Thyroxine and Seasonality in American Tree Sparrows

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The common observation that thyroidectomy blocks the transition from the breeding season to the nonbreeding season has led to the concept that thyroid hormone plays a key role in the termination of seasonal reproduction. Dr. Wilson's conceptual model, based on extensive studies on the obligately photoperiodic American tree sparrow (Spizella arborea), envisions a more pervasive role of thyroid hormone in seasonality and includes thyroid hormone-dependent control of gonadal growth and postnuptial molt as well as thyroid hormone-dependent control of gonadal regression. Recent data from his laboratory support the hypothesis that thyroxine (is the tissue-active thyroid hormone that) acts in the brain to program photostimulated American tree sparrows for seasonality. Thyroxine-dependent programming occurs early during photostimulation and resembles an organizational-like process in that thyroxine is not later required for expression of seasonality. Knowing where thyroxine acts in the brain is essential for advancing understanding of the physiological basis of seasonality. Using an acute model system, wherein a single injection of thyroxine on the first day of photostimulation programs thyrodectomized male American tree sparrows for seasonality, coupled with immunocytochemical and autoradiographic techniques, he expects to reveal potential "programmable" cells. Revealing such cells will provide a starting point for exploring the neural networks and/or glial connections that facilitate thyroxine-dependent programming of seasonality and for studying the cellular mechanism of thyroid hormone action.