Ludwig van Beethoven poignantly expressed the perceptual and social burden of hearing loss in an 1801 letter to a friend stating, ?But that jealous demon, my wretched health, has put a nasty spoke in my wheel?for the last three years my hearing has become weaker and weaker. My ears continue to hum and buzz day and night. Sometimes I can scarcely hear a person who speaks softly?but if anyone shouts I can?t bear it. Heaven alone knows what is to become of me.? Beethoven?s self-described maladies can be identified as tinnitus, threshold shift and hyperacusis, respectively. Hyperacusis presents as two distinct neurological disorders: i) ?noxicusis?, in the form of excruciating sound-triggered ear pain or ii) a generalized auditory hypersensitivity that makes even moderately intense sounds seem uncomfortably loud. The neurobiological causes of this second, more common, type of hyperacusis have yet to be defined. This project will develop a mouse model of noise-induced hearing loss to reveal neural circuit changes that cause auditory perceptual hypersensitivity. Studies pursuant to Aim 1 will develop a suite of head-fixed operant behavioral assays to track the emergence of perceptual hypersensitivity following noise-induced high-frequency hearing loss. Studies in Aim 2 will use chronic 2-photon calcium imaging of genetically targeted excitatory and inhibitory neurons in auditory cortex to pinpoint the emergence of cortical hyperactivity relative to perceptual hypersensitivity. Complementary single unit electrophysiology studies will contrast cortical hyperexcitability elicited with acoustic stimuli versus optogenetic stimuli that bypass the ear and brainstem to directly activate neurons in the auditory thalamus.
Aim 3 will test the hypothesis that auditory cortex hyperexcitability is necessary and sufficient for auditory perceptual hypersensitivity by expressing stabilized step function opsins to temporarily induce or reverse cortical hyperexcitability independent of hearing loss. Studies in Aim 4 will address the distributed downstream effects of excess central gain by tracking the emergence of noise- induced hyperexcitability in descending cortical efferents as well as local cell bodies in the amygdala and dorsal cortex of the inferior colliculus. By tracking the precise chronology of hyperexcitability within and beyond the auditory pathway alongside sound-triggered defensive behaviors such as freezing, it will be possible to identify a direct link between sensory plasticity and disorders of anxiety and stress that are commonly observed in individuals with hyperacusis. This association can be causally tested by inducing or reversing cortical hyperexcitability and noting a potential reversal in subcortical makers of excess loudness growth. Taken together, this proposal will leverage modern neuroscience tools to perform causal hypothesis testing on neural circuit changes that underlie a common hearing disorder. Sensory hypersensitivity is also a core phenotype of migraine as well as neurodevelopmental disorders including Autism and Fragile X syndrome. Identifying the biological signatures of over-powered cortical amplification would open up new treatment strategies, with far- ranging implications for hearing impairment and other related neurological disorders.
For many individuals with sensory impairment or neurodevelopmental disorders, moderate stimuli can be perceived as overwhelmingly intense. This work will identify neurobiological signatures of hyperacusis, an auditory hypersensitivity disorder that often accompanies migraine, Autism, Fragile X syndrome and sensorineural hearing loss. This work would identify neural circuit mechanisms for hyperacusis and open new long-term therapeutic avenues for a common neurological disorder that has no widely accepted treatment.
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