Circadian rhythms in the auditory system

Circadian clocks have been found in numerous cell types and tissues of the body, allowing organisms to coordinate their daily biological functions. They generate cycles of behavioral and physiological processes with a period length of ~24h, which are synchronized to the rotation of the Earth and the subsequent daily changes in illumination, temperature and humidity. The circadian system evolved in order to ensure anticipation and adaptation to these predictable environmental changes, thereby optimizing physiological functions. The current view is that all the cellular clocks of the body are organized in a hierarchical system, which consists of a central master clock and a network of peripheral clocks found in different organs. The circadian organization facilitates temporal control of physiological functions and ensures tissue homeostasis. Prior to this thesis, the consequences of circadian regulation in the auditory system had not been deeply explored. The studies presented here are aiming to investigate the role of the circadian rhythmicity in regulating auditory function. We focused on mechanisms underlying noise sensitivity, involving neurotrophic factors and glucocorticoid hormones. Our results demonstrate that circadian rhythms play a significant role in modulating auditory sensitivity to noise trauma. Mice were found to be more vulnerable to night noise exposure, which triggered permanent hearing loss, whereas day-exposed mice recovered to normal hearing thresholds. This diurnal variation was associated with the presence of a circadian cochlear clock and the circadian control of the brain-derived neurotrophic factor (BDNF). We next found a molecular clock machinery in a central auditory structure, the inferior colliculus (IC), which demonstrated a differential response to day or night noise trauma, and was independent from that of the cochlear clock. Focusing on the cochlear clock, we next identified self-sustained single-cell oscillators originating from sensory and neuronal populations. Cellular clocks were tonotopically arranged, suggesting that networks of individual oscillators may organize circadian rhythms along the length of the cochlea. Finally, we examined the interaction between glucocorticoids and the cochlear clock in regulating the diurnal sensitivity to noise. We found that the absence of circadian glucocorticoid rhythms abolished the greater vulnerability to noise trauma at night, as hearing thresholds recovered completely. This response was linked to glucocorticoid-dependent control of inflammatory cochlear genes. Finally, treatment with the synthetic glucocorticoid dexamethasone at day time, but not at night, protected against noise damage, highlighting the importance of endogenous glucocorticoid rhythms on the effects of otoprotective drugs. In summary, sensitivity to noise insults is greater at specific phases of the circadian cycle, both at behavioral and molecular level and is mediated through complex interaction between circadian clocks, BDNF and glucocorticoids. Overall this thesis is describing a novel feature of the auditory system that would likely have major clinical implications

Circadian Regulation of Cochlear Sensitivity to Noise by Circulating Glucocorticoids

The cochlea possesses a robust circadian clock machinery that regulates auditory function. How the cochlear clock is influenced by the circadian system remains unknown. Here, we show that cochlear rhythms are system driven and require local Bmal1 as well as central input from the suprachiasmatic nuclei (SCN). SCN ablations disrupted the circadian expression of the core clock genes in the cochlea. Because the circadian secretion of glucocorticoids (GCs) is controlled by the SCN and GCs are known to modulate auditory function, we assessed their influence on circadian gene expression. Removal of circulating GCs by adrenalectomy (ADX) did not have a major impact on core clock gene expression in the cochlea. Rather it abolished the transcription of clock-controlled genes involved in inflammation. ADX abolished the known differential auditory sensitivity to day and night noise trauma and prevented the induction of GABA-ergic and glutamate receptors mRNA transcripts. However, these improvements were unrelated to changes at the synaptic level, suggesting other cochlear functions may be involved. Due to this circadian regulation of noise sensitivity by GCs, we evaluated the actions of the synthetic glucocorticoid dexamethasone (DEX) at different times of the day. DEX was effective in protecting from acute noise trauma only when administered during daytime, when circulating glucocorticoids are low, indicating that chronopharmacological approaches are important for obtaining optimal treatment strategies for hearing loss. GCs appear as a major regulator of the differential sensitivity to day or night noise trauma, a mechanism likely involving the circadian control of inflammatory responses

Differential Phase Arrangement of Cellular Clocks along the Tonotopic Axis of the Mouse Cochlea Ex Vivo

Topological distributions of individual cellular clocks have not been demonstrated in peripheral organs. The cochlea displays circadian patterns of core clock gene expression [1, 2]. PER2 protein is expressed in the hair cells and spiral ganglion neurons of the cochlea in the spiral ganglion neurons [1]. To investigate the topological organization of cellular oscillators in the cochlea, we recorded circadian rhythms from mouse cochlear explants using highly sensitive real-time tracking of PER2::LUC bioluminescence. Here, we show cell-autonomous and self-sustained oscillations originating from hair cells and spiral ganglion neurons. Multi-phased cellular clocks were arranged along the length of the cochlea with oscillations initiating at the apex (low-frequency region) and traveling toward the base (high-frequency region). Phase differences of 3 hr were found between cellular oscillators in the apical and middle regions and from isolated individual cochlear regions, indicating that cellular networks organize the rhythms along the tonotopic axis. This is the first demonstration of a spatiotemporal arrangement of circadian clocks at the cellular level in a peripheral organ. Cochlear rhythms were disrupted in the presence of either voltage-gated potassium channel blocker (TEA) or extracellular calcium chelator (BAPTA), demonstrating that multiple types of ion channels contribute to the maintenance of coherent rhythms. In contrast, preventing action potentials with tetrodotoxin (TTX) or interfering with cell-to-cell communication the broad-spectrum gap junction blocker (CBX [carbenoxolone]) had no influence on cochlear rhythms. These findings highlight a dynamic regulation and longitudinal distribution of cellular clocks in the cochlea