T3- and T4-induced rates of metamorphosis in immature and sexually mature larvae of Ambystoma tigrinum (Amphibia: Caudata)

Thyroxine-induced metamorphosis in the axolotl (Ambystoma mexicanum)

The axolotl (Ambystoma mexicanum) has remained an important model for regeneration and developmental biology for over a century. Although axolotls in captive-bred colonies usually exist in an aquatic form, they retain the ability to undergo metamorphosis following exposure to thyroid hormone. Here we present a robust method for inducing metamorphosis in adult axolotls that results in high survivability and produces terrestrial animals that can be maintained in long-term captivity.

Pedomorphosis revisited: thyroid hormone receptors are functional in Necturus maculosus

Heterochrony, a difference in developmental timing, is a central concept in modern evolutionary biology. An example is pedomorphosis, retention of juvenile characteristics in sexually mature adults, a phenomenon largely represented in salamanders. The mudpuppy (Necturus maculosus) is an obligate pedomorphic amphibian, never undergoing metamorphosis. Thyroid hormone induces tissue transformation in metamorphosing species and this action is mediated by nuclear thyroid hormone (TH) receptors (TRs). The absence of metamorphosis in Necturus has been attributed to a resistance to TH action as treatment with exogenous TH fails to induce transformation. The failure to metamorphose could be due to the lack of TR expression in target tissues, or to a loss of TR function. Toward understanding the molecular basis for the failure of Necturus tissues to respond to TH, and the ultimate cause for the expression of the obligate pedomorphic life history, we characterized the structure, function, and expression of TR genes in Necturus. Strikingly, we found that Necturus TRalpha and TRbeta genes encode fully functional TR proteins. These TRs bind both DNA and TH and can transactivate target genes in response to TH. Both TRalpha and TRbeta are expressed in various tissues. TH treatment in vivo induced expression in the gill of some but not all genes known to be activated by TH in anuran larvae, caused whole organism metabolic effects, but induced no external morphological changes in adults or larvae. Thus, Necturus possesses fully functional TRs and its tissues are not generally resistant to the actions of TH. Rather, the absence of metamorphosis may be due to the loss of TH-dependent control of key genes required for tissue transformation.

Effects of steroids and dioxin (2,3,7,8-TCDD) on the developing wolffian ducts of the tiger salamander (Ambystoma tigrinum)

This study was undertaken to investigate effects of the prototypical dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on steroid-dependent development of the wolffian ducts of an amphibian, the tiger salamander (Ambystoma tigrinum). Larvae with immature gonads and undeveloped mullerian ducts were injected with the steroid hormones estradiol (E2), dihydrotestosterone (DHT), or vehicle alone. Additionally, steroid-treated and vehicle-control larvae were immersed in sub-lethal solutions of technical grade TCDD (0, 0.0003, 0.003, 0.03, 0.3, and 3.0 microg TCDD/L). Both steroid treatments stimulated hypertrophy of the wolffian duct epithelium and an increase in mean epithelial cell size. Only DHT treatment stimulated epithelial cell proliferation. TCDD stimulated wolffian duct hypertrophy through an increase in mean epithelial cell size. TCDD acted as an androgen agonist on wolffian duct epithelial area and epithelial cell size. TCDD had no effect on wolffian duct epithelium among E2-injected animals. Stimulatory effects on cell size were observed at 0.0003 microg/L TCDD in saline-injected animals and at 0.003 microg/L TCDD in DHT-injected animals. Both E2 and DHT stimulated growth of the wolffian ducts early in development. Technical grade TCDD alone mimics E2 and DHT action but exhibits an androgen-agonistic action in the presence of exogenously administered DHT. Implications of possible interactions between TCDD and xenosteroids are discussed.

Acceleration of Ambystoma tigrinum metamorphosis by corticotropin-releasing hormone

Previous work of others and ours has shown that corticotropin-releasing hormone (CRH) is a positive stimulus for thyroid and interrenal hormone secretion in amphibian larvae and that activation of CRH neurons may mediate environmental effects on the timing of metamorphosis. These studies have investigated CRH actions in anurans (frogs and toads), whereas there is currently no information regarding the actions of CRH on metamorphosis of urodeles (salamanders and newts). We tested the hypothesis that CRH can accelerate metamorphosis of tiger salamander (Ambystoma tigrinum) larvae. We injected tiger salamander larvae with ovine CRH (oCRH; 1 microg/day; i.p.) and monitored effects on metamorphosis by measuring the rate of gill resorption. oCRH-injected larvae completed metamorphosis earlier than saline-injected larvae. There was no significant difference between uninjected and saline-injected larvae. Mean time to reach 50% reduction in initial gill length was 6.9 days for oCRH-injected animals, 11.9 days for saline-injected animals, and 14.1 days for uninjected controls. At the conclusion of the experiment (day 15), all oCRH-injected animals had completed metamorphosis, whereas by day 15, only 50% of saline-injected animals and 33% of uninjected animals had metamorphosed. Our results show that exogenous oCRH can accelerate metamorphosis in urodele larvae as it does in anurans. These findings suggest that the neuroendocrine mechanisms controlling metamorphosis are evolutionarily conserved across amphibian taxa.

A correlation of thyroid hormone receptor gene expression with amphibian metamorphosis

The expression of the thyroid hormone (TH) receptor genes alpha (TR alpha) and beta (TR beta) in Xenopus laevis begins after the embryo hatches. The TR alpha mRNA increases throughout the premetamorphosis stage of tadpole development, is maximal by prometamorphosis, and falls after climax of metamorphosis to a lower level in frogs. The TR beta mRNA is barely detectable during premetamorphosis. In synchrony with the onset of endogenous TH synthesis by the thyroid gland (prometamorphosis), the level of TR beta mRNA rises in parallel with endogenous TH, reaching a peak at the climax of metamorphosis (stage 61) and drops to approximately 10% of its peak level after metamorphosis. As suggested by this correlation, exogenous TH up-regulates TR beta mRNA as much as 20-fold during premetamorphosis, whereas TH up-regulates TR alpha mRNA by approximately 2-fold during the same period. Up-regulation of TR beta mRNA is the earliest response to exogenous TH by competent tadpoles yet detected.

Temperature, thyroxine, and induced metamorphosis in tadpoles of a primitive frog, Ascaphus truei

To understand the environmental factors and endocrine mechanisms which regulate the extended period (2 to 4 years) of larval development in Ascaphus, the effect of temperature and thyroxine (T4 by immersion) on metamorphic changes of the tail, limb buds, operculum, anal fold, oral disc, and teeth were studied in premetamorphic and prometamorphic tadpoles. (1) Control tadpoles did not undergo metamorphic change at any of the temperatures tested. (2) When tadpoles were kept in thyroxine solutions at 5 degrees, there were no changes in developmental morphology, and tadpoles kept at 18 and 20 degrees showed significant metamorphic changes in 5 to 6 days, but there was a high mortality at the higher T4 concentrations. The optimum temperature is about 10 degrees, and significant changes were completed in 3 weeks. (3) The premetamorphic tadpoles were less sensitive to T4 than prometamorphic tadpoles. The most responsive tissues are teeth and operculum, and the tail tissue shows the lowest sensitivity. (4) Duration of exposure to hormone and concentration also affect the growth or regression of larval tissue.

Seasonal variations in spermatogenesis, testicular weights, vasa deferentia, and androgen levels in neotenic male tiger salamanders, Ambystoma tigrinum

Significant seasonal changes were observed in spermatogenesis, testicular weights, vasa deferentia, and cloacal development as well as in plasma androgen levels (testosterone and dihydrotestosterone). Androgen levels were highest in the fall and lowest during the period of spermatogenesis. Low levels of androgens were also observed during the breeding season. Diameter of the vasa deferentia corresponded to presence of stored spermatozoa and not to plasma androgen levels. Testicular weights were independent of androgen levels and stage of spermatogenesis. Maximal testicular weight occurred at the onset of spermiation. Hypertrophy of the epithelial lining of the vasa deferentia and the condition of the cloacal gland complex were correlated closely to plasma androgen levels. Comparison of spermatogenetic events, androgen-dependent sex accessory structures, and plasma androgen levels suggests either differential release of pituitary gonadotropins on a seasonal basis or variable testicular responsiveness to gonadotropins.

Difference of the in vivo responsiveness to thyrotropin stimulation between the neotenic and metamorphosed axolotl, Ambystoma mexicanum: failure of prolactin to block the thyrotropin-induced thyroxine release

Basal and TSH-induced plasma concentrations of T4 have been measured by radioimmunoassay in the neotenic and metamorphosed male axolotl Ambystoma mexicanum both before and after an ovine prolactin pretreatment. All injections are made into the vena abdominalis. Basal levels of T4 are low in neotenes (85 +/- 19 pg/ml) and somewhat higher in metamorphosed Ambystoma (171 +/- 39 pg/ml), but are increased during metamorphosis (1094 +/- 138 pg/ml). Following injection of 5 mU bovine TSH circulating levels of T4 are raised about 4 times in neotenes, but more than 50 times in metamorphose animals. Three intravenous injections, each of 640 mU prolactin and given, respectively, 24 and 13 hr before and simultaneously with 5 mU TSH, do not inhibit the TSH-induced release in both experimental groups. In the metamorphosed Ambystoma again a more than 50-fold T4 increase is present, whereas in neotenes a 10-fold TSH-induced T4 release is seen, which is more pronounced than before the prolactin treatment. It is concluded that in A. mexicanum ovine prolactin does not block a TSH-induced T4 release and that any antagonistic action with thyroid hormones is not mediated through the thyroid gland.

Role of thyroid hormones in the normal and glucocorticosteroid hormone-induced evolution of carbamoyl-phosphate synthase (ammonia) activity in axolotl liver

1. In axolotl liver, the activity of carbamoyl-phosphate synthase (ammonia), expressed per mg liver protein, decreases to a minimum at 5 months of age, then increases to a maximum at 8 months of age which is followed by a decrease again. The initial decrease between 3 and 5 months of age appears to be largely due to an increase in non-carbamoyl-phosphate synthase protein and the following increase between 5 and 8 months of age to a relative increase of carbamoyl-phosphate synthase protein. 2. Treatment of the animals with triiodothyronine causes an increase in carbamoyl-phosphate synthase activity, the extent of which is dependent upon hormone concentration and age of the animal. After 8 months of age no increase of enzyme occurs upon thyroid hormone treatment, although metamorphosis occurs. 3. Glucocorticosteroid hormones stimulate carbamoyl-phosphate synthase activity 2-to 3-fold in animals older than 6 months. However, in animals younger than 6 months, low concentrations of thyroid hormone, insufficient to induce metamorphosis, are necessary as permissive agents. 4. The stimulatory effects of high concentrations of thyroid hormones (T3) on carbamoyl-phosphate synthase appear to be mediated via a stimulatory effect on glucocorticosteroid biosynthesis. 5. The natural rise in enzyme activity between 5 and 8 months of age seems to be due to a rise in the concentration of circulating glucocorticosteroid hormones.