The Role of the Thyroid Axis in Fish

In all vertebrates, the thyroid axis is an endocrine feedback system that affects growth, differentiation, and reproduction, by sensing and translating central and peripheral signals to maintain homeostasis and a proper thyroidal set-point. Fish, the most diverse group of vertebrates, rely on this system for somatic growth, metamorphosis, reproductive events, and the ability to tolerate changing environments. The vast majority of the research on the thyroid axis pertains to mammals, in particular rodents, and although some progress has been made to understand the role of this endocrine axis in non-mammalian vertebrates, including amphibians and teleost fish, major gaps in our knowledge remain regarding other groups, such as elasmobranchs and cyclostomes. In this review, we discuss the roles of the thyroid axis in fish and its contributions to growth and development, metamorphosis, reproduction, osmoregulation, as well as feeding and nutrient metabolism. We also discuss how thyroid hormones have been/can be used in aquaculture, and potential threats to the thyroid system in this regard.

The relationship between ingested thyroid hormones, thyroid homeostasis and iodine metabolism in humans and teleost fish

Oral l-thyroxine (T4) therapy is used to treat human hypothyroidism but T4 fed to teleost fish does not raise plasma thyroid hormone (TH) levels nor induce growth, even though oral 3,5,3'-triiodo-l-thyronine (T3) is effective. This suggests a major difference in TH metabolism between teleosts and humans, often used as a starting thyroid model for lower vertebrates. To gain further insight on the proximate (mechanistic) and ultimate (survival value) factors underlying this difference, the several steps in TH homeostasis from intestinal TH uptake to hypothalamic-hypophyseal regulation were compared between humans and teleosts, and following dietary TH challenges. A major proximate factor limiting trout T4 uptake is a potent constitutive thiol-inhibited intestinal complete T4 deiodination that is ineffective for T3. At the hepatic level, T4 deiodination, conjugation and extensive biliary excretion with negligible T4 enterohepatic recycling can further block teleost T4 uptake to plasma. Such protection of plasma T4 from dietary T4 may be particularly critical for piscivorous fish consuming thyroid tissue, rich in T4 but not T3. It would prevent disruption by unregulated ingested T4 of the characteristic acute and transient changes in teleost plasma T4 due to diel rhythms, food intake and stress-related factors. These marked natural short-term fluctuations in teleost plasma T4 levels are enabled by the relatively small and rapidly-cleared plasma T4 pool, stemming largely from properties of the plasma T4-binding proteins. Humans, however, due mainly to plasma T4-binding globulin, have a relatively massive circulating pool of T4 and an extremely well-buffered free T4 level, consistent with the major TH role in regulating basal metabolic rate. Furthermore, this large well-buffered and slowly-cleared plasma T4 pool, in conjuction with enterohepatic recycling and relaxation of hypothalamic-hypophyseal negative feedback, allows humans to temporarily 'store' ingested T4 in plasma, thereby sparing endogenous TH secretion and conserving thyroidal iodine reserves. Indeed, iodine conservation is likely the key ultimate factor determining the divergent evolution of the human and teleost systems. For humans, ingested iodine in the form of I^-, or TH and their derivatives, is the sole iodine source and may be limiting in many environments. However, most freshwater teleosts, in addition to their ability to assimilate dietary I^-, can derive sufficient I^- from their copious gill irrigation, with no selective advantage in absorbing dietary T4 which would disrupt their natural acute and transient changes in plasma T4. Thus T4 may act also as a vitamin (vitamone) in humans but not in teleosts; in contrast, T3, naturally ingested at much lower levels, may act as a vitamone in both humans and teleosts.

RNA-seq and microRNA-seq analysis of Japanese flounder (Paralichthys olivaceus) larvae treated by thyroid hormones

Flatfish pigmentation is a complex process, affected by environmental factors including light, nutrients, and hormones. Of those, the thyroid hormone has been reported to increase the albinism rate of Japanese flounder (Paralichthys olivaceus). However, the underlying mechanism remains unclear. In the present study, triiodothyronine (T3), thyroxine, and thiourea were introduced into P. olivaceus larvae from 16 to 57 days after hatching (DAH). By comparison of albinism rate, T3 treatment and control larvae of 42 DAH were chosen for mRNA and miRNA high-throughput sequencing analyses. A total of 337 miRNAs were identified via miRNA-seq, and 12 miRNAs exhibited significantly differential expression patterns in D42_T3 versus D42_Con (TPM > 10, fold change ≥ 1.5 or ≤ 0.67 and q ≤ 0.05). These differentially expressed miRNAs targeted 3658 putative genes, which further enriched to 10 GO terms (q < 0.05). RNA-seq identified 146 differentially expressed genes (DEGs) in D42_T3 versus D42_Con (|log₂ fold change| > 1 and q < 0.005), including pigmentation-related genes such as the receptor tyrosine-protein kinase erbB-3, pro-opiomelanocortin A, and melanotransferrin, and the growth-related gene somatotropin. These DEGs were significantly enriched to 15 GO terms and 8 KEGG pathways (q < 0.05), which included several sugar metabolic pathways (glycolysis/gluconeogenesis and the pentose phosphate pathway). Integrated analysis revealed that 26 overlapping genes between DEGs and mRNAs were targeted by miRNAs. Furthermore, seven mRNA-miRNA pairs exhibited reversed regulation patterns. This provides important clues to understand the role of thyroid hormones in flatfish pigmentation.

The organophosphate pesticide -OP- malathion inducing thyroidal disruptions and failures in the metamorphosis of the Senegalese sole, Solea senegalensis

Background

Organophosphate pesticides-OP-, like malathion, can alter the normal functioning of neuro-endocrine systems (e.g., hypothalamus-pituitary-thyroid-HPT- axis), and to interfere on the thyroidal homeostasis. Through direct interactions with thyroid receptors, an/or indirectly via up-stream signalling pathways, from the HPT axis (i.e., negative feedback regulation), malathion possess the ability to affect integrity of thyroidal follicular tissue, and it can also block or delay its hormonal functioning. This insecticide can alter the majority of the ontogenetic processes, inducing several deformities, and also provoking decreases in the growth and survival patterns. The present study has been performed to determine the sublethal effects of malathion during the first month of life of the Senegalese sole, Solea senegalensis, and it is mainly focused on the metamorphosis phase. Different transcript expression levels (i.e. thyroid receptors, matrix and bone -Gla-proteins) and immunohistochemical patterns (i.e. thyroid hormones, osteocalcin, cell proliferation) have been analysed during the most critical phases of the flatfish metamorphosis, that is, through differentiation of thyroid system and skeletal development, migration of the eye, and further adaptation to benthic behaviours.

Results

In early life stages of the Senegalese sole, the exposure to the highest concentration of malathion (6.25 μg/L) affected to the growth patterns, showing the exposed individuals, a reduction around 60 and 92% of the total length and the dry weigth, respectively. In paralell, a significant reduction of the thyroid follicles (i.e., size and number) it was also been recorded, in a dose-dependent way. Abnormal phenotypes induced in the exposed larvae, did not complete the process of metamorphosis, and displayed several morphological abnormalities and developmental disorders, which were mainly associated with the eye migration process, and with thyroidal and skeletal disorders (i.e., transcriptional and protein changes of thyroid hormones and receptors, and of matrix and bone Gla proteins distribution), that conduced to an inadequate adaptation to the benthic life.

Conclusions

In the Senegalese sole, the majority of the ontogenetic alterations induced by the exposure to malathion were mainly associated to the metamorphosis period, which is a thyroid-driven proccess. In fact, most crucial and transitional ontogenic events, appeared notably disturbed, for e.g., thyroid gland differentiation and functioning, migration of eye, skeletal development and benthonic behaviors.

Malathion exposure induces the endocrine disruption and growth retardation in the catfish, Clarias batrachus (Linn.)

Many hormones are known for their role in the regulation of metabolic activities and somatic growth in fishes. The present study deals with the effects of malathion (an organophosphorous pesticide) on the levels of metabolic hormones that are responsible for promotion of somatic and ovarian growth of the freshwater catfish, Clarias batrachus. Malathion treatment for thirty days drastically reduced the food intake and body weight of fish. These fish also exhibited a great avoidance to food. Exposure of catfish to malathion reduced the levels of thyroxine (T(4)), triiodothyronine (T(3)), growth hormone (GH), insulin like growth factor-I (IGF-I), testosterone (T) and estradiol-17β (E(2)) in a dose dependent manner during all the studied reproductive phases, in general, except that malathion increased the level of GH during the quiescence phase. Significant reduction in muscle and hepatic protein content also occurred in the malathion-treated fish. Malathion exposure induced lipolysis too in the liver and muscle. The results thus support that malathion treatment disrupts the endocrine functions and the olfactory sensation responsible for food intake and gustatory feeding behavior, which ultimately leads to retardation of fish growth.

Thyroid function in growth-hormone-transgenic coho salmon (Oncorhynchus kisutch)

We measured growth rate, plasma thyroxine (T4) and 3,5,3'-triiodothyronine (T3) concentrations, and liver and whole-brain T4 and T3 deiodination activities in yearling non-transgenic coho salmon, Oncorhynchus kisutch (Walbaum, 1792), fed a satiation ration (NTS) and in growth-hormone (GH)-transgenic salmon fed for 63 d with either a satiation ration (TS) or pair-fed the satiation ration consumed by NTS fish (TNT). Daily feed intake and specific growth rate for TS fish were significantly enhanced and approximately double those for TNT and NTS fish. There were no differences among groups in plasma T4 concentration or liver T4 outer-ring deiodination activity, but for both TS and TNT fish, plasma T3 concentrations were higher and liver T4 and T3 inner-ring deiodination activities were lower than for NTS fish. Whole-brain deiodination activities did not differ between TS and NTS fish. We conclude that the elevated plasma T3 concentrations of GH-transgenic salmon neither are driven by elevated plasma T4 concentrations nor are they the result of increased hepatic conversion of T4 to T3 by outer-ring deiodination. Instead they can be explained, at least in part, by reduced hepatic degradation of T3 to 3,3'-diiodothyronine by inner-ring deiodination. These changes in T4 and T3 metabolism are tightly linked to the GH-transgenic state and not to food intake or growth rate.

Differential effects of thyroxine on metabolic enzymes and other macromolecules in a freshwater teleost

The effects of thyroxine (T(4)) on citrate synthase (CS), glucose 6-phosphate dehydrogenase (G6-PDH), lactate dehydrogenase (LDH), DNA, RNA, and protein of various tissues were studied to elucidate the hormonal control of metabolism in a freshwater catfish, Clarias batrachus. T(4) did not produce any significant effect on DNA content of the fish. The CS, RNA, and protein contents of brain, liver, and skeletal muscle of the fish exposed to thiourea for 28 days decreased approximately 50-58% as compared to their levels in control individuals. Injection of T(4) to thiourea-exposed fish produced about three-fold increases in CS, RNA, and protein. These macromolecular inductions by T(4) were blocked by actinomycin D or cycloheximide. This suggests T(4)-induced de novo synthesis of macromolecules and stimulation of aerobic capacity. However, the activities of G6-PDH and LDH of brain, liver, and skeletal muscle of the fish exposed to thiourea increased two times that of the activities in control individuals. Administration of T(4) to thiourea-exposed fish reduced LDH and G6-PDH activities by about 64-74%, which reflects T(4)-dependent inhibition in anaerobic power and selective anabolic activities of the HMP pathway. These differential effects of T(4) on some metabolic enzymes and other important macromolecules may be to meet the other T(4)-induced responses in the freshwater catfish.

Response of rainbow trout gill Na+-ATPpase to T(3) and NaCl administration

The effect of the administration of commercial diets supplemented with 9 mg kg(-1) 3,5,3'-triiodo-l-thyronine (T(3)) or 10% (w/w) NaCl was evaluated on the ouabain-insensitive Na+-ATPase activity in rainbow trout gill microsomes. The trial, carried out following the seasonal trend from March to mid-May, included a treatment phase in freshwater and a subsequent transfer to brackish water (22 per thousand salinity) where trout were not treated. pH dependence, apparent Km values for Mg(2+) and Na+, and Hill coefficients evaluated throughout the trial for Na+-ATPase were generally not affected by the treatments and habitat change. In comparison with the control group, in both treated groups, Na+-ATPase activity was lower during the freshwater phase and higher after brackish-water transfer. As compared with untreated trout, gill (Na++K+)-ATPase activity during the freshwater phase was stimulated by NaCl treatment and also by T(3) treatment after transfer to brackish water. The results indicate that NaCl and T(3) administration act differently on the two ATPase activities involved in Na+ regulation and suggest a prevalent role of Na+-ATPase activity in hypoosmotic conditions.

The effects of photoperiod on growth rate and circulating thyroid hormone levels in the red drum, Sciaenops ocellatus: evidence for a free-running circadian rhythm of T(4) secretion

Previous studies with red drum and other species have indicated that diurnal rhythms of circulating thyroid hormones (thyroxine, T(4), and 3-5-3'-triiodo-L-thyronine, T(3)) are synchronized to the light cycle, and not to time of feeding. In this study we set out to address the effects of various lighting regimes on thyroid hormone levels in the red drum. The first experiment was undertaken to determine the effects of long and short photoperiods on diurnal thyroid hormone rhythms, growth rate, feed efficiency and food consumption. Red drum raised under a long photoperiod (16L:8D) grew significantly larger and exhibited greater feed efficiency than their short photoperiod (8L:16D) counterparts. There were no changes in food consumption or the diurnal profile of plasma thyroid hormones, e.g. increased peak amplitude or duration, that would explain this increase in growth rate and feed efficiency. The second experiment was undertaken to determine if diurnal thyroid hormone rhythms in the red drum originate from an endogenous circadian clock. To address this question, red drum were housed under a 12L:12D photoperiod and fed once daily at variable times before the lighting was switched to constant dim illumination for up to 3 days. The rhythm of circulating T(4) levels persisted for two complete cycles with constant amplitude in fish that continued to be fed during constant dim illumination, and did not appear to entrain to feeding. The T(4) rhythm also persisted for three complete cycles under constant conditions in feed-restricted fish, although with a diminished amplitude over time. To our knowledge, these data provide the first evidence for a free-running circadian rhythm of plasma T(4) levels in a fish. These findings implicate the involvement of an endogenous circadian clock that determines when the hypothalamo-pituitary-thyroid axis is activated.

Hormonal status and phagocytic activity in sea bream infected with vibriosis

Serum taken from female, sexually mature, silver sea bream (Sparus sarba) displaying either no symptoms of vibriosis, mild infection, severe infection or moribundity were assayed for a number of key hormones. Serum cortisol levels were not significantly different among symptomless, mildly- and severely-infected groups, whereas moribund fish displayed hypercortisolemia with a 14-fold increase in serum cortisol in comparison to symptomless fish. Serum estradiol levels were significantly reduced 19-fold in mildly-infected fish and remained at a low level as infection progressed, whereas serum testosterone increased gradually during vibriosis with a 1.8-fold increase in moribund groups in comparison to symptomless groups. Both serum thyroxine (T(4)) and triiodothyronine (T(3)) gradually decreased during vibriosis being 26- and 2.8-fold lower, respectively, in moribund fish in comparison to symptomless fish. The non-specific immune response, as determined by phagocytic activity, was also assessed using macrophages isolated from the pronephros and spleen of infected fish. Phagocytic indices significantly increased in mildly- and severely-infected fish and then decreased from these stimulated levels in moribund fish.

Salmon thyroid-stimulating hormone: isolation, characterization, and development of a radioimmunoassay

Coho salmon (Oncorhynchus kisutch) thyroid-stimulating hormone (TSH) was isolated by ethanol extraction of pituitary glands from mature coho salmon. Extraction was followed by gel-filtration chromatography on Sephadex G-100 superfine, anion-exchange chromatography on a Whatman DE-52 column, and finally by reverse-phase high-performance liquid chromatography. Fractions were monitored for TSH content by a homologous in vivo bioassay and by immunoblots using anti-human TSH beta-subunit antisera. In vivo treatment of coho salmon parr with coho salmon TSH caused a dose-dependent increase in plasma thyroxine level similar to that induced by bovine TSH. The N-terminal sequence (25 residues) of the salmon TSH beta subunit has 56% sequence identity to that of human TSH beta subunit and is identical to the deduced amino acid sequence of trout TSH beta subunit. The N-terminal sequence (25 residues) of the salmon TSH alpha subunit is identical to gonadotropin alpha-II subunit. Molecular sizes of the alpha and beta subunits are 18,000 and 24,000 daltons, respectively, as estimated by SDS-PAGE. Antiserum generated against salmon TSH, which was preadsorbed with alpha subunit using an alpha-subunit affinity column, detected only salmon TSH beta subunit by immunoblot and specifically stained thyrotropin-producing cells of the pituitary gland. A homologous radioimmunoassay (RIA) was developed using purified salmon TSH standard, iodinated TSH beta subunit, and antiserum generated against salmon TSH. Cross-reactivities of GTH I, GTH II, GTH I beta and GTH II beta subunits, alpha subunit, growth hormone, prolactin, and somatolactin were less than 1%. Displacement curves for serial dilutions of plasma and pituitary extracts of various salmonid species, as well as coho salmon pituitary cell culture medium, were parallel to the coho salmon TSH standards. In contrast, plasma of hypophysectomized juvenile coho salmon and pituitary extracts of Pacific tomcod (Microgadus proximus) did not displace bound radiolabeled salmon TSH. Finally, in vivo injection of juvenile coho salmon with triiodothyronine decreased plasma TSH levels, whereas the goitrogen, methimazole, increased plasma TSH levels. Injection of gonadotropin-releasing hormone agonist did not alter plasma TSH. These data suggest that the RIA is specific for TSH and confirm a negative-feedback relationship between the thyroid hormones and TSH.

Quantification of salmon alpha- and thyrotropin (TSH) beta-subunit messenger RNA by an RNase protection assay: regulation by thyroid hormones

In order to study salmon thyroid-stimulating hormone (TSH), we designed a highly specific, sensitive, and rapid RNase protection assay (RPA) for quantification of steady-state levels of salmon TSH beta-subunit mRNA expression. The cDNA encoding the beta-subunit of TSH was isolated from coho salmon pituitary total RNA by reverse-transcriptase PCR, partially sequenced, and used as template for synthesizing a radioactively labeled, sequence-specific, antisense probe, and sense standard for the RPA. This assay, along with a similar RPA previously designed for coho salmon total alpha-subunit mRNA, was used to examine the effects of feeding T3 (0, 10, 100 micrograms/g) and methimazole (a thyroid inhibitor) (2.5 mg/g) on TSH subunit gene expression after 2 and 4 weeks. The low dose of T3 (10 micrograms/g) caused no change in TSH beta mRNA after 2 and 4 weeks and a transient increase in alpha mRNA after 2 weeks, followed by no significant effect after 4 weeks. The high dose of T3 (100 micrograms/g) caused a decrease in TSH beta mRNA after 4 weeks and no change in total alpha mRNA after 2 and 4 weeks. In contrast, methimazole treatment caused significant increases in both TSH beta mRNA (250%) and alpha mRNA (50%) levels after 4 weeks. These findings confirm that, as in mammals, TSH alpha- and beta-subunit expression in teleosts may be differentially regulated by negative feedback from the thyroid hormones.

Response of rainbow trout gill (Na(+)+K (+))-ATPase and chloride cells to T 3 and NaCl administration

With the aim of comparing the effects of oral T3 and NaCl administration on trout hypoosmoregulatory mechanisms, three groups of rainbow trout (Oncorhynchus mykiss Walbaum) held in freshwater (FW) were fed a basal diet (C), the same diet containing 8.83 ppm of 3,5,3'-triiodo-L-thyronine (T3) (T) or 10% (w/w) NaCl (N) respectively for 30 d. They were then transferred to brackish water (BW) for 22 d and fed on diet C. Gill (Na(+)+K(+))-ATPase activity and its dependence on ATP, Na(+) and pH, number of gill chloride cells (CC), serum T3 level as well as fish growth, condition factor (K) and mortality were evaluated. During the FW phase, as compared to C trout, T trout showed a two fold higher serum T3 level, had unchanged gill (Na(+)+K(+))-ATPase activity and increased CC number, whereas N trout showed higher gill (Na(+)+K(+))-ATPase activity and CC number. At the end of the experiment the enzyme activity was in the order T>N>C groups and all groups showed similar CC number. Both treatments changed the enzyme activation kinetics by ATP and Na(+). A transient increase in K value occurred in N group during the period of salt administration. In BW, T and N groups had higher and lower survival than C group respectively. Other parameters were unaffected by the treatments. This trial suggests that T3 administration promotes the development of hypoosmoregulatory mechanisms of trout but it leaves the (Na(+)+K(+))-ATPase activity unaltered till the transfer to a hyperosmotic environment.

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.

Comparison of 3,5,3'-triiodo-L-thyronine and L-thyroxine absorption from the intestinal lumen of the fasted rainbow trout, Oncorhynchus mykiss

The absorptions of 3,5,3'-triiodo-L-thyronine (T3) and L-thyroxine (T4) from the intestinal lumen of the rainbow trout were compared in vivo. Tracer doses of [(125)I]T4 ((+)T4) or [(125)I]T3 ((*)T3) were injected through an anal cannula into the duodenum of trout fasted for 3 days at 12°C, and radioactivity was measured in blood and tissues at 4-48 h. (*)T3 was removed more extensively than (*)T4 from the intestinal lumen and more radioactivity was absorbed into the blood and tissues of u+T3-injected trout than (*)T4-injected trout. HPLC analysis showed that a high proportion of the radioactivity in the plasma, liver, kidney and intestinal lumen of (*)T3-injected trout remained as the parent (*)T3. However, in (*)T4-injected trout most plasma radioactivity was in the form of (125)I(-), and by 24 h a high proportion of luminal radioactivity was (125)I(-). By 48 h, over 4% of the injected (*)T3 and 1% of the injected (*)T4 dose resided in the gall bladder, primarily as derivatives of (*)T3 or (*)T4. We conclude that T3 is absorbed more effectively than T4 from the intestinal lumen of fasted trout, indicating the potential for an enterohepatic T3 cycle.

Interactive effects of high stocking density and triiodothyronine-administration on aspects of the in vivo intermediary metabolism and in vitro hepatic response to catecholamine and pancreatic hormone stimulation in brook charr, Salvelinus fontinalis

Brook charr (Salvelinus fontinalis) were maintained at one of two stocking densities (SD) (30 or 120 kg/m3) and fed either a control or a T3-supplemented (20 mg/kg) diet for 30 days in order to investigate possible interactive effects of SD and T3-administration on growth, feeding rate, food conversion efficiency, and hepatic and dark muscle enzyme activity. In addition, liver slices were incubated in vitro for 6 h with epinephrine, norepinephrine, isoproterenol, propranolol, insulin, glucagon, or somatostatin to evaluate possible SD-T3 interactive effects on hepatic responses to hormonal stimulation. Maintaining the fish at high SD appeared to increase the clearance rate of T3 from the T3-supplemented group. There was no clear evidence of SD-T3 interactive effects on growth rate, feeding rate, or food conversion efficiency, although T3-administration decreased food conversion efficiency, and high SD decreased growth and feeding rates. Of the hepatic enzymes studied, HOAD, malic enzyme, G6PDH, CS, PFK, HK, and GDH activities all showed changes suggestive of interactive SD-T3 effects. Although hepatic FBPase was stimulated by both high SD and T3-administration, there was no evidence of interactive SD-T3 effects. Dark muscle HOAD, CS, and PFK also showed SD-T3-related responses; dark muscle malic enzyme, G6PDH, HK, and GDH were unaffected by either altered SD or T3-administration. Prior treatment of the fish with T3 and high SD had significant effects on free fatty acid (ffa) release to the medium and on hepatic lipid content, but had no effect on the responses to the various endocrine agents used. Glucose release from liver slices of fish stocked at high density (both T3-supplemented and controls) was higher than that of the fish stocked at low density; with the exception of insulin and glucagon, glucose release was similar in all pre-treatment groups. The insulin- and glucagon-stimulated changes in glucose release seen in the fish fed non-supplemented diets were not found in the two groups of fish fed the T3-supplemented diets. High SD and/or T3-administration induced significant lowering of hepatic glycogen content, but there was no effect of pre-treatment on the response to any of the endocrine agents used. The data show a marked effect of SD on energy partitioning processes in brook charr and the animal's ability to respond to T3-stimulation, but provided no evidence of such effects on the liver response to the various agents used.

The acute influence of ingested thyroid hormones on hepatic deiodination pathways in the rainbow trout, Oncorhynchus mykiss

Juvenile rainbow trout were fed once daily with trout pellets supplemented with L-thyroxine (T4) or 3,5,3'-triiodo-L-thyronine (T3) and the effects on plasma T4 and T3 levels and hepatic 5'-monodeiodinase (5'D) activity determined after 1, 2, or 3 days. In all cases T3 (12 ppm) elevated plasma T3 and caused a significant reduction in the functional level (Vmax) of 5'D with no change in enzyme affinity (Km). After 3 daily meals (3 ppm T3), 5'D activity fell to 52% of control levels. In most instances plasma T4 was increased. In contrast, ingestion of T4 for 3 days at levels up to 48 ppm did not modify hepatic 5'D. This may reflect either insensitivity of the 5'D system to T4 or poor T4 uptake from the gut, as plasma T4 levels were influenced to a small extent by T4 ingestion. HPLC analyses showed that dietary T3 supplements (0, 3, 6, or 12 ppm) for 3 days acted in a dose-dependent manner, not only to suppress T3 formation from T4 by outer-ring deiodination, but also to promote inner-ring deiodination of T4 to 3,3',5'-triiodo-L-thyronine (reverse T3) and outer-ring deiodination of T3. In conclusion, the present data indicate that in the face of a T3 challenge there is a rapidly responding hepatic autoregulation of T3 production, achieved by a complex coordinated regulation of several different iodothyronine deiodination pathways.

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.

Influence of dietary lipid and carbohydrate levels and chronic 3,5,3'-triiodo-L-thyronine treatment on thyroid function in immature rainbow trout, Oncorhynchus mykiss

The influence of varying dietary levels of nonprotein energy sources (lipid, L; carbohydrate, C) and 3,5,3'-triiodo-L-thyronine (T3) on thyroid function in immature rainbow trout was studied. Three diets of equivalent available energy content and identical nutrient composition, except for dissimilar concentrations of L and C (diet 1, L = 7%, C = 28.3%; diet 2, L = 13%, C = 14.9%; diet 3, L = 19%, C = 1.5%), were each supplemented with 0, 4, 8, or 12 ppm T3 and fed to satiation to trout at 6.5 +/- 0.5 degrees on a 12-hr photoperiod for 12 weeks. Dietary L and C concentrations did not influence plasma total L-thyroxine (T4) or T3 levels, indices of free T4 or free T3 levels, hepatic T4 5'-monodeiodinase (5'D) activity, capacity or affinity of hepatic nuclear T3 receptors, or thyroid follicle epithelial cell height. T3 treatment elevated total and free T3 levels and decreased 5'D activity (Vmax) in approximate proportion to T3 dose, and without effect on plasma total or free T4 levels or T3 receptor properties. However, thyroid follicle epithelial cell height was depressed at 8 or 12 ppm dietary T3. In trout reverted for 20 days to a T3-free diet from a T3 (12 ppm) diet, plasma total T3 levels fell to 30% of those of control trout (0 ppm T3 throughout). It was concluded that, under our experimental conditions, (i) trout thyroid function was refractory to dietary concentrations of L and C, (ii) the primary response to T3 supplementation was suppressed hepatic 5'D level and T3 production, which was sustained for at least 20 days after T3 treatment ceased, and (iii) despite causing a probable indirect decrease in thyroidal secretion, T3 did not modify the set point of the hypothalamo-hypophyseal-thyroid axis based on plasma total or free T4 levels.

Stimulation of thyroid function by several pituitary hormones results in an increase in plasma thyroxine and reverse triiodothyronine in tilapia (Tilapia nilotica)

In this study, intravenous injection of several doses of porcine follicle stimulating hormone (pFSH: 0.002, 0.01, 0.05, and 0.5 micrograms/g body wt), bovine TSH (bTSH: 0.5 micrograms/g body wt), and ovine growth hormone (oGH: 0.04, 0.02, and 0.4 microgram/g body wt) stimulated an increase in plasma thyroxine (T4) and reverse triiodothyronine (rT3) in tilapia. This effect occurred in a dose-dependent manner. pFSH was the most potent in stimulating thyroid function. The dose of 0.002 microgram pFSH/g body wt increased plasma levels of T4 over control levels (2.59 +/- 0.16 ng/ml) about 2.5-fold within 4 hr, whereas a concentration of 0.5 micrograms/g body wt caused a great and prolonged increase of T4 and rT3 levels. Control levels (2.59 +/- 0.16 ng/ml for T4 and 40.37 +/- 8.60 pg/ml for rT3) were increased 19- and 22-fold respectively, over 24 hr. An increase of T4 and rT3 levels occurred also after injection of total hypophyseal extract and Con A II glycoprotein fraction of a tilapia pituitary homogenate, whereas the protein fraction failed to alter plasma concentrations of T4 and rT3. rT3 levels were also significantly increased at 2 hr, but not at 1 hr, after injection of T4. Basal T3 levels (1.90 +/- 0.22 ng/ml) were reduced by half over 24 hr in all experiments. These results suggest the existence, in tilapia, of a 5-D pathway deiodination of T4 which is pituitary independent. Stimulation of T4 release is always followed by an increase in plasma rT3 levels.

Growth hormone stimulates hepatic thyroxine 5'-monodeiodinase activity and 3,5,3'-triiodothyronine levels in rainbow trout (Salmo gairdneri)

Intraperitoneal injection of rainbow trout (Salmo gairdneri) with 0.4 microgram/g of human growth hormone (hGH) increased plasma levels of 3,5,3'-triiodo-L-thyronine (T3) and Vmax of the hepatic microsomal 5'-monodeiodinase enzyme (5'D) that converts thyroxine (T4) to T3, with no effect on Km or plasma T4 levels. A dose of 0.4 microgram hGH/g increased both plasma T3 and Vmax as early as 8 hr postinjection (pi). Maximal levels for both parameters occurred at 24 hr pi and significant stimulation was sustained to 48 hr pi. In trout injected with hGH over a dose range of 0.1-1.0 microgram/g and sampled at 24 hr pi, both Vmax and plasma T3 increased in a dose-dependent manner. It is concluded that the increase in the level of functional hepatic 5'D may contribute to the rapid hGH-induced elevation in plasma T3.

The relationship between T3 production and energy balance in salmonids and other teleosts

Extrathyroidal T4 5'-monodeiodination, demonstrated in several teleost species, generates T3 which binds more effectively than T4 to putative nuclear receptors and is probably the active thyroid hormone. T4 to T3 conversion is sensitive to the physiological state and provides a pivotal regulatory link between the environment and thyroid hormone action. T3 generation is enhanced in anabolic states (positive energy balance or conditions favoring somatic growth; food intake or treatment with androgens or growth hormone) and is suppressed in catabolic states (negative energy balance or conditions not favoring somatic growth; starvation, stress, or high estradiol levels associated with vitellogenesis). In fish, as in mammals, thyroidal status may be finely tuned to energy balance and through T3 production regulate energy-demanding processes, which in fish include somatic growth, development and early gonadal maturation.

Enhanced survival in striped bass fingerlings after maternal triiodothyronine treatment

Elevation of the triiodothyronine (T3) content of striped bass (Morone saxatilis) eggs by maternal T3 injection confirms the uptake of T3 by oocytes. The resulting offspring were influenced favorably by the T3, as seen in quantitative indices of development. As reported previously, larvae from T3-supplemented eggs raised under laboratory conditions exhibited increased body area, length, dry weight, and rates of swimbladder inflation and survival, compared to controls. Also, the T3 content of unfertilized oocytes correlated positively and highly significantly with survival to two weeks of age within individual cohorts (Brownet al., 1988). In the present study, the survival of experimental and control striped bass was monitored through the fingerling stage, under hatchery production conditions. The rate of recovery of maternally T3-treated cohorts from pond-culture was approximately fourfold that of controls. The striking effects of T3 enrichment of eggs on offspring indicate the potential contribution of maternal hormones in striped bass development, and suggest possible applications in aquaculture.

Circadian studies of plasma cortisol, thyroid hormone, protein, glucose and ion concentration, liver glycogen concentration and liver and spleen weight in rainbow trout, Salmo gairdneri Richardson

1. Although there are many reports in the literature of circadian rhythms in plasma hormone and metabolite levels, the data are highly variable between research groups. The present study attempts to re-examine whether circadian rhythm in plasma cortisol, thyroid hormone, ions, glucose and protein levels and liver glycogen levels are evident in rainbow trout, Salmo gairdneri, and determine whether there is a significant correlation between any of the measured variables. 2. Significant fluctuations throughout the day were found in all measured variables; although these fluctuations appear to be a normal component in the homeostatic function of rainbow trout, their timing was neither consistent nor predictable. 3. "Circadian-like" patterns were observed in levels of plasma cortisol, glucose, Mg2+ and K+ concentrations and liver glycogen concentration. 4. Seasonal variations in these circadian-like rhythms were found in liver glycogen and plasma cortisol, Mg2+ and K+ concentrations. 5. Plasma cortisol and glucose concentrations were affected by time of feeding. 6. There were significant correlations between plasma thyroid hormone and plasma protein levels, but no other significant correlation between any of the measured variables was found.

Effects of triiodothyronine and propylthiouracil on thyroid function and smoltification of coho salmon (Oncorhynchus kisutch)

Yearling coho salmon (Oncorhynchus kisutch) were fed diets containing triiodothyronine (T3; 4 or 12gmg), propylthiouracil (PTU; 1.5 or 6.0 mg/g), or both T3 (12μg/g) and PTU (1.5 mg/g) from January 10 to May 29. Plasma T4 and T3, concentrations were maintained within normal physiological limits in all groups of treated fish. Increases in plasma thyroxine (T4) occurred in late April in groups receiving the high dose of T3 or PTU, or the combination of T3, and PTU. Peaks of plasma T4 occurred in May in the other groups. Thyroid follicle epithelial cell height was increased in the groups in the following order: highest; PTU (6.0 mg/g), PTU (1.5 mg/g), PTU + T3, control, T3 (4gmg/g), T3 (12μg/g); lowest. In March specific binding of T3 by liver nuclei was not affected by treatment with T3 (12μg/g) but was decreased 30% by treatment with PTU (6.0μg/g). Body growth, food conversion efficiency, and pigmentary changes were increased by T3 (12μg/g), decreased dose-dependently by PTU, and unaffected by the other dietary treatments. All treatments altered body proximate composition. Food consumption and the ability to osmoregulate in seawater were decreased in fish fed 6.0 mg/g PTU, but were unaffected by the other dietary treatments. These results suggest that during smoltification: 1) Thyroid hormones may be involved in control of thyroidal function and body growth and pigmentary changes, 2) Hypo-osmoregulatory competence is not further stimulated by exogenous T3 and 3) PTU does not block T4 synthesis but may block T3, action by interfering with nuclear T3 binding.

Does oral 3,5,3 '-triiodo-l-thyronine affect dietary glucose utilization and plasma insulin levels in rainbow trout (Salmo gairdneri)?

A factorial experiment was conducted to determine the effect and interaction of dietary carbohydrate level and triiodo-L-thyronine (T3) supplementation on the growth, physiological response and plasma insulin and cortisol levels of rainbow trout. The oral administration of T3 significantly increased the growth, protein efficiency ratio and feed efficiency of trout, indicating an increased protein and perhaps energy utilization in these fish. However, T, administration did not significantly increase the utilization of dietary glucose as an energy source by the trout. Similarly, the administration of T3 did not significantly affect plasma insulin levels in either the fed or the fasted trout. Plasma insulin levels were significantly higher in fed trout reared on the non-T3 supplemented high carbohydrate diet in comparison to trout reared on the low carbohydrate diets. This indicates that increased dietary carbohydrate stimulates increased insulin secretion in the trout. Therefore, although rainbow trout are not insulin-deficient, they can still be considered a diabetic-like animal due to their poor glucose tolerance. Plasma cortisol levels were not affected by diet composition and altered plasma glucose levels.

Effects of various doses of thyroxine and triiodothyronine on the metamorphosis of flounder (Paralichthys olivaceus)

Two experiments were carried out to clarify the effects of thyroxine (T4) and triiodothyronine (T3) on flounder metamorphosis. In the first experiment, three doses of T4 were administered to premetamorphic flounder larvae together with thiourea (TU). Thiourea treatment (30 ppm in ambient seawater) alone caused metamorphic stasis in the fish, although the beginning of the right eye migration was not completely inhibited. T4 at 100 and 10 ppb in ambient seawater induced metamorphosis in the thiourea-treated larvae. The fish of these groups completed translocation of the right eye to the left side and resorption of elongated dorsal fin rays. However, 1 ppb T4 failed to induce metamorphic climax. The elongated dorsal fin rays continued to grow until the end of the experiment in the group treated with TU alone and in combination with 1 ppb T4. In the second experiment, premetamorphic flounder larvae were treated with two doses of T4 and three doses of T3. The T3 treatment was several times more potent than T4 treatment in the induction of metamorphosis. The results of the present study suggest that the metamorphic events of flounder larvae are dependent on the thyroid hormone concentration, and the relative biological potency of T4 and T3 on flounder metamorphosis is similar to that observed in mammals and amphibia.

Nutrition, hormones and growth

A review is presented relating to the effects of hormones on food intake and nutritional state and also of the effects of food intake and nutritional state on hormone activity. These interactions and their effect on fish growth are described.Growth hormone, thyroid hormones, insulin, and gonadal steroids have all been implicated in the normal growth processes of fish and their metabolic actions have been related to the nutritional state of the animal. Furthermore, variations in plasma levels and hormone application appear to affect food intake and food conversion.Anabolic steroids and their possible involvement in aquaculture practice has been an area of active investigation for the past few years. The current situation is reviewed, particularly regarding relevance to protein intake.

The influence of 3,3',5-triiodo-L-thyronine on growth survival and body composition of slow-growing development elvers (Anguilla rostrata L.)

Effect of 3,3',5-triiodo-L-thyronine (T3) on survival, growth and body composition (protein, fat and ash) of slow-growing (Anguilla rostrata L.) elvers was investigated. A significant increase was found in the mean weight of slow growing elvers receiving diets containing 20 ppm T3 and held in the low water turnover system. A significant increase was also found in the weight of slow-growing elvers receiving diets containing 40 or 60 ppm T3 and held in the high water turnover system. Percentage protein (dry weight basis) was significantly correlated to body weight, but was not significantly correlated to concentration of T3 in the diet. There were no significant differences between the total fat content of slow-growing elvers when compared to the total body fat of control elvers.

Photoperiod-dependent negative feedback effects of thyroid hormones in Fundulus heteroclitus

In Fundulus heteroclitus, an annual cycle in the response of the thyroid to ovine thyroid-stimulating hormone (oTSH) is characterized by maximal thyroxin (T4) secretion in mid-winter and minimal T4 secretion in summer. Four daily injections of oTSH, given in winter caused serum T4 to plateau at elevated levels for several days, while in summer fish similar treatment resulted in far more fluctuating titers of serum T4; maximum levels were similar in both groups. The difference in sustenance rather than magnitude of Peak T4 led to an examination of the negative feedback effects of thyroid hormones as they might relate to these seasonal changes. Radioiodine uptake by thyroid follicles served as a simple, but effective bioassay for endogenous TSH. Fish collected in summer were more sensitive to negative feedback of T3 than those collected in winter; feedback effects of T4 in the two groups were not significantly different. The effects of specific photoperiods on negative feedback sensitivity to T3 and T4 were also tested. Exposure of winter fish for one month to long days (LD 14:10) enhanced the degree of reduction of iodine uptake caused by T4 in the aquarium water (10 micrograms/100 ml). Negative feedback in short-day (LD 8:16) winter fish was not demonstrated. It is concluded that long days increase and short days diminish the negative feedback sensitivity of the hypothalamus-pituitary axis to thyroid hormones in F. heteroclitus. Such photoperiodically induced changes may act to aid in the year-round maintenance of T4 levels necessary for seasonal adaptation and survival.