Expression of neuropeptide Y in olfactory ensheathing cells during prenatal development

Neuropeptide Y (NPY) is expressed in a special type of glial cell, the olfactory ensheathing cells, that surround the axons of olfactory sensory neurons on their way from the olfactory epithelium to the glomeruli in the olfactory bulb. The expression of NPY in ensheathing cells was examined during prenatal development of the olfactory system by using immunohistochemistry and in situ hybridization. NPY expression was compared with the expression of growth associated protein-43, olfactory marker protein, the low-affinity nerve growth factor receptor (p75) and S-100, factors expressed in the olfactory system at known stages of development. NPY-like immunoreactivity (NPY-LI) and NPY mRNA expression was first detected in the olfactory nerve layer of the olfactory bulb at embryonic day 15. From embryonic day 16 and onward, a clear segregation could be observed in the intensity of both NPY-LI and NPY mRNA expression within the olfactory nerve layer. NPY expression was most intense in the inner part of the olfactory nerve layer. In the outer olfactory nerve layer, a clear decrease in NPY expression was observed. The inner olfactory nerve layer, showing high NPY expression, did not stain for S-100 or p75. However, NPY-LI was found to coexist with S-100-LI from the outer olfactory nerve layer until the olfactory epithelium and with p75-LI in cells surrounding the olfactory nerve. These results show that NPY is expressed in ensheathing cells before olfactory sensory neurons mature and the formation of the glomerular layer starts. NPY might be involved in the guidance, growth, or both, of olfactory sensory axons toward their target glomeruli in the olfactory bulb or have a function in the maturation of the olfactory sensory neurons. (C) 2000 Wiley-Liss, Inc.