Effect of a light pulse during the dark on photoperiodic regulation of the rate of thyroxine‐induced, spontaneous, and prolactin‐inhibited metamorphosis in Rana pipiens tadpoles

Abstract

Since Rana pipiens tadpoles injected with thyroxine (T4) early in the dark develop more slowly than those injected in the light, we studied the effect of giving a light pulse of 1 hr early in the dark. Tadpoles injected under a 7.5-W red light bulb in a darkened room with 0.2 microgram T4 daily at 2200 hr went through metamorphosis faster on a 12L:3D:1L:8D cycle with a light pulse after injection than on a 12L:12D cycle without a light pulse, and even faster on a 12L:1.5D:1L:9.5D cycle with a light pulse before the injection. Thus a 1-hr light pulse counteracted the metamorphic delay resulting from administration of T4 in the dark, and set in motion the conditions that resulted in a more rapid response to an injection of T4. However, a 1-hr light pulse in the early dark had no effect on growth and development of older or younger untreated tadpoles or those constantly immersed in 30 micrograms/liter T4. Larvae on 21L:3D with T4 injection in the dark and on 12L:3D:1L:8D with T4 injection at 0700 hr just before the start of the main light phase progressed faster than 12L:3D:1L:8D with injection at 2200 hr in the dark before only a 1-hr light pulse. Thus the length of the light phase immediately after T4 injection was significant. There was no difference on 12L:12D and 12L:3D:1L:8D cycles in the effectiveness of daily injections of 10 micrograms prolactin (PRL) in the early dark at 2200 hr in promoting tail growth or antagonizing tail resorption induced by T4 immersion. Under these conditions, PRL utilization did not appear to be inhibited by the light pulse.

Influence of Photoperiod, Daylength, and Feeding Schedule on Tadpole Growth and Development

“…T4 was most effective in promoting metamorphosis when it was injected late in the dark or in the early or mid light phase, or if tadpoles were immersed in it for a daily 8 hr span at least partly in the light (18). Moreover, a light pulse of 1 hr, before or after an injection of T4, counteracted the metamorphic delay resulting from administration of the hormone early in the dark (19), suggesting that light sets in motion the conditions that result in a more rapid response to T4. Light might influence T4 metabolism, binding, transport, or activity within the cell, perhaps through the mediation of melatonin, a pineal hormone secreted in darkness (17) and known to affect the thyroid axis at some point (11).…”

Efeito do fotoperíodo sobre o desenvolvimento de girinos de rã-touro (Rana catesbeiana Shaw, 1802)

“…Trabalhos realizados por Wright et al (1988) demonstraram que a fase longa de luz acelerou a metamorfose em girinos de R. pipiens, com a administração de T 4 (tiroxina). Este resultado difere daquele encontrado no presente estudo, que mostrou ser a fase longa de luz o pior tratamento para este parâmetro.…”

“…Porém, os resultados diferem daqueles encontrados por Eichler (1976), citado por Delgado et al (1987), observando que girinos de R. pipiens mantidos sob luz constante apresentaram tamanhos menores em relação àqueles submetidos ao fotoperíodo 12L:12D. Wright et al (1988) verificaram que a metamorfose ocorreu mais rapidamente em girinos de R. pipiens mantidos na fase longa de luz 12L:3D:1L:8D, em comparação à fase curta 12L:12D, seguida da administração de T 4 . Embora não tenha utilizada a administração de T 4 , observou-se que o regime 12L:12D superou os demais tratamentos quanto à metamorfose.…”

Melatonin accelerates metamorphosis in Xenopus laevis

“…Although Prl is an important growth-promoting factor in amphibians, high exogenous levels of Prl delay tail regression, hind limb growth, and gill regression [Wright et al, 1988;Gray and Janssens, 19901. The effects of exogenous Prl on metamorphosis are specific to the developmental stage at the time of administration [Wright et al, 19941.…”