Caffeine Exposure During Late Chick Embryogenesis Alters Hatchability and Plasma Thyroid Hormone Levels

Abstract

To evaluate the consequences of exposure to caffeine during the final week of embryogenesis, we treated chick embryos with caffeine (20 μmol/egg) on day 15 and examined them and the hatchlings' behavior. Of the control embryos, more than 85% hatched on day 21, whereas 75% of the caffeine-treated embryos hatched on day 20. The caffeine hatchlings did not have significant malformations. The caffeine hatchlings on day 20 showed a significantly decreased tissue weight (mg) of the telencephalon (391.7±7.5 vs. 451.8±4.6), optic lobe (139.6±2.1 vs. 157.9±4.9), cerebellum (106.9±2.4 vs. 126.8±3), and brain stem (171.4±4.1 vs. 203.1±7.5) compared to the controls on day 21. The weights did not reach the control levels during the experimental period. The hatchlings' behaviors were evaluated using the imprinting ability test (assessing recognition and memory) and social behavior tests (aggregation, belongingness, and vocalization activity). These were designed to examine their willingness to find each other and form a group. None of the behavior tests showed significant differences between the caffeine-treated and the control hatchlings. Marked elevation of the duodenum alkaline phosphatase activity and the plasma triiodothyronine level were observed before hatching during normal development. The caffeine-treated group showed the increase in parallel 1 day earlier. Since thyroid hormone is involved in brain and gut maturation during the final week of embryogenesis, it is suggested that caffeine exposure accelerated the elevation of the hormone, which affects the maturation of brain quality related to imprinting and behavior and necessary gut maturation in hatchlings.