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

First-in-Class Selenium-Containing Potent Serotonin Receptor 5-HT6 Agents with a Beneficial Neuroprotective Profile against Alzheimer's Disease

Alzheimer's disease (AD) has a complex and not-fully-understood etiology. Recently, the serotonin receptor 5-HT6 emerged as a promising target for AD treatment; thus, here a new series of 5-HT6R ligands with a 1,3,5-triazine core and selenoether linkers was explored. Among them, the 2-naphthyl derivatives exhibited strong 5-HT6R affinity and selectivity over 5-HT1AR (13-15), 5-HT7R (14 and 15), and 5-HT2AR (13). Compound 15 displayed high selectivity for 5-HT6R over other central nervous system receptors and exhibited low risk of cardio-, hepato-, and nephrotoxicity and no mutagenicity, indicating its "drug-like" potential. Compound 15 also demonstrated neuroprotection against rotenone-induced neurotoxicity as well as antioxidant and glutathione peroxidase (GPx)-like activity and regulated antioxidant and pro-inflammatory genes and NRF2 nuclear translocation. In rats, 15 showed satisfying pharmacokinetics, penetrated the blood-brain barrier, reversed MK-801-induced memory impairment, and exhibited anxiolytic-like properties. 15's neuroprotective and procognitive-like effects, stronger than those of the approved drug donepezil, may pave the way for the use of selenotriazines to inhibit both causes and symptoms in AD therapy.

Impact of the Substitution Pattern at the Basic Center and Geometry of the Amine Fragment on 5-HT6 and D3R Affinity in the 1H-Pyrrolo[3,2-c]quinoline Series

Salt bridge (SB, double-charge-assisted hydrogen bonds) formation is one of the strongest molecular non-covalent interactions in biological systems, including ligand-receptor complexes. In the case of G-protein-coupled receptors, such an interaction is formed by the conserved aspartic acid (D3.32) residue and the basic moiety of the aminergic ligand. This study aims to determine the influence of the substitution pattern at the basic nitrogen atom and the geometry of the amine moiety at position 4 of 1H-pyrrolo[3,2-c]quinoline on the quality of the salt bridge formed in the 5-HT₆ receptor and D₃ receptor. To reach this goal, we synthetized and biologically evaluated a new series of 1H-pyrrolo[3,2-c]quinoline derivatives modified with various amines. The selected compounds displayed a significantly higher 5-HT₆R affinity and more potent 5-HT₆R antagonist properties when compared with the previously identified compound PZ-1643, a dual-acting 5-HT₆R/D₃R antagonist; nevertheless, the proposed modifications did not improve the activity at D₃R. As demonstrated by the in silico experiments, including molecular dynamics simulations, the applied structural modifications were highly beneficial for the formation and quality of the SB formation at the 5-HT₆R binding site; however, they are unfavorable for such interactions at D₃R.

Multitargeting nature of muscarinic orthosteric agonists and antagonists

Muscarinic receptors (mAChRs) are typical members of the G protein-coupled receptor (GPCR) family and exist in five subtypes from M₁ to M₅. Muscarinic receptor subtypes do not sufficiently differ in affinity to orthosteric antagonists or agonists; therefore, the analysis of receptor subtypes is complicated, and misinterpretations can occur. Usually, when researchers mainly specialized in CNS and peripheral functions aim to study mAChR involvement in behavior, learning, spinal locomotor networks, biological rhythms, cardiovascular physiology, bronchoconstriction, gastrointestinal tract functions, schizophrenia, and Parkinson’s disease, they use orthosteric ligands and they do not use allosteric ligands. Moreover, they usually rely on manufacturers’ claims that could be misleading. This review aimed to call the attention of researchers not deeply focused on mAChR pharmacology to this fact. Importantly, limited selective binding is not only a property of mAChRs but is a general attribute of most neurotransmitter receptors. In this review, we want to give an overview of the most common off-targets for established mAChR ligands. In this context, an important point is a mention the tremendous knowledge gap on off-targets for novel compounds compared to very well-established ligands. Therefore, we will summarize reported affinities and give an outline of strategies to investigate the subtype’s function, thereby avoiding ambiguous results. Despite that, the multitargeting nature of drugs acting also on mAChR could be an advantage when treating such diseases as schizophrenia. Antipsychotics are a perfect example of a multitargeting advantage in treatment. A promising strategy is the use of allosteric ligands, although some of these ligands have also been shown to exhibit limited selectivity. Another new direction in the development of muscarinic selective ligands is functionally selective and biased agonists. The possible selective ligands, usually allosteric, will also be listed. To overcome the limited selectivity of orthosteric ligands, the recommended process is to carefully examine the presence of respective subtypes in specific tissues via knockout studies, carefully apply “specific” agonists/antagonists at appropriate concentrations and then calculate the probability of a specific subtype involvement in specific functions. This could help interested researchers aiming to study the central nervous system functions mediated by the muscarinic receptor.

Multifunctional Arylsulfone and Arylsulfonamide-Based Ligands with Prominent Mood-Modulating Activity and Benign Safety Profile, Targeting Neuropsychiatric Symptoms of Dementia

The current pharmaceutical market lacks therapeutic agents designed to modulate behavioral disturbances associated with dementia. To address this unmet medical need, we designed multifunctional ligands characterized by a nanomolar affinity for clinically relevant targets that are associated with the disease pathology, namely, the 5-HT_2A/6/7 and D₂ receptors. Compounds that exhibited favorable functional efficacy, water solubility, and metabolic stability were selected for more detailed study. Pharmacological profiling revealed that compound 11 exerted pronounced antidepressant activity (MED 0.1 mg/kg), outperforming commonly available antidepressant drugs, while compound 16 elicited a robust anxiolytic activity (MED 1 mg/kg), exceeding comparator anxiolytics. In contrast to the existing psychotropic agents tested, the novel chemotypes did not negatively impact cognition. At a chronic dose regimen (25 days), 11 did not induce significant metabolic or adverse blood pressure disturbances. These promising therapeutic-like activities and benign safety profiles make the novel chemotypes potential treatment options for dementia patients.

N-Skatyltryptamines-Dual 5-HT6R/D2R Ligands with Antipsychotic and Procognitive Potential

A series of N-skatyltryptamines was synthesized and their affinities for serotonin and dopamine receptors were determined. Compounds exhibited activity toward 5-HT1A, 5-HT2A, 5-HT6, and D2 receptors. Substitution patterns resulting in affinity/activity switches were identified and studied using homology modeling. Chosen hits were screened to determine their metabolism, permeability, hepatotoxicity, and CYP inhibition. Several D2 receptor antagonists with additional 5-HT6R antagonist and agonist properties were identified. The former combination resembled known antipsychotic agents, while the latter was particularly interesting due to the fact that it has not been studied before. Selective 5-HT6R antagonists have been shown previously to produce procognitive and promnesic effects in several rodent models. Administration of 5-HT6R agonists was more ambiguous-in naive animals, it did not alter memory or produce slight amnesic effects, while in rodent models of memory impairment, they ameliorated the condition just like antagonists. Using the identified hit compounds 15 and 18, we tried to sort out the difference between ligands exhibiting the D2R antagonist function combined with 5-HT6R agonism, and mixed D2/5-HT6R antagonists in murine models of psychosis.

Activation and blockade of 5-HT6 receptor in the medial septum-diagonal band recover working memory in the hemiparkinsonian rats

Working memory impairment is a common symptom occurred in Parkinson's disease (PD). The medial septum-diagonal band (MS-DB) complex and 5-HT₆ receptor are involved in modulation of cognition. However, their roles in working memory in PD are still unknown. Here, we used behavioral, neurochemical and immunohistochemical approaches to assess the role of MS-DB 5-HT₆ receptor in working memory in unilateral 6-hydroxydopamie (6-OHDA)-induced PD rats. Intra-MS-DB injection of 5-HT₆ receptor agonist WAY208466 (3, 6 and 12 μg/rat) enhanced working memory and increased dopamine (DA) and noradrenaline (NA) levels in the medial prefrontal cortex (mPFC) and hippocampus in sham and 6-OHDA-lesioned rats. The dose that produced significant effect on working memory in 6-OHDA-lesioned rats was lower than that in sham rats, indicating hypersensitivity of 5-HT₆ receptor after lesioning. Intra-MS-DB injection of 5-HT₆ receptor antagonist SB258585 (2, 4 and 8 μg/rat) alleviated working memory deficits and increased DA level in the mPFC and hippocampus and NA level in the mPFC in 6-OHDA-lesioned rats while having no effect in sham rats, suggesting that SB258585 did not change normal cognitive status. These results suggest that activation and blockade of MS-DB 5-HT₆ receptor recovered working memory in 6-OHDA-lesioned rats, which is probably related to changes in monoamine levels in the mPFC and hippocampus.

Dual-acting agents for improving cognition and real-world function in Alzheimer's disease: Focus on 5-HT6 and D3 receptors as hubs

To date, there are no interventions that impede the inexorable progression of Alzheimer's disease (AD), and currently-available drugs cholinesterase (AChE) inhibitors and the N-Methyl-d-Aspartate receptor antagonist, memantine, offer only modest symptomatic benefit. Moreover, a range of mechanistically-diverse agents (glutamatergic, histaminergic, monoaminergic, cholinergic) have disappointed in clinical trials, alone and/or in association with AChE inhibitors. This includes serotonin (5-HT) receptor-6 antagonists, despite compelling preclinical observations in rodents and primates suggesting a positive influence on cognition. The emphasis has so far been on high selectivity. However, for a multi-factorial disorder like idiopathic AD, 5-HT6 antagonists possessing additional pharmacological actions might be more effective, by analogy to "multi-target" antipsychotics. Based on this notion, drug discovery programmes have coupled 5-HT6 blockade to 5-HT4 agonism and inhibition of AchE. Further, combined 5-HT6/dopamine D3 receptor (D3) antagonists are of especial interest since D3 blockade mirrors 5-HT6 antagonism in exerting broad-based pro-cognitive properties in animals. Moreover, 5-HT6 and dopamine D3 antagonists promote neurocognition and social cognition via both distinctive and convergent actions expressed mainly in frontal cortex, including suppression of mTOR over-activation and reinforcement of cholinergic and glutamatergic transmission. In addition, 5-HT6 blockade affords potential anti-anxiety, anti-depressive and anti-epileptic properties, and antagonising 5-HT6 receptors may be associated with neuroprotective ("disease-modifying") properties. Finally D3 antagonism may counter psychotic episodes and D3 receptors themselves offer a promising hub for multi-target agents. The present article reviews the status of "R and D" into multi-target 5-HT6 and D3 ligands for improved treatment of AD and other neurodegenerative disorders of aging. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Management of Dementia-Related Psychosis, Agitation and Aggression: A Review of the Pharmacology and Clinical Effects of Potential Drug Candidates

Along with cognitive decline, 90% of patients with dementia experience behavioral and psychological symptoms of dementia, such as psychosis, aggression, agitation, and depression. Atypical antipsychotics are commonly prescribed off-label to manage certain symptoms, despite warnings from the regulatory agencies regarding the increased risk of mortality associated with their use in elderly patients. Moreover, these compounds display a limited clinical efficacy, mostly owing to the fact that they were developed to treat schizophrenia, a disease characterized by neurobiological deficits. Thus, to improve clinical efficacy, it has been suggested that patients with dementia should be treated with exclusively designed and developed drugs that interact with pharmacologically relevant targets. Within this context, numerous studies have suggested druggable targets that might achieve therapeutically acceptable pharmacological profiles. Based on this, several different drug candidates have been proposed that are being investigated in clinical trials for behavioral and psychological symptoms of dementia. We highlight the recent advances toward the development of therapeutic agents for dementia-related psychosis and agitation/aggression and discuss the relationship between the relevant biological targets and their etiology. In addition, we review the compounds that are in the early stage of development (discovery or preclinical phase) and those that are currently being investigated in clinical trials for dementia-related psychosis and agitation/aggression. We also discuss the mechanism of action of these compounds and their pharmacological utility in patients with dementia.

Characterization of Behavioral, Signaling and Cytokine Alterations in a Rat Neurodevelopmental Model for Schizophrenia, and Their Reversal by the 5-HT6 Receptor Antagonist SB-399885

Post-weaning social isolation of rats produces neuroanatomical, neurochemical and behavioral alterations resembling some core features of schizophrenia. This study examined the ability of the 5-HT₆ receptor antagonist SB-399885 to reverse isolation-induced cognitive deficits, then investigated alterations in hippocampal cell proliferation and hippocampal and frontal cortical expression of selected intracellular signaling molecules and cytokines. Male Lister hooded rats (weaned on post-natal days 21-24 and housed individually or in groups of 3-4) received six i.p. injections of vehicle (1% Tween 80, 1 mL/kg) or SB-399885 (5 or 10 mg/kg) over a 2-week period starting 40 days post-weaning, on the days that locomotor activity, novel object discrimination (NOD), pre-pulse inhibition of acoustic startle and acquisition, retention and extinction of a conditioned freezing response (CFR) were assessed. Tissue was collected 24 h after the final injection for immunohistochemistry, reverse-phase protein microarray and western blotting. Isolation rearing impaired NOD and cue-mediated CFR, decreased cell proliferation within the dentate gyrus, and elevated hippocampal TNFα levels and Cdc42 expression. SB-399885 reversed the NOD deficit and partially normalized CFR and cell proliferation. These effects were accompanied by altered expression of several members of the c-Jun N-terminal Kinase (JNK) and p38 MAPK signaling pathways (including TAK1, MKK4 and STAT3). Although JNK and p38 themselves were unaltered at this time point hippocampal TAK1 expression and phosphorylation correlated with visual recognition memory in the NOD task. Continued use of this neurodevelopmental model could further elucidate the neurobiology of schizophrenia and aid assessment of novel therapies for drug-resistant cognitive symptoms.

Serotonin 5-HT6 Receptor Antagonists in Alzheimer's Disease: Therapeutic Rationale and Current Development Status

Alzheimer's disease (AD) is the most common cause of dementia in elderly people. Because of the lack of effective treatments for this illness, research focused on identifying compounds that restore cognition and functional impairments in patients with AD is a very active field. Since its discovery in 1993, the serotonin 5-HT₆ receptor has received increasing attention, and a growing number of studies supported 5-HT₆ receptor antagonism as a target for improving cognitive dysfunction in AD. This article reviews the rationale behind investigations into the targeting of 5-HT₆ receptors as a symptomatic treatment for cognitive and/or behavioral symptoms of AD. In addition to describing the available clinical evidence, this article also describes the purported biochemical and neurochemical mechanisms of action by which 5-HT₆ receptor antagonists could influence cognition, and the preclinical data supporting this therapeutic approach to AD. A large number of publications describing the development of ligands for this receptor have come to light and preclinical data indicate the procognitive efficacy of 5-HT₆ receptor antagonists. Subsequently, the number of patents protecting 5-HT₆ chemical entities has continuously grown. Some of these compounds have successfully undergone phase I clinical studies and have been further evaluated in clinical phase II trials with variable success. Phase II studies have also revealed the potential of combining 5-HT₆ receptor antagonism and cholinesterase inhibition. Two of these antagonists, idalopirdine and RVT-101, have been further developed into ongoing phase III clinical trials. Overall, 5-HT₆ receptor antagonists can reasonably be regarded as potential drug candidates for the treatment of AD.

Extended N-Arylsulfonylindoles as 5-HT₆ Receptor Antagonists: Design, Synthesis & Biological Evaluation

Based on a known pharmacophore model for 5-HT₆ receptor antagonists, a series of novel extended derivatives of the N-arylsulfonyindole scaffold were designed and identified as a new class of 5-HT₆ receptor modulators. Eight of the compounds exhibited moderate to high binding affinities and displayed antagonist profile in 5-HT₆ receptor functional assays. Compounds 2-(4-(2-methoxyphenyl)piperazin-1-yl)-1-(1-tosyl-1H-indol-3-yl)ethanol (4b), 1-(1-(4-iodophenylsulfonyl)-1H-indol-3-yl)-2-(4-(2-methoxyphenyl)piperazin-1-yl)ethanol (4g) and 2-(4-(2-methoxyphenyl)piperazin-1-yl)-1-(1-(naphthalen-1-ylsulfonyl)-1H-indol-3-yl)ethanol (4j) showed the best binding affinity (4b pK_i = 7.87; 4g pK_i = 7.73; 4j pK_i = 7.83). Additionally, compound 4j was identified as a highly potent antagonist (IC₅₀ = 32 nM) in calcium mobilisation functional assay.

Effects of ST1936, a selective serotonin-6 agonist, on electrical activity of putative mesencephalic dopaminergic neurons in the rat brain

The serotonin-6 (5-HT6) receptor is the most recently discovered serotonin receptor, and it represents an increasingly promising target for improving cognition in both normal and disease states. Recently, a new selective 5-HT6 receptor agonist, 2-(5 chloro-2-methyl-1H-indol-3-yl)-N,N-dimethylethanamine (ST1936), with nanomolar affinity for 5-HT6 receptors was described. We performed in-vivo electrophysiological studies to investigate the physiological role of 5-HT6 receptors in the control of the function of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA). Extracellular single-unit recordings were performed from putative dopamine-containing neurons in the SNc and VTA of anesthetised rats. In the SNc, acute systemic administration of ST1936 had no effects on basal firing activity of these dopamine neurons; however, in the VTA, ST1936 induced either dose-related increases (45% of cells) or decreases in basal activity of these dopaminergic neurons. Local application of ST1936 into the VTA caused excitation in all of the dopamine neurons, but had no effects on non-dopamine VTA neurons. Both effects of systemic and microiontophoretic ST1936 were completely reversed by the potent and selective 5-HT6 receptor antagonist 5-chloro-N-(4-methoxy-3-piperazin-1-ylphenyl)-3-methyl-2- benzothiophene-sulfonamide (SB271046). Systemic application of another 5-HT6 agonist, 2-(1-{6-chloroimidazo[2,1-b] [1,3]thiazole-5-sulfonyl}-1H-indol-3-yl)ethan-1-amine (WAY-181187), induced dose-dependent inhibition of these VTA dopaminergic neurons. ST1936 and WAY-181187 appear to have different effects on these VTA dopaminergic neurons, potentially due to different mechanisms of action or to the complexity of 5-HT6 receptor functions. Our data demonstrate the need for further investigations into the use of 5-HT6 receptor agonists to control cognitive disfunction, such as in schizophrenia and depression.

Deconstructing 5-HT6 receptor effects on striatal circuit function

Medium spiny neurons (MSNs) constitute 95% of neurons in the dorsal striatum subdivided into direct (striatonigral) and indirect (striatopallidal) pathways. Whereas D1 and D2 receptors and several neuropeptides, including dynorphin and enkephalin, are differentially expressed in these neurons, 5-hydroxytryptamine 6 receptors (5-HT6) are expressed in both pathways. Previous results demonstrate that concurrent 5-HT6 receptor overexpression in MSNs of both pathways in the dorsomedial striatum (DMS) interferes with instrumental learning and that 5-HT6 overexpression in the dorsolateral striatum (DLS) relieves rats from inflexible habitual behaviors. We hypothesized that 5-HT6 receptor-mediated co-activation of both pathways interferes with the differential activation/inhibition of direct/indirect pathways by dopamine. To test this idea, we cloned novel viral vectors to selectively overexpress 5-HT6 receptors in direct or indirect pathway MSNs to deconstruct their role in modulating instrumental learning and habitual responding. We found that increasing 5-HT6 receptor expression in either direct or indirect pathway MSNs of the posterior DMS selectively enhanced or impaired initial acquisition of a discrete instrumental learning task respectively, though all rats were ultimately able to learn the task. In a separate set of experiments, 5-HT6 receptor overexpression in indirect pathway MSNs of the DLS facilitated behavioral flexibility in rats overtrained on a repetitive pressing task using a variable interval schedule of reinforcement, during an omission contingency training session and subsequent probe testing. Together these findings further the notion that 5-HT6 signaling causes balanced activation of opposing MSN pathways by serotonin in sub-regions of the dorsal striatum allowing for more reflective modalities of behavior.

Partial agonist efficacy of EMD386088, a 5-HT6 receptor ligand, in functional in vitro assays

BACKGROUND

Over recent years, the 5-hydroxytryptamine6 (5-HT6) receptor has emerged as a promising molecular target which interacts with several central nervous system acting drugs. In animal models, both agonists and antagonists of this receptor exhibit equivalent potency and efficacy as potential antidepressants, anxiolytics and anti-obesity or anti-dementia drugs. EMD386088 (5-chloro-2-methyl-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole hydrochloride) has been described as a high affinity 5-HT6 receptor ligand with a full agonist activity and with moderate affinity for 5-HT3 sites.

METHODS

We have extended these data by broadening its profile for other, not yet tested, monoaminergic, GABA(A), opioid μ receptors and serotonin transporter (SERT) and we have conducted functional in vitro assays; i.e., measurement of cAMP by homogeneous TR-FRET immunoassay and HTRF method made by CEREP as well as aequorin-based calcium flux assay.

RESULTS

In two in vitro models based on cAMP formation, maximal efficacy values for EMD386088 were 65 and 31%, for in house and CEREP experiments, respectively. In a model based on calcium response, the studied compound showed 46% of maximal serotonin (5-HT) signal. EMD386088 antagonizes 5-HT response in increasing concentrations from 10(-9) to 10(-6) M.

CONCLUSIONS

The present in vitro findings confirm that EMD386088 is a selective 5-HT6 receptor ligand with moderate affinity for 5-HT3 sites only and it behaves as a potent partial agonist of 5-HT6 receptor with varying levels of agonist intrinsic activity, depending on a method employed. In view of these results, caution is recommended in the interpretation of pharmacological in vivo studies with EMD386088.

5-HT6 receptors and Alzheimer's disease

During the past 20 years, the 5-HT6 receptor has received increasing attention and become a promising target for improving cognition. Several studies with structurally different compounds have shown that not only antagonists but also 5-HT6 receptor agonists improve learning and memory in animal models. A large number of publications describing the development of ligands for this receptor have come to light, and it is now quite evident that 5-HT6 receptors have great pharmaceutical potential in terms of related patents. However, 5-HT6 receptor functionality is much more complex than initially defined. According to the existing data, different cellular pathways may be activated, depending on the drug being used. This article reviews preclinical and clinical evidence of the effects that 5-HT6 receptor compounds have on cognition. In addition, the biochemical and neurochemical mechanisms of action through which 5-HT6 receptor compounds can influence cognition will be described. Overall, several 5-HT6-targeted compounds can reasonably be regarded as powerful drug candidates for the treatment of Alzheimer's disease.

Scintillation proximity assay in lead discovery

BACKGROUND

Scintillation proximity assay (SPA) is a homogeneous scintillant bead-based platform for the measurement of biological processes and plays an important role in the identification of active chemical entities in drug discovery.

OBJECTIVE

The design and development of solid-phase SPA approaches are examined and compared with alternative non-radiometric fluorescence-based technologies.

METHODS

This review provides background on the principle of SPA and its application to biomolecular interactions from a variety of biological sources.

CONCLUSION

The SPA approach is well suited to the demands of commercial high volume automation and assay miniaturization for target-based high-throughput screening campaigns on synthetic and natural product libraries as well as for benchtop characterization and confirmation studies. In the near future, innovations in the way SPA and fluorescence-based screening strategies are multiplexed will improve our comprehensive understanding of cellular system biology and dramatically advance the lead discovery process for the treatment of complex target-related disorders.

Atypical antipsychotic-induced weight gain: insights into mechanisms of action

Prescriptions for second-generation antipsychotics (SGAs) have surpassed those for first-generation agents in the treatment of schizophrenia and bipolar disorder. While SGAs have the benefit of a much reduced risk of causing movement disorders, they have been associated with weight gain and metabolic effects. These adverse reactions are not uncommon, and threaten to have a significant impact on the patient's health over the long-term treatment that the patient requires. Currently, the aetiology of these effects is not known. This article reviews the data exploring the weight gain phenomenon. The literature was reviewed from searches of PubMed and the references of major articles in the field. The SGAs present a heterogeneous risk for weight gain. In addition, different individuals receiving the same drug can exhibit substantially different weight changes. This pattern suggests that a group of factors are associated with the weight gain phenomenon rather than a single mechanism. Coupled with the genetic profile that the patient brings to the treatment, the risk for SGA-induced weight gain will be different for different drugs and different individuals. Targets for exploration of the weight gain phenomenon include receptor interactions involving serotonin, histamine, dopamine, adrenergic, cannabinoid and muscarinic receptors. The association of SGA-induced weight gain and the role of orexigenic and anorexigenic peptides are reviewed. Also, a brief discussion of genetic factors associated with SGA-induced weight gain is presented, including that of the serotonin 5-HT(2C) receptor gene (HTR2C) and the cannabinoid 1 receptor gene (CNR1). The most promising data associated with SGA-induced weight gain include investigations of the histamine H(1), 5-HT(2A), 5-HT(2C), muscarinic M(3) and adrenergic receptors. In addition, work in the genetic area promises to result in a better understanding of the variation in risk associated with different individuals.

ST1936 stimulates cAMP, Ca2+, ERK1/2 and Fyn kinase through a full activation of cloned human 5-HT6 receptors

5-HT(6) receptor is one of the most recently cloned serotonin receptors, and it might play important roles in Alzheimer's disease, depression, and learning and memory disorders. Availability of only very few 5-HT(6) receptor agonists, however, does not allow examining their contribution in psychopharmacological processes. Therefore, a new 5-HT(6) receptor agonist, ST1936, was synthesized. ST1936 binds to human 5-HT(6) receptors with good affinity (K(i)=28.8 nM). ST1936 also exhibited some moderate binding affinity for 5HT(2B), 5HT(1A), 5HT(7) receptors and adrenergic α receptors. ST1936 behaved as a full 5-HT(6) agonist on cloned cells and was able to increase Ca(2+) concentration, phosphorylation of Fyn kinase, and regulate the activation of ERK1/2 that is a downstream target of Fyn kinase. These effects were completely antagonized by two 5-HT(6) receptor antagonists, SB271046 and SB258585. The other 5-HT(6) receptor agonist, WAY181187 also increased Fyn kinase activity. These results suggest that both ST1936 and WAY181187 mediate 5-HT(6) receptor-dependent signal pathways, such as cAMP, Fyn and ERK1/2 kinase, as specific agonists.

5-HT(6) receptor and cognition

Since its discovery in 1993 and subsequent development of selective antagonists, a growing number of studies support the use of serotonin 5-HT(6) receptor antagonism as a promising mechanism for treating cognitive dysfunction. Lately, several studies with structurally different compounds have shown that not only antagonists, but also 5-HT(6) receptor agonists improve learning and memory in animal models. There is even an antagonist, SB-742457, that has completed phase II trials for the treatment of Alzheimer's disease. In addition to describe preclinical and clinical evidence of the effect of 5-HT(6) receptor compounds on cognition, this article will also focus on the purported biochemical and neurochemical mechanisms of action by which 5-HT(6) receptor compounds could influence cognition in health and disease.

Metabolic side effects of antipsychotic drug treatment--pharmacological mechanisms

Obesity and metabolic syndrome, with increased risk of eventual cardiovascular disease and type II diabetes, are significant problems for patients receiving antipsychotic drugs and are likely contribute to their decreased life expectancy. Several drug-related mechanisms may contribute to these problems, including effects both influencing food intake and on glucose and lipid metabolism. The metabolic consequences of different antipsychotic drugs vary substantially; these variations reflect differences in receptor pharmacology and provide clues as to the underlying pharmacological mechanisms. The two drugs with the greatest effects on body weight, olanzapine and clozapine, also have high affinity for the 5-HT2C and histamine H1 receptors, which implicate these receptors in antipsychotic-induced weight gain, while peripheral M3 muscarinic receptor antagonism as well as central 5-HT2C effects may contribute to obesity-independent diabetes. Other receptor mechanisms may have additive or synergistic effects; dopamine D2 receptor antagonism can enhance 5-HT2C-mediated effects on food intake, as well as influencing lipid and glucose metabolism via disinhibition of prolactin secretion. Pharmacogenetic associations of drug-induced weight gain with 5-HT2C receptor and leptin gene polymorphisms, among others, have provided further clues. Elevated leptin secretion in the absence of a decrease in food intake indicates drug-induced leptin insensitivity in the hypothalamus. The minimal weight gain seen with ziprasidone and aripiprazole may reflect their having further pharmacological effects that protect against changes in food intake and related metabolic factors. Understanding the pharmacology of metabolic consequences of current antipsychotic drug treatment is clearly the key to developing improved pharmacotherapies that avoid these problematic and limiting adverse effects.

Activation of the 5-HT(6) receptor attenuates long-term potentiation and facilitates GABAergic neurotransmission in rat hippocampus

The 5-HT(6) receptor is predominantly expressed in the CNS and has been implicated in the regulation of cognitive function. Antagonists of the 5-HT(6) receptor improve cognitive performance in a number of preclinical models and have recently been found to be effective in Alzheimer's disease patients. Systemic administration of 5-HT(6) antagonists increases the release of acetylcholine and glutamate in the frontal cortex and dorsal hippocampus. In contrast, the selective 5-HT(6) agonist, WAY-181187, can elicit robust increases in extracellular levels of GABA. The reported behavioral and neurochemical effects of 5-HT(6) receptor ligands raise the possibility that the 5-HT(6) receptor may modulate synaptic plasticity in the hippocampus. In the present study, selective pharmacological tools were employed to determine the effect of 5-HT(6) receptor activation on long-term potentiation (LTP) in brain slices containing area CA1 of the hippocampus. While having no effect on baseline synaptic transmission, the results demonstrate that the selective 5-HT(6) agonist, WAY-181187, attenuated LTP over a narrow dose range (100-300 nM). The increase in the slope of the field excitatory post synaptic potential (fEPSP) caused by theta burst stimulation in brain slices treated with the most efficacious dose of WAY-181187 (200 nM) was 80.1+/-4.0% of that observed in controls. This effect was dose-dependently blocked by the selective 5-HT(6) antagonist, SB-399885. WAY-181187 also increased the frequency of spontaneous GABA release in area CA1. As assessed by measuring and evaluating spontaneous inhibitory postsynaptic currents (sIPSCs), 200 nM WAY-181187 increased sIPSC frequency by 3.4+/-0.9 Hz. This increase in GABA sIPSCs was prevented by the selective 5-HT(6) antagonist SB-399885 (300 nM). Taken together, these results suggest that the 5-HT(6) receptor plays a role in the modulation of synaptic plasticity in hippocampal area CA1 and that the regulation of GABAergic interneuron activity may underlie the cognition enhancing effects of 5-HT(6) antagonists.