Indexed on: 12 Jan '99Published on: 12 Jan '99Published in: General and Comparative Endocrinology
To explore the role of the thyroid in the control of seasonality, photosensitive female American tree sparrows (Spizella arborea) were thyroidectomized (THX), moved to long days, and given daily injections of thyroxine (T4) for 3 weeks; THX and thyroid-intact (THI) controls received daily injections of alkaline vehicle. Birds were retained on long days 4 additional weeks and then moved to constant light and given T4 in drinking water for 5 weeks in order to test for photorefractoriness. Endpoints were ovarian mass, molt score, and hypothalamic cGnRH-I (chicken gonadotropin-releasing hormone I) content; data were collected as independent measures at intervals of 1 to 5 weeks. THX females given T4 replacement therapy (THXT4 females) exhibited all components of seasonality (i.e., photoperiodic ovarian growth, photorefractoriness, and postnuptial molt), as did THI females. THX females not given replacement T4 were aseasonal: They showed only minor thyroid-independent photoperiodic ovarian growth; they remained photosensitive, despite chronic photostimulation; and they did not initiate postnuptial molt. Collectively, these observations support, and extend to female tree sparrows, our model of seasonality in male tree sparrows, wherein control circuits are programmed for gonadal growth, photorefractoriness, and postnuptial molt by interactive effects of long days and thyroid hormone during the first 3 weeks of photostimulation. The unexpected finding that constant light and/or exogenous T4 extended the simulated breeding seasons of some THI females prompted us to investigate the nature and expression of photorefractoriness. Our approach was to evaluate the same endpoints as before in chronically photostimulated THI females either retained on long days, with or without T4 in drinking water, or moved to constant light, with or without T4. The results showed that exposure to constant light (not T4) near the end of a simulated breeding season can-though usually it does not-temporarily extend the breeding season and prevent the onset of postnuptial molt. It remains unclear whether these perturbations caused by constant light reflect a transitional relative photorefractoriness or merely a delay in the onset of absolute photorefractoriness. In either case, the thyroid- and daylength-dependent programs controlling seasonality in female American tree sparrows have a heretofore undemonstrated potential for limited temporal flexibility.