Changes in serum thyroxine and triiodothyronine concentrations during metamorphosis of the Southern Hemisphere Lamprey Geotria australis, and the effect of propylthiouracil, triiodothyronine and environmental temperature on serum thyroid hormone concentrations of ammocoetes

Function and Evolution of Nuclear Receptors in Environmental-Dependent Postembryonic Development

Nuclear receptors (NRs) fulfill key roles in the coordination of postembryonal developmental transitions in animal species. They control the metamorphosis and sexual maturation in virtually all animals and by that the two main environmental-dependent developmental decision points. Sexual maturation and metamorphosis are controlled by steroid receptors and thyroid receptors, respectively in vertebrates, while both processes are orchestrated by the ecdysone receptor (EcR) in insects. The regulation of these processes depends on environmental factors like nutrition, temperature, or photoperiods and by that NRs form evolutionary conserved mediators of phenotypic plasticity. While the mechanism of action for metamorphosis and sexual maturation are well studied in model organisms, the evolution of these systems is not entirely understood and requires further investigation. We here review the current knowledge of NR involvement in metamorphosis and sexual maturation across the animal tree of life with special attention to environmental integration and evolution of the signaling mechanism. Furthermore, we compare commonalities and differences of the different signaling systems. Finally, we identify key gaps in our knowledge of NR evolution, which, if sufficiently investigated, would lead to an importantly improved understanding of the evolution of complex signaling systems, the evolution of life history decision points, and, ultimately, speciation events in the metazoan kingdom.

Molecular and thyroid hormone binding properties of lamprey transthyretins: The role of an N-terminal histidine-rich segment in hormone binding with high affinity

Transthyretin (TTR) is a plasma thyroid hormone (TH) binder that emerged from an ancient hydroxyisourate hydrolase by gene duplication. To know how an ancient TTR had high affinity for THs, molecular and TH binding properties of lamprey TTRs were investigated. In adult serum, the lipoprotein LAL was a major T3 binder with low affinity. Lamprey TTRs had an N-terminal histidine-rich segment, and had two classes of binding sites for 3,3',5-triiodo-L-thyronine (T3): a high-affinity and a low-affinity site. Mutant TTRΔ_3-11, lacking the N-terminal histidine-rich segment, lost the high-affinity T3 binding site. [¹²⁵I]T3 binding to wild type TTR and mutant TTRΔ_3-11, was differentially modulated by Zn²⁺. Zn²⁺ contents of wild type TTR were 7-10/TTR (mol/mol). Our results demonstrate that lamprey TTR is a Zn²⁺-dependent T3 binder. The N-terminal histidine-rich segment may be essential for neo-functionalization (i.e., high-affinity T3 binding activity) of an ancient TTR after gene duplication.

Lamprey metamorphosis: Thyroid hormone signaling in a basal vertebrate

As one of the most basal living vertebrates, lampreys represent an excellent model system to study the evolution of thyroid hormone (TH) signaling. The lamprey hypothalamic-pituitary-thyroid and reproductive axes overlap functionally. Lampreys have 3 gonadotropin-releasing hormones and a single glycoprotein hormone from the hypothalamus and pituitary, respectively, that regulate both the reproductive and thyroid axes. TH synthesis in larval lampreys takes place in an endostyle that transforms into typical vertebrate thyroid tissue during metamorphosis; both the endostyle and follicular tissue have all the typical TH synthetic components found in other vertebrates. Furthermore, lampreys also have the vertebrate suite of peripheral regulators including TH distributor proteins (THDPs), deiodinases and TH receptors (TRs). Although at the molecular level the components of the lamprey thyroid system are ancestral to other vertebrates, their functions have been largely conserved. TH signaling as it relates to lamprey metamorphosis represents a particularly interesting phenomenon. Unlike other metamorphosing vertebrates, lamprey THs increase throughout the larval period, peak prior to metamorphosis and decline rapidly at the onset of metamorphosis; patterns of deiodinase activity are consistent with these increases and declines. Moreover, goitrogens (which suppress TH levels) initiate precocious metamorphosis, and exogenous TH treatment blocks goitrogen-induced metamorphosis and disrupts natural metamorphosis. Despite this clear physiological difference, TH action via TRs is consistent with higher vertebrates. Based on observations that TRs are upregulated in a tissue-specific fashion during morphogenesis and the finding that lamprey TRs upregulate genes via THs in a fashion similar to higher vertebrates, we propose the following hypothesis for further testing. THs have a dual role in lampreys where high TH levels promote larval feeding and growth and then at the onset of metamorphosis TH levels decrease rapidly; at this time the relatively low TH levels function via TRs in a fashion similar to that of other metamorphosing vertebrates.

Change of body height is regulated by thyroid hormone during metamorphosis in flatfishes and zebrafish

Flatfishes with more body height after metamorphosis should be better adapted to a benthic lifestyle. In this study, we quantified the changes in body height during metamorphosis in two flatfish species, Paralichthys olivaceus and Platichthys stellatus. The specific pattern of cell proliferation along the dorsal and ventral edge of the body to allow fast growth along the dorsal/ventral axis might be related to the change of body height. Thyroid hormone (T4 and T3) and its receptors showed distribution or gene expression patterns similar to those seen for the cell proliferation. 2-Mercapto-1-methylimidazole, an inhibitor of endogenous thyroid hormone synthesis, inhibited cell proliferation and decreased body height, suggesting that the change in body shape was dependent on the local concentration of thyroid hormone to induce cell proliferation. In addition, after treatment with 2-mercapto-1-methylimidazole, zebrafish larvae were also shown to develop a slimmer body shape. These findings enrich our knowledge of the role of thyroid hormone during flatfish metamorphosis, and the role of thyroid hormone during the change of body height during post-hatching development should help us to understand better the biology of metamorphosis in fishes.

Alterations along the Hypothalamic-Pituitary-Thyroid Axis of the Zebrafish (Danio rerio) after Exposure to Propylthiouracil

In the past, various approaches have been developed to detect adverse effects of pollutants on the thyroid of vertebrates, most of these with special emphasis on the South African clawed frog, Xenopus laevis. Although fish are primarily affected by thyroid-disrupting chemicals, studies into alterations of the thyroid of fish are scarce. Therefore, effects of the reference compound propylthiouracil on histopathology of the thyroid axis were analyzed in a modified early life-stage test with zebrafish (Danio rerio) exposed to propylthiouracil. The test substance induced dose-dependent alterations of thyroidal tissue concomitant with increases in the number of surrounding blood vessels. Despite this massive proliferation of the thyroid, zebrafish were not able to maintain thyroxin concentrations. The pituitary was affected displaying significant alterations in thyroid-stimulating hormone cell counts. Quantitative evaluation of pituitary surface areas revealed a dose-dependent increase of adenohypophyseal tissue. Distinct histopathological effects may contribute to a more easy identification and interpretation of alterations induced by thyroid-disrupting chemicals.

KClO(4) inhibits thyroidal activity in the larval lamprey endostyle in vitro

An in vitro experimental system was devised to assess the direct effects of the goitrogen, potassium perchlorate (KClO(4)), on radioiodide uptake and organification by the larval lamprey endostyle. Organification refers to the incorporation of iodide into lamprey thyroglobulin (Tg). Histological and biochemical evidence indicated that endostyles were viable at the termination of a 4h in vitro incubation. A single iodoprotein, designated as lamprey Tg, was identified in the endostylar homogenates by polyacrylamide gel electrophoresis and Western blotting. Lamprey Tg was immunoreactive with rabbit anti-human Tg serum and had an electrophoretic mobility similar to that of reduced porcine Tg. When KClO(4) was added to the incubation medium, both iodide uptake and organification by the endostyle were significantly reduced relative to controls as determined by gamma counting, and gel-autoradiography and densitometry, respectively. Western blotting showed that KClO(4) significantly lowered the total amount of lamprey Tg in the endostyle. Based on the results of this in vitro investigation, we conclude that KClO(4) acts directly on the larval lamprey endostyle to inhibit thyroidal activity. These data support a previous supposition from in vivo experimentation that KClO(4) acts directly on the endostyle to suppress the synthesis of thyroxine and triiodothyronine, resulting in a decrease in the serum levels of these two hormones.

Changes in intestinal and hepatic thyroid hormone deiodination during spontaneous metamorphosis of the sea lamprey, Petromyzon marinus

We measured microsomal low-K(m) outer-ring deiodination (ORD) and inner-ring deiodination (IRD) activities for thyroxine (T(4)) and 3, 5,3'-triiodothyronine (T(3)) in intestine and liver in nonmetamorphosing (undersized) larvae, immediately premetamorphic larvae, animals in stages 1-7 of metamorphosis, and immediately postmetamorphic sea lampreys (Petromyzon marinus). For intestine: T(4)ORD activity was relatively low in nonmetamorphosing larvae, increased in premetamorphic individuals, was highest in stages 1 and 2 and was very low during stages 3-7; T(4)IRD activity was negligible until stage 3 but increased 4.7-fold through stages 3 to 7 such that T(4)IRD activity was 14 times T(4)ORD activity at stage 6; T(3)ORD activity was undetectable; T(3)IRD activity was not measured through stages 3-7 but correlated with T(4)IRD activity at other stages. For liver: deiodination was only measured up to stage 2 and in postmetamorphic animals; in contrast to intestine, T(4)ORD activity fell to low levels at stage 2 and was low during postmetamorphosis; T(4)IRD and T(3)IRD activities were very low and uninfluenced by developmental stage; T(3)ORD activity was undetectable. We conclude that (1) deiodination activity is usually much higher in intestine than in liver, (2) intestinal ORD and IRD activities change reciprocally so that ORD predominates in early metamorphosis but IRD predominates in mid and late metamorphosis, and (3) changes in intestinal deiodination may contribute to the characteristic depression of plasma T(4) and T(3) levels during spontaneous metamorphosis. J. Exp. Zool. 286:305-312, 2000.

Temperature and KClO(4)-induced metamorphosis in the sea lamprey (Petromyzon marinus)

Larval sea lampreys (Petromyzon marinus) were exposed to either a warm (18 degrees C) or a cold (3 degrees C) water temperature and either with (treated) or without (untreated) the presence of potassium perchlorate (KClO4). After 23 weeks, larvae were examined for signs of metamorphosis and serum samples were collected to assay thyroxine (T4) and 3,5,3'-triiodothyronine (T3) concentrations. Water temperature did not significantly affect serum T4 or T3 concentrations in untreated larvae and no metamorphosis occurred in these groups. Serum T4 concentrations were not significantly different between the two temperature groups treated with KClO4. However, serum T3 concentrations were significantly higher in the cold water, KClO4-treated larvae (5.4 nmol/l) than in the warm water, KClO4-treated larvae (1.2 nmol/l). KClO4 treatment at a warm water temperature induced metamorphosis in all larvae and resulted in serum T4 and T3 concentrations which were 66 and 95% lower, respectively, than untreated larvae in warm water. Despite having significantly lower serum T4 and T3 concentrations (73 and 80%, respectively) than untreated cold water larvae, metamorphosis was not observed in cold water, KClO4-treated larvae. The results of this study indicate that warm water is a requirement for the successful induction of metamorphosis with KClO4, and provide further evidence of water temperature as an important factor in the metamorphosis of lampreys.

Spontaneous and induced metamorphosis in the American brook lamprey, Lampetra appendix

We described the seven stages of spontaneous metamorphosis in the American brook lamprey (Lampetra appendix) and assessed the importance of size as a determinant of spontaneous and induced metamorphosis. Serum concentrations of the thyroid hormones (TH) thyroxine (T4) and triiodothyronine (T3) were measured in metamorphosing and nonmetamorphosing L. appendix. The sequence of stages in metamorphosis and changes in the relative lengths of most body regions were consistent with data reported for other lamprey species. However, premetamorphic and metamorphosing L. appendix in the early stages of metamorphosis (1-3) were much larger in size (at least 155 mm and 5.40 g) than has been observed for other lamprey species. Serum concentrations of T4 and T3 were high by the end of the larval period and declined significantly by stage 2 of metamorphosis. Larvae greater than or equal to 130 mm in length were treated with either potassium perchlorate (KClO4; 0.01 and 0.05%) or 10 mg/L propylthiouracil (PTU; 0.0001%) for 117 days from September to January to determine if metamorphosis could be induced by these goitrogens. Both concentrations of KClO4 successfully induced metamorphosis in L. appendix, but the incidence of metamorphosis (62%) was much lower than reported in sea lampreys (98%) of comparable size. Serum concentrations of T4 and T3 declined by 64-76 and 93-96% relative to control values, respectively, in metamorphosing and nonmetamorphosing L. appendix treated with KClO4. PTU elicited declines of 55% for T4 and 80% for T3, but only one animal metamorphosed. Based on these data, we conclude that a decline in serum TH levels is necessary for metamorphosis in L. appendix, but not sufficient by itself to trigger the process.

Study of the relationship between thyroid hormones and lipid metabolism during KClO4-induced metamorphosis of landlocked lamprey, Petromyzon marinus

This study examines the role of thyroid hormones (TH) (thyroxine and triiodothyronine) in regulating lipid metabolism of landlocked larval sea lampreys, Petromyzon marinus. Larvae were treated with either thyroxine (0.5 or 1 mg l(-1) water) or triiodothyronine (0.25 or 1 mg l(-1) water) in the presence or absence of the goitrogen, potassium perchlorate (KClO4) (0.05% w/v), for 4, 8, and 16 weeks. Treatment with KClO4 alone, which induced metamorphosis after 8 weeks and lowered plasma TH levels, reduced hepatic and renal total lipid content after 8 weeks of treatment. KClO4-induced lipid depletion after the 8-week treatment was supported by an increased rate of hepatic lipolysis, as indicated by increased triacylglycerol lipase activity. Furthermore, reduced lipogenesis in the liver was indicated by decreased hepatic acetyl-CoA carboxylase and diacylglycerol acyltransferase (DGAT) activities, and by decreased renal DGAT activity following 8 weeks of KClO4 treatment. Treatment of larvae for 4 weeks with TH alone resulted in either no change or a slight increase of lipid in the liver and kidney. TH treatments in combination with KClO4 failed to induce metamorphosis and, after up to 8 weeks, several TH treatments blocked changes in lipid content and enzyme activity associated with KClO4-induced metamorphosis. These experimental results suggest that TH deficiency during metamorphosis may promote lipid catabolism, while the presence of TH tends to protect/promote lipid reserves, perhaps favoring the larval condition. The actions of TH and KClO4 on metamorphosis-associated lipid metabolism in sea lampreys may be direct, permissive, and/or indirect via other factors.

Blocking of KC1O4-induced metamorphosis in premetamorphic sea lampreys by exogenous thyroid hormones (TH); effects of KC1O4 and TH on serum TH concentrations and intestinal thyroxine outer-ring deiodination

Immediately premetamorphic larval sea lampreys (Petromyzon marinus) (>/=120 mm in length) were treated for 4, 8, or 16 weeks with one of two concentrations of either exogenous thyroxine (T4; 1 or 0.5 mg/L) or 3,5,3'-triiodothyronine (T3; 1 or 0.25 mg/L) in the presence or absence of the goitrogen potassium perchlorate (KC1O4; 0.05%) as well as with KC1O4 alone. Larvae from all treatments were examined for signs of metamorphosis, changes in serum T4 and T3 concentrations (serum T4 and serum T3), and changes in intestinal T4 outer-ring (5') deiodination to T3 (T4ORD). KC1O4 depressed both serum T4 and T3 and induced metamorphosis in 80% of larvae treated for 8 weeks or longer. However, neither effect was observed in larvae exposed to KC1O4 combined with either thyroid hormone (TH). These data confirm previous suggestions that exogenous TH blocks KC1O4-induced metamorphosis by elevating serum TH concentrations, and provide evidence that declines in serum TH concentrations are mandatory for precocious metamorphosis. Serum T4, but not serum T3, was elevated following exogenous T4 treatment in the presence or absence of KC1O4. This maintenance of serum T3 at control levels, in the presence of a T4 challenge, was not due to decreases in intestinal T4ORD activity, since T4ORD activity was not affected by any treatments in the study. Exogenous T3 elevated both serum T4 and T3. However, serum T3 in T3-treated larvae decreased with time, suggesting a stringent T3 regulation. Elevation of serum T4 following T3 treatment may have been a result of either inhibition of T4 metabolism, or stimulation of T4 secretion by the endostyle. Based on these results, we conclude that (i) exogenous TH blocks KClO4-induced metamorphosis in sea lampreys and (ii) serum T3 is maintained at control levels despite elevations in serum T4 (its immediate precursor), but this does not involve any changes in intestinal T4ORD activity.

Thyroid hormone deiodination in various tissues of larval and upstream-migrant sea lampreys, Petromyzon marinus

Properties and activities of four potential thyroid hormone (TH) monodeiodinating pathways (T4ORD, L-thyroxine (T4) outer-ring (5') deiodination to 3,5,3'-triiodo-L-thyronine (T3); T4IRD, T4 inner-ring (5) deiodination to 3,3',5'-triiodo-L-thyronine (reverse T3); T3ORD, T3 outer-ring deiodination to 3,5-diiodo-L-thyronine; T3IRD, T3 inner-ring deiodination to 3,3'-diiodo-L-thyronine) were studied in microsomes of liver, kidney, muscle, and intestine of unmetamorphosed larvae and nontrophic upstream-migrant (spawning-phase) sea lampreys, Petromyzon marinus. T4ORD properties (pH optimum, dithiothreitrol cofactor requirement, apparent K(m), substrate preference and potency of potential inhibitors) were similar in most respects to those described previously for teleosts. T4ORD activity was detected in all larval tissues examined and was highest in intestine. In upstream migrants, T4ORD was also greatest in intestine, but low in muscle and kidney and undetectable in liver. T3ORD activity was not found in any tissue of either developmental stage. T4IRD and T3IRD activities were negligible in larval tissues, but present in kidney and particularly intestine of upstream migrants. We conclude that depending on developmental/physiological state, sea lampreys possess low-K(m) outer-ring and inner-ring monodeiodinases, which in most respects correspond functionally with those of teleosts. However, in contrast to teleosts, deiodination is particularly active in larval intestine, perhaps reflecting the release from the endostyle of TH into the lumen of the alimentary canal.

The effects of exogenous thyroxine (T4) or triiodothyronine (T3), in the presence and absence of potassium perchlorate, on the incidence of metamorphosis and on serum T4 and T3 concentrations in larval sea lampreys (Petromyzon marinus L.)

Larval sea lampreys (Petromyzon marinus) measuring 100-119 mm in length were exposed to thyroxine (T4; 10 mg liter-1) or 3,5,3'-triiodothyronine (T3; 1 mg liter-1) in the presence and absence of the goitrogen potassium perchlorate (KClO4; 0.01%), for 4-24 weeks. Every 4 weeks, treated and untreated (control) groups of sea lampreys were examined for external signs of metamorphosis and serum was assayed for T4 and T3 concentrations. Precocious metamorphosis was observed following 8, 12, and 24 weeks of KClO4 treatment; however, metamorphosis was not observed in any control, or T4-, T3-, T4+KClO4-, and T3+KClO4-treated larvae. In addition, serum T4 and T3 concentrations were 62 and 72% lower in KClO4-treated individuals than in control animals, respectively. Treatment with exogenous thyroid hormones (TH), in the presence or absence of KClO4, resulted in serum T4 concentrations which were significantly greater (1.2- to 58-fold) than those of the controls in all sampling periods except one, but serum T3 concentrations were not significantly elevated in more than 50% of the cases. TH+KClO4 treatments produced serum T3 concentrations which were significantly greater than those of KClO4-treated animals and never less than those of controls. These data indicate that larval sea lampreys have a tremendous capacity to take up and store exogenous T4 in their serum, but the uptake and/or serum storage of T3 appears to be stringently regulated. Also, the absence of both metamorphosis and a decline in serum TH concentrations in TH+KClO4-treated animals suggests that a decline in serum TH concentrations may be an essential factor contributing to the induction of metamorphosis by KClO4.

Temporal changes in plasma thyroid hormone, growth hormone and free fatty acid concentrations, and hepatic 5'-monodeiodinase activity, lipid and protein content during chronic fasting and re-feeding in rainbow trout (Oncorhynchus mykiss)

Temporal changes in growth, plasma thyroid hormone, cortisol, growth hormone (GH) and non-esterified fatty acid (NEFA) concentrations, hepatic T3 content and hepatic 5'-monodeiodinase activity were measured in rainbow trout (Oncorhynchus mykiss) subjected to a sustained fast for up to eight weeks, and during a four-week re-feeding period. The purpose of the study was to examine aspects of the endocrine control of energy partitioning processes characteristic of short-term (acute; fasting) and long-term (chronic; starvation) food-deprivation states in fish, and to explore the role of the thyroid hormones, cortisol and GH in the energy repartitioning that takes place during an acute anabolic (re-feeding) state following chronic food deprivation.Differences in growth rate between fed and fasted groups were evident after two weeks, but significant weight loss by the fasted groups was not evident until between four and six weeks into the fast. Hepatosomatic indices (HSIs) were significantly reduced in the fasted fish within seven days, and as early as two days in one study; recovery of the HSI in fasted fish was evident within three days of re-feeding. Liver protein content (expressed as % wet weight) was consistently depressed in the fasted fish in only one of the three studies. Liver total lipid content (expressed as % wet weight) was depressed in the fasted fish within two days of food deprivation. Because of the rapid and sustained decrease in the HSI of fasted fish, the hepatic total protein and lipid reserves, when considered on a body weight basis, were markedly lowered within the first few days of the fast. Plasma GH concentrations exhibited a bi-modal pattern of change, with a transient fall in levels, followed by a sustained increase in fasted fish. The indicators of interrenal activity were suggestive of a depressed pituitary-interrenal axis in fasted animals; plasma cortisol levels were elevated to levels of fed animals within one day of re-feeding. The indicators of thyroid hormone economy (plasma thyroid hormone levels, liver triiodothyronine content, hepatic 5'-monodeiodinase (MD) activity, thyroid epithelial cell height) were similarly indicative of a depressed pituitary-thyroid axis in fasted animals, with recovery to levels of the fed animals within one week. Despite the compensatory changes in accumulation of reserves (as indicated by a compensatory increase in HSI), there were no apparent compensatory changes in any of the endocrine parameters evident during the re-feeding period.

In vivo effects of the steroid analogue RU486 on some aspects of intermediary and thyroid metabolism of brook charr, Salvelinus fontinalis

The glucocorticoid analogue RU486 was administered by intraperitoneal injection to brook charr (Salvelinus fontinalis) to further explore the role of cortisol on aspects of intermediary and thyroid hormone metabolism of the species. RU486 significantly elevated the hepatosomatic index, hepatic G3PDH activity, and hepatic glycogen content, but was without effect on hepatic protein content, hepatic FPBase activity, or plasma glucose concentration. However, the stressor-related increase in plasma glucose concentration that was evident in brook charr 24 h following handling and injection was suppressed in RU486-treated groups. The distribution volume, turnover rates, and metabolic clearance rates of [3H]cortisol were similar in RU486- and vehicle-treated groups. Plasma T3 and T4 concentrations were similar in RU486- and vehicle-treated groups, but hepatic T3 production and hepatic T3 content were lower in RU486-treated fish; TSH had no effect on hepatic T3 content of vehicle-treated brook charr but significantly increased T3 content in the RU486-treated group. These observations support the concept of a role of cortisol in the control of peripheral monodeiodination of T4 in salmonid fish and suggest that RU486 may be a useful drug for evaluating the role of cortisol in fish in vivo.

Chronic fasting reduces the response of the thyroid to growth hormone and TSH, and alters the growth hormone-related changes in hepatic 5'-monodeiodinase activity in rainbow trout, Oncorhynchus mykiss

Chronically fasted rainbow trout (Oncorhynchus mykiss) had significantly lower plasma L-thyroxine (T4) and triiodo-L-thyronine (T3), and higher plasma growth hormone (GH) concentrations than fed animals. Fasted and fed trout were administered bovine thyrotropic hormone (bTSH), native ovine GH (oGH), or recombinant human GH (rhGH) alone, or GH in combination with bTSH to further study the effects of food deprivation on the activity of the pituitary-thyroid axis and on the control of hepatic T3 production. Although the fasted rainbow trout retained the ability to respond to bTSH challenge, the resultant elevation in plasma T4 concentration was significantly lower than that of fed animals; there was no plasma T3 response to bTSH challenge in either fed or fasted trout, except for a significant elevation in fed bTSH-injected fish and a significant depression in fed saline-injected fish sampled 2.5 hr after the injection. GH when administered alone had no significant effect on plasma T4 concentrations of either fed or fasted animals, and stimulated an increase in plasma T3 concentration and an increased hepatic T3 content only in the fed fish, despite a significant stimulation by both oGH and rhGH of in vitro hepatic 5'-monodeiodinase activity (MDA) in both fed and fasted groups. bTSH appeared to suppress rhGH- and oGH-stimulated MDA in fasted groups, and rhGH-stimulated MDA in fed trout. The data suggest that chronic fasting induced a down-regulation of the response of thyroid tissue to bTSH challenge, and of the GH-stimulation of T3 production, in vivo, although in vitro hepatic MDA was elevated following GH administration to both fed and fasted rainbow trout.