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

Using immunohistochemistry in combination with confocal laser scanning micr oscopy, we studied the ontogeny of neuropeptide Y-Y1 receptor (Y1-R) expres sion in the trigeminal system of the rat. The study was limited to the nerv e fibers innervating the mystacial pad and the trigeminal ganglia. In the t rigeminal 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 in nervating the follicles of the mystacial vibrissae were first observed at E 18. 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 l ocalization of the Y1-R at this developmental stage. The transient expressi on of the Y1-R during prenatal mystacial pad development suggests a role fo r the Y1-R in the functional development of the vibrissae. J. Comp. Neurol. 429:183-191, 2001. (C) 2001 Wiley-Liss, Inc.