Studies on thyroid hormones and their binding in bullfrog tadpole plasma during metamorphosis

Analyses of mRNA Expression Levels of Pituitary Hormones, Their Hypothalamic Regulating Factors, and Receptors Involved in Metamorphosis with Special Reference to the Summer and Winter Seasons

Bullfrog (Rana catesbeiana) larvae inhabiting the main island of Japan overwinter as preclimax animals, whereas the larvae that reached climax in summer complete metamorphosis. We analyzed the mRNA expression levels of the adenohypophyseal hormones, hypothalamic hormones, and their receptors that are involved in controlling metamorphosis in tadpoles at various developmental stages available in summer and winter in order to understand the hormonal mechanism regulating metamorphosis progression. Corticotropin-releasing factor (CRF) and thyrotropin β-subunit (TSHβ) mRNA expression was enhanced as they reached the climax stage in metamorphosing summer tadpoles, although type 2 CRF receptor (CRFR2) mRNA levels demonstrated a tendency of elevation, indicating the activation of the hypothalamo-hypophyseal axis for stimulating the release of thyroid hormone in summer. Arginine vasotocin (AVT) mRNA levels were elevated as metamorphosis progressed, but mRNA expression levels were not synchronized with those of proopiomelanocortin (POMC) and V1b-type AVT receptor (V1bR). The elevation of mRNA levels of prolactin (PRL) 1A and type 3 thyrotropin-releasing hormone receptor (TRHR3), but not of thyrotropin-releasing hormone (TRH) precursor mRNA levels, was noted in climactic tadpoles, indicating that PRL mRNA levels are not simply dependent on the expression levels of TRH precursor mRNA. In the preclimactic larvae captured in winter, which are in metamorphic stasis, mRNA levels of pituitary hormones, hypothalamic factors, and their receptors remained low or at levels similar to those of the larvae captured in summer. These results indicate the relationship between the mRNA expression of metamorphosis-related factors and the seasonal progression/stasis of metamorphosis.

Relationship between serum thyroid hormones and their associated metabolites, and gene expression bioindicators in the back skin of Rana [Lithobates] catesbeiana tadpoles and frogs during metamorphosis

Anuran metamorphosis is characterized by profound morphological changes including remodeling of tissues and organs. This transition is initiated by thyroid hormones (THs). However, the current knowledge of changing levels of THs during metamorphosis relies on pooled samples using methods known for high variability with sparse reporting of measured variation. Moreover, establishing a clear linkage between key gene expression bioindicators and TH levels throughout the metamorphic process is needed. Using state-of-the-art ultra-high performance liquid chromatography isotope-dilution tandem mass spectrometry, we targeted 12 THs and metabolites in the serum of Rana [Lithobates] catesbeiana (n=5-10) across seven distinct postembryonic stages beginning with premetamorphic tadpoles (Gosner stage 31-33) and continuing through metamorphosis to a juvenile frog (Gosner stage 46). TH levels were related to TH-relevant gene transcripts (thra, thrb, and thibz) in back skin of the same individual animals. Significant increases from basal levels were observed for thyroxine (T4) and 3,3',5-triiodothyronine (T3) at Gosner stage 41, reaching maximal levels at Gosner stage 44 (28 ± 10 and 2.3 ± 0.5 ng/mL, respectively), and decreasing to basal levels in juvenile frogs. In contrast, 3,5-diiodothyronine (T2) increased significantly at Gosner stage 40 and was maintained elevated until stage 44. While thra transcript levels remained constant and then decreased at the end of metamorphic climax, thrb and thibz were induced to maximal levels at Gosner stage 41, followed by a decrease to basal levels in the froglet. This exemplifies the exquisite timing of events during metamorphosis as classic early response genes are transcribed in anticipation of peak TH concentrations. The distinct T2 concentration profile suggests a biological role of this biomolecule in anuran postembryonic development and an additional aspect that may be a target of anthropogenic chemicals that can disrupt anuran metamorphosis and TH signalling. Hence, as a second aim of the study, we set out to find additional bioindicators of metamorphosis, which can aid future investigations of developmental disruption. Using a sensitive nanoLC-Orbitrap system an untargeted analysis workflow was applied. Among 6,062 endogenous metabolites, 421 showed metamorphosis-dependent concentration dynamics. These potential bioindicators included several carnitines, prostaglandins and some steroid hormones.

Oxidative Stress Parameters in Goitrogen-Exposed Crested Newt Larvae (Triturus spp.): Arrested Metamorphosis

Thiourea is an established disruptor of thyroid hormone synthesis and is frequently used as an inhibitor of metamorphosis. The changes caused by thiourea can affect processes associated with the oxidative status of individuals (metabolic rate, the HPI axis, antioxidant system). We investigated the parameters of oxidative stress in crested newt (Triturus spp.) larvae during normal development in late larval stage 62 and newly metamorphosed individuals, and during thiourea-stimulated metamorphosis arrest in individuals exposed to low (0.05%) and high (0.1%) concentrations of thiourea. Both groups of crested newts exposed to thiourea retained their larval characteristics until the end of the experiment. The low activities of antioxidant enzymes and the high lipid peroxidation level pointed to increased oxidative stress in larvae at the beginning of stage 62 as compared to fully metamorphosed individuals. The activities of catalase (CAT) and glutathione-S-transferase (GST) and the concentration of sulfhydryl (SH) groups were significantly lower in larvae reared in aqueous solutions containing thiourea than in newly metamorphosed individuals. The high thiourea concentration (0.1%) affected the antioxidative parameters to the extent that oxidative damage could not be avoided, contrary to a lower concentration. Our results provide a first insight into the physiological adaptations of crested newts during normal development and simulated metamorphosis arrest.

Amphibian Hormones, Calcium Physiology, Bone Weight, and Lung Use Call for a More Inclusive Approach to Understanding Ossification Sequence Evolution

Skeleton plays a huge role in understanding how vertebrate animals have diversified in phylogeny, ecology and behavior. Recent evo-devo research has used ossification sequences to compare skeletal development among major groups, to identify conserved and labile aspects of a sequence within a group, to derive ancestral and modal sequences, and to look for modularity based on embryonic origin and type of bone. However, questions remain about how to detect and order bone appearances, the adaptive significance of ossification sequences and their relationship to adult function, and the utility of categorizing bones by embryonic origin and type. Also, the singular focus on bone appearances and the omission of other tissues and behavioral, ecological and life history events limit the relevance of such analyses. Amphibians accentuate these concerns because of their highly specialized biphasic life histories and the exceptionally late timing, and high variability of their ossification sequences. Amphibians demonstrate a need for a whole-animal, whole-ontogeny approach that integrates the entire ossification process with physiology, behavior and ecology. I discuss evidence and hypotheses for how hormone mediation and calcium physiology might elicit non-adaptive variability in ossification sequence, and for adaptive strategies to partition larval habitats using bone to offset the buoyancy created by lung use. I also argue that understanding plasticity in ossification requires shifting focus away from embryonic development and adult function, and toward postembryonic mechanisms of regulating skeletal growth, especially ones that respond directly to midlife environments and behaviors.

Developmental regulation of gene expression in the thyroid gland of Xenopus laevis tadpoles

Thyroid hormones (TH) are the primary morphogen regulating amphibian metamorphosis. However, knowledge about molecular mechanisms regulating thyroid gland activity in anuran tadpoles is very scarce. In this study, we characterized gene expression profiles in thyroids of Xenopus laevis tadpoles during spontaneous metamorphosis. Using real-time PCR, elevated expression of slc5a5, tpo, tshr, and sar1a mRNAs was detected at late prometamorphic and climax stages. For dio2 and dio3 but not dio1, developmental regulation of thyroidal expression was evident from a strong up-regulation at late stages. Conversely, expression of the DNA replication markers mcm2 and pcna declined at climax stages. The presence of functional feedback mechanisms at premetamorphic stages was examined in two experiments. Stage 52 tadpoles were exposed for 72 h to 1.0 microg/l thyroxine (T4). This treatment caused reduced mRNA expression of slc5a5, tpo, and dio2, whereas no significant changes were detectable for tshr expression in thyroids and tshb expression in the pituitary. In another experiment, stage 46 tadpoles were treated with 20 mg/l sodium perchlorate (PER) for 5 and 10 days. Within this period of time, control tadpoles developed to stages 50 and 52, respectively. PER treatment resulted in up-regulation of slc5a5, tpo, and tshr mRNAs at both time points and increased dio2 mRNA expression at day 10. Effects of PER on thyroid histology were only apparent on day 10. Together, our analyses of thyroidal gene expression demonstrate a marked developmental regulation for functional markers of thyroid activity, two deiodinases as well as for DNA replication markers. Expression patterns detected in PER- and T4-treated tadpoles indicate that functional feedback signaling controlling thyroid activity is already active during premetamorphosis.

Thyroid hormones inhibit frog corticotropin-releasing factor-induced thyrotropin release from the bullfrog pituitary in vitro

Due to the lack of a radioimmunoassay (RIA) system for amphibian thyrotropin (TSH), no direct evidence that thyroid hormone suppresses the release of TSH from the amphibian pituitary has been obtained. However, we recently developed an RIA for bullfrog (Rana catesbeiana) TSH and thus were able to study the effect of thyroid hormone on the release of TSH from the bullfrog pituitary. Enzymatically dispersed pituitary cells of larval, juvenile, and adult bullfrogs were cultured in the absence or presence of 100 nM corticotropin-releasing factor of bullfrog origin (fCRF), which is known to be a potent stimulator of the release of TSH. The amount of spontaneously released TSH was higher in late prometamorphic and climactic tadpoles than in early prometamorphic larvae and juvenile and adult frogs. Pituitary cells from tadpoles at metamorphic climax responded to fCRF to release much more TSH than those from early and late prometamorphic tadpoles and juvenile and adult frogs. In all cases, the fCRF (100 nM)-induced, but not the basal, release of TSH was significantly suppressed by 1 nM triiodothyronine (T(3)) and 1000 nM thyroxine (T(4)), when examined using adult pituitary cells. The suppressive effect of thyroid hormones was revealed to be dependent on their concentrations.

Low-temperature arrest of the triiodothyronine-dependent transcription in Rana catesbeiana red blood cells

We examined possible molecular mechanisms for the low-temperature arrest of T3-induced Rana catesbeiana metamorphosis. Scatchard plots revealed that the ratios of maximum binding capacity/dissociation constant for high-affinity sites of tadpole serum proteins for T3 at 20 and 28 C was 3.3-4.6 times less than that at 4 C, due to the decrease in maximum binding capacity values. Kinetic studies of T3 uptake into tadpole red blood cells demonstrated that the ratio of maximum uptake rate/Michaelis constant at 23 C was approximately 13 times greater than that at 4 C. The process of intracellular transport of T3 into the nucleus was not arrested at 4 C. The ratio of T3 incorporated into nuclei to that taken up into red blood cells was not significantly different at 4, 20, and 28 C, indicating the absence of temperature-sensitive sites in this process. T3 binding to the T3 receptors alpha and beta were not temperature sensitive at least at 4 and 20 C. Transcription of the tr genes, early primary T3 response genes, was activated by 10 nM T3 at 20 and 28 C but was barely detected at 4 C. These results indicate that the major molecular event causing the low-temperature arrest of amphibian metamorphosis occurs after T3 entry into the nucleus but before or during the transcriptional activation of the tr genes. Plasma proteins binding T3 and the cellular thyroid hormone uptake system on the plasma membrane may contribute to the slowing of the incorporation of T3 into nucleus at 4 C by decreasing the uptake velocity of T3.

Development of radioimmunoassay for bullfrog thyroid-stimulating hormone (TSH): effects of hypothalamic releasing hormones on the release of TSH from the pituitary in vitro

A bullfrog (Rana catesbeiana) thyroid-stimulating hormone (TSH) beta-subunit (TSHbeta) antiserum was produced by employing a C-terminal peptide synthesized on the basis of the amino acid sequence deduced from bullfrog TSHbeta cDNA. Immunohistochemical studies revealed that the bullfrog adenohypophyseal cells that immunologically reacted with the anti-bullfrog TSHbeta corresponded to those positively stained with an antiserum against human (h) TSHbeta. The antiserum was used for the development of a specific and sensitive radioimmunoassay (RIA) for the measurement of bullfrog TSH. The sensitivity of the RIA was 0.75+/-0.07ng TSH/100microl assay buffer. The interassay and intraassay coefficients of variation were 7.6 and 5.3%, respectively. Several dilutions of pituitary homogenates of larval and adult bullfrogs, or medium in which bullfrog pituitary cells were cultured, yielded dose-response curves that were parallel to the standard curve. Bullfrog prolactin, growth hormone, luteinizing hormone, follicle-stimulating hormone, and alpha-subunit derived from glycoprotein hormones did not react in this assay. Immunoassayable TSH in the pituitary culture medium was confirmed to exist in the form of TSHbeta coupled with the alpha-subunit by an immunoprecipitation experiment using the TSHbeta antiserum and an alpha-subunit antiserum. TSH released from pituitary cells into the medium was also confirmed to possess a considerable activity in stimulating the release of thyroxine from the thyroid glands of larval bullfrogs in vitro. The effects of hypothalamic hormones such as mammalian gonadotropin-releasing hormone (mGnRH), ovine corticotropin-releasing hormone (oCRH), and thyrotropin-releasing hormone (TRH) on the release of TSH by dispersed anterior pituitary cells of the bullfrog larvae and adults were also studied. CRH markedly stimulated the release of TSH from both adult and larval pituitary cells. Both TRH and GnRH moderately stimulated the release of TSH from adult pituitary cells but not from the larval cells. This is the first report on the development of an RIA for amphibian TSH, which has provided the direct evidence that the release of TSH from the amphibian pituitary is enhanced by the hypothalamic releasing hormones such as CRH, TRH, and GnRH.

Developmental and diel changes in plasma thyroxine and plasma and ocular melatonin in the larval and juvenile bullfrog, Rana catesbeiana

Diel variation in plasma thyroxine (T(4)), and plasma and ocular melatonin was studied in Rana catesbeiana tadpoles and postmetamorphic froglets on 12:12 and 6:18 light/dark (LD) regimens. A progressive rise in plasma T(4) initiates metamorphosis while melatonin can modulate metamorphic progress. Changes in the phase of the rhythms of these two hormones during development might influence the hormonal regulation of metamorphosis. The hormones studied exhibited LD cycle-specific diel fluctuations except in froglet plasma T(4) and all hormones at prometamorphosis on 6L:18D. On 12L:12D, plasma T(4) and ocular melatonin peaked during the scotophase at prometamorphosis and early climax, whereas the plasma melatonin acrophase shifted from the light to the dark at climax. A nocturnal peak of plasma melatonin closely correlated with the onset and offset of dark appeared in the froglet, while the peak of ocular melatonin shifted to the light. Compared to 12L:12D, the peaks of the diel fluctuations on 6L:18D occurred later than on 12L:12D in synchrony with an earlier onset, and increase in length, of the scotophase. The phase of the hormone rhythms changed during metamorphosis in such a way that the peaks of melatonin had a different relationship to the T(4) peaks as development proceeded. On both LD cycles, the 24-h mean of plasma T(4) rose at climax and fell in the froglet whereas plasma melatonin decreased at climax and then rose to a high level in the froglet. After only minor changes during metamorphosis, froglet ocular melatonin levels decreased on 12L:12D and increased on 6L:18D. The findings indicate that the hormonal flux during metamorphosis has circadian aspects, which might explain variations in the response to exogenous hormone treatment at different times of the day and LD cycle-specific timing of development. A fall in plasma melatonin at climax appears to be as much a part of the hormonal changes of metamorphosis as a rise in plasma T(4).

Effect of diethylstilbestrol on thyroid hormone binding to amphibian transthyretins

Transthyretin (TTR) is responsible for a major part of the binding of thyroid hormone to proteins in the plasma in amphibian tadpoles. To characterize the binding properties of amphibian TTRs, the effects of 17 hydrophobic signaling molecules, including 6 endocrine disruptors, on 3,5,3'-l-[(125)I]triiodothyronine ([(125)I]T(3)) binding to plasma proteins were examined in bullfrog Rana catesbeiana tadpoles. T(3) was the most potent competitive inhibitor among the 11 natural biological ligands studied, with an ID(50) of 8 nM. Diethylstilbestrol (DES) was the most powerful inhibitor among the 6 endocrine disruptors studied, with an ID(50) of 20 nM. Similar inhibitions of [(125)I]T(3) binding by these compounds were obtained when purified recombinant Xenopus and Rana TTRs were analyzed. Scatchard analysis revealed that Xenopus and Rana TTRs each possessed a single class of binding site for T(3), with a K(d) of 262 and 1.9 nM, respectively, at 0 degrees C. DES, at a concentration of 200 nM, induced the uptake of [(125)I]T(3) into Rana red blood cells suspended in Rana plasma from prometamorphic stages XIII-XV, when TTR is present in plasma. DES induced the uptake of [(125)I]T(3) into red blood cells to a lesser extent when they were suspended in Rana plasma from metamorphic climax stage XXIV, in which the level of TTR was lower than in plasma from the prometamorphic tadpoles. These results indicate that amphibian TTRs have the ability to bind DES with similar affinity to T(3), the natural ligand, and raise the possibility that DES binding to TTR might induce the temporary elevation of the free concentration of plasma T(3) followed by acceleration of cellular T(3) uptake.

Cloning of bullfrog thyroid-stimulating hormone (TSH) beta subunit cDNA: expression of TSHbeta mRNA during metamorphosis

A thyroid-stimulating hormone beta subunit (TSHbeta) cDNA encoding both signal peptide and mature TSHbeta molecule was cloned from a cDNA library constructed from total RNA of the bullfrog (Rana catesbeiana) adenohypophysis. The bullfrog TSHbeta mRNA was estimated by Northern blot analysis to be approximately 1 kb. The deduced amino acid sequence showed 40-61% homologies with the sequences of TSH beta subunits of other vertebrates. Using the cDNA as a probe, we measured changes in mRNA expression in metamorphosing tadpoles of R. catesbeiana. The TSH beta subunit mRNA level increased progressively throughout prometamorphic stages, reaching its maximum at the end of prometamorphosis. The maximum level was maintained throughout early and mid climax, declining at late climax. These results, together with previously obtained data on plasma prolactin and pituitary prolactin mRNA levels, as well as thyroid hormone levels, are discussed in relation to metamorphic changes occurring in the bullfrog larvae.

Metamorphosis and the amphibian immune system

Studies of the ontogeny of immunity in a limited number of representative amphibians have shown that while the immune systems of the larval forms are competent to defend against potential pathogens in the temporary ponds they inhabit, they are not equivalent to the mature immune systems that develop after metamorphosis. Metamorphosis is a critical time of transition when increased concentrations of metamorphic hormones, principally thyroid hormones (TH) and corticosteroid hormones (CH), orchestrate the loss or reorganization of many tissues and organ systems, including the immune system. Immune system reorganization may serve to eliminate unnecessary lymphocytes that could be destructive if they recognized newly emerging adult-specific antigens on the adult tissues. Increased corticosteroids during metamorphosis appear to induce apoptosis of susceptible lymphocytes. This cell death can be inhibited in vitro or in vivo by the corticosteroid receptor antagonist, RU486. A coordinate increase in both TH and CH at metamorphosis may be common to all amphibians that undergo metamorphosis. Current evidence suggests that the central hypothalamic mediator that induces pituitary production of both thyroid-stimulating hormone and adrenocorticotropic hormone in larval amphibians is corticotropin-releasing hormone. Most amphibians probably survive the temporary immunosuppression associated with metamorphosis with no deleterious effects. However, it is hypothesized that if environmental stressors result in the induction of metamorphosis at a less than optimal body size and state of immune maturation, the immune system destruction would be more significant, and the amphibians could be at greater risk of infection and death.

Distribution and reciprocal interactions of 3H-melatonin and 125I-thyroxine in peripheral, neural, and endocrine tissues of bullfrog tadpoles

Tissue distribution of 125I-thyroxine (T4) and 3H-melatonin and the effect of each hormone on the tissue content of the other were studied because previous work indicated that melatonin antagonized metamorphosis through peripheral, as well as thyroidal effects. Late pre- to prometamorphic Rana catesbeiana tadpoles on an 18 light:6 dark cycle were used for injection of hormones in vivo or to supply tissues for in vitro hormone administration. Labeled melatonin uptake was highest in intestine, ventral skin and pituitary; lowest in thyroid and brain and intermediate in hindlimb, tail and gills. The tissue content of labeled T4 was distributed in nearly the same way, except that the thyroid level was relatively higher, and pituitary lower, than that of labeled melatonin. The pineal, studied only in the tracer T4 experiments, had the highest content of labeled T4 of all tissues. Simultaneous injection of either 0.007 or 0.2 microgram T4 increased 3H-melatonin uptake into peripheral tissues that undergo major metamorphic changes but not into neural or endocrine organs. In contrast, 0.033, 3.75 or 15 micrograms melatonin had no significant influence on the content of 125I-T4 in any tissue studied in vivo. Results of in vitro labeling of selected tissues were generally in agreement with the in vivo work except that the 125I-T4 content of intestinal segments from late prometamorphic larvae was lower in melatonin-treated than in control groups. The results suggest that peripheral tissues are a major site for T4-melatonin interactions and that T4 may modulate its own action through influencing melatonin levels in target tissues and perhaps in the thyroid. Because melatonin had no effect on tissue T4 content in young tadpoles, retardation of metamorphic events by melatonin does not seem to involve modulation of T4 availability to the tissues.

Changes in thyroid hormone concentrations and total contents through ontogeny in three anuran species: evidence for daily cycles

Three anuran species (Rana perezi, Xenopus laevis, and Bufo calamita) of different phylogenetic origins and ecological habitats have been studied during ontogeny with respect to day/night changes in whole-body concentrations and total content of extrathyroidal thyroxine (T4) and triiodothyronine (T3). There were no significant day/night changes in thyroid hormones (TH) during embryonic stages. Daily cycles in TH with higher nocturnal values appeared during premetamorphosis in R. perezi and X. laevis. Cyclicity disappears for T3, while it is reversed for T4, in prometamorphic R. perezi and X. laevis. In contrast, there were significantly higher T3 (0.74 +/- 0.13 ng/g) and T4 (2.08 +/- 0.54 ng/g) levels at night in prometamorphic B. calamita. Significant daily changes in T3 and T4 with higher nocturnal values (T3, 788.29 +/- 118.38 pg/g; T4, 1.95 +/- 0.4 ng/g) were again seen in X. laevis at the end of climax, while in B. calamita low TH values appeared at early scotophase and there were no significant changes in R. perezi at this time. Similar daily profiles were observed for TH whole-body concentrations and total contents.

Effects of exogenous triiodothyronine on fast axonal transport during tadpole metamorphosis

Bullfrog tadpoles at metamorphic stages V, X and XVIII were immersed in 25 nM triiodothyronine (T3) to assess whether the 4-5 fold increase in fast axonal transport (FAxT) previously observed during this span of spontaneous metamorphosis (1) could be mimicked by precocious application of thyroid hormone. The trend initially observed was for T3 to stimulated [35S]methionine incorporation into lumbar DRG and inhibit incorporation in tail DRG. Both effects, however, appeared to be exerted primarily on satellite cells rather than neurons since most of the T3-induced changes in DRG were of a similar magnitude to those in the respective nerve trunks. Findings consistent with this observation resulted from use of the retrogradely transported lectin, ricin120, to determine the proportion of DRG incorporation occurring in neurons. When incorporation of [35S]methionine in lumbar DRG neurons was examined, T3 had no stimulatory effect at any of the metamorphic stages examined. When FAxT was assessed as [35S]protein accumulating proximal to a nerve trunk ligature, and expressed as a percentage of newly-synthesized protein in lumbar DRG neurons, no stimulatory effect of T3 was detected. The question remains whether the changes in FAxT in peripheral neurons observed during spontaneous metamorphosis may be induced by circulating hormones other than T3 or are secondary to changes in the target tissues.

Skeletal morphogenesis in the urodele skull: III. Effect of hormone dosage in TH-induced remodeling

This study examines the dosage dependency of thyroid hormone (TH)-mediated remodelling in the cranial skeleton of the hemidactyliine plethodontid urodele, Eurycea bislineata. One set of experiments quantifies morphogenetic responses in 21 tissues for four size-age classes of larvae immersed in four different T₄ concentrations. A second set varies both the period and concentration of T₄ treatment to evaluate the effect of different TH profiles on adult tissue shape. The tissues surveyed in this study exhibit a 100-fold range in TH sensitivity. Those in regressive morphogenesis have tissue-specific sensitivities which correlate with the timing of their remodelling in natural development: bone resorption is more sentitive than cartilage resorption and is initiated earlier in metamorphosis. In contrast, the TH sensitivities of tissues in progressive morphogenesis vary within each tissue type and even within some tissues, and they do not correlate with timing in natural development. Some explanation for this discrepancy is offered by the constant spatial and temporal relationships between nasal cartilage and dermal bone, which suggest that some TH-mediated ossification may additionally require induction by cartilage. Also, the failure of nasolacrimal duct morphogenesis at all but the lowest dosage correlates with the inductdion of integumentary changes that may preclude duct formation. Variable T₄ treatments produce no effect upon the adult skull, other than loss of the nasolacrimal duct and/or foramen. These results have two developmental implicatons. First, the dosage dependencies of the nasolacrimal duct, ossification sequences, and cranial remodelling patterns all support a TH profile with exceptionally low levels at larval stages and at least a 100-fold increase at metamorphosis. Second, a small change in the rate of TH activity has the potential to effect a large-scale rearranggement and restructuring of TH-dependent remodelling. The lack of such transformations in metamorphic plethodontids suggests that TH activity is highly conserved in this group. © 1995 Wiley-Liss, Inc.

Anterior pituitary and adrenal cortical hormones accelerate or inhibit tadpole hindlimb growth and development depending on stage of spontaneous development or thyroxine concentration in induced metamorphosis

The effect of prolactin, growth hormone, and various adrenal corticoids on hindlimb growth, development, and differentiation was studied in Rana pipiens larvae. Experiments were performed at different stages of spontaneous development and during metamorphosis induced in premetamorphic tadpoles by various concentrations of exogenous T4. Prolactin at 10 micrograms/day inhibited the limb at spontaneous premetamorphosis, had no effect at prometamorphosis or when administered with 3.8 nM T4, and synergized with T4 at 63 nM T4 and above. Growth hormone (10 or 20 micrograms/day) promoted limb growth and development during premetamorphosis but had no effect on spontaneous or induced metamorphosis thereafter, nor did it stimulate limb epidermal differentiation. The adrenal corticoids inhibited limb growth and epidermal cell proliferation during pre- and prometamorphosis but had no effect on limb morphogenesis or differentiation. The depressive effect of corticoids during spontaneous metamorphosis is at least partly through thyroid inhibition since hydrocortisone significantly reduced follicle cell height, lumen diameter, and cell proliferation in the thyroid. During induced metamorphosis, steroids (0.29 microM), especially corticosterone and aldosterone, antagonized the effect of 0.38 to 1.2 nM T4 on the limb. All steroids except deoxycorticosterone synergized with 3.8 nM T4, and at 31 nM T4, approximating the climax level with permeability factors taken into account, all corticoids synergized with T4 to promote limb growth and development. Aldosterone antagonized T4 at a higher T4 level than the other corticoids. The effect of all steroids except corticosterone was also corticoid dose-dependent. The results show the importance of the T4 concentrations in interactions of T4 with other hormones and suggest a scheme for hormonal control of limb growth and morphogenesis during metamorphosis. During premetamorphosis growth hormone synergizes with low endogenous T4 to promote initial limb growth and development while prolactin opposes this action. During prometamorphosis, as growth hormone and prolactin become ineffective corticosteroids begin to synergize with the rising level of endogenous T4. At climax, prolactin also augments the action of T4 to bring about rapid hindlimb growth.

Asynchrony of changes in tissue and plasma thyroid hormones during the parr-smolt transformation of coho salmon

The relationship between plasma thyroid hormone concentrations and the thyroid hormone concentrations in selected tissues was examined throughout the spring during the typical course of parr-smolt transformation in coho salmon (Oncorhynchus kisutch) in fresh water and also in coho salmon moved prematurely to seawater. The thyroid hormones thyroxine (T4) and triiodothyronine (T3) were extracted from brain, liver, and muscle tissue. The T4 and T3 concentrations in the extracts and plasma were measured by radioimmunoassay. The peak in plasma T4 occurred in late April; however, the concentration of T4 in the brain and liver increased before levels of T4 in plasma increased. During the rise in plasma T4, the T4 content in muscle decreased. Plasma T3 concentrations were unchanged in March and April, but decreased in May. Transfer to seawater eliminated the late April peak in plasma T4 levels, indicating suppressed thyroid activity; however, the tissues of salmon in seawater contained more T3 than tissues of salmon in fresh water at this time. These findings indicate complex peripheral regulation of thyroidal status in this teleost and represent the first bridge between compartmental models of thyroid hormone kinetics and actual measurement of tissue pools of thyroid hormones in an ectothermic vertebrate. In summary, tissue concentrations of thyroid hormones did not echo plasma concentrations, indicating that thyroidal status cannot be inferred from plasma data alone.

Binding of thyroxine and 3,5,3'-triiodothyronine to trout plasma lipoproteins

The plasma vectors of thyroid hormones (TH) in trout have been characterized. Plasma components were separated by density gradient ultracentrifugation after first labeling binding sites with trace levels of radioactive hormones, both in vivo and in vitro. Lipoproteins play only a minor role in humans but are major carriers of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) in trout plasma. More than 67% of T4 and 89% of T3 were bound to lipoproteins (density less than 1.210 g/ml), predominantly to high-density lipoproteins (HDL), regardless of the nutritional status of the animals. The percentage of hormone bound to very-low-density lipoproteins, on the other hand, was proportional to their concentration and thus to nutritional status. T3 and T4 could also bind to vitellogenin, a very-high-density lipoprotein, which could transfer TH to the yolk of oocytes. Homologous ligand displacement indicated that T3 could bind to at least two classes of saturable sites in the plasma. In addition, plasma HDL were the major binding sites with low affinity (1.7 +/- 0.4 x 10(5) M-1) but with high capacity (3.1 +/- 0.3 x 10(-5) M). In conclusion, these results show that lipoproteins are the principal binding sites of TH in trout plasma.

Sequential up-regulation of thyroid hormone beta receptor, ornithine transcarbamylase, and carbamyl phosphate synthetase mRNAs in the liver of Rana catesbeiana tadpoles during spontaneous and thyroid hormone-induced metamorphosis

During both spontaneous and thyroid hormone (TH)-induced metamorphosis, the Rana catesbeiana tadpole undergoes postembryonic developmental changes in its liver which are necessary for its transition from an ammonotelic larva to a ureotelic adult. Although this transition ultimately results from marked increases in the activities and/or de novo synthesis of the urea cycle enzymes, the precise molecular means by which TH exerts this tissue-specific response are presently unknown. Recent reports, using RNA from whole Xenopus laevis tadpole homogenates and indirect means of measuring TH receptor (TR) mRNAs, suggest a correlation between the up-regulation of TR beta-mRNAs and the general morphological changes occurring during amphibian metamorphosis. To assess whether or not this same relationship exists in a TH-responsive tissue, such as liver, we isolated and characterized a cDNA clone containing the complete nucleotide sequence for a R. catesbeiana urea cycle enzyme, ornithine transcarbamylase (OTC), as well as a genomic clone containing a portion of the hormone-binding domain of a R. catesbeiana TR beta gene. Through use of these homologous sequences and a heterologous cDNA fragment encoding rat carbamyl phosphate synthetase (CPS), we directly determined the relative levels of the TR beta, OTC, and CPS mRNAs in liver from spontaneous and TH-induced tadpoles. Our results establish that TH affects an up-regulation of mRNAs for its own receptor prior to up-regulating CPS and OTC mRNAs. Moreover, results with cultured tadpole liver demonstrate that TH, in the absence of any other hormonal influence, can affect an up-regulation of both the TR beta and OTC mRNAs.

The thyroid hormone receptor gene (c-erbA alpha) is expressed in advance of thyroid gland maturation during the early embryonic development of Xenopus laevis

The c-erbA proto-oncogene encodes the thyroid hormone receptor, a ligand-dependent transcription factor which plays an important role in vertebrate growth and development. To define the role of the thyroid hormone receptor in developmental processes, we have begun studying c-erbA gene expression during the ontogeny of Xenopus laevis, an organism in which thyroid hormone has well-documented effects on morphogenesis. Using polymerase chain reactions (PCR) as a sensitive assay of specific gene expression, we found that polyadenylated erbA alpha RNA is present in Xenopus cells at early developmental stages, including the fertilized egg, blastula, gastrula, and neurula. By performing erbA alpha-specific PCR on reverse-transcribed RNAs from high-density sucrose gradient fractions prepared from early-stage embryos, we have demonstrated that these erbA transcripts are recruited to polysomes. Therefore, erbA is expressed in Xenopus development prior to the appearance of the thyroid gland anlage in tailbud-stage embryos. This implies that erbA alpha/thyroid hormone receptors may play ligand-independent roles during the early development of X. laevis. Quantitative PCR revealed a greater than 25-fold range in the steady-state levels of polyadenylated erbA alpha RNA across early stages of development, as expressed relative to equimolar amounts of total embryonic RNA. Substantial increases in the levels of erbA alpha RNA were noted at stages well after the onset of zygotic transcription at the mid-blastula transition, with accumulation of erbA alpha transcripts reaching a relative maximum in advance of metamorphosis. We also show that erbA alpha RNAs are expressed unequally across Xenopus neural tube embryos. This differential expression continues through later stages of development, including metamorphosis. This finding suggests that erbA alpha/thyroid hormone receptors may play roles in tissue-specific processes across all of Xenopus development.

The thyroid hormone receptor gene (c-erbA alpha) is expressed in advance of thyroid gland maturation during the early embryonic development of Xenopus laevis

The c-erbA proto-oncogene encodes the thyroid hormone receptor, a ligand-dependent transcription factor which plays an important role in vertebrate growth and development. To define the role of the thyroid hormone receptor in developmental processes, we have begun studying c-erbA gene expression during the ontogeny of Xenopus laevis, an organism in which thyroid hormone has well-documented effects on morphogenesis. Using polymerase chain reactions (PCR) as a sensitive assay of specific gene expression, we found that polyadenylated erbA alpha RNA is present in Xenopus cells at early developmental stages, including the fertilized egg, blastula, gastrula, and neurula. By performing erbA alpha-specific PCR on reverse-transcribed RNAs from high-density sucrose gradient fractions prepared from early-stage embryos, we have demonstrated that these erbA transcripts are recruited to polysomes. Therefore, erbA is expressed in Xenopus development prior to the appearance of the thyroid gland anlage in tailbud-stage embryos. This implies that erbA alpha/thyroid hormone receptors may play ligand-independent roles during the early development of X. laevis. Quantitative PCR revealed a greater than 25-fold range in the steady-state levels of polyadenylated erbA alpha RNA across early stages of development, as expressed relative to equimolar amounts of total embryonic RNA. Substantial increases in the levels of erbA alpha RNA were noted at stages well after the onset of zygotic transcription at the mid-blastula transition, with accumulation of erbA alpha transcripts reaching a relative maximum in advance of metamorphosis. We also show that erbA alpha RNAs are expressed unequally across Xenopus neural tube embryos. This differential expression continues through later stages of development, including metamorphosis. This finding suggests that erbA alpha/thyroid hormone receptors may play roles in tissue-specific processes across all of Xenopus development.

Gonadal hormones inhibit the induction of metamorphosis by thyroid hormones in Xenopus laevis tadpoles in vivo, but not in vitro

Although the major hormones controlling amphibian metamorphosis are those of the thyroid, other hormones, notably prolactin and the adrenal steroids, modulate the effects of thyroid hormones (TH). Some authors report that the gonadal steroids stimulate the metamorphic actions of TH whereas others report inhibition. The aims of the present study were to determine the effects of gonadal steroids on TH-induced metamorphosis in Xenopus laevis and to determine the site of action of these steroids. In all cases, hormones were added to the water in which the tadpoles were swimming. The gonadal steroids, testosterone and 17 beta-estradiol, inhibited triiodothyronine (T3)-induced metamorphosis in living, premetamorphic tadpoles of X. laevis. Both steroids, at 3.4 microM, prevented the reduction in body weight and the shrinkage of head and alimentary canal brought about by 1 nM T3. In contrast, 3.4 microM corticosterone stimulated T3-induced metamorphosis. Addition of 100 nM T3 to the medium induced a large reduction in size of X. laevis tails cultured in vitro. The antagonistic effects of testosterone were not reproduced in such cultures, whereas the synergistic action of corticosterone was maintained. Testosterone had no effect upon the specific binding of T3 to X. laevis tail tissue, whereas corticosterone increased such binding. These findings indicate that, while corticosterone stimulates the metamorphic actions of T3 by acting directly in the peripheral tissues, the gonadal steroids, particularly testosterone, inhibit T3 by acting at a more central site. Prolactin is known to antagonize the metamorphic actions of T3 and one such central action could be the stimulation of prolactin synthesis. However, testosterone inhibited the prometamorphic actions of bromocriptine, which stimulates metamorphosis by inhibiting production of prolactin. Thus the central action of testosterone is unlikely to be a stimulation of prolactin production.

Neuronal overload in the developing anuran lateral motor column in response to limb removal and thyroid hormone

The lateral motor column (LMC) in the anuran spinal cord normally undergoes a dramatic reduction in motor neuron number during development. At least two factors influence this process: the limb target which is required for the progression of cell loss, and thyroid hormone, a requisite for metamorphosis. This study has examined the relative and combined effects of limb amputation and exogenous thyroxine, initiated at the onset of normal rapid cell loss in Rana pipiens tadpoles, in regulating neuron number in the lumbosacral LMC. Thyroxine treatment or unilateral limb amputation temporarily resulted in significantly more LMC neurons than in untreated controls. Extraordinary numbers of motor neurons persisted through metamorphic climax when both treatments were combined. Population sizes frequently exceeded the maximum number of neurons observed prior to the onset of natural cell loss. Moreover, thyroxine-treated tadpoles contained increased numbers of mitotic figures in the ventricular zone of the spinal cord and significantly more newly generated cells in the LMC, as revealed by 3H-thymidine autoradiography. These findings suggest that thyroxine-potentiated mitogenesis promotes greater numbers of new motor neurons to the LMC while, simultaneously, target removal delays the loss of extant cells. It is proposed that this interaction effectively maintains an immature state in the LMC so that neuronal "decisions" for survival and the consequent loss of target-deprived neurons are postponed far longer than previously reported.

Premetamorphic effects of thyroid hormones on tadpole sensory ganglia

Tadpoles at premetamorphic stages of development were used to compare the precocious responses of lumbar dorsal root ganglia (DRG) and hindlimb bud (tissues destined for growth) with the responses of tail DRG and tail muscle (tissues destined for resorption) following exogenous administration of triiodothyronine (T3) and thyroxine (T4). Responses to intraperitoneal (i.p.) hormone treatment were assessed at varying times by injection of [3H]leucine i.p. and determination of 3H-labeled protein in tissues after an additional 1.5 hr. Incorporation of [3H]leucine in lumbar DRG and hindlimb bud was markedly stimulated by either hormone. T3 and T4 effects were both maximal at 0.3 nmol/g body wt although, as examined in lumbar DRG, the response to T4 was more rapid and of lesser magnitude than that to T3. By contrast, incorporation in tail DRG and tail muscle was significantly depressed in response to T3 and was unaffected by T4. Co-injection of T3 and T4 (either 1:1 or 1:6 as occurs during the peak surge of circulating thyroid hormones during metamorphic climax) did not produce an additive effect; the hindlimb bud response was reduced while the lumbar DRG, tail DRG and tail muscle responses to the individual hormones were virtually eliminated. The present data suggest that the responses of lumbar and tail sensory neurons to thyroid hormones parallel the responses of their peripheral target tissues.

Immunohistochemical localization of thyrotropic cells during amphibian morphogenesis: a stereological study

TSH-like producing cells have been studied using the peroxidase-antiperoxidase (PAP) immunohistochemical technique and rabbit anti-human thyrotropin (TSH) antiserum in the pars distalis of five species of amphibian anuran tadpoles at different stages of development as well as in 1-year-old postmetamorphic animals. This cell type has been identified and located in the pars distalis ventral zone of Bufo calamita showing no changes in its distribution pattern throughout development, and in Rana perezi where TSH-like cells were located in the central zone of the gland in premetamorphic stages, occupying the ventral zone at the end of metamorphosis. No immunoreaction has been observed in the other species studied (Hyla meridionalis, Alytes cisternasii, and Pelobates cultripes). Morphometry and stereology were used to evaluate the changes observed in pars distalis volume and TSH immunoreactive cells during development. Pars distalis volume increased during the larval growth period and decreased throughout the metamorphic climax. TSH volume density and cellular area showed different evolutions in the species studied. The correlation between these parameters suggests a greatly proliferative period followed by an increase in cellular size. The changes observed in TSH total volume as well as the physiological data found by other authors lead us to conclude that there are two phases in amphibian development: first a period of TSH storage (pre- and prometamorphosis) and second a period of TSH release at metamorphic climax.

Triiodothyronine increases translatable albumin messenger RNA in Rana catesbeiana tadpole liver

The effects of both 3,5,3'-triiodo-L-thyronine and spontaneous metamorphosis on Rana catesbeiana liver mRNA were studied using in vitro translation of isolated liver poly(A)+ RNA in a rabbit reticulocyte lysate system. Conventional phenol extraction methods yielded degraded RNA due to high levels of endogenous ribonucleases released upon homogenization of Rana catesbeiana liver. Isolation of intact total RNA was achieved using the potent ribonuclease denaturant, guanidinium thiocyanate. Adult bullfrog serum albumin was purified to homogeneity and a monospecific antibody was elicited against it. A serum protein of 23,000 daltons that migrated near serum albumin on a 6% native gel was also purified to homogeneity. A monospecific antibody was also raised against this protein. Both antibodies were used to quantitatively immunoprecipitate the in vitro translation products of poly(A)+ RNA isolated at intervals following a single injection of triiodothyronine or during various stages of spontaneous amphibian metamorphosis. Triiodothyronine caused a sevenfold increase in translatable albumin mRNA and a threefold increase in translatable mRNA for the 23,000 dalton protein. These increases are consistent with a nuclear initiated mechanism for thyroid hormone action during amphibian metamorphosis.

Evidence for hepatic involvement in the regulation of amphibian development by prolactin

Hormonal control of amphibian development involves thyroid hormones (TH), which promote metamorphosis, and prolactin (PRL), which antagonizes the effects of TH and promotes larval growth. Although the liver is not considered to be a regulator of developmental processes such as metamorphosis, it secretes a PRL-synergizing factor (synlactin) in response to PRL. We explored the possibility that the liver may participate in the antimetamorphic actions of PRL in Rana catesbeiana. Bullfrog tadpoles, in which release of endogenous PRL was suppressed by injections of bromocryptine to induce metamorphic changes including tail regression, received hormone-containing implants in various sites. PRL implants in the spleen to deliver hormone directly to the liver via the hepatic portal drainage not only prevented tail regression but actually caused a substantial increase in the height of the tail fin. PRL implanted in other sites or GH implanted in the spleen was much less effective. The liver of animals with intrasplenic PRL implants secreted more synlactin in vitro than that of tadpoles with subcutaneous PRL implants. Young grass frogs were injected with ovine (o) GH or oPRL to determine effects on hepatic synlactin secretion. Although the GH stimulated body growth it did not induce the liver to secrete synlactin. By contrast, PRL treatment did stimulate hepatic secretion of synlactin without stimulating body growth. These results indicate that the liver of pre- and postmetamorphic animals can be stimulated by PRL to secrete synlactin. Furthermore, the antimetamorphic actions of PRL in tadpoles appears to be mediated, at least in part, by an action on the liver. Synlactin may mediate this hepatic effect.

Regulation by thyroid hormones of terminal differentiation in the skeletal dorsal muscle. II. Urodelan amphibians

In the urodelan amphibian Pleurodeles waltlii, spontaneous anatomical metamorphosis was correlated with an increase in the serum level of thyroxine (T4). It was also accompanied by a change in the myofibrillar ATPase profile of the dorsal skeletal muscle; fibers of larval type were gradually replaced by the adult fiber types I, II A, and II B. Likewise, a myosin isoenzymic transition was observed in dorsal muscle, larval isomyosins were replaced by adult isoforms. In a related species, Ambystoma mexicanum, in which no spontaneous external metamorphosis occurs under standard conditions, the serum T4 level was shown to remain low. During further development, the myofibrillar ATPase profile acquired the adult fiber types, but a high percentage of immature fibers of type II C persisted. Myosin isoenzymic transition was also incomplete; larval isoforms were still distinguished in the neotenic adults. In experimental hypothyroidian P. waltlii, no external metamorphosis occurred; the myofibrillar ATPase profile was of the immature type, and the larval isomyosins persisted. Triiodothyronine induced experimental anatomical metamorphosis in A. mexicanum; only limited changes in the myofibrillar ATPase profile resulted from the treatment, but a complete myosin isoenzymic transition was observed. These results tend to indicate that a moderate increase in the level of thyroid hormone is sufficient to induce the differentiation of adult fiber types, together with the production of adult myosin isoforms in the skeletal dorsal muscle of amphibians, while a pronounced increase would be necessary for repressing the initial larval features.

Adenohypophysial-thyroid activity of the tiger salamander, Ambystoma tigrinum, as a function of metamorphosis and captivity

The aim of this study was to determine the timing of adenohypophysial activation during metamorphosis of the tiger salamander, Ambystoma tigrinum. It consisted of two parts: 1) determination of plasma thyroid hormone concentrations and analysis of thyroid gland histology as a function of metamorphic stage and 2) analysis of the time-course of uptake of 125I by the thyroids during metamorphosis as an indicator of endogenous thyrotropin (TSH) levels. Significant increases in both triiodothyronine (T3) and thyroxine (T4) first were evident at the onset of metamorphic climax (stage II). Maximum levels of both hormones were not observed, however, until the completion of gill resorption (stage VII). No changes in thyroid histology were observed that could be unambiguously related to metamorphic transformation. The thyroids accumulated 125I in a slow but linear fashion in premetamorphic larvae (stage I). However, uptake exhibited a rapid peak during early climax (stage II), before maximum concentrations of thyroid hormones were observed. In addition, uptake was maintained above premetamorphic levels at stage VII, in conjunction with maximum levels of T4 and T3. Captivity alone produced a small but significant increase in plasma concentrations of T3. It produced no significant effect on either thyroid histology or uptake of 125I. These results indicate that adenohypophysial activation occurs rapidly and is maximal at the onset of metamorphic climax.

Immunofluorescent detection and localization of thyroxine in blood of adult amphibians

Indirect immunofluorescent staining was used to detect and localize thyroxine (T4) in blood smears from different species of adult amphibians, namely, Rana pipiens, Rana catesbeiana, Bufo marinus, Xenopus laevis, and Notopthalmus viridescens. Fluorescence, indicative of T4, was observed in both plasma and erythrocytes (RBC) from all individuals of the five species studied. It was weak and diffuse in the plasma and in the cytoplasm of the RBC but was intense in the nuclei (especially the nuclear perimeter) of these cells. The finding of intracellular T4 suggests that thyroid hormone may be of some physiological importance in adult amphibians.

An analysis of the influence of thyroid hormone on the synthesis of proteins in the tail fin of bullfrog tadpoles

By incubation of explants of tail fin from tadpoles of Rana catesbeiana in a solution of 35S-methionine for 4 h, newly synthesized proteins were labeled isotopically. After separation by two-dimensional polyacrylamide gel electrophoresis, those proteins were visualized by fluorography. Exposure of explants to culture medium containing thyroxine (T4) (150 nM) increased the incorporation of 35S-methionine into several proteins with 48 h. Effects of T4 on the relative abundance of two of these newly synthesized proteins were detected after 8 h of hormonal treatment. Very similar patterns of newly synthesized proteins were observed when proteins from explants of tail fin removed from tadpoles at metamorphic climax and immediately incubated with 35S-methionine were compared with proteins produced in fin derived from premetamorphic animals. These results are interpreted to indicate that both treatment of explants with T4 and elevation of endogenous levels of thyroid hormones during spontaneous metamorphosis increased the relative rates of synthesis of several identical proteins. The potential involvement of those proteins in early phases of metamorphic action which eventually lead to cell death and resorption is discussed.

Elevation of plasma aldosterone levels of tadpoles at metamorphic climax

Plasma aldosterone concentrations in Rana catesbeiana tadpoles during metamorphosis were determined by radioimmunoassay. Aldosterone levels were relatively low (0.7-1.4 ng/ml) during pre- and prometamorphosis. At the onset of climax, aldosterone concentration increased markedly and reached a peak (10.9 ng/ml) at stage XXII. Subsequently, aldosterone levels declined and at the end of metamorphosis (stage XXV), the average concentration was 3.9 ng/ml plasma. Administration of ACTH elevated plasma aldosterone levels in hypophysectomized tadpoles. When hypophysectomized animals were kept in thyroxine (T4), plasma aldosterone concentrations also increased. Combined ACTH and T4 treatment caused a marked increase in plasma aldosterone concentration. The possible involvement of thyroid hormone in the elevation of plasma aldosterone during metamorphosis is discussed.

Metamorphic rate in Rana pipiens larvae treated with thyroxine or prolactin at different times in the light/dark cycle

Premetamorphic Rana pipiens tadpoles at Stages V to VIII on a 12L/12D cycle with photoperiod from 0800 to 2000 hr were treated with 30 micrograms/liter thyroxine (T4) by immersion for various daily 8-hr spans, or by daily intraperitoneal injection at different times with 0.1 to 10 micrograms T4 or 10 micrograms prolactin (PRL), in order to see if the rate of metamorphosis varied with the time of hormone treatment. T4 was most effective in promoting tail resorption and hindlimb growth and development if tadpoles were immersed at least partly in the light or if the hormone was injected late in the dark or in the early or mid light phase. T4 was least effective in accelerating metamorphosis when immersion was totally in the dark or injection early in the dark. PRL antagonized metamorphosis just as well at any time of the day except that tail fin growth was faster when PRL was administered early in the dark. The findings show circadian variations in target tissue responsiveness to exogenous T4 that depend on the occurrence of light during T4 immersion or following T4 injection. The data suggest rhythmicity in the secretion of endogenous T4 in the tadpole and a diurnal effect on T4 utilization by target cells.

Synthesis and storage of prolactin in the pituitary gland of bullfrog tadpoles during metamorphosis

Changes in prolactin (PRL) levels and PRL-producing activities in the pituitary gland of tadpoles during metamorphosis were studied. PRL concentration in the pituitary gland as measured by a homologous radioimmunoassay was relatively low during pre- and prometamorphosis and early climax (2.5-3.5 micrograms/100 micrograms protein). PRL levels increased thereafter and reached maximum (7.3 micrograms/100 micrograms protein) at late climax (stage 24). It was also demonstrated that the pituitary cells which immunologically reacted with the antiserum to bullfrog PRL increased in number as metamorphosis proceeded. Measurement of incorporation of [3H]leucine into pituitary PRL in vitro revealed that elevation of PRL synthesis took place in the tadpoles which had reached climax. Radioimmunoassay and bioassay of PRL in the pituitary glands of adult frogs and tadpoles indicated that there was no difference in immunological and biological properties between adults and larvae.

Changes in plasma thyroxine levels during and after spontaneous metamorphosis in a natural population of the green frog, Rana clamitans

Plasma thyroxine (T4) levels were measured by radioimmunoassay in a spontaneously metamorphosing population of green frog, Rana clamitans, tadpoles. In accordance with species previously studied, a sharp rise in plasma T4 was seen at metamorphic climax, followed by a decline at final transformation. Unlike other species studied thus far, T4 levels were also moderately high in growing tadpoles (stages X-XIII). These early elevated levels may be due to daily rhythmicity in thyroid secretion coupled with the restricted hours of blood collection used in this study.

Plasma T4 and T3 levels in naturally metamorphosing Eurycea bislineata (Amphibia; Plethodontidae)

We measured the circulating T4 and T3 levels in the plethodontid salamander Eurycea bislineata at various stages of metamorphosis using radioimmunoassay (RIA). Seven distinct metamorphic stages were defined based on specific developmental events concerning the remodeling and differentiation of skeletal elements. Special effort was made to study individual variation in the levels of plasma thyroid hormones. For this reason we did not pool serum from several specimens. The RIA was conducted in aliquots of 2 microliters (T4) and 5 microliters (T3), with minimum detectable levels of 100 ng/dl (T4) and 20 ng/dl (T3). In agreement with previous studies on other amphibians, we found metamorphosis in E. bislineata to be accompanied by a sharp increase in the circulating plasma levels of T3 and T4. No hormones were detectable in the larval and adult stages. Our technique allowed for simultaneous measurement of T3 and T4 levels in some individuals. These data indicated that, although both the onset of the production of the two thyroid hormones is simultaneous, T3 remains in the system longer than T4. However, at all metamorphic stages a large proportion of specimens did not exhibit any measurable levels of T3 and/or T4. These results underscore the need to reassess the mode of operation and production of thyroid hormones in amphibian metamorphosis.

Immunofluorescent detection and localization of thyroxine in blood of Rana catesbeiana from early larval through metamorphic stages

Indirect immunofluorescent staining was used to detect and localize thyroxine (T4) in blood smears from individual Rana catesbeiana tadpoles in almost every stage of larval development. Earlier radioimmunoassays revealed a surge of T4 in the blood plasma during metamorphic stages, but plasma T4 concentrations in earlier stages were either very low or below the minimal detectable limits of the assays. With the present immunofluorescent method, T4 was found in plasma of tadpoles throughout the entire larval period from early limb bud stages to the end of metamorphosis. Moreover, T4 was also found in association with cytoplasm and nuclei of red blood cells, particularly nuclei of the new population of adult red cells differentiating during metamorphic stages. In conclusion, thyroid hormone is present in both blood plasma and erythrocytes of R. catesbeiana from early through late stages of larval development.

Tadpole erythrocytes: luminescent properties with dark field microscopy

To characterize and classify erythrocytes of ranid tadpoles, alcohol-fixed blood smears were studied with dark field illumination. All preclimax stage (limb bud-foot) Rana castesbeiana tadpoles from ponds in Massachusetts had red blood cells that were polymorphic. The majority of cells (88%) showed a bright, granular luminescence varying from white to blue-grey, whereas, cytoplasm of the other cells was smooth, black, and nonluminescent. On the other hand, tadpoles in similar stages from other species (Rana clamitans, Mass. and Rana pipens, Vermont) and from R. catesbeiana tadpoles from other locations (Wisconsin and North Carolina) had no observable cytoplasmic luminescence in any of their red blood cells. Moreover, as Mass. R. catesbeiana underwent metamorphic climax their luminescent cells disappeared and were replaced by small, round, dark, nonluminescent cells, precursors of the oval, nonluminescent erythrocytes characteristic of adult frogs. Cells with black nonluminescent cytoplasm generally contained nuclei which were luminescent. In conclusion, two main types of red blood cells-those with and those without cytoplasmic luminescence-are distinguishable with dark field microscopy. Luminescence of the cells varies with species, geographic location, and developmental stage of the tadpoles.

Free T4 and T3 in relation to total hormone, free hormone indices, and protein in plasma of rainbow trout and arctic charr

The percentages of total L-thyroxine (TT4) and total 3,5-3'-triiodo-L-thyronine (TT3) in the free form (%FT4 and %FT3) were measured in plasma from trout or charr by equilibrium dialysis at 12 degrees and by an index employing miniature G-25 Sephadex columns at room temperature. Dialysis estimates of both %FT4 and %FT3 were highly correlated with their respective column indices (rT4 = 0.84; rT3 = 0.92) validating the latter as a rapid and convenient method for monitoring alterations in plasma T4 or T3 binding. Binding depended on both pH and buffer composition. In charr, %FT4 (usual range 0.15-0.28%) exceeded %FT3 (usual range 0.09-0.17%); similar values were obtained for trout. The %FT4 values were much higher and the %FT3 values somewhat lower than those reported for mammals and birds. However, when TT4 and TT3 were taken into account, mean estimated concentrations of charr free T4 (FT4) and free T3 (FT3) were respectively 5.0 and 3.2 fmol/ml. These values are comparable to those in birds and mammals and suggest that FT4 and FT3 correspond more closely between various vertebrates than either TT4 and TT3 or %FT4 and %FT3. Plasma protein levels were correlated inversely with %FT4 (r = -0.54) and with %FT3 (r = -0.81) and correlated directly with TT4 (r = 0.45) and TT3 (r = 0.64). Despite these trends, FT4 and FT3 were highly correlated with their respective total hormone levels (r T4 = 0.95; rT3 = 0.94). Thus, for this charr population at least, TT4 and TT3 are representative of peripheral thyroidal status, as judged by the theoretically more physiologically relevant FT4 and FT3 levels.

The development of ACTH-like substance during tadpole metamorphosis

The interrenals of tadpoles of Rana catesbeiana at various developmental TK stages were frozen, sectioned, and prepared histochemically for assessment of delta 5-3 beta-hydroxysteroid dehydrogenase (delta 5-3 beta-HSD) activity. The interrenals of hypophysectomized (HX) tadpoles from the same batch of eggs as control tadpoles were prepared similarly for delta 5-3 beta-HSD activity following a series of intraperitoneal injections of different concentrations of mammalian ACTH. The histochemically stained sections of control and HX tadpoles were scanned with a computerized microscope spectrophotometer for diformazan absorbance. Two regression equations were obtained: Y = 0.1460 + 0.0015X and Y' = 0.1494 + 0.0893X', where Y or Y' = delta 5-3 beta-HSD activity absorbance (for the control and HX tadpoles, respectively), X = developmental stages of control, and X' = exogenous ACTH dosage administered to the HX tadpoles. Taking absorbance of delta 5-3 beta-HSD activity as the common factor, a curve of concentrations of ACTH-like substance against TK stages was made. The developing concentration of ACTH-like substance was very low at first and then rose during metamorphic climax. The trend of the development of ACTH-like substance was discussed from the view point of thyroid and delta 5-3 beta-HSD activities.

Binding of dexamethasone by hepatic, intestine, and tailfin cytosol in Rana catesbeiana tadpoles during spontaneous and triiodothyronine-induced metamorphosis

The binding of the synthetic glucocorticoid dexamethasone (dex) to Rana catesbeiana tadpole liver, intestine, and tailfin cytosol during both spontaneous and triiodothyronine (T3)-induced metamorphosis has been examined. No change was observed in the dissociation constant (KD) in the liver or intestine during either spontaneous or T3-induced metamorphosis compared with liver and intestine cytosol from the frog. The binding capacity (N) in liver cytosol of premetamorphic tadpoles (14.33 x 10(-14) mol dex/mg protein) was not significantly different from that found during prometamorphosis (stage XVIII) (11.50 x 10(-14) mol dex/mg protein) and in the adult frog (19.24 x 10(-14) mol dex/mg protein). Following the onset of metamorphic climax, however, there were significant reductions in N in liver cytosol, reaching a nadir at stage XXIV (0.38 x 10(-14) mol dex/mg protein). Binding capacity in premetamorphic tadpole intestine (19.60 x 10(-14) mol dex/mg protein) was significantly reduced following premetamorphosis . Values did not return to premetamorphic values in the frog intestine (6.54 x 10(-14) mol dex/mg protein) as occurred in the frog liver, nor were values significantly reduced following the onset of metamorphic climax (10.43 x 10(-14) mol dex/mg protein) when compared with prometamorphosis (11.58 x 10(-14) mol dex/mg protein). The binding capacity in the tailfin cytosol did not deviate from premetamophic tadpole values (11.61 x 10(-14) mol dex/mg protein) through stage XXI (9.68 x 10(-14) mol dex/mg protein), the last stage in which sufficient tissue was available for analysis.

Corticoids augment nuclear binding capacity for triiodothyronine in bullfrog tadpole tail fins

The effect of aldosterone and corticosterone on T3 binding to the nuclear fraction of the tail fin of bullfrog tadpoles was investigated in vitro. Both corticoids (10(-7) - 10(-5) M) added to the incubation medium increased T3 binding in a dose-dependent manner. The aldosterone-induced increase in T3-binding was blocked by cycloheximide, an inhibitor of protein synthesis, and actinomycin D, an inhibitor of RNA synthesis. Scatchard analysis revealed that 10(-6) M aldosterone and corticosterone increased the maximum binding capacity for T3 by 60 and 41%, respectively, and that the corticoids did not alter the value of the dissociation constant. The significance of the finding is discussed in relation to metamorphosis.