Impact of cochlear ablation on calretinin and synaptophysin in the gerbil anteroventral cochlear nucleus before the hearing onset

Developmental expression of calretinin in the mouse cochlea

This study investigated the expression of calretinin (CR) in the mouse cochlea from embryonic day 17 (E17) to adulthood through immunofluorescence. At E17, CR immunoreactivity was only detected in the inner hair cells (IHCs). At E19, the IHCs and spiral ganglion neurons (SGNs) begin to express CR. At birth, CR immunoreactivity was confined primarily to the IHCs and the majority of the SGNs, as identified by TUJ1, both the cytoplasm and the nucleus of SGNs exhibited CR positivity. At postnatal day 2 (P2), auditory nerve fibers reaching the IHCs were stained for CR. CR continued to be expressed in the IHCs, whereas only single row of outer hair cells (OHCs) were positive for CR. By P5, CR expression was evident in IHCs and the three rows of OHCs, with SGNs soma and their neurite projections also displaying CR immunoreactivity. From P8 through adulthood, CR expression persisted in the SGNs and their afferent neurite projections to the IHCs, as well as in IHCs and OHCs. Dual labeling of CR with afferent nerve marker neurofilament 200 (NF200) demonstrated that NF 200-positive SGN somas were encompassed by CR-labeled plasma membrane of SGNs, and NF 200 was co-localized with CR in the afferent nerve fibers innervating the IHCs. We also described the expression of peripherin, a marker for type II SGNs, in the mouse cochlea at various postnatal stages. Peripherin showed a distinct spatio-temporal expression compared to CR in auditory nerve fibers. No co-expression of peripherin and CR was detected in adult. Dynamic expression patterns of CR in the embryonic and postnatal cochlea supported its roles in cochlear development.

Impact of cochlear ablation on calbindin and synaptophysin in the gerbil medial nucleus of the trapezoid body before hearing onset

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.