Plasma prolactin (PRL) concentration increases markedly at late metamorphic climax (stage XXIV of Taylor and Kollros (1946)) in Rana catesbeiana. Two series of experiments were performed in order to elucidate the mechanisms involved in this phenomenon. First, thyroidectomized tadpoles (stages XVI-XVIII) were kept in tap water with or without thyroxine (T4). Tadpoles in tap water did not develop beyond stage XX and showed no sign of hypersecretion of PRL by the time T4-treated tadpoles reached stage XXIV. In thyroidectomized tadpoles treated with T4, plasma PRL levels showed a conspicuous rise at stage XXIV. Second, the pituitary stalk was transected, or the adenohypophysis was ectopically autografted at early climax (stage XXII). These tadpoles metamorphosed as intact controls did, but their plasma PRL levels remained low. Treatment of operated tadpoles with T4 failed to elevate plasma PRL levels. It is concluded that hypothalamic stimulation is necessary for the elevation of plasma PRL levels at the late climax stage and that the stimulatory system develops in the hypothalamus under the influence of thyroid hormone.
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