Influence of photoperiod and melatonin administration on growth and metamorphosis in Discoglossus pictus larvae

Exposure to artificial light at night during the larval stage has delayed effects on juvenile corticosterone concentration in American toads, Anaxyrus americanus

Artificial Light At Night (ALAN) is an environmental stressor that can disrupt individual physiology and ecological interactions. Hormones such as corticosterone are often responsible for mediating an organism's response to environmental stressors. We investigated whether ALAN was associated with a corticosterone response and whether it exacerbated the effects of another common stressor, predation. We tested for consumptive, non-consumptive, and physiological effects of ALAN and predator presence (dragonfly larvae) on a widespread amphibian, the American toad (Anaxyrus americanus). We found predators had consumptive (decreased survival) and non-consumptive (decreased growth) effects on larval toads. ALAN did not affect larval toads nor did it interact with the predator treatment to increase larval toad predation. Despite the consumptive and non-consumptive effects of predators, neither predators nor ALAN affected corticosterone concentration in the larval and metamorph life-stages. In contrast to studies in other organisms, we did not find any evidence that suggested ALAN alters predator-prey interactions between dragonfly larvae and toads. However, there was an inverse relationship between corticosterone and survival that was exacerbated by exposure to ALAN when predators were absent. Additionally, larval-stage exposure to ALAN increased corticosterone concentration in juvenile toads. Our results suggest the physiological effects of ALAN may not be demonstrated until later life-stages.

Artificial light at night decreases metamorphic duration and juvenile growth in a widespread amphibian

Artificial light at night (ALAN) affects over 20% of the earth's surface and is estimated to increase 6% per year. Most studies of ALAN have focused on a single mechanism or life stage. We tested for indirect and direct ALAN effects that occurred by altering American toads' (Anaxyrus americanus) ecological interactions or by altering toad development and growth, respectively. We conducted an experiment over two life stages using outdoor mesocosms and indoor terraria. In the first phase, the presence of ALAN reduced metamorphic duration and periphyton biomass. The effects of ALAN appeared to be mediated through direct effects on toad development, and we found no evidence for indirect effects of ALAN acting through altered ecological interactions or colonization. In the second phase, post-metamorphic toad growth was reduced by 15% in the ALAN treatment. Juvenile-stage ALAN also affected toad activity: in natural light, toads retreated into leaf litter at night whereas ALAN toads did not change behaviour. Carry-over effects of ALAN were also present; juvenile toads that had been exposed to larval ALAN exhibited marginally increased activity. In this time frame and system, our experiments suggested ALAN's effects act primarily through direct effects, rather than indirect effects, and can persist across life stages.

Some like it hot? Developmental differences in Yellow-bellied Toad (Bombina variegata) tadpoles from geographically close but different habitats

The key for the long-term survival of species is their potential to respond to changing conditions. These reactions are usually species-specific and may vary between populations. The Yellow-bellied Toad (Bombina variegata (L., 1758)) occurs in forested and open areas. We wanted to know whether tadpoles react plastically to different environmental conditions, and if so, whether reaction norms are species, population, or season specific. In a common garden experiment, we compared developmental traits (developmental time, size, body condition) of metamorphs from different habitats (forest vs. quarry) in close geographic proximity. Tadpoles from both habitats grew up under shaded and sunny conditions. The experiments were run during early and late breeding season. We detected different developmental strategies between populations, concerning treatments and season on a microgeographic scale. Tadpoles with quarry origin developed faster and reached larger body sizes, at the expense of lower body condition. Major risks affecting tadpole’s survival in the open habitat are high temperatures and high desiccation. Forest tadpoles were comparatively smaller in size, but showed higher plasticity and higher body condition. Under changing climatic conditions, quarry population may reach temperatures above their thermal limits. In contrast, forest conditions may mitigate increasing temperatures. Forest populations could be better adapted to future climate change.

Expression of green fluorescent protein in insect larvae and its application for heterologous protein production

Many eukaryotic proteins have been successfully expressed in insect cells infected with a recombinant baculovirus derived from the Autographa californica nuclear polyhedrosis virus (AcNPV). There are, however, disadvantages with this cell-based system when carried out in suspension cultures at high bioreactor volume (e.g., limited oxygen transfer, susceptibility to contamination, high cost). These problems can be avoided by using whole larvae as the "reactors." There are, however, other problems encountered with larvae, one being their inaccessibility for product sampling. To combat this problem, we have investigated the expression of green fluorescent protein (GFP) as a reporter molecule in Trichoplusia ni insect larvae. A high production level of GFPuv (1.58 mg per larva, 26% of total protein) was obtained, enabling the rapid and non-invasive monitoring of GFP. Bright green light was emitted directly from the large opaque carcasses ( approximately 30mm) after illumination with UV light. Based on the green light intensity and a correlation between intensity and GFP mass, we determined the optimal harvest time (c.a. approximately 3 days post-infection). In parallel experiments, we expressed human interleukin-2 (IL-2) from another recombinant baculovirus with an almost identical expression profile. Since both GFP and IL-2 were rapidly degraded by protease activity during the fourth day post-infection (another disadvantage with larvae), we found an accurate determination of harvest time was critical. Correspondingly, our results demonstrated that GFP was an effective on-line marker for expression of heterologous protein in insect larvae.

Ontogeny of central melatonin receptors in tadpoles of the anuran Rana perezi: modulation of dopamine release

The objective of this work was to study melatonin receptors in the eye and the brain and their possible functionality in the ontogeny of Rana perezi. The binding of 2-[(125)I]melatonin increases throughout embryonic larval development in both tissues. The most pronounced increase takes place at the end of premetamorphosis and during early prometamorphosis. This rise coincides temporarily with the appearance of the rhythmic melatonin-synthesizing capacity in the retina. In the three studied developmental stages (32G, 40G and 49-50G), melatonin-binding sites are coupled to G proteins and become functional. Moreover, melatonin inhibits dopamine (DA) release by the eyecups and brain of R. perezi tadpoles in vitro (stage 40G). Thus, the modulation of DA release could be one mechanism by which melatonin interacts with hormones, like prolactin and thyroxine that are involved in the regulation of anuran development and metamorphosis. Finally, we show that melatonin decreases K(+)-evoked cAMP content in the frog retina in vitro, suggesting that the effect of melatonin on DA release in the frog retina is mediated by the inhibition of this intracellular messenger.

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).

Melatonin, diel rhythms, and metamorphosis in anuran amphibians

Melatonin is present in picogram quantities in the plasma of anuran tadpoles, although the origin of circulating melatonin is not known. Melatonin may have a role in metamorphosis because it is a thyroid antagonist, whose level falls at the metamorphic climax when the thyroid hormones peak. Melatonin rhythms in plasma and eyes are entrained to the light/dark (LD) cycle and affected by temperature. Consequently, melatonin could transduce environmental information to regulate endocrine periodicity and larval circadian organization and influence metamorphic rate. Contradictory results of exogenous melatonin treatment may be largely due to a regulation of the plasma melatonin level which rapidly clears high melatonin concentrations and which can even result in lower circulating melatonin than in controls. Excess exogenous melatonin concentrates in tissues and glands, although the fall in melatonin at metamorphic climax does not occur by this mechanism. There may be thyroxine (T(4))-melatonin interactions at the tissue level that affect metamorphic progress. The rhythms of T(4) and the corticosteroids are also entrained to the LD cycle, and these rhythms, and those of melatonin, change during development, in a specific way on each LD cycle. Differences in the direction and magnitude of phase shifts during development place the peaks of thyroid modulators, such as the corticosteroids and melatonin, in different relationships to the T(4) peaks, which could be an important aspect of the hormonal regulation of metamorphosis.

The decrease in plasma melatonin at metamorphic climax in Rana catesbeiana (bullfrog) tadpoles is induced by thyroxine

Melatonin decreases in the plasma of Rana catesbeiana (bullfrog) tadpoles at the climax of metamorphosis when the thyroxine (T(4)) level peaks. Since melatonin inhibited thyroid function in vitro, it would be of interest to determine if the decline in plasma melatonin permits greater thyroid hormone secretion, or if the increasing levels of T(4) cause the climactic decrease in plasma melatonin. The reciprocal effects of administering T(4) or melatonin just prior to metamorphic climax were examined in tadpoles kept at 22 degrees C on an 18L:6D cycle. If melatonin functions as a thyroid antagonist at later metamorphic stages, administration of melatonin should decrease plasma T(4), whereas if T(4) causes the decline in plasma melatonin, T(4) treatment of tadpoles prior to climax should induce the climactic melatonin decrease prematurely. Once daily injection of 40 microg melatonin for 5 days at 19.30 h had no effect on metamorphic progress, or on plasma T(4) or melatonin levels, except for a transient rise in melatonin just after the injection. Immersion in 2.2x10(-4) M melatonin for 6 days accelerated metamorphosis and decreased plasma melatonin, but had no effect on plasma T(4). Administration of T(4) by injection of 0.2 microg, or immersion in a 6.3x10(-8) M solution accelerated metamorphosis more than melatonin immersion, raised plasma T(4) to climax levels, and induced a decrease in plasma melatonin. We conclude that rapid clearance of exogenous melatonin from the circulation in these experiments did not allow it to affect plasma T(4), and that there is clear evidence that the rise in T(4) induces the climax decrease in plasma melatonin. The finding that immersion in a high level of melatonin can lower plasma melatonin and accelerate metamorphosis, whereas a single daily injection does not, provides an explanation for some of the contradictory reports in the literature concerning melatonin's effect on tadpole metamorphic progress.

Direct influence of melatonin on the thyroid and comparison with prolactin

Melatonin administered in vivo had previously been shown to inhibit thyroid cell proliferation and subsequent in vitro thyroxine (T(4)) secretion in anuran tadpoles. Melatonin in vitro also directly reduced the sensitivity of the thyroid to thyrotropin (TSH). The present work sought to determine whether melatonin directly affected baseline, unstimulated T(4) secretion, and to compare its effect with that of prolactin (PRL). Thyroids from larval Rana catesbeiana or adult Rana pipiens were incubated in control or melatonin (0.01 to 100 microg/ml) media. Melatonin directly inhibited T(4) secretion by thyroids from both tadpoles and frogs at all concentrations of melatonin used and at both prometamorphic and climax tadpole stages. PRL, used in vitro at 10 microg/ml, did not influence the response of the thyroid to TSH (0.2 microg/ml) in young tadpoles, or the baseline secretion of T(4) by thyroids at any stage of larval life except climax, when T(4) secretion was significantly decreased by the third day of culture. Thus although both melatonin and PRL have been shown to antagonize the action of T(4) in vitro, and to decrease metamorphic rate, melatonin is a much more effective thyroid gland inhibitor than PRL.

Melatonin accelerates metamorphosis in Xenopus laevis

Delayed metamorphosis associated with large body size has been observed in Woodhousei fowleri tadpoles reared in continuous dark (DD). To evaluate the mechanism by which DD delayed metamorphosis, light-cycle exposure was controlled and thyroxine (T4), melatonin, or drugs that alter prolactin (Prl) concentrations were given to Xenopus laevis tadpoles. It was hypothesized that exogenous melatonin would delay metamorphosis and increase body size, and that elevation of Prl concentrations would have effects similar to melatonin exposure. Xenopus laevis tadpoles were randomized to three light conditions [light/dark (LD, 12 h/12 h), DD, and continuous light (LL)] and subgroups in each light condition were treated with T4, melatonin, bromocriptine (Bro), haloperidol (Hal), or no drug. Each subgroup included 12 tadpoles. Drugs were administered in the water either continuously or daily from 07.00 to 19.00 h (Intermittent). Measurements of total length, leg length, and stage of metamorphosis were obtained at regular intervals. DD resulted in delayed metamorphosis, while LL did not. T4 accelerated metamorphosis as expected, countering the delaying effects of DD. In contrast to the hypothesis, melatonin accelerated metamorphosis and impaired body size compared to controls. Intermittent Hal also accelerated metamorphosis, while Bro delayed it. In DD, both T4 and melatonin led to increased tadpole size in contrast to their counterparts in LD or LL. Delayed metamorphosis in DD is not caused by increased melatonin production. Melatonin and Hal (as given in this study) accelerate metamorphosis. Melatonin acceleration of metamorphosis may occur through alteration of the concentration of prolactin.

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.

The effects of photoperiod and different dosages of melatonin on metamorphic rate and weight gain in Xenopus laevis tadpoles

This study examined the effects of photoperiod and three different dosages of melatonin on the rate of metamorphosis and weight gain in Xenopus laevis. Exposure of larvae to 23L:1D resulted in lower mean body weight and a retarded metamorphic rate in comparison to larvae exposed to 1L:23D. Larvae reared in either photoperiod and treated with exogenous melatonin demonstrated a dose-dependent suppression of weight attained, with short photoperiod larvae showing a more dramatic effect. Analysis of growth patterns indicate that photoperiod and exogenous melatonin have a greater effect on weights prior to Nieuwkoop and Faber stage 56 in the development of Xenopus. Larvae exposed to 1L:23D and exogenous melatonin metamorphosed at an accelerated rate when compared to larvae exposed to 23L:1D and the same dosages of melatonin. In both photoperiod regimens an exogenous melatonin concentration of 45 micrograms/100 ml resulted in an accelerated metamorphic rate, whereas 225 and 450 micrograms/100 ml retarded metamorphic rate of larvae in comparison to controls.

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.

Effects of melatonin on gonadal steroids and glucose plasma levels in frogs (Rana perezi and Rana temporaria)

The effect of melatonin treatment on the Gonosomatic Index (GSI), ovarian germinal epithelium, plasma estradiol and testosterone levels was studied in Rana perezi females in December. No significant changes were observed in GSI, estradiol, and testosterone levels in melatonin treated animals when compared with saline injected controls, but the percentage of previtellogenic follicles decreased in animals treated with melatonin (100 micrograms). The effect of melatonin treatment on glucose level was studied in Rana perezi females in December and Rana temporaria males in February. In Rana perezi no significant differences were observed between melatonin treated and control animals; however, significant reductions by melatonin treatment were obtained in Rana temporaria. The possibility that the inhibitory effect of melatonin can be observed only when gonadal function and metabolism are stimulated by temperature is discussed.

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.