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

Abstract

Premetamorphic Rana pipiens tadpoles at Stages V to VIII on a 12L/12D cycle with photoperiod from 0800 to 2000 hr were treated with 30 μg/liter thyroxine (T 4) by immersion for various daily 8-hr spans, or by daily intraperitoneal injection at different times with 0.1 to 10 μg T 4 or 10μg prolactin (PRL), in order to see if the rate of metamorphosis varied with the time of hormone treatment. T 4 was most effective in promoting tail resorption and hindlimb growth and development if tadpoles were immersed at least partly in the light or if the hormone was injected late in the dark or in the early or mid light phase. T 4 was least effective in accelerating metamorphosis when immersion was totally in the dark or injection early in the dark. PRL antagonized metamorphosis just as well at any time of the day except that tail fin growth was faster when PRL was administered early in the dark. The findings show circadian variations in target tissue responsiveness to exogenous T 4 that depend on the occurrence of light during T 4 immersion or following T 4 injection. The data suggest rhythmicity in the secretion of endogenous T 4 in the tadpole and a diurnal effect on T 4 utilization by target cells.