Spontaneous bursting activity is already generated in the cochlea before hearing onset and represents an important condition of the functional and anatomical organization of auditory brainstem nuclei. In the present study, cochlea ablation induced changes were characterized in auditory brainstem nuclei indirectly innervated by auditory nerve fibers before hearing onset. In Meriones unguiculatus immunohistochemical labeling of calbindin-D28k (CB) and synaptophysin (SYN) were performed. The influence of cochlea-ablation on CB or SYN was analyzed by considering their differential immunoreaction during development. During the normal postnatal development, CB was first detected in somata of the medial nucleus of the trapezoid body (MNTB) at postnatal day (P)4. The immunoreaction increased gradually in parallel to the appearance of CB-immunoreactive terminal fields in distinct superior olivary complex (SOC) nuclei. Cochlear removal at P5 or P9 in animals with 24 and 48 h survival times resulted in an increase in somatic CB-labeling in the lesioned MNTB including terminal fields compared to the non-lesioned MNTB. SYN-immunolabeling was first detected at P0 and began to strongly encircle the MNTB neurons at P4. A further progression was observed with age. Cochlear ablation resulted in a significant reduction of SYN-labeled MNTB areas of P5-cochlea-ablated gerbils after 48 h post-lesion. In P9 cochlea-ablated gerbils, a redistribution of SYN-positive terminals was seen after 24 and 48 h. Taken together, the destruction of cochlea differentially influences CB- and SYN-labeling in the MNTB, which should be considered in association with different critical periods before hearing onset.
Keywords: Auditory brainstem; Calcium-binding proteins; Cochlear removal; Sensitive period; Superior olivary complex; Synaptic terminals.
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