7 receptors were studied using polyclonal antibodies. cAMP concentration after 5-HT7 receptor stimulation with 5-carboxamidotryptamine (5-CT, a 5-HT7 agonist) was tested by enzyme immunoassay, PKA activity was estimated by kinase consumption of ATP.

RESULTS

Immunocytochemistry and immunofluorescence revealed the presence of 5-HT7 receptors in corneal epithelial and endothelial cells. Stimulation of corneal 5-HT7 receptors with 5-CT revealed a dose-dependent increase in intracellular cAMP concentration in corneal epithelium (0.01-0.34 pmol/ml) and endothelium (0.01-0.19 pmol/ml) between 10(-10) and 10(-7) mg/ml 5-CT (p = 0.001) with maximal stimulation from 10(-7) to 10(-3) mg/ml 5-CT (0.30 ± 0.03 and 0.18 ± 0.01 pmol/ml, respectively). Incubation with 10(-6) mg/ml SB269970 (a selective 5-HT7 antagonist) blocked 5-CT-induced cAMP increase in corneal epithelial (0.03 pmol/ml) and endothelial cells (0.02 pmol/ml; p = 0.001). Stimulation of corneal 5-HT7 receptors with 5-CT revealed a dose-dependent increase in PKA activity between 10(-10) and 10(-8) mg/ml 5-CT in corneal epithelium and endothelium (<1 to >99%; p = 0.013 and p = 0.017, respectively) with maximal stimulation from 10(-8) to 10(-4) mg/ml (>99%) 5-CT.

CONCLUSIONS

Our data demonstrate that native corneal epithelial and endothelial cells express a functional 5-HT7 receptor positively coupled to adenylyl cyclase and PKA formation. However, at the present time, the physiological role of 5-HT receptors and the cAMP-PKA pathway in the cornea remains a matter of speculation.

Genome-wide association study of theta band event-related oscillations identifies serotonin receptor gene HTR7 influencing risk of alcohol dependence

Event-related brain oscillations (EROs) represent highly heritable neuroelectrical correlates of human perception and cognitive performance that exhibit marked deficits in patients with various psychiatric disorders. We report the results of the first genome-wide association study (GWAS) of an ERO endophenotype-frontal theta ERO evoked by visual oddball targets during P300 response in 1,064 unrelated individuals drawn from a study of alcohol dependence. Forty-two SNPs of the Illumina HumanHap 1 M microarray were selected from the theta ERO GWAS for replication in family-based samples (N = 1,095), with four markers revealing nominally significant association. The most significant marker from the two-stage study is rs4907240 located within ARID protein 5A gene (ARID5A) on chromosome 2q11 (unadjusted, Fisher's combined P = 3.68 × 10⁻⁶). However, the most intriguing association to emerge is with rs7916403 in serotonin receptor gene HTR7 on chromosome 10q23 (combined P = 1.53 × 10⁻⁴), implicating the serotonergic system in the neurophysiological underpinnings of theta EROs. Moreover, promising SNPs were tested for association with diagnoses of alcohol dependence (DSM-IV), revealing a significant relationship with the HTR7 polymorphism among GWAS case-controls (P = 0.008). Significant recessive genetic effects were also detected for alcohol dependence in both case-control and family-based samples (P = 0.031 and 0.042, respectively), with the HTR7 risk allele corresponding to theta ERO reductions among homozygotes. These results suggest a role of the serotonergic system in the biological basis of alcohol dependence and underscore the utility of analyzing brain oscillations as a powerful approach to understanding complex genetic psychiatric disorders.

Studies on 1-arylpiperazine derivatives with affinity for rat 5-HT7 and 5-HT1A receptors

Several 1-aryl-4-(2-arylethyl)piperazine derivatives were synthesized and tested in-vitro for their binding affinity for 5-HT7 and 5-HT1A receptors. These compounds displayed 5-HT7 receptor affinity ranging between Ki = 474 nm and Ki = 8.2 nm, besides high affinity for the 5-HT1A receptor. Intrinsic activity of the most potent compounds was assessed. 4-[2-(3-Methoxyphenyl)ethyl]-1-(2-methoxyphenyl)piperazine (16) and 1-(1,2-benzisoxazol-3-yl)-4-[2-(3-methoxyphenyl)ethyl]piperazine (20) (Ki = 24.5 and 8.2 nm, respectively) behaved as partial agonist and full agonist, respectively, when tested for 5-HT7 receptor-mediated relaxation of substance P-induced guinea-pig ileum contraction.

A novel corticotropin-releasing factor receptor splice variant exhibits dominant negative activity: a putative link to stress-induced heart disease

A growing body of experimental and clinical studies supports a strong association between psychological stress and cardiovascular disease. An important endogenous cardioprotective role in heart physiology has been attributed to corticotropin-releasing factor receptor type 2beta (CRFR2beta). Here, we report the isolation of cDNA from mouse (m) heart encoding a novel CRFR2beta splice variant. Translation of this insertion variant (iv)-mCRFR2beta isoform produces a 421-aa protein that includes a unique C-terminal cytoplasmic tail. Our functional analysis and cellular localization studies demonstrated that when coexpressed with wild-type mCRFR2beta, iv-mCRFR2beta significantly inhibited the wild-type mCRFR2beta membrane expression and its functional signaling by ER-Golgi complex retention, suggesting a dose-dependent dominant negative effect. Interestingly, mice exposed to a 4-wk paradigm of chronic variable stress, a model of chronic psychological stress in humans, presented significantly lower levels of mCRFR2beta and higher levels of iv-mCRFR2beta mRNA expression in their hearts, compared to nonstressed control mice. The dominant-negative effect of iv-mCRFR2beta and its up-regulation by psychological stress suggest a new form of regulation of the mCRFR2beta cardioprotective effect and a potential role for this novel isoform in stress-induced heart disease.

Isolation and characterization of new exon 11-associated N-terminal splice variants of the human mu opioid receptor gene

Alternative splicing of the mu opioid receptor genes to create multiple mu receptor subtypes has been demonstrated in animals and humans. Previously, we identified a number of C-terminal variants in mice, rats and human, followed by several N-terminal variants associated with a new upstream exon in mice (exon 11). Behavioral studies in exon 11 knockout mice suggest an important role for the exon 11 variants in the analgesic actions of heroin and morphine-6beta-glucuronide, but not morphine or methadone. We now have identified a homologous human exon 11 and three similar human exon 11-associated variants, suggesting conservation of exon 11 and its associated variants across species. hMOR-1i has an additional 93 amino acids at the tip of the N-terminus but is otherwise identical to hMOR-1. When expressed in Chinese hamster ovary cells, the additional 93 amino acids in hMOR-1i had little effect on opioid binding, but significantly altered agonist-induced G-protein activation. hMOR-1G1 and hMOR-1G2 predicted six transmembrane domain variants, similar to those seen in mice. The regional expression of these exon 11-associated variants, as determined by RT-PCR, varied markedly, implying region-specific alternative splicing. The presence of exon 11-associated variants in humans raises questions regarding their potential role in heroin and morphine-6beta-glucuronide actions in people as they do in mice.

Serotonin pharmacology in the gastrointestinal tract: a review

Serotonin (5-hydroxytryptamine or 5-HT) plays a critical physiological role in the regulation of gastrointestinal (GI) function. 5-HT dysfunction may also be involved in the pathophysiology of a number of functional GI disorders, such as chronic constipation, irritable bowel syndrome and functional dyspepsia. This article describes the role of 5-HT in the enteric nervous system (ENS) of the mammalian GI tract and the receptors with which it interacts. Existing serotonergic therapies that have proven effective in the treatment of GI functional disorders and the potential of drugs currently in development are also highlighted. Advances in our understanding of the physiological and pathophysiological roles of 5-HT in the ENS and the identification of selective receptor ligands bodes well for the future development of more efficacious therapies for patients with functional GI disorders.

Molecular biology of 5-HT receptors

Serotonin (5-hydroxytryptamine; 5-HT) is a monoamine neurotransmitter whose effects are mediated by at least 13 distinct G protein-coupled receptors (GPCRs) of the type A family which includes the monoamine receptors and a combination of ligand-gated ion channels (5-HT3) of the Cys loop family which constitutes heteropentamers. 5-HT receptors are currently divided into seven classes (5-HT1 to 5-HT7), based on structural, transductional and operational features. While this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity is supported by the existence of a great number of splice and editing variants for several 5-HT receptors, their possible modulation by accessory proteins and chaperones, as well as their potential to form homo or heteromers both at the GPCR and at the ligand-gated channel level.

Selective 5-HT6 receptor ligands: progress in the development of a novel pharmacological approach to the treatment of obesity and related metabolic disorders

The increasing global prevalence of obesity unequivocally demonstrates that neither behavioural (diet and exercise) nor pharmacological approaches to this health problem are working. In this area of high unmet clinical need, the 5-HT6 receptor has generated enormous interest amongst academic and pharmaceutical industry scientists as a molecular target for the development of a new generation of safe and more effective anti-obesity drugs. In this review, we have described the major developments that have occurred in the fields of the medicinal chemistry and pharmacology of 5-HT6 ligands, with particular emphasis on their potential application as novel anti-obesity drugs. The last 5 years have witnessed an increasing understanding of the 5-HT6 receptor and its structural requirements that has produced an explosion in the number and diversity of novel, highly selective 5-HT6 receptor agonists, partial agonists and antagonists that have been designed and synthesized. In animal models, 5-HT6 receptor ligands of all functional types have been shown to decrease food intake when given acutely and chronically, to evoke profound and sustained weight-loss in obese animals, and concomitantly to improve a number of cardio-metabolic risk factors. Comparator studies in obese animal models, which are highly predictive of clinical outcomes, indicate that 5-HT6 ligands may have the potential to be more efficacious in the treatment of obesity than the current generation of anti-obesity drugs.

Differential profile of typical, atypical and third generation antipsychotics at human 5-HT7a receptors coupled to adenylyl cyclase: detection of agonist and inverse agonist properties

5-HT(7) receptors are present in thalamus and limbic structures, and a possible role of these receptors in the pathology of schizophrenia has been evoked. In this study, we examined binding affinity and agonist/antagonist/inverse agonist properties at these receptors of a large series of antipsychotics, i.e., typical, atypical, and third generation compounds preferentially targeting D(2) and 5-HT(1A) sites. Adenylyl cyclase (AC) activity was measured in HEK293 cells stably expressing the human (h) 5-HT(7a) receptor isoform. 5-HT and 5-CT increased cyclic adenosine monophosphate level by about 20-fold whereas (+)-8-OH-DPAT, the antidyskinetic agent sarizotan, and the novel antipsychotic compound bifeprunox exhibited partial agonist properties at h5-HT(7a) receptors stimulating AC. Other compounds antagonized 5-HT-induced AC activity with pK (B) values which correlated with their pK (i) as determined by competition binding vs [(3)H]5-CT. The selective 5-HT(7) receptor ligand, SB269970, was the most potent antagonist. For antipsychotic compounds, the following rank order of antagonism potency (pK (B)) was ziprasidone > tiospirone > SSR181507 > or = clozapine > or = olanzapine > SLV-314 > SLV-313 > or = aripiprazole > or = chlorpromazine > nemonapride > haloperidol. Interestingly, pretreatment of HEK293-h5-HT(7a) cells with forskolin enhanced basal AC activity and revealed inverse agonist properties for both typical and atypical antipsychotics as well as for aripiprazole. In contrast, other novel antipsychotics exhibited diverse 5-HT(7a) properties; SLV-313 and SLV-314 behaved as quasi-neutral antagonists, SSR181507 acted as an inverse agonist, and bifeprunox as a partial agonist, as mentioned above. In conclusion, the differential properties of third generation antipsychotics at 5-HT(7) receptors may influence their antipsychotic profile.

Bivalent ligand approach on 4-[2-(3-methoxyphenyl)ethyl]-1-(2-methoxyphenyl)piperazine: Synthesis and binding affinities for 5-HT7 and 5-HT1A receptors

We here report on the synthesis and binding properties at 5-HT(7) and 5-HT(1A) receptors of ligands 3-12, that were designed according to the 'bivalent ligand' approach. Two moieties of the 5-HT(7)/5-HT(1A) ligand 4-[2-(3-methoxyphenyl)ethyl]-1-(2-methoxyphenyl)piperazine (1) were linked through their 3-methoxy substituent by polymethylene chains of variable length, with the aim to increase the affinity for 5-HT(7) receptor and the selectivity over 5-HT(1A) receptors. In the best cases, the dimers showed affinities for 5-HT(7) receptors as high as the monomer with no improvement in selectivity. Some dimers displayed 5-HT(1A) receptor affinities slightly higher than monomer 1.

Structure−Activity Relationship Study on N-(1,2,3,4-Tetrahydronaphthalen-1-yl)-4-aryl-1-piperazinehexanamides, a Class of 5-HT7 Receptor Agents. 2

Here we report the synthesis of N-(1,2,3,4-tetrahydronaphthalen-1-yl)-4-aryl-1-piperazinealkylamides 16-29 that were designed to elucidate both structure-affinity and -activity relationships for the 5-HT7 receptor, by targeting the substituent in 2-position of the aryl linked to the piperazine ring. The affinities of 16-29 for 5-HT7, 5-HT1A, 5-HT2A, and D2 receptors were assessed by radioligand binding assays. The intrinsic activities at the 5-HT7 receptor of the most potent compounds were determined. A series of substituents covering a wide range of electronic, steric, and polar properties was evaluated, revealing a key role on 5-HT7 receptor affinity and intrinsic activity. Certain lipophilic substituents (SCH3, CH(CH3)2, N(CH3)2, CH3, Ph) led to high-affinity agonists, whereas OH and NHCH3 substituents switched intrinsic activity toward antagonism. 4-[2-(1-Methylethyl)phenyl]-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (19), 4-(2-diphenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (21), and 4-(2-dimethylaminophenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (22) were identified as potent 5-HT7 receptor agonists (Ki = 0.13-1.1 nM, EC50 = 0.90-1.77 microM), showing selectivity over 5-HT1A, 5-HT2A, and D2 receptors.

Reanalysis of constitutively active rat and human 5-HT7(a) receptors in HEK-293F cells demonstrates lack of silent properties for reported neutral antagonists

The present study reinvestigated a series of 5-HT receptor antagonists at both constitutively active rat and human 5-HT7(a) receptors in HEK-293F cells using the cAMP signalling pathway as a functional read-out. Both rat and human 5-HT7(a) receptors were expressed in similar amounts ([3H]-LSD binding: 1.0 to 1.1 pmol/mg protein). Attenuation of basal cAMP formation by the inverse agonist SB-691673 (1 microM) was slightly larger by the human 5-HT7(a) (-73+/-3 %) than rat 5-HT7(a) receptor (-62+/-3 %). The 5-HT receptor antagonists investigated here displayed systematically inverse agonism. While methiothepin and SB-269970 displayed similar negative intrinsic activity to SB-691673 at the rat 5-HT7(a) receptor, the compounds SB-258719, mesulergine and metergoline displayed some lower negative intrinsic activity (between -38 and -49%). Inverse agonist properties were observed with potencies fitting with their respective binding pIC50 values and pKB values as estimated from antagonist studies with 5-HT. With the exception of SB-258719 and mesulergine, which remained a partial inverse agonist at the human 5-HT7(a) receptor, the other compounds behaved with a similar Emax value to the full inverse agonist SB-691673. In conclusion, none of the 5-HT receptor antagonists investigated displayed silent properties at the rat or human 5-HT7(a) receptor, when these are expressed in a system allowing detection of constitutive activity. They appear to be partial to full inverse agonists, further illustrating that an antagonist is preferentially an inverse agonist when investigated under constitutively active receptor conditions.

5-Hydroxytryptamine receptors in the human cardiovascular system

The human cardiovascular system is exposed to plasma 5-hydroxytryptamine (5-HT, serotonin), usually released from platelets. 5-HT can produce harmful acute and chronic effects. The acute cardiac effects of 5-HT consist of tachycardia (preceded on occasion by a brief reflex bradycardia), increased atrial contractility and production of atrial arrhythmias. Acute inotropic, lusitropic and arrhythmic effects of 5-HT on human ventricle become conspicuous after inhibition of phosphodiesterase (PDE) activity. Human cardiostimulation is mediated through 5-HT4 receptors. Atrial and ventricular PDE3 activity exerts a protective role against potentially harmful cardiostimulation. Chronic exposure to high levels of 5-HT (from metastatic carcinoid tumours), the anorectic drug fenfluramine and its metabolites, as well as the ecstasy drug 3,4-methylenedioxymethamphetamine (MDMA) and its metabolite 3,4-methylenedioxyamphetamine (MDA) are associated with proliferative disease and thickening of cardiac valves, mediated through 5-HT2B receptors. 5-HT2B receptors have an obligatory physiological role in murine cardiac embryology but whether this happens in humans requires research. Congenital heart block (CHB) is, on occasion, associated with autoantibodies against 5-HT4 receptors. Acute vascular constriction by 5-HT is usually shared by 5-HT1B and 5-HT2A receptors, except in intracranial arteries which constrict only through 5-HT1B receptors. Both 5-HT1B and 5-HT2A receptors can mediate coronary artery spasm but only 5-HT1B receptors appear involved in coronary spasm of patients treated with triptans or with Prinzmetal angina. 5-HT2A receptors constrict the portal venous system including oesophageal collaterals in cirrhosis. Chronic exposure to 5-HT can contribute to pulmonary hypertension through activation of constrictor 5-HT1B receptors and proliferative 5-HT2B receptors, and possibly through direct intracellular effects.

Novel interaction between the human 5-HT7 receptor isoforms and PLAC-24/eIF3k

Three 5-HT(7) receptor isoforms are expressed in rat and man, which differ in the amino acid sequence of their C-terminus. Thus far, no changes have been observed in the pharmacological profile of all three isoforms. To further elucidate the signal transduction pathway specific for these receptor variants, we screened for possible interacting proteins of the C-terminus of the h5-HT(7(a)) variant in a human foetal brain cDNA library. Using a yeast two-hybrid assay, we isolated PLAC-24/eIF3k as a possible interacting candidate. The association of PLAC-24 with all three receptor variants was observed and further reconfirmed in vivo by co-immunoprecipitation of PLAC-24 with the full-length receptor isoforms in transfected COS-7 cells. Studies with different deletion mutants of the receptor showed that the interaction between PLAC-24 and the receptor is not restricted to the C-terminus of the receptor. PLAC-24/eIF3k consists of 3 domains: an N-terminal HAM domain, a central WH domain and a C-terminal tail. We generated different domain constructs of PLAC-24, which indicated that the HAM and WH domain both interact with the 5-HT(7(a)) receptor. Overexpression of PLAC-24 in HEK293 cells, stably expressing the h5-HT(7(a)) receptor, caused a threefold augmentation in the expression levels of the receptor. Co-localisation studies in COS-7 cells showed that PLAC-24 relocates from the nucleus and perinuclear sites towards the plasma membrane upon co-expression with the receptor. On the other hand, the expression of domain variants of PLAC-24 seems to block the translocation of the receptor towards the membrane. These observations suggest that PLAC-24 may play a role in the transport and the stabilisation of newly synthesised 5-HT(7) receptor towards the plasma membrane.

Monoamine receptors in human corneal epithelium and endothelium

PURPOSE

Monoamine receptors are found throughout the body. Reports about the presence of monoamine receptors in the human cornea are inconsistent.

METHODS

Immunohistochemistry, immunofluorescence and immunoblotting were used to localize monoamine receptor sites on human corneal epithelium and endothelium.

RESULTS

Antibodies to alpha-1, beta-1 and beta-2 adrenergic receptors and to D1-like and 5HT-7 receptors were bound in corneal epithelium. Antibodies to alpha-1, alpha-2A, beta-1 and beta-2 adrenergic receptors and to 5HT-7 receptors were bound in corneal endothelium.

CONCLUSIONS

Our data demonstrate the presence of several monoamine receptors in the human cornea. These receptors may play a role in the regulation of fluid transport or corneal homeostasis.

Glycogen metabolism in rat ependymal primary cultures: regulation by serotonin

Ependymal primary cultures are a model for studying ependymal energy metabolism. Intracellular glycogen is built up in the cultures dependent on culture age and the presence of glucose and glutamate. This energy store is mobilized upon glucose withdrawal, stimulation with isoproterenol, forskolin or serotonin and after uncoupling of oxidative phosphorylation from ATP production. Serotonin regulates ependymal glycogen metabolism predominantly via 5-HT receptor (5-HTR) 7, which elicits an increase in the level of ependymal cyclic AMP. Although the most abundant mRNAs for serotonin receptors are those of 5-HTR 2B and 5-HTR 3A, ependymal cells in primary culture do not respond to serotonin with an increase in their concentration of cytosolic calcium ions. The mRNAs of 5-HTRs 1A, 6, 1B, 5B, 7, 1/2C and 5A are also detectable in order of decreasing abundance. The mRNAs for 5-HTRs 1D, 1F, 3B and 4 are absent from the cultured cells. The ability of serotonin to mobilize ependymal glycogen depends on the culture age and the time allowed for glycogen buildup. During glycogen buildup time, glutamate is consumed by the cells. An increased ability of 5-HT to mobilize ependymal glycogen stores is noticed after the depletion of glutamate from the glycogen buildup medium. In ependymal primary cultures, cilia are colocalized with glycogen phosphorylase isozyme BB, while the MM isoform is not expressed. It is known from the literature that an increase in the concentration of cytosolic cAMP in ependymal cells leads to a decrease in ciliary beat frequency. Therefore, the present data point towards a function for ependymal glycogen other than supplying energy for the movement of cilia.

Paroxetine is effective in desensitizing 5-HT1A receptor function in adult offspring exposed prenatally to cocaine

RATIONALE

Desensitization of postsynaptic 5-HT(1A) receptors may be responsible for the therapeutic effectiveness of serotonin selective uptake inhibitors (SSRIs). As prenatal cocaine exposure produces long-term deficits in 5-HT neurons in offspring, it may alter the ability of postsynaptic 5-HT(1A) receptors to be desensitized by chronic paroxetine.

OBJECTIVES

The aim of the study is to determine (1) prenatal cocaine-induced changes in 5-HT(1A) receptor function and (2) the effectiveness of chronic treatment with paroxetine to produce 5-HT(1A) receptor desensitization in adult offspring exposed to cocaine in utero.

METHODS

Pregnant rats received saline or (-)cocaine (15 mg/kg, s.c.) twice daily from gestational days 13 through 20. Adult male offspring from each of prenatal groups were treated with saline or paroxetine (10 mg/kg/day; i.p.) for 14 days. Eighteen hours post-treatment, rats were challenged with saline or the 5-HT(1A) receptor agonist (+)8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.04 or 0.2 mg/kg, s.c.). Plasma oxytocin, adrenocorticotropic hormone (ACTH), corticosterone, renin and prolactin were determined.

RESULTS

Prenatal cocaine exposure did not alter 5-HT(1A) receptor-mediated neuroendocrine responses. Paroxetine treatment desensitized 5-HT(1A) receptor-mediated increases in oxytocin, ACTH and corticosterone to a comparable extent in all offspring and reduced the E(max) for ACTH only in prenatal cocaine-exposed offspring. Cortical [(3)H]-8-OH-DPAT- or [(3)H]-WAY100635-labeled 5-HT(1A) receptors were unaltered by prenatal cocaine or subsequent paroxetine treatment.

CONCLUSIONS

Postsynaptic 5-HT(1A) receptor function is unaltered by prenatal cocaine exposure and paroxetine can effectively desensitize 5-HT(1A) receptor function in adult cocaine-exposed offspring. These data suggest that paroxetine may be clinically effective in treating mood disorders in adults exposed in utero to cocaine.

Molecular analysis of the interaction between the intracellular loops of the human serotonin receptor type 6 (5-HT6) and the alpha subunit of GS protein

The serotonin type 6 (5-HT(6)) receptor is a G-protein coupled receptor (GPCR) coupled to a stimulatory G-protein (G(S)). To identify the structural basis for the interaction of the 5-HT(6) receptor with the G(S) protein, we have dissected the interaction between GST-fusion proteins containing the second intracellular loop (iL2), the third intracellular loop (iL3), or the C-terminal tail of the 5-HT(6) receptor and the alpha subunit of G(S) (Galpha(S)). The direct interaction of iL3 and Galpha(S) was demonstrated by co-immunoprecipitation. Furthermore, the kinetic parameters of the interaction between iL3 and Galpha(S) were measured by surface plasmon resonance, and the apparent dissociation constant was determined to be 0.9 x 10(-6)M. In contrast, the second intracellular loop and C-terminal tail regions showed negligible affinity to Galpha(S). The critical residues within the iL3 region for the interaction with Galpha(S) were identified as conserved positively charged residues near the C-terminus of iL3 by measuring the cellular levels of cAMP produced in response to 5-HT stimulation of cells transfected with 5-HT(6) receptor mutants.

Differential inverse agonist efficacies of SB-258719, SB-258741 and SB-269970 at human recombinant serotonin 5-HT7 receptors

Recombinant 5-hydroxytryptamine 5-HT7 receptors are known to express constitutive, i.e., agonist-independent activity. Nonselective ligands, like methiothepin, ritanserin or clozapine behave as full inverse agonists at 5-HT7 receptors. The aim of the present study was to evaluate the degree of inverse agonist activity of three selective 5-HT7 receptor antagonists ((R)-3,N-dimethyl-N-[1-methyl-3-(4-methyl-piperidin-1-yl)propyl]benzene sulfonamide or SB-258719, R-(+)-1-(toluene-3-sulfonyl)-2-[2-(4-methylpiperidin-1-yl)ethyl]-pyrrolidine or SB-258741 and (R)-3-(2-(2-(4-methylpiperidin-1-yl)ethyl)-pyrrolidine-1-sulfonyl)-phenol or SB-269970) in the same model. cAMP accumulation was measured in intact Chinese hamster ovary (CHO) cells expressing human recombinant 5-HT7a receptors. In these cells, 5-HT stimulated cAMP levels and a series of ligands antagonized the effect of 5-HT with a 5-HT7 receptor-like profile. SB-258719 had no inverse agonist activity, SB-258741 behaved as a partial inverse agonist and SB-269970 was a quasi-full inverse agonist (as compared to methiothepin). The inverse agonist effect of SB-269970 was antagonized in a concentration-dependent manner by SB-258719. The widespread spectrum of inverse agonist activities shown by these compounds should help assessing the physiological relevance of constitutive 5-HT7 receptor activity in native tissues.

Serotonin 5-HT7 receptors coupled to induction of interleukin-6 in human microglial MC-3 cells

Brain serotonin 5-HT(7) receptors are known to be expressed in neurons and astrocytes. We now report the presence of these receptors in a third type of cell, microglial cells. 5-Hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine (5-MeOT) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) induced concentration-dependent stimulations of cAMP accumulation in the human microglial MC-3 cell line. The maximal effect of 5-HT was 3.4+/-0.3-fold stimulation (mean+/-S.E.M., n=5) above basal levels. The rank order of agonist potency (pEC50 values) was 5-CT (7.09)>5-HT (6.13)>or=5-MeOT (5.78)>>8-OH-DPAT (ca. 5). The effect of 5-CT was inhibited in a concentration-dependent manner by the selective 5-HT7 receptor antagonist SB-269970 (pA2 value 9.03). Western blot analysis revealed the presence of immunoreactive bands corresponding to the human 5-HT7 receptor in extracts of MC-3 cells. The presence of two splice variants of the 5-HT7 receptor (5-HT7(a/b)) was visualized by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis with specific primers. In real-time PCR studies, the mRNA for interleukin-6 (IL-6) was found to be increased by 2.5-fold in MC-3 cells after 1 h incubation with 5-CT (1 microM) and this effect was fully blocked by the 5-HT7 receptor antagonist SB-269970 (1 microM). These data show that functional 5-HT7 receptors are present in human microglial MC-3 cells, suggesting that they are involved in neuroinflammatory processes.

Differential agonist-mediated internalization of the human 5-hydroxytryptamine 7 receptor isoforms

The human 5-hydroxytryptamine 7 (5-HT(7)) serotonin receptor is a class A G-protein coupled receptor that has three isoforms, 5-HT(7(a)), 5-HT(7(b)), and 5-HT(7(d)), which are produced by alternative splicing. The 5-HT(7) receptors are expressed in discrete areas of the brain and in both vascular and gastrointestinal smooth muscle. Central nervous system 5-HT(7) receptors may play a role in mood and sleep disorders. 5-HT(7) receptors show high affinity for a number of antidepressants and typical and atypical antipsychotics. We report here that the human 5-HT(7(d)) isoform expressed in human embryonic kidney (HEK) 293 cells exhibits a pattern of receptor trafficking in response to agonist that differ from 5-HT(7(a)) or 5-HT(7(b)) isoforms. We employed a modification of a live cell-labeling technique to demonstrate that surface 5-HT(7(d)) receptors are constitutively internalized in the absence of agonist. This is in contrast to 5-HT(7(a)) and 5-HT(7(b)) isoforms, which do not show this profound agonist-independent internalization. Indeed, the 5-HT(7(d)) isoform displays this internalization in the presence of a 5-HT(7) -specific antagonist. In addition, the human 5-HT(7) isoform shows a diminished efficacy in stimulation of cAMP-responsive reporter gene activity in transfected cells compared with 5-HT(7(a)) or 5-HT(7(b)) receptors expressed at comparable levels. Thus, the carboxy-terminal tail of 5-HT(7(d)), which is the longest among known human 5-HT(7) isoforms, may contain a motif that interacts with cellular transport mechanisms that is distinct from 5-HT(7(a)) and 5-HT(7(b)).

Novel 3-aminochromans as potential pharmacological tools for the serotonin 5-HT7 receptor

The synthesis of novel C6-aryl substituted derivatives of 3-(dimethylamino)chroman is described. The novel derivatives display 5-HT(7) receptor affinities that varies from nM to muM, indicating that this small set of derivatives constitute a novel and interesting starting point for further structure-serotonin 5-HT(7) activity relationship (SAR) studies.

Evidence for 5-HT2B and 5-HT7 receptor-mediated relaxation in pulmonary arteries of weaned pigs

This study characterizes the relaxant response to 5-hydroxytryptamine (5-HT) in prostaglandin F(2alpha) (PGF(2alpha))-precontracted pulmonary arteries of weaned pigs. In arterial rings with intact endothelium, the relaxation to 5-HT was biphasic. The high affinity component of relaxation to 5-HT (0.1-10 nM) was abolished by mechanical removal of the endothelium or after the addition of L: -NAME (200 microM), and was inhibited by the 5-HT(2B/2C) receptor antagonist SB 206553 (1 microM), but not the 5-HT(2C) receptor antagonist SB 242084 (0.1 microM). Endothelium-intact arteries were also relaxed by the selective 5-HT(2B) receptor agonist BW 723C86 (pD(2) 7.7). The relaxant response to BW 723C86 was inhibited by 1 microM SB 206553 (pK(B) 6.8). The low affinity component of relaxation to 5-HT (>/=30 nM) remained unaffected after mechanical removal of the endothelium or the addition of L: -NAME. In endothelium-denuded arterial rings, 5-HT, 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine (5-MeOT), and frovatriptan produced monophasic relaxations with pD(2) values of 6.5, 7.5, 5.9, and 4.7 respectively. Relaxant responses to the agonists were antagonized by the selective 5-HT(7) receptor antagonist SB 269970 (pK(B) 8.2-8.9). The relaxant response to the potent 5-HT(7) receptor agonist 5-CT was also antagonized by methiothepin (pK(B) 9.6), pimozide (pK(B) 8.2), mesulergine (pK(B) 7.7), methysergide (pK(B) 7.4), clozapine (pK(B) 7.6), and spiperone (pK(B) 7.4). The estimated pK(B) values argue in favor of an involvement of 5-HT(7) receptors in the direct vasorelaxant action of 5-HT in the pulmonary arteries of weaned pigs. The relaxant response to 5-CT was associated with an increase in cAMP that was surmountably antagonized by SB 269970 (pK(B) 8.6). The present in vitro bioassay can be used to characterize new drugs with potential agonist or antagonist properties at functional 5-HT(7) receptors.

Structure−Affinity Relationship Study onN-(1,2,3,4-Tetrahydronaphthalen-1-yl)-4-Aryl-1-Piperazinealkylamides, a New Class of 5-Hydroxytryptamine7Receptor Agents

A series of N-(1,2,3,4-tetrahydronaphthalen-1-yl)-4-aryl-1-piperazinealkylamides was prepared and their affinity for serotonin (5-hydroxytryptamine, 5-HT) 5-HT7, 5-HT(1A), and 5-HT(2A) receptors was measured by in vitro binding assays. In relation to 5-HT7 receptor affinity, receptor binding studies indicated that (i) the optimal alkyl chain length was five methylenes, (ii) an unsubstituted 1,2,3,4-tetrahydronaphthalenyl nucleus was preferred, and (iii) the substitution pattern of the aryl ring linked to the piperazine ring played a crucial role. Several compound with high affinity for 5-HT7 receptors were identified. Among them, 4-(2-methoxyphenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (28), 4-(2-acetylphenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (34), 4-(2-methylthiophenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (44), 4-(2-hydroxyphenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (46), and 4-(2-methylphenyl)-N-(1,2,3,4-tetrahydronaphthalen-1-yl)-1-piperazinehexanamide (49) were assayed for the 5-HT7 receptor-mediated relaxation of substance P-induced guinea pig ileum contraction. Compounds 28, 44, and 49 behaved as full agonists and compound 34 as a partial agonist, whereas derivative 46 acted as an antagonist. Among the compounds presented here, it emerged that 44 was identified as a potent 5-HT7 receptor agonist (Ki = 0.22 nM, EC50 = 2.56 microM), endowed with selectivity over 5-HT(1A) and 5-HT(2A) receptors (200-fold and >1000-fold, respectively).