Autoradiographic reevaluation of the binding properties of 125I-[Leu31,Pro34]peptide YY and 125I-peptide YY3-36 to neuropeptide Y receptor subtypes in rat forebrain

I-125-[Leu(31),Pro(34)]peptide YY (PYY) and I-125-PYY3-36, initially described as selective neuropeptide Y Y-1 and Y-2 receptor ligands, respectively, were recently shown to label also Y-4 and Y-5 receptors. We used receptor autoradiography to assess whether these ligands can be reliably used to investigate the various neuropeptide Y receptors in rat forebrain, in most of the brain regions examined tin coronal sections at the level of dorsal hippocampus), specific I-125-[Leu(31),pro(34)]PYY binding was completely inhibited by 1 mu M BIBP-3226, a selective Y-1 receptor ligand, but unaffected by 10 nM rat pancreatic polypeptide, selectively inhibiting Y-4 receptors, suggesting that Y-4 receptors are present in negligible numbers compared with Y-1 receptors in the areas examined. Significant numbers of BIBP-3226-insensitive (125)l-[Leu(31),Pro(34)]PYY binding sites were measured in the CA3 subfield of the hippocampus only, possibly representing Y, receptors. I-125-PYY3-36 binding was unchanged by 1 mu M BIBP-3226, whereas a population of I-125-PYY3-36 binding sites was sensitive to 100 nM [Leu(31),Pro(34)]neuropeptide Y, likely representing Y-5 receptors. The possibility of distinguishing between Y-2 and Y-5 receptors using (125)l-PYY3-36 as radioligand was validated by their different regional distribution and their distinct changes 24 h after kainate seizures, i.e., binding to Y-5 receptors was selectively decreased in the outer cortex, whereas binding to Y-2 receptors was enhanced in the hippocampus. Thus, the use of selective unlabeled compounds is required for distinguishing the various receptor subtypes labeled by I-125-[Leu(31),Pro(34)]PYY and I-125-PYY3-36 in rat brain tissue.