Triiodothyronine Exerts Direct Cell-Specific Regulation of Thyrotropin-Releasing Hormone Gene Expression in the Hypothalamic Paraventricular Nucleus*

Abstract

Thyroid hormone administered systemically exerts negative feedback control of biosynthesis of the TRH pro-hormone in the hypothalamic paraventricular nucleus (PVN), the origin of neurons that regulate anterior pituitary TSH secretion, but not in any other group of TRH-synthesizing neurons in the brain. To determine whether this response is mediated by direct effects on PVN neurons, we studied the effect of unilateral stereotaxic implants of L-T3 into the anterior hypothalamus on the concentration of pro-TRH mRNA and pro-TRH in the PVN of hypothyroid rats. Because hypothalamic-pituitary-thyroid function is also regulated by central catecholamines, we also determined the effect of unilateral ablation of ascending catecholaminergic fibers to one side of the PVN by stereotaxic injection of 6-hydroxydopamine or transection of ascending catecholaminergic pathways. T3-implanted hypothyroid animals showed a marked reduction in pro-TRH mRNA and immunoreactive pro-TRH in medial parvocellular neurons of the PVN on the same side as the implant, but not in contralateral PVN neurons or TRH-synthesizing neurons in other hypothalamic regions. In contrast, hypothyroid animals implanted with pellets of hormonally inactive 3,5-diiodo-L-thyronine showed intense symmetric hybridization and immunoreaction product in both wings of the PVN. Despite marked unilateral reduction in the catecholamine innervation to the PVN, no reduction in pro-TRH mRNA or immunoreactive pro-TRH was observed in the PVN on the affected side compared to that on the unaffected side. These studies demonstrate that negative feedback regulation of thyroid hormone occurs directly on TRH neurons and is restricted only to those in the PVN tuberoinfundibular system.

American Thyroid Association Guide to Investigating Thyroid Hormone Economy and Action in Rodent and Cell Models

“…However, paraffin embedding interferes with RNA quality, thus better signal-to-noise ratio and analysis of gene expression are obtained using fixed frozen tissue. For example, in situ hybridization has been used to define TRH as a negatively T 3 -regulated gene in the hypothalamus (486)(487)(488)(489) and to study the cortex/dentate gyrus expression of RC3/neurogranin, which is positively regulated by T 3 (396).…”

“…A related approach is to implant T 3 crystals stereotaxically into discrete hypothalamic areas. This method has been used to define the direct role played by T 3 in the TRH negative feedback mechanism (487,488) and to study the photoperiod-mediated regulation of seasonal energy balance and reproduction in the Siberian hamster (517). Furthermore, the direct role played by a number of neuropeptides on the HPT axis has been studied using the ICV route; for example, aMHS, neuropeptide-Y, and agouti-related protein (518)(519)(520)(521)(522).…”

α-Melanocyte-Stimulating Hormone Is Contained in Nerve Terminals Innervating Thyrotropin-Releasing Hormone-Synthesizing Neurons in the Hypothalamic Paraventricular Nucleus and Prevents Fasting-Induced Suppression of Prothyrotropin-Releasing Hormone Gene Expression

“…Every fourth section of the PV N was hybridized with a 1241 base pair single-stranded 35 S-UTP-labeled cRNA probe for pro-TRH as described previously (Dyess et al, 1988;Kakucska et al, 1992). Serial sections taken from the arcuate nucleus were hybridized with a labeled 260 base pair single-stranded antisense 35 S-UTP RNA probe for AGRP, generously provided by Dr. G. Barsh (Stanford University School of Medicine, Stanford, CA) and previously characterized by Ollmann et al (1997).…”

A model for chronic, intrahypothalamic thyroid hormone administration in rats

“…We did not observe any change in Trh mRNA expression measured by qPCR in a punch of the PVN region 28 days after placement of T 3 Ps. A previous study using a hypothyroid rat model showed that unilateral T 3 crystalline implants within the anterior hypothalamus induced a marked reduction of Trh mRNA only in the medial parvocellular division of PVN as shown by in situ hybridization within 4 days (Dyess et al 1988). Moreover, a study by Murphy et al (2012) showed a mild suppression of hypothalamic Trh mRNA assessed by in situ hybridization 6 weeks after T 3 implants in the hypothalamus of Siberian hamsters.…”