Auditory nerve synapses persist in ventral cochlear nucleus long after loss of acoustic input in mice with early-onset progressive hearing loss

Abstract

Perceptual performance in persons with hearing loss, especially those using devices to restore hearing, is not fully predicted by traditional audiometric measurements designed to evaluate the status of peripheral function. The integrity of auditory brainstem synapses may vary with different forms of hearing loss, and differential effects on the auditory nerve-brain interface may have particularly profound consequences for the transfer of sound from ear to brain. Loss of auditory nerve synapses in ventral cochlear nucleus (VCN) has been reported after acoustic trauma, ablation of the organ of Corti, and administration of ototoxic compounds. The effects of gradually acquired forms deafness on these synapses are less well understood. We investigated VCN gross morphology and auditory nerve synapse integrity in DBA/2J mice with early-onset progressive sensorineural hearing loss. Hearing status was confirmed using auditory brainstem response audiometry and acoustic startle responses. We found no change in VCN volume, number of macroneurons, or number of VGLUT1-positive auditory nerve terminals between young adult and older, deaf DBA/2J. Cell-type specific analysis revealed no difference in the number of VGLUT1 puncta contacting bushy and multipolar cell body profiles, but the terminals were smaller in deaf DBA/2J mice. Transmission electron microscopy confirmed the presence of numerous healthy, vesicle-filled auditory nerve synapses in older, deaf DBA/2J mice. The present results suggest that synapses can be preserved over a relatively long time-course in gradually acquired deafness. Elucidating the mechanisms supporting survival of central auditory nerve synapses in models of acquired deafness may reveal new opportunities for therapeutic intervention.

Keywords: Auditory nerve; Deafness; Endbulb; Hearing loss; Ventral cochlear nucleus.

Figure 1

Auditory brainstem responses (ABR) thresholds for DBA/2J mice as a function of age. A) individual DBA/2J mice (filled circles) and average CBA/CaJ thresholds at 2 months of age (large open circles). DBA/2J mice show a profound hearing loss by 5 months of age. Click thresholds are in dB peak equivalent level. Tone thresholds are in dB sound pressure level (SPL). B) Examples of ABR waveforms from a young, hearing DBA/2J and an older, deaf DBA/2J mouse evoked by broadband clicks.

Figure 2

Acoustic startle responses (ASR) to short bursts of broadband noise presented in quiet or continuous 60 dB SPL broadband noise in 1 and 2 month old DBA/2J (A, B) and CBA/CaJ (C, D) mice. The horizontal line near zero indicates the level of background activity when the mouse is at rest. Error bars indicate standard deviation.

Figure. 3

Stereological analysis of VCN. Ventral cochlear nucleus (VCN) volume (A), large cell number (B), and number of VLGUT1-positive auditory nerve puncta (C) are similar in young hearing and older deaf DBA/2J mice. Error bars indicate standard deviation.

Figure 4

VGLUT1 immunolabeling in DBA/2J mouse VCN. Numerous VGLUT1-positive terminals in hearing and deaf DBA/2J mice contact VCN bushy, multipolar, and octopus cells.

Figure 5

Transmission electron micrographs reveal ultrastructural details of auditory nerve synapses in DBA/2J mice. A) Profiles of axosomatic endbulb synapses appear normal in young hearing DBA/2J mice, displaying short, curved postsynaptic densities (psd), numerous clear large round synaptic vesicles, and glial sheaths (glia). B) Axosomatic endbulb synapse profiles appeared similar, but smaller in deaf DBA/2J mice.