Distribution of [Leu 31,Pro 34]NPY-sensitive, BIBP3226-insensitive [ 125I]PYY(3–36) binding sites in rat brain: possible relationship to Y 5 NPY receptors

Abstract

Recently, using molecular cloning approaches, three new neuropeptide Y (NPY)/peptide YY (PYY) receptors have been described in rodent brain, with pharmacological profiles that differ from the three previously described Y 1, Y 2 and Y 3 NPY receptors and the Y 4 pancreatic polypeptide- (PP-) preferring receptor. Two of these new receptors are spice variants and are called Y 5 receptors, whilst a third receptor has been called Y 6 and has been suggested to be expressed only in the mouse. In the absence of a totally selective Y 5 and/or Y 6 radioligands, we have examined [ 125I]PYY(3–36) binding, which binds Y 2 and Y 5/Y 6 receptors, using homogenate assays and quantitative receptor autoradiography to study the distribution of the three newly discovered Y 5/Y 6 receptors by masking binding to Y 1 receptors with high concentrations of the non-peptidergic selective Y 1 antagonist, BIBP3226, and using either [Leu 31,Pro 34]NPY or human PP to mask binding to Y 5 and Y 6 receptors, leaving binding to Y 2 receptors. Using this approach, [ 125I]PYY(3–36) labels a small population of Y 1 receptors and a larger population of binding sites that are insensitive to BIBP3226, human PP and [Leu 31,Pro 34]NPY, presumed to be Y 2 receptors. There was also [ 125I]PYY(3–36) binding to sites sensitive to NPY, human PP and [Leu 31,Pro 34]NPY, but insensitive to BIBP3226, located in the hypothalamus, amygdala, hippocampus and thalamus. As one of the recently cloned Y 5 receptors is synthesized in these regions, as shown by in-situ hybridization techniques, we suggest that the small population of [ 125I]PYY(3–36) binding sites which are sensitive to human PP and [Leu 31,Pro 34]NPY, but insensitive to BIBP3226, may represent binding to Y 5 receptors. We have been unable, however, to visualize a smaller population of Y 6 receptors which are labelled by [ 125I]PYY 3–36 and sensitive to [Leu 31,Pro 34]NPY, but not to BIBP3226 and human PP, confirming that the murine Y 6 receptor does not appear to be expressed in rat brain.