Thyroxine treatment induces changes in hypothalamic gonadotrophin-releasing hormone characteristic of photorefractoriness in starlings (Sturnus vulgaris).

Photosensitive intact male starlings were transferred from short days (8L:16D) to 11L:13D for 16 weeks, and were therefore sexually mature. Experimental groups were (i) held under 11L:13D and given exogenous thyroxine dissolved in the drinking water for 6 weeks or (ii) given thyroxine for 6 weeks and then transferred from 11L:13D to long days (18L:6D) for a further 6 weeks, while control groups were transferred to long days (18L:6D) either (iii) for 6 or (iv) for 12 weeks, or were (v) maintained under 11L:13D throughout. Changes in testicular size and plumage molt were monitored at regular intervals during the 12-week period. At the end of the experiment, the birds were killed and hypothalamic gonadotropin-releasing hormone (Gn-RH) content and testicular mass were measured. Treatment with exogenous thyroxine caused rapid testicular regression followed by plumage molt, and after 6 weeks hypothalamic Gn-RH content was much reduced, to an even greater extent than that in control birds exposed to long days for 6 weeks. After 6 weeks of thyroxine treatment, withdrawal of exogenous thyroxine and exposure to long days for a further 6 weeks caused no increase in testicular size, and caused a further reduction in hypothalamic Gn-RH content to a level similar to that in controls after 12 weeks of exposure to long days. The results confirm previous findings that thyroxine induces a state of photorefractoriness in sexually mature starlings and show for the first time that the treatment mimics the effect of long days in reducing Gn-RH content in the hypothalamus.