Thyroid hormone deficiency during zebrafish development impairs central nervous system myelination.

Thyroid hormones are messengers that bind to specific nuclear receptors and regulate a wide range of physiological processes in the early stages of vertebrate embryonic development, including neurodevelopment and myelogenesis. We here tested the effects of reduced T3 availability upon the myelination process by treating zebrafish embryos with low concentrations of iopanoic acid (IOP) to block T4 to T3 conversion. Black Gold II staining showed that T3 deficiency reduced the myelin density in the forebrain, midbrain, hindbrain and the spinal cord at 3 and 7 dpf. These observations were confirmed in 3 dpf mbp:egfp transgenic zebrafish, showing that the administration of IOP reduced the fluorescent signal in the brain. T3 rescue treatment restored brain myelination and reversed the changes in myelin-related gene expression induced by IOP exposure. NG2 immunostaining revealed that T3 deficiency reduced the amount of oligodendrocyte precursor cells in 3 dpf IOP-treated larvae. Altogether, the present results show that inhibition of T4 to T3 conversion results in hypomyelination, suggesting that THs are part of the key signaling molecules that control the timing of oligodendrocyte differentiation and myelin synthesis from very early stages of brain development.

Metal brain bioaccumulation and neurobehavioral effects on the wild rodent Liomys irroratus inhabiting mine tailing areas.

Ecotoxicological studies are necessary in order to evaluate the effects of environmental exposure of chemicals on wild animals and their ecological consequences. Particularly, neurobehavioral effects of heavy metal elements on wild rodents have been scarcely investigated. In the present study, we analyzed the effect of metal bioaccumulation (Pb, As, Mg, Ni, and Zn) in the brain and in the liver on exploratory activity, learning, memory, and on some dopaminergic markers in the wild rodent Liomys irroratus living inside mine tailings, at Huautla, Morelos, Mexico. We found higher Pb concentration but lower Zn in striatum, nucleus accumbens, midbrain, and hippocampus in exposed animals in comparison to rodents from the reference site. Exposed rodents exhibited anxious behavior evaluated in the open field, while no alterations in learning were found. However, they displayed slight changes in the memory test in comparison to reference group. The neurochemical evaluation showed higher levels of dopamine and 5-hydroxyindolacetic acid in midbrain, while lower levels of metabolites dihydroxyphenyl acetic acid and homovanillic acid in striatum of exposed rodents. In addition, mRNA expression levels of dopaminergic D2 receptors in nucleus accumbens were lower in animals from the mining zone than in animals from the reference zone. This is the first study that shows that chronic environmental exposure to metals results in behavioral and neurochemical alterations in the wild rodent L. irroratus, a fact that may comprise the survival of the individuals resulting in long-term effects at the population level. Finally, we suggest the use of L. irroratus as a sentinel species for environmental biomonitoring of mining sites.

Knock-Down of Specific Thyroid Hormone Receptor Isoforms Impairs Body Plan Development in Zebrafish.

The role of thyroid hormones (THs) in development has been extensively studied, however, the specific molecular mechanisms involved are far from being clear. THs act by binding to TH nuclear receptors (TR) that act as ligand-dependent transcription factors to regulate TH-dependent gene expression. Like vertebrates, zebrafish express different isoforms of functional Tr alpha and beta, some of which can bind alternative ligands like 3,5-T2. In this study, we first analyzed the effects of exogenous T3 and 3,5-T2 exposure during embryogenesis. The percentage of affected embryos was similar to those vehicle-injected, suggesting that the early exposure to low TH levels is not sufficient to elicit effects upon the phenotype of the embryo. We then generated crispants for four isoforms of thr to learn more about the role of these receptors in early development. We found that crispant larvae from thraa and a newly identified l-thrb+, but not thrab and canonical thrb1 showed profound deleterious effects upon symmetry and laterality, suggesting early novel roles for these Tr isoforms in the body plan developmental program. Since critical events that determine cell fate start in the late gastrula, we tested if some genes that are expressed during early developmental stages could indeed be TH targets. We identify early development genes, like sox10 and eve, that were specifically over-expressed in thraa and l-thrb+ crispants, suggesting that these specific thr isoforms function as transcription repressors for these genes, while transcription of zic and ets appear to be thraa and l-thrb+-mediated, respectively. Overall, present results show that TH signaling participates in early zebrafish development and identify Tr isoform-specific mediated regulation of early gene expression.

Differential responses of the somatotropic and thyroid axes to environmental temperature changes in the green iguana.

Growth hormone (GH), together with thyroid hormones (TH), regulates growth and development, and has critical effects on vertebrate metabolism. In ectotherms, these physiological processes are strongly influenced by environmental temperature. In reptiles, however, little is known about the direct influences of this factor on the somatotropic and thyroid axes. Therefore, the aim of this study was to describe the effects of both acute (48h) and chronic (2weeks) exposure to sub-optimal temperatures (25 and 18°C) upon somatotropic and thyroid axis function of the green iguana, in comparison to the control temperature (30-35°C). We found a significant increase in GH release (2.0-fold at 25°C and 1.9-fold at 18°C) and GH mRNA expression (up to 3.7-fold), mainly under chronic exposure conditions. The serum concentration of insulin-like growth factor-I (IGF-I) was significantly greater after chronic exposure (18.5±2.3 at 25°C; 15.92±3.4 at 18°C; vs. 9.3±1.21ng/ml at 35°C), while hepatic IGF-I mRNA expression increased up to 6.8-fold. Somatotropic axis may be regulated, under acute conditions, by thyrotropin-releasing hormone (TRH) that significantly increased its hypothalamic concentration (1.45 times) and mRNA expression (0.9-fold above control), respectively; and somatostatin (mRNA expression increased 1.0-1.2 times above control); and under chronic treatment, by pituitary adenylate cyclase-activating peptide (PACAP mRNA expression was increased from 0.4 to 0.6 times). Also, it was shown that, under control conditions, injection of TRH stimulated a significant increase in circulating GH. On the other hand, while there was a significant rise in the hypothalamic content of TRH and its mRNA expression, this hormone did not appear to influence the thyroid axis activity, which showed a severe diminution in all conditions of cold exposure, as indicated by the decreases in thyrotropin (TSH) mRNA expression (up to one-eight of the control), serum T4 (from 11.6±1.09 to 5.3±0.58ng/ml, after 2weeks at 18°C) and T3 (from 0.87±0.09 to 0.05±0.01ng/ml, under chronic conditions at 25°C), and Type-2 deiodinase (D2) activity (from 992.5±224 to 213.6±26.4fmolI(125)T4/mgh). The reduction in thyroid activity correlates with the down-regulation of metabolism as suggested by the decrease in the serum glucose and free fatty acid levels. These changes apparently were independent of a possible stress response, at least under acute exposure to both temperatures and in chronic treatment to 25°C, since serum corticosterone had no significant changes in these conditions, while at chronic 18°C exposure, a slight increase (0.38 times above control) was found. Thus, these data suggest that the reptilian somatotropic and thyroid axes have differential responses to cold exposure, and that GH and TRH may play important roles associated to adaptation mechanisms that support temperature acclimation in the green iguana.

3,5-Diiodothyronine-mediated transrepression of the thyroid hormone receptor beta gene in tilapia. Insights on cross-talk between the thyroid hormone and cortisol signaling pathways.

T3 and cortisol activate or repress gene expression in virtually every vertebrate cell mainly by interacting with their nuclear hormone receptors. In contrast to the mechanisms for hormone gene activation, the mechanisms involved in gene repression remain elusive. In teleosts, the thyroid hormone receptor beta gene or thrb produces two isoforms of TRß1 that differ by nine amino acids in the ligand-binding domain of the long-TRß1, whereas the short-TRß1 lacks the insert. Previous reports have shown that the genomic effects exerted by 3,5-T2, a product of T3 outer-ring deiodination, are mediated by the long-TRß1. Furthermore, 3,5-T2 and T3 down-regulate the expression of long-TRß1 and short-TRß1, respectively. In contrast, cortisol has been shown to up-regulate the expression of thrb. To understand the molecular mechanisms for thrb modulation by thyroid hormones and cortisol, we used an in silico approach to identify thyroid- and cortisol-response elements within the proximal promoter of thrb from tilapia. We then characterized the identified response elements by EMSA and correlated our observations with the effects of THs and cortisol upon expression of thrb in tilapia. Our data show that 3,5-T2 represses thrb expression and impairs its up-regulation by cortisol possibly through a transrepression mechanism. We propose that for thrb down-regulation, ligands other than T3 are required to orchestrate the pleiotropic effects of thyroid hormones in vertebrates.

The variable region of iodothyronine deiodinases directs their catalytic properties and subcellular localization.

The stereospecific removal of iodine from thyroid hormones is an essential first step for T3 action and is catalyzed by three different deiodinases: D2 and D3 remove iodine only from the outer or inner ring, respectively, whereas D1 catalyzes both pathways. We used in silico predictions from vertebrate deiodinase sequences to identify two domains: the N-terminal variable region (VR) containing the transmembrane, hinge and linker domains, and the conserved or globular region (CR). Given the high sequence and structural identity of the CR among paralogs as well as of the VR among orthologs but not paralogs, we hypothesized that both the catalytic properties and the subcellular localization rely on the VR. We used shark D2 and D3 as templates to build the chimeric enzymes D2VR/D3CR and D3VR/D2CR. Biochemical characterization revealed that D3VR/D2CR has inner-ring deiodination activity and T3 as preferred substrate, whereas D2VR/D3CR showed no deiodinating activity. Also, D2VR/D3CR and D3VR/D2CR reside in the endoplasmic reticulum and plasmatic membrane, respectively, as do their D2 and D3 wild-type counterparts. We conclude that the VR determines the subcellular localization and is critical in defining the catalytic properties and activity of thyroid hormone deiodinases.

Iodine nutrition and thyroid function assessment in childbearing age women from Queretaro, Mexico.

OBJECTIVE: To assess iodine nutrition and thyroid function in Mexican childbearing age women. METHODS: 101 childbearing age women (21.7 ± 3.5 years) randomly selected from the university student population participated in this cross-sectional study. TSH, thyroid hormones, anti-thyroid antibodies, thyroid volume, iodine intake, and urinary iodine concentration (UIC) were assessed. The knowledge about the importance of iodine in nutrition was also evaluated by using questionnaires. RESULTS: TSH median (interquartile range) value was 1.9 (1.4-2.5) mIU/L, while FT4 median value was 9.0 (8.3- 9.6) µg/dL. The median FT3 and total rT3 values were 3.3 pg/mL and 40.1 ng/dL, respectively. The prevalence of subclinical hypothyroidism (serum TSH >4.5 mIU/L) and of positive anti-thyroid antibodies were 2.9% and <5.9%, respectively. Median thyroid volume was 5.6 mL and none of the subjects were diagnosed with goiter. Median urinary iodine concentration was 146 (104-180) µg/L. As for the knowledge of iodine nutrition, only 37.6% considered that a pregnant woman needs more dietary iodine than a non pregnant woman, while 43.6% recognized that the lack of iodine can cause mental retardation in children. CONCLUSIONS: Prevalence of thyroid test function abnormalities was low in this population and the median UIC indicates adequate iodine intake. We also found a poor knowledge about the importance iodine nutrition in the studied population.

Iodine nutrition and thyroid function assessment in child bearing age women from Queretaro, Mexico / Estado nutricio en yodo y función tiroidea en mujeres en edad reproductiva de Querétaro, México

Objective: To assess iodine nutrition and thyroid function in Mexican childbearing age women. Methods: 101 childbearing age women (21.7 ± 3.5 years) randomly selected from the university student population participated in this cross-sectional study. TSH, thyroid hormones, anti-thyroid antibodies, thyroid volume, iodine intake, and urinary iodine concentration (UIC) were assessed. The knowledge about the importance of iodine innutrition was also evaluated by using questionnaires. Results: TSH median (interquartile range) value was1.9 (1.4-2.5) mIU/L, while FT4 median value was 9.0 (8.3-9.6) μg/dL. The median FT3 and total rT3 values were 3.3pg/mL and 40.1 ng/dL, respectively. The prevalence of subclinical hypothyroidism (serum TSH >4.5 mIU/L) and of positive anti-thyroid antibodies were 2.9% and <5.9%,respectively. Median thyroid volume was 5.6 mL and none of the subjects were diagnosed with goiter. Median urinary iodine concentration was 146 (104-180) μg/L. As for the knowledge of iodine nutrition, only 37.6% considered that a pregnant woman needs more dietary iodine than a non pregnant woman, while 43.6% recognized that the lack of iodine can cause mental retardation in children. Conclusions: Prevalence of thyroid test function abnormalities was low in this population and the median UIC indicates adequate iodine intake. We also found a poor knowledge about the importance iodine nutrition in the studied population (AU)

Objetivo: Evaluar el estado nutricional en yodo y la función tiroidea en mujeres mexicanas en edad reproductiva. Métodos: 101 mujeres universitarias en edad reproductiva(21,7 ± 3,5 años) fueron seleccionadas al azar para participar en este estudio transversal. Se evaluaron los niveles séricos de tirotropina, hormonas tiroideas, anticuerpos anti-tiroideos, volumen tiroideo, consumo de yodo y yoduria. También se evaluó el conocimiento sobre la importancia del yodo en la nutrición. Resultados: La mediana (rango intercuartilar) de tirotropina fue de 1,9 (1,4-2,5) mIU/L, mientras que para T4libre fue de 9,0 (8,3-9,6) μg/dL. Los valores de la mediana de T3 libre y T3 reversa fueron de 3,3 pg/mL y 40,1 ng/dL, respectivamente. La prevalencia de hipotiroidismo subclínico fue 2,9% (tirotropina sérica >4,5 mUI/L). La prevalencia de anticuerpos antitiroideos positivos fue <5,9%.La mediana del volumen tiroideo fue de 5,6 mL y no se diagnosticaron mujeres con bocio. La mediana (rangointercuartilar) de la yoduria fue de 146 (104-180) μg/L. En cuanto al conocimiento de la importancia del yodo en la nutrición, el 37,6% consideró que las mujeres gestantes requieren más yodo en la dieta que las no gestantes, mientras que el 43,6% reconoció que la deficiencia de yodo puede causar retraso mental en los infantes. Conclusiones: Se encontró una baja prevalencia de alteraciones en las pruebas de función tiroidea, mientras que la mediana de la yoduria indicó un adecuado consumo de yodo. También se encontró un conocimiento bajo acerca de la importancia del yodo en la nutrición (AU)

Inhibition of intrathyroidal dehalogenation by iodide.

Iodide is a trace element and a key component of thyroid hormones (TH). The availability of this halogen is the rate-limiting step for TH synthesis; therefore, thyroidal iodide uptake and recycling during TH synthesis are of major importance in maintaining an adequate supply. In the rat, the thyroid gland co-expresses a distinctive pair of intrathyroidal deiodinating enzymes: the thyroid iodotyrosine dehalogenase (tDh) and the iodothyronine deiodinase type 1 (ID1). In the present work, we studied the activity of these two dehalogenases in conditions of hypo- and hyperthyroidism as well as during acute and chronic iodide administration in both intact and hypophysectomized (HPX) rats. In order to confirm our observations, we also measured the mRNA levels for both dehalogenases and for the sodium/iodide symporter, the protein responsible for thyroidal iodide uptake. Our results show that triiodothyronine differentially regulates tDh and ID1 enzymatic activities, and that both acute and chronic iodide administration significantly decreases rat tDh and ID1 activities and mRNA levels. Conversely, both enzymatic activities increase when intrathyroidal iodide is pharmacologically depleted in TSH-replaced HPX rats. These results show a regulatory effect by iodide on the intrathyroidal dehalogenating enzymes and suggest that they contribute to the iodide-induced autoregulatory processes involved in the Wolff-Chaikoff effect.

Molecular cloning and characterization of a type 3 iodothyronine deiodinase in the pine snake Pituophis deppei.

The three distinct but related isotypes of the iodothyronine deiodinase family: D1, D2, and D3, have been amply studied in vertebrate homeotherms and to a lesser extent in ectotherms, particularly in reptiles. Here, we report the molecular and kinetic characteristics of both the native and the recombinant hepatic D3 from the pine snake Pituophis deppei (PdD3). The complete PdD3 cDNA (1680 bp) encodes a protein of 287 amino acids (aa), which is the longest type 3 deiodinase so far cloned. PdD3 shares 78% identity with chicken and 71% with its other orthologs. Interestingly, the hinge domain in D3s, including PdD3, is rich in proline. This structural feature is shared with D1s, the other inner-ring deiodinases, and deserves further study. The kinetic characteristics of both native and recombinant PdD3 were similar to those reported for D3 in other vertebrates. True K(m) values for T(3) IRD were 9 and 11 nM for native and recombinant PdD3, respectively. Both exhibited a requirement for a high concentration of cofactor (40 mM DTT), insensitivity to inhibition by PTU (>2 mM), and bisubstrate, sequential-type reaction kinetics. In summary, the present data demonstrate that the liver of the adult pine snake P. deppei expresses D3. Furthermore, this is the first report of the cloning and expression of a reptilian D3 cDNA. The finding of hepatic D3 expression in the adult pine snake P. deppei is consistent with results in adult piscine species in which the dietary T(3) content seems to regulate liver deiodinase expression. Thus, our present results support the proposal that hepatic D3 in adult vertebrates plays a sentinel role in avoiding an inappropriate overload of exogenous T(3) secondary to feeding in those species that devour the whole prey.

Cloning and characterization of a type 3 iodothyronine deiodinase (D3) in the liver of the chondrichtyan Chiloscyllium punctatum.

Thyroid hormone bioactivity is finely regulated at the cellular level by the peripheral iodothyronine deiodinases (D). The study of thyroid function in fish has been restricted mainly to teleosts, whereas the study and characterization of Ds have been overlooked in chondrichthyes. Here we report the cloning and operational characterization of both the native and the recombinant hepatic type 3 iodothyronine deiodinase in the tropical shark Chiloscyllium punctatum. Native and recombinant sD3 show identical catalytic activities: a strong preference for T3-inner-ring deiodination, a requirement for a high concentration of DTT, a sequential reaction mechanism, and resistance to PTU inhibition. The cloned cDNA contains 1298 nucleotides [excluding the poly(A) tail] and encodes a predicted protein of 259 amino acids. The triplet TGA coding for selenocysteine (Sec) is at position 123. The consensus selenocysteine insertion sequence (SECIS) was identified 228 bp upstream of the poly(A) tail and corresponds to form 2. The deduced amino acid sequence was 77% and 72% identical to other D3 cDNAs in fishes and other vertebrates, respectively. As in the case of other piscivore teleost species, shark expresses hepatic D3 through adulthood. This characteristic may be associated with the alimentary strategy in which the protection from an exogenous overload of thyroid hormones could be of physiological importance for thyroidal homeostasis.

Raspado y alisado radicular con y sin curetaje gingival. en pacientes del área de periodoncia de la Facultad de Odontología de la Universidad de El Salvador / Scaling and root planing with and without gingival curettage. in patients from the periodontics area of the Faculty of Dentistry of the University of El Salvador

Esta investigación de tipo cuasi experimental, evalúa las técnicas de raspado y alisado radicular con y sin curetaje gingival en pacientes del área de periodoncia de la Facultad de Odontología de la Universidad de El Salvador, durante el periodo de septiembre 2006 a febrero 2007, para establecer la técnica más idónea en el tratamiento de la enfermedad periodontal. Se utilizaron 20 cuadrantes dentales para cada técnica, seleccionando 3 piezas de cada uno, obteniendo un total de 120 piezas analizadas. Se registraron porcentajes de placa dentobacteriana, promedio de hemorragia gingival, profundidad de bolsa periodontal y el nivel de inserción clínica periodontal durante cuatro controles semanales posteriores a los tratamientos, obteniéndose que el 36.67% de las piezas tratadas con raspado y alisado radicular sin curetaje gingival logró una ganancia de inserción final de 1.5 a 2 mm y solo el 25% de las piezas tratadas con raspado y alisado radicular con curetaje gingival se mantuvo en ese rango. Al aplicar la prueba estadística "T Student" a los resultados finales se logró comprobar la hipótesis, estableciendo que no hay diferencia entre las técnicas utilizadas, ya que al no retirar la pared blanda de la bolsa periodontal se ejerce menos daño tisular disminuyendo con esto el promedio de hemorragia gingival y el tiempo de cicatrización durante la etapa de recuperación, por lo cual el raspado y alisado radicular sin curetaje gingival es el tratamiento idóneo para obtener ganancia de inserción clínica en la fase inicial no quirúrgica del tratamiento de la enfermedad periodontal.

This quasi-experimental research evaluates scaling and root planing techniques with and without gingival curettage in patients from the periodontics area of ​​the Faculty of Dentistry of the University of El Salvador, during the period from September 2006 to February 2007, to establish the most suitable technique in the treatment of periodontal disease. 20 dental quadrants were used for each technique, selecting 3 pieces of each one, obtaining a total of 120 pieces analyzed. Percentages of dentobacterial plaque, average gingival hemorrhage, periodontal pocket depth and level of periodontal clinical insertion were recorded during four weekly post-treatment controls, obtaining that 36.67% of the pieces treated with scaling and root planing without gingival curettage achieved a final insertion gain of 1.5 to 2 mm and only 25% of the pieces treated with scaling and root planing with gingival curettage remained in this range. By applying the statistical test "Student's T" to the final results, the hypothesis was verified, establishing that there is no difference between the techniques used, since by not removing the soft wall of the periodontal pocket, less tissue damage is exerted, thereby reducing the average gingival bleeding and healing time during the recovery stage, so scaling and root planing without gingival curettage is the ideal treatment to obtain clinical attachment gain in the initial non-surgical phase of periodontal disease treatment

Halometabolites and cellular dehalogenase systems: an evolutionary perspective.

We review the role of iodothyronine deiodinases (IDs) in the evolution of vertebrate thyroidal systems within the larger context of biological metabolism of halogens. Since the beginning of life, the ubiquity of organohalogens in the biosphere has provided a major selective pressure for the evolution and conservation of cellular mechanisms specialized in halogen metabolism. Among naturally available halogens, iodine emerged as a critical component of unique developmental and metabolic messengers. Metabolism of iodinated compounds occurs in the three major domains of life, and invertebrate deuterostomes possess several biochemical traits and molecular homologs of vertebrate thyroidal systems, including ancestral homologs of IDs identified in urochordates. The finely tuned cellular regulation of iodometabolite uptake and disposal is a remarkable event in evolution and might have been decisive for the explosive diversification of ontogenetic strategies in vertebrates.

The liver of Fundulus heteroclitus expresses deiodinase type 1 mRNA.

The presence of a type 1 deiodinase (D1) in the liver of teleosts has been a controversial issue. Recently we characterized the deiodinase activity in rainbow trout and killifish liver and found that the liver of both species co-expresses the two enzymes (D1 and D2) that catalyze the outer ring-deiodinating pathway. We here report the cloning and characterization of an mRNA from the liver of the killifish Fundulus heteroclitus that encodes a D1 (FhD1). The cDNA amplified by RT-PCR from F. heteroclitus liver is 1314 nt long and encodes a protein of 248 aa. It contains a TGA codon in its open reading frame and a selenocysteine insertion sequence in its 3(') untranslated region, consistent with the structure of a selenoenzyme mRNA. The deduced peptide sequence is 73% identical to that encoded by the tilapia D1 cDNA cloned from kidney and 46% identical to the D1s reported in other vertebrates. Northern blot analysis shows that FhD1 mRNA is expressed in F. heteroclitus liver, consistent with prior biochemical evidence for hepatic D1 activity. Furthermore, heterologous expression of the FhD1 cDNA resulted in a protein with properties similar to the D1-like activity in F. heteroclitus liver. The cloned enzyme, like the native species, is relatively insensitive to inhibition by PTU, but mutation of Ser-159 in FhD1 to the Pro residue found in D2 and D3 isoforms increased the sensitivity to PTU. Our results show that, under basal conditions, killifish liver indeed expresses a D1 enzyme that is homologous to mammalian D1s, establishing this as a useful model in which to study the regulation of D1 and D2 concurrently.

Environmental salinity selectively modifies the outer-ring deiodinating activity of liver, kidney and gill in the rainbow trout.

We here analyzed the effect of a mild hyperosmotic challenge on the activities of deiodinases type I (D1) and II (D2) in the trout liver, and D1 in kidney and gill, two organs involved in osmoregulation. FW-adapted immature rainbow trout were transferred to 5 per thousand SW and killed 0.5, 1, 2, 4, 8 12, 24 and 48 h post-transfer (PT). Fish maintained in FW served as controls. Hepatic, renal and branchial D1 and hepatic D2 activities were assessed as well as circulating levels of T(3), T(4) and cortisol. Hyperosmotic challenge elicited significant and sustained decreases in kidney D1 and liver D2 activities at 8 h PT, which returned to control values at 48 h PT. In contrast, liver and gill D1 activities exhibited no significant change throughout the study. Also, significant increases in circulating T(4) at 2-4 and 48 h PT were observed. Circulating T(3) remained unmodified until 24-48 h PT, when it rose sharply. Simultaneously, cortisol showed a trend towards increase during the initial 4 h PT, which attained significance at 48 h PT. The present findings demonstrate that a mild hypertonic challenge is sufficient to elicit responses in the trout thyroidal axis. Hormonal changes in the circulatory compartment are in accordance with those previously described for migratory salmonids. A novel aspect of our findings is the organ-specific differential response exhibited by ORD-enzymes when trout are exposed to a mildly different osmotic environment. Our findings further establish the uniqueness of fish thyroid physiology, and can be of value in further understanding the evolutionary aspects of this ORD family of deiodinases.