AUTHOR=Ogawa Satoshi , Ng Kai W., Xue Xiaoyu , Ramadasan Priveena N., Sivalingam Mageswary , Levavi-Sivan Berta , Lin Haoran , Parhar Ishwar 

TITLE=Thyroid Hormone Upregulates Hypothalamic kiss2 Gene in the Male Nile Tilapia, Oreochromis niloticus

JOURNAL=Frontiers in Endocrinology

VOLUME=4

YEAR=2013

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2013.00184

DOI=10.3389/fendo.2013.00184

ISSN=1664-2392

ABSTRACT=<p>Kisspeptin has recently been recognized as a critical regulator of reproductive function in vertebrates. During the sexual development, kisspeptin neurons receive sex steroids feedback to trigger gonadotropin-releasing hormone (GnRH) neurons. In teleosts, a positive correlation has been found between the thyroid status and the reproductive status. However, the role of thyroid hormone in the regulation of kisspeptin system remains unknown. We cloned and characterized a gene encoding kisspeptin (<italic>kiss2</italic>) in a cichlid fish, the Nile tilapia (<italic>Oreochromis niloticus</italic>). Expression of <italic>kiss2</italic> mRNA in the brain was analyzed by <italic>in situ</italic> hybridization. The effect of thyroid hormone (triiodothyronine, T<sub>3</sub>) and hypothyroidism with methimazole (MMI) on <italic>kiss2</italic> and the three GnRH types (<italic>gnrh1</italic>, <italic>gnrh2</italic>, and <italic>gnrh3</italic>) mRNA expression was analyzed by real-time PCR. Expression of thyroid hormone receptor mRNAs were analyzed in laser-captured kisspeptin and GnRH neurons by RT-PCR. The <italic>kiss2</italic> mRNA expressing cells were seen in the nucleus of the lateral recess in the hypothalamus. Intraperitoneal administration of T<sub>3</sub> (5 μg/g body weight) to sexually mature male tilapia significantly increased <italic>kiss2</italic> and <italic>gnrh1</italic> mRNA levels at 24 h post injection (<italic>P</italic> &lt; 0.001), while the treatment with an anti-thyroid, MMI (100 ppm for 6 days) significantly reduced <italic>kiss2</italic> and <italic>gnrh1</italic> mRNA levels (<italic>P</italic> &lt; 0.05). <italic>gnrh2</italic>, <italic>gnrh3</italic>, and thyrotropin-releasing hormone mRNA levels were insensitive to the thyroid hormone manipulations. Furthermore, RT-PCR showed expression of thyroid hormone receptor mRNAs in laser-captured GnRH neurons but not in <italic>kiss2</italic> neurons. This study shows that GnRH1 may be directly regulated through thyroid hormone, while the regulation of Kiss2 by T<sub>3</sub> is more likely to be indirect.</p>