Generation of Thyrotropin-Releasing Hormone Receptor 1-Deficient Mice as an Animal Model of Central Hypothyroidism

Abstract

To provide an animal model of central hypothyroidism, mice deficient in the TRH-receptor 1 (TRH-R1) gene were generated by homologous recombination. The pituitaries of TRH-R1-/- mice are devoid of any TRH-binding capacity, demonstrating that TRH-R1 is the only receptor localized on TRH target cells of the pituitary. With the exception of some retardation in growth rate, TRH-R1-/- mice appear normal, but compared with control animals they exhibit a considerable decrease in serum T(3), T(4), and prolactin (PRL) levels but not in serum TSH levels. In situ hybridization histochemistry and real-time RT-PCR analysis revealed that in adult TRH-R1-/- animals TSHbeta-mRNA expression is not impaired whereas PRL mRNA and GH mRNA levels are considerably reduced compared with control mice. The numbers of thyrotropes, somatotropes, and lactotropes, however, are not affected by the deletion of the TRH-R1 gene. The mutant mice are fertile, and the dams nourish their pups well, indicating that TRH is not a decisive factor for suckling-induced PRL release. In situ hybridization and quantitative RT-PCR analysis, furthermore, revealed that, as in control animals, pituitary PRL-mRNA expression in TRH-R1-/- is considerably increased during lactation, albeit strongly reduced as compared with lactating control animals.

60 YEARS OF NEUROENDOCRINOLOGY: TRH, the first hypophysiotropic releasing hormone isolated: control of the pituitary–thyroid axis

“…The importance of the effects of TRH on TSH glycosylation and activity (Weintraub et al 1989) has been demonstrated by comparing the phenotypes of TRH-KO, THRb-KO, and the double mutant. Increased TSH serum levels but reduced TSH bioactivity accounts for the low circulating T 4 concentration (Nikrodhanond et al 2006), similar to that observed in humans with hypothalamic hypothyroidism (Beck-Peccoz et al 1985), or in TRHR1 K/K mice that have normal TSH levels but low circulating T 3 and T 4 concentrations (Rabeler et al 2004). D1-, D2-, and D1D2-KO show compensatory mechanisms in the interplay between hypophysiotropic TRHergic neurons, pituitary TSH expression and release, which in combination maintain serum levels of T 3 stable despite altered serum concentrations of T 4 and TSH (Abdalla & Bianco 2014, Galton et al 2014.…”

“…This discrepancy may be explained by CART which inhibits PRL release and its expression is upregulated in hypophysiotropic TRH neurons by cold but not by suckling (Sánchez et al 2007). TRH and TRHR1 KO mice have shown that while TRH is necessary to sustain PRL secretion during lactation, pups from KO dams grow normally, suggesting that TRH is not essential for suckling-induced PRL release (Rabeler et al 2004.Contrary to data showing that anterior pituitary PPII does not regulate the response of thyrotroph response to TRH, there is evidence that in lactotrophs the intensity of TRH action is under PPII control. PPII is expressed in lactotrophs and its knockdown or inhibition enhances TRH-induced PRL release (Cruz et al 2008).…”

“…This discrepancy may be explained by CART which inhibits PRL release and its expression is upregulated in hypophysiotropic TRH neurons by cold but not by suckling (Sánchez et al 2007). TRH and TRHR1 KO mice have shown that while TRH is necessary to sustain PRL secretion during lactation, pups from KO dams grow normally, suggesting that TRH is not essential for suckling-induced PRL release (Rabeler et al 2004.…”

“…K/K mice pituitaries are devoid of any TRH-binding capacity, which suggested that TRHR1 is the only pituitary receptor (Rabeler et al 2004).…”

“…A second TRHR (TRHR2) was later cloned and found to be expressed mainly in the brain (O'Dowd et al 2000). TRHR1K/K mice pituitaries are devoid of any TRH-binding capacity, which suggested that TRHR1 is the only pituitary receptor (Rabeler et al 2004).TRH binds to TRHR1 at various residues of the extracellular (low binding affinity) and of the transmembrane (high binding affinity) domains. The extracellular site is proposed to be the initial place of interaction, which accounts for the low binding affinity and slow transformation to a tightly bound conformation with movement of TRH to the transmembrane site (Engel & Gershengorn 2007).…”