Thyroid Response to Peripheral Endocrine Factors: Neuropeptide Y Influences Thyroid Function in the Reptile Podarcis siculus.

Neuropeptide Y (NPY) is a small signalling molecule produced by neurons through the cleavage of a precursor protein. It generally binds to and activates G protein-coupled receptors to modulate complex homeostatic processes and behaviours in animals. Mammals provide definitive proof of the role of NPY in the thyroid axis, but in reptiles, this link is unclear. We demonstrate that the thyroid axis responds to NPY administration in a dose-dependent manner, with a reduction in plasma TRH and TSH concentrations, and an increase in plasma T3 and T4 levels 2 and 24 h after administration, suggesting that NPY may activate the thyroid axis. This increase in thyroid hormones is supported by morphological findings in the thyroid gland, which show clear signs of stimulation demonstrated by a dose-dependent increase in the height of the follicular epithelium and the presence of numerous resorption vacuoles. Moreover, we investigated the 5-T4 ORD (type II) Monodeiodinase activity at the hepatic level, showing that NPY increased hepatic T3 levels and decreased hepatic T4 levels, and suggesting an alternative mode of signalling by NPY on peripheral biosynthesis of thyroid hormones. Our study helps to address the current lack of research in the field of endocrinology concerning the effects of NPY on metabolism and thyroid function.