Autoradiographic Reevaluation of the Binding Properties of 125 I‐[Leu 31 ,Pro 34 ]Peptide YY and 125 I‐Peptide YY 3‐36 to Neuropeptide Y Receptor Subtypes in Rat Forebrain

125 I‐[Leu 31 ,Pro 34 ]peptide YY (PYY) and 125 I‐PYY 3‐36 , 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 (in coronal sections at the level of dorsal hippocampus), specific 125 I‐[Leu 31 ,Pro 34 ]PYY binding was completely inhibited by 1 μ M BIBP‐3226, a selective Y 1 receptor ligand, but unaffected by 10 n M 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 I‐[Leu 31 ,Pro 34 ]PYY binding sites were measured in the CA3 subfield of the hippocampus only, possibly representing Y 5 receptors. 125 I‐PYY 3‐36 binding was unchanged by 1 μ M BIBP‐3226, whereas a population of 125 I‐PYY 3‐36 binding sites was sensitive to 100 n M [Leu 31 ,Pro 34 ]neuropeptide Y, likely representing Y 5 receptors. The possibility of distinguishing between Y 2 and Y 5 receptors using 125 I‐PYY 3‐36 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 125 I‐[Leu 31 ,Pro 34 ]PYY and 125 I‐PYY 3‐36 in rat brain tissue.