Coordinated expression and regulation of deiodinases and thyroid hormone receptors during metamorphosis in the Japanese flounder (<Emphasis Type="Italic">Paralichthys olivaceus</Emphasis>)

In vertebrates, thyroid hormone receptors (TRs) and deiodinases are essential for developmental events driven by the thyroid hormones (THs). However, the significance of deiodinases during the metamorphosis of the Japanese flounder (Paralichthys olivaceus) remains unclear. Moreover, regulation and response of the TRs and deiodinases to THs in this fish are poorly understood. Therefore, we detected the expression patterns of THs, deiodinases, and TRs in drug-treated larvae and untreated larvae of P. olivaceus by using enzyme-linked immunosorbent assay and quantitative real-time PCR during P. olivaceus metamorphosis. To further understand the roles of these elements, a rescue assay was performed. Our results show the importance of THs, TRs, and deiodinases in flatfish metamorphosis. Our results also confirm that D1 and D2 activate THs and D3 plays the opposite and complementary role. Moreover, we demonstrated that both TRα and TRβ have important but different roles during P. olivaceus metamorphosis.     

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

Serum thyroid hormone concentrations were measured during the seven stages of metamorphosis (1–7) of the southern hemisphere lamprey, Geotria australis. The respective mean concentrations ± SEM of serum thyroxine (T₄) and triiodothyronine (T₃) fell from 31.73 ± 4.09 and 5.06 ± 0.70 nM in large ammocoetes sampled in February, at the time when metamorphosis was initiated, to 4.54 ± 0.36 and 1.03 ± 0.12 nM at stage 5. Although there was a small, but significant, recovery of serum T₄ concentrations during stages 6 and 7, no such corresponding statistically significant rise occurred in serum T₃ concentrations. Serum thyroid hormone concentrations in ammocoetes sampled during the period when metamorphosis was taking place, exhibited a marked seasonal increase between February and May–June (late autumn/early winter); serum T₃ and T₄ concentrations peaked in May–June and were, respectively, > 2 fold and > 8 fold higher than those recorded for samples in late February (mid summer). By mid-July the serum T₄ and T₃ levels had declined from the peak values. Ammocoetes taken from streams at 16°C in June and acclimated to aquaria water at 25°C or 6°C had significantly lower serum T₃ and T₄ concentrations at the higher temperature, and also a lower serum T₄, but not T₃ concentration, at the lower temperature. Treatment of separate groups of ammocoetes with either propylthiouracil or T₃ for 70 days significantly depressed and raised respectively, the serum thyroid hormone and hepatic T₃ concentrations and caused significant changes in the body weight, but did not induce the onset of metamorphosis.     

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.     

Involvement of Gonadotropin-Releasing Hormone in Thyroxine Release in Three different forms of Teleost Fish: Barfin Founder, Masu Salmon and Goldfish

Effects of gonadotropin-releasing hormone (GnRH) on thyroxine (T₄) release in vivo and in vitro were studied in barfin flounder Verasper moseri, masu salmon Oncorhynchus masou and goldfish Carassius auratus. Seabream GnRH (sbGnRH) at a dose of 200 ng/50 g body weight (BW) significantly increased plasma T₄ levels 1 h after the in vivo injection in the barfin flounder, but thereafter the levels normalized. Salmon GnRH (sGnRH) significantly increased plasma T₄ levels l h after the injection with a significant return to initial levels in male masu salmon and male goldfish. In contrast, sGnRH and cGnRH-II in barfin flounder, and cGnRH-II in male masu salmon and male goldfish were not effective in stimulating T₄ release. To clarify direct involvement of GnRH in T₄ release, dissected lower jaw including scattered thyroid follicles was incubated with sbGnRH (1 μg/well) in barfin flounder, and with two doses (0.1 and 1 μg/well) of sGnRH in masu salmon and goldfish in vitro. T₄ concentrations of control were stable during 24 h. Incubation of lower jaw with high dose (1 μg/well) of GnRH significantly (P<0.05) increased T₄ concentrations of incubation medium at 1 h in all experimental fishes. These results indicate that direct stimulation of T₄ secretion by GnRH occurs widely in teleost fish.     

Effects of thyroxine and thiourea on the metamorphosis of coral trout grouper Plectropomus leopardus

The contribution of the thyroid hormone to the metamorphosis of the coral trout Plectropomus leopardus was examined. Juveniles, aged 35 days, were exposed to thyroxine (T4, 0.1 p.p.m) and thiourea (TU; inhibitor of thyroid hormone synthesis, 30 p.p.m) for 13 days. All the fish in the T4-treated group had completed metamorphosis (second metamorphosis, pigmentation and resorption of fin spines) within three days after the beginning of the experiment (day 3), whereas it took 13 days for this to occur in the control fish. Moreover, the fish in the TU group had not completed metamorphosis even by day 13. Settling behavior was significantly stimulated by T4 treatment, as was the resorption of the dorsal and pelvic fin spines, the reduction of serration on the spines, and the transition of body color into an opaque reddish hue. Fish in the TU group had longer dorsal and pelvic spines, retarded pigmentation and an abnormal black coloration. These results suggest that thyroid hormone accelerates most changes during the second metamorphosis in the coral trout.     

Thyroid hormones in eggs of various freshwater,marine and diadromous teleosts and their changes during egg development

Thyroid hormone concentrations in unfertilized eggs of 26 species of various freshwater, marine and diadromous teleosts were examined, together with changes in their concentrations during egg development in some species. Significant quantities of both thyroxine (T₄) and triiodothyronine (T₃) were found in eggs of all species examined. Mean T₄ and T₃ concentrations in eggs varied from 0.04 (marbled sole) to 15.00 ng/g (chum salmon), and from 0.07 (goldfish) to 9.95 ng/g (Pacific herring), respectively. T₄ concentrations were significantly greater than T₃ concentrations in eggs of most freshwater fishes, whereas T₃ concentrations were greater in seawater fishes. During the course of development, thyroid hormones in eggs decreased markedly before hatching. These findings suggest that thyroid hormones are consistently present in teleost eggs, and thus may play an important role in the egg development.     

Thyroid hormone modulation of ovarian recrudescence of air-breathing catfish Clarias gariepinus

In the present study, thiourea-induced thyroid hormone depletion and thyroxine (T₄) ‘overdose’ were used as a strategy to understand the influence of thyroid hormones on ovarian recrudescence of juvenile (3-months-old), immature (8-months-old) and adult (1-year-old) air-breathing catfish, Clarias gariepinus. Thiourea-induced thyroid hormone depletion in juvenile catfish impaired ovarian development, but no significant effect was observed in immature catfish and during late stage of ovarian recrudescence of mature catfish. T₄ treatment in females undergoing late stages of ovarian recrudescence induced rapid oocyte growth by promoting its early entry into maturational phase as evident from the presence of more number of vitellogenic and post-vitellogenic follicles, decrease in aromatse immunoreactivity and reduced estradiol–17β levels. Hence, thyroid hormones have an important role to play during early stages of ovarian development and vitellogenesis of catfish and also indicating that thyroid has a stage dependent effect on ovary.     

Effect of triiodothyronine (T3) in the larval development of the barber goby Elacatinus figaro

The barber goby Elacatinus figaro is an endangered fish, endemic to Brazil and very important to the aquarium trade. One of the bottlenecks for its production in captivity is the larviculture, which is characterized by high mortality rates, especially in the first week after hatching, and prior to metamorphosis. The experiment evaluated the effect of triiodothyronine hormone (T₃) on survival, growth and metamorphosis of barber goby. Larvae of 14 days after hatching (DAH) were immersed in three concentrations of T₃, in triplicate: TC (control) – 0; T01 – 0.01; T025 – 0.025 and T05 – 0.05 mg/L. An additional replicate of each treatment was performed for sampling for the histological analysis of the thyroid follicles in 24 DAH larvae. The survival rates in TC, T01, T025 and T05 were 24%, 54%, 36% and 37% respectively, without significant differences between the treatments (p > 0.05). Regarding larval growth, the highest length values were obtained in TC. In T025 and T05, larval metamorphosis was anticipated in up to 11 days in relation to TC and T01. In T01 larvae, follicles were numerous with the presence of reabsorption vesicles in the colloid periphery, indicating increased production of thyroid hormones (THs), associated with the larval metamorphosis process. In T05, a follicle reduction in number and size was observed, given evidence of the end of metamorphosis. The use of 0.025 and 0.05 mg/L of T₃ anticipated metamorphosis in barber goby larvae, shortening the larviculture period for this species.     

Dietary effects on thyroid hormones in the red drum,Sciaenops ocellatus

Juvenile red drum (Sciaenops ocellatus) were cultured at 25°C on a variety of diets and blood sampled over eight weeks to examine the relationship between growth and plasma thyroid hormone levels. Maximum growth rates were achieved on formulated experimental diets and a simulated natural shrimp diet. Associated with these maximal rates was a significant increase in triidothyronine (T₃), but no consistent change in thyroxine (T₄). Reduced rations of diets resulted in low growth rates associated with significantly lowered levels of T₃ but not T₄. To determine whether weight gain could be increased by application of exogeneous hormone, diets were supplemented with T₃ or T₄ at 2, 10, and 50 mg hormone/kg diet. Significantly elevated T₃ was induced by supplementation with 10 and 50 mg T₃/kg diet, although there were no indications of an anabolic effect of T₃ incorporation, and 50 mg T₃/kg diet was in fact associated with decreased weight gain. Incorporation of T₄ into diets had no effect on growth or T₃, and had effects on T₄ which were small and inconsistent, indicating that T₄ may not be effectively absorbed from the gut. No difference was found in response to hormone feeding between low (6 ppt) or high (35 ppt) water salinity. T₃ levels thus appear to closely parallel growth in fish on unsupplemented diets, whereas T₄ which were small and manipulation. Supplementation with T₃ is not an effective means of stimulating growth in red drum fed optimum diets. Whereas thyroid hormones may function to regulate intermediary metabolism in red drum, elevated endogenous thyroid hormone levels appear adequate to supply tissue needs during juvenile growth in culture.     

Thyroid hormone deficiency in abnormal larvae of the Japanese flounder <Emphasis Type="Italic">Paralichthys olivaceus</Emphasis>

ABSTRACT:   In large-scale rearing of juveniles of the Japanese flounder Paralichthys olivaceus, a certain morphological abnormality occurred spontaneously in 4% of the fish. These fish showed a slight but clearly different appearance from any developmental stage of this species, and did not settle when all the other juveniles in the same tank completed metamorphosis and had settled. From comparisons of external and internal structures between the normal and the abnormal fish, the abnormality was attributed to unbalanced progress of metamorphosis, mainly due to metamorphic stasis. The thyroid of the abnormal fish was apparently activated morphologically. In addition, serum thyroxine (T4) concentrations in the abnormal fish were reduced to less than 1/10 of that of normal fish. After 14 days of T4 treatment (0.1 p.p.m) of the abnormal fish, all the abnormal characteristics disappeared, and the fish recovered to normal, suggesting normal responsiveness to thyroid hormones in peripheral tissues, whereas thiourea treatment (30 p.p.m., 14 days) further delayed metamorphosis. These results suggest that these abnormal fish were suffering from thyroid hormone deficiency, and were unable to secrete a sufficient amount of thyroid hormone to complete metamorphosis.     

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.     

Development of thyroid gland and changes in thyroid hormone levels in Leptocephali of Japanese Eel (Anguilla japonica)

Development of the thyroid gland of the Japanese eel (Anguilla japonica) was studied with the use of tank-reared fish. A single thyroid follicle was first found in larvae at 29 days post hatching (dph), total length (TL) 12 mm. Until reaching 25 mm in TL (100 dph), leptocephali had one or two follicles per individual. The inner colloid of the follicles was weakly immuno-positive against the anti-thyroxine (T₄) antibody. The number of thyroid follicles and the immunoreactivity later increased as the larvae grew. Thyroid hormones (TH) T₄ and triiodothyronine (T₃) were not detected in premetamorphic larvae by radioimmunoassays, but became detectable during metamorphosis. The maximum level of T₄ was seen in fish at the end of metamorphosis and in just-metamorphosed juveniles, whereas T₃ reached the highest level during metamorphosis and declined toward the end of metamorphosis. The results indicated that the thyroid gland first became active during metamorphosis in the development of eel.     

Seasonal changes in channel catfish thyroid hormones reflect increased magnitude of daily thyroid hormone cycles

Channel catfish (Ictalurus punctatus) in pond culture, sampled once per day, have been reported to exhibit significant seasonal cycles in the thyroid hormones thyroxine (T₄) and 3,5,3′-triiodothyronine (T₃), rising from levels generally below 2 ng/ml in January to above 8 ng/ml in July. To determine if daily thyroid hormone cycles underlie these seasonal changes, we blood sampled groups of 20 catfish (10 males and 10 females) in the morning (approx. 1 h after sunrise), midday, and evening (approx. 1.5 h before sunset) on January 9, April 4, and July 29. From January to July, pond temperatures rose from 7 ° to 32 °, associated with significant (p < 0.05) increases in mean fish weight (from 477 to 1052 g) and in monthly mean food consumption (from 34 to 474.7 g/kg fish). On all three dates, significantly (p < 0.05) greater levels of both hormones (except T₃ in April) were found in midday and evening compared to morning samples. In January, the daily change was small (from morning to midday, mean T₃ rose from 2.2 to 3.6 ng/ml and mean T₄ from 2.3 to 4.8 ng/ml), whereas in July it was considerably greater (from morning to evening, mean T₃ rose from 7.2 to 17.8 ng/ml, and T₄ from 9.0 to 22.4 ng/ml). No significant differences were found between midday and evening levels, or between males and females. Additionally, no seasonal phase-shifting of cycles was apparent. A subset of animals was examined to evaluate the potential contribution of peripheral mechanisms in generating these seasonal and daily cycles. Whereas we observed only minor changes in thyroid hormone binding to plasma proteins during any single day, a significant seasonal increase in the ratio of free T₄:free T₃ indices (from a mean of 1.3–1.5 in January to 2.0–2.1 in July) indicated enhanced T₃ binding by plasma proteins in July. Furthermore, in vitro hepatic T₄ and T₃ deiodination activities showed across dates no significant change in T₄ outer-ring deiodination to produce T₃ (ranging from a mean of 53.1 to 70.1 pmol T₄ deiodinated/h/mg microsomal protein), but a significant (p < 0.05) decrease in T₄ inner-ring deiodination to degrade T₄ to 3,3′5′-triiodothyronine (from a mean in January of 2.4 to 0.65 pmol T₄ deiodinated /h/mg protein in April) and a significant (p < 0.05) decrease in T₃ inner-ring deiodination to degrade T₃ to 3,3′-diiodothyronine (from a mean in January of 115.5 to 3.1 pmol T₄ deiodinated/h/mg protein in July). These results demonstrate that channel catfish under conditions of natural temperature and photoperiod exhibit robust daily cycles in total plasma T₄ and T₃ similar in magnitude to those reported for other fish species held under controlled laboratory conditions. These cycles maintain a similar phase throughout the year, indicating that apparent seasonal increases in thyroid hormones are not due to phase-shifting of daily cycles. However, seasonal studies sampling fish only in the morning would underestimate the magnitude of the annual changes in blood thyroid hormones. Thyroidal status, as judged from total plasma T₄ and T₃ levels in the afternoon, is greatest in July, coinciding with the postspawning peak in food consumption and growth. Enhanced T₃ plasma protein binding and a shift from predominantly hepatic inner-ring deiodination in winter to outer-ring deiodination in summer suggest that peripheral mechanisms contribute to the generation of these seasonal changes.     

Effects of thyroid hormone on gonadotropin-induced steroid production in medaka,Oryzias latipes,ovarian follicles

Blood and ovarian samples were collected at intervals of 4h prior to spawning time from medaka (Oryzias latipes) that were maturationally synchronized with artificial photoperiod (14h light: 10h dark). Plasma estradiol-17β (E₂) levels increased rapidly from 16h before spawning and peaked at 8h before spawning. Follicle-enclosed oocytes (ovarian follicles) at different stages of development were isolated from the ovaries and used to study the in vitro effects of thyroid hormone (triiodothyronine; T₃) on pregnant mare serum gonadotropin (GTH)-induced E₂ production. GTH at a concentration of 100 IU/ml stimulated E₂ production by ovarian follicles collected between 32 and 16h before spawning. At 32h before spawning, T₃ (5 ng/ml) administered along with GTH (100 IU/ml) resulted in a 3.5 fold increase in E₂ production, compared with GTH administered alone. These results suggest that T₃ can act on ovarian follicles directly to modulate GTH-stimulated E₂ production in the medaka.     

Effect of excess iodide on thyroid function of rainbow trout,Salmo gairdneri

The acute and chronic effects of excess iodide (KI or NaI) were studied on thyroid function of rainbow trout at 11±1°C. No Wolff-Chaikoff effect, characteristic of mammals, was observed and instead plasma L-thyroxine (T₄) levels increased 6 hr after a single iodide injection. Plasma 3,5,3′-triiodo-L-thyronine (T₃) did not change and by 24 hr plasma T₄ returned to normal. This iodide-induced elevation in plasma T₄ was probably not due to toxic effects demonstrated at higher NaI or KI doses. A single iodide injection also decreased the plasma iodide distribution space, decreased the fractional rate of plasma iodide loss and completely blocked thyroidal uptake of radioiodide. Injections of iodide over a 22-day period elevated plasma iodide 200X with no mortality and no influence on plasma T₄ or T₃. It is concluded that: (i) apart from the transient 6h increase in plasma T₄, trout thyroid function, as judged by plasma hormone levels, is insensitive to considerable iodide excess, (ii) non-invasive iodide suppression of thyroidal radioiodide recycling may be useful in kinetic studies of¹²⁵I-labeled thyroid hormones, and (iii) fundamental differences in intrathyroidal iodine metabolism appear to exist between mammals and fish.     

Effect of dietary vitamin A on Senegalese sole (Solea senegalensis) skeletogenesis and larval quality

The effects of different levels of vitamin A (VA) in Senegalese sole larval performance and development were evaluated by means of a dietary dose–response experiment using enriched Artemia metanauplii as a carrier of this micronutrient. Larvae were fed from 6 to 27 days post hatch (dph) with enriched Artemia containing graded levels of total VA (1.3, 2.1, 4.5 and 12.9 µg VA mg^− 1 DW). The content of VA in live prey directly affected its accumulation in larvae and early juveniles. Retinyl palmitate accumulated during larval ontogeny, whereas retinol showed the opposite trend, decreasing from hatching until 41 dph and then remaining constant until the end of the study.In metamorphic larvae (10 and 15 dph), VA did not affect the number of thyroid follicles or the intensity of the immunoreactive staining of T₃ and T₄. However, at older stages of development (post-metamorphic larvae: 20, 30, 41 and 48 dph), VA decreased the number of thyroid follicles but increased their mean size and enhanced T₃ and T₄ immunoreactive staining. A dietary excess of VA did not affect either larval performance in terms of growth and survival or the maturation of the digestive system. However, the most remarkable impact of this morphogenetic nutrient was detected during skeletal morphogenesis. Dietary VA accelerated the intramembranous ossification of vertebral centrums, which led to the formation of a supranumerary haemal vertebra and a high incidence of fused and compressed vertebrae in fish fed 2.1, 4.5 and 12.9 mg VA mg^− 1 DW. In addition, VA also affected those structures from vertebrae and caudal fin formed by chondral ossification, leading to defects in their shape and fusions with adjacent skeletal elements. In particular, the caudal fin was the region most affected by the dietary treatments. In order of importance, the bones with more developmental anomalies were the modified neural and haemal spines, epural, hypurals and parahypural. The impact of systemic factors such as thyroidal hormones in skeletogenesis should not be neglected since present results revealed that an excess of dietary VA affected the levels of T₃ and T₄, which might have affected bone formation and remodelling, leading to skeletal deformities.     

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.     

Does the time of feeding affect the diurnal rhythms of plasma hormone and glucose concentration and hepatic glycogen content of rainbow trout?

The diurnal rhythms of plasma glucose, cortisol, growth hormone (GH) and thyroid hormone (T₄, T₃) concentrations and hepatic glycogen content were measured in rainbow trout that had been entrained to a specific time of daily feeding (post-dawn, midday, pre-dusk); the purpose of the study was to investigate the significance of feeding time on hormones and metabolite patterns. Plasma GH, cortisol and T₄ concentrations all showed evidence of a diurnal rhythm in some treatment groups. There was a significant interaction between the time of feeding and plasma GH and cortisol concentration rhythms; for GH, this appeared to be related to the phase-shifting of the post-prandial increases in plasma GH concentrations, and for cortisol, the rhythms were only evident in fish fed in the post-dawn period [diurnal rhythms were not evident in treatment groups fed in at midday or pre-dusk]. Peak plasma T₄ concentrations were evident during the photophase in all three treatment groups; however, the time of feeding had a negligible effect on the timing of those peaks. There were no apparent diurnal rhythms of plasma T₃ and glucose concentrations, hepatic glycogen content or hepatosomatic index in any of the three treatment groups. To whom correspondence should be addressed     

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.     

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.