Electrophysiological correlates of sound processing in the limbic pathways and implications for tinnitus-related anxiety

Abstract

This thesis investigates the electrophysiological correlates of the hippocampus and medial prefrontal cortex to auditory activity, either in an animal model of tinnitus induced by highdose salicylate or in the response to loud broadband noise in mice during locomotion. The results are organized into four chapters with three experimental articles (chapter 1-3, with 2 published articles and one manuscript in preparation) and one book protocol (chapter 4, preprint). In the first article (Winne et al. 2019), we found that a high dose of salicylate (300mg/kg) induced type 2 theta (4-6Hz) oscillation in the ventral hippocampus (VHipp) as well as elicits anxiety-like behavior in mice. In addition, high dose salicylate administration abolished dorsal hippocampus (DHipp) type 1 theta (7-10Hz) correlation with running speed. In the second article (Winne et al. 2020), we showed that after pretreatment with salicylate, only young mice with preserved hearing exhibited anxiety-like behavior, compared to old mice. We hypothesize that old C57BL/6 mice might already have altered hearing due to agerelated hearing loss, and thus are less likely to present salicylate-induced tinnitus and tinnitusrelated anxiety. In young mice, an increase in type 2 theta oscillations and slow gamma (30- 60Hz) was observed in the VHipp, as well as the medial prefrontal cortex (mPFC), after salicylate treatment during the open field and in the elevated plus-maze. Furthermore, we also noticed an increased theta 2 coherence between VHipp and mPFC during the tasks. Lastly, pretreatment of mice with a single dose of the psychedelic substance 5-MeO-DMT prevented the emergence of anxiety-associated behaviors and the induction of type 2 theta and slow gamma after salicylate injection. Based on electrophysiological and behavioral evidence, articles 1 and 2 indicate that anxiety-related mechanisms are triggered in the salicylate-model of tinnitus. Chapter 3 focuses on mice with normal hearing, specifically how loud noise stimuli modulates limbic circuits, and the Reticular-limbic auditory pathway. Using combined approaches (silence/noise, chemo- and optogenetics), type 1 theta oscillations (7- 10Hz) of the DHipp were shown to be accelerated by loud broadband noise. Specifically, noise input relayed from the entorhinal cortex, and medial septum could modulate DHipp theta 1 oscillations while decreasing the activity of the auditory cortex did not affect this noncanonical pathway of loud noise processing. We also verified that the activation of this pathway increased coherence between the DHipp and medial prefrontal cortex. We hypothesize that increasing the activity of DHipp neurons through loud auditory stimuli could be a biological strategy to quickly identify dangerous environments and increase the alertness of animals in response to possible surrounding threats. Lastly, in Chapter 4 (Winne et al., 2021, pre-print), we discuss a technique of calcium imaging of freely moving animals, using miniaturised microendoscopes (‘miniscopes’) and describe in detail the lens and baseplate implant surgery for the UCLA Miniscope V3 and V4. We used a gradient index (GRIN) lens for the hippocampus and a combination of a prism and GRIN lens for the auditory and motor cortex, with adaptations from published work for an improved lens fixation (GRIN-relayPrism). Finally, we comment on the differences between analysis software. In conclusion, this thesis demonstrates electrophysiological correlates of salicylate-induced tinnitus that strengthen the neurobiological link between tinnitus and anxiety. More so, limbic pathways relaying loud noise information were also shown to modulate the activity of the hippocampus and the medial prefrontal cortex. Together these results evidence new electrophysiological signatures of sound processing in the limbic system.