Thyroid hormone acts centrally to programme photostimulated male american tree sparrows (Spizella arborea) for vernal and autumnal components of seasonality.

Thyroid hormone and long days interact to programme American tree sparrows (Spizella arborea) for seasonality (i.e. thyroid hormone-dependent photoperiodic gonadal growth, photorefractoriness, and postnuptial moult). This study explored in radiothyroidectomized (THX) males given thyroid hormone replacement therapy whether thyroid hormone acts within the brain and, additionally, the identity of the putative tissue-active thyroid hormone. The minimum dose (30 ng) of L-thyroxine (T4) that restored all components of seasonality when given i.c.v. daily during the first 21 days of photostimulation restored no component of seasonality when given s.c. The same dose of L-triiodothyronine (T3) also was ineffective when administered s.c., but restored photoperiodic testicular growth (though neither photorefractoriness nor postnuptial moult) when admiministered i.c.v. Three of seven birds given a 10-fold lower dose of T4 (3 ng) exhibited thyroid hormone-dependent photoperiodic testicular growth, albeit damped. The other four birds given 3 ng T4 and all birds given 3 ng T3 responded like THX controls, exhibiting only slight thyroid hormone-independent photoperiodic testicular growth. The highest dose (300 ng) of T3 restored all components of seasonality only when administered i.c.v. daily during the first 49 days of photostimulation. This demonstration in American tree sparrows is the first in any species that the thyroid-dependent transition from the breeding season to the non-breeding season can be effected by T3. The same dose of reverse T3 administered daily over the same 49 days restored photoperiodic testicular growth in only half of 10 subjects and photorefractoriness and moult in none. Collectively, the data support the hypothesis that thyroid hormone acts centrally to programme photostimulated male American tree sparrows for all components of seasonality. The most parsimonious interpretation of the data, including the threshold-like effect of 3 ng T4, favours T4 as the tissue-active thyroid hormone for vernal as well as autumnal events, but does not entirely exclude T3.