Thyroid hormones in blood plasma of developing salmon embryos

Thyroid signaling in immune organs and cells of the teleost fish rainbow trout (Oncorhynchus mykiss)

Thyroid hormones are involved in modulating the immune system in mammals. In contrast, there is no information on the role played by these hormones in the immune system of teleost fish. Here we provide initial evidence for the presence of active thyroid signaling in immune organs and cells of teleosts. We demonstrate that immune organs (head kidney and spleen) and isolated leukocytes (from head kidney and peripheral blood) of the rainbow trout (Oncorhynchus mykiss) express both thyroid receptor α (THRA) and β (THRB). Absolute mRNA levels of THRA were significantly higher than those of THRB. THRA showed higher expression in immune organs and isolated immune cells compared to the reference organ, liver, while THRB showed the opposite. In vivo exposure of trout to triiodothryronine (T3) or the anti-thyroid agent propylthiouracil (PTU) altered THR expression in immune organs and cells. Effect of T3 and PTU over the relative expression of selected marker genes of immune cell subpopulations was also studied. Treatments changed the relative expression of markers of cytotoxic, helper and total T cells (cd4, cd8a, trb), B lymphocytes (mIgM) and macrophages (csf1r). These findings suggest that the immune system of rainbow trout is responsive to thyroid hormones.

Systemic thyroid hormone is necessary and sufficient to induce ultraviolet-sensitive cone loss in the juvenile rainbow trout retina

Rainbow trout possess ultraviolet-sensitive (UVS) cones in their retina that degenerate naturally during development. This phenomenon can be induced with exogenous thyroxine [T4, a thyroid hormone (TH)] treatment. However, the previous T4 exposure experiments employed static water immersion; a method that could introduce confounding stress effects on the fish. Because of this, it was uncertain if T4 alone was sufficient to induce retinal changes or if stress-related hormones were also necessary to initiate this process. Furthermore, it was unclear whether endogenous T4 was the factor responsible for initiating natural UVS cone loss during development. The current study examined the role of systemic T4 on the juvenile rainbow trout retina using a slow-release implant. Exogenous T4 treatment resulted in SWS1 opsin downregulation and UVS cone loss after four weeks of exposure, signifying that T4 is sufficient to induce this process. Blocking endogenous T4 production with propylthiouracil (PTU, an anti-thyroid agent) attenuated SWS1 downregulation and UVS cone loss in the retina of naturally developing rainbow trout, suggesting that endogenous T4 is necessary to initiate retinal remodelling during development. Quantitative real-time RT-PCR analysis demonstrated that several TH-regulating components are expressed in the trout retina, and that expression levels of the TH receptor isoform TRbeta and the type 2 deiodinase (D2) change with T4 treatment. This suggests that T4 may act directly on the retina to induce UVS cone loss. Taken together, these results demonstrate that systemic TH is necessary and sufficient to induce SWS1 opsin downregulation and UVS cone loss in the retina of juvenile rainbow trout.

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.

Effect of hypothyroidism induced by propylthiouracil on ovarian function and structure in offspring from treated mothers (Rats)

The aim of the present study was to investigate the effects of hypothyroidism induced during the pre- and postnatal periods of life on ovarian function and structure in offspring (pups) 120 days of age. Three groups were used. In the prenatal group, treatment was given from conception to parturition. In the postnatal group, treatment was given from parturition to 25 days postpartum. Hypothyroidism was induced by administration of 0.1% 6-n-propyl-2-thiouracil (PTU) in the drinking water of mothers. Body weights of the offspring were measured weekly. In each group, ten offspring were sacrificed at 120 days of age. Postnatal PTU treated pups showed delay in eye opening, teething, fur development, and weaning (35-37 days) compared to control animals (28-30 days). Body weight of offspring in the postnatal PTU treatment group was significantly decreased (P < 0.001), while the prenatal PTU treatment group showed a significant increase (P < 0.0001) compared to control animals. There was a significant (P < 0.05) reduction in paired ovarian weight of offspring in the postnatal PTU treatment group compared to control animals. Diameter of the ovaries was not affected by any treatment. Regarding the morphometery, only offspring in the prenatal PTU treatment group showed a significant (P < 0.001) increase in the diameter of graafian follicles. No significant difference was observed in morphometery of the granulosa layer, primary, and developing follicles of control and all treated groups. Number of primary, developing, and graafian follicles of all the treated groups was similar to that of the control group. The corpora lutea of the postnatal PTU treated group contained a population of large numbers of luteal cells compared to the control group. The prenatal PTU treated group did not exhibit a profound effect on ovarian morphology, histology, and morphometery. No difference was found in the serum estradiol concentration of control and PTU treated groups. J. Exp. Zool. 293:407-413, 2002.

Characterization of a sea bream (Sparus aurata) thyroid hormone receptor-beta clone expressed during embryonic and larval development

A clone encoding thyroid hormone receptor-beta (TR-beta) was isolated from a sea bream (Sparus aurata) ovary cDNA library. Sea bream (sb)TR-beta is closely related to its counterparts from other vertebrates and, like them, preferentially binds T3 rather than T4. However, the putative sbTR-beta protein contains a nine-amino-acid insert that is also present in the corresponding proteins from flounder and salmon but absent in TR-betas from zebrafish and terrestrial vertebrates. Semiquantitative RT-PCR analysis showed that sbTR-beta transcripts begin to accumulate during gastrulation and increase markedly in quantity up to the period around hatch (ca. 40 h postfertilization) before declining slightly. In adult tissues, TR-beta mRNA was present in approximately equal quantities in heart, intestine, brain, kidney, skeletal muscle, liver, and gill. The significance of the relatively strong expression of TR-beta during sea bream embryogenesis is discussed.

Cloning of putative piscine (Sparus aurata) transthyretin: developmental expression and tissue distribution

cDNA encoding putative transthyretin (prealbumin, TTR) was cloned from liver of the marine fish Sparus aurata. The cDNA contains an open reading frame of 453 nt, encoding for a TTR precursor of 151 amino acids. The deduced amino acid sequence of S. aurata TTR shows identity of 54, 57.3 and 54.1% with lizard, chicken and rat TTR, respectively. Northern blot analysis revealed a TTR transcript of about 700 nt, highly expressed in liver, but also in skin. Low expression was detected in 12 other tissues by using RT-PCR. The ontogeny of TTR expression during early stages of larval development of S. aurata was examined by Northern blot analysis using poly(A+)RNA from larvae collected on different days after hatching. TTR mRNA was seen already on the first day after hatching and its steady-state levels increased from Day 15 onwards. Molecular cloning of a TTR-like cDNA from fish suggests that TTR evolved earlier in vertebrate development than previously thought. Furthermore, its expression in liver exceeds by several-fold that found in brain, yet high expression is also found in skin. These results suggest that in fish, liver is the main site of TTR synthesis, but that TTR may have an important function in fish skin.

Thyroid hormones in brown trout (Salmo trutta) reproduction and early development

Gravid brown trout (Salmo trutta) females were injected with various doses of a synthetic gonadotropin-releasing hormone analog (GnRHa), given with or without an injection of triiodothyronine (T3), in order to investigate the potential of T3 (a) to enhance the stimulatory effect of GnRHa on ovulation, and (b) to enhance the growth and survival of the produced progeny. From the time the hormonal treatments were initiated until ovulation was detected 5-38 days later, endogenous plasma T3 levels increased from an average of 3.6 to 11.6 ng ml(-1). Injection with 20 mg T3 kg(-1) body weight, further elevated plasma T3 levels at ovulation (16.0 ng ml(-1). Mean time to ovulation was reduced significantly in fish injected with 10 μg kg(-1) of GnRHa, whereas treatment with lower doses was ineffective. Injection with T3 did not enhance the ovulatory response of brown trout to GnRHa. Unfertilized eggs obtained from T3-injected females had a higher T3 content, suggesting a transfer of T3 from the maternal circulation into the oocytes. Maternal T3 injection had no effect on egg fertilization rates, embryo survival to eyeing and hatching, or the prevalence of abnormal larvae at the time of hatching. Length and weight gain of the progeny during yolk absorption was also not influenced by maternal T3 treatment. At the completion of yolk-sac absorption, progeny from females injected with T3 had a higher prevalence of skeletal abnormalities than controls. The results suggest that in teleosts like brown trout, which have high endogenous circulating T3 levels, treatment of females with T3 does not enhance responsiveness to GnRHa and it has the potential for deleterious effects on their offspring.

Investigations into the development of the pituitary gland-thyroid tissue axis and distribution of tissue thyroid hormone content in embryonic coho salmon (Oncorhynchus kisutch) from Lake Ontario

Total organism content of L-thyroxine (T4) and triiodo-L-thyronine (T3) were measured in the early developmental stages of a stock of Lake Ontario coho salmon from the egg to the yolk absorption stage. Whole organism T4 levels were constant between the egg and pre-hatch embryo stages, but fell progressively during yolk absorption. T3 levels were low from egg to eye-pigment appearance, but then increased prior to hatch and fell again during the post-hatch yolk absorption period.When expressed as ng/tissue, T4 content of the body compartment rose progressively between days 67 and 87 post-fertilization, whilst T4 content of the yolk compartment fell progressively during the same period; the pattern was not evident for tissue T3 content. When expressed as ng/g dry weight of tissue, the inverse relationship was found for T4, and T3 content of the body and yolk compartments decreased progressively and increased progressively, respectively during the same period, suggesting that thyroid hormones were selectively retained in the yolk compartment.Intensely "immunostained" (using anti-human β-TSH antibody) thyrotropic cells were present in small numbers in the pars distalis of the embryonic pituitary at the eye-pigment appearance stage, and the numbers increased markedly until the pre-hatch period.Administration of either bovine thyrotropic hormone (bTSH) or ovine growth hormone (oGH) had no effect on thyroid hormone content of larvae challenged during the yolk absorption period, suggesting that the thyroid tissue was not responsive to exogenous bTSH challenge at this time, and that oGH-sensitive 5'-monodeiodination was either not present or at levels that were too low to cause an elevation in total T3 content, or that the substrate levels were insufficient to permit a measureable increase in whole body T3 content.

Changes in brain gonadotropin-releasing hormone, pituitary and plasma gonadotropins, and plasma thyroxine during smoltification in chinook salmon (Oncorhynchus tschawytscha)

Concentrations of brain salmon gonadotropin-releasing hormone (sGnRH), plasma gonadotropin I (GTH I), and pituitary GTH I and GTH II were determined in yearling chinook salmon (Oncorhynchus tschawytscha) during the parr-smolt transformation in two successive seasons. There were significant elevations in brain sGnRH content from February to March in 1988, and from February to April in 1989. Increases in brain sGnRH content coincided with elevations in plasma thyroxine levels that occurred from February to March, 1988 and 1989. Plasma GTH levels were relatively constant (1-2 ng/ml) throughout the period of sampling. However, during 1988, plasma concentrations of GTH I decreased significantly between late March and early April. During 1989, plasma GTH I levels appeared to reach a peak (2 ng/ml) in mid-February, but otherwise remained near 1 ng/ml. Previous studies have shown that GTH II was not detectable in plasma at this stage. During 1989, pituitary GTH I concentrations were 50- to 70-fold higher than that of GTH II, and increased, though not significantly, from February through April. Although GTH II was detected in the pituitary by RIA, it is likely that the measurable levels are due to GTH I cross-reaction in the GTH II RIA. Histological examination of the gonads indicated that throughout smoltification the oocytes remained in the perinucleolar stage of oogenesis and the testes were in the spermatogonial stage of spermatogenesis. Although no observable changes in gametogenesis occurred, the changes in brain sGnRH content, plasma GTH I levels, and pituitary GTH content suggest that some changes in the hypothalamic-pituitary axis may occur during smoltification.

Role of thyroid hormones in tilapia larvae (Oreochromis mossambicus): I. Effects of the hormones and an antithyroid drug on yolk absorption, growth and development

Treatment of one-day old yolksac larvae of tilapia (Oreochromis mossambicus) by immersion in 0.05 ppm T4 or 0.01 ppm T3 significantly accelerated the differentiation and growth of all the fins, particularly pectoral and tail fins. Both the treatments also significantly accelerated yolk absorption and transition to free-swimming activity in the larvae. The treatments also significantly accelerated the growth of the larvae, with T3 at 0.01 ppm having a greater effect than T4 at 0.05 ppm. The yolk conversion efficiency was found not to be significantly affected by the hormone treatments but the treated larvae exhibited an increased heart beat, suggesting metabolic stimulation by the hormones.On the other hand, yolk absorption and free-swimming activity were significantly delayed in tilapia larvae immersed in 25 ppm solution of an antithyroid drug, phenylthiocarbamide (PTC). PTC also retarded the growth of the larvae. T4 (0.05 and 0.10 ppm) or T3 (0.01 and 0.02 ppm) therapy removed the PTC-inhibition,albeit not completely, suggesting that thyroid hormones are involved in the larval growth and development of tilapia.

Role of thyroid hormones in tilapia larvae (Oreochromis mossambicus): II. Changes in the hormones and 5'-monodeiodinase activity during development

Thyroid hormone profiles and 5'-monodeiodinase activity were determined in tilapia at different stages of early development. The results showed that both T4 and T3 were present in significant amounts in fertilized eggs. There was a steady decrease in both T4 and T3 levels during embryonic development. The levels continued to decline after hatching until around 7 days later when most of the yolk had been absorbed. The T4 level started to rise then, suggesting that the larval thyroid had begun to produce T4 at this time, which coincided with the period of faster growth of the larvae. The T3 level remained fairly constant until around 20 days after which it rose significantly. In vitro determination of 5'-monodeiodinase activity (5'-D activity) in the whole-body homogenates of larvae showed that the enzymatic conversion of T4 to T3 was not detectable in eggs and 3-day-old larvae but detected in 5-day-old and older larvae. There was a gradual increase in the Vmax as development proceeded indicating increasing 5'-D activity during larval development. The Km values did not differ significantly in the different stages of development. These results are discussed in relation to the growth and development of the larvae.

Salinity and temperature effects on whole-animal thyroid hormone levels in larval and juvenile striped bass, Morone saxatilis

Whole-animal thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels were measured in larval and juvenile striped bass, Morone saxatilis, reared for 10 days at one of three levels of salinity (equivalent to fresh water (FW), one-third seawater (1/3 SW), and seawater (SW) and two temperatures (15°C and 20°C). The striped bass were pre-metamorphic larvae, metamorphic larvae or juveniles. The short-term effects of seawater on plasma T4 levels of juvenile striped bass were also measured. Higher salinities increased T4 levels in premetamorphic larvae. In metamorphic larvae, SW and 1/3 SW increased T4 levels and SW increased T3 levels at 20°C. This response was eliminated in those at 15°C. Whole-animal thyroid hormone content was unaffected by salinity or temperature in juvenile striped bass, although significant fluctuations in plasma T4 levels occurred in those transferred to 1/3 SW and SW. The thyroid axis of striped bass responds to salinity and temperature as early as in the pre-metamorphic stage. Thyroid hormones may mediate the beneficial effects of salinity on larval striped bass growth and survival.

Changes in tissue and blood concentrations of thyroid hormones in developing chum salmon

The changes in tissue and blood concentrations of thyroxine (T4) and triiodothyronine (T3) were examined during development of the chum salmon (Oncorhynchus keta). Extraction methods previously established for tissue T4 were also validated for tissue T3, by parallel displacement curves to T3 standard in the radioimmunoassay and by the same elution patterns of immunoreactivity in a HPLC system. The T3 concentration of the eggs just after fertilization (4-9 ng/g) was lower than the T4 concentration (5-15 ng/g). Both T4 and T3 concentrations in the whole body decreased steadily during yolk absorption, primarily due to the decline of the hormone content in the yolk. Both T4 and T3 were detected in blood plasma at later stages of yolk absorption, and the plasma levels increased toward the end of yolk absorption. At the end of yolk absorption, when the larvae emerge from the gravel bed, a transient increase in whole body concentrations of T4 and T3 was observed. Plasma levels of T4 were always greater than the T3 levels. Thyroid follicles began to develop during the early stages of yolk absorption. These findings suggest important roles of maternal thyroid hormones for developing salmon embryos during yolk absorption.

Changes in thyroid hormone levels in eggs and larvae and in iodide uptake by eggs of coho and chinook salmon,Oncorhynchus kisutsch andO. tschawytscha

Developmental profiles of thyroxin (T4), triiodothyronine (T3) and radioactive iodide uptake were established for eggs and T4 and T3 profiles were established for larvae (whole-body, yolk-only and body-only) of coho and chinook salmon. T4 and T3 were consistently present in all samples. In eggs, hormone levels remained fairly constant in all cohorst for at least the first three weeks of incubation, but then fluctuated in both directions in some sample groups. Large increases in T4 (from 9 ng/g to 245 ng/g) were seen in 1985 chinook eggs 28 days after fertilization. Radioactive iodide uptake (which was used as a possible indicator of thyroxinogenesis) increased at least 10-fold in both 1986 coho and chinook eggs from 23-30 days after fertilization. T4 (62 ng/g) and T3 (393 ng/g) were found in the bodies of 28-day-old 1986 chinook embryos. In whole larvae, hormone levels varied depending upon the cohort studied. In general, initial body-only concentrations of both T4 and T3 decreased as body weight increased, but before yolksac resorption was completed, both thyroid hormone content and concentration increased (except for chinook T3). T4 and T3 content in larval yolk stayed constant as yolksac size decreased, resulting in increased thyroid hormone concentration in the yolksac. All of these data suggest that the initial source of thyroid hormones in coho and chinook salmon eggs is maternal, but that by approximately 3-4 weeks after fertilization, the developing embryos begin to produce their own thyroid hormones. After hatching, increases in tissue T4 and T3 concentration coupled with constant T4 and T3 content in diminishing yolksacs suggest that larvae also produce their own thyroid hormones; yolksac content then may reflect both the original maternal hormones and the larva-producted hormones.

Presence of thyroxine in eggs and changes in its content during early development of chum salmon, Oncorhynchus keta

In order to examine the role of thyroid hormones during salmonid development, techniques were developed for quantitative extraction of thyroxine from eggs, whole embryos, and alevins of chum salmon (Oncorhynchus keta) at various stages of development. Frozen eggs, embryos, alevins, or fry were homogenized in ice-cold methanol. The homogenate was centrifuged, and the supernatant was washed with a mixture of chloroform and 0.05% CaCl2. The aqueous layer was lyophilized, and the residue was redissolved in barbital buffer for thyroxine radioimmunoassay (RIA). Serial dilutions of the egg or tissue extracts gave inhibition slopes that were parallel to that of the thyroxine standard in the RIA. Immunoreactivity of the extracts coeluted with thyroxine standard in reverse-phase HPLC on an ODS column. Recovery of thyroxine from egg and tissue extracts was estimated from the recovery of 125I-labeled thyroxine added to the initial homogenates. Thyroxine content of eggs just after fertilization was 4-5 ng/egg, and this level was maintained until hatching. A decrease in thyroxine content was seen during yolk absorption. Total thyroxine increased to about 10 ng/fish, a level higher than that in the unfertilized egg, at the time of complete yolk absorption, and decreased within 10 days to a low level of 1 ng/fish. These findings are discussed in relation to the role of maternal thyroid hormones during early development and also to the onset of larval thyroid function.