The photoperiodic control circuit in euthyroid American tree sparrows (Spizella arborea) is already programmed for photorefractoriness by week 4 under long days

Abstract

The aim of this investigation was to determine whether spontaneous testicular regression in male American tree sparrows (Spizella arborea) that were thyroidectomized at week 4 of photostimulation manifests photorefractoriness, as it does in chronically photostimulated euthyroid controls. On the basis of our demonstration that exogenous thyroxine stimulates the gonads only when recipient birds are photosensitive, male tree sparrows were thyroidectomized at week 4 of photostimulation (20 h light:4 h dark) and given thyroxine periodically to assay for photosensitivity. When initiated at weeks 4, 7, 10, 13 and 16 of photostimulation, thyroxine replacement therapy had no effect on testis size until week 16, when most recipient birds showed robust testicular growth. The inductive effect of exogenous thyroxine at week 16 confirms that chronic thyroidectomy dissipates photorefractoriness and simulates the effect of short days. The failure of replacement thyroxine to halt spontaneous testicular regression between week 7 and week 13 establishes that spontaneous testicular regression after thyroidectomy manifests photorefractoriness. Moreover, the failure of replacement thyroxine to induce testicular growth between week 4 and week 7 indicates that by week 4 of photostimulation, at least 3 weeks before photoinduced testicular growth ends, male tree sparrows are programmed for photorefractoriness. This conclusion is strengthened by the finding that thyroidectomy at week 4 of photostimulation does not uncouple photorefractoriness and postnuptial moult, which in euthyroid tree sparrows are tightly linked. In another experiment, photosensitive thyroid-intact tree sparrows were moved from 8 h light:16 h dark to 20 h light:4 h dark and given exogenous thyroxine or vehicle through week 6 of photostimulation. Exogenous thyroxine augmented testicular growth.

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

“…Alternatively, the protracted long-day requirement may indicate that our model merely requires refinement. Thus, long days may have two effects on the vernal component of seasonality: an early, interactive programming effect (with THs or a TH-dependent metabolite or ligand) [12,[14][15][16][17][18]26] and a later, independent permissive effect (current study), which creates a milieu appropriate for full expression of programmed testicular growth. Our assigning a postprogramming permissive role to long days corroborates the conclusion of Harris and Turek [27], namely, that after the testes of thyroid-intact white-throated sparrows have grown to a functional size (cf.…”

“…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].…”

“…In previous experiments [14][15][16]18], we tested this model by withdrawing THs by THX before, at, or after the onset of photostimulation. Here we test it by withdrawing long days at times suggested by studies on thyroid-dependent programming in photostimulated male tree sparrows [15,16] and by Wingfield's [22] observation that a single long day stimulates gonadal growth in song sparrows.…”

“…Here we test it by withdrawing long days at times suggested by studies on thyroid-dependent programming in photostimulated male tree sparrows [15,16] and by Wingfield's [22] observation that a single long day stimulates gonadal growth in song sparrows.…”

Clues from wildlife to create an assay for thyroid system disruption.

“…We still do not know where THs or TH-dependent ligands exert their effect in the photoneuroendocrine system in the birds. Wilson and Reinert (35) suggest that THs interact somewhere between the photoreceptors and neuroendocrine cells in the brain. On the basis of what we do know in mammals, Bentley (33) points out that T 4 mediates nerve growth factor synthesis, which controls microtubule and axonal growth in the central nervous system, and this may in some way affect gonadotropinreleasing hormone secretion (32), which in turn influences LH/FSH secretion (33).…”