A Theoretical Basis for the Integration of Developmental and Thermoregulatory Roles of Thyroid Hormones in Vertebrata

Abstract

This review assesses the effects of thyroid hormones (T4) on the development and thermal response of ectothermic zebrafish (Danio rerio). Based on our results, the thyroxine hormone (alone or in combination with cortisol) regulates the early maturation of eyes, gut, swim bladder and growth of larval zebrafish. It was apparent that during development, thyroid hormones (T4) induced the proliferation and activation of mitochondria – the primary site for the production of heat and energy. The presence of mitochondria in gut and swim bladder suggest that nutrient uptake is facilitated, ion-transport mechanism is functional, energy expenditure is regulated, and heat is produced. We reviewed that thyroid hormone increases metabolism and regulates thermal responses in zebrafish during cold acclimation. However, we suggest that heat production indices such as mitochondrial proton leak and plasma membrane Na+ conductance should be measured in order to gain more evidence for thyroid hormone regulating the production of heat in ectotherms. On the other hand, it is interesting to note that uncoupling protein 1 (UCP1) – which is responsible for proton leak and heat production, has been detected in teleosts. In mammals, the mitochondria containing UCP 1 are found in brown adipose tissues – the primary site for thermogenesis. Teleost has only white adipose tissues and UCP1 but no brown adipose tissues have been detected. In mice, thyroid hormones were found to regulate conversion of white adipose tissues into brown adipose tissues, however, it is unclear whether this converted tissue can regulate heat production. This finding may provide insight that the appearance of UCP1 in teleosts may have evolved later in birds and mammals, which may have activated the mitochondrial uncoupling mechanism that is responsible for endothermy.