Chronic 5-HT6 receptor modulation by E-6837 induces hypophagia and sustained weight loss in diet-induced obese rats

Comparison of rimonabant and sibutramine treatment effects on food compulsion in rats

PURPOSE: To compare the therapeutic effect of rimonabant, a new drug which is a selective antagonist of CB1 receptors, with the sibutramine. METHODS: It is an experimental clinical trial, prospective, placebo controlled. Our test was performed in 38 rats, adults females with a hyper caloric diet. We collected their blood 3 times and weighted them once a week. We divided the rats in 3 groups: Rimonabant, Sibutramine and Control. Statistic analysis has been made through ANOVA test, Tukey test and t Student test. RESULTS: The Rimonabant group demonstrated a significant reduction of the weight increase in rats. The Sibutramine group showed a significant reduction on blood glycemia compared to Rimonabant group and Control group. CONCLUSIONS: Rimonabant showed to be more effective than Sibutramine by decreasing weight gain. Sibutramine has been more effective than Rimonabant and Control groups by decreasing the blood glycemia.

Identification of novel indanylsulfonamide guanylhydrazones as potent 5-HT6 serotonin receptor antagonists

Changing the N,N-(dimethylamino)ethyl side chain in the N-[3-(aminoethyl)inden-5-yl]sulfonamide 5-HT(6) serotonin receptor agonists 1 by a conformationally rigid guanylhydrazone moiety at the indene 3-position led to the identification of the title indanylguanylhydrazones 6, which exhibited excellent binding affinities and an antagonistic response at the 5-HT(6) receptor, with K(i) and IC(50) values in the nanomolar range (K(i) >or= 1.2 nM, IC(50) >or= 47 nM, and I(max) <or= 173%).

Cannabinoids for clinicians: the rise and fall of the cannabinoid antagonists

The endocannabinoid system has emerged as a significant player in the control of energy balance and metabolism, through its direct central and peripheral effects, as well as via its interaction with other appetite-regulating pathways. There is mounting evidence that the endocannabinoid system is overactive in obesity and were it possible to safely dampen-down the elevated endocannabinoid tone, lipid and carbohydrate profiles could be improved and weight loss induced. The series of randomised clinical trials showed reproducible beneficial effects on weight, HbA1c and lipid parameters, in addition to other cardiovascular risk factors. However, to date, clinical developments have been halted because of psychiatric side effects. Although recent evidence has highlighted the importance of an appetite-independent, peripheral mode of action, it is still unclear whether selectively blocking the peripheral system could potentially solve the problem of the central side effects, which thus far has led to the demise of the cannabinoid antagonists as useful pharmaceuticals. In this concise review, we summarise the data on the metabolic effects of the cannabinoid pathway and its antagonists.

Indene-based frameworks targeting the 5-HT6 serotonin receptor: ring constraint in indenylsulfonamides using cyclic amines and structurally abbreviated counterparts

Further studies in quest of 5-HT(6) serotonin receptor ligands led to the design and synthesis of a few selected examples of N-(inden-5-yl)sulfonamides with a ring-constrained aminoethyl side chain at the indene 3-position, some of which exhibited a high binding affinity, such as the pyrrolidine analogue 28 (K(i)=3nM). Moreover, the structurally abbreviated N-(inden-5-yl)sulfonamides showed K(i) values > or = 43 nM, which indicates that neither the N,N-aminoethyl nor the conformationally restricted aminoethyl side arm at the indene 3-position are required for binding. Selected compounds were then tested in a functional cAMP stimulation assay and found to act as 5-HT(6) antagonists, although with moderate potency at the micromolar level.

Pharmacological targeting of the serotonergic system for the treatment of obesity

The attenuation of food intake as induced by an increase in serotonergic (5-hydroxytryptamine, 5-HT) efficacy has been a target of antiobesity pharmacotherapies. However, the induction of tolerance and/or side-effects limited the clinical utility of the earliest serotonin-related medications. With the global prevalence of obesity rising, there has been renewed interest in the manipulation of the serotonergic system as a point of pharmacological intervention. The serotonin(2C) receptor (5-HT(2C)R), serotonin(1B) (rodent)/serotonin(1Dbeta) (human) receptor (5-HT(1B/1Dbeta)R) and serotonin(6) receptor (5-HT(6)R) represent the most promising serotonin receptor therapeutic targets. Canonical serotonin receptor compounds have given way to a myriad of novel receptor-selective ligands, many of which have observable anorectic effects. Here we review serotonergic compounds reducing ingestive behaviour and discuss their clinical potential for the treatment of obesity.

Impact of regional 5-HT depletion on the cognitive enhancing effects of a typical 5-ht(6) receptor antagonist, Ro 04-6790, in the Novel Object Discrimination task

RATIONALE

Selective 5-ht(6) receptor antagonists like Ro 04-6790 prolong memory in many rodent preclinical paradigms, possibly by blocking tonic 5-HT-evoked GABA release and allowing disinhibition of cortico-limbic glutamatergic and cholinergic neurones. If this is the case, behavioural responses to Ro 04-6790 should be abolished by depletion of endogenous 5-HT, and selective lesions of dorsal raphé (DR) or median raphé (MR) 5-HT pathways would allow the neuroanatomical substrates underlying the cognitive effects of 5-ht(6) receptor antagonists to be elucidated.

OBJECTIVES

This study compared the effect of Ro 04-6790 on novel object discrimination (NOD) before and after sham or 5,7-dihydroxytryptamine (5,7-DHT)-induced lesions produced by injection into the lateral ventricles (LV), DR or MR.

MATERIALS AND METHODS

NOD tests used a 4 h inter-trial interval (ITI) and Ro 04-6790 (10 mg kg(-1) i.p.) was administered 20 min before the familiarization trial. Brain region-specific 5-HT depletion was assessed by high performance liquid chromatography with electrochemical detection (HPLC-ED).

RESULTS

Widespread LV or selective MR, but not DR lesions, abolished the ability of Ro 04-6790 to delay natural forgetting. Successful performance of all lesioned rats in subsequent 'drug-free' NOD tests using a 1 h ITI excluded the possibility of any confounding effects on visual acuity or motivation.

CONCLUSIONS

The ability of Ro 04-6790 to prolong object recognition memory requires blockade of MR 5-HT function. Because DR lesions did not produce the expected depletion of striatal 5-HT an additional contribution of DR inputs to this region cannot be completely excluded.

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.

Design, synthesis and preliminary screening of novel 3-(2-N,N-dimethylaminoethylthio) indole derivatives as potential 5-HT(6) receptor ligands

The synthesis and potential 5-hydroxytryptamine(6) receptor (5-HT6R) antagonist activity of a novel series of N-arylsulfonyl-3-(2-N,N-dimethylaminoethylthio) indoles has been reported. The molecular modeling, synthesis and in-vitro radioligand binding data of this series are discussed. The present article describes 37 derivatives of the title series. It was observed that the increased side-chain length with the insertion of a sulfur atom did not lead to the loss of binding affinity of these compounds, although the affinities were reduced. The compounds exhibited moderate affinity and selectivity to human 5-HT6 receptors.

Actions of novel agonists, antagonists and antipsychotic agents at recombinant rat 5-HT6 receptors: a comparative study of coupling to G alpha s

Though 5-HT6 receptors are targets for the treatment of schizophrenia and other psychiatric disorders, the influence of drugs upon signal transduction has not been extensively characterized. Herein, we employed a Scintillation Proximity Assay (SPA)/antibody-immunocapture procedure of coupling to G alpha s to evaluate the interaction of a broad range of novel agonists, antagonists and antipsychotics at rat 5-HT(6) receptors stably expressed in HEK293 cells. Serotonin (pEC(50), 7.7) increased [35S]GTP gamma S binding to G alpha s by ca 2-fold without affecting binding to Gi/o or Gq. LSD (9.2), 5-MeODMT (7.9), 5-CT (7.0) and tryptamine (6.1) were likewise full agonists. In contrast, the novel sulfonyl derivatives, WAY181,187 (9.1) and WAY208,466 (7.8), behaved as partial agonists and attenuated the actions of 5-HT. SB271,046 and SB258,585 abolished activation of G alpha s by 5-HT with pKb values of 10.2 and 9.9, respectively, actions mimicked by the novel antagonist, SB399,885 (10.9). SB271,046 likewise blocked partial agonist properties of WAY181,187 and WAY208,466 with pKb values of 9.8 and 9.0, respectively. 5-HT-stimulated [35S]GTP gamma S binding to G alpha s was antagonised by various antipsychotics including olanzapine (7.8), asenapine (9.1) and SB737,050 (7.8), whereas aripiprazole and bifeprunox were inactive. Further, antagonist properties of clozapine (8.0) were mimicked by its major metabolite, N-desmethylclozapine (7.9). In conclusion, the novel ligands, WAY208,466 and WAY181,187, behaved as partial agonists at 5-HT6 receptors coupled to G alpha s, while SB399,885 was a potent antagonist. Though 5-HT6 receptor blockade is not indispensable for therapeutic efficacy, it may well play a role in the functional actions of certain antipsychotic agents.

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.

Stable expression of adenylyl cyclase 2 leads to the functional rescue of human 5-HT6 receptor in a CHODUKX cell line

INTRODUCTION

The generation and selection of recombinant cell lines specifically designed to express high picomolar levels of heterologous G-protein-coupled receptors can lead to loss of ligand-dependent functional activity. As a result, the clonal selection of a suitable host model and/or lower receptor expression levels within the same cell system becomes important especially when a functional assay is necessary to evaluate the pharmacological potencies of ligands at the receptor site. To address this question, we examined the utility of various signal transducers to restore the functional capacity of a high expressing human 5-HT(6) receptor CHODUKX system.

METHODS

The plasmids for human 5-HT(6) receptor and full-length human G(s), G(olf) and rat adenylyl cyclase isoforms 2 (rAC2) and 5 were obtained by PCR. The h5-HT(6) receptor pHTop plasmid was stably transfected into a CHODUKX cell line to generate an h5-HT(6) expressing clone. h5-HT(6) CHODUKX cells were transfected with signaling components and functional cAMP responses measured. rAC2 was selected to generate a double stable h5-HT(6) receptor/rAC2 pHTop CHODUKX line.

RESULTS

The h5-HT(6) receptor CHODUKX line was a high receptor expressor (>2 pmol/mg protein) but an extremely poor ligand-dependent functional responder, failing to produce the appropriate cAMP signal upon addition of selective agonists. We found that stable co-expression of rAC2 with h5-HT(6) receptor in the CHODUKX cell line displayed dose-dependent cAMP accumulation following agonist treatment. The pharmacological profile of several agonists in the h5-HT(6) receptor/rAC2 cell line was consistent with an h5-HT(6)-like receptor-mediated event.

DISCUSSION

We provide evidence for restoration of functional capacity in a heterologous G(s)-coupled 5-HT(6)/AC2 CHODUKX expression system. We discuss the broader value of a stable AC2-expressing CHODUKX cell line in which the generation of high expressing GPCR receptor/AC2 lines can retain their functional responsiveness and provide pharmacological drug comparisons between the same host line for screening purposes and measurement of multiple cellular parameters.