Acute effect of thyroid hormones in mimicking photoperiodically induced release of gonadotropins in Japanese quail

Summary

Photoperiodic induction occurs in Japanese quail after exposure to a single long day and this leads to a wave of pituitary LH secretion which lasts for up to 10 days. Pharmacological doses of thyroid hormones mimic this photoperiodic response if given to quail on short days, the magnitude and duration of the rise in LH and FSH output being dose-dependent. Thyroxine (T₄) is some 7 times more potent than tri-iodothyronine (T₃). There is no effect of T₄ on LH secretion in quail already on long days although such birds can increase LH output markedly if treated with Gn-RH. Testosterone prevents the initial rise in LH secretion following T₄ but does not block the long-term effect, suggesting that T₄ acts high in the photoneuroendocrine chain to mimic long days. The first rise in LH secretion following T₄ injection takes place about 24 h after the injection and the time-scale of secretion is quite similar to that seen when quail are exposed to a long day. T₄ elicits a rise in LH secretion even if the quail are maintained in darkness. However, T₄ does not act simply as ‘light’ for if it is given at the exact time when birds are in a photoinducible state (i.e. 12–16 h after dawn) the rise in LH secretion still occurs 24 h later.