Distribution of [Leu,Pro]NPY-sensitive, BIBP3226-insensitive [I]PYY(3–36) binding sites in rat brain: possible relationship to Y5 NPY receptors

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 Y₁, Y₂ and Y₃ NPY receptors and the Y₄ pancreatic polypeptide- (PP-) preferring receptor. Two of these new receptors are spice variants and are called Y₅ receptors, whilst a third receptor has been called Y₆ and has been suggested to be expressed only in the mouse. In the absence of a totally selective Y₅ and/or Y₆ radioligands, we have examined [¹²⁵I]PYY(3–36) binding, which binds Y₂ and Y₅/Y₆ receptors, using homogenate assays and quantitative receptor autoradiography to study the distribution of the three newly discovered Y₅/Y₆ receptors by masking binding to Y₁ receptors with high concentrations of the non-peptidergic selective Y₁ antagonist, BIBP3226, and using either [Leu³¹,Pro³⁴]NPY or human PP to mask binding to Y₅ and Y₆ receptors, leaving binding to Y₂ receptors. Using this approach, [¹²⁵I]PYY(3–36) labels a small population of Y₁ receptors and a larger population of binding sites that are insensitive to BIBP3226, human PP and [Leu³¹,Pro³⁴]NPY, presumed to be Y₂ receptors. There was also [¹²⁵I]PYY(3–36) binding to sites sensitive to NPY, human PP and [Leu³¹,Pro³⁴]NPY, but insensitive to BIBP3226, located in the hypothalamus, amygdala, hippocampus and thalamus. As one of the recently cloned Y₅ 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 [Leu³¹,Pro³⁴]NPY, but insensitive to BIBP3226, may represent binding to Y₅ receptors. We have been unable, however, to visualize a smaller population of Y₆ receptors which are labelled by [¹²⁵I]PYY₃–36 and sensitive to [Leu³¹,Pro³⁴]NPY, but not to BIBP3226 and human PP, confirming that the murine Y₆ receptor does not appear to be expressed in rat brain.