The role of activity of the dorsal cochlear nucleus in tinnitus perception in mice

Abstract

Noise-induced tinnitus is the perception of an acute or chronic phantom sound, without a physical source, caused by over-exposure to loud noise. Tinnitus patients that report bothersome tinnitus often suffer from increased anxiety, stress, lack of concentration, insomnia and depression, conditions that can severely decrease quality of life. There are still no cure or a reliable treatment option for tinnitus highlighting that further studies of cellular mechanisms behind the generation and maintenance of tinnitus are much needed. The dorsal cochlear nucleus (DCN), a region of the brainstem known to integrate somatosensory and auditory input, has been identified as a potential key structure in the generation of tinnitus. However, there is still a lack of studies investigating the effect of manipulating subgroups of DCN neurons in tinnitus in vivo. Here, we first show how optogenetic stimulation of DCN cells expressing proteins for activating (CaMKIIα-ChR2) or inhibiting (CaMKIIα-eArch3.0) DCN neurons expressing the genetic promoter calcium/calmodulindependent protein kinase type II α can modulate different DCN units responses to simple sounds. Our results show that these neurons are highly interesting for modulating sound processing of the auditory brainstem. Next, in mice with noise-induced tinnitus, we show that our noise exposure does not lead to hearing loss, but changes the perception of a silent gap in background noise before an acoustic startle. This indicates that mice have tinnitus in a specific frequency that is matched by the background sound tested. Next we show that chemogenetic inhibition of DCN cells expressing CaMKIIα-hM4Di, a synthetic protein that lowers excitability of neurons, can significantly decrease tinnitus (p = 0.038, n = 11 mice), compared to control mice that showed no improvement in tinnitus (control virus; CaMKIIα-eYFP, p = 0.696, n = 7 mice). Unit recordings confirmed chemogenetic inhibition of CaMKIIα-hM4Di positive DCN cell activity. Specifically, decreasing DCN units excitability using chemogenetic g-protein receptors activated by low dose clozapineN-oxide decreased DCN firing frequency, modulated best frequency and tuning width of unit response to sound. Next, to examine if this population is necessary for generating noise-induced tinnitus, CaMKIIα-hM4Di positive DCN neurons were instead chemogenetically inhibited during tinnitus induction (n = 6 experimental and 6 control mice), but such manipulation did not prevent tinnitus, nor did chemogenetic inhibition 2 weeks later decreased tinnitus perception. Our results suggest that CaMKIIα-hM4Di positive cells in the DCN are not crucial for tinnitus induction but play a significant role in maintaining tinnitus in mice. This study adds understanding of the role of a subgroup of DCN cells in tinnitus, pointing to a potential target for tinnitus interventions.