Pharmacological characterization of I-125-1229U91 binding to Y1 and Y4 neuropeptide Y/peptide YY receptors

1229U91 (GW1229 or GR231118) [IIe,Glu,Pro,Dpr,Tyr, Arg,Leu,Arg,Tyr-NH2)2 cy clic (2,4'),(2'4)-diamide] has been reported by several research groups to be a potent antagonist at the Y1 neuropeptide Y (NPY) receptor subtype. How ever, 1229U91 also displaces I-125-peptide YY (PYY) with high affinity from the Y4 subtype. Previously, we reported that 1229U91 had full agonist prop erties for the Y4 receptor. To characterize the pharmacological properties of 1229U91 directly, we had it radioiodinated with the chloromine-T method. I-125-1229U91 bound to cell lines expressing the human Y1 and Y4 receptors with high affinity. The K-d and B-max for I-125-1229U91 binding to Y1 were 14.9 pM and 1458 fmol/mg protein, respectively. The Y4 receptor bound I-12 5-1229U91 with a K-d of 12.5 pM and a B-max of 1442 fmol/mg protein. When c ompeting I-125-1229U91 binding from Y1 and Y4 receptors, a similar rank ord er of potency was observed: 1229U91 > [Leu(31),pro(34)]-NPY greater than or equal to [Leu(31),pro(34)]-PYY > PYY greater than or equal to NPY > NPY(2- 36) > PYY(3-36). Pancreatic polypeptide (PP) potently displaced I-125-1229U 91 from the Y4 receptor, but displayed little affinity for Y1. In autoradio graphic studies with rat brain sections, I-125-1229U91 bound with a distrib ution similar to that reported for the Y1 receptor when localized with I-12 5-[Leu(31),Pro(34)]-PYY. Brain regions exhibiting binding sites for I-125-P P were not detected with this radioligand. Those include the interpeduncula r nucleus and the periventricular nucleus of the hypothalamus. Furthermore, I-125-labeled rat PP was not displaced from these areas with 10 nM 1229U91 . Thus, I-125-1229U91 is a high affinity Y1 and Y4 radioligand and binds wi th a distribution in the rat brain consistent with the localization of the Y1 receptor.