Structure-Activity Relationships of Constrained Phenylethylamine Ligands for the Serotonin 5-HT2 Receptors

Serotonergic ligands have proven effective drugs in the treatment of migraine, pain, obesity, and a wide range of psychiatric and neurological disorders. There is a clinical need for more highly 5-HT₂ receptor subtype-selective ligands and the most attention has been given to the phenethylamine class. Conformationally constrained phenethylamine analogs have demonstrated that for optimal activity the free lone pair electrons of the 2-oxygen must be oriented syn and the 5-oxygen lone pairs anti relative to the ethylamine moiety. Also the ethyl linker has been constrained providing information about the bioactive conformation of the amine functionality. However, combined 1,2-constriction by cyclization has only been tested with one compound. Here, we present three new 1,2-cyclized phenylethylamines, 9–11, and describe their synthetic routes. Ligand docking in the 5-HT_2B crystal structure showed that the 1,2-heterocyclized compounds can be accommodated in the binding site. Conformational analysis showed that 11 can only bind in a higher-energy conformation, which would explain its absent or low affinity. The amine and 2-oxygen interactions with D3.32 and S3.36, respectively, can form but shift the placement of the core scaffold. The constraints in 9–11 resulted in docking poses with the 4-bromine in closer vicinity to 5.46, which is polar only in the human 5-HT_2A subtype, for which 9–11 have the lowest affinity. The new ligands, conformational analysis and docking expand the structure-activity relationships of constrained phenethylamines and contributes towards the development of 5-HT₂ receptor subtype-selective ligands.     

5-HT3 Receptors

5-Hydroxytryptamine type 3 (5-HT₃) receptors are cation-selective Cys loop receptors found in both the central and peripheral nervous systems. There are five 5-HT₃ receptor subunits (A–E), and all functional receptors require at least one A subunit. Regions from noncontiguous parts of the subunit sequence contribute to the agonist-binding site, and the roles of a range of amino acid residues that form the binding pocket have been identified. Drugs that selectively antagonize 5-HT₃ receptors (the “setrons”) are the current gold standard for treatment of chemotherapy-induced and postoperative nausea and vomiting and have potential for the treatment of a range of other conditions.     

The Influence of Gender and Age on Neonatal Rat Hypothalamic 5-HT1A and 5-HT2A Receptors

1.Rat hypothalamic 5-hydroxytryptamine (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) concentrations are transiently sexually differentiated in the second week postpartum (pp), with higher levels in the female. In this report we investigate the possibility that 5-HT receptors may also exhibit sexual dimorphism in the neonatal period.2.5-HT1A and 5-HT2A receptors were quantitated by radioligand binding of [³H]ketanserin and [³H]8-OH DPAT, respectively, in hypothalamus and amygdala from male and female rats at days 8–16 pp.3.There was no sexual dimorphism or change in the density of 5-HT2A binding in hypothalamus or amygdala over days 8–16 pp. There was also no sexual dimorphism of 5-HT1A receptors.4.There was an increase in 5-HT1A receptor density in both the hypothalamus and the amygdala. In the hypothalamus, but not the amygdala, this increase was interrupted on day 14 by a decrease in 5-HT1A receptors, which we suggest may be of physiological significance in modifying the eventual pattern of adult agonistic activity.5.The results suggest that the sexual dimorphism in 5-HT turnover is predominantly presynaptic, relating to altered synthesis and/or release, and is not of sufficient magnitude or duration to produce adaptive responses in postsynaptic 5-HT1A or 5-HT2A receptors.     

Agonistic Properties of Cannabidiol at 5-HT1a Receptors

Cannabidiol (CBD) is a major, biologically active, but psycho-inactive component of cannabis. In this cell culture-based report, CBD is shown to displace the agonist, [3H]8-OH-DPAT from the cloned human 5-HT1a receptor in a concentration-dependent manner. In contrast, the major psychoactive component of cannabis, tetrahydrocannabinol (THC) does not displace agonist from the receptor in the same micromolar concentration range. In signal transduction studies, CBD acts as an agonist at the human 5-HT1a receptor as demonstrated in two related approaches. First, CBD increases [35S]GTPγS binding in this G protein coupled receptor system, as does the known agonist serotonin. Second, in this GPCR system, that is negatively coupled to cAMP production, both CBD and 5-HT decrease cAMP concentration at similar apparent levels of receptor occupancy, based upon displacement data. Preliminary comparative data is also presented from the cloned rat 5-HT2a receptor suggesting that CBD is active, but less so, relative to the human 5-HT1a receptor, in binding analyses. Overall, these studies demonstrate that CBD is a modest affinity agonist at the human 5-HT1a receptor. Additional work is required to compare CBD’s potential at other serotonin receptors and in other species. Finally, the results indicate that cannabidiol may have interesting and useful potential beyond the realm of cannabinoid receptors.     

Structure-Based Discovery of Selective Serotonin 5-HT1B Receptor Ligands

1. Download : Download high-res image (256KB)
2. Download : Download full-size image

     

Asymmetric Clustering Index in a Case Study of 5-HT1A Receptor Ligands

The automatic clustering of chemical compounds is an important branch of chemoinformatics. In this paper the Asymmetric Clustering Index (Aci) is proposed to assess how well an automatically created partition reflects the reference. The asymmetry allows for a distinction between the fixed reference and the numerically constructed partition. The introduced index is applied to evaluate the quality of hierarchical clustering procedures for 5-HT_1A receptor ligands. We find that the most appropriate combination of parameters for the hierarchical clustering of compounds with a determined activity for this biological target is the Klekota Roth fingerprint combined with the complete linkage function and the Buser similarity metric.     

Molecular dynamics of buspirone analogues interacting with the 5-HT1A and 5-HT2A serotonin receptors

Three-dimensional (3-D) models of the human serotonin 5-HT1A and 5-HT2A receptors were constructed, energy refined, and used to study the interactions with a series of buspirone analogues. For both receptors, the calculations showed that the main interactions of the ligand imide moieties were with amino acids in transmembrane helix (TMH) 2 and 7, while the main interactions of the ligand aromatic moieties were with amino acids in TMH5, 6 and 7. Differences in binding site architecture in the region of highly conserved serine and tyrosine residues in TMH7 gave slightly different binding modes of the buspirone analogues at the 5-HT1A and 5-HT2A receptors. Molecular dynamics simulations of receptor-ligand interactions indicated that the buspirone analogues did not alter the interhelical hydrogen bonding patterns upon binding to the 5-HT2A receptor, while interhelical hydrogen bonds were broken and others were formed upon ligand binding to the 5-HT1A receptor. The ligand-induced changes in interhelical hydrogen bonding patterns of the 5-HT1A receptor were followed by rigid body movements of TMH2, 4 and 6 relative to each other and to the other TMHs, which may reflect the structural conversion into an active receptor structure.     

Design and synthesis of dual 5-HT1A and 5-HT7 receptor ligands

5-HT_1A and 5-HT₇ receptors have been at the center of discussions recently due in part to their major role in the etiology of major central nervous system diseases such as depression, sleep disorders, and schizophrenia. As part of our search to identify dual targeting ligands for these receptors, we have carried out a systematic modification of a selective 5HT₇ receptor ligand culminating in the identification of several dual 5-HT_1A and 5-HT₇ receptor ligands. Compound 16, a butyrophenone derivative of tetrahydroisoquinoline (THIQ), was identified as the most potent agent with low nanomolar binding affinities to both receptors. Interestingly, compound 16 also displayed moderate affinity to other clinically relevant dopamine receptors. Thus, it is anticipated that compound 16 may serve as a lead for further exploitation in our quest to identify new ligands with the potential to treat diseases of CNS origin.     

Novel 5-HT(1A/1B/1D) receptors antagonists with potent 5-HT reuptake inhibitory activity

Novel 2-methyl-5-quinolinyl-1-piperazinylalkyl-3,4-dihydro-2H-1,4-benzoxazin-3-ones showing high affinities for the 5-HT(1A/1B/1D) receptors coupled with potent 5-HT reuptake inhibitory activity have been discovered. This is the first report describing docking of the lead compound 6-{2-[4-(2-methyl-5-quinolinyl)-1-piperazinyl]ethyl}-2H-1,4-benzoxazin-3(4H)-one 1, into a model of the 5-HT transporter and the 5-HT(1A) receptor model.     

Effect of Dopaminergic D1 Receptors on Plasticity Is Dependent of Serotoninergic 5-HT1A Receptors in L5-Pyramidal Neurons of the Prefrontal Cortex

Major depression and schizophrenia are associated with dysfunctions of serotoninergic and dopaminergic systems mainly in the prefrontal cortex (PFC). Both serotonin and dopamine are known to modulate synaptic plasticity. 5-HT_1A receptors (5-HT_1ARs) and dopaminergic type D1 receptors are highly represented on dendritic spines of layer 5 pyramidal neurons (L5PyNs) in PFC. How these receptors interact to tune plasticity is poorly understood. Here we show that D1-like receptors (D1Rs) activation requires functional 5HT_1ARs to facilitate LTP induction at the expense of LTD. Using 129/Sv and 5-HT_1AR-KO mice, we recorded post-synaptic currents evoked by electrical stimulation in layer 2/3 after activation or inhibition of D1Rs. High frequency stimulation resulted in the induction of LTP, LTD or no plasticity. The D1 agonist markedly enhanced the NMDA current in 129/Sv mice and the percentage of L5PyNs displaying LTP was enhanced whereas LTD was reduced. In 5-HT_1AR-KO mice, the D1 agonist failed to increase the NMDA current and orientated the plasticity towards L5PyNs displaying LTD, thus revealing a prominent role of 5-HT_1ARs in dopamine-induced modulation of plasticity. Our data suggest that in pathological situation where 5-HT_1ARs expression varies, dopaminergic treatment used for its ability to increase LTP could turn to be less and less effective.     

Novel aryloxy-8-azabicyclo[3.2.1]oct-3-enes with 5-HT transporter and 5-HT1A affinity

Joining aryl 8-azabicyclo[3.2.1]oct-3-enes with aryloxyethanes and aryloxypropanes produces novel series of compounds 11 and 12 with potent 5-HT-T affinity and moderately potent 5-HT(1A) affinity. Moreover, several of these compounds possess functional 5-HT(1A) antagonism. Optimal compounds are, 4-indolyloxyethane 21, 4-indolyloxypropanes 25, and 27, which possess potent 5-HT-T affinity (5-HT-T K(i): 21: 1.2nM, 25: 0.54nM, 27: 0.38nM) and good 5-HT(1A) affinity/antagonism (5-HT(1A)K(i), [(35)S]GTPgammaS: E(max) (%): 21: 111.1nM, 0%; 25: 173.2nM, 0%; 27: 107nM, 0%).     

5-HT2 Receptors Facilitate JC Polyomavirus Entry

The human JC polyomavirus (JCPyV) causes the rapidly progressing demyelinating disease progressive multifocal leukoencephalopathy (PML). The disease occurs most often in individuals with AIDS but also occurs in individuals receiving immunomodulatory therapies for immune-related diseases such as multiple sclerosis. JCPyV infection of host cells requires the pentasaccharide lactoseries tetrasaccharide c (LSTc) and the serotonin receptor 5-hydroxytryptamine (5-HT) receptor 5-HT_2AR. While LSTc is involved in the initial attachment of virus to cells via interactions with VP1, the mechanism by which 5-HT_2AR contributes to infection is not clear. To further define the roles of serotonin receptors in infection, HEK293A cells, which are poorly permissive to JCPyV, were transfected with 14 different isoforms of serotonin receptor. Only 5-HT₂ receptors were found to support infection by JCPyV. None of the other 11 isoforms of serotonin receptor supported JCPyV infection. Expression of 5-HT₂ receptors did not increase binding of JCPyV to cells, but this was not unexpected, given that the cells uniformly expressed the major attachment receptor, LSTc. Infection of these cells remained sensitive to inhibition with soluble LSTc, confirming that LSTc recognition is required for JCPyV infection. Virus internalization into HEK293A cells was significantly and specifically enhanced when 5HT₂ receptors were expressed. Taken together, these data confirm that the carbohydrate LSTc is the attachment receptor for JCPyV and that the type 2 serotonin receptors contribute to JCPyV infection by facilitating entry.     

5-HT Receptors: Subtypes and Second Messengers

Abstract

Our knowledge about 5-HT (serotonin, 5-hydroxytryptamine) receptors has gained significantly over the recent few years. The discovery of selective ligands and the use of new techniques have led to a significant increase in the number of recognised receptors subtypes. The present status of awareness is largely related to the use of radioligand binding studies, autoradiography, second messenger analysis and more recently, molecular biological techniques. Three main families of 5-HT receptors, of which subtypes have been described, are now accepted. This heterogeneity is further substantiated by the cloning of the cDNA's of three different 5-HT receptors. This article reviews some of the recent developments which led to the characterisation of 5-HT receptor subtypes.     

Specific Involvement of Central 5-HT1A Receptors in the Mediation of Male Rat Ejaculatory Behavior

The aminotetralin 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), pharmacologically characterized as a 5-HT_1A receptor agonist, produces a pronounced decrease in ejaculation latency in the male rat. Stimulation of 5-HT receptors by a pharmacologically induced increase in the synaptic availability of 5-HT has been shown to produce the opposite effect. The 8-OH-DPAT-induced decrease in ejaculation latency is specific for this compound, and some chemically related ergot derivatives. In this paper we review the evidence in support for stimulation of serotonergic auto-receptors of the 5-HT_1A receptor subtype as a mechanism of action for effects by 8-OH-DPAT on male rat ejaculatory behavior. We also present the questions posed by the fact that quinpirole and lisuride both produce 8-OH-DPAT-like effects on male rat ejaculatory behavior. The effects by quinpirole, lisuride or 8-OH-DPAT are not sensitive to pretreatment with the DA D_2/3 receptor antagonist raclopride. Continued studies will show whether the effects of quinpirole and lisuride can be related to stimulation of 5-HT_1A receptors, or if all these compounds have as yet undefined common properties.     

Role of Somatodendritic and Postsynaptic 5-HT1A Receptors on Learning and Memory Functions in Rats

Memory impairment is a major problem afflicting mankind. The association between memory functions and neurotransmitter functions is of great interest for understanding brain function. Serotonergic pathways play an important role in the modulation of memory functions but the importance of its receptor types and subtypes on memory functions is still unclear. Activation and blockade of various serotonin (5-HT) receptors has been reported to alter cognitive processes and 5-HT receptor antagonism could be beneficial in the treatment of cognitive diseases. The role of 5-HT on memory functions is complicated. Among the 5-HT receptors subtypes, 5-HT(1A) receptors are of special interest because these receptors are present in the brain areas involved in learning and memory functions such as hippocampus and cortex. The present study was therefore designed to investigate the effect of activation and blockade of somatodendritic and/or postsynaptic 5-HT(1A) receptor on learning and memory functions in rats using modified version of water maze. In this study, 8-OH-DPAT (8-hydroxy-2-(di-N-propylamino) tetralin) at 0.3?mg/kg significantly enhanced learning acquisition (LA), short-term memory (STM) and long term memory (LTM) of rats pre-injected with saline suggesting that the activation of pre-synaptic 5-HT(1A) receptors by its agonist enhanced the memory functions of rats. Conversely, rats injected with 8-OH-DPAT at 1.0?mg/kg exhibited impaired LA and STM and had no effect on LTM. It was also shown in this study that blockade of 5-HT(1A) receptors by spiperone enhanced LA, had no effect on STM but impaired the LTM, which showed that the blockade of 5-HT(1A) receptors by its antagonist exerts different effect on different types of memory. This study suggests that 5-HT(1A) receptor could be used as a significant pharmacological target for the treatment of CNS diseases. Unraveling the role of serotonin in cognition and memory disorders could provide better therapy and it may lead to new insights in our understandings of learning and memory.     

A Novel Molecular Design for a Hybrid Phage-DNA Construct Against DKK1

Nucleic acid immunization has recently exhibited a great promise for immunotherapy of various diseases. However, it is now clear that powerful strategies are imminently needed to improve their efficiency. In this regard, whole bacteriophage particles have been described as efficient DNA vaccine delivery vehicles, capable of circumventing the limitations of naked DNA immunization. Moreover, phage particles could be engineered to display specific peptides on their surfaces. Given these inherent characteristics of phages, we have designed a novel hybrid phage-DNA immunization vector using both M13 and pAAV plasmid elements. Following the construction and in vitro confirmation of the designed vectors, they were used for comparative mice immunization, carrying the same DNA sequence. The results indicated the efficacy of the designed hybrid phage particles, to elicit higher humoral immunity, in comparison to conventional DNA-immunization vectors (pCI). In light of these findings, it could be concluded that using adeno-associated virus (AAV) expression cassette along with displaying TAT peptide on the surface of the phage particle could be deemed as an appealing strategy to enhance the DNA-immunization and vaccination efficacy.     

Design and synthesis of new benzimidazole-arylpiperazine derivatives acting as mixed 5-HT1A/5-HT3 ligands

A series of new benzimidazole-arylpiperazine derivatives III were designed, synthesized and evaluated for binding affinity at serotoninergic 5-HT(1A) and 5-HT(3) receptors. Compound IIIc was identified as a novel mixed 5-HT(1A)/5-HT(3) ligand with high affinity for both serotonin receptors and excellent selectivity over alpha(1)-adrenergic and dopamine D(2) receptors. This compound was characterized as a partial agonist at 5-HT(1A)Rs and a 5-HT(3)R antagonist, and was effective in preventing the cognitive deficits induced by muscarinic receptor blockade in a passive avoidance learning test.