Influence of Melatonin on the Rate of Rana pipiens Tadpole Metamorphosis In Vivo and Regression of Thyroxine-Treated Tail Tips In Vitro: (tadpole/metamorphosis/thyroxine/melatonin/tissue culture).

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 (15 µg) retarded tail growth and hindlimb development on 18L6D but had no significant effect on 6L18D. During induced metamorphosis (30 µg/liter T4 ), melatonin injections retarded tail resorption on 18L6D and accelerated it on 6L18D, but did not affect the hindlimb. When melatonin was injected during T4 immersion at different times in the photophase on 18L6D (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 vitro induced to resorb by 0.2 µg/ml T4 in Niu-Twitty solution regressed more slowly in the presence of melatonin (10 or 15 µg/ml) than with T4 alone on both 6L18D and 18L6D. 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.