Influence of cortisol on the larval bullfrog thyroid axis in vitro and in vivo and on plasma and ocular melatonin.

Adrenal (interrenal) steroids have an important role in amphibian development, antagonizing the metamorphic changes induced by the thyroid at first and then synergizing with the thyroid hormones as their level rises during metamorphosis. Because most of the studies of corticoids at metamorphosis have focused on peripheral tissues, we investigated the effect of cortisol (hydrocortisone; HC) in vitro and in vivo on the thyroid of Rana catesbeiana (bullfrog) tadpoles on 12:12 light/dark (LD) cycles. Plasma and ocular melatonin, which is altered by changes in thyroxine (T(4)) levels, were also assayed in some experiments. Thyroids from premetamorphic tadpoles secreted less T(4) into culture media when incubated with 10 micrograms/ml HC and 0.2 micrograms/ml ovine thyrotropin (TSH) than with TSH alone and when cultured in the absence of TSH following 5 days of 10-micrograms HC injections, indicating that HC inhibited the thyroid at young stages. The effect of 10 micrograms/ml HC at older stages was investigated by culturing thyroids and pituitaries separately on the first day in control or HC media and then incubating the thyroids on the second day in homologous pituitary-conditioned media as a bioassay for pituitary TSH. HC had no effect on baseline T(4) secretion by the thyroids of prometamorphic or climax tadpoles on the first day but increased T(4) secretion over the control on the second day. Thyroids cultured with TSH and HC showed no increase in T(4) secretion over the control TSH group on the second day, indicating that, in the previous experiments, HC had enhanced pituitary secretion of TSH, rather than the response of the thyroid to TSH. In vivo, 5 days of injections of 10 micrograms HC increased plasma T(4) at prometamorphosis and decreased it at climax. There was no marked effect of HC on plasma or ocular melatonin levels. The findings showed that the nature of the effect of HC on the thyroid axis changes during metamorphosis from inhibition at early stages to a positive influence at prometamorphosis and finally to a negative effect on the T(4) level in the plasma at climax.