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

doi:10.1016/S0968-0896(00)00307-2

Bioorganic & Medicinal Chemistry

Volume 9, Issue 4, April 2001, Pages 881-895

https://doi.org/10.1016/S0968-0896(00)00307-2 Get rights and content

Abstract

Three-dimensional (3-D) models of the human serotonin 5-HT_1A and 5-HT_2A 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-HT_1A and 5-HT_2A 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-HT_2A receptor, while interhelical hydrogen bonds were broken and others were formed upon ligand binding to the 5-HT_1A receptor. The ligand-induced changes in interhelical hydrogen bonding patterns of the 5-HT_1A 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.

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Introduction

The serotonin receptors (except for the 5-HT₃ receptor) are members of the rhodopsin family of G-protein-coupled receptors (GPCRs).¹ Recently, an X-ray crystal structure of rhodopsin at 2.8 Å resolution was reported.² This is the first experimental structure of a GPCR at an atomic resolution, and the organisation of the seven transmembrane helices (TMHs) confirms the suggested α-carbon template for the TMHs in the rhodopsin family of GPCRs.³ GPCRs are integral membrane proteins consisting of seven transmembrane-spanning α-helices (TMHs), connected by three intracellular (Is) and three extracellular (Es) loops forming a central core. The available data indicate that binding of small ligands (neurotransmitters, drugs) to GPCRs involves amino-acids within the central core.³

Clinical trials have shown that buspirone, which is structurally unrelated to the benzodiazepines, has anxiolytic and anti-depressive properties.4, 5, 6 Buspirone is a relatively potent but non-selective partial agonist at 5-HT_1A serotonin receptors,⁴ and its anxiolytic effects are assumed to be related to its 5-HT_1A receptor affinity.4, 5, 6 In general, the buspirone analogues have some important advantages over other groups of anxiolytics, such as the benzodiazepines; they do not cause a sedative effect, the toxicity is relatively small, they are not addictive, and they have no associated withdrawal syndrome.⁷ The introduction of buspirone as a clinically efficacious anxiolytic drug has focused attention on the 5-HT_1A receptor as a possible molecular site for anxiolytic drug action, and several buspirone analogues have been synthesised and tested for their 5-HT_1A receptor affinities.⁸

There are indications that 5-HT_2A, 5-HT_2B and 5-HT_2C receptors may also be targets for anxiolytic drugs.9, 10 Mixed 5-HT_2A/2C receptor antagonists, like ritanserin, have anxiolytic properties in animal models¹¹ and in clinical trials.¹² Ligands combining agonist properties at presynaptic 5-HT_1A receptors with antagonist properties at 5-HT_2A/2C receptors may have greater efficacy than buspirone in producing anxiolytic effects.¹³ To rationalise the design of new compounds with a therapeutic potential in the treatment of anxiety, more information about the structural determinates for 5-HT_1A and 5-HT_2A/2C receptor specific binding, and about the conformational states of the receptor induced by different ligands is necessary.

Computational chemistry techniques have proved to be valuable tools for a better understanding of the molecular events involved in drug specificity and selectivity and for the study of protein dynamics. In the present study, three-dimensional (3-D) models of the human serotonin 5-HT_1A and 5-HT_2A receptors were constructed by computational chemistry techniques based on the suggested α-carbon template for the TMHs in the rhodopsin family of GPCRs.³ The models were used to study structural determinants for 5-HT_1A and 5-HT_2A receptor selectivity of buspirone and three of its analogues (Table 1), and to study the different conformational states of the receptors induced by the buspirone analogues.

Section snippets

Conformational analysis of the ligands

In order to obtain low-energy conformers that might interact with the receptors, the conformational space of the ligand molecules (Table 1) were explored by molecular dynamics (MD) simulations. Conformers obtained during MD simulations were analysed, and the conformers with a structure in accordance with the biophore model of the 5-HT_1A receptor or the 5-HT_2A receptor (Fig. 1) were energy-minimised. The results of the conformational analysis of ligands ( $1$ )–( $4$ ) (Table 1) are shown in Table 2.

Discussion

The present models of 5-HT_1A and 5-HT_2A serotonin receptors were constructed with the helical parts organised according to the suggested α-carbon atom template of GPCRs.³ This template was confirmed by the recent experimental X-ray structure of bovine rhodopsin at 2.8 Å resolution.² At 2.8 Å resolution, it is not possible to identify internal hydrogen bonds and side-chain torsional angels from the X-ray density map, and the experimental model must be considered as relatively crude. Several

Conclusion

The present models of the 5-HT_1A and 5-HT_2A receptors indicated that an asparagine in TMH1 (Asn54 in 5-HT_1A, Asn92 in 5-HT_2A), aspartic acid in TMH2 (Asp82 in the 5-HT_1A receptor, Asp120 in 5-HT_2A), a serine in TMH3 (Ser123 in 5-HT_1A and Ser162 in 5-HT_2A), asparagine in TMH7 (Asn396 in 5-HT_1A, Asn375 in 5-HT_2A) and proline in TMH7(Pro397 in 5-HT_1A and Pro376 in 5-HT_2A) form interhelical contacts that constrain TMH1, 2, 3 and 7 relative to each others. Simulations of receptor–ligand interactions

Methods

Molecular mechanic energy minimisation (MM) and MD simulations were performed with the AMBER 5.0 all atom force field.³⁹ Explicit solvent molecules were not included in the calculations, and a distance-dependent dielectric function (ε=4r, r: inter-atomic distance) was used to include the solvent effects. MM energy minimisation of ligand molecules and of averaged receptor–ligand complexes after MD simulations were performed using 0.002 kcal/mol Å for the norm of the energy gradient as convergence

Acknowledgements

This work was supported by grants from the Pharmaceutical Research Institute, Warsaw, Poland, and by computer time on the HP RISC supercomputer at the University of Tromsø, Norway.

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

Abstract

Introduction

Section snippets

Conformational analysis of the ligands

Discussion

Conclusion

Methods

Acknowledgements

Neuropharmacology

J. Mol. Biol.

Trends Pharmacol. Sci.

J. Mol. Biol.

J. Mol. Biol.

J. Biol. Chem.

T.I.P.S.

J. Biol. Chem.

Curr. Opin. Biotech

FEBS Lett.

Brain Res. Mol. Brain Res.

Biochem. Pharmacol.

Science

Drugs

CNS Drugs

CNS Drugs

J. Med. Chem.

J. Med. Chem.

Br. J. Pharmacol.

Drug Develop. Res.

Psychopharnacology

J. Pharmacol. Exp. Ther.

Arch. Pharm. Pharm. Med. Chem.

Molecular dynamics of buspirone analogues interacting with the 5-HT_1A and 5-HT_2A serotonin receptors