Premetamorphic effects of thyroid hormones on tadpole sensory ganglia

Tadpoles at premetamorphic stages of development were used to compare the precocious responses of lumbar dorsal root ganglia (DRG) and hindlimb bud (tissues destined for growth) with the responses of tail DRG and tail muscle (tissues destined for resorption) following exogenous administration of triiodothyronine (T3) and thyroxine (T4). Responses to intraperitoneal (i.p.) hormone treatment were assessed at varying times by injection of [3H]leucine i.p. and determination of 3H-labeled protein in tissues after an additional 1.5 hr. Incorporation of [3H]leucine in lumbar DRG and hindlimb bud was markedly stimulated by either hormone. T3 and T4 effects were both maximal at 0.3 nmol/g body wt although, as examined in lumbar DRG, the response to T4 was more rapid and of lesser magnitude than that to T3. By contrast, incorporation in tail DRG and tail muscle was significantly depressed in response to T3 and was unaffected by T4. Co-injection of T3 and T4 (either 1:1 or 1:6 as occurs during the peak surge of circulating thyroid hormones during metamorphic climax) did not produce an additive effect; the hindlimb bud response was reduced while the lumbar DRG, tail DRG and tail muscle responses to the individual hormones were virtually eliminated. The present data suggest that the responses of lumbar and tail sensory neurons to thyroid hormones parallel the responses of their peripheral target tissues.