EFFECTS OF THYROXINE (T-4) OR TRIIODOTHYRONINE (T-3) REPLACEMENT THERAPY ON THE PROGRAMMING OF SEASONAL REPRODUCTION AND POSTNUPTIAL MOLT IN THYROIDECTOMIZED MALE AMERICAN TREE SPARROWS (SPIZELLA-ARBOREA) EXPOSED TO LONG DAYS

This study tested the hypothesis that T-3 (triiodothyronine) is the ti ssue-active ''seasonality'' hormone by determining whether T-3 could m imic T-4 (thyroxine) and program photostimulated thyroidectomized (THX ) male American tree sparrows (Spizella arborea) for three components of seasonality (i.e., full-blown testicular growth, photorefractorines s, and postnuptial molt). Photosensitive males were radiothyroidectomi zed, transferred to long days 4 weeks later, and administered 14 daily injections (sc) of alkaline saline (V) containing 0.1, 1, or 10 mu g T-4 or T-3. THX and thyroid-intact (THI) controls received only V. Aft er 5 additional weeks on long days, all birds were tested for photosen sitivity/photorefractoriness. Periodically during the experiment, prim ary flight feathers were scored for molt, and testis length was monito red by laparotomy. As an independent measure of reproductive (i.e., ph otosensitive vs. photorefractory) state, hypothalami collected at the end of the experiment were assayed for cGnRH-I (chicken gonadotropin-r eleasing hormone I) content. Like THI controls, THX males administered 1 or 10 mu g T-4 exhibited full-blown testicular growth and then regr ession, initiated molt, and had low hypothalamic cGnRH-I, indicating t hat photostimulated birds that received mid-or high-dose T-4 replaceme nt therapy had been programmed for all three components of seasonality . On the other hand, both THX controls and THX males administered low- dose (0.1 mu g) T-3 replacement therapy exhibited only modest testicul ar growth, signifying that neither group had been programmed for any c omponent of seasonality. By contrast, photostimulated THX males that r eceived 0.1 mu g T-4, Or 1 Or 10 mu g T-3, were programmed for testicu lar growth, but not for photorefractoriness or molt. Collectively, the se results show that subcutaneously administered T-3 mimicked T-4 impe rfectly and suggest either that T-3 does not program photostimulated m ale tree sparrows for photorefractoriness and postnuptial molt, or tha t T-3 does not cross the blood-brain barrier as efficiently as does T- 4. (C) 1997 Wiley-Liss, Inc.