Molecular interactions of agonist and inverse agonist ligands at serotonin 5-HT_2C G protein-coupled receptors: computational ligand docking and molecular dynamics studies validated by experimental mutagenesis results

<div><p>To understand molecular determinants for ligand activation of the serotonin 5-HT_2C G protein-coupled receptor (GPCR), a drug target for obesity and neuropsychiatric disorders, a 5-HT_2C homology model was built according to an adrenergic β₂ GPCR (β₂AR) structure and validated using a 5-HT_2B GPCR crystal structure. The models were equilibrated in a simulated phosphatidyl choline membrane for ligand docking and molecular dynamics studies. Ligands included (2<i>S</i>, 4<i>R</i>)-(–)-<i>trans</i>-4-(3’-bromo- and trifluoro-phenyl)-<i>N</i>,<i>N</i>-dimethyl-1,2,3,4-tetrahydronaphthalene-2-amine (3’-Br-PAT and 3’-CF₃-PAT), a 5-HT_2C agonist and inverse agonist, respectively. Distinct interactions of 3’-Br-PAT and 3’-CF₃-PAT at the wild-type (WT) 5-HT_2C receptor model were observed and experimental 5-HT_2C receptor mutagenesis studies were undertaken to validate the modelling results. For example, the inverse agonist 3’-CF₃-PAT docked deeper in the WT 5-HT_2C binding pocket and altered the orientation of transmembrane helices (TM) 6 in comparison to the agonist 3’-Br-PAT, suggesting that changes in TM orientation that result from ligand binding impact function. For both PATs, mutation of 5-HT_2C residues S3.36, T3.37, and F5.47 to alanine resulted in significantly decreased affinity, as predicted from modelling results. It was concluded that upon PAT binding, 5-HT_2C residues T3.37 and F5.47 in TMs 3 and 5, respectively, engage in inter-helical interactions with TMs 4 and 6, respectively. The movement of TMs 5 and 6 upon agonist and inverse agonist ligand binding observed in the 5-HT_2C receptor modelling studies was similar to movements reported for the activation and deactivation of the β₂AR, suggesting common mechanisms among aminergic neurotransmitter GPCRs.</p></div

An Orally Active Phenylaminotetralin-Chemotype Serotonin 5‑HT₇ and 5‑HT_1A Receptor Partial Agonist That Corrects Motor Stereotypy in Mouse Models

Stereotypy (e.g., repetitive hand waving) is a key phenotype of autism spectrum disorder, Fragile X and Rett syndromes, and other neuropsychiatric disorders, and its severity correlates with cognitive and attention deficits. There are no effective treatments, however, for stereotypy. Perturbation of serotonin (5-HT) neurotransmission contributes to stereotypy, suggesting that distinct 5-HT receptors may be pharmacotherapeutic targets to treat stereotypy and related neuropsychiatric symptoms. For example, preclinical studies indicate that 5-HT₇ receptor activation corrects deficits in mouse models of Fragile X and Rett syndromes, and clinical trials for autism are underway with buspirone, a 5-HT_1A partial agonist with relevant affinity at 5-HT₇ receptors. Herein, we report the synthesis, <i>in vitro</i> molecular pharmacology, behavioral pharmacology, and pharmacokinetic parameters in mice after subcutaneous and oral administration of (+)-5-(2′-fluorophenyl)-<i>N</i>,<i>N</i>-dimethyl-1,2,3,4-tetrahydronaphthalen-2-amine ((+)-5-FPT), a new, dual partial agonist targeting both 5-HT₇ (<i>K</i>_i = 5.8 nM, EC₅₀ = 34 nM) and 5-HT_1A (<i>K</i>_i = 22 nM, EC₅₀ = 40 nM) receptors. Three unique, heterogeneous mouse models were used to assess the efficacy of (<i>+</i>)-5-FPT to reduce stereotypy: idiopathic jumping in C58/J mice, repetitive body rotations in C57BL/6J mice treated with the NMDA antagonist, MK-801, and repetitive head twitching in C57BL/6J mice treated with the 5-HT₂ agonist, DOI. Systemic (<i>+</i>)-5-FPT potently and efficaciously reduced or eliminated stereotypy in each of the mouse models without altering locomotor behavior on its own, and additional tests showed that (+)-5-FPT, at the highest behaviorally active dose tested, enhanced social interaction and did not cause behaviors indicative of serotonin syndrome. These data suggest that (<i>+</i>)-5-FPT is a promising medication for treating stereotypy in psychiatric disorders