Influence of Melatonin on the Rate of Rana pipiens Tadpole Metamorphosis In Vivo and Regression of Thyroxine‐Treated Tail Tips In Vitro

Abstract

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.

Modulation of cell cycle phases by various circadian and noncircadian light/dark schedules and evidence implicating melatonin

“…Through such mechanisms, no doubt, the LD regimen exerts influence on the rate of tadpole metamorphosis, which differs on each of the schedules studied (Wright et al, 1988a;Wright et al, 1990a), so that transformation is coordinated with external conditions favorable to survival of the juvenile frog. In this connection, it is relevant that the thyroid hormones induce metamorphosis and it has been recently shown in vitro that melatonin directly antagonizes the effect of thyroxine at the peripheral level (Wright et al, 1991). The present work also showed, in a vertebrate animal, the specific phases of the cell cycle affected by a particular alteration in the LD schedule.…”

Melatonin and thermoregulation in ectothermic vertebrates: a review

“…These antigonadal effects, in combination with MEL's seasonal rhythm of secretion, are thought to modulate and synchronize reproductive behavior with environmental cues (Underwood 1981(Underwood , 1985aRismiller and Heldmaier 1987;Crews et al 1988;Mendonça et al 1995). Various other actions of MEL include its role in regulating circadian activity rhythms, melanosome aggregation, thyroid function and activity, and free-radical scavenging (e.g., Wright et al 1991;Krotewicz et al 1992;Krotewicz and Lewinski 1994;Cagnoli et al 1995;Reiter et al 1995;Hadley 1996;Reiter 1996;Wright et al 1996;Hyde and Underwood 2000). MEL can also modulate an animal's sensitivity to temperature extremes by reducing thermal tolerance (Erskine and Hutchison 1982).…”