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

Response of the thyroid axis and appetite-regulating peptides to fasting and overfeeding in goldfish (Carassius auratus)

The thyroid axis is a major regulator of metabolism and energy homeostasis in vertebrates. There is conclusive evidence in mammals for the involvement of the thyroid axis in the regulation of food intake, but in fish, this link is unclear. In order to assess the effects of nutritional status on the thyroid axis in goldfish, Carassius auratus, we examined brain and peripheral transcripts of genes associated with the thyroid axis [thyrotropin-releasing hormone (TRH), thyrotropin-releasing hormone receptors (TRH-R type 1 and 2), thyroid stimulating hormone beta (TSHβ), deiodinase enzymes (DIO2, DIO3) and UDP-glucoronsyltransferase (UGT)] and appetite regulators [neuropeptide Y (NPY), proopiomelanocortin (POMC), agouti-related peptide (AgRP) and cholecystokinin (CCK)] in fasted and overfed fish for 7 and 14 day periods. We show that the thyroid axis responds to overfeeding, with an increase of brain TRH and TSHβ mRNA expression after 14 days, suggesting that overfeeding might activate the thyroid axis. In fasted fish, hepatic DIO3 and UGT transcripts were downregulated from 7 to 14 days, suggesting a time-dependent inhibition of thyroid hormone degradation pathways. Nutritional status had no effect on circulating levels of thyroid hormone. Central appetite-regulating peptides exhibited temporal changes in mRNA expression, with decreased expression of the appetite-inhibiting peptide POMC from 7 to 14 days for both fasted and overfed fish, with no change in central NPY or AgRP, or intestinal CCK transcript expression. Compared to control fish, fasting increased AgRP mRNA expression at both 7 and 14 days, and POMC expression was higher than controls only at 7 days. Our results indicate that nutritional status time-dependently affects the thyroid axis and appetite regulators, although no clear correlation between thyroid physiology and appetite regulators could be established. Our study helps to fill a knowledge gap in current fish endocrinological research on the effects of energy balance on thyroid metabolism and function.

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.

Wild fish from the Bay of Quinte Area of Concern contain elevated tissue concentrations of PCBs and exhibit evidence of endocrine-related health effects

The Bay of Quinte (BOQ) is an Area of Concern listed under the Great Lakes Water Quality Agreement. The presence of dioxins and dioxin-like PCBs in fish in the BOQ AOC has led to restrictions on fish consumption by humans, which is a beneficial use impairment. Adult yellow perch (Perca flavescens) and brown bullhead (Ameiurus nebulosus) were sampled from Trenton, Belleville, and Deseronto (reference site) in the BOQ. A suite of hormone assays and various measures of exposure and/or sublethal health effects were used to assess the health status of fish of both species and sex. Condition factor, hepatosomatic index, ethoxyresorufin-O-deethylase activity, circulating steroid and thyroid hormones, thyroid activation, oocyte size distribution, spermatogenic cell stages, and plasma vitellogenin were among the endpoints that were significantly (p < 0.05) affected by location. Many of those effects corresponded with significantly (p < 0.05) greater tissue concentrations of polychlorinated biphenyls (PCBs) at Belleville and Trenton. Hepatic extracts from brown bullhead sampled from Trenton had significantly (p < 0.05) greater binding activity to the androgen receptor and sex steroid binding protein. Taken together, these data and preliminary data from a concomitant study suggest that PCBs are likely being hydroxylated in vivo, resulting in enhanced bioactivity at endocrine receptors and measurable health responses. The present study supports the growing body of evidence that PCBs and their metabolites can affect fish thyroid and steroid hormone systems.

Testing an ecophysiological mechanism of morphological plasticity in pupfish and its relevance to conservation efforts for endangered Devils Hole pupfish

Imperiled species that have been translocated or established in captivity can show rapid alterations in morphology and behavior, but the proximate mechanisms of such phenotypic changes are rarely known. Devils Hole pupfish (Cyprinodon diabolis) are endemic to a single desert pool and are characterized by a small body, large head and eyes, and lack of pelvic fins. To lessen the risk of extinction, additional populations of C. diabolis were established in artificial refuges. Yet, pupfish in these refuges rapidly shifted to a larger body, smaller head and eyes, and greater body depth. Here we examined how food availability and temperature, which differ between these habitats, influence morphological development in closely related Amargosa River pupfish (Cyprinodon nevadensis amargosae). We were interested in knowing whether these environmental factors could developmentally shift Amargosa River pupfish toward the morphology typical of pupfish in Devil's Hole. By regulating food ration, we created groups of pupfish with low, medium and high growth rates. Pupfish with low growth showed proportionally larger head and eyes, smaller body depth, and reduction in pelvic fin development. Elevated temperature further inhibited pelvic fin development in all treatments. Pupfish in the low growth group also showed reduced levels of thyroid hormone, suggesting a possible physiological mechanism underlying these morphological changes. To test this mechanism further, pupfish were reared with goitrogens to pharmacologically inhibit endogenous thyroid hormone production. Pupfish given goitrogens developed larger heads and eyes, shallower bodies, and reduced pelvic fins. Taken together, our results suggest that changes in environmental factors affecting the growth and thyroid hormone status of juvenile pupfish may play a developmental role in generating the morphological differences between C. diabolis in Devil's Hole and the refuges. These findings illustrate the need to incorporate a mechanistic understanding of phenotypic plasticity into conservation strategies to preserve imperiled fishes.

The Effects of Methimazole on Development of the Fathead Minnow, Pimephales promelas, from Embryo to Adult

The importance of thyroid hormones in regulating early developmental processes of many amphibian and fish species is well known, but the impacts of exposure to disrupters of thyroid homeostasis during the embryo-larval-juvenile transitions are unclear. To investigate these impacts, fathead minnows, Pimephales promelas, were exposed to a model thyroid axis disrupter, methimazole, an inhibitor of thyroid hormone synthesis, at control (0), 32, 100, and 320 mug/l, starting at <24-h postfertilization, for 28, 56, and 83/84 days postfertilization (dpf). Thyroid disruption was evident at 28 dpf, when survival was significantly reduced by 32 or 100 mug/l methimazole concomitant with a reduced thyroxine (T(4)) content. However, the T(3) content of these fish was similar to that of control fish, and body mass was unaffected (as in all groups), suggesting compensatory mechanisms overcame reduced T(4) synthesis. At the highest concentration of methimazole (320 mug/l), activation of feedback mechanisms on the hypothalamic-pituitary-thyroid axis was suggested by the normal T(4) content after 28 dpf exposure to methimazole, although triiodothyronine (T(3)) content of these fish was significantly reduced. The generally less pronounced disruption of thyroid hormone homeostasis after 56 days exposure to methimazole also suggests compensatory mechanisms in juvenile/adult fish that may regulate T(4) content, despite exposure to methimazole at 32 or 100 mug/l (in fish held in 320 mug/l methimazole, the T(4) content was significantly higher than in controls). Whole body T(3) content at 56 dpf was significantly depressed only in fish held in 100 mug/l methimazole. By 83/84 dpf, length, body mass, and thyroid hormone concentrations were similar in all experimental groups and controls, indicating that adult fish may achieve regulation of their thyroid axis despite prolonged exposures to thyroid disruptors throughout early development.

Central regulation of thyroidal status in a teleost fish: nutrient stimulation of T4 secretion and negative feedback of T3

Several experiments were conducted to investigate the dynamics of central regulation of thyroid function in the red drum, Sciaenops ocellatus, by manipulating a well-characterized circadian rhythm of T(4) secretion. In the first experiment, red drum were reared under either a long (16L:8D) or short (8L:16D) photoperiod and fed at the same time relative to dawn. The same feeding time under different photoperiods maintained the same phase relationship between T(4) cycles under each photoperiod. This suggests that the circadian clock that determines when the hypothalamus-pituitary-thyroid (HPT) axis is activated is comprised of a feeding-entrained oscillator and a light-entrained oscillator that interact to determine the phase of the T(4) rhythm. Additionally, the amplitude of the main T(4) peak of the cycle was inversely related to the frequency of feeding, while the duration of the main T(4) peak was directly related to feeding frequency under a long photoperiod. Feeding time appears to modify the diurnal profile of circulating T(4) by stimulating post-prandial T(4) secretion that subsequently results in negative feedback on the HPT axis to regulate thyroidal status. In following experiments, red drum immersed in T(3), in lieu of a meal at a specific time that would diminish the main T(4) peak, exhibited a dose-dependent decline in amplitude of the T(4) cycle. This demonstrates that T(3) can exert negative feedback on the HPT axis of red drum to maintain appropriate thyroid hormone concentrations. These data are consistent with a dynamic and physiologically important central component of the regulation of thyroid function in fish.

Hypothyroidism induces type I iodothyronine deiodinase expression in tilapia liver

In the current study, the authors examined the effects of experimentally induced hypothyroidism on peripheral thyroid hormone metabolism and growth in two closely related tilapia species: the Nile tilapia (Oreochromis niloticus) and the slower growing black tilapia (Sarotherodon melanotheron). Hypothyroidism, induced by administration of 0.2% methimazole through the food, significantly decreased plasma T(3) and T(4) in both species. This decrease in circulating thyroid hormones was accompanied by an increase in hepatic type II deiodinase (D2) and a decrease in hepatic type III deiodinase (D3). Hepatic type I deiodinase (D1), which is barely expressed in euthyroid tilapia, was significantly upregulated during hypothyroidism. The changes in hepatic D1 and D2 enzyme activity were paralleled by changes in D1 and D2 mRNA levels, indicating pretranslational regulation. Hypothyroidism also resulted in severe growth retardation that was accompanied by an increase in condition factor. Because hyperthyroidism has been shown to decrease the condition factor, these results suggest that thyroid hormones play an essential role in the control of proportional body growth in fish. The authors conclude that (1) hepatic D1 expression is induced by hypothyroidism in tilapia, (2) the changes in hepatic iodothyronine deiodinases during hypothyroidism in tilapia are predominantly regulated at a pretranslational level, and (3) thyroid hormones are involved in the control of proportional body growth in fish.

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.

Characterization of hepatic low-K(m) outer-ring deiodination in red drum (Sciaenops ocellatus)

The more biologically active thyroid hormone 3,5,3'-triiodothyronine (T(3)), is primarily derived from peripheral deiodination of thyroxine (T(4)). We characterized hepatic deiodination for a commercially important, warm water teleost fish, the red drum (Sciaenops ocellatus). Low K(m) outer-ring deiodination (ORD) activity was determined by production of free iodide ((125)I) upon incubation of hepatic microsomes with radiolabeled T(4). HPLC analysis demonstrated that (125)I, and T(3) were produced in equal amounts, thereby validating 125I as a measure of T(3) production. A small amount of 3,3',5'-triiodothyronine (reverse T(3)) was also produced by inner-ring deiodination. Production of (125)I was linear over a range of 0--100 microg protein/ml and for incubations of 30 min--4 h. Maximal ORD activity was measured at pH 6.6, 50 mM dithiothreitol (DTT) and an incubation temperature of 20 degrees C. Double reciprocal plots demonstrated that the average apparent K(m) was 5.1 nM and the average V(max) was 3.7 pmol T(4) converted/h per mg protein. ORD was not inhibited by propylthiouracil but was 50% inhibited by 90 microM of iodoacetic acid and 7 microM of gold thioglucose. The substrate analog preference was T(4) = tetraiodoacetic acid = reverse T(3) > triiodoacetic acid >> T(3). In relation to other tissues, ORD for liver>gill>intestine>kidney. Similar hepatic deiodination activity was present in adult wild, aquacultured and laboratory-reared red drum, but in adult wild red drum the optimum temperature was higher. Red drum hepatic low-K(m) deiodination activity appears to most closely resemble rainbow trout hepatic and mammalian Type II deiodination. Evidence of inner-ring T(4) deiodination suggests a more active hepatic iodothyronine catabolic pathway than in other teleost species.

The effects of photoperiod and feeding on the diurnal rhythm of circulating thyroid hormones in the red drum, Sciaenops ocellatus

Available data in cyprinid and salmonid species indicate that nutrient intake sustains thyroidal rhythmicity and that time of feeding may influence the amplitude, but not the phase, of diurnal thyroid hormone cycles. Several experiments were conducted to characterize the nature of thyroidal rhythmicity in a more derived perciform teleost, the red drum. These studies were designed to test the following hypotheses: (1) that feeding time will alter the amplitude of the thyroid hormone rhythm without altering its phase and (2) that food deprivation will diminish the amplitude of the thyroid hormone rhythm. Circulating T(4) levels in this species exhibit high-amplitude diurnal rhythms, whereas circulating T(3) levels fluctuate within a more narrow range. Fish were reared under a 12L:12D photoperiod and fed 5% body weight once daily either at dawn or at dusk. Feeding time had no discernible effect on the phase of the T(4) cycle, but altered the amplitude of the cycle. Dawn-fed fish had significantly greater mean peak levels of T(4) than dusk-fed fish, although there was no difference in daily mean levels in both groups of fish. When red drum were deprived of food, significant declines in plasma glucose, HSI, and liver glycogen content occurred within 3 days. When red drum were sampled once per day after 3, 7, or 11 days of food deprivation there were no consistent changes in circulating T(4) and T(3) levels compared to those of fed controls. However, significant declines in circulating T(4) and T(3) levels in response to food deprivation were detected with a diurnal sampling protocol. Within 3 days of food deprivation, T(4) levels were significantly reduced compared to those in fed controls and not significantly different from T(4) levels after 10 days of food deprivation. T(3) levels exhibited a stepwise decline in circulating levels during food deprivation. These data indicate that both feeding time and nutrient status exert their effects on thyroid hormone rhythms by modifying the amplitude of these cycles. These data also underscore the importance of incorporating a consideration of endocrine rhythmicity into sampling protocols.

Effects of long-term temperature acclimation on thyroid hormone deiodinase function, plasma thyroid hormone levels, growth, and reproductive status of male Atlantic cod, Gadus morhua

The recent collapse of the Northwestern Atlantic cod fisheries has coincided with a cooling of water temperatures. During this time the condition factor of cod has been poor. The objective of the present study was to determine the effects of long-term temperature acclimation on growth reproduction and thyroid function in laboratory held Atlantic cod (Gadus morhua). One of the key parameters used to assess thyroid function is the peripheral metabolism of L-thyroxine (T4) by microsomal deiodinase enzymes. Deiodinase function has not been described for gadid fish. T4 outer-ring deiodinating activity (apparent K(m) 1-2 nM) was confined primarily to liver. Its properties resembled those for hepatic T4ORD activity of other teleosts and the mammalian type II deiodinase. The T4ORD activity of cod liver exceeded that of salmonids and could explain the high plasma T3 levels (10-18 ng/ml), which were 2-5 times greater than T4 levels. T4 and T3 inner-ring deiodination was confined mainly to brain. In order to determine the effects of long-term temperature acclimation on cod, somatic growth, reproduction, and thyroidal status were assessed monthly in 400-900-g satiation-fed male Atlantic cod captured in June from the St. Lawrence Estuary and then acclimated from August to the following June under a natural photoperiod at 2-4 degrees C (LT) or 6-10 degrees C (HT). Reproductive status was determined from the gonadosomatic index (GSI), plasma testosterone (T) and 11-ketotestosterone (11-KT) levels, and the appearance of milt; thyroidal status was determined from plasma T4 and 3,5,3'-triiodo-L-thyronine (T3) levels and hepatic T4ORD activity to produce biologically active T3. Testis maturation (high levels of 1 and 11-KT, and milt release) occurred in April and May and was uninfluenced by acclimation temperature. LT cod grew more slowly than HT cod. Differences in body weight were particularly evident from December to February. In conclusion, (i) cod possess outer- and inner-ring deiodinase activities, predominating respectively in liver and brain, and with properties resembling those of other teleosts, (ii) T4ORD activity of liver is unusually high and may account for the high plasma T3 levels in this species, (iii) T4ORD activity tends to increase during periods of increased somatic growth, and (iv) chronic acclimation of male cod to 2-4 degrees C, as opposed to 6-10 degrees C, decreases somatic growth but does alter circulating levels of thyroid hormones and androgens and it does not change the time of sexual maturation.

Effect of feeding time on hepatic nucleic acid, plasma T3, T4, and GH concentrations in rainbow trout

The effect of feeding time (dawn, midnight) on the growth performance and daily pattern of liver nucleic acid concentrations, plasma thyroid hormone, and growth hormone concentrations was studied in immature rainbow trout (Oncorhynchus mykiss, mean individual weight: 55 g). Fish were held in groups of 20 individuals (12 groups per treatment) and maintained in flow-through tanks supplied with river water under natural photoperiod. Food pellets (Aqualim, 49% crude protein) were delivered daily over a 30-min period by means of belt feeders. Growth performance and protein retention efficiency were higher for trout fed at dawn than for those fed at midnight, whereas both groups of fish ingested the same total amount of feed. All parameters studied showed significant daily variations. The daily patterns of liver RNA concentrations, RNA/DNA, and protein/DNA ratios were significantly different between fish fed at dawn and at midnight, indicative of a higher liver protein synthesis in the trout fed at dawn. On average, plasma thyroid hormone levels were higher and plasma GH concentrations were lower in trout fed at dawn compared with those fed at midnight. The hormonal patterns were only affected by feeding time when the fish were fed at dawn. These results suggested that the observed differences in growth and protein retention efficiency were linked to the observed differences in plasma hormone levels.

Effects of dietary thyroid hormones on the red drum (Sciaenops ocellatus)

Four separate 8-week feeding trials were conducted to assess the effects of supplementing semipurified diets with either triiodothyronine (T3) or thyroxine (T4) at 0, 2, 10, and 50 mg/kg on growth and body composition of juvenile red drum (Sciaenops ocellatus) held in artificial brackish water (6‰) and artificial seawater (32‰). At both levels of salinity, increasing doses of T3 resulted in fish with reduced weight gain, feed efficiency, condition factor (weight × 100/length(3)), and muscle ratio (muscle weight × 100/body weight), as well as a lighter body color. Significant (p < 0.05) effects of T3 on the proximate composition of whole body, liver, and muscle were variable, generally reflecting decreased lipid and protein storage in liver and muscle, respectively. The two highest doses of T3 given to seawater adapted fish increased survival. Dietary T4 supplementation had no distinctive effects on appearance, growth or proximate body composition. These results indicate that whereas T3 may function to regulate protein and lipid metabolism in red drum, dietary supplementation with T3 leads to a hyperthyroidism-induced catabolic state. The elevated endogenous thyroid hormone levels found in fish fed optimal diets may thus adequately supply tissue needs during juvenile growth.