Search

Abstract: In the present study, morphological changes leading to complete metamorphosis have been induced in the neotenic axolotl Ambystoma mexicanum using a submetamorphic dose of T4 together with an injection of corticotropin-releasing hormone (CRH). An injection of CRH alone is ineffective in this regard presumably due to a lack of thyrotropic stimulation. Using this low hormone profile for induction of metamorphosis, the deiodinating enzymes D2 and D3 known to be present in amphibians were measured in liver and brain 24h following an intraperitoneal injection. An injection of T4 alone did not influence liver nor brain D2 and D3, but dexamethasone (DEX) or CRH alone or in combination with T4 decreased liver D2 and D3. Brain D2 activity was slightly increased with a higher dose of DEX, though CRH did not have this effect. A profound synergistic effect occurred when T4 and DEX or CRH were injected together, in the dose range leading to metamorphosis, increasing brain D2 activity more than fivefold. This synergistic effect was not found in the liver. It is concluded that brain T3 availability may play an important role for the onset of metamorphosis in the neotenic axolotl. [Copyright &y& Elsevier]

Entanglement of functions between the adrenal (or interrenal) and thyroid axis has been well described for all vertebrates and can be tracked down up to the level of gene expression. Both thyroid hormones and corticosteroids may induce morphological changes leading to metamorphosis climax in the neotenic Mexican axolotl (Ambystoma mexicanum). In a first series of experiments, metamorphosis was induced with an injection of 25 μg T4 on three alternate days as judged by a decrease in body weight and tail height together with complete gill resorption. This injection also resulted in elevated plasma concentrations of T3 and corticosterone. Previous results have indicated that the same dose of dexamethasone (DEX) is ineffective in this regard (Gen. Comp. Endocrinol. 127 (2002) 157). In a second series of experiments low doses of T4 (0.5 μg) or DEX (5 μg) were ineffective to induce morphological changes. However, when these submetamorphic doses were injected together, morphological changes were observed within one week leading to complete metamorphosis. It is concluded that thyroid hormones combined with corticosteroids are essential for metamorphosis in the axolotl and that only high doses of either thyroid hormone or corticosteroid can induce morphological changes when injected separately. [Copyright &y& Elsevier]

Recent research has shown that in the chicken important interactions take place between the adrenal and the thyroidal axis both at the central and the peripheral level. In vivo as well as in vitro experiments showed that ovine corticotropin-releasing hormone (oCRH) clearly increases thyrotropin (TSH) secretion in late embryonic and early posthatch chicks. In vivo experiments in older chickens, however, suggested that this response might disappear at a later stage. Therefore we started to study in detail the ontogeny of the TSH releasing activity of oCRH using the in vitro perifusion technique. Several embryonic stages (E14, E16, and E18) as well as posthatch stages (C1, C8, C22, and adult chickens) were included in the study. We also investigated the possible regulatory role of somatostatin (SRIH) in this specific endocrine function of CRH. The perifusion studies show that CRH stimulated the TSH release at all stages tested. The 10 and 100 nM oCRH doses were almost equally effective at the early embryonic stages while in most posthatch stages the higher oCRH dose was significantly more effective than the lower one. The stimulation factor, representative for the relative increase in TSH secretion following oCRH challenge, was high at early embryonic stages and clearly lower in adult animals. This seemed to be related to an age-dependent increase in basal TSH secretion levels. In both embryonic (E19) and posthatch (C8) chicks a pretreatment of the pituitaries with SRIH lowered the sensitivity of the thyrotropes to an oCRH challenge. This effect was more pronounced in the posthatch chicks compared to the embryos. The results show that CRH is capable of stimulating the TSH secretion during the entire life cycle of the chicken and that SRIH may play an important role in the fine-tuning of this response by lowering the sensitivity of the thyrotropes to CRH. [Copyright &y& Elsevier]

Abstract: The finding that thyrotropin-releasing hormone does not always act as a thyrotropin (TSH)-releasing factor in non-mammalian vertebrates has led researchers to believe that another hypothalamic factor may exhibit this function. In representatives of all non-mammalian vertebrate classes, corticotropin-releasing hormone (CRH) appears to be a potent stimulator of hypophyseal TSH secretion, and might therefore function as a common regulator of both the thyroidal and adrenal/interrenal axes. CRH exerts its dual hypophysiotropic action through two different types of CRH receptors. Thyrotropes express type 2 CRH receptors, while CRH-induced corticotropin (ACTH) secretion is mediated by type 1 CRH receptors on the corticotropic pituitary cells. The stimulating effect of CRH on both TSH and ACTH release has profound consequences for the peripheral action of both hormonal axes. The simultaneous stimulation of the thyroidal and adrenal/interrenal axes by CRH, possibly fine-tuned by differential regulation of the expression of the different CRH receptor isoforms, provides a potential mechanism for developmental plasticity. [Copyright &y& Elsevier]

We report the cloning of the complete coding sequence of the putative chicken type 2 corticotropin-releasing hormone receptor (CRH-R2) by rapid amplification of cDNA ends (RACE). The chicken CRH-R2 is a 412-amino acid 7-transmembrane G protein-coupled receptor, showing 87% identity to the Xenopus laevis and Oncorhynchus keta CRH-R2s, and 78–80% to mammalian CRH-R2s. The distribution of CRH-R2 mRNA was studied by RT-PCR analysis and compared to CRH-R1 distribution. Both CRH-R1 and CRH-R2 mRNA are expressed in the main chicken brain parts. In peripheral organs, CRH-R1 mRNA shows a more restricted distribution, whereas CRH-R2 mRNA is expressed in every tissue investigated, indicating that a number of actions of CRH and/or CRH-like peptides remain to be discovered in the chicken as well as in other vertebrates. [Copyright &y& Elsevier]

In the current study, the authors examined the type 1 (D1), type 2 (D2), and type 3 deiodinase (D3) activity and mRNA expression patterns in thyroid, lung, brain, pituitary, heart, liver, spleen, gonads, skin, muscle, intestine, Fabricius'' bursa, and kidney during the last week of chicken embryonic development and the first 2 days posthatch. The D3 was the most widely expressed, occurring in all examined tissues. Also, the D1 knows a widespread distribution, although no D1 activity or mRNA expression could be detected in the brain, the thyroid, the muscle, and the skin. In contrast, the D2 has a much more restricted expression pattern, since the brain is the only organ where, prior to hatching, both in vitro D2 activity and D2 mRNA expression can be detected. Taken together, these results demonstrate that during the last week of chicken embryonic development, the majority of tissues express D3, together with either D1 or D2, indicating that each tissue possesses the necessary tools to regulate local thyroid hormone levels at least partly independent from T3 and T4 levels in plasma. In addition, the deiodinase expression data could be correlated to certain thyroid hormone dependent tissue-specific developmental events. This strongly suggests that in birds, as in mammals and amphibians, the correct spatial and temporal expression of iodothyronine deiodinases are essential for normal embryonic development. [Copyright &y& Elsevier]

In amphibians, there is a close interaction between the interrenal and the thyroidal axes. Hypothalamic corticotropin-releasing hormone or related peptides stimulate thyroidal activity by increasing thyrotropin synthesis and release, while corticosterone accelerates both spontaneous and thyroid hormone-induced metamorphosis. One of the mechanisms that is thought to contribute to this acceleration is a corticosterone-induced change in peripheral deiodinating activity. The present experiments were designed to investigate further the effects of glucocorticoid treatment on amphibian deiodinase activities and to explore the possible role of these effects in metamorphosis. Neotenic axolotls (Ambystoma mexicanum) were treated either acutely or chronically with dexamethasone (DEX) and changes in type II and type III iodothyronine deiodinase (D2 and D3) activities were studied in liver, kidney, and brain. In addition, gill length, tail height, and body weight were measured at regular intervals in the chronically treated animals in search of metamorphosis-related changes. A single injection of 50 μg DEX decreased hepatic D3 activity (6–48 h) while it increased D2 activity in brain (6–48 h) and to a lesser extent in kidney (24 h). These changes were accompanied by an increase in plasma T3 levels (48 h). Samples taken during chronic treatment with 20 or 100 μg DEX showed that both hepatic D2 and D3 activities were decreased on day 26, while renal D3 activity was decreased but only in the 20 μg dose group. All other deiodinase activities were not different from those in control animals. At 25 days, all DEX-treated axolotls showed a clear reduction in gill length, tail height, and body weight, changes typical of metamorphosis. Prolongation of the treatment up to 48 days resulted in complete gill resorption by days 44–60. Although probably several mechanisms contribute to these DEX-induced metamorphic changes, the interaction with thyroid function via a sustained downregulation of hepatic D3 may be one of them. [Copyright &y& Elsevier]

All tissues of the embryonic saltwater crocodile (Crocodylus porosus) gradually increased in weight during development except for lung tissue, which had a peak weight of 1.09 g at day 67, thereafter decreasing in weight. The brain was a relatively large organ. Deiodinase activities in liver, kidney, lung, heart, gut, and brain from day 29 to day 77 of development of the saltwater crocodile were investigated. High-Km reverse triiodothyronine (rT3) outer ring deiodination (ORD) activity was present in all tissues except the brain. Activity ranged from 559 ± 51.3 pmol rT3 deiodinated/mg protein/min in the liver at day 77 to below 10 pmol rT3 deiodinated/mg protein/min in gut, lung, and heart tissue. rT3 ORD increased during development in the liver and kidney but decreased in the gut and lung. Activity in the heart was very low (less than 2 pmol rT3 deiodinated/mg protein/min) and did not change during development. Low-Km thyroxine (T4) ORD in liver and kidney tissue had peaks of activity around day 49 of incubation (0.52 and 0.09 fmol T4 deiodinated/mg protein/min, respectively). After day 49, T4 ORD activity in these tissues decreased. T4 ORD activity in gut, lung, and heart was very low (less than 0.04 fmol T4 deiodinated/mg protein/min), with activity in lung increasing slightly during the rest of development. T4 ORD activity in the brain increased toward day 77 (0.14 ± 0.03 fmol T4 deiodinated/mg protein/min), illustrating its importance in local triiodothyronine (T3) production during brain development. T3 inner ring deiodination activity was present only in the embryonic liver and peaked at day 49 (10.1 fmol T3 deiodinated/mg protein/min), after which activity decreased. [Copyright &y& Elsevier]

The distribution and characterization of outer ring deiodination (ORD) using reverse triiodothyronine (rT3) and thyroxine (T4) as substrates is reported in microsomes of liver, kidney, lung, heart, gut, and brain tissues from juvenile saltwater crocodiles (Crocodylus porosus). In lung and heart only small amounts of rT3 ORD and T4 ORD were detected, while in brain only a small amount of T4 ORD was detected. More detailed characterization studies could be performed on liver, kidney, and gut microsomes. Reverse T3 outer ring deiodination (rT3 ORD) was the predominant activity in liver and kidney microsomes. The properties of crocodile liver and kidney rT3 ORD, such as preference for rT3 as substrate, a dithiothreitol (DTT) requirement of 10 mM, inhibition by propylthiouracil (PTU), and Michaelis–Menten (Km) constant in the micromolar range, correspond to the properties previously reported for a type I deiodinase. The temperature optimum for rT3 ORD was between 30 and 35°. There was also rT3 ORD activity in gut microsomes, along with what appeared to be a type II-like, low-Km deiodinase with a substrate preference for T4. There was also a small amount of T4 ORD activity in liver and kidney microsomes. Liver T4 ORD, like a type II deiodinase, had a preference for T4 as substrate at low substrate concentrations and a DTT requirement of 15 mM and was insensitive to PTU. However, at high substrate concentrations the predominant activity was of the type I deiodinase nature. T4 ORD in liver had an optimal incubation temperature of 30 to 35°. Gut microsomal T4 ORD was also type II-like at low substrate concentrations and type I-like at high substrate concentrations. Gut T4 ORD had an optimal incubation temperature of 25 to 30° and a DTT requirement of 20 mM DTT. Kidney microsomal T4 ORD had the same optimal temperature and DTT requirement as that in gut microsomes; however, there was no competition by low substrate concentrations. These results suggest that ORD in juvenile saltwater crocodile kidney is most likely exclusively catalyzed by a type I-like deiodinase. Liver and gut ORD, in contrast, is catalyzed by two enzymes, with a predominance of a type I-like deiodinase in liver and a type II-like deiodinase in gut. Low-Km T3 IRD activity could not be detected in any tissues of the juvenile saltwater crocodile. [ABSTRACT FROM AUTHOR]