Orexin Signaling in the Mouse Cochlea

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The research undertaken for this proposal is significant because it represents the first investigation of the role of Orexin signaling in the cochlea and as such, will result in novel basic knowledge of a new signaling pathway related to cochlear function. We will also define potential safety concerns regarding impacts to hearing health associated with the use of an insomnia drug that is already used by a significant portion of the US population. The Orexins are a family of two peptides that in the brain are exclusively expressed in hypothalamic neurons. Both Orexin receptors, however, are widely dispersed throughout the brain. Defective Orexin signaling has been linked to sporadic nonfamilial narcolepsy, the most common form of narcolepsy, and therefore most research on Orexin signaling targets sleep/wake behavior. Using immunohistochemistry, we show that the Orexin pre-prohormone, both mature Orexins peptides, and their receptors are expressed in the cochlea. We demonstrate that Orexin A/B pre-prohormone gene null mice (thus lacking all Orexin ligands) do not recover ABR thresholds following noise exposures that induce classic TTS in wild type mice. This is a translationally significant finding because Ox1R and Ox2R are targets of Orexin receptor antagonist pharmaceuticals such as Suvorexant (Belsomra®) marketed to treat insomnia. Approximately 30% of adults in the US report symptoms of insomnia and 4% reported prescription sleep aid use in the month prior to a 2013 CDC poll. While sleep aids are meant for brief duration use, they are often used for long periods of time (months to years). This is especially true of aged individuals and those suffering from any of numerous other conditions (PTSD, anxiety, etc.). A major question is whether a latent risk exists in using Orexin-targeting insomnia medications- do these drugs leave patients at risk for hearing dysfunction, especially if taken consistently over time that would result in incomplete clearance of the drug? This work is further translationally relevant because sleep aids are often used by older individuals, who as a population suffer from abnormal sleep patterns/insomnia and also typically are experiencing normal age-related hearing dysfunction. Our goal for this project is to: 1) verify and expand our preliminary data obtained with Orexin ligand nulls by demonstrating which receptor (Ox1R , Ox2R, or both) contribute to the loss of hearing following moderate-level sound exposures; and 2) begin an assessment of the mechanisms underlying the observed dysfunction associated with loss of Orexin signaling. We will use a combination of morphological (immunostaining for afferent synapses under IHCs), physiological (ABRs and DPs), and protein expression (cytokine arrays) analyses to investigate the role of Orexin signaling in maintenance of hearing sensitivity following noise exposures. We will also examine the noise-induced local immune responses of the cochlea by assessing the inflammatory state and numbers of immune cells infiltrating the cochlea following Orexin receptor genetic manipulation and noise exposure.