SALVANTE, K.G.*; WILLIAMS, T.D.: Hormonal control of reproduction: Effects of corticosterone on timing of laying, egg size, clutch size, and yolk precursor levels

Egg formation involves major adjustments in lipid metabolism, including an upregulation of total lipoprotein synthesis, and production of a specific yolk-targeted very-low density lipoprotein (VLDL), and vitellogenin (VTG). Corticosterone (B) is known to be involved in regulating many aspects of lipid metabolism in non-reproductive animals, including increased fat deposition in the liver and adipose tissue and hyperglycemia. We therefore investigated whether B is involved in regulating or modulating lipoprotein production and metabolism during yolk formation, and the effect of B treatment on reproductive output (egg size, clutch size, and timing of laying), in zebra finches (Taeniopygia guttata). Egg-laying females had significantly lower stress-induced plasma B levels compared with non-breeders (8.8 vs. 18.1 ng/ml), although there was no difference in basal B levels (3.2 vs. 3.3 ng/ml). Egg-laying females were treated with exogenous corticosterone using silastic implants, and this elevated plasma B to high physiological levels (23.5 � 7.8 ng/ml). B-treated females were less likely to lay eggs and also had a longer laying interval compared with control females. Effects of B on egg size and clutch size, as well as on plasma levels of the two yolk precursors, VTG and VLDL, will also be described.