Improved behavioural flexibility in chick by thyroid hormone

Professor Koichi Homma, Faculty of Pharmaceutical Sciences, Teikyo University, Japan Abstract

Filial imprinting in birds is the process of forming a social attachment during a sensitive or critical period, restricted to the first few days after hatching. Newly hatched chicks follow the first conspicuous moving object and learn the color and shape of the object and become attached to it. We are investigating the molecular mechanism of the sensitive period using chicks as a model to memorize the plastic bricks for their mother. We showed that the thyroid hormone determines the start of the sensitive period. Imprinting training causes rapid inflow of thyroid hormone (T3) into brain, converted from circulating plasma thyroxine. The hormone thus initiates and extends the sensitive period to last more than 1 week. Even in non-imprinted chicks whose sensitive period has ended, exogenous T3 enables imprinting. It can also confer “Memory Priming” (MP). Once chicks have achieved MP, it is maintained for long periods, driving subsequent other learning. MP endowed by imprinting or exogenous T3 clearly enhanced behavioral flexibility in task-switching and reversal learning paradigms in which chicks adjust to the switch or reversal of task contingencies. This suggests that avian species may develop highly cognitive functions via MP in order to adapt to environmental changes. It is possible that the sensitive period for learning closes only if MP is not conferred at an appropriate time of development. Our study elucidates the critical role of imprinting to subsequent learning as being governed by the acute action of thyroid hormone.

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