The role of the hypothalamus-pituitary-thyroid axis in appetite regulation of goldfish (Carassius auratus)

Abstract

This thesis aimed to understand the role that the hypothalamus-pituitary-thyroid (HPT) axis plays in appetite regulation of goldfish (Carassius auratus). I altered nutritional and thyroid statuses to measure the response of thyroid axis components and appetite-regulating peptides. I predicted that fasting would downregulate the thyroid axis and trigger an orexigenic response, while overfeeding would upregulate the thyroid axis and trigger an anorexigenic response. Additionally, I predicted that hyperthyroid conditions would lead to negative feedback of the thyroid axis and an orexigenic response, whilst opposite under hypothyroid conditions. I uncovered for both experiments that the thyroid axis in goldish is most responsive to overfeeding and hyperthyroidism. Overfeeding led to a time-dependent increase in central thyroid transcripts while fasting decreased thyroid hormone degradation peripherally with no central response, no treatment altered levels of thyroid hormone in circulation. Hyperthyroidism resulted in negative feedback to the pituitary, but not hypothalamus, and did not lead to an increase in food intake despite an increase in the levels of thyroxine. The thyroid inhibitor, propylthiouracil, did not induce hypothyroidism or alter the expression of any thyroid axis transcript. Appetite-regulating peptides correlated weakly to changes in the thyroid, suggesting an overall poor association in goldfish between appetite regulation and thyroid status.