[³H]UR-JG102-A Radiolabeled Cyclic Peptide with High Affinity and Excellent Selectivity for the Neuropeptide Y Y₄ Receptor

The family of human neuropeptide Y receptors (YRs) comprises four subtypes (Y₁R, Y₂R, Y₄R, and Y₅R) that are involved in the regulation of numerous physiological processes. Until now, Y₄R binding studies have been predominantly performed in hypotonic sodium-free buffers using ¹²⁵I-labeled derivatives of the endogenous YR agonists pancreatic polypeptide or peptide YY. A few tritium-labeled Y₄R ligands have been reported; however, when used in buffers containing sodium at a physiological concentration, their Y₄R affinities are insufficient. Based on the cyclic hexapeptide UR-AK86C, we developed a new tritium-labeled Y₄R radioligand ([³H]UR-JG102, [³H]20). In sodium-free buffer, [³H]20 exhibits a very low Y₄R dissociation constant (K_d 0.012 nM). In sodium-containing buffer (137 mM Na⁺), the Y₄R affinity is lower (K_d 0.11 nM) but still considerably higher compared to previously reported tritiated Y₄R ligands. Therefore, [³H]20 represents a useful tool compound for the determination of Y₄R binding affinities under physiological-like conditions.