Analysis of Hypothalamic TRH Neurons in Regulating Thyroid Hormone Levels

Thyroid hormone (TH) is a major regulator of development and metabolism. An important mechanism controlling TH production is the negative feedback at the hypothalamic and pituitary level and it has been suggested that thyroid hormone receptor β (TRβ) is the main mediator of TH actions in the hypothalamic paraventricular nucleus (PVN). Nevertheless, the direct actions of TH and TRβ in the negative regulation of TRH have yet to be demonstrated in vivo. Here we used two approaches to investigate the TRH neuron. First, we used a chemogenetic tool to directly investigate the role of TRH neurons on the regulation of thyroid hormone levels. Mice expressing Cre-recombinase in TRH neurons received bilateral injections of the activating designer receptors exclusively activated by designer drugs (DREADD) directly into the PVN. Activation of TRH neurons produced a rapid and sustained increase in circulating TSH levels in both males and females. TSH levels increased approximately 10-fold from baseline within 15 minutes of injection of CNO, returning to baseline within 2.5 hours. TH levels were increased approximately 2-fold in males and females. Therefore, using a chemogenetic approach, we were able to directly evaluated the role of PVN TRH neurons on the control of thyroid activity, for the first time. Next, we generated mice deficient in TRβ specifically in neurons expressing melanocortin 4 receptor (MC4R), which overlaps with TRH expression in the PVN. Knockout mice (KO) developed normally and showed no change in TH and TSH levels. TRH mRNA levels in the PVN of KO mice were similar to control mice. To investigate if the deletion of TRβ in the PVN changes the sensitivity of the HPT axis to T3, mice were rendered hypothyroid and given increasing doses of T3 for 2 weeks. Results show no difference in TRH mRNA or serum TSH between controls and KO. Surprisingly, despite the presence of detectable genomic recombination on the TRβ gene in the PVN, there was no difference in TRβ mRNA expression between control and KO mice, suggesting that either MC4R-positive neurons do not express TRβ or they represent a very small population of TRβ-positive cells in the PVN. Present data show that TRH neuron activation rapidly stimulates TSH release and increases TH levels, demonstrating a major role of these neurons in the regulation of the hypothalamic-pituitary-thyroid (HPT) axis. Nevertheless, deletion of TRβ from MC4R neurons had no major effect on either TRH or TH levels in in mice. Additionally, TRβ in MC4R-positive TRH neurons in the PVN is not necessary for TH-induced suppression of TRH mRNA. Although further studies are necessary, these data suggest that there are distinct populations of hypophysiotropic TRH neurons in the PVN, some of which are not regulated by thyroid hormone and TRβ.