Vol. 293, Issue 1, 275-280, April 2000

Pharmacological Characterization of ¹²⁵I-1229U91 Binding to Y1 and Y4 Neuropeptide Y/Peptide YY Receptors

Douglas A. Schober, Susan L. Gackenheimer, Mark L. Heiman and Donald R. Gehlert

Lilly Neuroscience (D.A.S., S.L.G., D.R.G.) and Lilly Endocrine Research (M.L.H.), Lilly Research Laboratories, Eli Lilly and Co., Lilly Corporate Center, Indianapolis, Indiana

1229U91 (GW1229 or GR231118) [lle,Glu,Pro,Dpr,Tyr, Arg,Leu,Arg,Tyr-NH₂)2 cyclic (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. However, 1229U91 also displaces ¹²⁵I-peptide YY (PYY) with high affinity from the Y4 subtype. Previously, we reported that 1229U91 had full agonist properties for the Y4 receptor. To characterize the pharmacological properties of 1229U91 directly, we had it radioiodinated with the chloromine-T method. ¹²⁵I-1229U91 bound to cell lines expressing the human Y1 and Y4 receptors with high affinity. The K_d and B_max for ¹²⁵I-1229U91 binding to Y1 were 14.9 pM and 1458 fmol/mg protein, respectively. The Y4 receptor bound ¹²⁵I-1229U91 with a K_d of 12.5 pM and a B_max of 1442 fmol/mg protein. When competing ¹²⁵I-1229U91 binding from Y1 and Y4 receptors, a similar rank order of potency was observed: 1229U91 > [Leu³¹,Pro³⁴]-NPY  [Leu³¹,Pro³⁴]-PYY > PYY  NPY > NPY(2-36) > PYY(3-36). Pancreatic polypeptide (PP) potently displaced ¹²⁵I-1229U91 from the Y4 receptor, but displayed little affinity for Y1. In autoradiographic studies with rat brain sections, ¹²⁵I-1229U91 bound with a distribution similar to that reported for the Y1 receptor when localized with ¹²⁵I-[Leu³¹,Pro³⁴]-PYY. Brain regions exhibiting binding sites for ¹²⁵I-PP were not detected with this radioligand. Those include the interpeduncular nucleus and the periventricular nucleus of the hypothalamus. Furthermore, ¹²⁵I-labeled rat PP was not displaced from these areas with 10 nM 1229U91. Thus, ¹²⁵I-1229U91 is a high affinity Y1 and Y4 radioligand and binds with a distribution in the rat brain consistent with the localization of the Y1 receptor.