This study elucidated the structure-activity relationships (SAR) of three distinct opioid ligand series at the μ-opioid receptor (μ-OR) and investigated the ligand-dependent role of arrestin in downstream signaling. Radioligand binding assays across 13 compounds revealed that high-affinity μ-OR binding is not restricted to a single chemical scaffold. For fentanyl analogs, SAR analysis identified four key structural determinants: the N-phenethyl group (R1) is essential for μ-OR binding, the piperidine 4-position (R2) is sterically constrained and tolerates only small substituents such as carbomethoxy, the acyl side chain (R3) tolerates diverse chemical modifications including alkyl, alkoxy, and heteroaryl groups without substantial loss of affinity, and para-substitution on the phenethyl ring (R4) with electron-withdrawing groups like fluorine is well-tolerated. Analysis of non-fentanyl scaffolds (2-Methyl AP-237, W-15, and brorphine) demonstrated that specific side-chain functionalization, rather than core scaffold architecture, primarily determines binding potency. Comparison of classical opioids further revealed that structural flexibility, exemplified by methadone, can confer superior binding affinity compared to the rigid morphine scaffold. Consistent with this structural diversity, the contribution of arrestins to ERK activation varied substantially across compounds, suggesting that μ-OR-biased signaling is not dictated exclusively by intrinsic receptor properties but emerges from the interplay between receptor conformation and ligandspecific structural features. Collectively, our results provide a molecular framework for understanding opioid pharmacology with important implications for rational drug design aimed at minimizing adverse effects while maintaining analgesic efficacy.

The N-methyl-D-aspartate receptor (NMDA-R) subunit GluN2B is abundantly expressed in brain regions critical for synaptic plasticity and cognitive processes. This study investigated the structure-activity relationships (SAR) of NMDA-R ligands using GluN2B as a molecular target. Thirty potential NMDA-R antagonists were categorized into two structural classes: 1-(1-phenylcyclohexyl) amines (series A) and α-amino-2-phenylcyclohexanone derivatives (series B). In series A compounds, the phenyl ring and R1 substituents were positioned at the carbon center of the cyclohexyl ring, with R2 substituents at the para- or meta-positions of the phenyl ring. SAR analysis revealed optimal binding affinity when R1 was carbonyl (C=O) and R2 was 4-methoxy (4-OMe). Series B compounds featured a cyclohexanone scaffold with NH-R1 at the α-position and a phenyl ring bearing R2 substituents at ortho-, meta-, or para-positions. Maximum binding affinity was achieved with R1 as hydrogen (H) and R2 as hydroxyl (OH). Compounds were assessed for GluN2B-mediated ERK activation to evaluate potential metabotropic signaling properties. Approximately 50% of the compounds demonstrated ERK activation through a non-ionotropic signaling cascade involving Src, phosphatidylinositol 3-kinase, and protein kinase C. This study elucidated key structural determinants for NMDA-R binding and characterized a novel metabotropic signaling pathway. Notably, our findings suggest that compounds acting as antagonists at the ionotropic site may simultaneously function as agonists through non-ionotropic mechanisms.

Although histone deacetylase 6 (HDAC6) is considered a therapeutic target for Alzheimer’s disease (AD), its role in cholinergic dysfunction in AD patients remains unclear. This study investigated the effects of (E)-3-(2-(4-fluorostyryl)thiazol-4-yl)-N-hydroxypropanamide (4-FHA), a new synthetic HDAC6 inhibitor, on cognitive and memory impairments in a scopolamine-induced-AD mouse model. Behaviorally, 4-FHA improved scopolamine-induced memory impairments in the Y-maze, passive avoidance, and Morris water maze tests. In addition, 4-FHA ameliorated scopolamine-induced cognitive impairments in the novel object recognition and place recognition tests. Furthermore, 4-FHA increased acetylation of α-tubulin (a major HDAC6 substrate); the expression of BDNF; and the phosphorylation of ERK 1/2, CREB, and ChAT in the hippocampus of scopolamine-treated mice. In summary, according to our data 4-FHA, an HDAC6 inhibitor, improved the cognitive and memory deficits of the AD mouse model by normalizing BDNF signaling and synaptic transmission, suggesting that 4-FHA might be a potential therapeutic candidate for AD.

The N-methyl-D-aspartate receptor (NMDA-R) subunit GluN2B is abundantly expressed in brain regions critical for synaptic plasticity and cognitive processes. This study investigated the structure-activity relationships (SAR) of NMDA-R ligands using GluN2B as a molecular target. Thirty potential NMDA-R antagonists were categorized into two structural classes: 1-(1-phenylcyclohexyl) amines (series A) and α-amino-2-phenylcyclohexanone derivatives (series B). In series A compounds, the phenyl ring and R1 substituents were positioned at the carbon center of the cyclohexyl ring, with R2 substituents at the para- or meta-positions of the phenyl ring. SAR analysis revealed optimal binding affinity when R1 was carbonyl (C=O) and R2 was 4-methoxy (4-OMe). Series B compounds featured a cyclohexanone scaffold with NH-R1 at the α-position and a phenyl ring bearing R2 substituents at ortho-, meta-, or para-positions. Maximum binding affinity was achieved with R1 as hydrogen (H) and R2 as hydroxyl (OH). Compounds were assessed for GluN2B-mediated ERK activation to evaluate potential metabotropic signaling properties. Approximately 50% of the compounds demonstrated ERK activation through a non-ionotropic signaling cascade involving Src, phosphatidylinositol 3-kinase, and protein kinase C. This study elucidated key structural determinants for NMDA-R binding and characterized a novel metabotropic signaling pathway. Notably, our findings suggest that compounds acting as antagonists at the ionotropic site may simultaneously function as agonists through non-ionotropic mechanisms.

Although histone deacetylase 6 (HDAC6) is considered a therapeutic target for Alzheimer’s disease (AD), its role in cholinergic dysfunction in AD patients remains unclear. This study investigated the effects of (E)-3-(2-(4-fluorostyryl)thiazol-4-yl)-N-hydroxypropanamide (4-FHA), a new synthetic HDAC6 inhibitor, on cognitive and memory impairments in a scopolamine-induced-AD mouse model. Behaviorally, 4-FHA improved scopolamine-induced memory impairments in the Y-maze, passive avoidance, and Morris water maze tests. In addition, 4-FHA ameliorated scopolamine-induced cognitive impairments in the novel object recognition and place recognition tests. Furthermore, 4-FHA increased acetylation of α-tubulin (a major HDAC6 substrate); the expression of BDNF; and the phosphorylation of ERK 1/2, CREB, and ChAT in the hippocampus of scopolamine-treated mice. In summary, according to our data 4-FHA, an HDAC6 inhibitor, improved the cognitive and memory deficits of the AD mouse model by normalizing BDNF signaling and synaptic transmission, suggesting that 4-FHA might be a potential therapeutic candidate for AD.

The N-methyl-D-aspartate receptor (NMDA-R) subunit GluN2B is abundantly expressed in brain regions critical for synaptic plasticity and cognitive processes. This study investigated the structure-activity relationships (SAR) of NMDA-R ligands using GluN2B as a molecular target. Thirty potential NMDA-R antagonists were categorized into two structural classes: 1-(1-phenylcyclohexyl) amines (series A) and α-amino-2-phenylcyclohexanone derivatives (series B). In series A compounds, the phenyl ring and R1 substituents were positioned at the carbon center of the cyclohexyl ring, with R2 substituents at the para- or meta-positions of the phenyl ring. SAR analysis revealed optimal binding affinity when R1 was carbonyl (C=O) and R2 was 4-methoxy (4-OMe). Series B compounds featured a cyclohexanone scaffold with NH-R1 at the α-position and a phenyl ring bearing R2 substituents at ortho-, meta-, or para-positions. Maximum binding affinity was achieved with R1 as hydrogen (H) and R2 as hydroxyl (OH). Compounds were assessed for GluN2B-mediated ERK activation to evaluate potential metabotropic signaling properties. Approximately 50% of the compounds demonstrated ERK activation through a non-ionotropic signaling cascade involving Src, phosphatidylinositol 3-kinase, and protein kinase C. This study elucidated key structural determinants for NMDA-R binding and characterized a novel metabotropic signaling pathway. Notably, our findings suggest that compounds acting as antagonists at the ionotropic site may simultaneously function as agonists through non-ionotropic mechanisms.

Although histone deacetylase 6 (HDAC6) is considered a therapeutic target for Alzheimer’s disease (AD), its role in cholinergic dysfunction in AD patients remains unclear. This study investigated the effects of (E)-3-(2-(4-fluorostyryl)thiazol-4-yl)-N-hydroxypropanamide (4-FHA), a new synthetic HDAC6 inhibitor, on cognitive and memory impairments in a scopolamine-induced-AD mouse model. Behaviorally, 4-FHA improved scopolamine-induced memory impairments in the Y-maze, passive avoidance, and Morris water maze tests. In addition, 4-FHA ameliorated scopolamine-induced cognitive impairments in the novel object recognition and place recognition tests. Furthermore, 4-FHA increased acetylation of α-tubulin (a major HDAC6 substrate); the expression of BDNF; and the phosphorylation of ERK 1/2, CREB, and ChAT in the hippocampus of scopolamine-treated mice. In summary, according to our data 4-FHA, an HDAC6 inhibitor, improved the cognitive and memory deficits of the AD mouse model by normalizing BDNF signaling and synaptic transmission, suggesting that 4-FHA might be a potential therapeutic candidate for AD.

The type-1 cannabinoid receptor (CB 1R) is a potential therapeutic target in several pathological conditions, including neuropsychological disorders and neurodegenerative diseases. Owing to their structural diversity, it is not easy to derive general structure–activity relationships (SARs) for CB 1R ligands. In this study, CB 1R ligands were classified into six structural families, and the corresponding SAR was determined for their affinities for CB 1R. In addition, we determined their functional activities for the activation of extracellular signal-regulated kinases (ERKs). Among derivatives of indol-3-yl-methanone, the highest ligand affinity was observed when a pentyl and a naphthalenyl group were attached to the N1 position of the indole ring and the carbon site of the methanone moiety, respectively. In the case of adamantane indazole-3-carboxamide derivatives, the presence of fluorine in the pentyl group, the substituent at the N1 position of the indazole ring, strongly increased the affinity for CB 1R. For (naphthalen-1-yl) methanone derivatives, the presence of 4-alkoxynaphthalene in the methanone moiety was more beneficial for the affinity to CB 1R than that of a heterocyclic ring. The functional activities of the tested compounds, evaluated through ERK assay, were correlated with their affinity for CB 1R, suggesting their agonistic nature. In conclusion, this study provides valuable insight for designing novel ligands for CB 1R, which can be used to control psychiatric disorders and drug abuse.

Numerous psychotropic and addictive substances possess structural features similar to those of β-phenethylamine (β-PEA). In this study, we selected 29 β-PEA derivatives and determined their structure–activity relationship (SAR) to their ability to inhibit dopamine (DA) reuptake; conducted docking simulation for two selected compounds; and identified their potential functionals. The compounds were subdivided into arylethylamines, 2-(alkyl amino)-1-arylalkan-1-one derivatives and alkyl 2-phenyl-2-(piperidin-2-yl)acetate derivatives. An aromatic group, alkyl group, and alkylamine derivative were attached to the arylethylamine and 2-(alkyl amino)-1-arylalkan-1-one derivatives. The inhibitory effect of the compounds on dopamine reuptake increased in the order of the compounds substituted with phenyl, thiophenyl, and substituted phenyl groups in the aromatic position; compounds with longer alkyl groups and smaller ring-sized compounds at the alkylamine position showed stronger inhibitory activities. Docking simulation conducted for two compounds, 9 and 28, showed that the (S)-form of compound 9 was more stable than the (R)-form, with a good fit into the binding site covered by helices 1, 3, and 6 of human dopamine transporter (hDAT). In contrast, the (R, S)-configuration of compound 28 was more stable than that of other isomers and was firmly placed in the binding pocket of DAT bound to DA. DAinduced endocytosis of dopamine D2 receptors was inhibited when they were co-expressed with DAT, which lowered extracellular DA levels, and uninhibited when they were pretreated with compound 9 or 28. In summary, this study revealed critical structural features responsible for the inhibition of DA reuptake and the functional role of DA reuptake inhibitors in regulating D2 receptor function.

Among 14 subtypes of serotonin receptors (5-HTRs), 5-HT 2AR plays important roles in drug addiction and various psychiatric disorders. Agonists for 5-HT 2AR have been classified into three structural groups: phenethylamines, tryptamines, and ergolines. In this study, the structure-activity relationship (SAR) of phenethylamine and tryptamine derivatives for binding 5-HT 2AR was determined. In addition, functional and regulatory evaluation of selected compounds was conducted for extracellular signal-regulated kinases (ERKs) and receptor endocytosis. SAR studies showed that phenethylamines possessed higher affinity to 5-HT 2AR than tryptamines. In phenethylamines, two phenyl groups were attached to the carbon and nitrogen (R 3 ) atoms of ethylamine, the backbone of phenethylamines. Alkyl or halogen groups on the phenyl ring attached to the β carbon exerted positive effects on the binding affinity when they were at para positions. Oxygen-containing groups attached to R 3 exerted mixed influences depending on the position of their attachment. In tryptamine derivatives, tryptamine group was attached to the β carbon of ethylamine, and ally groups were attached to the nitrogen atom. Oxygen-containing substituents on large ring and alkyl substituents on the small ring of tryptamine groups exerted positive and negative influence on the affinity for 5-HT 2AR, respectively. Ally groups attached to the nitrogen atom of ethylamine exerted negative influences. Functional and regulatory activities of the tested compounds correlated with their affinity for 5-HT 2AR, suggesting their agonistic nature. In conclusion, this study provides information for designing novel ligands for 5-HT 2AR, which can be used to control psychiatric disorders and drug abuse.