Distribution and reciprocal interactions of 3H-melatonin and 125I-thyroxine in peripheral, neural, and endocrine tissues of bullfrog tadpoles (original) (raw)
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Development, Growth and Differentiation, 1991
Metamorphosis of Rana pipiens tadpoles may be retarded when the light phase of the light/dark (LD) cycle is shortened or when thyroxine (T4) is given in the dark because melatonin peaks during the dark. Injection of premetamorphic tadpoles in spontaneous metamorphosis with melatonin (1 5 pg) retarded tail growth and hindlimb development on 18L:6D but had no significant effect on 6L: 18D. During induced metamorphosis (30 pg/liter T4), melatonin injections retarded tail resorption on 18L : 6D and accelerated it on 6L : 18D, but did not affect the hindlimb. When melatonin was injected during T4 immersion at different times in the photophase on 18L : 6D (L onset 0800 hr), tail regression was retarded by melatonin at 1430 or 2030 hr. At 0830 hr, shrinkage of tail length was accelerated whereas tail height was not affected. Tail tips in vifro induced to resorb by 0.2 pg/ml T4 in Niu-Twitty solution regressed more slowly in the presence of melatonin (10 or 15pg/ml) than with T4 alone on both 6L: 18D and 18L:6D. The findings implicate melatonin in LD cycle effects on tadpole metamorphic rate in vivo, show the importance of the time of melatonin injections, and indicate that melatonin antagonizes the metamorphic action of T4 at the tissue level.
Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 2004
The diel fluctuations in plasma thyroxine (T 4 ) and plasma and ocular melatonin entrain to the light/dark (LD) cycle in the bullfrog tadpole, although the phase of the rhythms changes during development. Previous studies on the rhythmicity of these hormones were conducted under various LD cycles, but with a constant temperature, raising the question of the role of the natural thermocycle in determining the phase of the rhythms, and the changes that occur in the hormone levels and rhythms during late metamorphosis. To study this question, tadpoles were acclimated to simulated natural conditions of 14.5L:9.5D with a corresponding thermocycle in which the thermophase was 28 8C and the cryophase was 18 8C, or to the same thermocycle under constant light (24L). On both photoregimens, the diel fluctuations changed between prometamorphosis and metamorphic climax. However, more statistically significant rhythms, as indicated by the cosinor, occurred on 14.5L:9.5D than on 24L. At climax on the LD cycle, all hormones peaked around the same time in the late scotocryophase, whereas on 24L, plasma T 4 peaked in the thermophase and plasma and ocular melatonin peaks occurred some distance from each other early in the cryophase. The earlier peaks of plasma and ocular melatonin on 24L were due to a transient rise in these hormones at the onset of the cryophase, which was not sustained in the absence of an LD cycle. On 14.5L:9.5D with a corresponding thermocycle, the hormone rhythms had nearly the same phases as was found in previous work on 12L:12D at a constant temperature of 22 8C, allowing for minor phase shifting due to the photocycle differences, indicating that in this species laboratory studies on constant temperature give valid results even in the absence of a thermocycle. The findings show that the phases of the hormone rhythms are determined by the LD cycle although the onset of the cryophase, in the absence of a photocycle, may exert some influence on the nighttime rise in melatonin. The developmental rise in plasma T 4 , and drop in plasma melatonin, occurred on both 14.5L:9.5D and 24L, indicating, taken together with previous work, that these climactic changes were independent of temperature and light cycling. D
Comp Biochem Physiol Pt a, 2004
The diel fluctuations in plasma thyroxine (T4) and plasma and ocular melatonin entrain to the light/dark (LD) cycle in the bullfrog tadpole, although the phase of the rhythms changes during development. Previous studies on the rhythmicity of these hormones were conducted under various LD cycles, but with a constant temperature, raising the question of the role of the natural thermocycle in determining the phase of the rhythms, and the changes that occur in the hormone levels and rhythms during late metamorphosis. To study this question, tadpoles were acclimated to simulated natural conditions of 14.5L:9.5D with a corresponding thermocycle in which the thermophase was 28 °C and the cryophase was 18 °C, or to the same thermocycle under constant light (24L). On both photoregimens, the diel fluctuations changed between prometamorphosis and metamorphic climax. However, more statistically significant rhythms, as indicated by the cosinor, occurred on 14.5L:9.5D than on 24L. At climax on the LD cycle, all hormones peaked around the same time in the late scotocryophase, whereas on 24L, plasma T4 peaked in the thermophase and plasma and ocular melatonin peaks occurred some distance from each other early in the cryophase. The earlier peaks of plasma and ocular melatonin on 24L were due to a transient rise in these hormones at the onset of the cryophase, which was not sustained in the absence of an LD cycle. On 14.5L:9.5D with a corresponding thermocycle, the hormone rhythms had nearly the same phases as was found in previous work on 12L:12D at a constant temperature of 22 °C, allowing for minor phase shifting due to the photocycle differences, indicating that in this species laboratory studies on constant temperature give valid results even in the absence of a thermocycle. The findings show that the phases of the hormone rhythms are determined by the LD cycle although the onset of the cryophase, in the absence of a photocycle, may exert some influence on the nighttime rise in melatonin. The developmental rise in plasma T4, and drop in plasma melatonin, occurred on both 14.5L:9.5D and 24L, indicating, taken together with previous work, that these climactic changes were independent of temperature and light cycling.
General and Comparative Endocrinology, 1996
The thyroid gland controls the progress of metamorphosis, although other hormones influence metamorphic rate, including melatonin, which may coordinate metamorphosis with seasonal and light conditions. Melatonin directly antagonized the action of thyroxine (T 4 ) in promoting regression of tadpole tail tips in vitro, and this study sought to determine if it affects the thyroid axis of tadpoles as well. In an experiment sampling at 8-hr intervals for 24 hr, after melatonin treatment (15 mg/day for 12 days) of premetamorphic Rana pipiens tadpoles at approximately 1100 hr on 18L:6D, thyroid follicle cell height and lumen diameter were lowered by melatonin, but follicle cell proliferation was not significantly depressed. In a second experiment conducted under the same conditions, but sampling at 3-hr intervals for 24 hr, melatonin significantly lowered follicle cell labeling index and suppressed its ultradian (7.6 hr) rhythm, while shifting the peak of follicle lumen diameter to the dark instead of the light. Thus, melatonin tended to depress the thyroid of young tadpoles and suppress or shift its rhythms. Melatonin (10 mg/day for 5 days) injected into prometamorphic Rana catesbeiana tadpoles at 1930 hr on 18L:6D significantly altered subsequent in vitro thyroid function as determined by radioimmunoassay of media collected at intervals for 54 hr from cultured thyroids of injected control and melatonin groups, and a noninjected control group. Melatonin decreased T 4 secre-tion during the first 30 hr, but not during the last 24 hr of culture, suppressed 3,5,38-triiodothyronine (T 3 ) secretion for 12 hr, and then raised T 3 output into the media above the control for the remainder of the culture period, increasing the T 3 :T 4 ratio. Injection alone increased both T 3 and T 4 secretion for the first 30 hr, but did not change the T 3 :T 4 ratio. The findings show that exogenous melatonin administered in vivo significantly modulated thyroid activity and morphometry directly and/or indirectly and comprise the first demonstration of an effect of melatonin on the amphibian thyroid gland. r 1996 Academic Press, Inc.
2013
The origin of circulating melatonin has been studied in Rana catesbeiana tadpoles and Rana pipiens frogs on a 12L:12D cycle. Previous work had indicated that in tadpoles the eyes were a significant source of scotophase plasma melatonin, so we investigated the contribution of the pineal gland in tadpoles and the eyes and the pineal gland in frogs. The findings showed that in both larvae and adults the pineal gland and lateral eyes secrete nearly equal amounts of melatonin into the blood as determined by destruction of the pineal gland and eye extirpation. Pinealectomy and ophthalmectomy together reduce the mid-scotophase blood plasma level to that of photophase, so that the scotophase rise in melatonin is solely due to secretion of the pineal gland and the eyes. These procedures singly or in combination do not change the low mid-photophase level of plasma melatonin. The tadpole digestive tract was studied as a possible source of the photophase, and residual scotophase, plasma melatonin. The gut was found to contain a high level of melatonin, not changed by pineal and eye removal. However, gut extirpation did not change the plasma melatonin level in photophase or scotophase. The findings indicate
Journal of Experimental Zoology, 1989
The effects of several neuropeptides on in vitro thyrotropin (TSH) secretion by pituitaries of the frog Rana catesbeiana were studied at two stages of development. The relative concentration of TSH secreted into the medium was determined by bioassay using in vitro thyroxine (T4) production by thyroids from adult grass frogs, Rana pipiens. Pituitaries from tadpoles in Taylor-Kollros stages 17-19 responded to doses of ovine corticotropin-releasing hormone (CRH) ranging from 10 t o 1,000 ngiml; the same glands did not respond t o doses of thyrotropin-releasing hormone (TRH) between 10 and 1,000 ngiml. A single dose of 1,000 ngiml rat growth hormonereleasing hormone (GHRH) had no effect on TSH release. Pituitaries from postmetamorphic frogs (stage 25) exposed to 1,000 ng/ml ovine CRH secreted concentrations of TSH that were higher on average than controls (four of seven responded), and two of seven responded to 100 ng/ml TRH. These observations raise the possibility for a common central regulator of the thyroid and interrenal axes in metamorphosis in which a CRH-like molecule might act as an important neuroendocrine stimulus for both corticotropin and thyrotropin secretion. Furthermore, these and previous results with several species of adult anurans (R.J. Denver: Gen. Comp. Endocrinol., 72:383-393) suggest that pituitary TSH responsiveness t o TRH develops in frogs after metamorphic climax.
Effect of Melatonin on the Response of the Thyroid to Thyrotropin Stimulationin Vitro
General and Comparative Endocrinology, 1997
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 (T 4 ) action at the tissue level and directly inhibited baseline T 4 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 g/ml) used in the culture of tadpole thyroids stimulated T 4 secretion as much as frog pituitaries, but more than late premetamorphic tadpole pituitaries. There was no significant effect of 1 to 15 g/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 g/ml melatonin tended to depress T 4 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 g/ml TSH, or TSH and 10 or 100 g/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 T 4 was inhibited significantly by 10 g/ml melatonin at only two of the early media collections. In contrast, 100 g/ml melatonin significantly depressed T 4 secretion in response to
Influence of exogenous thyroxine on plasma melatonin in juvenile Atlantic salmon ( Salmo salar
Comparative Biochemistry and Physiology B-biochemistry & Molecular Biology, 2004
One of the most clearly defined endocrine changes during the parr–smolt transformation of anadromous salmonids is an increase in plasma levels of thyroid hormones. The role of pineal hormone melatonin in timing and synchronisation of smoltification is widely discussed. The effect of administration of exogenous thyroxine (T4) on plasma melatonin was investigated in juvenile Atlantic salmon (Salmo salar) at the early stages of parr–smolt transformation. Fish were kept in fresh water under simulated-natural photoperiod and exposed to exogenous T4. Fish were sampled at 12.00 and 24.00 h from treatment and control tanks, 2 and 14 days after treatment started. Plasma melatonin and l-thyroxine were measured using RIA and competitive enzyme immunoassay, respectively. After 2 days of T4 treatment, marked difference in plasma melatonin concentration measured at 12.00 and 24.00 h was still observed in both groups. However, 2-week exposure to T4 caused a reduction in night-time plasma melatonin level and thus, probably, inhibited melatonin related time-keeping system in juvenile salmon. Additional studies are needed to clarify the mechanism of the described phenomenon.
Journal of Comparative Physiology A, 2005
11 Abstract The objective of this work was to study mela-12 tonin receptors in the eye and the brain and their pos-13 sible functionality in the ontogeny of Rana perezi. The 14 binding of 2-[ 125 I]melatonin increases throughout 15 embryonic larval development in both tissues. The most 16 pronounced increase takes place at the end of premeta-17 morphosis and during early prometamorphosis. This 18 rise coincides temporarily with the appearance of the 19 rhythmic melatonin-synthesizing capacity in the retina. 20 In the three studied developmental stages (32G, 40G and 21 49-50G), melatonin-binding sites are coupled to G 22 proteins and become functional. Moreover, melatonin 23 inhibits dopamine (DA) release by the eyecups and brain 24 of R. perezi tadpoles in vitro (stage 40G). Thus, the 25 modulation of DA release could be one mechanism by 26 which melatonin interacts with hormones, like prolactin 27 and thyroxine that are involved in the regulation of 28 anuran development and metamorphosis. Finally, we 29 show that melatonin decreases K + -evoked cAMP con-30 tent in the frog retina in vitro, suggesting that the effect 31 of melatonin on DA release in the frog retina is mediated 32 by the inhibition of this intracellular messenger. 33 Keywords Melatonin receptors AE Tadpole AE 34 Dopamine AE Brain AE Eye Abbreviations: DA: Dopamine AE DHBA: 3,4-Dihydroxybenzylamine AE GTPcS: S,Guanosine 5¢-O-[c-thiotriphosphate] AE [ 125 I]Mel: 2-[ 125 I]iodomelatonin AE