Joint migration of gonadotropin-releasing hormone (GnRH) and neuropeptide Y (NPY) neurons from olfactory placode to central nervous system.

The olfactory epithelium in vertebrates generates the olfactory sensory neurons and several migratory cell types. Prominent among the latter are the gonadotropin-releasing hormone (GnRH) neurons that differentiate within the olfactory epithelium during embryogenesis and migrate along the olfactory nerve to the central nervous system. We initiated studies to characterize additional neuronal phenotypes of olfactory epithelial derivation. Neuropeptide Y (NPY) neurons are functionally related to the reproductive axis, modulating the release of GnRH and directly enhancing GnRH-induced luteinizing hormone (LH) secretion from gonadotrophs. We demonstrate that a population of migratory NPY neurons originates within the olfactory epithelium of the chick. At stage 25, NPY-positive fibers, but not cells, were detected in the epithelium and the nerve. By stages 28-34, NPY neurons and processes were present in the olfactory epithelium, olfactory nerve, and at the junction of the olfactory nerve and forebrain. In these regions the number of NPY neurons increased until stage 30 and then declined as development progressed. Electron microscopic immunocytochemistry confirmed the neuronal phenotype of the NPY-positive cells. The origin and migratory nature of some of these NPY cells was confirmed by double-label immunocytochemical detection of NPY and GnRH. A large percentage of the NPY-cells coexpressed the GnRH peptide. Between stages 28 and 34 single- and double-labeled NPY and GnRH neurons were found side by side along the GnRH migratory route emanating from the nasal epithelium, along the olfactory nerve, and into the ventral forebrain. These data suggest that an NPY population originates in the olfactory epithelium and migrates into the central nervous system during embryogenesis. By stage 42, no NPY/GnRH double-labeled cells were detected.