Development of dynorphin-immunoreactivity in the chick cochlear nucleus

The purpose of the present study was to investigate the development of dynorphin-immunoreactivity (DYN-I) in the chick cochlear nucleus magnocellularis (NM) from embryos through young adults using an antibody to dynorphin-B (DYN-B). Auditory brainstem tissue from White Leghorn chicks from embryonic day 13 (E13) to posthatch day 22 (P22) was examined under the light microscope after using standard immunohistochemical procedures. DYN-I first appeared in NM at E16 as short flat structures surrounding NM neurons. The density of DYN-I terminals appeared to increase during development, reaching a peak at P6, and then declining until few DYN-I terminals were seen at P22. Thus, DYN-I appears to be expressed during a restricted time period. Because DYN-I terminals appear after the auditory end-bulbs of Held have already formed around NM neurons, it is unlikely that DYN is involved in their development. Additionally, DYN-I terminals appeared to be randomly distributed with no spatial gradient within NM at any of the ages examined. Thus, DYN-B is unlikely to be related to the tonotopic gradient that exists in NM. Although DYN-B appears to be contained within the auditory end-bulbs of Held, its function remains unknow