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Introduction Thyrotropin-releasing hormone (TRH) is a neuroactive tripeptide ( l -pyroglutamyl- l -histidyl- l -prolinamide; pGlu-His-ProNH 2 ), that was originally isolated from porcine and ovine hypothalami as a first
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Laboratory of Endocrine Physiology, Laboratory of Neurophysiology, Carlos Chagas Filho Biophysic Institute, Biology Institute, State University of Rio de Janeiro, Rio de Janeiro 20551‐030, Brazil
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(TRH) and TSH immunostaining, the in vivo TSH content of the pituitary gland, and in vitro TSH secretion after TRH stimulation in adult rats that had been nicotine-exposed in postnatal life. To our knowledge, no study has been specifically designed
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) and thyrotrophin-releasing hormone (TRH) respectively, coupled with increased thyroid size, is responsible for the changes in T 4 secretion ( Scanes et al. 1987 ). However, TRH is not the only factor controlling TSH secretion in birds
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/agouti-related peptide (AgRP)) ones ( Cheung et al. 1997 , Ahima et al. 1999 , Elias et al. 1999 ). Orexigenic and anorexigenic neuron populations from the ARC project to thyrotropin-releasing hormone (TRH)- and corticotrophin-releasing factor (CRF
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, TRH A very comprehensive review on TRH, its overall biology, including its non-hypophysiotropic activities, biosynthesis, and biology of non-TRH fragments from the precursor, was published in 1999 ( Nillni & Sevarino 1999 ). The small
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-iodinated/h mg of protein. Protein was measured by the method described by Bradford (1976) . In vitro TRH-stimulated TSH release Pituitaries of C and BRO groups were quickly dissected out. The anterior pituitary was separated from the posterior pituitary and
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understanding regarding the degree of hypothyroidism previously reported in EO animal models. Because circulating thyrotropin (TSH) was unaltered in adult small litter (SL) rats, we evaluated the thyrotropin release hormone (TRH) in the hypothalamus, the TSH in
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Thyrotrophin releasing hormone (TRH)-immunoreactive peptides have been quantified in canine serum, hypothalamus, liver, pancreas, adrenal, thyroid, prostate, testis, epididymis and semen by TRH radioimmunoassay, SP-Sephadex C-25 cation exchange chromatography, Sephadex G-10 exclusion chromatography and high pressure liquid chromatography. The total concentration of TRH and TRH-like peptides was highest in the hypothalamus, followed by liver, adrenal, pancreas, thyroid, prostate, epididymis, testis and serum. All of the TRH immunoreactivity (TRH-IR) within extracts of the hypothalamus was due to TRH. On the other hand, nearly all of the TRH-IR of extracts of liver, thyroid, prostate, epididymis, testis and semen was due to TRH-homologous peptides. Adrenal and pancreatic extracts contained a greater proportion of TRH in relation to the TRH-homologous peptides. Extracts of dog serum and semen were found to contain a TRH-binding substance which reduced the retention of added TRH by cation exchangers. The half-time of disappearance (t ½) of synthetic TRH incubated at 23 °C in 10% (w/v) homogenates in 0·15 m-NaCl–0·05 m-phosphate buffer, pH 7·5, ranged from 22±10 (s.d.) min for liver to 120 ±58 min for thyroid. The short t ½ for TRH added to dog liver homogenates contrasted with a previous report that dog liver is essentially free of TRH-degrading activity.
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Pyroglutamyl-N 3im-methyl-histidyl-prolineamide (methyl-thyrotrophin releasing hormone, methyl-TRH) is a potent synthetic analogue of TRH. N 3im-Methyl-histidine is present in mammalian brain and it has been suggested that methyl-TRH is a physiological releasing hormone normally present in the hypothalamus. A non-gradient cation-exchange chromatography system that uses SP-Sephadex C-25 and completely resolves methyl-TRH and TRH has been developed. Because methyl-TRH cross-reacts in the immunoassay for TRH, this assay was used to measure TRH and methyl-TRH in the chromatographic fractions. By this means it has been demonstrated that the amount of methyl-TRH present in the rat is less than 0·025 ng/hypothalamus.
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Thyrotrophin releasing hormone-degrading activity (TRH-DA) is present in plasma, hypothalamus, pituitary gland, liver and kidney of adults of several species. Each of these tissues contains more than one TRH-degrading enzyme but only one, pyroglutamate aminopeptidase, isolated from the blood, is a TRH-specific enzyme.
The aim of this study was to describe the developmental pattern of TRH-DA in the plasma, hypothalamus and liver and the role of tri-iodothyronine (T3) in the development of TRH-DA in the rat. Based on the hypothesis that thyroid hormones stimulate plasma TRH-DA in adult rats, degradation of TRH was studied in hypo- or hyperthyroid rats induced by 6-n-propyl-2-thiouracil or T3 respectively.
Tritiated TRH was incubated with plasma and homogenates of hypothalamus or liver. After separation of degradation products by thin-layer chromatography, the amount of degraded [3H]TRH (pmol/50 μl plasma or homogenate) was taken as a comparative index of TRH-DA.
Plasma TRH-DA was not detectable before day 9 while hypothalamic and hepatic TRH-DA was already active at birth. Furthermore, the maturation pattern of total TRH-DA was different in plasma compared with other tissues and T3 appeared to play a significant role in its development.
The absence of plasma TRH-DA in the neonatal period, its special thyroid-dependent developmental pattern and the presence of a specific TRH-degrading enzyme in adult blood suggest a physiological regulatory role for this activity.
