Thyroid involvement in photoperiodically induced testicular and body weight cycles of the black-headed bunting,Emberiza melanocephala

Abstract

In the black-headed bunting, Emberiza melanocephala, thyroidectomy suppressed the amplitude of light-induced testicular growth and accelerated the onset of photorefractoriness. Extirpation of the thyroid glands during progressive phase inhibited further testicular growth and led to immediate testicular regression when performed during peak phase of a photostimulated cycle. The inhibitory influence of thyroidectomy on light-induced testicular growth was fully reversed by daily im injection of L-T4 at a dose level of 2 μg/bird/day. Neither thyroidectomy nor L-T4 had any effect on the regressing testes of thyroidectomized buntings. Body weight which was about 26 g increased to about 37 g within 15 days of photostimulation by 20L/4D. Removal of thyroid glands suppressed light-induced increase in body weight and resulted in premature decline. The inhibitory influence of thyroidectomy on body weight depended upon lipid reserves at the time of operation, an effect which could be reversed by daily im treatments of L-T4 at dose level 2 μg/bird/day. It is suggested that in the black-headed buntings thyroid hormones are required for sustaining light-induced increase in the testes and body weight and may not necessarily be involved in the development of photorefractory states—gonadal and metabolic. © 1992 Wiley-Liss, Inc.