AUTHOR=Nabi Ghulam , Hao Yinchao , Liu Xuelu , Sun Yanfeng , Wang Yang , Jiang Chuan , Li Juyong , Wu Yuefeng , Li Dongming 

TITLE=Hypothalamic–Pituitary–Thyroid Axis Crosstalk With the Hypothalamic–Pituitary–Gonadal Axis and Metabolic Regulation in the Eurasian Tree Sparrow During Mating and Non-mating Periods

JOURNAL=Frontiers in Endocrinology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2020.00303

DOI=10.3389/fendo.2020.00303

ISSN=1664-2392

ABSTRACT=<p>Reproduction is an energetically costly phenomenon. Therefore, to optimize reproductive success, male birds invest enough energetic resources for maintaining well-developed testes. The hypothalamic–pituitary–thyroid (HPT) axis in birds can crosstalk with the hypothalamic–pituitary–gonadal (HPG) axis, thus orchestrating both the reproduction and metabolism. However, until now, how the free-living birds timely optimize both the energy metabolism and reproduction via HPT-axis is not understood. To uncover this physiological mechanism, we investigated the relationships among body mass, testis size, plasma hormones including thyroid-stimulating hormone (TSH), thyroxine (T<sub>4</sub>), triiodothyronine (T<sub>3</sub>), metabolites including glucose (Glu), triglyceride (TG), total cholesterol (TC), uric acid (UA), diencephalon mRNA expressions of type 2 (<italic>Dio2</italic>) and 3 (<italic>Dio3</italic>) iodothyronine deiodinase enzymes, thyrotropin-releasing hormone (<italic>TRH</italic>), thyroid-stimulating hormone (<italic>TSH)</italic>, gonadotropin-releasing hormone <italic>I</italic> (<italic>GnRH-I)</italic>, and gonadotropin-inhibitory hormone (<italic>GnIH</italic>) in a male Eurasian tree sparrow (ETS, <italic>Passer montanus</italic>). We found significantly larger testis size; elevated diencephalon <italic>Dio2</italic> and <italic>TRH</italic> mRNA expressions, plasma T<sub>3</sub>, and UA levels; and significantly lowered Glu, TG, and TC levels during mating relative to the non-mating stages in male ETSs. However, <italic>Dio3, TSH, GnRH-I</italic>, and <italic>GnIH</italic> mRNA expression did not vary with the stage. Furthermore, life-history stage dependent variation in plasma T<sub>3</sub> had both direct effects on the available energy substrates and indirect effects on body mass and testis size, indicating a complex regulation of metabolic pathways through the HPT- and HPG-axes. The identified differences and relationships in mRNA expression, plasma T<sub>3</sub> and metabolites, and testis size in male ETSs contribute to our understanding how free-living birds adjust their molecular, endocrinal, and biochemical features to orchestrate their reproductive physiology and metabolism for the maintenance of well-developed testes.</p>