Thyroid Gland in Regulation of Annual Reproduction and Oxidative Metabolism of a Tropical Bird, Perdicula Asiatica:Role of Melatonin and Environmental Factors

As a transducer of the environmental factors the pineal gland, together with the metabolically active thyroid gland, plays a major role in control of reproduction during different times of the year, in response to the changing environmental conditions of the tropical zone unlike in the temperate zone. Our avian model P. asiatica is a long day breeder. It is reproductively active during summer and quiescent during winter months. In this study we investigated the role of thyroid gland super-imposed by melatonin in the regulation of the annual male reproduction in this bird. The metabolically active thyroid gland presented a functional parallelism with the testicular activity suggesting that thyroid hormone is essential for reproductive activity and related metabolic energy for avian species. Our data also suggest an inhibitory effect of melatonin on thyroid gland function [weight, thyroxine (T3/T4) level and thymidine kinase activity, THK] both during active and inactive phases of the reproductive cycle. However, this inhibitory effect was less evident during the active phase. The low level of melatonin during the reproductively active phase might be due to long days of summer which prevented the birds from being hyperthyroidic. Therefore, we suggest that the level of melatonin serves as a physiological check to control the seasonal reproductive activities of gonads and thyroid which synergistically play most important physiological roles in energy metabolism of these seasonally breeding bird P. asiatica.