Effects of bilateral and unilateral ophthalmectomy on plasma melatonin in Rana tadpoles and froglets under various experimental conditions.

The effect of ophthalmectomy (enucleation) on plasma melatonin in Rana tadpoles and froglets was studied under various experimental conditions to determine if ocular melatonin is released into the circulation from the eyes and to study the factors which might affect this process. Where operations occurred in early or mid-photophase on a 12 light:12 dark (12L:12D) cycle (light onset at 08:00 h), sampling in mid-light and mid-dark revealed that scotophase plasma melatonin was reduced at all developmental stages, with the more significant effects occurring before metamorphic climax. Experiments sampling prometamorphic tadpoles six times in a 24h period on 18L:6D, 12L:12D, or 6L:18D five days after enucleation also showed a significant lowering of plasma melatonin in the dark, so that the scotophase peak was virtually eliminated on all the LD cycles.

Developmental and diel profiles of plasma corticosteroids in the bullfrog, Rana catesbeiana.

Corticosteroids synergize with the thyroid hormone (TH) at late metamorphic stages and might have a role in the hormonal regulation of amphibian metamorphosis. This role could be influenced by diel fluctuations, particularly if the peak of the plasma corticoids changed in relation to the TH peaks. Diel variation in plasma corticosteroids was studied in Rana catesbeiana prometamorphic and climax tadpoles on 18:6, 12:12 and 6:18 light:dark (LD) cycles.

Developmental and diel changes in plasma thyroxine and plasma and ocular melatonin in the larval and juvenile bullfrog, Rana catesbeiana.

Diel variation in plasma thyroxine (T(4)), and plasma and ocular melatonin was studied in Rana catesbeiana tadpoles and postmetamorphic froglets on 12:12 and 6:18 light/dark (LD) regimens. A progressive rise in plasma T(4) initiates metamorphosis while melatonin can modulate metamorphic progress. Changes in the phase of the rhythms of these two hormones during development might influence the hormonal regulation of metamorphosis.