A One-Time Injection of Thyroxine Programmed Seasonal Reproduction and Postnuptial Molt in Chronically Thyroidectomized Male American Tree Sparrows Spizella arborea Exposed to Long Days

Thyroid hormones and seasonal reproductive neuroendocrine interactions

“…First, exogenous treatment with thyroid hormones has been shown to mimic the effects of long-day conditions. Additionally, thyroid gland removal stops the development of photorefractoriness (i.e., insensitivity to previous stimulatory day-length) in birds and if carried out during the refractory period leads to its spontaneous dissipation (Follett & Nicholls 1988, Goldsmith & Nicholls 1992, Wilson & Reinert 1995, Bentley et al 1997. Most of the molecular analyses described below have focused upon the involvement of thyroid hormones in long-day induced gonadotropin secretion and it is still unclear exactly how the thyroid is involved in refractoriness (refer to Dawson 1993, Dawson et al 2001.…”

Effect of Withdrawing Long Days from Male American Tree Sparrows (Spizella Arborea): Implications for Understanding Thyroid-Dependent Programming of Seasonal Reproduction and Postnuptial Molt1

“…On the other hand, THX at or before the onset of photostimulation markedly impedes photoinduced testicular growth. Taken together, the time-and day length-dependent effects of THX indicate that maximum testis size is a thyroid-dependent characteristic that is programmed during the first week of photostimulation [12,14,16,18]. Thereafter, the tacit program plays out, even in the absence of the thyroid.…”

“…Our studies on obligately photoperiodic American tree sparrows (Spizella arborea) lead us to conclude that THs and long days interact to program males for the vernal component of seasonality, as well as the autumnal component [12,[14][15][16][17][18]. By performing thyroidectomy (THX) before, at, or after the onset of photostimulation, we have demonstrated that programming for testicular growth occurs during the first week of photostimulation [16,18] and is independent of programming for photorefractoriness and postnuptial molt, which occurs between Weeks 1 and 3 [15,16,18].…”

“…These results provide strong support for the concept that vernal and autumnal events are independently controlled [1]. Furthermore, we have shown that thyroxine (T 4 ), administered s.c. to thyroidectomized male tree sparrows either as a one-time injection (100 ,ug) on the first day of photostimulation [12] or as 14 daily injections (1 or 10 Rg each) during the first 2 wk of photostimulation [17], later restores all three components of seasonality in the proper sequence. These findings, generated against a backdrop of protracted long-day stimulation, argue against a permissive role for T 4 (i.e., an essential, nonregulatory presence [9,19]) in the control of seasonality, a role that others [9,10,20,21] have suggested.…”

“…Among the birds and mammals that have been studied, thyroid hormones (THs) are required for the transition from the breeding season to the nonbreeding season [4][5][6][7][8][9][10][11][12][13]. Our studies on obligately photoperiodic American tree sparrows (Spizella arborea) lead us to conclude that THs and long days interact to program males for the vernal component of seasonality, as well as the autumnal component [12,[14][15][16][17][18].…”