Asynchrony of changes in tissue and plasma thyroid hormones during the parr-smolt transformation of coho salmon

Teleost Metamorphosis: The Role of Thyroid Hormone

In most teleosts, metamorphosis encompasses a dramatic post-natal developmental process where the free-swimming larvae undergo a series of morphological, cellular and physiological changes that enable the larvae to become a fully formed, albeit sexually immature, juvenile fish. In all teleosts studied to date thyroid hormones (TH) drive metamorphosis, being the necessary and sufficient factors behind this developmental transition. During metamorphosis, negative regulation of thyrotropin by thyroxine (T4) is relaxed allowing higher whole-body levels of T4 that enable specific responses at the tissue/cellular level. Higher local thyroid cellular signaling leads to cell-specific responses that bring about localized developmental events. TH orchestrate in a spatial-temporal manner all local developmental changes so that in the end a fully functional organism arises. In bilateral teleost species, the most evident metamorphic morphological change underlies a transition to a more streamlined body. In the pleuronectiform lineage (flatfishes), these metamorphic morphological changes are more dramatic. The most evident is the migration of one eye to the opposite side of the head and the symmetric pelagic larva development into an asymmetric benthic juvenile. This transition encompasses a dramatic loss of the embryonic derived dorsal-ventral and left-right axis. The embryonic dorsal-ventral axis becomes the left-right axis, whereas the embryonic left-right axis becomes, irrespectively, the dorsal-ventral axis of the juvenile animal. This event is an unparalleled morphological change in vertebrate development and a remarkable display of the capacity of TH-signaling in shaping adaptation and evolution in teleosts. Notwithstanding all this knowledge, there are still fundamental questions in teleost metamorphosis left unanswered: how the central regulation of metamorphosis is achieved and the neuroendocrine network involved is unclear; the detailed cellular and molecular events that give rise to the developmental processes occurring during teleost metamorphosis are still mostly unknown. Also in flatfish, comparatively little is still known about the developmental processes behind asymmetric development. This review summarizes the current knowledge on teleost metamorphosis and explores the gaps that still need to be challenged.

Perchlorate exposure does not modulate temporal variation of whole-body thyroid and androgen hormone content in threespine stickleback

Previously we showed that exposure of threespine stickleback (Gasterosteus aculeatus) to the endocrine disruptor perchlorate results in pronounced structural changes in thyroid and gonad, while surprisingly, whole-body thyroid hormone concentrations remain unaffected. To test for hormone titer variations on a finer scale, we evaluated the interactive effects of time (diel and reproductive season) and perchlorate exposure on whole-body contents of triiodothyronine (T3), thyroxine (T4), and 11-ketotestosterone (11-KT) in captive stickleback. Adult stickleback were exposed to 100ppm perchlorate or control water and sampled at 4-h intervals across the 24-hday and at one time-point (1100h) weekly across the reproductive season (May-July). Neither whole-body T3 nor T4 concentration significantly differed across the day in control or perchlorate treated stickleback. Across the reproductive season, whole-body T3 concentration remained stable while T4 significantly increased. However, neither hormone concentration was significantly affected by perchlorate, verifying our previous studies. The concentration of whole-body 11-KT, a major fish androgen, displayed significant diel variation and also steadily declined across the reproductive season in untreated males; perchlorate exposure did not influence the concentration of 11-KT in either diel or reproductive season schedules. Diel and reproductive season variations in 11-KT content in male stickleback are likely related to reproductive physiology and behavior. The observed increase in T4 content across the reproductive season may be reflective of increased energy investment in reproduction near the end of the life cycle.

The pituitary-thyroid axis during the parr-smolt transformation of Coho salmon, Oncorhynchus kisutch: quantification of TSH β mRNA, TSH, and thyroid hormones

The objective of this investigation was to quantify pituitary thyroid stimulating hormone (TSH) β mRNA, pituitary and plasma TSH and plasma thyroid hormone levels during the parr-smolt transformation of Coho salmon that occurs in spring from February to May. The status of the pituitary-thyroid axis was assessed using an RNase protection assay for pituitary TSH β mRNA and radioimmunoassays for salmon pituitary and plasma TSH and thyroid hormones. TSH β mRNA was highest during late winter (February) (4.9 pg/μg DNA) and gradually declined during spring (2.3 pg/μg DNA). In contrast, pituitary and plasma TSH levels showed a small, but statistically non-significant change during smoltification. Despite minimal change in plasma TSH levels, characteristically large increases in plasma T4 (January-3.3 ng/ml to April-10.2 ng/ml) and significant, but modest increases in plasma T3 (February-2.4 ng/ml to April-5.8 ng/ml) were observed. Regression analysis showed a significant positive relationship between plasma T4 and T3 and negative relationship between plasma T3 and pituitary TSH β mRNA. However, all other relations were not significant. These data suggest a significant role for peripheral regulation (i.e. T4-T3 conversion, change in tissue sensitivity, hormone degradation rate) as well as evidence of central regulation via negative feedback at the level of the pituitary gland in regulation of thyroid activity in salmon. Furthermore, the increased thyroid sensitivity to TSH (shown previously), in the face of relatively constant plasma TSH levels, may be the major factor responsible for the increased thyroid activity observed during smoltification.

cDNA cloning and expression analysis of thyroid hormone receptor in the coho salmon Oncorhynchus kisutch during smoltification

Two types of cDNA clones encoding thyroid hormone receptor (TR) genes, TRalpha and TRbeta, were isolated from the coho salmon Oncorhynchus kisutch. The two genes each consists of multiple isoforms, derived from different loci or by an alternative splicing. RT-PCR revealed a broad localization of the TRalpha and TRbeta transcripts that both expressed in brain, gill, liver, muscle, kidney, ovary, skin and eyeball. During the early stages of the smoltification period, the mRNA level of the TR transcripts showed a large individual variability at the maximum of 10-fold. In contrast, as the plasma thyroid hormone level elevated at the middle of the period, the variation in the expression level became much smaller at 2-fold. After the completion of the smoltification, the mRNA accumulation level of the TR genes rapidly decreased. These findings suggest that the dynamics of the receptor (TR) should be taken account in addition to the availability of the hormone ligands (T3 and T4) to understand the underlying mechanisms of the smoltification causing numerous physiological and morphological changes in salmonids.

The hypothalamic-pituitary-thyroid (HPT) axis in fish and its role in fish development and reproduction

Bony fishes represent the largest vertebrate class and are a very diverse animal group. This chapter provides a thorough review of the available scientific literature on the thyroid system in these important vertebrate animals. The molecular components of the hypothalamic-pituitary-thyroid (HPT) axis in this group correspond closely to those of mammals. The thyroid tissue in the fishes is organized as diffuse follicles, with a few exceptions, rather than as an encapsulated gland as is found in most other vertebrate species. The features of this diffuse tissue in fishes are reviewed with an emphasis on feedback relationships within the HPT axis, the molecular biology of the thyroid system in fishes, and comparisons versus the thyroid systems of other vertebrate taxa. A review of the role of thyroid hormone in fish development and reproduction is included. Available information about the HPT axis in fishes is quite detailed for some species and rather limited or absent in others. This review focuses on species that have been intensively studied for their value as laboratory models in assays to investigate disruption in normal function of the thyroid system. In addition, in vitro and in vivo assay methods for screening chemicals for their potential to interfere with the thyroid system are reviewed. It is concluded that there are currently no in vitro or in vivo assays in fish species that are sufficiently developed to warrant recommendation for use to efficiently screen chemicals for thyroid disruption. Methods are available that can be used to measure thyroid hormones, although our ability to interpret the causes and implications of potential alterations in T4 or T3 levels in fishes is nonetheless limited without further research.

Evidence that thyroid hormone induces olfactory cellular proliferation in salmon during a sensitive period for imprinting

Salmon have long been known to imprint and home to natal stream odors, yet the mechanisms driving olfactory imprinting remain obscure. The timing of imprinting is associated with elevations in plasma thyroid hormone levels,with possible effects on growth and proliferation of the peripheral olfactory system. Here, we begin to test this idea by determining whether experimentally elevated plasma levels of 3,5,3′-triiodothyronine (T3)influence cell proliferation as detected by the 5-bromo-2′-deoxyuridine(BrdU) cell birth-dating technique in the olfactory epithelium of juvenile coho salmon (Oncorhynchus kisutch). We also explore how natural fluctuations in thyroxine (T4) relate to proliferation in the epithelium during the parr-smolt transformation. In both studies, we found that BrdU labeled both single and clusters of mitotic cells. The total number of BrdU-labeled cells in the olfactory epithelium was significantly greater in fish with artificially elevated T3 compared with placebo controls. This difference in proliferation was restricted to the basal region of the olfactory epithelium, where multipotent progenitor cells differentiate into olfactory receptor neurons. The distributions of mitotic cluster sizes differed significantly from a Poisson distribution for both T3 and placebo treatments, suggesting that proliferation tends to be non-random. Over the course of the parr-smolt transformation, changes in the density of BrdU cells showed a positive relationship with natural fluctuations in plasma T4. This relationship suggests that even small changes in thyroid activity can stimulate the proliferation of neural progenitor cells in the salmon epithelium. Taken together, our results establish a link between the thyroid hormone axis and measurable anatomical changes in the peripheral olfactory system.

Dietary accumulation and biochemical responses of juvenile rainbow trout (Oncorhynchus mykiss) to 3,3',4,4',5-pentachlorobiphenyl (PCB 126)

Juvenile rainbow trout (Oncorhynchus mykiss) (initial weights 2-5 g) were exposed to three dietary concentrations (0, 12.4 and 126 ng g(-1), wet weight) of a 14C-labelled 3,3',4,4',5-pentachlorobiphenyl (PCB 126) for 30 days followed by 160 days of clean food. We assessed bioaccumulation, histology (liver and thyroid) and biochemical responses (liver ethoxyresorufin-O-deethylase (EROD), liver vitamins (retinoids and tocopherol) and muscle thyroid hormone levels) along with growth and survival. The half-life of PCB 126 in the rainbow trout ranged from 82 to 180 days while biomagnification factors (BMF) ranged from 2.5 to 4.1 providing further evidence that PCB 126 is among the most bioaccumulative PCB congeners. Toluene extractable 14C declined with time in the trout suggesting the possibility of some biotransformation and/or covalent bonding with biological macromolecules. The threshold for liver EROD induction by PCB 126 was approximately 0.1 ng g(-1) (wet weight). EROD activities in the low- and high treatments were 9 and 44 times greater than control, respectively, and remained elevated throughout the experiment. EROD activity was correlated with whole body concentrations of PCB 126 although there was evidence of EROD activity suppression in the highly exposed fish. Liver didehydroretinoids and tocopherol concentrations were depressed by the high PCB 126 dose after 30 days exposure. Initially, muscle concentrations of thyroxine (T4) and triiodo-L-thyronine (T3) declined as the fish grew during the experiment, and exposure to PCB 126 accelerated the growth related decline. More information is needed to assess the functional significance of the reduced muscular stores of thyroid hormones. Despite the changes in liver EROD, liver vitamins and muscle thyroid hormones, liver and thyroid histology in trout examined after 30 days exposure and growth parameters were unaffected by PCB 126. This indicates that the functional competences of the physiological factors associated with growth were maintained under the experimental conditions.

Thyroid of lake sturgeon, Acipenser fulvescens. I. Hormone levels in blood and tissues

The authors measured thyroid hormone (TH) levels in plasma, whole carcass, and tissues of cultured 2-year-old immature lake sturgeon held in fresh water and in serum of adults at spawning time from the Winnipeg River. Circulating thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels were low (T4 approximately 0.3 ng/ml, T3 approximately 0.2 ng/ml) in all cultured fish and most adults, but a few of the latter had exceptionally high T4 and T3 levels. The percentages of blood TH found in erythrocytes were 19.5% (T4), 6.1% (T3) and 6.9% (reverse T3 = rT3). Equilibrium dialysis showed much higher percentages of plasma free (F) FT4 (1.1%), FT3 (0.4%), and FrT3 (3,3',5'-triiodothyronine = rT3, 3.0%) for sturgeon than for rainbow trout, indicating more limited TH binding to sturgeon plasma sites. However, concentrations of FT4 and FT3 were close to those reported for salmonids. T3 levels exceeded T4 levels in most extrathyroidal tissues of cultured sturgeon but in most cases were less than 0.1 ng/g and 10 to 100 times lower than reported for salmonids; only the whole brain T3 concentration (5.6 ng/g) approached that of salmonids. The digested thyroid contained 21.3 ng T3/g and 2.4 ng T4/g. The authors conclude that lake sturgeon have a low circulating reserve of bound TH but have FT4 and FT3 concentrations close to those of salmonids. The high thyroidal T3:T4 ratio and low tissue T4 levels suggest that, in contrast to teleosts studied to date, the thyroid may be a significant direct source of T3, the primary TH in sturgeon tissues. High serum T4 and T3 levels in some sturgeon at spawning time may suggest a thyroid role in reproduction.

Timing of metamorphosis and the onset of the negative feedback loop between the thyroid gland and the pituitary is controlled by type II iodothyronine deiodinase in Xenopus laevis

Two important features of amphibian metamorphosis are the sequential response of tissues to different concentrations of thyroid hormone (TH) and the development of the negative feedback loop between the pituitary and the thyroid gland that regulates TH synthesis by the thyroid gland. At the climax of metamorphosis in Xenopus laevis (when the TH level is highest), the ratio of the circulating precursor thyroxine (T4) to the active form 3,5,3'-triiodothyronine (T3) in the blood is many times higher than it is in tissues. This difference is because of the conversion of T4 to T3 in target cells of the tadpole catalyzed by the enzyme type II iodothyronine deiodinase (D2) and the local effect (cell autonomy) of this activity. Limb buds and tails express D2 early and late in metamorphosis, respectively, correlating with the time that these organs undergo TH-induced change. T(3) is required to complete metamorphosis because the peak concentration of T4 that is reached at metamorphic climax cannot induce the final morphological changes. At the climax of metamorphosis, D2 expression is activated specifically in the anterior pituitary cells that express the genes for thyroid-stimulating hormone but not in the cells that express proopiomelanocortin. Physiological concentrations of T3 but not T4 can suppress thyrotropin subunit beta gene expression. The timing and the remarkable specificity of D2 expression in the thyrotrophs of the anterior pituitary coupled with the requirement for locally synthesized T3 strongly support a role for D2 in the onset of the negative feedback loop at the climax of metamorphosis.

Parr-smolt transformation in Atlantic salmon: thyroid hormone deiodination in liver and brain and endocrine correlates of change in rheotactic behavior

We tested the hypothesis that metabolism of thyroid hormones by the brain of Atlantic salmon (Salmo salar) changes when rheotactic behavior reverses during parr-smolt transformation (PST). We measured brain and liver thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3) outer-ring deiodination (ORD) and inner-ring deiodination (IRD) activities and plasma T4 and T3 levels in Atlantic salmon held under natural photoperiod in fresh water at 10°C in the spring of 1993 and 1994. We also measured changes in T4, T3, and cortisol levels during the change in rheotactic behavior. Condition factor decreased while salinity tolerance improved from mid-March to late April. The turbidity-induced transition from upstream to downstream swimming occurred in mid to late April. The main changes in brain deiodination were reduced T3IRD (1993 study) and elevated T4ORD (1994 study). In both years, a high ratio of T4ORD/T3IRD activities in the brain indicated an increased potential for T3 production in the brain during advanced PST. Liver deiodination profiles differed between years, but during advanced PST the low T4ORD activity and low T4ORD/T3IRD activity ratio suggested a low potential for hepatic, and hence systemic, T3 production. However, plasma T4 was increased in downstream swimmers at 1 d (1993) and 4 h (1994) after the turbidity increase. Since at this time brain deiodination pathways were poised towards T3 production, the surge in plasma T4 would likely increase local T3 formation in brain. We conclude that during PST there is no major change in hepatic deiodination and hence probably no major change in systemic T3 availability. But deiodination properties in brain during late PST indicate the potential for local T3 formation. This may be significant when plasma T4 increases at the time of downstream migration.

Effect of experimental 3,5,3'-triiodothyronine hyperthyroidism on thyroid hormone deiodination in brain regions and liver of rainbow trout, Oncorhynchus mykiss

We studied the effect of 3,5,3'-triiodothyronine (T3) hyperthyroidism, induced by 12 ppm T3 in food for 10 days, on the low-Km activities of thyroxine (T4) outer-ring deiodination (ORD) to form T3, T4 inner-ring deiodination (IRD) to form 3,3',5'-triiodothyronine (reverse T3 (rT3)), T3ORD to form 3,5-diiodothyronine (3,5-T2), and T3IRD to form 3,3'-diiodothyronine (3,3'-T2) in six brain regions and in liver of immature rainbow trout (Oncorhynchus mykiss) at 12°C. Throughout the brain, T4ORD activity of control trout was uniformly low and T3ORD activity was negligible. T4IRD and T3IRD activities were about 5-fold and 50-fold greater, respectively, than T4ORD activity and were higher in the optic lobes, hypothalamus, and telencephalon/olfactory bulbs than in the medulla or cerebellum. T3 treatment doubled the plasma T3 level with no change in plasma T4 level and reduced T4ORD and T4IRD activities in all brain regions but did not alter T3IRD activity or the negligible T3ORD activity. Relative to controls, T3 treatment reduced liver T4ORD activity 6-fold, increased T4IRD activity 8-fold, and increased T3IRD activity 12-fold. We conclude that (i) there are regional differences in trout brain T4IRD and T3IRD activities but not in T4ORD activity, indicating spatial variation in brain T4 and T3 catabolism, (ii) in response to a mild T3 challenge the brain deiodination pathways do not undergo the same autoregulatory adjustments as those in liver, and (iii) a T3 challenge reduces brain T4IRD activity with no change in T3IRD activity, which suggests that the two IRDs may be controlled by separate deiodinases.

Plasma and tissue thyroxine and triiodothyronine contents in sublethally stressed, aluminum-exposed brown trout (Salmo trutta)

Immature female brown trout, Salmo trutta, were exposed to pH 5.0 soft water in the presence or absence of aluminum (Al) at 12.5 micrograms liter-1 and their plasma concentrations and tissue contents of thyroxine (T4) and triiodothyronine (T3) were compared with those of a control group of trout held in pH 7.0 soft water. After 120 hr, plasma cortisol, glucose, T4, and T3 concentrations were greater in the Al-exposed trout than in trout exposed to acid conditions alone, indicating that although the Al conditions were sublethal, a significant stress response was elicited. Significant increases in liver T4 content, liver 5'-monodeiodinase activity and liver T3 content indicated increased hepatic T4 to T3 conversion in the Al-exposed trout. The T4 contents of brain, gill filaments, white muscle, heart ventricle, caudal kidney, and ovary were not significantly altered by Al exposure. The T3 content of caudal kidney and ovary were significantly lower in Al-exposed trout than in control fish in neutral water but were unchanged in the brain, gill filaments, heart ventricle, and white muscle of these trout. The present data support previous observations of increased plasma T3 concentrations in sublethally Al-exposed brown trout and indicate that at least part of the increased plasma T3 concentration is due to an increased hepatic uptake of T4 and monodeiodination to T3. However, analysis of nonhepatic tissue T3 content gave no indication of increased T3 production by these tissues.

Thyroid hormone concentrations in the gonads of wild chum salmon during maturation

Changes in gonadal and plasma concentrations of thyroid hormones were examined at various stages of maturation in chum salmon (Oncorhynchus keta) caught in the Bering Sea and the Bay of Alaska. Plasma concentrations of thyroxine (T4) were less than 5 ng ml(-1), and those of 3,5,3'-triiodo-L-thyroxine (T3) were less than 2 ng ml(-1) I in both males and females, regardless of the degree of sexual maturity or the gonadosomatic index (GSI).There was no clear relationships between circulating thyroid hormone levels and tissue levels. The ovarian T4 concentrations were undetectable (less than 0.2 ng g(-1)) or less than 2 ng g(-1) when GSI was lower than 1%, but increased thereafter and reached a plateau of 8-10 ng g(-1) when GSI became 2%. The ovarian T3 concentrations were about 5 ng g(-1) when GSI was 1%, increased to a maximum level (20 ng g(-1)) when GSI was about 2%, and decreased to a constant level of 10 ng g(-1) thereafter. The T4 and T3 content in single oocyte increased proportionally to the oocyte volume, indicating a constant incorporation of the hormones into the oocyte.The T4 concentrations in the testis were 1 ng g(-1) or less regardless of the GS1. On the other hand, the T3 concentrations were highest (15 ng g(-1)) when the GSI was less than 1%, decreased thereafter when spermatocytes appeared in the testis, and became about 5 ng g(-1) I in testes containing spermatozoa, raising the possibility of a role for T3 during early gamete and/or gonad maturation of testes.