Transient prenatal expression of NPY-Y1 receptor in trigeminal axons innervating the mystacial vibrissae

Using immunohistochemistry in combination with confocal laser scanning microscopy, we studied the ontogeny of neuropeptide Y‐Y1 receptor (Y1‐R) expression in the trigeminal system of the rat. The study was limited to the nerve fibers innervating the mystacial pad and the trigeminal ganglia. In the trigeminal ganglia, Y1‐R‐immunoreactive (IR) neurons were first observed at E16.5. At this same stage some nerve fibers in the trigeminal ganglia also exhibited Y1‐R‐like immunoreactivity (LI). Strongly Y1‐R‐IR nerve fibers innervating the follicles of the mystacial vibrissae were first observed at E18. After double labeling, the Y1‐R‐LI was found to be colocalized with the neuronal marker protein gene product 9.5. At P1 only weak labeling for the Y1‐R was found around the vibrissae follicles, whereas the neurons in the trigeminal ganglia were intensely labeled. The same was true for the adult rat, but at this stage no Y1‐R labeling at all was observed in nerve fibers around the vibrissal follicles. These results strongly support an axonal localization of the Y1‐R at this developmental stage. The transient expression of the Y1‐R during prenatal mystacial pad development suggests a role for the Y1‐R in the functional development of the vibrissae. J. Comp. Neurol. 429:183–191, 2001. © 2000 Wiley‐Liss, Inc.