Thyroid hormone-dependent seasonality in American tree sparrows (Spizella arborea): Effects of GC-1, a thyroid receptor β-selective agonist, and of iopanoic acid, a deiodinase inhibitor

To explore the role of TH in the control of seasonality [i.e., photoperiodic testicular growth, photo refractoriness, and postnuptial (prebasic) molt] in American tree sparrows (Spizella arborea), we performed experiments in which THX males were simultaneously photostimulated and given TH replacement therapy. In the first experiment, equimolar concentrations (1X= 1.3 nmol) of T4, T3, or GC-1, an iodine-free TRβ agonist, were administered s.c. daily during the first 21 days of photostimulation. Two additional THX groups received GC-1 at 0.1X or 10X, and THX and THI control groups received vehicle. In the second experiment, T4 or T3, alone or in combination with the deiodinase inhibitor IOP, was injected i.m. twice daily during the first 14 days of photostimulation. In both experiments, end points were testis length and molt score. In the first experiment, THI birds given vehicle and THX birds given T4 replacement therapy exhibited all three components of seasonality. THX birds given T3 or GC-1 (1X or 10X) showed a subdued photoperiodic testicular response, but they did not become photorefractory or initiate molt. THX birds that received 0.1X GC-1 or vehicle exhibited none of the components of seasonality. These data are consistent with the hypothesis that photoperiodic testicular growth, a vernal component of seasonality, is a TRβ-mediated response and suggest that T4 may activate TRβ more efficiently than does T3 or GC-1. By contrast, the failure both of T3 and of GC-1, but not of T4, to program photostimulated THX males for photorefractoriness and postnuptial molt suggests that autumnal components of seasonality may be TRα-mediated responses solely to T4. In the second experiment, IOP administered alone had no significant impact on seasonality. THX birds that received T4 with or without IOP showed all components of seasonality, whereas birds that received T3 with or without IOP showed only photoperiodic testicular growth. These results challenge the widely held view that T4 is merely a prohormone for T3 and support the emerging view that T4 has intrinsic hormonal activity. Because IOP augmented the photoperiodic testicular response in T3-treated THX birds, T3 may act either independently or codependently with T4 in programming vernal seasonal events.