WRIGHT, M.L.*; FRANCISCO, L.L.; VISCONTI, R.F.; RAMAH, B.L.; College of Our Lady of the Elms, Chicopee,MA; College of Our Lady of the Elms, Chicopee,MA; College of Our Lady of the Elms, Chicopee,MA; College of Our Lady of the Elms, Chicopee,MA: Contribution of the eye to circulating melatonin in anuran larvae and froglets.
Although plasma melatonin (ML) originates mainly from the pineal gland in mammals, the retina of the eye, and perhaps other organs, contribute to circulating ML in many nonmammalian vertebrates. Little is known about the secretion sites of plasma ML in the amphibia. The rhythms of plasma ML undergo phase changes during late stages of bullfrog tadpole metamorphosis, and there is a substantial decrease in the plasma level of ML at climax. Ocular ML rhythms are often inverse to those of plasma ML in larvae and froglets, which suggests that the eyes may contribute to circulating ML. Initial opthalmectomy experiments on Rana catesbeiana and Rana pipiens tadpoles on a 12L:12D lighting regimen indicated that nearly all the plasma ML came from the eyes at premetamorphosis, whereas only about 75% was of retinal origin by late prometamorphosis. In postmetamorphic froglets, about 70% of the photophase and 28% of the scotophase ML originated from the eyes. Opthalmectomized climax tadpoles, moreover, had higher plasma T4 than controls. The findings indicate that there are developmental changes in ML-secreting sites during metamorphosis, and suggest that ML inhibits the thyroid even at late metamorphic stages.
