Characterization and localization of nerve growth factor receptors in the embryonic otic vesicle and cochleovestibular ganglion.

We have investigated the possibility that nerve growth factor (NGF) may play a role in the development of the inner ear. Primordia of the inner ear, the otic vesicle (OV) and cochleovestibular ganglion (CVG), were isolated from 72-hr (stage 19-20) quail embryos and examined for the presence of NGF receptors. Quantitative binding studies revealed that both OV and CVG exhibited specific 125I-NGF binding; levels of nonspecific binding were 6 to 26% of total binding. Scatchard analysis yielded a linear plot, indicating the presence of a single class of NGF receptor. The average binding constant (Kd) was 8.0 nM for OV and 8.6 nM for CVG, corresponding to the low affinity (site II) NGF receptor. Examination of light microscopic radioautographs indicated that most of the specific 125I-NGF binding was located in the ventromedial wall of the OV, with little or no binding in the lateral wall and endolymphatic primordia. These studies were corroborated by microdissection of OV, in which 70% of the radioactivity was found to be localized in the medial half of the OV. In CVG, specific 125I-NGF binding was more concentrated in the cochlear portion of the ganglion, with silver grains primarily over areas containing support cells and immature neurons. Quantitative binding studies with isolated cochlear and vestibular ganglia obtained from 144-hr (stage 29-30) quail embryos revealed that the cochlear ganglion exhibited three times more specific 125I-NGF binding than the vestibular ganglion. The presence of NGF receptors on OV and CVG suggests that these structures are responsive to and/or dependent upon NGF. The following paper (J. Represa and P. Bernd, 1989, Dev. Biol. 134) examines the question of whether NGF serves either as a mitogen, a survival factor, or a differentiation factor in this system.