Brain and behavior development in hypothyroid zebrafish

Thyroid hormones are required for vertebrate development, and disruption of the thyroid system in developing embryos can result in a large range of morphological, physiological, and behavioral changes. As thyroid hormones are known to interact with multiple systems during development, our goal was to show that mild hypothyroidism can cause changes to the brain and behavior associated with other neuroendocrine axes. This study showed that treatment with methimazole (MMI), a goitrogenic thyroid hormone inhibitor, produced characteristic thyroid axis feedback changes in the hypothalamus and anterior pituitary comparable to studies in other vertebrate models. These changes included upregulated thyrotropin-releasing hormone and thyroid-stimulating hormone mRNA expression. This is a novel finding and strengthens the use of zebrafish in developmental neuroendocrinology studies assessing the thyroid system, and is a good first step to understanding thyroid regulation in fishes. Furthermore, significant upregulation of prolactin expression and downregulation of oxytocin mRNA was observed in methimazole-treated embryos. In addition to embryonic brain changes, our analysis of larval behavior in treated and controls revealed that early exposure to low-dose MMI impacts development of anxiety-related behaviors, but not social or visual behaviors. Interestingly, these effects were prolonged beyond exposure times. However, larvae recovered and produced normal behavior resembling controls 10 days following removal from treatment. This shows that early, low-dose MMI treatment causes prolonged, but not permanent, changes to behavior. This is the first evidence that thyroid state plays a role in behavioral development in zebrafish. Taken together, we conclude that changes to the thyroid system during zebrafish development produce embryos with increased prolactin and decreased oxytocin that later translate to increased displays of anxiety.