Diurnal and seasonal variations in thyroid function of neotenic tiger salamanders (Ambystoma tigrinum)

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).

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.

Daily changes in thyroid activity in the frog Rana perezi: variation with season

Plasma triiodothyronine (T3) and thyroxine (T4) levels, as well as thyroid free (f) and bound (b) thyroid hormones (TH) content, were determined by radioimmunoassay (RIA) in adult Rana perezi frogs during a 24 h cycle in winter, spring, summer, and autumn. Significant daily changes in plasma T3 levels were present in all the seasons except for winter, being the lowest values observed during the scotophase. In contrast, plasma T4 only showed significant changes in spring, following a similar pattern to the one described for T3. Thyroid fT3 content did present day/night significant changes only in spring showing high contents at early scotophase. Mean fT4 content was higher at the beginning of light phase than during the rest of daily photocycle in spring and autumn, but significant differences appeared only in autumn. Regarding the thyroid bound content of TH, bT3, and bT4 presented significant daily changes in spring and autumn. However, different profiles were observed in these two seasons. High bound contents were found at early photo- and scotophase with lower values at late dark phase in spring, whereas higher contents were detected at this time in autumn. The present results indicate the existence of seasonally changing daily fluctuations in thyroid activity in Rana perezi and it seems that an interaction between photoperiod and temperature plays a role in the regulation of these daily changes.

Thyroxine and triiodothyronine in plasma and thyroids of the neotenic and metamorphosed axolotl Ambystoma mexicanum: influence of TRH injections

Circulating levels of T3 and T4, as well as T3 and T4 content of the thyroid glands were measured by radioimmunoassay in the neotenic and metamorphosed axolotl Ambystoma mexicanum. In the two experiments which were performed plasma T4 concentrations were more elevated in metamorphosed axolotls, especially in the first experiment (2.12 +/- 0.40 ng/ml vs. 369 +/- 30 pg/ml). T3 plasma values which were only estimated in the second experiment were about five times higher in metamorphosed animals (63.2 +/- 7.4 pg/ml vs. 12.5 +/- 0.8 pg/ml). Also the thyroid hormone content of the glands was higher after metamorphosis. Nevertheless the neotenic gland still contained considerable amounts of T3 (14.7 +/- 1.8 ng and 48.3 +/- 4.8 ng/thyroid, respectively, in the first and second experiment) and T4 (530 +/- 61 ng; 2173 +/- 291 ng/thyroid). Because of the higher T3/T4 ratio found in the plasma compared to the thyroid gland, it was suggested that circulating T3 may be derived partly from peripheral T4 conversion, mainly after metamorphosis. An intravenous injection of 10 micrograms synthetic TRH was able to induce a very significant increase of the plasma T4 concentration (which was maintained during 24 hr) in the metamorphosed axolotls of the first experiment, however, not in those of the second experiment nor in the neotenic animals. Following an injection of 10 mU bovine TSH (first experiment) circulating levels of T4 were raised in both groups. The opposing TRH results could be related with the different control levels of T4 in the two experiments. However, the results indicate that TRH is capable of functioning as a possible thyrotropin-releasing factor in the metamorphosed axolotl.

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.

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.

Persistence of a circadian rhythmicity for thyroid hormones in plasma and thyroid of hibernating male Rana ridibunda

The presence and circadian rhythmicity of thyroid hormones was studied in plasma and the thyroid gland of male Rana ridibunda before and during hibernation. Hibernating January frogs do have a lower T3 and T4 content of their thyroid gland whereas plasma levels of T3 are maintained and of T4 increased compared to fed September or October frogs. It seems likely that the increased photoperiod in January will be responsible for this increased T4 secretion, since controlled laboratory experiments performed in December did not reveal any influence of low temperature on circulating T3 or T4 levels. Also feeding does not influence circulating levels and thyroid content of thyroid hormones in frogs kept at room temperature during the month of January. A circadian rhythmicity of T3 and T4 in the thyroid gland is present in fed October frogs and in non fed December frogs acclimated at 5 degrees C for 12 days with an acrophase for T3 at approximately 1500 h and for T4 at around 1900 h, whereas in plasma only T3 does have circadian variations (acrophase about midnight) but not T4. When December frogs are acclimated to room temperature for 12 days, frogs are active again, but do not eat and have a lower body weight than frogs hibernating at 5 degrees C. Their T3 content of the thyroid gland has disappeared, but T4 thyroid content and plasma levels of T3 and T4 are maintained. As in hibernating frogs, no circadian variations in T4 plasma concentrations are present whereas the circadian thyroid T4 rhythm disappears. At the same time a dampening in rhythmicity for plasma T3 as judged by the significantly lower amplitude occurs. It is concluded that the persistence of circulating levels of thyroid hormones and of a circadian cyclicity for T3 in plasma in non feeding hibernating frogs may reflect the special metabolic state e.g. availability of food reserves in these animals.

Seasonal and daily plasma corticosterone rhythms in American toads, Bufo americanus

Concentrations of corticosterone were measured in the plasma of American toads, Bufo americanus, on a seasonal basis using a radioimmunoassay technique. Two populations of toads, maintained under different light conditions, were monitored to observe the effects of photoperiod on the seasonal rhythm of plasma corticosterone. Under a natural photoperiod toads demonstrated a rhythm consisting of a spring peak and a fall peak in corticosterone concentration. Toads maintained under a 12L:12D photoperiod all year round demonstrated a similar rhythm with peaks in the spring and fall. This suggests that an endogenous (circannual) rhythm of corticosterone may be playing an important role in the seasonal change of overt behavior and physiology of Bufo americanus. A daily rhythm of corticosterone was also detected in toads when blood samples were taken every 4 hr. When compared to a previously published circadian rhythm study of locomotor activity, the surge in corticosterone concentration for the day occurred at 1730 just prior to the peak in locomotor activity.