Increased brain neuropeptide Y1 and Y2 receptor binding in NPY knock out mice does not result in increased receptor function

The brain neuropeptide Neuropeptide Y (NPY) is an important modulator of a number of centrally mediated processes including feeding, anxiety-like behaviors, blood pressure and others. NPY produces its effects through at least four functional G-protein coupled receptors termed Y1, Y2, Y4 and Y5. In the brain, the Y1 and Y2 receptor subtypes are the predominant receptor population. To better understand the roles of NPY, genetically modified mice lacking NPY were produced but lacked the expected phenotypes. These mice have previously been reported to have a marked increase in Y2 receptor binding. In the present study, we found an upregulation of both Y1 and Y2 receptor binding and extended these findings to the female. These increases were as large as 10-fold or greater in many brain regions. To assess functional coupling of the receptors, we performed agonist-induced [35S]GTPγS autoradiography. In the mouse brain, the Y1/Y4/Y5 agonist Leu31,Pro34-NPY increased [35S]GTPγS binding with a regional distribution consistent with that produced when labeling adjacent sections with [125I]-Leu31,Pro34-PYY. In a few brain regions, minor increases were noted in the agonist-induced binding when comparing knock out mice to wild type. The Y2 agonist C2-NPY stimulated [35S]GTPγS binding in numerous brain areas with a regional distribution similar to the binding observed with [125I]-PYY3-36. Again, no major increases were noted in the functional activation of Y2 receptors between knock out and wild type mice. Therefore, the increased Y1 and Y2 binding observed in the NPY knock out mice does not represent an increase in NPY receptor mediated signaling and is likely due to an increase in spare (uncoupled) receptors.