Abstract
Cochlear hair cells receive efferent signals from neurons located in the superior olivary nuclear complex (SONs). The efferent signals are considered to alter sound sensitivity of inner hair cells by changing the length of outer hair cells. Little is known, however, about the mechanisms how the inhibitory efferent fibers form synapses with hair cells during the development. To address these questions, we cultured explants from SONs together with organ of Corti in the same collagen gel matrix, and examined how the efferent fibers were guided to the organ of Corti and formed synapses with their hair cells in the co-culture system. Small sections (approximately (0.5mm)3) dissected from SONs of embryonic or newborn rats were embedded in collagen matrix and were incubated in a serum-free medium supplemented with neurotrophins. These explants of SONs extended thousands of fine neurites in a BDNF-dependent manner. Immunostain against GAD revealed that these neurites contained many GABA-ergic ones. Co-culture of explants of SONs with sections of organ of Corti showed that many neurites from SONs made convergence to several regions on the organ of Corti at which outer hair cells were located. Many GABA-ergic fibers were identified among such fibers. These observations indicated that hair cells of the late embryonic and newborn stages were able to attract efferent fibers from the SONs neurons. [Jpn J Physiol 55 Suppl:S160 (2005)]
