Nuclear receptors for L-triiodothyronine in quail liver

Sensitive periods for social development: Interactions between predisposed and learned mechanisms

We analysed research that makes use of precocial species as animal models to describe the interaction of predisposed mechanisms and environmental factors in early learning, in particular for the development of social cognition. We also highlight the role of sensitive periods in this interaction, focusing on domestic chicks as one of the main animal models for this field. In the first section of the review, we focus on the emergence of early predispositions to attend to social partners. These attentional biases appear before any learning experience about social stimuli. However, non-specific experiences occurring during sensitive periods of the early post-natal life determine the emergence of these predisposed mechanisms for the detection of social partners. Social predispositions have an important role for the development learning-based social cognitive functions, showing the interdependence of predisposed and learned mechanisms in shaping social development. In the second part of the review we concentrate on the reciprocal interactions between filial imprinting and spontaneous (not learned) social predispositions. Reciprocal influences between these two sets of mechanisms ensure that, in the natural environment, filial imprinting will target appropriate social objects. Neural and physiological mechanisms regulating the sensitive periods for the emergence of social predispositions and for filial imprinting learning are also described.

Thyroid hormones correlate with resting metabolic rate, not daily energy expenditure, in two charadriiform seabirds

Thyroid hormones affect in vitro metabolic intensity, increase basal metabolic rate (BMR) in the lab, and are sometimes correlated with basal and/or resting metabolic rate (RMR) in a field environment. Given the difficulty of measuring metabolic rate in the field—and the likelihood that capture and long-term restraint necessary to measure metabolic rate in the field jeopardizes other measurements—we examined the possibility that circulating thyroid hormone levels were correlated with RMR in two free-ranging bird species with high levels of energy expenditure (the black-legged kittiwake, Rissa tridactyla, and thick-billed murre, Uria lomvia). Because BMR and daily energy expenditure (DEE) are purported to be linked, we also tested for a correlation between thyroid hormones and DEE. We examined the relationships between free and bound levels of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) with DEE and with 4-hour long measurements of post-absorptive and thermoneutral resting metabolism (resting metabolic rate; RMR). RMR but not DEE increased with T3 in both species; both metabolic rates were independent of T4. T3 and T4 were not correlated with one another. DEE correlated with body mass in kittiwakes but not in murres, presumably owing to the larger coefficient of variation in body mass during chick rearing for the more sexually dimorphic kittiwakes. We suggest T3 provides a good proxy for resting metabolism but not DEE in these seabird species.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Birds and Its Role in Bird Development and Reproduction

This article reviews thyroid function and its hypothalamic-pituitary-thyroid (HPT) axis control in birds with emphasis on the similarities and differences in thyroid function compared to mammals and other vertebrate classes. Thyroid hormones are important in metabolism and the thermogenesis required for homeothermy in birds, as in mammals, the other homeothermic class of vertebrates. Thyroid hormones play important roles in development and growth in birds, as is the case for all vertebrate classes. The developmental effects of thyroid hormones in birds are presented in the context of differences in precocial and altricial patterns of development and growth with emphasis on oviparous development. The sections on thyroid hormone actions include discussion of effects on the development of a number of tissue types as well as on seasonal organismal processes and interactions of the thyroid axis with reproduction. The current picture of how environmental chemicals may disrupt avian thyroid function is relatively limited and is presented in the context of the assessment endpoints that have been used to date. These endpoints are categorized as thyroid and HPT axis endpoints versus target organ endpoints. The final section discusses two recommended assay protocols, the avian two-generation toxicity assay and the avian one-generation assay, and whether these protocols can evaluate thyroid disruption in birds.

Reprint of "Avian thyroid development and adaptive plasticity" [Gen. Comp. Endocrinol. 147, 93-101]

Precocial and altricial modes of avian development are characterized by different degrees of maturation and physiological capabilities at hatching. In precocial birds, thyroid function and its control are well developed during the latter part of incubation and hatchlings exhibit metabolic responses to cooling and relatively mature sensory and locomotor capabilities. In altricial birds, thyroid function shows little maturation until after hatch as also is the case for thermoregulatory, sensory, and motor functions. This review describes the patterns of precocial and altricial thyroid development, their hypothalamic-pituitary control, extrathyroidal control of hormone activation and deactivation, and target tissue effects during development. Our knowledge is greatest for precocial galliform birds although the organismal picture of thyroid development has been investigated in several altricial avian species.

Avian thyroid development and adaptive plasticity

Precocial and altricial modes of avian development are characterized by different degrees of maturation and physiological capabilities at hatching. In precocial birds, thyroid function and its control are well developed during the latter part of incubation and hatchlings exhibit metabolic responses to cooling and relatively mature sensory and locomotor capabilities. In altricial birds, thyroid function shows little maturation until after hatch as also is the case for thermoregulatory, sensory, and motor functions. This review describes the patterns of precocial and altricial thyroid development, their hypothalamic-pituitary control, extrathyroidal control of hormone activation and deactivation, and target tissue effects during development. Our knowledge is greatest for precocial galliform birds although the organismal picture of thyroid development has been investigated in several altricial avian species.

Temporal expression of thyroid hormone receptor ?1 in the liver of the lizardPodarcis sicula

The effects of thyroid hormones on metabolism and development are mediated by thyroid hormone receptors (TRs). To gain a better understanding of the potential role of thyroid hormone receptors in the liver of the lizard Podarcis sicula, we have evaluated the expression of TRs during the more critical periods of the annual variations of thyroid activity. The results obtained have indicated that in the liver of the lizard P. sicula there are three transcripts: mRNA of 5.0 kb for TRalpha1, mRNA of 2.6 kb for TRalpha2, and 6.0 kb band, which represent unprocessed heteronuclear RNA, encoding unspliced primary transcripts of RNA prior to their processing into the mature TRalpha1 and TRalpha2. By means of slot-blot, we are able to determine that there is a change in the expression of TRs that occurs in the liver during the annual cycle of thyroid activity. A major expression registers in May, when the lizard thyroid gland shows the maximal activity. The combination of molecular biology with immunohistochemistry revealed that hepatic cells were also TRalpha IR positive. Particularly intense immunostaining was present in the cell nuclei of animals sacrified in May. These observations suggest that in lizard P. sicula the thyroid hormone (T3) might regulate hepatic activity, modulating TR mRNA levels.

Hypothyroidism and nutritional status alter 3,5,3'-triiodothyronine (T3) receptors in isolated liver nuclei of Anabas testudineus (Bloch)

T3 receptors are ligand-dependent transcriptional activators that can stimulate gene expression resulting in growth and development. In this study T3 binding sites on isolated liver nuclei are demonstrated in the teleost Anabas testudineus. The effect of hypothyroidism and feeding of polyunsaturated fatty acids (PUFA) on T3 receptor number and binding affinity was also studied. Hypothyroidism caused an increase in T3 receptor number and a decrease in receptor affinity. PUFA feeding also increased the T3 receptor number without any significant change in its receptor affinity.

Thyroid function in embryonic and early posthatch chickens and quail

Technological advances, especially radioimmunoassays, have led to good descriptive information about the timing and pattern of development of the thyroid gland (TG) and circulating thyroid hormone (TH). Immunocytochemical studies in combination with more traditional techniques such as gland ablation and hormone replacement have revealed the time of appearance of each hormone in the axis, the relative amounts of hormone present in each gland at different developmental stages, and a general picture of the pattern of maturation of the hypothalamic-pituitary-thyroid axis. Studies of the pituitary control of the TG remain limited by the lack of adequately specific techniques for measuring avian thyroid-stimulating hormone. Our understanding of peripheral TH dynamics is progressing as a result of iodothyronine deiodinase assays but full understanding will require more elaborate studies that adequately characterize the enzyme system(s) in each case. Molecular techniques have made powerful strides in identifying the gene responsible for producing a protein that appears to be the triiodothyronine receptor. Receptor assays have revealed the pattern of changes in receptor-binding capacities during development but have not yet revealed how binding of hormone to the receptor triggers cellular activity. Molecular genetic techniques are being used to reveal the mechanisms whereby some examples of developmental events (e.g., malic enzyme synthesis) are induced by TH. Although it is not yet possible to assess the value of molecular studies in this area for developing practical applications, these techniques are revealing the fundamental biological roles of TH in growth and development.

Comparative thyroid function in adult Japanese quail and ring doves: influence of dietary iodine availability

Thyroid function was studied in Japanese quail, Coturnix japonica, and Ring doves, Streptopelia risoria, when both were fed the same dietary iodine (I; 930 micrograms I/kg). We also compared thyroid function in groups of doves receiving low I (less than 100 micrograms I/kg) or moderate I (930 micrograms I/kg). We measured thyroid gland (TG) weight, TG stable I content, TG 125I uptake, and 125I labeling of thyroid hormones. Triiodothyronine (T3) and thyroxine (T4) concentrations in TGs and serum were also determined. Our results indicate that doves and quail receiving the same dietary I show similar serum T3 (the presumed metabolically active hormone) and TG functional state but that there are some differences between the species in the way in which this functional state is achieved. We also assessed the effects of differences in I availability on thyroid function in doves. With low dietary I doves show decreases in some measures of thyroid function (reduced serum T4 and TG-hormone stores) compared to doves with moderate I but maintain a comparable level of serum T3. This regulation of T3 appears to be independent of serum T4 or TG-hormone stores.

Nuclear receptors for L-triiodothyronine in trout erythrocytes

We have developed an in vitro assay to evaluate saturable specific binding of triiodothyronine (T3) by erythrocyte (RBC) nuclei isolated from rainbow trout. Our results indicate that the nuclei contain a T3-saturable protein which binds T3 with temperature, and pH dependency, high T3 affinity (Ka = 1.6 X 10(9) M-1), and relative thyroid hormone (TH) analog affinities (TRIAC greater than T3 greater than T4 greater than rT3 greater than T2) which are characteristic of TH receptors in other vertebrates. Our estimate of the maximal T3 binding capacity (MBC) of nuclei isolated from heterogeneous populations of RBCs at different maturational stages (MBC = 3.6 fM/mg DNA; 13 sites/nucleus) was 10-100 times lower than would be expected of a TH-responsive tissue. Differential cell counts revealed that 1% of the RBCs in our trout were immature (pro-RBCs). Pro-RBCs, in contrast to mature RBCs, contain abundant heterochromatin, mitochondria, and polyribosomes, and synthesize hemoglobin. Evaluation of binding data for RBC nuclei taken from trout in which erythropoiesis was stimulated by prior bleeding indicated that MBC was directly proportional to the absolute number of pro-RBC nuclei in the incubation. Our maximum estimate of MBC for pro-RBC nuclei (458 fmol/mg DNA; 1781 sites/nucleus) falls within the range of MBC values reported for other vertebrate TH-responsive tissues. These data indicate that RBCs of rainbow trout contain a nuclear protein (putative receptor) which binds T3 with characteristics similar to the TH receptor of higher vertebrates, and that nuclear T3 binding may be diminished during RBC maturation.

Thyroid hormone binding to isolated trout (Salmo gairdneri) liver nuclei in vitro: binding affinity, capacity, and chemical specificity

Isolated nuclei from trout hepatocytes demonstrated in vitro high-affinity and low-capacity binding for 3,5,3'-tri-iodo-L-thyronine (T3). Binding was reversible; the dissociation rate was 5.7 X 10(-3) min-1 at 15 degrees C. Linear Scatchard plots suggested a single class of noncooperative sites (Kd = 1.4 +/- 0.10 X 10(-10) M; maximum binding capacity (MBC) = 62 +/- 10 fmol/mg DNA). After correction for site degradation, site occupancy by endogenous T3, and the dissociation rate of endogenous bound T3, the MBC was 106 +/- 16 fmol/mg DNA. The T3 affinity exceeded slightly that of the hepatocyte nucleus of the rat; the MBC was lower than for most other vertebrates. The relative binding affinities of thyroid hormone (TH) analogs for the T3 site were: TRIPROP greater than TRIAC greater than methyl-bridged T3 greater than TETRAPROP greater than T3 greater than TETRAC greater than TRIFORM greater than 3,5-dibromo, 3'-isopropyl thyronine greater than L-thyroxine (T4) greater than DL-T4 greater than 3'-isopropyl 3',5'-dimethyl thyronine greater than reverse T3 greater than 3,5-T2. MIT and DIT did not bind at all. This structure-affinity profile was similar but not identical to that of rat liver, indicating considerable but not complete evolutionary conservation of site structure. Parallel studies of T4 binding also indicate a single class of noncooperative sites (Kd = 7.2 +/- 2.4 X 10(-10) M). Both the MBC and the structure-affinity profile for T4 corresponded to those for T3. These observations, combined with the ability of excess T3 or T4 to completely displace both labeled T3 or T4, support a previous suggestion that in teleosts T3 and T4 bind to the same class of nuclear sites. These sites probably represent TH receptors.

Thyroid development in altricial ring doves, Streptopelia risoria

The development of thyroid function in altricial ring doves was assessed by measuring thyroid hormone concentrations in the serum, hormone content of the thyroid glands and hepatic 5'-monodeiodinase activity. Thyroid function is low at hatching and increases during the first eight days while the nestlings are ectothermic and completely dependent on parental care. The rate of increase of serum hormone concentrations slows after Day 8; hormone concentrations are stable by Day 15 and for the remainder of the nestling and early fledgling periods while locomotor ability, feeding self-sufficiency and thermoregulatory ability are maturing. Increases in serum hormone concentrations precede increases in thyroidal hormone content. T3/T4 ratios in serum are much higher than those of stored hormones in the thyroid. Deiodination of T4 to T3 is important in T3 production throughout development but most so in the early nestling stages.