Low Basicity as a Characteristic for Atypical Ligands of Serotonin Receptor 5-HT2

Serotonin receptors are extensively examined by academic and industrial researchers, due to their vital roles, which they play in the organism and constituting therefore important drug targets. Up to very recently, it was assumed that the basic nitrogen in compound structure is a necessary component to make it active within this receptor system. Such nitrogen interacts in its protonated form with the aspartic acid from the third transmembrane helix (D3x32) forming a hydrogen bond tightly fitting the ligand in the protein binding site. However, there are several recent studies that report strong serotonin receptor affinity also for compounds without a basic moiety in their structures. In the study, we carried out a comprehensive in silico analysis of the low-basicity phenomenon of the selected serotonin receptor ligands. We focused on the crystallized representatives of the proteins of 5-HT_1B, 5-HT_2A, 5-HT_2B, and 5-HT_2C receptors, and examined the problem both from the ligand- and structure-based perspectives. The study was performed for the native proteins, and for D3x32A mutants. The investigation resulted in the determination of nonstandard structural requirements for activity towards serotonin receptors, which can be used in the design of new nonbasic ligands.

Chiral Recognition of Flexible Melatonin Receptor Ligands Induced by Conformational Equilibria

N-anilinoethylamides are a class of melatoninergic agents with the aniline portion mimicking the indole ring of the natural ligand and the ethylamide chain reproducing that of melatonin. The simplest compound in this class, N-{2-[(3-methoxyphenyl)methylamino]ethyl}acetamide (UCM793), has nanomolar binding affinity for MT₁ and MT₂ membrane receptors. To explore the effect of chain conformation on receptor binding, a methyl group was inserted on the methylene alpha or beta to the amide nitrogen and conformational equilibria were investigated by NMR spectroscopy and molecular dynamics simulations. Receptor affinity was conserved only for the beta-methyl derivative, which also showed significant stereoselectivity, with the (S) enantiomer being the eutomer. Molecular dynamics simulations, validated by NMR spectroscopy, showed that the beta-methyl group affects the conformational preferences of the ethylamide chain. Docking into the receptor crystal structure provides a rationale for the observed chiral recognition, suggesting that the (S)-beta-methyl group favors the conformation that better fits the receptor binding site.

[A profile of antidepressive effects of agomelatine and a current view on the mechanism of its action]

Agomelatine is one of the latest antidepressants (melatoninergic agonists) with a new mechanism of action. From the positions of classical monoaminoergic theory, tts mechanism of action is difficult to understand, because the drug increases the levels of monoamines and neurotrophic factors, while not affecting their reuptake and negative feedback, which control neurotransmission level. Besides the effect on suprachiasmatic nucleus, a relevant role in the mechanism of action of agomelatine plays its special functionally selective (with regard to intracellular signaling pathways) interaction with heteromeric complexes of serotonin 5-НТ2С and melatonin MT2 receptors in the hippocampus and cerebral cortex. Agomelatine is competitive to other modern antidepressants in the efficacy assessed by the percentage of complete responders and superior in the total frequency of remissions. Compared to other SSRI antidepressants, agomelatine is more effective for anhedonia. In these cases, agomelatine increases the level of brain-derived neurotrophic factor (BDNF) in the blood of responders.