Testicular growth in American tree sparrows occurs in spring in response to increasing daylength. By early summer, male tree sparrows become unresponsive to long days (i.e., they become photorefractory), and their testes regress. This project explores the physiological mechanisms that turn seasonal breeding on and off. The first phase tests the hypotheses that (1) a thyroid hormone (T4) simulates longs days (and thyroidectomy, short days), and (2) for photorefractoriness to be induced, T4 must be available when male tree sparrows experience long days. The second phase examines the hypothesis that brain receptors for excitatory amino acids (e.g., L-glutamate) transmit processes photic information (from brain photoreceptors, the biological clock, and/or elsewhere) to neurons that control the secretion of reproductive hormones from the pituitary gland. The ultimate goal of the project is to strengthen and extend understanding of the physiological basis of seasonal reproduction.//
