Susceptibility of developing cochlear nucleus neurons to deafferentation-induced death abruptly ends just before the onset of hearing

To investigate the ability of developing cochlear nucleus (CN) neurons to survive in the absence of afferent input, left cochlear removals were performed on gerbils at 2 day intervals from postnatal (P)3 to P11, and at P18 and P93. After a 3 month postsurgical survival period, Nissl-stained frontal sections through the brainstem were analyzed under the light microscope. CN volume, anteroventral cochlear nucleus (AVCN) neuron cross-sectional area, and total number of neurons in the CN were measured on both sides of the brain. Mean volume reduction of the deafferented CN relative to the intact CN ranged between 76% in the P3 group to 33% in the P11 group and did not differ significantly between P11 and P93. Cochlear removal at all ages reduced AVCN neuron cross-sectional area by approximately 40% in the deafferented CN relative to the intact CN, except for the P93 group where neuron atrophy was significantly less severe (23% mean reduction). Massive loss of CN neurons (>50% of the intact side) was observed following cochlear removal performed during the first postnatal week. However, between P7 and P9, neurons in all areas of the CN lose susceptibility to deafferentation-induced neuron death. No significant neuron loss was observed following cochlear removal after P7. This study shows that an abrupt transition in the ability of CN neurons to survive in the absence of afferent input is coincident with events leading to the onset of hearing.