ESTROGEN-MEDIATED PLASTICITY ON HAIR CELL POPULATIONS IN THE INNER EAR AND LATERAL LINE OF MIDSHIPMAN FISH AND ZEBRAFISH

Coty William Jasper

doi:10.7273/000007055

Vertebrates utilize sensory information such as sound detection to avoid danger, gather resources, and enhance breeding success. However, loss of information processing, such as the detection of sound, can reduce individual fitness. In mammals, loss of sound transducing hair cells is permanent and results in irreversible hearing loss. In non-mammalian vertebrates, hair cells can be regenerated following damage, or even added to tissues over time. Additionally, in some non-mammalian vertebrates, sound detection is plastic and can be enhanced seasonally. In the plainfin midshipman (Porichthys notatus) (midshipman), reproductive fish have decreased auditory thresholds at low frequencies associated with male mating calls. In females, plasticity of sound detection is likely regulated by endogenous estradiol and can be stimulated with exogenous estradiol treatment. Additionally, reproductive females have a greater density of hair cells within the main auditory organ of the inner ear, otherwise known as the saccule. While we know that estrogen signals enhance sound encoding in midshipman, we do not fully understand the underlying mechanism(s). My thesis research provides greater mechanistic understanding of how estrogen stimulates auditory plasticity. I found that summer females have increased cell proliferation within the saccule compared to winter, suggesting that cell proliferation is responsible for increased hair cell density. I also found that estradiol is sufficient to increase saccular hair cells in non-reproductive females. I then compared gene expression of pathways governing cell turnover within the inner ear and found that the saccule and utricle likely have unique mechanisms to establish similar physiological plasticity. Although midshipman are an effective model of auditory plasticity, pharmacological study of hair cell addition is not possible within their inner ear. In my thesis I determined if larval zebrafish could be used as a pharmacological model of estrogen induced-morphological plasticity. I found that endogenous estrogen signals through both nuclear and membrane-bound receptors to influence hair cell number. My findings demonstrate that larval zebrafish are an effective model to pharmacologically study how estrogen alters hair cell number in sensory tissues. Overall, my findings contribute to our understanding of estrogen mediated auditory plasticity by showing that, in females, seasonal hair cell addition is likely instigated by a proliferative mechanism. I have also shown that estrogen increases hair cell number within zebrafish, suggesting that estrogen and hair cell interactions are a fundamental feature in vertebrates capable of hair cell addition. These findings will advance our knowledge of auditory plasticity.

ESTROGEN-MEDIATED PLASTICITY ON HAIR CELL POPULATIONS IN THE INNER EAR AND LATERAL LINE OF MIDSHIPMAN FISH AND ZEBRAFISH

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