In response to three highly conserved neuropeptides, neuropeptide Y (NPY), peptide YY, and pancreatic polypeptide (PP), four G protein–coupled receptors mediate multiple essential physiological processes, such as food intake, vasoconstriction, sedation, and memory retention. Here, we report the structures of the human Y ₁ , Y ₂ , and Y ₄ receptors in complex with NPY or PP, and the G _i1 protein. These structures reveal distinct binding poses of the peptide upon coupling to different receptors, reflecting the importance of the conformational plasticity of the peptide in recognizing the NPY receptors. The N terminus of the peptide forms extensive interactions with the Y ₁ receptor, but not with the Y ₂ and Y ₄ receptors. Supported by mutagenesis and functional studies, subtype-specific interactions between the receptors and peptides were further observed. These findings provide insight into key factors that govern NPY signal recognition and transduction, and would enable development of selective drugs.

Fluorescence-labeled receptor ligands have emerged as valuable molecular tools, being indispensable for studying receptor–ligand interactions by fluorescence-based techniques such as high-content imaging, fluorescence microscopy, and fluorescence polarization. Through application of a new labeling strategy for peptides, a series of fluorescent neurotensin(8–13) derivatives was synthesized by attaching red-emitting fluorophores (indolinium- and pyridinium-type cyanine dyes) to carbamoylated arginine residues in neurotensin(8–13) analogues, yielding fluorescent probes with high NTS₁R affinity (pK_i values: 8.15–9.12) and potency (pEC₅₀ values (Ca²⁺ mobilization): 8.23–9.43). Selected fluorescent ligands were investigated by flow cytometry and high-content imaging (saturation binding, kinetic studies, and competition binding) as well as by confocal microscopy using intact CHO-hNTS₁R cells. The study demonstrates the applicability of the fluorescent probes as molecular tools to obtain, for example, information about the localization of receptors in cells and to determine binding affinities of nonlabeled ligands.

On the basis of the long-known prototypic pharmacophore 3-(1H-imidazol-4-yl)propylguanidine (SK&F 91486, 2), monomeric, homodimeric, and heterodimeric bisalkylguanidine-type histamine H₂ receptor (H₂R) agonists with various alkyl spacers were synthesized. Aiming at increased H₂R selectivity of the ligands, the imidazol-4-yl moiety was replaced by imidazol-1-yl, 2-aminothiazol-5-yl or 2-amino-4-methylthiazol-5-yl according to a bioisosteric approach. All compounds turned out to be partial or full agonists at the h/gp/rH₂R. The most potent analogue, the thiazole-type heterodimeric ligand 63 (UR-Po461), was a partial agonist (E_max = 88%) and 250 times more potent than histamine (pEC₅₀: 8.56 vs 6.16, gpH₂R, atrium). The homodimeric structures 56 (UR-Po395) and 58 (UR-Po448) exhibited the highest hH₂R affinities (pK_i: 7.47, 7.33) in binding studies. Dimeric amino(methyl)thiazole derivatives, such as 58, generated an increased hH₂R selectivity compared to the monomeric analogues, e.g., 139 (UR-Po444). Although monomeric ligands showed up lower affinities and potencies at the H₂R, compounds with a short alkylic side chain like 129 (UR-Po194) proved to be highly affine hH₄R ligands.