This thesis explores thyroid hormone (TH) effects on adult neuroendocrine brain and pituitary in goldfish (Carassius auratus) to address the hypothesis that THs regulate the reproductive neuroendocrine axis through effects on gamma-aminobutyric acid (GABA), a neurotransmitter that stimulates gonadotropin release in fish. The experimental approach was to increase circulating levels of the TH 3,5,3'-triiodothyronine (T3) in goldfish through waterborne exposures, and measure effects on gene expression, enzyme activity and hormone levels in comparison to controls. In sexually regressed males, T3 exposure decreased gene expression of GABA-synthetic and degradative enzymes (glutamic acid decarboxylase and GABA-transaminase, respectively) in telencephalon in a time- and dose-dependent manner but not in hypothalamus. GABA-transaminase activity was not affected by this T3 challenge. In sexually mature males and females, T3 treatment resulted in increased follicle stimulating hormone beta subunit expression in pituitary in females only, and had no effect on luteinizing hormone (LH) beta subunit expression or circulating LH levels in eider sex. Further experiments with GABA agonists indicated that T3 exposure had no effect on GABA-mediated LH release. TH receptor expression was affected in a tissue-specific manner by T3 exposure, appearing to be autoregulated in pituitary, but unaffected in neuroendocrine brain regions. TH deiodinase expression in brain and pituitary and thyroid stimulating hormone beta subunit expression in pituitary was affected in a manner reflective of negative feedback by T3 to reduce T3 levels in tissues and in circulation to a set-point. Gene expression profiling with cDNA microarrays indicated a large-scale decrease in gene expression in adult male goldfish telencephalon in response to T3. This included genes encoding proteins with functions central to general metabolic activity including mRNA splicing and proteasomal protein turnover, as well as neuroendocrine signalling relevant to reproduction and neurogenesis. In conclusion, THs have the potential to regulate the reproductive neuroendocrine axis through minor effects on GABA function in neuroendocrine brain and sex-dependent effects on glycoprotein hormone subunit gene expression, except for LH beta, in pituitary. Moreover, gene expression in adult teleost brain is responsive to TH, which is critical to understanding the potential for TH regulation of the reproductive neuroendocrine axis in teleosts.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/29942 |
| Date | January 2009 |
| Creators | Wiens, Susanna Claire |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 180 p. |
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