5-ht6 receptors as emerging targets for drug discovery

Age-Related Changes in Central Nervous System 5-Hydroxytryptamine Signalling and Its Potential Effects on the Regulation of Lifespan

Serotonin or 5-hydroxytryptamine (5-HT) is an important neurotransmitter in the central nervous system and the periphery. Most 5-HT (~99%) is found in the periphery where it regulates the function of the gastrointestinal (GI) tract and is an important regulator of platelet aggregation. However, the remaining 1% that is found in the central nervous system (CNS) can regulate a range of physiological processes such as learning and memory formation, mood, food intake, sleep, temperature and pain perception. More recent work on the CNS of invertebrate model systems has shown that 5-HT can directly regulate lifespan.This chapter will focus on detailing how CNS 5-HT signalling is altered with increasing age and the potential consequences this has on its ability to regulate lifespan.

5-HT Receptors and the Development of New Antidepressants

Serotonin modulates several physiological and cognitive pathways throughout the human body that affect emotions, memory, sleep, and thermal regulation. The complex nature of the serotonergic system and interactions with other neurochemical systems indicate that the development of depression may be mediated by various pathomechanisms, the common denominator of which is undoubtedly the disturbed transmission in central 5-HT synapses. Therefore, the deliberate pharmacological modulation of serotonergic transmission in the brain seems to be one of the most appropriate strategies for the search for new antidepressants. As discussed in this review, the serotonergic system offers great potential for the development of new antidepressant therapies based on the combination of SERT inhibition with different pharmacological activity towards the 5-HT system. The aim of this article is to summarize the search for new antidepressants in recent years, focusing primarily on the possibility of benefiting from interactions with various 5-HT receptors in the pharmacotherapy of depression.

Time-Dependent Effects of Acute Handling on the Brain Monoamine System of the Salmonid Coregonus maraena

The immediate stress response involves the activation of the monoaminergic neurotransmitter systems including serotonin, dopamine and noradrenaline in particular areas of the fish brain. We chose maraena whitefish as a stress-sensitive salmonid species to investigate the influence of acute and chronic handling on the neurochemistry of monoamines in the brain. Plasma cortisol was quantified to assess the activation of the stress axis. In addition, we analyzed the expression of 37 genes related to the monoamine system to identify genes that could be used as markers of neurophysiological stress effects. Brain neurochemistry responded to a single handling (1 min netting and chasing) with increased serotonergic activity 3 h post-challenge. This was accompanied by a modulated expression of monoaminergic receptor genes in the hindbrain and a significant increase of plasma cortisol. The initial response was compensated by an increased monoamine synthesis at 24 h post-challenge, combined with the modulated expression of serotonin-receptor genes and plasma cortisol concentrations returning to control levels. After 10 days of repeated handling (1 min per day), we detected a slightly increased noradrenaline synthesis and a down-regulated expression of dopamine-receptor genes without effect on plasma cortisol levels. In conclusion, the changes in serotonergic neurochemistry and selected gene-expression profiles, together with the initial plasma cortisol variation, indicate an acute response and a subsequent recovery phase with signs of habituation after 10 days of daily exposure to handling. Based on the basal expression patterns of particular genes and their significant regulation upon handling conditions, we suggest a group of genes as potential biomarkers that indicate handling stress on the brain monoamine systems.

Acute EPA-induced learning and memory impairment in mice is prevented by DHA

Eicosapentaenoic acid (EPA), an omega-3 fatty acid, has been widely used to prevent cardiovascular disease (CVD) and treat brain diseases alone or in combination with docosahexaenoic acid (DHA). However, the impact of EPA and DHA supplementation on normal cognitive function and the molecular targets of EPA and DHA are still unknown. We show that acute administration of EPA impairs learning and memory and hippocampal LTP in adult and prepubescent mice. Similar deficits are duplicated by endogenously elevating EPA in the hippocampus in the transgenic fat-1 mouse. Furthermore, the damaging effects of EPA are mediated through enhancing GABAergic transmission via the 5-HT₆R. Interestingly, DHA can prevent EPA-induced impairments at a ratio of EPA to DHA similar to that in marine fish oil via the 5-HT_2CR. We conclude that EPA exhibits an unexpected detrimental impact on cognitive functions, suggesting that caution must be exercised in omega-3 fatty acid supplementation and the combination of EPA and DHA at a natural ratio is critical for learning and memory and synaptic plasticity.

Acute administration of EPA impairs learning and memory and hippocampal LTP in mice that was mediated through enhancing GABAergic transmission via the 5-HT₆R. DHA can prevent EPA-induced impairments at a ratio of EPA to DHA similar to that in marine fish oil via the 5-HT_2CR.

Development of New Methods Needs Proper Evaluation-Benchmarking Sets for Machine Learning Experiments for Class A GPCRs

New computational approaches for virtual screening applications are constantly being developed. However, before a particular tool is used to search for new active compounds, its effectiveness in the type of task must be examined. In this study, we conducted a detailed analysis of various aspects of preparation of respective data sets for such an evaluation. We propose a protocol for fetching data from the ChEMBL database, examine various compound representations in terms of the possible bias resulting from the way they are generated, and define a new metric for comparing the structural similarity of compounds, which is in line with chemical intuition. The newly developed method is also used for the evaluation of various approaches for division of the data set into training and test set parts, which are also examined in detail in terms of being the source of possible results bias. Finally, machine learning methods are applied in cross-validation studies of data sets constructed within the paper, constituting benchmarks for the assessment of computational methods developed for virtual screening tasks. Additionally, analogous data sets for class A G protein-coupled receptors (100 targets with the highest number of records) were prepared. They are available at http://gmum.net/benchmarks/ , together with script enabling reproduction of all results available at https://github.com/lesniak43/ananas .

Dysregulation of 5-hydroxytryptamine 6 receptor accelerates maturation of bone-resorbing osteoclasts and induces bone loss

Rationale: Characterizing the regulation of bone-resorbing osteoclasts is central to the understanding of the pathogenesis and treatment of bone diseases, such as osteoporosis and periodontitis. 5-hydroxytryptamine (5-HT) has drawn considerable attention for its role in bone; however, it remains unknown whether the intracellular signaling of 5-HT receptors (5-HTRs) is linked to any of the regulatory mechanisms in osteoclasts. Herein, we report 5-HT₆R to be a key regulatory receptor for osteoclastogenesis.

Methods: In order to explore the critical role of 5-HT₆R in bone-resorbing osteoclasts, in vitro experiments were performed using mouse whole bone marrow cells isolated from femora and tibiae and In vivo animal experiments were performed using 5-HT₆R-deficient (5-HT₆R^KO-/-) mice, bone resorption mice model, and osteoporosis mice model.

Results: Compared to other 5HTRs, activation of 5-HT₆R relatively increased TRAP (tartrate-resistant acid phosphatase) activity during osteoclastogenesis. 5-HT₆R^KO(-/-) mice and 5-HT₆R^KO(-/-) osteoclast lineages presented with an abnormal phenotype and impaired osteoclastogenesis and impaired osteoclastogenesis. Activation of 5-HT₆R increased the number of TRAP-positive multinuclear osteoclasts, actin ring formation, and expression of early osteoclast markers with osteoclast lineage commitment. Intracellular 5-HT₆R signaling was found to be linked to RhoA GTPase activation and was involved in the maturation of osteoclasts. This signaling pathway also showed enhanced bone destruction after lipopolysaccharide (LPS) administration in mice. Furthermore, inhibition of 5-HT₆R-mediated RhoA GTPase signaling protected against ovariectomy(OVX)-induced bone loss in mice.

Conclusion: Taken together, our findings place the 5-HT₆R system in a new context of osteoclast lineages in both an in vitro and in vivo system, and also it may offer a novel molecular target for the treatment of bone diseases.

Marine Inspired 2-(5-Halo-1H-indol-3-yl)-N,N-dimethylethanamines as Modulators of Serotonin Receptors: An Example Illustrating the Power of Bromine as Part of the Uniquely Marine Chemical Space

In previous studies, we have isolated several marine indole alkaloids and evaluated them in the forced swim test (FST) and locomotor activity test, revealing their potential as antidepressant and sedative drug leads. Amongst the reported metabolites to display such activities was 5-bromo-N,N-dimethyltryptamine. Owing to the importance of the judicious introduction of halogens into drug candidates, we synthesized two series built on a 2-(1H-indol-3-yl)-N,N-dimethylethanamine scaffold with different halogen substitutions. The synthesized compounds were evaluated for their in vitro and in vivo antidepressant and sedative activities using the mouse forced swim and locomotor activity tests. Receptor binding studies of these compounds to serotonin (5-HT) receptors were conducted. Amongst the prepared compounds, 2-(1H-indol-3-yl)-N,N-dimethyl-2-oxoacetamide (1a), 2-(5-bromo-1H-indol-3-yl)-N,N-dimethyl-2-oxoacetamide (1d), 2-(1H-indol-3-yl)-N,N-dimethylethanamine (2a), 2-(5-chloro-1H-indol-3-yl)-N,N-dimethylethanamine (2c), 2-(5-bromo-1H-indol-3-yl)-N,N-dimethylethanamine (2d), and 2-(5-iodo-1H-indol-3-yl)-N,N-dimethylethanamine (2e) have been shown to possess significant antidepressant-like action, while compounds 2c, 2d, and 2e exhibited potent sedative activity. Compounds 2a, 2c, 2d, and 2e showed nanomolar affinities to serotonin receptors 5-HT_1A and 5-HT₇. The in vitro data indicates that the antidepressant action exerted by these compounds in vivo is mediated, at least in part, via interaction with serotonin receptors. The data presented here shows the valuable role that bromine plays in providing novel chemical space and electrostatic interactions. Bromine is ubiquitous in the marine environment and a common element of marine natural products.

Computational approaches to the design of novel 5-HT6 R ligands

5-Hydroxytryptamine (5-HT, serotonin) subtype 6 receptor (5-HT6 receptor, 5-HT6 R) belongs to a 5-HT subclass of a relatively wide G protein-coupled receptor (GPCR) family. Accumulated biological data indicate that 5-HT6 R antagonists and agonists have a great potential for the treatment of neuropathological disorders, such as Parkinson's disease, Alzheimer's disease, and schizophrenia. A number of painstaking efforts have been made toward the design of novel 5-HT6 R ligands; however, there are still no drugs that successfully passed all the clinical trials and entered the market, except for several multimodal ligands. Novel active molecules are strongly needed to progress this development forward. The in silico drug design has some benefits compared with the other rough approaches in terms of thoroughness and predictive accuracy; therefore, it can be effectively used as a solid foundation for the design of novel 5-HT6 R ligands with high potency and selectivity. Here, we provide an overview of the reported computational approaches to the design of novel 5-HT6 R ligands.

Serotonin receptor of type 6 (5-HT6) in human prefrontal cortex and hippocampus post-mortem: an immunohistochemical and immunofluorescence study

Given the paucity of data on the distribution of serotonin (5-HT) receptors of type 6 (5-HT(6)) in the human brain, the aim of this study was to investigate their distribution in postmortem human prefrontal cortex, striatum and hippocampus by either immunohistochemical or immunofluorescence techniques. The brain samples were obtained from 6 subjects who had died for causes not involving primarily or secondarily the CNS. The 5-HT(6) receptor distribution was explored by the [(125)I]SB-258585 binding to brain membranes followed by immunohistochemical and immunofluorescence evaluations. A specific [(125)I]SB-258585 binding was detected in all the regions under investigation, whilst the content in the hippocampus and cortex being about 10-30 times lower than in the striatum. Immunohistochemistry and double-label immunofluorescence microscopy experiments, carried out in the prefrontal cortex and hippocampus only, since data in the striatum were already published, showed the presence of 5-HT(6) receptors in both pyramidal and glial cells of prefrontal cortex, while positive cells were mainly pyramidal neurons in the hippocampus. The heterogeneous distribution of 5-HT(6) receptors provides a preliminary explanation of how they might regulate different functions in different brain areas, such as, perhaps, brain trophism in the cortex and neuronal firing in the hippocampus. This study, taking into account all the limitations due to the postmortem model used, represents the starting point to explore the 5-HT(6) receptor functionality and its sub-cellular distribution.

Modulation of midbrain dopamine neurotransmission by serotonin, a versatile interaction between neurotransmitters and significance for antipsychotic drug action

Schizophrenia has been associated with a dysfunction of brain dopamine (DA). This, so called, DA hypothesis has been refined as new insights into the pathophysiology of schizophrenia have emerged. Currently, dysfunction of prefrontocortical glutamatergic and GABAergic projections and dysfunction of serotonin (5-HT) systems are also thought to play a role in the pathophysiology of schizophrenia. Refinements of the DA hypothesis have lead to the emergence of new pharmacological targets for antipsychotic drug development. It was shown that effective antipsychotic drugs with a low liability for inducing extra-pyramidal side-effects have affinities for a range of neurotransmitter receptors in addition to DA receptors, suggesting that a combination of neurotransmitter receptor affinities may be favorable for treatment outcome.This review focuses on the interaction between DA and 5-HT, as most antipsychotics display affinity for 5-HT receptors. We will discuss DA/5-HT interactions at the level of receptors and G protein-coupled potassium channels and consequences for induction of depolarization blockade with specific attention to DA neurons in the ventral tegmental area (VTA) and the substantia nigra zona compacta (SN), neurons implicated in treatment efficacy and the side-effects of schizophrenia, respectively. Moreover, it has been reported that electrophysiological interactions between DA and 5-HT show subtle, but important, differences between the SN and the VTA which could explain (in part) the effectiveness and lower propensity to induce side-effects of the newer atypical antipsychotic drugs. In that respect the functional implications of DA/5-HT interactions for schizophrenia will be discussed.

Physical interaction of Jab1 with human serotonin 6 G-protein-coupled receptor and their possible roles in cell survival

The 5-HT(6) receptor (5-HT(6)R) is one of the most recently cloned serotonin receptors, and it plays important roles in Alzheimer disease, depression, and learning and memory disorders. However, unlike the other serotonin receptors, the cellular mechanisms of 5-HT(6)R are poorly elucidated relative to its significance in human brain diseases. Here, using a yeast two-hybrid assay, we found that the human 5-HT(6)R interacts with Jun activation domain-binding protein-1 (Jab1). We also confirmed a physical interaction between 5-HT(6)R and Jab1 using glutathione S-transferase pulldown, fluorescence resonance energy transfer, co-immunoprecipitation, and immunocyto(histo)chemistry assays. The manipulation of Jab1 expression using Jab1 small interference RNA decreased 5-HT(6)R-mediated activity and cell membrane expression of 5-HT(6)R, whereas overexpression of Jab1 produced no significant effect. In addition, we demonstrated that the activation of 5-HT(6)R induced the translocation of Jab1 into the nucleus and increased c-Jun phosphorylation and the interaction between Jab1 and c-Jun. Furthermore, we found that 5-HT(6)R and Jab1 were up-regulated in middle cerebral artery occlusion-induced focal cerebral ischemic rats and in cultured cells exposed to hypoxic insults, suggesting possible protective roles for 5-HT(6)R and Jab1. These findings suggest that Jab1 provides a novel signal transduction pathway for 5-HT(6)R and may play an important role in 5-HT(6)R-mediated behavior changes in the brain.

An update on the role of the 5-hydroxytryptamine6 receptor in cognitive function

As the 5-hydroxytryptamine(6) (5-HT(6)) receptor is almost exclusively expressed in the CNS, particularly in areas associated with learning and memory, many studies have examined its role in cognitive function in the rodent, as reviewed herein. Most studies, in healthy adult rats, report that 5-HT(6) receptor antagonists enhance retention of spatial learning in the Morris water maze, improve consolidation in autoshaping tasks and reverse natural forgetting in object recognition. Antagonists appear to facilitate both cholinergic and glutamatergic neurotransmission, reversing scopolamine- and NMDA receptor antagonist-induced memory impairments. Recent reports show that the 5-HT(6) receptor antagonist, PRX-07034, restores the impairment of novel object recognition produced in rats reared in social isolation, a neurodevelopmental model producing behavioural changes similar to several core symptoms seen in schizophrenia. The 5-HT(6) receptor antagonist, Ro 04-6790, modestly improved reversal learning in isolation reared but not group-housed controls in the water maze. Ro 04-6790 also improved novel object discrimination both in adult rats that received chronic intermittent phencyclidine and drug-naïve 18-month-old rats. However, more information on their effect in animal models of schizophrenia and Alzheimer's disease is required. Several selective high-affinity 5-HT(6) receptor agonists developed recently also improve object discrimination and extra-dimensional set-shifting behaviour. Thus both 5-HT(6) receptor agonist and antagonist compounds show promise as pro-cognitive agents in pre-clinical studies but the explanation for their paradoxical analogous effect is currently unclear, and is discussed in this article.

Pro-cognitive effects of 5-HT6 receptor antagonists in the social recognition procedure in rats: implication of the frontal cortex

RATIONALE

5-HT6 receptor antagonists improve cognitive processes in rodents. However, their site(s) of action remains unexplored and their influence upon social memory has been little investigated.

OBJECTIVES

We examined the influence of 5-HT6 receptor ligands upon social memory in rats by use of systemic or local administration into the frontal cortex (FCX), striatum, or nucleus basalis magnocellularis (NBM).

MATERIALS AND METHODS

The social recognition test is based upon the ability of an adult rat to recognize a younger conspecific during the second of two 5-min sessions. In a procedure without an inter-session interval, the actions of drugs alone and the ability to reverse "amnesia" induced by the muscarinic antagonist, scopolamine (1.25 mg/kg, s.c.), were examined. The potential promnesic effect of drugs was also investigated in another procedure where a spontaneous deficit of recognition was induced by a 120-min inter-session interval.

RESULTS

The 5-HT6 receptor agonist, WAY-181187 (10.0 mg/kg, i.p.), significantly impaired social recognition. This effect was abolished by the 5-HT6 receptor antagonists, SB-271046 (20.0 mg/kg, i.p.) and SB-258585 (10.0 mg/kg, i.p.). These agents also abolished scopolamine-induced amnesia (10.0 and 2.5 mg/kg, i.p., respectively) and reversed the delay-induced deficit (10.0-20.0 and 2.5-10.0 mg/kg, i.p., respectively). WAY-181187 into the FCX significantly impaired social recognition (0.16-0.63 microg/side). Conversely, SB-271046 into the FCX (2.5-5.0 microg/side), but neither into the striatum nor the NBM, significantly reversed spontaneous deficit.

CONCLUSION

These results indicate that 5-HT6 receptors modulate social recognition by actions in the FCX and underpin their pertinence as targets for the treatment of psychiatric disorders in which cognitive function is compromised.

The synthesis and biological activity of pentafluorosulfanyl analogs of fluoxetine, fenfluramine, and norfenfluramine

The trifluoromethyl group of fluoxetine 1 and fenfluramine and norfenfluramine, 2 and 3, was substituted by the pentafluorosulfanyl group. On examination of the efficacy of the pentafluorosulfanyl containing compounds as inhibitors of 5-hydroxytryptamine receptors, it was found that substitution could lead to enhanced selectivity and in the case of the pentafluorosulfanyl analog of fenfluramine, 18, it significantly enhanced potency against the 5-HT(2b), 5-HT(2c), and 5-HT(6) receptors.

Endogenous serotonin excites striatal cholinergic interneurons via the activation of 5-HT 2C, 5-HT6, and 5-HT7 serotonin receptors: implications for extrapyramidal side effects of serotonin reuptake inhibitors

The striatum is richly innervated by serotonergic afferents from the raphe nucleus. We explored the effects of this input on striatal cholinergic interneurons from rat brain slices, by means of both conventional intracellular and whole-cell patch-clamp recordings. Bath-applied serotonin (5-HT, 3-300 microM), induced a dose-dependent membrane depolarization and increased the rate of spiking. This effect was mimicked by the 5-HT reuptake blockers citalopram and fluvoxamine. In voltage-clamped neurons, 5-HT induced an inward current, whose reversal potential was close to the K(+) equilibrium potential. Accordingly, the involvement of K(+) channels was confirmed either by increasing extracellular K(+) concentration and by blockade of K(+) channels with barium. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) profiling demonstrated the presence of 5-HT2C, 5-HT6, and 5-HT7 receptor mRNAs in identified cholinergic interneurons. The depolarization/inward current induced by 5-HT was partially mimicked by the 5-HT2 receptor agonist 2,5-dimethoxy-4-iodoamphetamine and antagonized by both ketanserin and the selective 5-HT2C antagonist RS102221, whereas the selective 5-HT3 and 5-HT4 receptor antagonists tropisetron and RS23597-190 had no effect. The depolarizing response to 5-HT was also reduced by the selective 5-HT6 and 5-HT7 receptor antagonists SB258585 and SB269970, respectively, and mimicked by the 5-HT7 agonist, 5-CT. Accordingly, activation of either 5-HT6 or 5-HT7 receptor induced an inward current. The 5-HT response was attenuated by U73122, blocker of phospholipase C, and by SQ22,536, an inhibitor of adenylyl cyclase. These results suggest that 5-HT released by serotonergic fibers originating in the raphe nuclei has a potent excitatory effect on striatal cholinergic interneurons.

Social isolation and expression of serotonergic neurotransmission-related genes in several brain areas of male mice

Early-life events influence brain development and evoke long-lasting behavioral consequences. Postweaning social isolation in rodents induces emotional and neurochemical alterations similar to those observed among some human psychopathologies. Central serotonergic neurotransmission is intimately involved in the observed adjustments, but the impact of social deprivation on serotonergic gene expression is unknown. We investigated the effects of prolonged early social isolation on emotion-related behaviors and 5-hydroxytryptamine (5-HT)-related gene transcription in mice. After weaning, male C57BL/6J mice were reared singly or in groups of four for 6 weeks. Gene expression of 5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT(2C), 5-HT(3A), 5-HT(6) and 5-HT(7) receptors and of 5-HT transporter and tryptophan hydroxylase-2 was determined by quantitative real-time polymerase chain reaction in distinct brain areas. Single-housed mice were hyperactive in a novel environment and showed signs of aggressive behavior. Housing condition did not alter weight gain or body temperature. Isolation markedly reduced transcription of all postsynaptic 5-HT receptors in the prefrontal cortex and reduced 5-HT(1B), 5-HT(2A) and 5-HT(2C) in both hypothalamus and midbrain. In contrast, the only alteration in the hippocampus was 5-HT(6) overexpression. Neither 5-HT transporter nor synthetic enzyme gene transcription differed between housing conditions. In conclusion, early social isolation in mice induces robust changes in postsynaptic 5-HT receptors gene transcription, motor hyperactivity and behavioral disinhibition. The overall pattern of decreased gene expression in the prefrontal cortex highlights its high vulnerability to environment. Furthermore, this is the first study to present a general representation of 5-HT-related gene expression in specific brain areas after social isolation and identifies novel candidates that may be critical for underlying molecular mechanisms.

The effects of pharmacological blockade of the 5-HT(6) receptor on formalin-evoked nociceptive behaviour, locomotor activity and hypothalamo-pituitary-adrenal axis activity in rats

5-hydroxytryptamine (5-HT) mediates behavioural and neuroendocrine responses to noxious or stressful stimuli. 5-HT(6) receptors are expressed in brain regions involved in nociceptive processing, however, their role in nociception is unknown. Here we demonstrate that acute, systemic administration of the 5-HT(6) receptor antagonist, 5-chloro-N-(4-methoxy-3-benzothio-phenesulfonamide (SB-271046), reduces formalin-evoked nociceptive behaviour and increases plasma corticosterone. SB-271046 dose-dependently reduced pre-formalin distance moved, rearing, grooming and defecation. These data provide the first evidence for 5-HT(6) receptor-mediated regulation of nociception and hypothalamo-pituitary-adrenal axis activity in a model of persistent pain although effects on locomotor activity demand that the putative antinociceptive effect of SB-271046 be interpreted with some caution.

Memory consolidation and amnesia modify 5-HT6 receptors expression in rat brain: An autoradiographic study

Traditionally, the search for memory circuits has been centered on examinations of amnesic and AD patients, cerebral lesions and, neuroimaging. A complementary alternative might be the use of autoradiography with radioligands. Indeed, ex vivo autoradiographic studies offer the advantage to detect functionally active receptors altered by pharmacological tools and memory formation. Hence, herein the 5-HT(6) receptor antagonist SB-399885 and the amnesic drugs scopolamine or dizocilpine were used to manipulate memory consolidation and 5-HT(6) receptors expression was determined by using [(3)H]-SB-258585. Thus, memory consolidation was impaired in scopolamine and dizocilpine treated groups relative to control vehicle but improved it in SB-399885-treated animals. SB-399885 improved memory consolidation seems to be associated with decreased 5-HT(6) receptors expression in 15 out 17 brain areas. Scopolamine or dizocilpine decreased 5-HT(6) receptors expression in nine different brain areas and increased it in CA3 hippocampus or other eight areas, respectively. In brain areas thought to be in charge of procedural memory such basal ganglia (i.e., nucleus accumbens, caudate putamen, and fundus striate) data showed that relative to control animals amnesic groups showed diminished (scopolamine) or augmented (dizocilpine) 5-HT(6) receptor expression. SB-399885 showing improved memory displayed an intermediate expression in these same brain regions. A similar intermediate expression occurs with regard to amygdala, septum, and some cortical areas in charge of explicit memory storage. However, relative to control group amnesic and SB-399885 rats in the hippocampus, region where explicit memory is formed, showed a complex 5-HT(6) receptors expression. In conclusion, these results indicate neural circuits underlying the effects of 5-HT(6) receptor antagonists in autoshaping task and offer some general clues about cognitive processes in general.

No evidence for binding of clozapine, olanzapine and/or haloperidol to selected receptors involved in body weight regulation

The underlying mechanisms of antipsychotic (AP)-induced weight gain are unknown, but both central and peripheral AP target receptors could potentially be involved. This study used radioligand binding assays to compare the binding affinities of clozapine, olanzapine and haloperidol for candidate receptors potentially involved in AP-induced weight gain. Selected candidates derived from known pathways involved in body weight regulation included receptors classified as anorexigenic (bombesin receptor subtype 3, calcitonin gene-related peptide receptor, cholecystokinin receptor, melanocortin-4 receptor, neurotensin receptor 1) or orexigenic (cannabinoid receptor 1, galanin 1 receptor, melanin-concentrating hormone receptor (MCHR), neuropeptide Y1 receptor) as well as receptors involved in physiological actions related to digestion and fluid homeostasis (angiotensin II type 1 receptor, bradykinin B2 receptor, endothelin receptor, neurokinin 1 receptor, vasoactive intestinal polypeptide receptor 1). Clozapine, olanzapine and haloperidol exhibited negligible affinities to all of these receptors except for the MCHR (Ki=501 nM; haloperidol). With respect to other candidates from (neuro)transmitter systems already suggested to be involved in AP-induced weight gain, the binding profile of olanzapine resembled that of clozapine, with high affinity (Ki<10 nM) for serotonin (5-HT) 5-HT2A, 5-HT2C and 5-HT6, muscarinic M1 and histamine H1 receptors. In contrast, the binding profile of haloperidol was substantially different (high affinity only for the dopamine D1 receptor). In conclusion, we have not identified a novel binding site of the two investigated atypical AP that could contribute to the induced weight gain.

A null mutation of the serotonin 6 receptor alters acute responses to ethanol

The anatomical distribution and pharmacology of serotonin 6 receptors (5-HT6Rs) implicate them as contributors to the serotonergic regulation of complex behavior. To complement the limited range of pharmacological tools available to examine 5-HT6R function, we have generated a mouse line bearing a constitutive null mutation of the 5-HT6R gene. No perturbations of baseline behavior were noted in a wide array of assays pertinent to multiple neurobehavioral processes. However, 5-HT6R mutant mice demonstrated reduced responses to the ataxic and sedative effects of ethanol. No differences in ethanol metabolism were evident between wild-type and 5-HT6R mutant mice. These findings implicate 5-HT6Rs in the serotonergic modulation of responses to ethanol.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Serotonin 5-HT2A and 5-HT6 receptors in the prefrontal cortex of Alzheimer and normal aging patients

BACKGROUND

It has been hypothesized that alterations of the serotonergic system contribute to neuropsychiatric symptoms in Alzheimer disease (AD). Cellular expressions of the two serotonergic receptors 5-HT2A and 5-HT6 have therefore been determined by immunohistochemistry in the prefrontal cortex of patients with AD (n=6) and normal age-matched controls (n = 7).

RESULTS

In normal aging patients, 5-HT2A label was mainly observed in large pyramidal cells, but to a lesser extent also in small pyramidal cells and in stellate cells of cortical layers II-VI. In AD, a similar distribution was observed, but density of positive cells was significantly reduced by 33%. In aging control patients, the 5-HT6 receptor was expressed by pyramidal cells and occasional stellate cells, not only of layers II-V, but also of layer I, where a distinct label was observed in neurons and surrounding fibers. 5-HT6 receptor expression in AD patients had the same pattern, but was significantly decreased by 40%.

CONCLUSION

Our results indicate that a decline in neurons expressing 5-HT2A, but also 5-HT6 receptors may play a role in the etiopathology of neuropsychiatric symptoms in AD.

5-HT6 receptors: a novel target for cognitive enhancement

Over the past decade, there has been increasing interest in the role of serotonin 6 (5-HT6) receptors in higher cognitive processes such as memory. Polymorphisms of the 5-HT6 receptor have been implicated in syndromes that affect cognition, such as schizophrenia and dementia. Manipulation of 5-HT6 receptor activity alters the transmission of several neurotransmitters important in memory: acetylcholine and glutamate, as well as dopamine, ã-aminobutyric acid (GABA), epinephrine (E), and norepinephrine (NE). Several 5-HT6 antagonists have been developed, advancing the understanding of the relationship between 5-HT6 blockade and memory consolidation in diverse learning paradigms. There is also evidence that 5-HT6 receptor activity affects anxiety behaviors and may be involved in the pathophysiology of schizophrenia. Several clinically useful atypical antipsychotics and antidepressants have 5-HT6 affinity, but recently developed selective 5-HT6 receptor antagonists may present attractive, new therapeutic options for several types of disease states.

Binding of isotryptamines and indenes at h5-HT6 serotonin receptors

To determine if the indolic nitrogen atom is required for the binding of N(1)-benzyltryptamines at h5-HT(6) serotonin receptors, several isotryptamines and indene analogs were examined. The affinity of 3-benzyl-N(1)-(N,N-dimethylaminoethyl)indole (5, K(i)=32nM) and 1-benzyl-3-(N,N-dimethylaminoethyl)indene (11, K(i)=3nM) indicates that the indolic nitrogen atom is not essential for binding.

A Three-Dimensional Pharmacophore Model for 5-Hydroxytryptamine6 (5-HT6) Receptor Antagonists

Forty-five structurally diverse 5-hydroxytryptamine(6) receptor (5-HT(6)R) antagonists were selected to develop a 3D pharmacophore model with the Catalyst software. The structural features for antagonism at this receptor are a positive ionizable atom interacting with Asp(3.32), a hydrogen bond acceptor group interacting with Ser(5.43) and Asn(6.55), a hydrophobic site interacting with residues in a hydrophobic pocket between transmembranes 3, 4, and 5, and an aromatic-ring hydrophobic site interacting with Phe(6.52).

Inhibition of perforant path input to the CA1 region by serotonin and noradrenaline

Bath-applied monoamines-dopamine (DA), serotonin (5-HT), and noradrenaline (NE)-strongly suppress the perforant path (PP) input to CA1 hippocampal region with very little effect on the Schaffer collaterals (SC) input. The effect of DA action on PP field excitatory postsynaptic potential (fEPSP) has been characterized in detail, but relatively little is known about the NE and 5-HT effects. Here we show that the maximal inhibition of the PP fEPSP by NE is approximately 55%, whereas 5-HT inhibition is weaker ( approximately 35%). The half-maximal inhibitory concentration of both 5-HT and NE is approximately 1 muM. Neither NE nor 5-HT affected paired-pulse facilitation, suggesting that the effect is not presynaptic. This is in contrast to DA, which does have a presynaptic effect. The NE effect was blocked by alpha2 antagonists, whereas the alpha1 antagonist corynanthine and beta-antagonist propranolol were ineffective. The effect of 5-HT was mimicked by the agonist, 5-carboxamidotryptamine maleate (5-CT), and not affected by adrenergic and dopaminergic antagonists. To determine the 5-HT receptors involved, we tested a number of 5-HT antagonists, but none produced a complete suppression of the 5-HT effect. Of these, only the 5-HT7 and 5-HT2 antagonists produced weak but significant inhibition of 5-HT effect. We conclude that NE inhibits the PP fEPSP through postsynaptic action on alpha2-adrenoceptors and that 5-HT7, 5-HT2, and some other receptor may be involved in 5-HT action in PP.

Differential involvement of 5-HT(1B/1D) and 5-HT6 receptors in cognitive and non-cognitive symptoms in Alzheimer's disease

Growing evidence suggests that a compromised serotonergic system plays an important role in the pathophysiology of Alzheimer's disease (AD). We assessed the expression of 5-HT(1B/1D) and 5-HT(6) receptors and cholinacetyltransferase (ChAT) activity in post-mortem frontal and temporal cortex from AD patients who had been prospectively assessed for cognitive function using the Mini-Mental State Examination (MMSE) and behavioral changes using the Present Behavioral Examination (PBE). 5-HT(1B/1D) and 5-HT(6) receptor densities were significantly reduced in both cortical areas. 5-HT(1B/1D) receptor density was correlated to MMSE decline in the frontal cortex, supporting its implication in memory impairment. The best predictor for lowered 5-HT(6) receptor density in the temporal cortex was the PBE measure of overactivity. The 5-HT(6)/ChAT ratio was related to aggression both in the frontal and temporal cortex. Therefore, antagonists acting at 5-HT(6) receptors could be useful in the treatment of non-cognitive symptoms associated to AD.

Effect of the acute and chronic administration of the selective 5-HT6 receptor antagonist SB-271046 on the activity of midbrain dopamine neurons in rats: An in vivo electrophysiological study

This study examined the effect of the acute and repeated per os (p.o.) administration of the selective 5-HT(6) receptor antagonist SB-271046, on the number, as well as the firing pattern of spontaneously active dopamine (DA) neurons in the rat substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in anesthetized male Sprague-Dawley rats. This was accomplished using the technique of extracellular in vivo electrophysiology. A single p.o. administration of either 1, 3, or 10 mg/kg of SB-271046 did not significantly alter the number of spontaneously active SNC DA neurons per stereotaxic electrode tract compared to vehicle-treated animals. The acute administration of either 1 or 3 mg/kg of SB-271046 did not significantly alter the number of spontaneously active VTA DA neurons. In contrast, a significant decrease in the number of spontaneously active VTA DA neurons was observed after a single administration of 10 mg/kg of SB-271046 compared to vehicle-treated animals. The acute p.o. administration of SB-271046 significantly altered the firing pattern parameters of all (bursting + nonbursting DA neurons) DA neurons, particularly those in the VTA, compared to vehicle-treated animals. The repeated p.o. administration (once per day for 21 days) of 1, 3, or 10 mg/kg of SB-271046 did not significantly alter the number of spontaneously active VTA DA neurons compared to vehicle-treated animals. The repeated administration of 3 or 10 mg/kg of SB-271046 significantly increased the number of spontaneously active SNC DA neurons compared to vehicle controls. Overall, the repeated administration of SB-271046 had relatively little effect on the firing pattern of midbrain DA neurons. The results obtained following the chronic administration of SB-271046 show that this compound has a profile different from that of typical or atypical antipsychotic drugs in this model. Clinical studies are required to understand what role 5-HT(6) receptor blockade might eventually play in the treatment of schizophrenia.

Serotonin receptors: their key role in drugs to treat schizophrenia

Serotonin (5-HT)-receptor-based mechanisms have been postulated to play a critical role in the action of the new generation of antipsychotic drugs (APDs) that are usually referred to as atypical APDs because of their ability to achieve an antipsychotic effect with lower rates of extrapyramidal side effects (EPS) compared to first-generation APDs such as haloperidol. Specifically, it has been proposed by Meltzer et al. [J. Pharmacol. Exp. Ther. 251 (1989) 238] that potent 5-HT2A receptor antagonism together with weak dopamine (DA) D2 receptor antagonism are the principal pharmacologic features that differentiate clozapine and other apparent atypical APDs from first-generation typical APD. This hypothesis is consistent with the atypical features of quetiapine, olanzapine, risperidone, and ziprasidone, which are the most common treatments for schizophrenia in the United States and many other countries, as well as a large number of compounds in various stages of development. Subsequent research showed that 5-HT1A agonism may be an important consequence of 5-HT2A antagonism and that substitution of 5-HT1A agonism for 5-HT2A antagonism may also produce an atypical APD drug when coupled with weak D2 antagonism. Aripiprazole, the most recently introduced atypical APD, and a D2 receptor partial agonist, may also owe some of its atypical properties to its net effect of weak D2 antagonism, 5-HT2A antagonism and 5-HT1A agonism [Eur. J. Pharmacol. 441 (2002) 137]. By contrast, the alternative "fast-off" hypothesis of Kapur and Seeman [Am. J. Psychiatry 158 (2001) 360] applies only to clozapine and quetiapine and is inconsistent with the "slow" off rate of most atypical APDs, including olanzapine, risperidone and ziprasidone. 5-HT2A and 5-HT1A receptors located on glutamatergic pyramidal neurons in the cortex and hippocampus, 5-HT2A receptors on the cell bodies of DA neurons in the ventral tegmentum and substantia nigra and GABAergic interneurons in the cortex and hippocampus, and 5-HT1A receptors in the raphe nuclei are likely to be important sites of action of the atypical APDs. At the same time, evidence has accumulated for the important modulatory role of 5-HT2C and 5-HT6 receptors for some of the effects of some of the current APDs. Thus, 5-HT has joined DA as a critical target for developing effective APDs and led to the search for novel drugs with complex pharmacology, ending the exclusive search for single-receptor targets, e.g., the D3 or D4 receptor, and drugs that are selective for them.

5-HT6 receptor antagonist SB-271046 enhances extracellular levels of monoamines in the rat medial prefrontal cortex

The present study investigated the neurochemical effects of the selective 5-HT(6) receptor antagonist SB-271046 in the rat medial prefrontal cortex (mPFC). The effect of SB-271046 on extracellular levels of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in the mPFC was examined using in vivo microdialysis in the freely moving rat. SB-271046 (10 mg/kg, p.o.) produced a significant increase in extracellular levels of both DA and NE without altering 5-HT neurotransmission. These results further support the rationale for the use of 5-HT(6) receptor antagonists in the treatment of cognitive dysfunction associated with psychiatric diseases.

5-HT6 receptor binding sites in schizophrenia and following antipsychotic drug administration: autoradiographic studies with [125I]SB-258585

The 5-hydroxytryptamine (5-HT; serotonin)-6 receptor (5-HT6R) is a putative target of atypical antipsychotic drugs and its mRNA expression is altered in schizophrenia. [125I]SB-258585 is a selective 5-HT6R antagonist which has been well characterized for use in the rat brain. The present study evaluated its suitability for receptor autoradiography in the human brain and its application to quantitative studies. The affinity (K(d) approximately 1.2 nM) and relative distribution of binding sites (striatum >> cortex approximately hippocampus) were similar to the rat. The distribution of [125I]SB-258585 binding in these regions was also consistent with that of 5-HT6R mRNA, determined in parallel using in situ hybridization. [125I]SB-258585 binding site densities were measured in dorsolateral prefrontal cortex of 20 patients with chronic schizophrenia and compared with 17 normal subjects. No differences were seen between groups. Neither were [125I]SB-258585 binding site densities affected in the frontal cortex or striatum of rats following 2 weeks' administration of the antipsychotic drugs haloperidol, chlorpromazine, olanzapine, risperidone, or clozapine. In summary, [125I]SB-258585 is a suitable radioligand for studies of human brain 5-HT6R binding sites and shows that their distribution is broadly similar to that of the rodent. The lack of effect of schizophrenia or antipsychotic drug administration on [125I]SB-258585 binding suggests that an altered receptor density does not contribute to any involvement which the 5-HT6R may have in the disease or its treatment.