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

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).

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.

Effect of melatonin on the response of the thyroid to thyrotropin stimulation in vitro

Thyroidal-melatonin interactions are of particular importance to amphibian development since the thyroid controls the progress of metamorphosis while melatonin may coordinate its rate with prevailing environmental conditions. Melatonin antagonized thyroxine (T4) action at the tissue level and directly inhibited baseline T4 secretion in culture, so the present work sought to determine if it antagonized the response of the thyroid to thyroid stimulating hormone (TSH) as well. A preliminary experiment showed that, in Rana pipiens, the concentration of TSH (0.2 microg/ml) used in the culture of tadpole thyroids stimulated T4 secretion as much as frog pituitaries, but more than late premetamorphic tadpole pituitaries. There was no significant effect of 1 to 15 microg/ml melatonin in TSH-containing thyroid cultures of various Rana species of tadpoles and frogs in experiments with media collected once every 24 or 48 hr, although 15 microg/ml melatonin tended to depress T4 secretion. In a final experiment, a higher melatonin concentration was used as well as more frequent media collections. Thyroids from prometamorphic Rana catesbeiana tadpoles were cultured in L-15 media with periodic stimulation by 0.2 microg/ml TSH, or TSH and 10 or 100 microg/ml melatonin. Media were collected at the end of two 3-hr TSH pulses, and every 8 hr thereafter for the next 3 days. Melatonin was administered until the end of Day 2 while TSH was given only on Day 2 in addition to the original 3-hr pulses. The secretion of T4 was inhibited significantly by 10 microg/ml melatonin at only two of the early media collections. In contrast, 100 micro;g/ml melatonin significantly depressed T4 secretion in response to TSH at all but one interval and completely inhibited the thyroidal response to TSH reintroduced into the media on Day 2. The findings suggest that a high concentration of melatonin is inhibitory to the thyroidal response to TSH, but that lower concentrations do not significantly overcome the TSH stimulus.

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.

DNA synthesis is unaffected but subsequent cell division is delayed in tadpole hindlimb epidermis when thyroxine is given in the dark

In Rana pipiens tadpoles, thyroxine (T4) injection in the early light promotes faster hindlimb growth and development than injection early in the dark. T4 also stimulates a significant increase in cell proliferation in the basal epidermal cells of the limb. Using autoradiography, we studied the timing of the T4-induced rise in the labeling and mitotic indices on 12L:12D cycles with 0.2 micrograms T4 injection early in the light or dark and on a 12L:3D:1L:8D cycle with a light pulse early in the dark shortly after a T4 injection. In all instances, the labeling index peaked at the same time after the start of T4, so diurnal differences in the binding or initial actions of T4 leading up to the entrance of epidermal cells into the DNA synthetic (S) phase of the cell cycle are not indicated. However, with T4 injection in the dark the subsequent mitotic index peak was delayed, to a greater extent on 12L:12D than on 12L:3D:1L:8D. Cells induced to proliferate following T4 injection in the dark evidently had longer cell cycles, probably at the expense of the S or G2 phase, than when T4 was given in the light.

Metamorphic rate as a function of the light/dark cycle in Rana pipiens larvae

1. The rate of development of Rana pipiens tadpoles in spontaneous and thyroxine (T4)-induced metamorphosis was studied on light/dark (LD) cycles in which the photophase was held constant while the scotophase was progressively extended or vice versa. 2. Metamorphic rate fluctuated in both types of experiments as the LD cycle lengthened. However, the pattern of resonance differed with the length of the photophase. For example, with an 8 hr light phase, development rate slowed and then increased as the cycle was extended from 24 to to 36 hr, whereas with a 12 hr photophase the reverse took place. 3. The findings are compatible with the occurrence of a rhythm of light sensitivity in photoperiodic time measurement in this amphibian. 4. From the viewpoint of hormonal mechanisms, it is suggested that photoperiodic effects on metamorphic rate result from different patterns of melatonin secretion under the various LD cycles, since melatonin can modify the action of T4 in metamorphosis.

Effect of a light pulse during the dark on photoperiodic regulation of the rate of thyroxine-induced, spontaneous, and prolactin-inhibited metamorphosis in Rana pipiens tadpoles

Since Rana pipiens tadpoles injected with thyroxine (T4) early in the dark develop more slowly than those injected in the light, we studied the effect of giving a light pulse of 1 hr early in the dark. Tadpoles injected under a 7.5-W red light bulb in a darkened room with 0.2 microgram T4 daily at 2200 hr went through metamorphosis faster on a 12L:3D:1L:8D cycle with a light pulse after injection than on a 12L:12D cycle without a light pulse, and even faster on a 12L:1.5D:1L:9.5D cycle with a light pulse before the injection. Thus a 1-hr light pulse counteracted the metamorphic delay resulting from administration of T4 in the dark, and set in motion the conditions that resulted in a more rapid response to an injection of T4. However, a 1-hr light pulse in the early dark had no effect on growth and development of older or younger untreated tadpoles or those constantly immersed in 30 micrograms/liter T4. Larvae on 21L:3D with T4 injection in the dark and on 12L:3D:1L:8D with T4 injection at 0700 hr just before the start of the main light phase progressed faster than 12L:3D:1L:8D with injection at 2200 hr in the dark before only a 1-hr light pulse. Thus the length of the light phase immediately after T4 injection was significant. There was no difference on 12L:12D and 12L:3D:1L:8D cycles in the effectiveness of daily injections of 10 micrograms prolactin (PRL) in the early dark at 2200 hr in promoting tail growth or antagonizing tail resorption induced by T4 immersion. Under these conditions, PRL utilization did not appear to be inhibited by the light pulse.