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Concept: 5-HT2B receptor


The prototypical hallucinogen LSD acts via serotonin receptors, and here we describe the crystal structure of LSD in complex with the human serotonin receptor 5-HT2B. The complex reveals conformational rearrangements to accommodate LSD, providing a structural explanation for the conformational selectivity of LSD’s key diethylamide moiety. LSD dissociates exceptionally slow from both 5-HT2BR and 5-HT2AR-a major target for its psychoactivity. Molecular dynamics (MD) simulations suggest that LSD’s slow binding kinetics may be due to a “lid” formed by extracellular loop 2 (EL2) at the entrance to the binding pocket. A mutation predicted to increase the mobility of this lid greatly accelerates LSD’s binding kinetics and selectively dampens LSD-mediated β-arrestin2 recruitment. This study thus reveals an unexpected binding mode of LSD; illuminates key features of its kinetics, stereochemistry, and signaling; and provides a molecular explanation for LSD’s actions at human serotonin receptors. PAPERCLIP.

Concepts: Signal transduction, Receptor, Serotonin, Agonist, 5-HT receptor, 5-HT2C receptor, 5-HT1A receptor, 5-HT2B receptor


Serotonin or 5-hydroxytryptamine (5-HT) regulates a wide spectrum of human physiology through the 5-HT receptor family. We report the crystal structures of the human 5-HT1B G protein-coupled receptor bound to the agonist antimigraine medications ergotamine and dihydroergotamine. The structures reveal similar binding modes for these ligands, which occupy the orthosteric pocket and an extended binding pocket close to the extracellular loops. The orthosteric pocket is formed by residues conserved in the 5-HT receptor family, clarifying the family-wide agonist activity of 5-HT. Compared with the accompanying structure of the 5-HT2B receptor, the 5-HT1B receptor displays a 3-angstrom outward shift at the extracellular end of helix V, resulting in a more open extended pocket that explains subtype selectivity. Together with docking and mutagenesis studies, these structures provide a comprehensive structural basis for understanding receptor-ligand interactions and designing subtype-selective serotonergic drugs.

Concepts: Signal transduction, Receptor, Serotonin, Agonist, 5-HT receptor, 5-HT2C receptor, 5-HT1B receptor, 5-HT2B receptor


Intermittent spinal serotonin receptor activation elicits phrenic motor facilitation (pMF), a form of spinal respiratory motor plasticity. Episodic activation of either serotonin type-2 (5-HT2) or 7 (5-HT7) receptors elicits pMF, although they do so via distinct cellular mechanisms known as the Q (5-HT2) and S (5-HT7) pathways to pMF. When co-activated, these pathways interact via mutual, cross-talk inhibition. Although we have a rudimentary understanding of mechanisms mediating cross-talk interactions between spinal 5-HT2 subtype-A (5-HT2A) and 5-HT7 receptor activation, we do not know if similar interactions exist between 5-HT2 subtype-B (5-HT2B) and 5-HT7 receptors. We confirmed that either spinal 5-HT2B or 5-HT7 receptor activation alone elicits pMF and tested the hypotheses that: 1) concurrent activation of both receptors suppresses pMF due to cross-talk inhibition; 2) 5-HT7 receptor inhibition of 5-HT2B receptor-induced pMF requires protein kinase A (PKA) activity; and 3) 5-HT2B receptor inhibition of 5-HT7 receptor-induced pMF requires NADPH oxidase (NOX) activity. Selective 5-HT2B and 5HT7 receptor agonists were administered intrathecally at C4 (3 injections, 5-min intervals) to anesthetized, paralyzed and ventilated rats. Whereas integrated phrenic nerve burst amplitude increased after selective spinal 5-HT2B or 5-HT7 receptor activation alone (i.e. pMF), pMF was no longer observed with concurrent 5-HT2B and 5-HT7 receptor agonist administration. With concurrent receptor activation, pMF was rescued by inhibiting either NOX or PKA activity, demonstrating their roles in cross-talk inhibition between these pathways to pMF. This report demonstrates cross-talk inhibition between 5-HT2B- and 5-HT7 receptor-induced pMF, and that NOX and PKA activity are necessary for that cross-talk inhibition.

Concepts: Signal transduction, Receptor antagonist, Serotonin, Agonist, G protein coupled receptors, Inverse agonist, 5-HT receptor, 5-HT2B receptor


The aim of the present study is to investigate the role and potential mechanisms of peripheral serotonin in postoperative intra-abdominal adhesion formation in mice. The caecum-rubbing operations were conducted for intra-abdominal adhesion formation modelling in wild-type and Tph1-/- mice. The deficiency of serotonin significantly decreased the adhesion scores, weight loss, and adhesion thickness as well as levels of collagen fibres and hydroxyproline in the adhesive tissues. The Tph1-/- mice exhibited a milder inflammatory response and oxidative stress in the adhesive tissues than did the wild-type mice. Moreover, the deficiency of serotonin reduced the levels of PAI-1 and fibrinogen, and raised the t-PA and t-PA/PAI levels in the peritoneal fluids. Moreover, the expressions of CD34, VEGF, TGF-β and 5-HT2B receptor in the adhesive tissues were significantly decreased in the Tph1-/- group mice. Furthermore, the Tph1-/- +5-HTP group showed more severe adhesions than did the Tph1-/- group mice, and the p-chlorophenylalanine (PCPA) could markedly alleviated the adhesion formation in the WT mice. In conclusion, the present study showed that peripheral serotonin regulated postoperative intra-abdominal adhesion formation by facilitating inflammation, oxidative stress, disorder of the fibrinolytic system, angiopoiesis and TGF-β1 expression via the 5-HT2B receptor in the adhesive tissues.

Concepts: Inflammation, Collagen, Fibrin, Fibrinolysis, Bowel obstruction, Serotonin, Adhesion, 5-HT2B receptor


Serotonin (5-HT) receptors are important in health and disease, but the existence of 14 subtypes necessitates selective ligands. Previously, the pulicatins were identified as ligands that specifically bound to the subtype 5-HT2B in the 500 nM to 10 μM range and that exhibited in vitro effects on cultured mouse neurons. Here, we examined the structure-activity relationship of 30 synthetic and natural pulicatin derivatives using binding, receptor functionality, and in vivo assays. The results reveal the 2-arylthiazoline scaffold as a tunable serotonin receptor-targeting pharmacophore. Tests in mice show potential antiseizure and antinociceptive activities at high doses without motor impairment.

Concepts: Signal transduction, Receptor, In vivo, In vitro, Serotonin, Agonist, Medicinal chemistry, 5-HT2B receptor


Pulmonary arterial hypertension (PAH), a condition characterized by pulmonary vasculature constriction and remodeling, involves dysregulation of the serotonin (5-HT) receptors 5-HT2A and 5-HT2B. A rat model of monocrotaline (MCT)-induced PAH was used to examine the potential beneficial effects of RP5063, a 5-HT receptor modulator. After a single 60 mg/kg dose of MCT, rats were gavaged twice-daily (b.i.d.) with vehicle, RP5063 (1, 3, or 10 mg/kg), or sildenafil (50 mg/kg) for 28 days. RP5063 at a dose as low as 1 mg/kg, b.i.d. reduced pulmonary resistance and increased systemic blood oxygen saturation. The highest dose of RP5063 (10 mg/kg, b.i.d.) reduced diastolic, systolic, and mean pulmonary pressure, right systolic ventricular pressure, ventilatory pressure, and Fulton’s index (ratio of right to left ventricular weight). Doses as low as 3 mg/kg RP5063, b.i.d. also increased weight gain and body temperature, suggesting an improvement in overall health of MCT-treated animals. Similar reductions in pulmonary, right ventricular, and ventilatory pressure, pulmonary resistance, and Fulton’s index as well as increased systemic blood oxygen saturation were observed in animals treated with the reference agent sildenafil at a higher dose (50 mg/kg, b.i.d.). Histological examination revealed that RP5063 produced dose-dependent reductions in pulmonary blood vessel wall thickness and proportion of muscular vessels, similar to sildenafil. RP5063 completely blocked MCT-induced increases in the plasma cytokines TNFα, ILβ, and IL-6 at all doses. In summary, RP5063 improved pulmonary vascular pathology and hemodynamics, right ventricular pressure and hypertrophy, systemic oxygen saturation, and overall health of rats treated with MCT.

Concepts: Blood, Heart, Blood vessel, Blood pressure, Vein, Pulmonary artery, Serotonin, 5-HT2B receptor


The serotonin receptor subtypes 2 comprises 5-HT2A, 5-HT2B and 5-HT2C that are Gq-coupled receptors and display distinct pharmacological properties. Although co-expressed in some brain regions and involved in various neurological disorders, their functional interactions have not been studied yet. We report that 5-HT2 receptors can form homo and heterodimers when expressed alone or co-expressed in transfected cells. Co-immunoprecipitation and bioluminescence resonance energy transfer studies confirmed that 5-HT2C receptors interact with either 5-HT2A or 5-HT2B receptors. Although heterodimerization with 5-HT2C receptors does not alter 5-HT2C Gαq-dependent inositol-phosphate signaling, 5-HT2A- or 5-HT2B-receptor-mediated signaling was totally blunted. This feature can be explained by a dominance of 5-HT2C on 5-HT2A and 5-HT2B receptor binding: in 5-HT2C-containing heterodimers, ligands bind and activate exclusively the 5-HT2C protomer. This dominant effect on the associated protomer was also observed in neurons, supporting a physiological relevance of 5-HT2 receptors heterodimerization in-vivo. Accordingly, exogenous expression of an inactive form of the 5-HT2C receptor in the Locus ceruleus is associated with decreased 5-HT2A-dependent noradrenergic transmission. These data demonstrate that 5-HT2 receptors can form functionally asymmetric heterodimers in-vitro and in-vivo that must be considered when analyzing the physiological or pathophysiological roles of serotonin in tissues where 5-HT2 receptors are co-expressed.

Concepts: Receptor, Serotonin, G protein coupled receptors, Norepinephrine, 5-HT receptor, 5-HT2C receptor, 5-HT2 receptor, 5-HT2B receptor


Wacker et al. report the crystal structure of LSD in complex with one of its major targets in the brain, the 5-HT2B receptor, the first such structure for any psychedelic drug. The results shed light on the molecular mechanisms underlying its ability to induce hallucinations with greater duration and potency than closely related compounds.

Concepts: Brain, Serotonin, Psychoactive drug, Controlled Substances Act, Psychedelics, dissociatives and deliriants, Timothy Leary, 5-HT2B receptor, Psychedelic music


Pancreatic β-cells synthesize and release serotonin (5 hydroxytryptamine, 5HT); however, the role of 5HT receptors on glucose stimulated insulin secretion (GSIS) and the mechanisms mediating this function is not fully understood. The aims of this study were to determine the expression profile of 5HT receptors in murine MIN6 β-cells and to examine the effects of pharmacological activation of 5HT receptor Htr2b on GSIS and mitochondrial function.

Concepts: Insulin, Receptor, Serotonin, Norepinephrine, 5-HT receptor, 5-HT2C receptor, 5-HT1B receptor, 5-HT2B receptor


Although 5-HT has been implicated in cholestatic itch and antinociception, two common phenomena in patients with cholestatic disease, the roles of 5-HT receptor subtypes are unclear. Herein, we investigated the roles of 5-HT receptors in itch and antinociception associated with cholestasis, which was induced by common bile duct ligation (BDL) in rats. 5-HT-induced enhanced scratching and antinociception to mechanical and heat stimuli were demonstrated in BDL rats. 5-HT level in the skin and spinal cord was significantly increased in BDL rats. Quantitative RT-PCR analysis showed 5-HT1B, 5-HT1D, 5-HT2A, 5-HT3A, 5-HT5B, 5-HT6, and 5-HT7 were up-regulated in peripheral nervous system and 5-HT1A, 5-HT1F, 5-HT2B, and 5-HT3A were down-regulated in the spinal cord of BDL rats. Intradermal 5-HT2, 5-HT3, and 5-HT7 receptor agonists induced scratching in BDL rats, whereas 5-HT3 agonist did not induce scratching in sham rats. 5-HT1A, 5-HT2, 5-HT3, and 5-HT7 agonists or antagonists suppressed itch in BDL rats. 5-HT1A agonist attenuated, but 5-HT1A antagonist enhanced antinociception in BDL rats. 5-HT2 and 5-HT3 agonists or antagonists attenuated antinociception in BDL rats. Our data suggested peripheral and central 5-HT system dynamically participated in itch and antinociception under cholestasis condition and targeting 5-HT receptors may be an effective treatment for cholestatic itch.

Concepts: Receptor antagonist, Serotonin, Agonist, G protein coupled receptors, Inverse agonist, Buprenorphine, 5-HT receptor, 5-HT2B receptor