The Hypothalamic Paraventricular Nucleus Is the Center of the Hypothalamic-Pituitary-Thyroid Axis for Regulating Thyroid Hormone Levels

Regulation of Thyroid Hormone Levels by Hypothalamic Thyrotropin-Releasing Hormone Neurons

Background: Thyrotropin-releasing hormone (TRH) neurons in the paraventricular nucleus of the hypothalamus (PVN) have been identified as direct regulators of thyrotropin (TSH) and thyroid hormone (TH) levels. They play a significant role in context of negative feedback by TH at the level of TRH gene expression and during fasting when TH levels fall due, in part, to suppression of TRH gene expression. Methods: To test these functions directly for the first time, we used a chemogenetic approach and activated PVN TRH neurons in both fed and fasted mice. Next, to demonstrate the signals that regulate the fasting response in TRH neurons, we activated or inhibited agouti-related protein (AgRP)/neuropeptide Y (NPY) neurons in the arcuate nucleus of the hypothalamus of fed or fasted mice, respectively. To determine if the same TRH neurons responsive to melanocortin signaling mediate negative feedback by TH, we disrupted the thyroid hormone receptor beta (TRβ) in all melanocortin 4 receptor (MC4R) neurons in the PVN. Results: Activation of TRH neurons led to increased TSH and TH levels within 2 hours demonstrating the specific role of PVN TRH neurons in the regulation of the hypothalamic-pituitary-thyroid (HPT) axis. Moreover, activation of PVN TRH neurons prevented the fall in TH levels in fasting mice. Stimulation of AgRP/NPY neurons led to a fall in TH levels despite increasing feeding. Inhibition of these same neurons prevented the fall in TH levels during a fast presumably via their ability to directly regulate PVN TRH neurons via, in part, the MC4R. Surprisingly, TH-mediated feedback was not impaired in mice lacking TRβ in MC4R neurons. Conclusions: TRH neurons are major regulators of the HPT axis and the fasting-induced suppression of TH levels. The latter relies, at least in part, on the activation of AgRP/NPY neurons in the arcuate nucleus. Interestingly, present data do not support an important role for TRβ signaling in regulating MC4R neurons in the PVN. Thus, it remains possible that different subsets of TRH neurons in the PVN mediate responses to energy balance and to TH feedback.

Kisspeptin Treatment Restores Ovarian Function in Rats with Hypothyroidism

Background: Hypothyroidism causes ovarian dysfunction and infertility in women, in addition to being associated with hyperprolactinemia and reduced hypothalamic expression of kisspeptin (Kp). However, it remains unknown whether and how Kp is able to reverse the ovarian dysfunction caused by hypothyroidism. Methods: Hypothyroidism was induced in adult female Wistar rats using 6-propyl-2-thiouracil for 3 months. In the last month, half of the animals received Kp10. Blood samples were collected for dosage of free thyroxine, thyrotropin (TSH), luteinizing hormone (LH), prolactin (PRL), progesterone (P₄), and estradiol (E₂), and uteruses and ovaries were collected for histomorphometry. Body and ovarian weight and the number of corpora lutea were also evaluated. Half of the brains were evaluated by immunohistochemistry to Kp, and the other half had the arcuate nucleus of hypothalamus (ARC) and preoptic area microdissected for gene evaluation of Kiss1, Nkb, Pdyn, and Gnrh1. The pituitary gland and corpora lutea were also dissected for gene evaluation. Results: Hypothyroidism kept the animals predominantly acyclic and promoted a reduction in ovarian weight, number of corpora lutea, endometrial thickness, number of endometrial glands, and plasma LH, in addition to increasing the luteal messenger RNA (mRNA) expression of Star and Cyp11a1 and reducing 20αHsd. An increase in plasma PRL and P₄ levels was also caused by hypothyroidism. Kp immunoreactivity and Kiss1 and Nkb mRNA levels in the ARC and Kiss1 in the anteroventral periventricular nucleus of hypothalamus were reduced in hypothyroid rats. Hypothyroid animals had lower pituitary gene expression of Gnrhr, Lhb, Prl, and Drd2, and an increase in Tshb. The treatment with Kp10 restored estrous cyclicality, plasma LH, ovarian and uterine morphology, and Cyp11a1, 3βHsd, and 20αHsd mRNA levels in the corpora lutea. Kp10 treatment did not alter gene expression for Kiss1 or Nkb in the ARC of hypothyroid rats. Nevertheless, Kp10 increased Lhb mRNA levels and reduced Tshb in the pituitary compared with the hypothyroid group. Conclusions: The present findings characterize the inhibitory effects of hypothyroidism on the hypothalamic-pituitary-gonadal axis in female rats and demonstrate that Kp10 is able to reverse the ovarian dysfunction caused by hypothyroidism, regardless of hyperprolactinemia.